IAEA-126

BASIC STRUCTURAL DESIGN PHILOSOPHY,CRITERIA AND SAFETY

OF CONCRETE REACTOR PRESSURE VESSELS

REPORT OF A PANELSPONSORED BY THE

INTERNATIONAL ATOMIC ENERGY AGENCYAND HELD IN VIENNA,9-13 FEBRUARY 1970

A TECHNICAL REPORT PUBLISHED BY THEINTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970

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FOFW0HD

A panel on"Baeic Structural Design Philosophy Criteria andSafety of Concrets Reactor Pressure Vessels" was held by theInternational Atomic Energy Agency on 9 to 13 February 19?0at Agency Headquarters. A total of 34 specialists representing14 countries and two international organizations participatedin the discussions.

Since the first two prestrsssed concrete reactor pressurevessels at Mar-coule, Prance, were built in 195^» the technologyof concrete pressure vessels for nuclear power reactor applicationhas-developed rapidly and there are now 15 vessels of this type inoperation or under construction» In addition, there are eightconcrete reactor pressure vessels known to be in the planningstage in the United Kingdom, France, U.S.A. and Federal Republic

è '

of Germany. It is also known that several other countrieshave already started very extensive studies and research forusing prestreesed concrete reactor pressure vessels.

Although the problem of vessel availability is not yetcritical for the PWH and BWH systems, it is clear that thepotential for continued growth in unit rating will ultimatelybe limited "by shop-fabricated vessels and that, at that time,either field fabrication of steel vessels or prestreesed concretevessels will bs required. Of these two alternatives, "based onexperience in the United Kingdom end France, and taken into considera-tion, integrated nuclear power station's concepts, prestreesedconcrete vessels now appear to offer excellent possibilities.

In view of the growing significance of this type of vesselin relation to the future of nuclear pov/er plants, the Agency-has organized a Panel on this subject with the"purpose ofreviewing the latest pertinent deveJ opme'nts, ' to facilitate theexchange of informations and, mainly, to discuss the variousaspacts of the design philosophy criteria,'economics and safetyof prestressed concrete pressure vessels as '#ell as to formulategeneral guidelines concerning the -objectives.

It is hoped that this collection of nine papers, togetherwith the conclusions and recommendations which were worked outby the Panel Meeting, will be of interest to reactor designersand to the authorities concerned with the safe working ofnuclear pressure vessels. The texts of these papers have beensupplied by the Panel members and no editing has "been done by theAgency.

The Agency is grateful to the authors of papers, to all theparticipants of the Panel for their contributions to the discussionsand, it would particularly like to ejtpress its thanks to -theChairman of the Panel, Mr. I. Bavidson, of the United KingdomAtomic Energy Authority, for his guidance of the discussionsin the most productive manner.

CONTENTS

. > General;. Analysis for Servies Load Conditions» Limit Design. Ul t i mat e Be s i gn, Subjects of Particular Interest for Further Research or

Dcvelopmewt Study

". . Structural Design Philosophy and Criteria for Concrete HeaetorVessels ~ U.S. Practice 17W. RcckenhauserContribution from the United Kingdom 53I. Davidson

. > Work on Reactor Pressure Vessels of Prestressed Concrete inYugoslavia (>1B. PetrovicProblems and Perspectives of Prestressed Concrete Pressui'eVessels - Franch Experiance up to 1970 o9D. Costes

;3, Teat Stand for the Prestressed Concrete Vessels Containingthe Keliusi Loop 85L.K. Komoli

•J. Ultimate Design, Experience fronn Small Dimension Models Testing 99P. Scotto

V. Report on the Starting of a Coordinated Work Programme forProstresssecl Concrete Reactor Preesxire Vessels in the FederalRepublic of Germany 115T , Jae ger

•'•', Design Philosophy and Criteria of Safety of Prestressed ConcretePressure Vessels ~ Practice in Czechoslovakia 179M. IDavidSwedish Development Work on Prestreased Concrete Pressure Vesselsfor Water Reactors 213S.

J-''22̂ _»i* Fro^r'i:ftne 243'L3I;2S~JZ> List of P":.ot:i cip'inhs £47

Sumaaary Report ^and Recoamendatiojggof. the Panel Meeting on/"Basic.Structural fiesiign PhirlQsophy_Criteria and Safety of Concrete Heaotor Pressure _Vgssels"

I. Genejral

1.1. Following the discussions of tra Panel which met in Vienna from9 to 13 February 1970 the present, report was prepared' with a viewto setting out the general requirements and principles whichappear to be applicable in this field. Attention is' drawn to theadvantage of farther work in certain areas, and recommendationsare made regarding further international collaboration. Becausethis is a rapidly developing field of study,, the present reportmust "be regarded as provisional.

1.2. Reactor vessels perfora the function of containing the nuolearreactor, the primary coolant, which is normally under pressure,and various othsr cbsponants and equipment essential to theoperation of the reactor. The vessel must perform this duty for

*its design life under all noriaal and foreseeable abnormal conditionsof operation, with a degree of reliability such as to precludeany -unacceptable risk to the public.

1.3. The vessel is loaded by the primary coolant pressure and by theeffect of temperature-induced strains in the various structuralcomponents. The basic principle of a prestressed concrets vesselis that, for a range of predetermined loads, including normaloperation, the concrete is maintained by the tendons in a state ofnet compression across any section of the vessel, The admissiblestate of stress and strain in the concrete may be influenced bypassive reinforcement.

1.4. The prestressed concrete structure may be furnished with featuressuch as penetrations and associated closures, an impermeable liner,insulation, a cooling system, passive reinforcement and means forpressure relief.

1.5» This application of prestressed concrete is novel in many respects}consequently, existing standards are of limited application,Progress is, however, being made in the preparation of nationaland international standards for PCRPVa.

1.6. Early development in this field was associated with gas-cooledreactors (OCRs). The potential, of the concspt f c .- other types,such as liquid cooled and/or moderated reactors, is now appreciated.It is not expected that the basic principles of PCRPV design willbe influenced by the type of reactor contained. Variations inengineering applications must be expected.

1,7» Sfce vessel must be capable of performing its function under certainpredetermined conditions of interaction between the contained reactorand the vessel. It follows that, not only must the vessel providethe necessary standard or' containment under predetermined reactorconditions, but there. must also be an acceptably low probabilitythat vessel behaviour will of itself induce an unsafe state in thereactor.

1.8. One incentive for adopting PCRPVs is the economic advantage to beexpected thereby. This advantage arises, for example, from:(a) the ability to contain large reactors or reactor systems

with the acceptance of high pressures?(b) simplification of a plant layout}(c) the fact that a highly developed steel fabrication industry

is not necessary,

1.9» Attractive features from the safet,y point of view include:(a) physical isolation of the stseJ prestressing tendons and

reinforcement from sources of heat and radiation and fromthe primary coolant;

(b) the high degree of redundancy in the preetressing system;

(c) the possibility in many cases that tendon loads mayba measured and reset;

(d) the possibility of removing tendons during operationfor inspection or replacement;

(e) inherent ability to withstand seismic shock.

1.10. 1-1 some cases the conventional practice of grouting tendons isadopted in order to provide sotae protection against corrosionand an alternative anchorage,, These .advantages must be setagainst the inability to test, inspect and replace individualtendons.

1.11.. Where a penetration as required bo provide access for services,equipment etc , it is customary to provide a purpose-built removableclosure. The design objective in such cases is that the standardof integrity of the closure and its attachment should be at leastas good as that of the main structure? the presence of a penetrationshould not prejudice the necessary integrity of the main structure,

1.12. Zt is necessary to provide some raeans of limiting ths effect on thestructure of an excessive rise in internal operating pressure. Theaiagnitude and rate of a postulated pressure rise can be determinedonly by reference to the characteristics of the particular reactorcontained. It is common practice to provide automatic ventingdevices, such as safety valves, for this purpose. An alternativeis to design the vessel so that it is self-venting by partialstructura] faiïure. It is generally recognized that this alternativecannot be relied upon at present*

1.13. It is customary and advantageous to make provision for verificationof .the state of the vessel. This may be done fey installed instrumenta-tion and/or by periodic in-service inspection» Such measures, whichare taken in a manner appropriate to the particular situation, serveto verify the vessel integrity ana to confirm the design criteria.

1.14. Design philosophy is based on the recognition of two or morephases in -vessel response to increasing pressure. Wie designobjective is to ensure that a particular response to the imposedload can b© achieved in each pbase and that this behaviour isconsistent with the appropriate operational and predeterminedfault conditions.

1.15. Over a range of pressure and temperature, including- normal operatingconditions, the vessel will respond to short-term variations inpressure in an elastic nurimîer, This facilitates machine analysisof stress and strain .in the structure, Longe r-ter-m stress andstrain are affected by shrinkage and creep of the concrete, re-laxation of tendons and possibly fatigue, ïn -fois range of responsethe affects of short- and long-ierifi behaviour can be combined todemonstrate that stresses and strains are limited to acceptablevalues.

1.16. Beyond the elastic range the response becomes increasingly inelasticand non-linear. Tho vessel would not be expected to enter this phaseexcept under the most severe overpressure fault conditions. In thisphase tho structure is stable but may experience permanent damage.It is in this phase of vessel response that some limit states occur,

1.17. The ultimate load condition, in which the vessel is incapable ofsustaining- any further Increase of internal pressure, is a furtherlimit state, Evaluation of tM ultimate load provides a measure ofthe factor of safety above design conditions.

1.18» The methods of design verification which are referred to in paragraphs1.14 to 1.17 are described more fully below* it should be noted thatit is the usual practice to use two different methods of designverification simultaneously.

II. Analyses foi*, Service Load_ Conditio.ns_2.1.Loading conditions

Stresses, strains and deflections in the vessel structureîîhould be analysed for all relevant combinations of mechanicaland thermal loads which can arise under normal service conditionsthroughout its life.

The tsndon forces adopted in each analysis should includeallowances for the most severe effects of friction and lossof prestress. Account should, be taken of all significantloadings applied to the structure, including stresses arisingas a result of construction procedures and normal operatingtransients. Any significant effects of penetrations and -lineron the vessel structure should be taker, into considération,

2.2.J Analysis

The analysis method selected for each loading condition shouldtake appropriate account of time and temperature dependentcharacteristics of the concrete and bsve clue regard for thecomplexity of the design and loading conditions and the accuracyrequired.

Each analysis should provide adequate details of the stressesinduced in the concrete, in the passive reinforcement and strainsin the liner to enable the acceptability of the design to beassessed.

For the purpose of the analyses covering the prestressingforces and dead loads at completion of construction, arid undertest pressure, the concrete may be assumed to be a linearelastic material»

For all other service load conditions, including start-up andshut-dowi both initially and at the end of the vessel life,the stress-strain characteristics used for the concrete shouldtake account of the age, temperature and time under load.

2_,j_._ Minimum design près tressNotwithstanding the acceptability of stresses in the concreteunder normal service load conditions a net cornpressive forceshould be maintained across any section of the vessel under apressure which exceeds the maximum normal service load pressureby a suitable margin.

2.4» Tendon anchorage zone designConcrete supporting tendon anchorages should be reinforced inaccordance with applicable codes v^here existing, 'The safety ofthe vessel structure is specially dependent upon the integrityof the prestressing tendon system. Suitable tests should becarried out on representative tendons and anchorages in combinationunder support conditions representative of those obtained inthe pressure vessel.

2.3. CrackingIt is considered that lire: ted cracking may be accepted provideddue regard is paid to any significant redistribution of the stresseswhich may arise and the integrity and leak tightness of theliner are not impaired.

2.6. Concrete gtrèse concentra.tiensWhere local concentrations of stress occur, due to the presenceof embedments or other discontinuities in the vessel geometry,these should be assessed individually. Where such stresses are

very local and can be shown to "be self-limiting, they may bedisregarded» Where they are more extensive, due regard shouldbe paid to the effects of increased creep rates or tensiJecracking on the distribution of stresses in the vessel concreteand the influence which they taay have on the strains in thevessel liner and the stress distribution arising in the vesselunder conditions of shut-down.

Comment on gialti-a.xj.a.ĵ jîompresslye stresses in concrete

2.7. A certain amount of evidence is available to demonstrate thatfor short-term loading, under multi-axial corepressive stresses,concrete can safely withstand higher compressave stresses thanare generally accepted under uniazial loading. It is generallyaccepted, however, that if one of the stresses approaches zero(or becomes marginal I?/ tensile) the effect can be nullified orreversed. There is a lack of knowledge regarding long-termloading unuer multi-axial compressive stresses.

2.8. In view of the above, it is recommended that for tne time being,if any advantage is to be taken of multi-axial compressive stressconditions?- The loading condition must be short-term only.

The minimum stress value irust ne suown to oe sulstantiallycompressive beyond all reasonable doubt.Careful attention must be paid to the effects of any increasedstrain on the stress distribution un

Ill,r Limit Design3.1. Five limit states are recognized for the present:

- Limit of instantaneous linear elastic response. Definesthe upper end of the range through which the overallresponse of the vessel to short-term loads remains essentiallylinear and reversible. Minor, localized clacking of theconcrete may occur.

- Limit of instantaneous, reversible overall structuralresponse. Similar to item 1. Defines the upper end ofthe range over which the vessel response remains reversiblealthough no longer linear.

- Limit of permissible deformation (short-term and/or long-term),Represents the largest oeformation under wbjch the containedreactor system wil] still function properly. The limitusually applies to penetrations und other such parts of thevease"1 where relatively close tolerances must "be preserved,

- Limit of" liner defect stability. Defines the upper end ofthe deflection regime in which liner integrity can bereasonably assured. For liner deformations beyond thisrange, such defects as may be present in the liner, maypropagate through the wall. Siraila^jy, highly stressedareas may lead to local liner faj'iure. This could resultin substantial leakage into the vessel concrete, andconsequently, to crack pressurisation,

- Ultimate strengtn limit. , As defjne in more detail inPart IV "Ultimate Design", tftis represents the ultimateload carrying capability of the vssaeT struct ire. It isevaluated wjtnout regard to the credibility or manner inwhich this load might come about.

3*2. The limit of linear elastic response and the limit ofpermissible permanent deformations are considered significantin terras of evaluating normal and upset loading conditions.An upset condition is defined as a transient operatingstate resulting from a single operator or equipment malfunction.Similarly, the limit of reversibility is related to the conceptof (potentially repetitive) emergency loading conditions»For reactor systems where major leakage from the vessel con-stitutes the design basis accident, that is, where linerfailure represents the faulted condition, the lirait of linerdefect stability 'becomes the significant consideration. Thus,the reactor systems design and vessel design 'interact directlyin the assessment of limit states three and four.

3.3. The designer, in evaluating the various limit states of the•vessel should provide suitable margins of safety with respectto the loading conditions and their probability of occurrence,against which the limits are assessed.

3.4. At present, quasi-elastic and visso-elasti.c analysis methods,as well as structural model tests are used to estimate theselimits. Because of the inherent uncertain ties, considerabledesign conservatism results, much of which may be unnecessary.

3.5» Finite element techniques are under development to predict vesselbehaviour in the aneiastic range. The methods consider crackingof the structure, but so far only on an axisymuzetrical basis,Chess finite element techniques should be expanded to threedimensions. In addition, meaningful failure criteria forconcrete under multiaxial loading, and defect stabilitycriteria for liner ftaiberiais, should be formulated. Furthermore,additional materials characterisations work, should be performedfor concrete in the elevated-temperature visco-elastic regime.

IV. Ultimate Design

,3 . Outline

The ultimate design is a. simple method of ensuring that thereis an adequate margin of safety between the design pressureand the pressure at which the vessel will fail. In calculatingthe failure pressure of the Tr3S^el it is assumed that the linerwill remain intact up to the failure pressure and that the over-load pressure is applied tu a new and cold vessel. The safetyfactor chosen should allow for the degree of predictabilityand the gradualness of the failure mode and should he adequateto cover any possible time-dependent deterioration of thevessel materials.

4+J?*, _ Re 1 e van oe

•fbte ultimate load calculations are not in^nded to representa realistic condition. The calculations do not take accountof possible gross liner rupture at a pressure somewhat lowerthan the vessel failure pressure wftich could, result in prematurevessel failure due to pressure acting on the cracked concrete.Keither does the approach consider a possible weakening of thevessel due to time dependent material degradation. The approachdoea not calculate the precise margin available against arealistic condition. It merely gives an indication of the marginavailable against a hypothetical over-preesuriaation under theoondit ': ons assumed aho ve .

4.3» Method of Gal cu] ation

The method of calculation depends on a knowledge of the mechanismsof failure of the vessel. This information is normally obtainedfrom appropriate model tests to destruction.

It is essential that the mechanism of failure is fully understoodand is shown to be progressive.

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IK the case of flft>ru.ra'i fa i lure , it- is posai r/e to calculate,

with smf f~ cie>i t accuracy, the r>ressure at v,*hlcV tho vesseJwill finally f a l ? ,

If a shear modo of fa i lure ayr l iea • tr» a s 'aY; , the xredic4 '!.bâ !t i tyoT this may be sufTioier.tly u r -ne r tu i r j to require a higher loadf .actor.

The moâel shou'H ix su.*Ti c i s / i ^ i y rspre-'sntati. '^ and its dimensionsso chosen thai th-3 resu'lvT ar-i not diKtcrtec by . inabi l i ty torepr'-jnent adc-.qii.ately such ,

It is important to ensure that tb.e design of penetrations is such.t.;;at the penetration and its interaction with the concrete doesnot reduce the load factor of the structure*

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V'._ . Sublets of ..p_a_yt.i.ouljâ ijn_tiejr̂ t._̂ ar]r../urthe]r. researoh ordevelopment study

The Panel has listed the subjects briefly described below, in noparticular order of importance, since it is felt that betterknowledge might result in greater economy with equal safetyor greater safety without increased cost.

Jĵ ,_ Pê jLvat:lou ojf_̂ currently._ opera ting PCRPYs

Vessels currently operating offer the best opportunity of checkingcertain assumptions and calculation methods. It would, therefore,be desirable if appropriate data could be obtained and made generallyavailable. Opportunities might arise during- commissioning tests,repeat pressure Lests and inservice inspections the latter should

• provide opportunities for examining tendon relaxation, tendoncondition, linsr and insulation conditions, concrete strains, etc,The instruments and facilities required for such purposes shouldbe considered so that provision may be made in the design specifi-cations; the inservice collection' of the data would be theresponsibility of the operating personnel.The development of instruments having suitable long-term stability -in so far as they are not already available - is recommended ae asubject of further study,

5•2, Concept of hot 1iner

In this concept the thermal insulation is placed outside theliner. The liner works at the temperature of the adjacent partof the primary fluid.

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The associated problems are:'a) Liner material?b) anchorage of 1 iner, buckling stability and fatigue in

general areas and at. special points and penetrations:c) insulation material?d) behaviour of the whole structure if the structural

concrete is at a high température.

3.3. External prestressing g^steme

With the trend towards increased pressures in reactor pressurevessels and tne adoption of PCRPVs for reactor systems otherthan GCHs, it is increasingly difficult to «achieve the high densitiesof hoop près tress required to satisfy the designer, A number ofexternal hoop prestresn systems have been developed recently or arestill in the development stage, ""he Panel feels that more informa-tion about thesa systems ought to be made availaole to vesseldesigners, including information about the following:a) Ultimate load behaviour5b) measurement of prestress levsl. and its adequate maintenance;c) corrosion protection5;d) effect of the system on station building design and schedule;e) economics,

cj.4» Load- oar ryingoapa o i ty analy si s

Methods of analysis of the load-carrying capacity of PCKPVs forintegrated reactor systeia designs developing towards geometricallymore complicated shapes may not give sufficiently good estimates.

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It is recommended that further development of methods beattempted with the aim of achieving more reliable toolsfor the ultimate carrying capacity analysis of FGRPVs.

5.» 5.» Effect ofa Jjĝ aĵ ^

The tendency of PCRPVs to become thicker- walled in relationto inner diameters has intensified the need for furtherinformation about possible pressures gradients in concretedue to porosity or cracks in the liner and concrets.

3.6 • Sliort~term and Ipng-̂ term sjr̂ n̂ ĵ oĵ ŝ ^

The short-term strength of concrete under triaxial loading conditionsat normal and elevated temperatures is not sufficiently well knonm.It is recommended that emphasis "be placed on the continuing of three-axial short-term strength testa. IJva-luation of the results of suchtests ehould lead to the formulation of statistically reliableparametric failure hypotheses for a range of typical concretes ofinterest for PGRPV construction.

The ratio of long-term to short-term strength of concrete is knownonly for the uniaxial états of stress (it is about 0.75-0.80).Almost nothing is known about this ratio for nsultiaxial states ofstress.It is recommended that investigations of the strength of concreteunder constant long-term multiaxial loading be performed. Theobjective should b© to correlate findings on the "strength loss*1under long-term loading referred to the short-terra strength, withconsideration of multiaxial states of stress.

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It is recommandée that in all caees the strains of the specimensbe measured. It is also recommended shat the effect of size ofspecimen "be eiaminad.It Is further considered that 1 jug-terre teats under slow cyclingof loading and temperature are of great importance.

5.7.» Cyclic Ipading of cĝ orĝ ta and

The future development of load-following high-temperature reactorswith direct-cycle gas turln ies may require that the coolant pressurein the PCBPVs - and porhapa the coolant temperature ~ be cycled. Therange of pressures may be L-stwesn 0.25 and 1.0, and fche total numberof cycles may 'be 25 000 in the life of the PCRPV.It is not yet clear what the above requirements would imply in termsof range of stress or strain cycles in the steel linsr, tha concreteor the attachments. It is, however, envisaged that these «ay besufficiently important to require further knowledge of the abilityof the concrete and steel liner to survive the cyclic loading inthe environment which will "be required,

Long-term strains in concrete.

Further studies of concrete material properties would bs useful,particularly in the following areas?a; Relaxation modulus and Pois£>n'e ratio, in th>. temperature

range and for concrete of interest to PCHPVs designers.Special emphasis should "be plsopd on the. current and predictedfuture trend towards higher temperatures and the so-called hotliner concepts}

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"b) interdependence of relaxation modulus and Poieson's ratiowith Hiul'ti axial stress (strain) fields;

c) moisture migration in mass concrete as affected by (high)temperatures, the presence of impermeable barriers andlocal injection, of water. Of particular interest is theinfluence of moisture on the oroperties of the concrete.

Radiation from the reactor core may cause deterioration in thesurrounding concrete, through the direct infjuence of irradiationon concrete properties and through a secondary effect, theproduction of beat with, ensuing temperature gradients. Someresults of irradiation tests on concrete specimens have beenpublished} they seem to indicate that significant damage mayoccur if the neutron or gamma dose is sufficiently large,Internal shielding may be sufficient to keep the irradiationdose within safe limits during the whole reactor life. Furtherwork on the effect of irradiation on concrete ia, therefore, required,

3 « 1 0 • JTendpn oorros ion

The structural integrity of a concrete pressure vessel depends, toa very great extent, on the integrity of the prestressing tendons.It îs, therefore, of vital importance to ensure that the tendonsdo not corrode. In the case of u.igrouted tendons, several commercialproducts are available to prevent corrosion. Many firms. r throughoutthe world - specialise in preventing the corrosion of ŝ ea] .in alltypes of environment. Research in this subject is generally conductedby specialist commercial companies whose proditcts should be thoroughlytested before acceptance. Attention should also be paid to thereliability of grouting,

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Tn concluding their Report, the Panel would like to make somerecommendations which they believe would be valuable in consoli-dating the work done by the Par;el and continuing it in thefu.ture.

1) It is recommended that the work initiated by the Pane]in this rapidly expanding field coiud "be continued withadvantage to the Member States if the IAEA would establish,under its au sin ces, a Working Croup ox* its equivalent onprestressed concrete reactor pressure vessels. Such aGroup, at subsequent meetings .and through other regularcontacts, coulr maice further progress regarding specificproblems, such as are listed in Part %

2) It is also recommended that the IAEA could usefully facilitatethe interchange of relevant information. For instance, theycould circulate a bibliographical listing of reports andpublications which were Rent to them. Also, it would beparticularly valuable to 'nave such a listing circulatednot only of research reports, but also of major experimentswhich are planned or a)ready in progress. It is envisagedthat the members of the Working Group oouM assist in thesetasks,

3} Finally, it is recommended that in certain fields, particularlythose mentionad in Part 5» it way be useful if the IAEA couldfacilitate the publication of review papers, and monographeon selected topics suggested from time to tirée by the WorkingGroup»

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STRUCTURAL BESIGÏÏ PHILOSOPHY ANS CRITEHIAPOH COECRETE BEACTQK VESSELS - U.S. PRACTICE

W. Rockenhauser

Prestreased concrete reactor pressure vessels technology isvery young and codified. Standards have not yet been established.However, efforts are being made in the U.S., as well as in Europe,to write uniform "tentative criteria*' or "recommended practices"which could eventually become tue codes for design ani coastructionof these vessels. Much of the material presented in the paper isdrawn from the content of tentative criteria and from the designstandards for the first U.S. PCPV, the Ft, St. Vrain reactor, Itis followed by a comparison of European and U.S. standards and abrief discussion of the relative merits of the various approaches,

A PCRV comprises five major constituents: the basic concretestructure, the post-tens!oning system, the nonprestressed reinforcement,the liner and the thermal control system (insulation and ooo]ingsystem). The paper concentrates on the first three components.

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1. INTRODUCTION AND DEFINITIONS

Concrète E.eactoj; Vessels as defined here, and as distinguishedfrom the secondary containaientG art structures which contain areactor or reactor (primary) system directly without the use ofanother intervening pressure barrJer. As such they are continuallysubjected to the pressure of the primary system whenever the reactoris in operation.

These vessels are commonly known as "piestressed concretepressure vessels for rp-îctoiV or more briefiy, as prestressed con-crete reactor vesseJs (PCEV's); hereafter this abréviation will beused. A PCRV comprises five major constituents: (1) the basicconcrete structure; (2) the post-tonsioning system; (3) the non-prestressed'reinforcement ; (f\ ) the Ijner; (5) tne thermal controlsystem (insulation and cooling system). This paper will be confinedto the first three», components.

PCRV technology is very young; and codified, across-the-boardstandards have not. yst been established. However, efforts are under-way in the U. S. 5 as well as in Europe, to write uniform "tentativecriteria" or "recommended practices'* which could eventually becomethe codes for design and construction of these vessels. Much of thematerial presented here is drawn from the content of these tentativecriteria and from thr> design standards for the first U. S. PCRV, theFt. St. Vrain reactor. It is followed by a comparison of Europeanwith U. S. standards and a brief discussion of the relative meritsof the various approaches.

2. PCRV STRUCTURAI, RESPONSE

The response of a correctly designed PCRV to an internal pressureload is illustrated in Figur? 1. The figure shiows three distinctregimes of structural behavior, which has in fact been observed onactual vessels. (Depending on the point of ineasurensent, the distinc-tion between the second and the third regime is not always as clear asshovvi in the figure).

The regimes can be explained qualitatively as follows:Regime 1. Starting from a compressive strain (Imposed by the

tendon system), the vessel overall response, i.e., strain, is linearlyelastic slightly beyond the point identified as RP on the ordinate.In this regime, minor cracking of the concrete may occur as a resultof thermal loads or local discontinuity stresses, but basically thevessel behaves as a monolithic structure.

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Pressure

ULTIMATE STRUCTURAL STRENGTH

DeformationFigure 1 Idealized General Load-deformation relationship of a PCRV

Regime 2. With further increase in pressure, the general tensilestress in the concrete reaches the point where major cracking commences.The concrete continues to crack throughout Regime. 2 with the structuralsteel still responding elastically. The overall response of the vesselis, therefore, still elastic (although no longer linearly so) and thevessel deformations are more or less reversible.

Regime 3. At even higher pressure, concrete will be extensivelycracked and the structural steel is loaded into its plastic range.Vessel deformation increases very rapidly wiTb increased pressures untilthe ultimate, strength of the structure is reached.

The foregoing simplified description assumes a short term responsein which time dependent deformations are not a factor. During the designlife of a vessel, considerable irreversible deformation (primarilyresulting from creep and shrinkage of the concrete) occurs. This resultsin a concomitant reduction in prestress which inust be factored into thedesign. Creep and shrinkage deformation interacts with the short termresponse of the vessel only to the extent that (because of the reductionin prestress) it changes the point at which the concrete enters thetensile regime under increasing load. Il: thus results in the somewhatlower end-of-life response curve also shown in Figure 1.

Thermal loads (thermal gradients through the. walls and heads) donot result in significant deformations in a structure as highly redundantas a PCRV. They have little direct effect on the response of a vesselto internal pressure although the presence of elastic thermal stressesmay result in a slight lowering of the pressure at which the vessel ceasesto be linearly elastic. Temperature does have an indirect effect, how-ever, in that it influences creep and shrinkage rates and thus increasesthe long term changes.

3. DESIGN PHILOSOPHY; DESIGN/ANALYSIS APPROACH

Design Requirements, Definitions of FajLlure

As in the case of other structures, a PCRV, to perform its functionadequately, must carry the imposed loads "safely", and its deformationsmust remain within allowable limits ur.der these Loads. In addition, itmust remain impervious to the. fluid it contains.

These requirements, imply three definite types of failure: (1) struc-ture failure; (2) excessive deformation ; (3) failure by leakage. Theintent of the design process, and the intent of the criteria on the basisof which one judges the design, is then to ensure that none of these typesof failures can occur during the service history of the structure.

22

Bas ic PCR VjDe s i grt Philo s o phy

To accomplish these aims, PCRV'ss (at .least to date) have beendesigned to be fully prestressed against the highest pressure which •the vessel is expected to encounter in seivice. The question ofwhich normal or abnormal pres&ure conditions need to be consideredin this context, is, of. course , related to design of the reactorsystem used inside the vessel, the pressure or absence of pressurerelief devices, the overall plant design, and other factors. Itcan, therefore, be discussed meaningfully only as part of the overallreactor system design philosophy.

In addition, the design usually intends to furnish some factorof over-strength beyond the highest anticipated load levels, regard-less of any credibility considerations. This is accomplished byproviding more structural steel than would be needed for service loadsalone.

Within the range of anticipated loads, vessel deformations areusually small enough as not to affect significantly the functioningof the PCRV. On the other hand, long-term viscoelastic response mustbe evaluated to ensure that creep and shrinkage deformation does notrender the vessel unusable. Aside from the associated reduction inprestress forces, the most critical factors in this respect tend to bethe position, alignment, and dimensional accuracy of penetrations usedfor refueling machinery or removable primary-system components.

Thermal loads are,, only partially balanced by prestressing (overand above the prestressing required for pressure) ; the remainder istaken into account fay appropriate use of bonded mild-steel reinforce-•rnent. The ratio of over-prestressing to reinforcing varies considerablybetween designs. However, the liner (and, therefore, the concreteimmediately adjacent) is kept in compression under all foreseeablenormal and abnormal operating conditions throughout the design life ofthe vessel, this compression, may actually become rather pronounced,particularly toward the end of the design life, and appropriate pre-cautions are therefore taken to avoid the possibility of liner buckling.Design Methods

In current U. S. practice, the design philosophy of Section 3 isimplemented by the simultaneous use of several design/analysis procedureswhich can be categorized under three main headings.

Elastic Ana^lysis^Cworkinjg _jsĵ gss_j esi gp }

The elastic response of the vessel to mechanical and thermal loadsis evaluated by appropriate methods and the stresses thus calculated arejudged against working stress criteria to ensure that the vessel responsewill be essentially elastic, under all credible combinations of operating»accident, and environmental loads.

23

Liaiit .Analysis

>,oad factors are applied to predicted JcaKimunt loads; thenallovible limit criteria are s.ttisfied for the«e factored luads,usinp apsropriat

TABLE I

LOADING O

a) Desd Loads

Weight of th% PGRV i t s e l f : we'lgbl of pur t s of v^ r sc ' . ^. variouscwjstjruct-iori sta[/fr !3 3no equipment subs:, ructurc-c; r-.;,ap->r^ri iy (e.g..,

).:i£ ccn-';i act.ion) or perwsïi^ni ly i,cn.,nu.d o^ or v*ti.ia the PCRV,

Caused by nquipineni moun ted 011 wr w i th in r h f t >'CRV - refuel ing machinery,ro ra f i rg TMchii.-af}' , pn r, ••,:"- I v

11

Caused by T"

Pressure LoadsCavity pressure may vary all the way from vacuum, to values above

the peak working pressure unc'er certain postulated abnormal conditions.European practice has been to design for a pressure level higher thanthe anticipated p*»ak working pr^ss ire by come arbitrary percentage.U. Sv design practice uses a reference pressure "TU''1 as the significantload. This is defined as the. highest pressure which uust he consideredin the design o£ trie pTrtJcu'ar reactor plant tx be housed inside thevesse). Up to this prefi^urt» rbe averr^p stress in the concrete acrossany section of the vess«.i nust be kept comnresBive.£;ĵ irj&nineota.l__Condit. ions

In the U. S. A. :>as beeow» established practice in nuclear powerplant design to consi-iei fw- severity levels t-iher. >~e d«»Jg*i eai thq»jiake or safeshutdown (Ksfu/'bancG is neutrally tak>'-n î9 nuit .'pie o£ c be def t i fu dis-turbancft "7hcre the n>uJ t ip] ic°t ion f-u*tT d«pcndp '»u xhe spec i f ic plantsite.

Locio[__Corcb 5 nat ion a

lu both the worKing srrecs d«si.go sad tîu. Jiuût. de*ipv>, certaincombinatirms of the .loading conditjn:;s of Tabit i. rMur,t be rcnbidered.In the working slxvss dcoif^» this iicludes ' l.i < oribf nations wbich canreasonably occur darL.g tne iiCe of the plant. Tht coc.birac ions are.lasted iu Tf»bJ.f ÏÎ. S imi lar ly» for lirait dostg.j purposes, specificload factor équations. wb3"ch wi iL be discussed in Section 1} h#-ve beententatively agreed upon. These s re intandoJ fc predict, the elasticlimit of the structure.

5. CRITERIA FOR ELASTIC BJ sWN

,a general., the cri teria for th« "Working *!trcs8 Lv*-3ign" areformulated fonventlonally in terms of «illuw bi«i stressée, expressed as.€r?ctions of standard ïiaterial ptrcper t i f tp . ïh-».se aïe c/Jincter or rubestrength for concrete azvd y ie ld or tensile atr^vi^lh lor P teoJ . Inlarge neasute, w*»tl established al.3owav1e "aiues «rf» u

TABLE IT

Load Combinations

• Period of Design Life

I,.- ^Construction

~*\ ^ iPressure Pr as tress Dead & PipeLive

| 1 Loads"~ """ '^' "•- \- '- ' - - -

2. Son-pressurized Vessel£_ter Completion ofPresrressiRgOpération

•

Reactions'

1

Wind- r • "

Tempera- Creep Shrinkageture______ : . • _ .-L. ._ _ .

/ / i /'j-.- _. _._... ..... . !..__.........

VF Effective »

* $3, Nou-pressurlaed Vessel j

ertcr Initî&I Heating V?. Effective /i

ColdSpring |

GoldSpring

*

. i /

ConcreteGeneral Consjlderationsof ConcreteAllowable concrete stresses quoted in AC1 318-63 generally are

expressed as .fractions of the 28-dsy cylinder strength f'c. Thisis du.;e for several reasons. One 1s just, time-honored practice,Another is the realization that structures are often fully loadedonly when they are considerably older than one month; the increasein strength over the 28-day value riien provides ? margin coveringuncertainties in analysis, material properties, loading conditionsand 30 on.

In the case of PCRVs, the luain service load (internal pressure)usually is applied only after the vessel is at least a year old; evenpost-tensioning may not occur for many months after fL;//;'! completionof the vessel. The- loads are known accurately, Moreover,, methodso£ analysis are much more refined than those used in everyday concretetechnology. Therefore, stresses are caJcalated with high accuracy;and prestress losses, particularly those occurring while the concreteis young, are calculated with great care. Consequently, tiui use ofthe 28-day strength as n clesjgu basis is o£ questionably validity andunnecessarily conversative. This is recognized, and ttept a^es UP to90 days have been specified (Section 8). The most ration»! basis wouJrtseem to be to use the strength of concrete at the time of post-tonstoo-ing.

Typical (28 day) concrète strengths for vessels completed or underconstruction f a.l 1 into the range of 4500 psi to 6000 pai.Types of Allowab 1 e Streajs es Jîpecil'ied

The stress analysis methods employed in the deuign of PCRV's(fini te-eietuent or f'inite-di f tecence calculauions) ^ Lve detailedresults on magnitude and orInatation of stresses at msny locationsthro'!.:.;hout the structure. ThereEora „ prlncipai-sf-esK criteria can bespecified directly and. no shear stras- allowables need 1o i>e given.

For the first PCRV's connp.rvrtti.ve .lilowafoîe ftveas values werespecified for "general stresses" (soiuewhat comparable to membranestresses in sheila) i.e., «tresses affecting an. area or voluisd largeenough to have overall structural significance. Different higher valueswere permitted for more- "local stresses" vrhich could be coiisidereidanologous to secondary or discontinuity stresses in stop! pressure vtigyodesign. The obvious difficulty in thiu Ls Kow to define quantitativelythe distinction between "general." and "locfjl" for a -struct.-or*? as coif-i>.lexas a PCRV. The trend now seems to be tc get away from "^e^craL strt-ss"criteria and to have only one limit., higher than the customary 0.45 f'c.which must be satisfied everywhere except at poinLs which are obviousstress concentrations.

Multiaxidlity EffectsMost of the concrete in a PCRV is subjected Lo multiaxial Load-

ing. It is well known that concrete strength increases with increasingdegree of confinement, and an effort is made to take advantage of thischaracteristic,

In a sense, even the early approach?*» which permitted higherlocal stresses can be viewed in this J ight , A more- systematic attemptis beinç m?de with the Pjirt St. Vrain vessel, wherr the. principal allowable v..jtnp res give stress L';f mined p^r Ap j i rndLx A. Whore high localstresses aie Incliceted adiacer,' r^ s r ruc tu r r l discun*- Jnui LifcS , and thusc ïear ïy result ivosi stress c OPO' LU. rat ion e f f e c t s , the values prediccsdhy eldstii. ana lysis are c i a t - regarded sxncc «..giiituu ing c^pcrjence hasshown tuat such local coiidltioiïo oc< or in Ail «•"••nctures wifhouL causingdetrimental e f f e c t s .

';ncrefe has a i-ensilo otreiigLh cf 1/12 Lo 1/8 nf the 28 day com-press, on strength For a 6000 p&J "Likrete, tlje I ensile sî.rt?ngf}> shouldtherefore bo 500 to 750 psi. ïyplrailv, the andésite concrete iut theFort St. Vrain PCRV has exhibited a Lenstie «freugth of around 600 osi,i.c», about__i/10 of the conp)es&iv*« streiigchs. éor f', - fOOO pal» thevalue S'T'̂ . givos an a] lovable tensiJ*.- stress uf 233

29

Reliance is «oL pioced on tot. tensile res.! SB tance of r.he concretewhen the computed tenslif stresses exceed 3/t'c, In such cases, it isassumed that concrete cracks will develop and, therefore, reinforcementis provided to carry the entire tensile load.

Bearing Stress Under Tendon Anchorake«" ——— • ——— • —— •.——--- — — —— -— — —— —- ——— — -- —— — ' —— - O.fj f .A,/ 0but not greater than f'

*

j 'ils allowable stress Is taken dirent.! y from Uie ACT -.3 IS, Paragraph2605., where A'b and A,., ;-jre as defined in ACT-318. Paragraph 2 GOO, forlow A'k/Atj ti.is formula results in conservative permissible values. Forlarge Afj /AL values the ion-uila roarers cxcossivtpe.rmip.'ïibio concretestresses. Therefore } the iv.axi.aum limit "f. ff'c is also specified whichagain is a very conservât i 12 VHÎUC. Guyon reports in hi H hook "̂î'estresŝ eCoiicî'et_«. ., Page 155. that t-,:r an inleriia.i t'rictjon angj.t- ol" 3'5a , a formuladeveJoped by 1-1. Cd.quof g'j.yc-c an uJ.tj'aate bearing nre.'î.surt- pqudl to about11 times tlu. uniaxiai coh>pr'.iss5ve H trees' tlu The c.rit crjc--1'» of 0.6 f „i,. _____ . ' '-vA!, /A., i.s wide.lv -.îaetl «j»d t'Uert- .-ire no kno-^a vea^oivw fo.' doubting itsc: ons erv& ti sm •.

Stetij

In general t.ÎHi convanLiOi i -^T prov-îBÎoi îS of AOT~^li< c;jn reodily beadopted. The- applicable paragr^pus ace a? follows:

/vppj .icdbj.i! i'ara.

Tensile 8 tresses

This implies

1003

For deformed bars No. 1! and wni.ilJcT" witha yield strength of 60,000 prf.i or mAllowable Strc-ôs . , . „ . . . .For "M otl'er reiuforc,au7ents :Allow^.blt; Sv.rt-.ss . . . . . . . . . . .

_Compre_s3ive_ Sj.rest.eR

"aragrauh .1102 i.s ime-decl ro -..icjiccompression of steal iu bendju^; .Tuni-bers. Although no! Jirocfi> Dralogous,che paragraph way be used i:i-re =IK veil.Thus, where steei is Latendo.d to fnucf-jo'.!as corapres s i ve r t; i nf or c emen l, :Allowable coaprcsuj on . . . . . . . . . .

2«i ,000 psi

?u>000 ps.I

1102

24,000 psi

s

Bond Stresses 1301Tendon_StressesTendon Wires - Allowable Mean Strèsses^The provisions of ACI-31S, Paragraph 2606» are met:

Temporary jacking forceAllowable stresses ...... r ... 0.8 £'

Immediately after transferor anchoringAllowable stress . . . . . . . . . . . 0.7 fg

Effective Prèstress 0.6 f1gAllowable stress . . . . . tb« smaller of

0.6 fsyIn addition, the maximum temporary jacking force is iiraited to 1.be

yield stress of (f }, and the maximum stress after anchoring is limitedto 0.9 f _t> These two limitations are intended to covet prestressingsteels which are not manufactured to the acquirements of AS'CM A-421, ystwould be acceptable for use in a PCRV, (ASTM A-A21 specifies a («Lnrwirnyield strength of 0.8 f5 }STendon Anchors

Ko; general criteria are available. It is specified that the actualend anchor assemblies must withstand, without failure, at least 1,2 xGUTS under static as well as cyclic loads; this strength to be demonstratedby appropriate tests.

6. CRITERIA FOR THERMAL LOADING AND THELASÏIC EFFECTS

Thermal loads (temperature gradients), creep aixd shrinkage areclosely interrelated. All three may cause stresses; in addition, creepmay relieve stresses caused by the other effects. To be most meaningful,a design calculation should, therefore, consider these, factor together,as well as their interaction, which is difficult at besi:.

Furthermore, relatively little quantitative information exists aboutthese phenomena, particularly at temperatures above the 150° - 200° Frange.

In view of this, several techniques are generally used simultaneouslywhen considering these factors in the design of a PGRV: i. Thermal loadsthemselves are Lirai ted to values known to be safe; ?.. Upper-bound esti-mates are made of the permanent deformations to be expected as a resultof creap and shrinkage;, 3. Stresses resulting from temperature gradients,creep and shrinkage are assessed on an elastic, nodifiad elastic, orviscoelastic basis and the calculated values are limitée* to values con-sidered safe and reasonable.

31

Criteria for Thermal LoadsFour quantities are usually specified by designers. Numerical

values vary somewhat from case to case, but fall within the rangesgiven below. The four parameters are:

o Effective concrete temperatureat (or close to) concrete-linerinterface 100 - 170 F

o Local maximum concretetemperature 130 - 200 F

o Maximum temperature differencebetween inside and outside ofconcrete 40 - 70 F

o Maximum rate of temperature riseduring startup Order of 5-20 F/week

Deformation CrjiteriaKo generally valid criteria can be given. Under t;he worst possi-*

ble combination of tolerance stack-up, anticipated load history, andcreep rates, deformations at. ead-of-life :nust b

ID U. S. practice, this has been replaced by a dual approach,both parts o£ which have been called limit design techniques. Actually,the first is a true limit design: the second is a variant of the pre-viously mentioned ultimate load analysis carried out for a different,and more meaningful purpose.

In view of this., it might have been more accurate to talk in thispresentation about four methods used simultaneously in the design ofa PCRV: elastic, viscoelastic, lirait, and ultiinate-joad design-analysis.

Limit Design (Prediction of E3-astic Limit)

As stated previous3.y, load factors are applied to predicted maximumloads; and stipulated limit criteria are then satisfied for certain com-binations of these factored loads (analogous to the manner in whichallowable stresses are satisfied ir\ the elastic design for all reasonablecombinations of actual loads),

The intent is to set the limit conditions and the correspondingcriteria such that compliance with them assures predictable (responseof the 'vessel. Local large deformations are uu.i-ike.ly and prematurefailure in the liner, piping., or internal equipment due to excessivevessel deformations should not occur.

The specific limit condition equations now in use are:a. BL H- RP •••>- £' -T XL

b. DL + 1.5 EP -f 1.5 TL

where

DL = dead loadKP =• reference pressureE' = load resulting from "design"

earthquakeTL - thermal load

Elastic response under Limit Conditions a is intended to ensure asatisfactory seismic design of the PCi'V. The reason for selection oflimi't Condition b is that some reserve load capacity should exist beyondthe maximum anticipated loads.

The allowable criteria corresponding tc the above limit conditionsare as f o3lows :

ConcretePrincipal Corspreasi.y^Stress 0.6 C f c

33

The use of an increased allowable concrete compression under LimitConditions a and b is justified by the philosophy implicit in the adop-tion of the limit design concept where it is intended to ensure elasticresponse at Reference Pressure and design temperature by demonstratingthat elastic response is to be expected even under higher loading condi-tions. Thus, the allowable level of concrete stress can be the elasticlimit in compression for concrete. îfewnisn^ ' states that concrete maybe P ^ected to respond elastically in uni axial compression up to about50 to 60% of die unLaxiaJ ultimate strength: hence, the use of the upperlimit 0.6 f'c sterna justified. ACI-318-63 aJso makes provision for auincrease in allowable stress of 33-1/2^ for improbable lord combinations.If this increase is applied to the concrete allowable used in elasticdesign (0.45 C f ), the quoted Ugure of 0.6 C F' is obtained.Bearing Stress Under TCP don Anchors 0.7 f 7 A.' /A,————— ——— —————————— —————— t. b b

The allowable stress permitted for bearing stresses io Increasedby only 17% over the elastic design allowable. This is a very conserva-tive approach considering tht maximum fotresset> possible ft these rein-forced and confined zones as. pointed out in the discussion on the allow-able bearing stress under tendon anchorages fcr operating lo«

The ultimate strength criteria of ACI-318 are directed primarilyto framed structures, where yield of reinforcement implies the formationof plastic hinges and collapse mechanisms. Yield in the wall reinforce-ment of the PCRV does not mean that a collapse mechanism has formed and,provided the deflection characteristics are investigated under increasingload, there is no reason to state that ultimate capacity has been reachedsince the tensile failure strength of reinforcement is much greater thanthe yield strength. Conventional analyses do not poc^pps this ability,making the attaining of yield stress a convenient point to halt an analysisalthough a reserve of strength exists beyond this point.

The allowable stresses for this conditions are:Concrete Principal Compressive Stress 0.85 fTendons 0.95 GUTSBonded reinforcement GUTS

except in heads whore steelstresses will be limited to 0.9 fsy

ID areas exhibiting primarily membrane action, the concrete willof course, be fully cracked. Where structural action is more complex,e.g.» in the heads, compressive stresses are limited to 0.85 f , avalue which is accepted bv A,CI-318-63 (Section 1900) as being applicableto members subjected to combined axial compression and bending. Whereit car» he clearly demons treated that a condition of triaxial compressivestress exists and the material is confined, an increase of this compressivestress allowable is justified similar to that provided under Limit Condi-tion 1, i.e. , 0.85 C f .c

b . Tendons and Bonded ReinforcementAs the ultimate limit condition is a definition of the minimum

cavity pressure required to fail the vessel, the use of minimum guaranteedultimate tensile strength seems warranted for bonded reinforcement. Àvalue of 0.95 GUTS is used for tendons. The use of this value is justi-fied by the results of European tests on full scale tendons reported inreference 27. Their authors report ultimate load tests on B.B.R.Vw 'unitscontaining 121 wires of 7 mm diameter. The results of tests on straighttendons indicated the development of 99.7% GUTS and the curved tendonsgave strengths of 96.5% GUTS.

The allowable stress of 0,9 f ̂ for bonded reinforcement in the heads,combined with the concrete allowab?! of 0.85 f, ensures that the thickplate nature of the structural response of the fèeads is not lost by theformation of large cracks. It also provides a margin of safety in thehead analysis which is necessary since the action of the heads is amenableto precise analysis than other portions of the structure.

35

8. COMPARISON OF VARIOUS DESIGN BASES

If one compares such design criteria information as has becomeavailable from different design organizations, considerable-variationwill be found. Design evaluation and progressive changes have takenplace within each organization as well.

Therefore, a single overall comparative evaluation cannot meaning-fully be made.

Table III is a compilation of parameters for which a comparison mayhave value. Brief explanatory notes follow:Pressures

In European practice,, the design pressure DP .(at which static balancebetween tendon force and internal pressure force occurs) is selected at10% above normal working pressure (NWP); and in the most recent Frenchvessels (Bugey and Fessenheim) as low as 6% above the NWP. This comparesto a corresponding value of 23% in the U. S. Moreover, in the U. S.practice this pressure (called Reference Pressure, RP), is higher thanthe highest pressure which can actually occur in the system. The Europeandesigns permit the maximum pressure to exceed the design pressure byanother 10%, giving a true maximum pressure of 1.21 x NWP and correspond-ing with it the possibility of net tension in the vessel walls, which maylead to partial cracking of the concrete. ,Load Factors

A basic characteristic of the European ultimate load evaluations, aswell as the limit conditions factored into the more recent French designs,is the a priori assumption of a certain mode of structural behavior orfailure respectively. This is exemplified by the analytical, approachesgiven in references 28 and 29. The analysis offers no method to ascertainwhether the postulated structural behavior would actually occur. Carryingout similar calculations for the Fort St. Vrain vessel would give ultimateload factors between 3.0 and 3.3.

In general, one can conclude that this type of. load 'factor evaluationis of limited use and can, in fact, be somewhat misleading since the resultdepends strongly on the assumptions and calculation techniques used.

The U. S. approach has been to base limit calculations as well asultimate load factor calculations on methods which mathematically pi-edict(rather then assume) the structural response of the vessel.-1 A correspond-ing numerical load factor of "« 2.5 x NWP has been selected.Concrete Temperatures

All temperature values shown in the table are sufficiently low asto eliminate the likelihood of significant material deterioration. U.K.practice parallels the British approach; both are somewhat more conserva-tive then the French designers*

36

TABLE III

\

P C . R . V . DESIGN COMPARISON- "

1 PQE55UQËNQBMAL WORklNGPGE55U2E (WWP)

maximum crediblysystem -pressures

, PCES5UÎSE (D R)BEFEGEMCSPJ2E.35UEE (CJP)INITIAL PEOOFdPIR)TE5T PCE5SUEE

2LLOAD FACTORUMiTCOMDîTICW la.tlAAJTCOMDtTSOK! îbUMSTCOWDITiOW L

FT 51 V2AINPSIG. W.W.R688

O345„_

845 1.23972 1.41

DL*IÎ3N\VP*E'+TLDL+L84H\YP*i5TL^(5EE TEXT) .PAGE vfl~25

3%KZNG°U*f!*"E 600OpREAMR£*at age ^days)

j EFFECTIVE4-.-COWCEETE TÊMR(F"}

5 STRESS „.„ .,.ceiTeaîA (PS.I.)TtJMPOSAKY PCIWC1PAL

"COWPBESSION. .___ __ ._. _ PBWCIPAL

TEMSfOWOP£J2AIING PS^tCIPAL

CGMPBES5IOWCQWTOEiSiON

(LOCAL)PeiMCIPAL

~ TEWSIOW

- - TCMDWWEABEAE1MG ;

SJEA8 AWCUOSS

6 LÏNF-P.. TUtC^WESS

130°

„_» ^-

) *^~""

u6C*^T:

Al lowab le. Stress es

In ail instances, it is recognized thai the established allovjabj.esof conventional s truc tarai technology cannot be used Verbatim in ameaningful manner. Higher values are permitted in compression Forlocalized stresses, or where concrete is confined. British designers ,are permitted to exceed the compressive allowables of Code U.S. C? 115in locations where two or three dimensional c oppressive stresses ar? pre-sent, provided the excess is justified "expérimentai. ly or in the light ofexisting knowledge". Also, calculated tonsile stressas up to 500 pai arepermitted provided the net force across tJie ve.ase.1 wall section regainsccrapressive.

In general, allowable tensile stresser, in unreinf creed areas rangefrom 'x̂ .̂ /T7" to 7.3/ET"".

Allowable comptess^va bearing stresse,--, at anchor a vary between0.6 f and 0.93 Ir ' ̂ 3; in conventional European practice/., In somePC8VT3crelatively large high-strength concrete anchor bearing blocksare used. While these would seem to reduce significantly the bearingloads ou the PCRV, the blocks th ems elves require considerable develop-ment, and no standards are available. ACT.-3iti (which is «adhered to byU. S, practice) appeals to be the only code prescribing allowable stressethat: account for tendon spacing and hole size.

Two other factors, uot listed on Table III, are worth

Relatively similar criteria appear to have bt?en used, by all designers,A summary comparison is given in Tabla TV. An obvions difference of opinionexists on the question of tendon grouting. The tendons in the Frenchvessels have been grouted. In British and U. S» practice, corrosion isachieved by other means, and tendons are J.&ft unjrroute:' so as to make tendonreplacement possible should it ever be required.

The U. S. criteria imply a conservative approach as regarda the usaof reinforcing steel, In. Great Britain, fch»? C.E.G.B. specifications con-tain no sf-ipulations regarding i:he use of bonded reinforcement. Actualpractice has varied considerably between different designs. The Oldburyvessels have very little bonded reinforcf.meiit and a h.iy.h degree of pre-s tress; the Wylfa vessels, oa the or.her hi-ind, coo tain a considerable ajcountof reinforcement:. Other cylindrical designs include a moderate proportionof bonded steel. The early French vessels alao had little rein forcement. .Later French designs have involved munh higher amounts of bonded niild steel,mainly for purposes of crack control,

38

TABLE IV

Crinaria for Prestre^sing Systems

LIMIT OR ITEM U, S.Practice

Européen

Average axial tensile stress duringjackingAverage, axial tensile stress afteranchoring% Elongation at failure in totaltendon length

30' of straight tendon10" wire length

Corrosion protection

TendonsType

EfficiencyStraight

Uirveci

Tubes

Anchor hardwareAnchor ultimate,percents oftendon ultimate

0.8 £'

0,7 £'

3.54.0

oil or greasesystem

unbonded

100%

95%Solid

0.85

0.7 £'

1.5 tc 3.0

cernant-sandgrout,and greasesystems

unbonded andbonded

100%

Solid andflexible

39

ConclusionAs staged in the introduction, professional groups, both here

and abroad, are working toward the- formulation of such criteria.Drafts are being circulated in France, the U. K. and the U. S. withinnuclear and PCRV oriented groups. It is anticipated that in the nextfew years, we will ^ee the emergence of reasonably unified PCRVcriteria standards.

40

APPENDIX A

Many attempts have been made to derive a single failurecriterion for concrete subjected to the many kinds of combined stresses, sofar without success. The leading candidate theories, Mohr's theory and theoctaii^dral shear stress theory, have been shown to be inappropriate byBresler and Pister (Réf. 1 and 2) and Bellamy (Réf. 3), respectively. Inthe latter work Bellamy makes the point that criteria based on the principalstresses themselves, rather than derivatives of them, are likely to be themost successful. The Mohr theory can be the basis of a satisfactory criterion(Réf. 4) for triaxial compression when the two lesser principal stresses areequal, but fails if one of them approaches zero (Refs. 2 and 5)(or becomesnegative). The point has been made by several investigators that the failurecriterion should be representable by a surface of revolution, or the like,in principal stress space whose axis is inclined equally with respect tothe three principal stress axes. No simply expressed surface of revolutionappears to be capable of representing results of triaxial compression testsin which the least principal stress varies from high values right down tozero, apparently because of a change in mode of failure in the low minimumprincipal stress r.egion which may be due to the large disparity between thetensile and compressive strengths of concrete.

For purposes of PCRV design, this difficulty can be over-come by use of a criterion which makes no pretense of indicating actualfailure throughout the range of its usefulness, but which does representfailure conditions reasonably well in regions where unnecessary conservatismwould be uneconomic, and which is conservative, or even extremely conserva-tive, in regions where economics are not affected. This approach basicallyresults in close prediction of failure in the region where the least principalstress is small and increasing conservatism as the value of the leastprincipal stress is increased. This characteristic is appropriate too inorder to make allowance for the somewhat uncertain effects of pore waterpressure on failure» the effect being small at the lower levels of confiningpressure, but becoming larger at larger confining pressures, as shown byAkroyd (Ref. 6).

Close prediction of failure in the region where the leastprincipal stress is small, demands careful consideration of the work thathas been done on failure of concrete in biaxial compression. Hilsdorf(Réf. 7) has reviewed and discussed quite comprehensively the existingknowledge on this subject and contributed one or two experimental results.Vile (Réf. 8) has also published work on this subject. Although Hilsdorfclaims that lower values of failing stress for specimens under biaxial com-pression will be found when more representative means of loading specimensare employed, it does not seem likely that this will be found to be asubstantial reduction in view of the awareness of this problem and the careta&en in this respect by Vile and Wastlund (Ref̂ 9)

41

The best available claua on failure under biaxial compression;by Vile (Re£. 8}, Wastlund (Réf. 9) and Weigler and Becker (Réf. 10), areplotted in FÏR. V.I,A.I as full-ling curves. The curve labeled "Vile" is theone drawn by him, though the scatter of his results would permit one to drawa curve almost equally well about halfway between his curve and the curve ofWeigler and Becker. The ooints representing equibiaxial compression on thecurves of Vile and of Weigier and Becker are quite well confirmed by Glomb(Réf. 11).

Chirm and Zimmerman (Réf. 12) have summarized prior inves-tigations of failure of concrete under triaxial compression and presentedtheir own work on this subject. Theirs appears to be the only investigationof failure of concrete for both the cases of axial pressure predominatingand confining pressure predominating. They also present data on the uniaxialcompressive strength of concrete remaining after it has been subjected to'high triaxial stresses. The outcome of all these investigations, includingChinn and Zimmerman's, is that for axial pressure predominating the maximumprincipal stress at failure is conservatively given by f£ plus four timesthe confining pressure. Ackroyd (Réf. 6) has found that this is only trueup to about 3 f£ for water saturated concrete « For the case of confiningpressure predominating Chinn and Zimmerman found that the -maximum principalstress at failure is approximately fco plus three times the axial pressurewhere f is the equibiaxial failing stress as determined in their tests.

The criterion chosen for use in PCRV analysis, valid forboth biaxial or triaxial compression is the paraboloidal expression(a,2 + c 2 + a 2) - (a o + o o + a a ) - .25 f (a + a + a ) - .75 f'2 - 0i 2 3 } 2 i 3 2 3 C I 2 3 ^where validity is confined to a < 3 f.* \ c

o. - Maximum principal stresso_ = Intermediate principal stresso «= Minimum pri^o.i-oal strp-?c'3Compression is negative, tension positive

For the case of biaxial compression, a£

For triaxial compression with axial pressure predominating,the criterion is plotted in Fig. .A.2 where it is compared with c^ = f£ H- 4 03,i.e., the two lesser principal stresses are equal. For triaxial compressionwith confining pressure predominating, i.e., the two greater principal stressesare equal. The criterion is plotted in Fig. ,.A.3 where it is compared witho as i.i5 f + 3 o3 where 1.15 f'c is tl-e equibiaxial stress predicted by thecriterion.

The assumed failure criterion represents the biaxial com-pressive strength of concrete fairly well and represents the triaxialstrength conservatively. The degree of conservatism increases as the con-fining stress, and thus the assumed failing stress, increases. The completecriterion is shown graphically in Fig. ,A.4.

ACI-318 sets an allowable stress value (0.45 f^) forextreme fibers without consideration of multiaxial stresses or the higherstresses that occur at discontinuities since such stresses are not predictedby the analysis methods used. The finite element methods used for the PCRVdesign predict in some detail the multiaxial stresses that occur throughoutthe vessel. These predicted stresses are then compared with acceptable ACI-318allowable values which are modified to take into account the multiaxial state•of stress by the utilization of the factor "C". The factor "C" is determinedas follows:

The failure criterion equation is solved for o , the maximumprincipal stress at assumed failure.

1. If all three principal stresses (ĉ , o , op are in compression:

calculate -jr anc* TTc ca

determine C « -rf from Fig. V.I.A.4c

[a | - 0.45 C f£

2. If a « tension, assume a « 00 ^

calculate -77 and -rr * 0c c

determine C - -rr f^om Fig. V.I.A.4c

|a,| £ 0.45 C f'

43

3. If o2 and 0- tension, o, •» a, =• 0C = 1

Jo ! ̂ 0.45 C f^

It is common design pr .ctice for ordinary biaxially stressedstructures to use the full allowable concrete compressive stress (0.45 f^)when the stress in a transverse direction is tension. Under such conditionsconcrete cracks will develop which will reduce the tensile stress fieldsin the concrete. The compressive stresses are carried by "compression colur.ns,"which cannot buckle because of the confinement provided by adjacent concrete.Lateral restrain may also be provided where bonded reinforcement is used.

In triaxially stressed structures where tension occurs, asimilar condition exists. Uniaxial tension with biaxial compression produces"compression planes" which are loaded biaxially in compression. Biaxialtension with uniaxial compression produces "compression columns" which sus-tain full uniaxial compressive loads providing confinement is provided.

It is recognized that this criterion depends on the resultsof test specimens being directly related to the structure where confinementis provided, and that biaxial determinations used as evidence in Fig. .A.Iare not further influenced by platen effects yet uncvaluated by the referencedresearchers. Although these assumptions are perhaps chailengeabie, they areused here in an effort to be as realistic as current failure technology permits.

44

ILr

WE1GLER & 3ECKÊR

0 0.2 0.4 0.6 0.8 1.0 1.2 1 . i* 1.6

f 'c

Fig, STRENGTHS OF CONCRETE STîRJKCTpP TO RTAXIAT.STRESS IN TERMS OF UNÏAXIAL COMPRESSIVE STRENGTH f '

,- - o

f 'c

Fig. .A .2—TK1AXIAL COMPRESSION WITH AXIAL PRESSURBPKWITNATTNG

46

r-1- O

3 -

2 -

0,2 0.6 0.8 1.0

ffc

Fig. .A.3—TRIAXIAL COMPRESSION WITH CONFINING PRESSUREPREDOMINATING

47

•Til-

REFERENCES

1. Bresler, B, and K, S. Pister, Faj lut e of Plain, Concrete,, under ̂ Combined.St_resses_, ASCE Trans. 122, pp. 1049-1068, 1957»

2. Bresler, B. and K. S, Pister, Str: ft&tĥ oj; Concret ejjnder ..Stresses^ AGI Journal 30 5 No. 3S p-t>, 321-345, 1958.

3. Bellamy, C. J., Str e ng t h o f Cone re te un de r Comb i ne d _S t r e sjs , AGI Journal58, No. 4, pp, 367-380, 1961.

4. Balmer, G, G. , Shearing . ..Strength pj L Conerevi:e .under _ljljgh Triaxlal Stres^s-. Compuca t ion pĵ Kohr̂ s ̂ Erive.̂ ope ..aj__a_ Ji'̂ IYJl» ^'" ^* Dept. of Interior,Bureau of Reclamation, Laboratory Report Ko. SP-23, pp. 1-13, 1949.

5. McHenry, D. and J. Karnis g t rerig th^ of n ̂ Corije re t e^ under Comb Ine.d Te ns_i l_e.rar.d^ompr_esslve.iiSt.res:s. Prôc. AGI -54, ppf 829-839, 1958.

6. Akroyd, T. S, W» , Concre te jande.r ̂ Tr iaxial^ , S t :res _s^ Magazine of ConcreteResearch 13, No. 39, pp. 111-118, 1901.

7. Hilsdorf, H., De ternunat io^-oj ̂t^&^J^y^al^§trejn.gtln_ p f Cone re te ,Deutscher Ausschuss fur Scahlbeton, Heft 173, Berlin, 1965.

8» VilGj G, W. D. , Strength _o_f... Concrete under^ Short- Term Static Biaxial^S.treAg.1 Paper P-2, International Conf. on the Structure of ConcreteImperial College, 1965.

9. Wastlund, G. , New Evidence Regarding the Basic Strength Properties ofConcrete,, Betong 1937, Keft 3, pp, 189-205»

10. Weigler, K. and Gf Becker, Exper .ing_nt .s or\ .the k_7racture and De j pjrmaticmBehavior of Concrete at Biaxial Loading» Deutscher Ausschuss furStahlbeton, Heft 157, Berlin, 1963.

11. G 1 crab, Die Ausnutzbarkelt Zwaiacb- ; tger^^Drtick^s^igkelt Des Bétons i,nFlachentragwerken, Congress F.I.P.S.L. Pa, 1, Berlin, 1958.

12. Chinn} J. and R. M. Ziranerrnan, B ehavi or _iof_ PI a .in ._CQRC.re it e^ynde r Var lous,.Hi.gjh_T.r_i.ax.lal....Cp_mpjr_aj_sl._qni ô_andjjnĵ ConjlAt̂ ô nŝ Air Force WeaponsLaboratory, Tech. Report WL TR 64-163, August 1965.

13. University of Illinois, Bulletin Sérias No. 405, Vol. 50t No. 29, 1952.

49

14. Heft 5/54, Vèro'f fentlichungen des Deutshen Stahlbau Verbandee, 1954,V

15. Heft 138, Deutsher Ausschuss flir Stahibeton, Uber die Grundlagen desVerbundes ZIrisehin Stahl und Bebern, Dr,-Juj. Callus Rehm,

16. England, G. L. , Lpns.._TcrTO 'Ifoe.mal.ĵ gejAeA-lX'Striic.t.uresit Conf. on Prestr'essed Concrete. Pressura Vessels , London,1967, Group p, papsr 34 ,

17. 0' Connor, H. £. and J, L. M. Morrison, The. ̂ Effect .of. Meaai.iiStre5S_onLjbguInternational Conferenceon Fatigua of Metals, AShfi, 1956,

18.' Design Data ~ Nelson Concrete Anchor Studs, Manual No. 21, Nelson StudWelding Division of Gregory Industries, Inc.j Lorain, Ohio,

19. Da vis, H» S.> ̂ fĴ c.ts_p̂ ._Htgĥ __Tretiip.e.jr.aturg_.j|xpp.&ure. .Qn.._pQ.ncy.&te, AmericanKuclear Society Meeting, Gatlinburg, Tennessee, June 1965.20. Cottrell, A. H., Theg.ry_̂ of .Jrj.̂ tl̂ Fraetj.ira _in^Stsel and Its, App.l icatlon

to^ Rad 1 a'c i on. %b r 1 t_t 1 ement . "̂ onferenc'e. on Brittle Fracture, p. 1,Culcheth Laboratories, England, November 1, 1957.

21. Pellinij W. S. and ?» P, Puzak, Practical Consideyatigns^inLaboratoryVessels., U. S. Naval Research Report 6030, November 1963.

22 » The_ TStru.c t ura 1 Us e of^Frest r e s sed... Concr e fcef ̂in̂ Buj. Id injgs.» The Council forCodes of Practice^ British Standards Institution, CP 115» 1959.23 « Sjate. _cf. .;grh_ê ^̂ Nuclear.À Critical Review of the Literature, ORNL-TM.-312.24. General Atomic Staff, P.re.s_tr.es_s_ed__C_o.ncr̂

General Atomic report GA-7097, October 25, 1966.23» General Atomic Staff, | 'res très s&ô i _ Cô rĵ tê React.or Ves _s e 1, Mode 1 2 1

General Atomic report GA~7150, November 4, 1966.

26, Newman, K. , The Structure .A?J.-..̂ n̂ 1̂ ee.?M̂ J'XCB̂ Î -̂-vP--CP.P.nS.̂ eĴ .eJ»Proceedings of luternational Symposium oa the Theory of Arch Dams,Southampton, April 1964.

27 . Ros, M, R. and ?, E< Speck, L£r̂ _Tend_on.g__for jr e ŝ ur e JVe.s.sjg I s_in.SiSiÊâL-̂ SSSïLâSâÊiSSS.» Conference -on Prestreseed Concrete PressureVessels, Hfjrch 1967 1 Group E^ Paper 25.

50

28. Harris, A. J. and J. D. Hay, Rupture Design . of. _ jth,e._0j4bury. VesselsConference on Prestressed Concrete Pressure Vessels, Institution ofCivil Engineers, London, March, 1967, Group F, Paper 29.

29. Finigan, A. , .Ult_ima te__ Analy s 1 sx._p £f .the,, Dufig,e fî ĝ ^̂ V̂ags.̂ !̂ . Conferenceon Prestressed Concrete Pressure Vessels» Institution of Civil Engineers,London, March, 1967, Group F, Paper 31.

30. Anthony, R. D. , Development: ̂ô Sj:_a.î:.iat.oir̂ JRe._(̂ u._iTreiine_n_tsj_for. Redactor Vessels,Conference on Prestressed Concrete Pressure Vessels, Institution of CivilEngineers, London, March, 1967, Group Bt Paper 9

51

CONTKIBUTIQN K30M THE UH1TED KINGDOM

I. David?on

The following organizations are concerned in the UK:~ twodesign and construction companies, two customer organizations, theInspectorate of Nuclear Installations» and the Atomic Energy Authority*

The basic philosophy in the UK is to provide adequate pressurerelief valves on each PGRV, to design for the working loads, and fora nominal ultimate load factor, to test models, to carry out a pressuretest and to monitor each vessel throughout its life.

A committee of the British Standards Institution has almostcompleted the first Code of Practice, in line with the philosophysummarised above. It is hoped to extond the Code to cover LimitState Design.

It is suggested that more knowledge about the long termproperties of concrete under working conditions would "be advantageous,and could be facilitated by international collaboration.

53

Introduction1. In the UK there are two reactor design and construction companies, BritishNuclear Design and Construction, and The Wuclear Power Group, within which thedesign and construction of PCRVs is carried out by Messrs. Taylor Woodrow andMessrs. KcAlpine respectively. These two companies have carried out extensivedevelopment programmes both theoretical and experimental, including the testingto destruction of numerous model pressure vessels, and have completed and arecurrently constructing several PCRVs. The two customer organisations, the CentralElectricity Generating Board and the South of Scotland Electricity Board,purchasethe PCRVs from the two construction groups, and the CSGB carries out theoreticalinvestigations into stress analysis and experimental work on the properties ofconcrete. The Inspectorate of Nuclear Installations, Ministry of Technology, isresponsible for issuing licences and, therefore, must be satisfied on all safetyaspects relating to PCRVs. The Atomic Energy Authority is also interested inmatters affecting the safety of nuclear power stations and their current PCRV exper:mental programme mainly examines structures and materials under overload conditions.2» In the UK to date *f PCRVs have been constructed; two of these at Oldbury havebeen in operation for some tv/o years and the other two at Wylfa are at present ueinjcommissioned. A further 8 vessels are at various stages of construction and willall be completed and commissioned within the next three or four years»Basic Philosophy and Current DesignCriteria3, The basic philosophy to ensure the safety of PCRVs is described below:

(a) Reactor safeguards including safety relief valves are provided such

that under any conceivable fault conditions the pressure inside thevessel cannot rise in excess of the safety valve setting of 1.10 xdesign pressure, the design pressure being 1.10 x working pressure»

54

(b) Comprehensive elastic and visco-elastic stress analyses are carriedout for all possible combinations of loadings to ensure that the stressesthroughout the life of the vessel remain within conservative acceptablelimits»

(c) An ultiraate load analysis is carried out to show that with increasingpressure at ambient temperature, and assuming that the liner remainsintact, the vessel is capable of sustaining at least 2-J x design pressurewithout failure»

(d) Fully instrumented model tests are carried out, at pressures up to atleast the ultimate pressure, to show that the design behaves as anticipatedby the calculations.

(e) A pressure test at 1.15 x design pressure is carried out on completionof construction to verify that the vessel as built behaves as predictedby the calculations and the model tests.

(f) The vessel is monitored throughout its life, paying particular attentionto the loads in the tendons and the freedom of these tendons from anyform of corrosion.

4. Although, at the time of the design of the first PCRVs in the UK (at Oldbury),-carried out in 1959 to 1961, computer programmes for stress analyses were notavailable, the first vessels being designed to an ultimate load criterion of not lessthan 3» subsequent checks with the computer programmes now available, both short terraelastic and long terra visco~elastic, have shown that the stresses in these firstvessels are within acceptable limits. Detailed stress analyses have been carriedout at the design stage on all subsequent vessels»5» The purpose of design criteria is to provide sufficient margins to cover threesorts of uncertainties?- loadings, calculations, and strengths. It is consideredthat the provision of relief valves and other safeguards in the UK type of gas-cooled, reactors removes any possibility of significant overpressure, and temperatureexcursions are necessarily slow» Therefore, the design criteria are required todeal only with errors in calculation and unforeseen loss of strength in the structure*The elastic plus visco-elastic design method, with appropriate code stresses, is

55

the classical and most direct way of providing such margins. The ultimate loadfactor is only a different way of providing the same margins because it is neverintended that the vessel will actually be subjected to the ultimate pressure»6. All the PCRVs built to date have been tested in the form of scale models,instrumented to measure strains and deflections and pressurised hydraulically inthe elastic regime and up to at least the intended ultimate load factor. In somecases thermal loads were also applied. It has been found that the elastic behaviouris in close agreement with the analysis for the cold model. Of courset temperaturegradients and creep have effects on the stresses which, whilst they can be estimatedby calculation, can not be completely measured in models.?. During the last 3 years a small Committee set up by the British StandardsInstitution has been drafting a British Standard for PCRVs based on current practicein the UK, and it is hoped that the final draft will be completed in a few monthsfrom now. The draft Standard covers in detail the design, construction, inspection,testing and surveillance during operation of the concrete pressure vessel completeincluding the concrete» prestressing tendons, bonded re-inforcement» liner, penetra-tions, closures, relief devices and permanent instrumentation. The insulation andcooling system are also included» but in brief outline only. The design sectionsett; out the requirements for the loadings to be considered and the stress analysesto be carried out, with guidance on the methods to be adopted for the short termelastic, the long term viaco-elastie and the ultimate load cases. Permissiblestresses for both the concrete and the steel are specified. Whilst much of theDesign Section of the Standard is of a mandatory nature there are, inevitably atthis present formative stage of PCRV design, some Sections which are of an advisorynature with further guidance being provided in the form of appendices. One of therequirements is that the design should be undertaken by a qualified engineerthoroughly experienced in the field of PCRVs.8. In preparing the Standard it has been appreciated that in the concrete structuresfield internationally there is a move towards adopting limit state design. CurrentBritish pressure vessel practice considers in effect two limit states, that of

56

permissible stresses under normal loading condition's and that of collapse of thestructure» As an alternative to the present design methods a more comprehensivelimit state approach is given in very brief outline only in the draft Standard,It is recognised that much more work is required to examine the application ofa full limit state approach to the design of PCRVs. The Drafting Committee intendto study this aspect further after publication of the present draft, with a viewto incorporating, in the form of an amendment, a fuller limit state approach shouldit be found to be practical.9. In the Materials Section reference has been made, where appropriate, to existingBri'L-i-ih 3U--ndar> s but the requirements relating -pacifically to PCRVs, particularly

on material selection, testing and quality control have been set out in some detail.Safety Considerations10. It is considered that the basic philosophy and criteria currently being usedin the UK, and which will shortly be published in the form of a British Standard,

provide adequate safeguards for PCRVs used in conjunction viith gas-cooled reactors

with designs incorporating safety relief valves and other devices to limit the

pressure within the vessel- The safety of the vessels depends on:-(a) comprehensive stress analysis confirmed by model, testing limiting the

stresb',o to acceptaole levels throughout the life of the vessel,

(i,) an adequate nsargin between normal loadings and those required to cause

failure, confirmed by stress analysis and model testing.

(c) careful selection of materials and control of quality throughoutconstruction.

(d) confirmation that the vessel as built behaves as predicted.

(e) knowledge of the behaviour of concrete throughout the life of thevessel.

(f) knowledge of the behaviour of the prestressing tendons throughout the

life of the vessel.

Lon g-1em 3e haviour11. As has already been said, the theoretical analysis of the long term behaviourof a PCJRV under mechanical and thermal loads is now feasible, provided the behaviour

57

of the concrete is known. la this respect the material cannot be divorced fromthe structurê because moisture movement, which is important as regards creep andshrinkaget is largely conditioned by the sise of the structure and by its teraperaturdistribution» Moreover, many of the experiments which are necessary cannot beperformed on a small scales and long times are required» Under these circumstancesthe value of large-scale long-term experiments is obvious, as from them some under-standing of the phenomena may be obtainsdj and ths ông-term behaviour of any otherconcrete in any other structure may b© deduced after relatively short-term tests»12* The failure of concrets under multi-axial stresses is a subject which hasengagod very much attention for many yeara^ but there still seems to be room fordiscussion concerning the safe long-tens multi-axial working stresses which mayb » appropriate in a thick-walled structure such as a PCRV» A further distinctionmay be appropriate as between mechanical loads and strain-induced loads.13» Concrete studies of the types just mentioned ara being carried out in severalcountries, and it could be of mutu