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L. Damiani et al. / Nuclear Engineering and Design265 (2013) 154163 155

Nomenclature

Symbol

D diameter (mm)

kHe helium thermal conductivity (W m1 K1)

p pitch between bayonets (mm)

WP5: safety and transient analysis;WP6: lead technology;

WP7: education and training.

Ansaldo Nucleare is involved in WP0 and leads the WP3. The

main objectives of WP3 are the design of a reference configura-

tion for the LFR industrial plant and the definition of the ALFRED

demonstrator reactor main features.

Regarding ALFRED, it is worth to remind that the project activi-

ties deal with the following objectives:

the definition of the main suitable characteristics and design

guidelines for the facility; the design of a scaled demonstrator fully representative of the

industrial size reactor; the use of components/technologies already availablein the short

term to be able to proceed in the near future to a detailed design

followed by the construction phase; theevaluation of safety aspects through a preliminary safety anal-

ysis; the demonstrator cost minimization.

About ALFRED, Ansaldo Nucleare is responsible for the design

of the primary system equipments (i.e. steam generator, primary

pump, reactor vessel, and reactor internals), and the develop-

ment of a passive, independent, redundant and diverse decay heat

removal system; in addition, Ansaldo Nucleare leads all the design

activities performed by the partners in the WP3, in particular the

development of the functional and mechanical design of the reac-tor control and shutdown systems and the primary fuel transfer

system.

Consistently with the LEADER objectives, the demonstrator

designshallbe characterized bya relatively lowpower(300MWth),

with a compact design to reduce the cost. For investment pro-

tection, it should be based as much as possible on simple and

removable components, operating at the lowest temperatures

compatible with the pure lead, chosen as primary coolant.

2. Main features of ALFRED

ALFRED has been conceived as a demonstration reactor with

300MW thermal power and a global efficiency higher than 40%,

depending on the secondary cycle configuration. The primary leadsystem has been designed as a compact pool covered by plenum of

Argon, lead circulation being assured by mechanical pumps neces-

sary to overcome a pressure loss of the loop expected to be about

1.5 bar. Lead temperature range is expected between 400 C and

480 C.

The fuel used is MOx with a cladding of T91 steel plus a coat-

ing; theclad designmaximum allowabletemperature is 550 C.The

main safety vessel material is austenitic stainless steel with eight

integrated steam generators.

The secondary fluid is water, entering the steam generator (SG)

at 335C and leaving it at 450 C, with a live steam design pres-

sure of 180 bar. Thesecondaryloopincludeseightsteam generators

submerged in the reactor pool and arranged around the core, each

equipped with an axial pump for lead circulation.

Fig. 1. ALFRED reactor block front view.

The safety approach adopted requires two independent, redun-

dant and diverse passive decay heat removal (DHR) systems. In

ALFRED each DHR system is composed by four independent loops

connected to one reactor SG. The unavailability of one DHR system

loop is taken into account with the design requirement to remove

the decay power with only three out of the four loops available, in

thehypothesis of lead primary loop operating in natural circulationcondition.

The first DHR system is based on the Isolation Condenser

concept, which has already been proposed and tested for SBWR

(Burgazzi, 2002) and investigated for LFR (Leoncini et al., 2009;

Damiani and Pini Prato, 2012).

The ALFRED reactor vessel is a cylindrical-shaped containment

closed at its bottom by a hemispherical cap, as shown in Fig. 1. It

is made of austenitic stainless steel and is located inside a cavity

in the ground for most of its height. The inner vessel assembly is

also cylindrical and from it eight ducts detach, bringing the lead

through the eight pumps to the SGs.

Regarding the secondary loop, the best option seems to be the

Rankine cycle operating in parallel with a desalination plant, able

to combine high efficiency and good behaviour during discontinu-

ous operation. Electricity and fresh water generation would not be

related, giving another important advantage. The main parameters

of ALFRED are indicated in Table 1.

3. The bayonet-tube steam generator

The bayonet-tube steam generator consists of a bundle of bayo-

nets organizedin an equilateral triangular cell immersed in thelead

vesselpool. The bayonet-tube (De Fur, 1975;Belloni et al., 2011) is a

vertical tube with external safetytube and internal insulating layer

and it is composed by 4 coaxial tubes, namely:

the slave tube;

the inner tube;

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156 L. Damiani et al. / Nuclear Engineering and Design265 (2013) 154163

Table 1

ALFRED main parameters.

Power 300 MWth

Net efficiency > 40%

Primary system Pool, compact

Primary circulation 4 Mechanical pumps

Primary pressure loss

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Table 2

SG thermal-hydraulic design data.

Working condition foreach SG

Primary loop Secondary loop

Fluid Lead Water

Design pressure Atmospheric pressure 180 bar

SG i nlet temperature 480 C 335 C

SG outlet temperature 400 C 450 C

Mass flow rate 3247.54 kg/s 24.068 kg/s

Exchanged p ower p er S G 37.5 MWth (300MWthdistributed over 8 SGs)

the gap between the slave and the inner tubes in order to minimize

the thermal power transferred to water. The three main configura-

tions tested are here reported, and they are named A, B and C

following the chronological order of investigation.

Bayonet configuration Case A

CaseA (Fig. 3) consists of four coaxial pipes sized, respectively,

9.52 mm (slave pipe), 12.7 mm (inner pipe), 19.05mm (interme-

diate pipe) and 25.4mm (external pipe). The thickness has been

set to 0.81mm for slave and inner tubes, to 1.88mm for the outer

tube and to 2.11mm for the outermost tube. For Case A, a pitch-to-diameter ratio of 1.355 (meaning a pitch equal to 34.417 mm)

has been postulated for the elementary equilateral triangular cell

of theSG. This information is neededfor thesettingof thebayonet

tube RELAP 5-3D model. Bayonet tube Case B

Case B (Fig. 4) is the same as Case A, except for the diameter of

theinnermost tube setto thesmallervalueof 6 mmdiameter. The

same pitch-to-diameter ratio and the same tubes thickness has

been assumed. This has entailed better exchange performances

but still not enough to justify the not commercially available size

of 6mm; therefore, this solution has been abandoned. Bayonet tube Case C

InCaseC(Fig.5), theexternaldiameterof thefour coaxial tubes

has been set to 9.52mm, 19.05 mm, 25.4mm and 31.73 mm; thewall thicknesses have been setto respectively1.07mm, 1.88mm,

Fig. 3. Bayonet tube configuration A.

Fig. 4. Bayonet tube configuration B.

1.88 mm and2.11mm thick.The pitch-to-diameter ratio is in this

case 1.42.

4.1. SG bayonet tube: RELAP 5 input model

Fig. 6 shows a scheme of the RELAP 5-3D model for the perfor-

mance analysis of the single bayonet tube. The bayonet component

has been simulated considering:

- the descending inner pipe, conveniently divided into two pipes(RELAP 5 components 015 and 017 indicated in Fig. 6) in order to

Fig. 5. Bayonet tube configuration C.

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Fig. 6. Single bayonet tube RELAP5-3D scheme.

account forthe thermal insulation provided by theslavetube;the

influence of inner pipe thermal insulation on the bayonet perfor-

mance has been in fact investigated, selecting different lengths of

the slave pipe;

- the small hemispherical cap at the bottom of the bayonet where

flow inverts (component 019);

- the riser annulus where evaporation takes place (component

021).

Thewatermass flowrate hasbeen imposed by thetime depend-

ent junction 01 2, while the time dependent volumes (011 and

0 25, respectively) provide the inlet temperature and the outlet

pressure.

The primary lead circuit is simplified and described by a

Relap5-3D pipe component (00 5) which simulates the elemen-

tary triangular cell by the setting of a proper hydraulic and thermal

diameter. Again, the lead mass flow rate has been imposed by a

time dependent junction (00 2), while the inlet temperature and

the outlet pressure have been set by the time dependent volumes

0 01 and 00 9, respectively.

Three heat structures (HS) arepresent in themodel.Two ofthem

(HS 001 and HS 002) thermally connect the inner descending pipe

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with the ascending annulus through three layers, which are T91

stainless steel, eventual insulating material and T91.

Thethirdone (HS003)thermally connects theprimary lead loop

with thesecondarysteamone andis composedby fivelayers,which

are, from the innermost to the outermost:

a fouling thickness water-side (50m thick and with a conduc-

tivity of 2 W/m K);

T91 stainless steel; the helium gap containing high conductivity particles; T91 stainless steel; the tantalum layer that protects the bayonet surface from the

flowing lead (50m thick with a conductivity of 57.5W/mK).

Since RELAP5 uses mainly heat transfer correlations for water,

an accurate control of the heat transfer coefficient on the lead side

of the bayonet has been necessary. Ushakovs Kirillovs, Mikityuks,

Mareskas and Dwyers correlations (OCED, 2007) all estimate a

heat transfer coefficient of about 10,000W/m2 K, ensuring that the

value of 9400W/m2 K calculated by the code is adequate and even

so more slightly conservative.

4.2. Four metres tube length case

At the beginning of the investigations performed, very conser-

vative values of the safety helium gap thermal resistance were

provided, with conductivity kHe ranging between 35 times and 55

times that of the helium: i.e., referring to the temperature range

of interest, between 6.35 and 10.54W/m K in the adverse case and

between 9.98 and16.56 W/mK in thefavourable case. Later, further

and more detailed information described the 55 times helium

conductivity value as fully reliable and even slightly conservative,

allowing hence a reduced number of simulations for the 5 and 6 m

case.

The initial 4 m length bayonet has been investigated referring

to the configurations A and B afore indicated. The results here

presented include a sensitivity analysis envisaging a tube with:

no insulation (slave tube absent); 3 m long water insulation (thermal conductivity varying in the

range 0.4110.494W/m K, depending on temperature); 3m long insulating paint insulation (thermal conductivity of

about 0.05 W/mK); 3m long vacuum insulation (thermal conductivity of about

0.0001 W/m K).

The four cases above listed have been studied for both low(35

times kHe) gap conductivityand high (55 timeskHe) gap conductiv-

ity but only the latter is reported, being the former not relevant.

Concerning the mass flow rates per bayonet for both water and

lead, they have been set to 0.029 kg/s and 3.823 kg/s respectively,values corresponding to an initial estimated number of 840 bayo-

nets of per SG. The results appear in Table 3.

The analysis of the results shows that:

(1) the better is the insulation, the higher is the exchanged power

and the fewer the number of necessary tubes;

(2) theabsence of droplets in theoutlet steam canbe achieved only

using the vacuum insulation;

(3) all the cases tested allow to reach a temperature widely too

low in comparison with the required 450C and would require

a higher number of tubes than expected. The low outlet tem-

perature is mainly due to the small tube length and hence

insufficient surface for heat exchange in the present bayonet

configuration.

On the basis of the considerations above reported, the 4 m long

configuration has been judged not suitable.

4.3. Five metres tube length case

The insufficient thermal exchange performed by the 4m long

tube hasrequiredan increase of theheat exchange surface, realized

by a longer bayonet.

The A and C bayonet configurations have then been inves-tigated in this case. The thermal inner tube insulation means have

beenkept the same as in the 4m case.

The results in Table 4 presented refer to a 5 m tube sensitivity

analysis envisaging:

no insulation (slave tube absent); 3 m long water insulation (thermal conductivity varying in the

range 0.4110.494W/m K, depending on temperature); 3 m long insulating paint insulation (thermal conductivity of

about 0.05 W/mK); 3m long vacuum insulation (thermal conductivity of about

0.0001 W/m K).

Table 4 outlines a moderate improvement in comparison with

the 4m case, while keeping a similar trend: the better is the insu-

lation, the higher is the heat exchange, with small differences

between vacuum and insulating paint. However, again only the

vacuum case assures the complete outlet steam dryness. Further

drawbacks are the relatively high number of tubes for the whole

reactor (more than 6600) and the low stiffness of the tube, due to

its high thinness and sharpness, which requires numerous spacers

to keep the structure in position.

As a consequence, the larger diameter configuration C has

been introduced, as it allows an increased heat exchange, thanks

to the more extended surface of the tube. The lead mass flow-rate

has been set to 6.367 kg/s while the water-steam has been set to

0.0471kg/s which both correspond to an approximate number of

510 tubes per SG.As expected, the results in Table 5 indicate that the exchanged

powerper bayonet widely increases (by about 55%), strongly reduc-

ing the number of tubes necessary. Also, the droplets absence in

the outlet steam is ensured even with the insulating paint. Since

the outlet steam temperature remains widely below the target of

450 C, an even larger exchange surface is necessary: therefore, the

6 m case has then been investigated.

4.4. Six metres tube length case

As in the previous cases, different inner pipe insulation geome-

tries have been investigated. Theinsulation lengths below 3 m have

been abandoned after the first trials and a 6 m insulation, that is as

long as the whole inner tube, has instead been introduced.

The same 4 insulation means described in the previous para-

graphs have been simulated. Also, only the highest conductivity

value for the safety gap (55 times helium conductivity) has been

adopted. For 6 m bayonets length, only the configuration C has

been investigatedwith the same mass flow rate values used for the

5 m long tube.

The results here presented include a 6 m tube with:

no insulation (slave tube absent); both 3 and 6 m long water insulation (thermal conductivityvary-

ingin therange0.4110.494 W/mK, depending on temperature); both 3 and 6 m long insulating paint insulation (thermal conduc-

tivity of about 0.05W/mK);

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Table 3

Performances in case of favourable gap conductivity (55 times kHe) case.

Four metrescase configuration A

Insulation Power per bayonet (kWth) Outlet Tsteam(C) Outlet static qual. Outlet void fraction Tubes for 37.5 MW

None 42.214 429.5 0.958 0.99453 888

Water 43.407 440.0 0.996 0.99944 864

Paint 43.460 441.0 1.000 0.99993 863

Vacuum 43.520 441.1 1.000 1.0000 862

Table 4

Five metres tubes configuration A: performance.

Five metrescase configuration A


None 43.922 448.0 0.979 0.99736 854

Water 45.466 457.0 0.996 0.99955 825

Paint 45.754 459.1 1.000 0.99995 820

Vacuum 45.794 459.4 1.000 1.00000 819

Table 5

Five metres tubes configuration C: performance.

Five metrescase configuration C


None 68.145 424.7 0.984 0.99775 550

Water 70.096 432.3 0.999 0.99987 535

Paint 70.283 432.3 1.000 1.00000 534

Vacuum 70.306 432.4 1.000 1.00000 533

both 3 and 6 m long vacuum insulation (thermal conductivity of

about 0.0001 W/m K).

Tables 6 and 7show that the 6m long bayonet is eventually

able to reach the required thermal power exchanged giving an

outlet temperature even slightly higher than required with both

3 and 6 m long insulation. Both the vacuum and the insulating

paint options are suitable to assure the dryness of the outlet

steam and the desired number of 510 bayonets. Evident advan-

tages in the construction process (a much easier and cheaper

construction) lead to abandon the 3m long slave as well as the

vacuum insulation. The considered geometry also makes very

difficult the vacuum conservation because of the likelihood of

leakages due to the high steam pressure (180bar). Moreover, a

6-metres-long slave tube enhances slight improvement in ther-

mal performances, visible in terms of exchanged power and outlet

temperature.

What exposed above shows that the most convenient configu-

ration is the option with a 6-metres-long bayonet equipped with

special paint insulation for the whole tube length. The absence of

droplets is testified by the outlet void fraction and static quality

both equal to 1. Furthermore, the slightly higher outlet tempera-

ture obtained can be useful to compensate possible heat losses in

the steam plenum and in the steam line.

5. SG design and analysis

After having sized the single bayonet, another investigation has

involved the whole SG in order to take account for the other non-

negligible components of the device, such as:

The feed-water line; The feed-water chamber; The upper stretch of the descending tube;

Table 6

Six metres bayonet with 3 m insulation: performance.

Six metrescase insulation 3m long

Insulation Power (kWth) Outlet Tsteam(C) Outlet static quality Outlet void fraction Tubes for 37.5 MW

None 70.396 438.9 0.976 0.99682 533

Water 73.552 451.2 0.999 0.99988 510

Paint 73.686 451.4 1.000 1.00000 509

Vacuum 73.720 451.5 1.000 1.00000 508

Table 7

Six metres bayonet with 6 m insulation: performance.

Six metrescase insulation 6m long

Insulation Power (kWth) Outlet Tsteam(C) Outlet static qual. Outlet void fraction Tubes for 37.5 MW

None 70.396 438.9 0.976 0.99682 533

Water 73.512 451.4 0.999 0.99988 510

Paint 73.765 451.9 1.000 1.00000 508

Vacuum 73.809 452.1 1.000 1.00000 508

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Theupperstretch of theannulus,whichpasses through theArgon

plenum just above lead and through the Helium plenum; The steam plenum; The steam line.

The only aim of this phase of the work has been the thermal

sizing of the heat exchangers; hence, no investigation about the

flow stability has been conducted.

5.1. Steam generator design data

According to the SG tube configuration illustrated in Section 3,

the investigation of the whole component started implementing all

the geometrical design data reported in Table 8 which rely on the

single bayonet analysis, i.e. a 6 m long configuration C.

Table 9 summarizes the hydraulic working condition for pri-

mary and secondary loops: the thermal cycle remains the same,

with the only modification of the mass flow-rates, to account for

the presence of all the 510 bayonet tubes.

5.2. Steam generator RELAP5 input model description

The above described SG has been modelled in order to simulate,

through theRELAP5 tool, a 100% power case andpredict therelatedperformances.

Fig. 7 shows the two subsystems in which the model is divided,

theprimary lead one(in red) andthe secondarywaterone (inblue).

As in the single bayonet investigation, the lead systemis composed

by only 5 elements having a simplified noding. The mass flow-rate

(3247.54kg/s) is again imposed by the time dependent junction

Fig. 7. Steam Generator RELAP5-3D nodding.

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Table 8

Steam generator design geometry.

Steam generator geometry Bayonet tube

Number of coaxial tubes 4

Slave tube outer diameter 9.52 mm

Slave tube thickness 1.07 mm

Inner tube outer diameter 19.05 mm

Inner tube thickness 1.88 mm

Outer tube outer diameter 25.4 mm

Outer tube thickness 1.88 mmOutermost tube (gap) outer diameter 31.73 mm

Outermost tube thickness 2.11 mm

Length of exchange 6 m

Argon plenum height 1 m

Helium plenum height 0.8 m

Steam plenum height 0.8 m

T91 plate height 0.25 m

Number of tubes 510

P/D (triangular array) 1.42

(component 10 1 in Fig. 7), whilst the inlet lead temperature is

imposed through the time dependent volume 1 0 0.

The water-steam system is fairly more complicated than the

previous singlebayonetinvestigation, as it includes somenew com-

ponents upstream of the descending pipe, i.e.: the feed-water line(202); the feed-water plenum (206) and the uppermost part of

the descending tube (212), that is the one passing through the

steam, the helium and the argon plena. The water mass flow rate

(24.068 kg/s) and its inlet temperature (335 C) are again imposed

by respectively time dependent junction 201 and time depend-

ent volume 200. Downstream of the previous model, three new

pipes (components 242 and 248) have been included in order to

simulate the additional bayonet length (not effective in terms of

heat exchange with the primary lead) introduced to account for

the arrangement of the bayonet tubes in the SG plates (see Fig. 2).

The steam line (pipe 252) entering eventually the time depend-

ent volume 254, which imposes the steam line current pressure,

completes the model water side.

Heat structures 00 1 and 00 4 represent the double externaltube wall and slave tube wall respectively, while the 00 2 and the

00 3 represent the slave tube wall in the extended portion of the

bayonets.

The double external wall of the bayonet thermally connects the

secondary fluid to the helium and argon plena through the heat

structures 00 5 and 00 6. All the thermal characteristics of the

materials have been kept the same as in the final configuration of

the single bayonet, previously discussed.

The bayonet tubes bundle has been modelled through one

collapsed pipe of the suitable cross-section crossed by the total

water mass flow rate; pressure drop and heat exchange are

characterized through the single tube hydraulic andthermal diam-

eter.

A tantalum thickness has also been simulated on the shellside, which protects bayonets from the erosion caused by primary

lead.

Table 9

SG working condition.

Working condition foreach SG

Primary loop Secondary loop

Fluid Lead Water

Pressure Atmospheric 180 bar

Temperature at the SG inlet 480 C 335 C

Temperature at the SG outlet 400 C 450 C

Mass flow rate 3247.54 kg/s 24.068 kg/s

Exchanged thermal power 37.5 MW

Table 10

Global steam generator performance.

Steam generator performance

Number of tubes (#) 510

Removed power (MW) 37.5

Core outlet lead temperature (C) 480.0

Core inlet lead temperature (C) 401.3

Feed-water temperature (C) 335.0

Immersed bayonet steam outlet temperature (C) 451.3

Steam plenum temperature (

C) 450.1Steam plenum void fraction () 1.000

Steam plenum static quality () 1.000

SG steam/water side global pressure drop (bar) 3.30

5.3. SG full power steady state condition

The investigation on the steam generator has focused on the

steady state condition of 100% power and therefore no transient

investigation and simulation have been conducted.

The model has been set up with initial conditions for tempera-

tures, pressures and mass flow rates approximately near those of

the steady state. The end of calculation has then been set to obtain

stabilization of the system properties in each component.

The predicted performances are shown in Table 10 and wellexplained by Figs. 810.

The total power exchanged is 37.5MW in accordance with the

target: thisvalue is calculatedby thecode through a control variable

which sums up the amount of power exchanged in each volume of

the bayonets. The lead temperature at the SG outlet, that is the

core inlet, is 401.3 C, a bit more than expected (400C) probably

Fig. 8. Steam generator temperatures profiles.

Fig. 9. Steam void fraction vs.S.G. Annulus length.

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Fig. 10. Static quality vs. S.G. Annulus length.

dueto theRELAP 5-3D correlation adopted forthe lead specific heat

capacity.

The predicted steam temperature is 451.3 C at the end of the

exchanging stretch of the bayonet and decreases down to 450.1

Cin the steam plenum due to the heat exchange with the cold water

entering the tubes. As shown in the plot ofFig. 8, approximately in

the first half metre of the annulus at the bottom of the bayonet, the

water reaches saturation.

The non-uniformity of the heat release has not been taken into

account in the present calculations. The spatial non-uniformity

existing in the system may lead to an increase of the bayonets

number required to obtain the desired power exchanged. Further

calculations, for example through CFD codes, are planned for the

future development of the component design.

As indicated in Fig. 8, the phase change takes place between

about 0.6 and 3m while the evaporation of the liquid droplets

present in thevapour phase is complete by3.6 m of exchanging sur-

face. This is well displayed in the Void Fraction (vapourvolume to

total-volume ratio) graph (Fig. 9) which also shows that the evapo-

ration begins after 0.6m. Static quality (vapourmass to total-mass

ratio) and voidfraction are equal to1 for the last 2.7 m, assuring the

superheated dry steam condition at the outlet.

6. Conclusions

The presented paper deals with the conceptual design of the

ALFRED lead demonstrator reactor steam generators, for which an

innovative bayonet tubes geometry has been proposed. The work

has first concerned the modelling through the RELAP 5-3D code of

a single bayonet tube and after of the whole steam generator.

Several bayonet configurations have been investigated, includ-

ing different lengths of the bayonet, different insulation methods

between inner and outlet tube and different lengths of the

insulation. The configuration finally chosen is a 6 m long bayonet

(referring to its active part submerged in the primary lead), with

a special paint 6 m long insulation of the inner tube.

The performance of the whole steam generator with its bundle

of 510 tubes has been finally investigated, assuring the attain-

ment of therequiredSG performances, namely: a 37.5MW thermal

power; a steam outlet temperature of 450 C; fully dry steam atthe

exit.

The simulations performed with the dedicated RELAP5-3D

Steam Generator model have provided satisfactory results, which

prove the feasibility of the bayonet tube steam generator for

ALFRED, the demonstrator of the industrial full size LFR plant.

The RELAP calculations carried out in the present work have

been useful for the conceptual design, at a preliminary level, of the

bayonet tube and the SG; however, a more thorough investigation

is required to complete the component design, for example with

the aid of a CFD solver, both for the single tube analysis (Sun and

Yang, 2013) and for the whole SG. In the second case, the analysis

will be carried out once the main components adjacent to the SG

are defined in detail, in order to allow the imposition of the correct

boundary conditions.

Investigations about structural and material resistance are still

to be conducted, leading then to an experimental phase which will

be performedwithin the LEADER consortium to complete the anal-

ysis.

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