consequences of hydrogen pick-up in fuel cladding

Consequences of hydrogen pick-up in fuel cladding A literature study

2010

Kristiina Sadian, Rickard Johansson and Christian Alex Uppsala University

5/25/2010

[CONSEQUENCES OF HYDROGEN PICK-UP IN FUEL CLADDING] den 25 maj 2010

Uppsala University | Abstract 1

ABSTRACT As long as fission nuclear power is used, efforts will be made to maximize revenues and increase the power

output to fuel input ratio. This includes higher burn-ups, uprating, and longer fuel cycles, resulting in higher

demands on the entire reactor. However, the focus of this study lies in the limitations of the fuel cladding,

more specifically, in the consequences of hydrogen pick-up.

Oxidation of the Zircaloy cladding liberates hydrogen, of which a part diffuses into the Zircaloy matrix and

eventually precipitates to zirconium hydride platelets. This degrades the mechanical properties of the Zircaloy,

and causes hydrogen embrittlement, delayed hydride cracking, and formation of hydride blisters, which in turn

might result in fission gas release, and other undesirable consequences.

Therefore, it is imperative that research continues to investigate known phenomena, which might behave

differently with new operating parameters. In Sweden, this is carried out by Studsvik, who recently prolonged

an international research project aiming to gain an in-depth knowledge of cladding related issues.

den 25 maj 2010 [CONSEQUENCES OF HYDROGEN PICK-UP IN FUEL CLADDING]

2 Abstract | Uppsala University

TABLE OF CONTENTS

Introduction and background .................................................................................................... 3

Purpose and aim of this study ............................................................................................................................. 3

Nuclear reactors in the world today .......................................................................................... 4

General cladding information .................................................................................................... 5

What is the cladding? .......................................................................................................................................... 5

PWR ..................................................................................................................................................................... 5

BWR ..................................................................................................................................................................... 5

CANDU................................................................................................................................................................. 5

Desired cladding properties ................................................................................................................................ 6

Cladding materials ............................................................................................................................................... 7

Fuel cladding materials used in Swedish reactors............................................................................................... 8

Cladding related problems ......................................................................................................... 8

Hydrogen pick-up ....................................................................................................................... 9

The hydrogen pick-up mechanism ...................................................................................................................... 9

Hydrogen content at different burn-ups ........................................................................................................... 10

Zirconium hydride properties ........................................................................................................................... 11

Effects of hydriding .................................................................................................................. 12

Hydrogen embrittlement .................................................................................................................................. 12

Delayed hydride cracking .................................................................................................................................. 12

Blistering ........................................................................................................................................................... 13

Incidents caused by hydriding in fuel claddings ....................................................................... 13

Transporting used up fuel ........................................................................................................ 14

Transporting damaged fuel claddings ............................................................................................................... 15

Fission gas release and radiation dosages ............................................................................... 15

Current Research and Development ........................................................................................ 16

Ongoing projects ............................................................................................................................................... 16

Cladding material .......................................................................................................................................... 16

SCIP ............................................................................................................................................................... 16

Bibliography .............................................................................................................................. 17


Uppsala University | Introduction and background 3

INTRODUCTION AND BACKGROUND A huge part of the research conducted on nuclear power has been focused on minimizing the downtime of the

reactors. A substantial part of the research conducted has been proven to be successful but there is still work

to be done. One issue that can cause a reactor shut down is the phenomenon of hydrogen pick-up in the fuel

cladding, which is the part of the fuel assembly as can be seen in Figure 3.The hydrogen can cause the cladding

to become brittle and crack which leads to radioactive material leaking out into the reactor water and in turn

cause the radiation levels outside the reactor to rise. When these radiation levels reach values that are

unacceptable the fuel assembly has to be replaced which cannot be done during operation. Changing the fuel

assembly thereby requires a shutdown of the reactor and the maintenance performed such as the cooling and

the fuel exchange typically requires the reactor to remain inactive for a week which of course is a very

expensive consequence.

Other reasons for the continued research in hydriding are the power uprate of the Swedish reactors and the

progress toward higher and higher burn-ups. The government has allowed uprating of Ringhals 1 and 2,

Oskarshamn 2 and 3, and Forsmark 1-3 and this power uprate has already begun to some extent. This, of

course, creates higher demands on the fuel cladding.

PURPOSE AND AIM OF THIS STUDY

This project is mainly a literary study with the purpose of creating an overview of the mechanisms and

consequences related to hydrogen uptake as well as giving insight to current research and yet unanswered

questions. It is a collaboration between Uppsala University and Vattenfall AB, with the purpose of giving

students an opportunity to apply their knowledge on “real” problems at the same time as Vattenfall benefits

from the students’ fresh perspectives, overall an exchange of in-depth knowledge and understanding. A visit

was made to Studsvik in Nyköping, together with Hans Henriksson, our supervisor, and Anna-Maria Wiberg,

both from Vattenfall. At Studsvik, experts in the research field of cladding related issues; Anna-Maria Alvarez,

Clara Anghel, Slava Griegoriev, held seminars as well as gave a guided tour of the laboratory facility for

mechanical testing. Furthermore, interviews have been made with senior specialists at Studsvik, Anna-Maria

Alvarez, at Westinghouse, Mats Dahlbäck, and at the Oskarshamn power plant, Gunnar Rönnberg.


4 Nuclear reactors in the world today | Uppsala University

NUCLEAR REACTORS IN THE WORLD TODAY There are 438

1 nuclear reactors in the world today (May 2010) operating in 30 countries, accounting for 15 %

of the world’s total electricity production. Table 1, below, shows general information about the nuclear

reactors in the world today. Worth noticing are the different types of nuclear reactors, water cooled and

moderated reactors stand for 91 % of the total amount of reactors.

TABLE 1. GENERAL INFORMATION ABOUT NUCLEAR REACTORS IN THE WORLD 20081.

Reactor type Main Countries Number GWe Fuel Coolant Moderator

Pressurised Water Reactor (PWR)

US, France, Japan, Russia,

China

265 251.6 enriched UO2 water water

Boiling Water Reactor (BWR)

US, Japan, Sweden

94 86.4 enriched UO2 water water

Pressurised Heavy Water Reactor 'CANDU' (PHWR)

Canada 44 24.3 natural UO2 heavy water

heavy water

Gas-cooled Reactor (AGR & Magnox)

UK 18 10.8 natural U (metal),

enriched UO2

CO2 graphite

Light Water Graphite Reactor (RBMK)

Russia 12 12.3 enriched UO2 water graphite

Fast Neutron Reactor (FBR)

Japan, France, Russia

4 1.0 PuO2 and UO2 liquid sodium

none

Other Russia 4 0.05 enriched UO2 water graphite

TOTAL 441 386.5

In Sweden, there are 10 reactors operating currently at three different locations; three BWRs at Oskarshamn,

three BWRs at Forsmark, and tree PWRs and one BWR at Ringhals. Figure 1 shows the ownerships of the

Swedish nuclear power plants where Vattenfall AB holds the majority ownership of Forsmark and Ringhals. In

Sweden during 2008, the total electricity production was 146 TWh of which nuclear power was 61,3 TWh which

corresponds to about 42%.2

FIGURE 1. OWNERSHIP OF NUCLEAR POWER PLANTS IN SWEDEN 20092 .

1 (World Nuclear Association)

2 (Svensk Energi)


Uppsala University | General cladding information 5

GENERAL CLADDING INFORMATION

WHAT IS THE CLADDING?

Cladding is used to keep fuel in place, which is necessary to control the reactor. It is also used to confine fission

gases so that they don’t escape out of the reactor core. Since the most common types of reactors are PWR,

BWR and CANDU, these claddings will be shortly discussed.

PWR

The enriched fuel pellets are inserted into Zircaloy tubes that are bundled together. The tubes are

approximately 1 cm in diameter and the fuel cladding gap, which is about 0.13 mm, is filled with helium gas to

improve the conduction of heat from the fuel to the cladding and to prevent pellet cladding interaction. One

assembly is about 4 m long and consists of 179-264 tubes. The reactor core is formed by 121-193 assemblies.4

BWR

The BWR fuel assemblies are similar to those used in PWRs. However, each BWR fuel assembly is put into a

canister to prevent local density variations, and thus prevent changes to neutronics and to thermal hydraulics.

The assemblies consist of 91, 92 or 96 fuel rods and between 368 and 800 bundles are loaded into the reactor.4

CANDU

The CANDU assemblies consist of unenriched (or low enriched) fuel pellets inserted in Zircaloy tubes that are

bundled together. Each assembly is about 0.5 m long and consists of 37 or 43 fuel rods and the reactors core

consists of 4500-6500 fuel assemblies.4

3 (Gerry d. Moan)

4 (Nuclear Fuel)

FIGURE 2. CANDU FUEL ASSEMBLY4.


6 General cladding information | Uppsala University

DESIRED CLADDING PROPERTIES

The fuel cladding is constructed to withstand the tough conditions and the strain it faces in a nuclear reactor. A

crack in the cladding could cause contamination of radioactive materials in the moderator, or worse yet, an

even more damaged cladding could cause release of the fuel into the moderator, thus losing control of the

fission processes. This of course creates extremely high requirements on the fuel cladding, that it should have

all the right properties. These attributes5 are:

Adequate σy at high T and during irradiation

Resist corrosion

Dimensionally stable

Predictable mechanical properties

High thermal conductivity

Low neutron capture cross-section

Easy to fabricate and install

Easy to reprocess

Low cost

Low demand on scarce resources

5 (Kulcinski)

FIGURE 3. THE LEFT ASSEMBLY IS A WESTINGHOUSE PWR FUEL DESIGN. THE MIDDLE ASSEMBLY IS A WESTINGHOUSE SVEA-

96 OPTIMA2 BWR FUEL.


Uppsala University | General cladding information 7

CLADDING MATERIALS

Naturally, the type of cladding needed depends on which type of reactor is being used as this determines the

operating parameters. Figure 4 shows operating temperature ranges for different reactors and claddings. The

first industrial-scale nuclear power plant in the world used magnox fuel cladding, a magnesium alloy. However,

disadvantages such as a very limited maximum temperature and a high reactivity with water, thus preventing

long-term storage under water, has in the long run disfavored these types of reactors and cladding. Other

popular claddings consist of stainless steel as well as aluminum. Stainless steel has found use in fast reactors

and in light water reactors where the neutron-capture cross section is less important. Aluminum on the other

hand has a low neutron-capture cross section but has inferior physical strength and poor corrosion resistance

at temperatures above 149°C.5

FIGURE 4. OPERATING TEMPERATURE RANGE FOR CLADDING MATERIAL FOR FISSION REACTORS6.

However, a vast majority of the reactors in use in the world today are water-cooled in which the element that

best fits the criteria for cladding is zirconium. Thus, in all water moderated reactors different zirconium alloys

are used. The beneficial properties of zirconium were first discovered by the U.S. Navy in the early 1950’s as

they were in search of a material with high corrosion resistance, high strength and low neutron capture cross-

section. Although zirconium is relatively expensive compared to other cladding materials, it has proved to be

extremely useful and thus developed to dominate the cladding market.

Commercial zirconium always contains a few weight percent of hafnium which has a neutron capture cross-

section in the order of 600 times that of zirconium. For this reason, hafnium has to be removed; nowadays

almost all of the hafnium can be extracted. To improve the properties of the cladding further, other elements

are added such as tin, nickel, chromium, iron and niobium. These elements make the cladding more resistant to

corrosion, reduce hydrogen uptake, reduce weight gain and improve creep properties.

Table 2, below shows the different additives in zirconium based alloys and the applications of the different

alloys.


8 Cladding related problems | Uppsala University

Table 2. Different zirconium based alloys, alloying elements and application6.

Material Application Typical Alloying element w%

Sn Nb Fe Cr Ni O

Zry-2 BWR 1,5 0 0,10-0,20 0,10-0,15 0,05 0,11

Zry-4 PWR 1,5 0 0,2 0,1 0 0,13

Zry-4, low Sn PWR 1,3 0 0,2-0,24 0,1 0 0,13

ELS 0.8 PWR 0,8 0 0,3 0,2 0 0

Zirlo PWR 1 1 0,1 0 0 0

M5 PWR 0 1 some 0 0 0,12

Duplex PWR 0,8 0 0,6 0,2 0 0,15

E110 WWER/RMBK 0 1 0 0 0 0,06

E125/Zr-2,5%Nb RMBK 0 2,5 0 0 0 0

Zr-2,5%Nb CANDU 0 2,5 0 0 0 0,12

E635/Anikulloy WWER/RMBK 1,2 1 0,4 0 0 0,1

For a more comparative overview of the most common types of the zirconium based alloys, see Figure 5. The

latest edition developed by Westinghouse is called OptZirlo7.

FIGURE 5. TYPICAL ALLOYING ELEMENTS IN COMMON ZIRCALOYS6,7.

FUEL CLADDING MATERIALS USED IN SWEDISH REACTORS

Swedish reactor types are boiling water reactors (BWR’s) and pressurized water reactors (PWR’s). In BWR’s,

Zircaloy-2 is the most commonly used material for fuel cladding whereas Zircaloy-4 is the preferred one in

PWR’s. Recently though, Zirlo is growing increasingly popular in the PWR type reactors. Zirlo is less prone to

hydriding than Zircaloy-4 but not as resistant to corrosion.8

CLADDING RELATED PROBLEMS There are two types of damages within the reactor of a power plant, primary and secondary. The primary

damages are those which can be caused without the influence of any other damages prior to that primary

damage in question. The secondary damages on the other hand cannot appear without being a result of a

primary damage. The problematic issues caused by hydrogen uptake in the fuel cladding today are of the

secondary type. The hydriding could also appear as a primary damage if the cladding hadn't been dried enough

during its construction. This is however an extremely rare scenario in the power plants of today since the issue

is now very well known among the nuclear fuel suppliers.

6 (Studsvik Seminar 16)

7 (Dahlbäck)

8 (Rönnberg)


Uppsala University | Hydrogen pick-up 9

As mentioned above, there are several problems that arise in association with the cladding. The most common

problems are debris damages, fretting, creep, corrosion and hydrogen uptake. Debris damages arise for

example in BWRs when metal sand from the sand blasting process get stuck on the cladding and later on in the

reactor cause fretting on the cladding due to vibrations8. In PWRs, a reoccurring issue is hydraulically induced

vibrations.8 Common to all types of reactors is creep which refers to movements or permanent deformations

of solid materials that result from long term exposure to high levels of stress and often high temperatures.

There are several different corrosion mechanisms of which shadow corrosion and nodular corrosion are of

interest here. Galvanic corrosion, called “shadow corrosion” in the nuclear industry, corresponds to a localized

enhanced corrosion of zircaloy when it is close to or in contact with

another metallic component that is usually more noble than the

zircaloy. This results in a corrosion surface on the zircaloy shaped as a

mirror image of the more noble component, hence the name shadow

corrosion. Figure 6 depicts this phenomenon, a cladding tube with a

mirror image of a control rod handle.9

Nodular corrosion refers to the formation of thick oxide spots on the

zircaloy cladding, as shown in Figure 7. Additives, such as iron, nickel

and chromium reduce these effects.9

Finally, the phenomenon that will

discussed in further detail in this report, hydrogen uptake and its effects and

consequences. Hydrogen appears as a biproduct of the oxidation of zircaloy, and

is readily abundant, bound in water molecules, in the moderator surrounding

the cladding. This hydrogen will travel into the zircaloy matrix and cause various

undesirable effects.

HYDROGEN PICK-UP It has long been known that hydrogen entering solid metals causes

embrittlement, which in turn possibly results in undesirable cracking. Even though research and development

has been conducted on the subject for several decades, many of the fundamental mechanisms are still not fully

understood, to a great extent due to the fact that there are too many factors playing a role.

THE HYDROGEN PICK-UP MECHANISM

What is known, is that the oxidation of zirconium liberates hydrogen ions as shown by the chemical reaction

below:

2 H2O+Zr ZrO2+4H- 9

It has been shown in an operating PWR that approximately 16 %10

of these hydrogen atoms are picked up by

the cladding. The hydrogen together with the zirconium creates hydride precipitates called zirconium hydrides,

or simply hydrides. At a certain point, depending mostly on temperature, the hydride precipitation solubility is

reached and hydride platelets will begin to form. Figure 8 below shows the solubility limit of hydrogen in

zircaloy as a function of temperature.

8 (Rönnberg)

9 (Studsvik AB)

10 (Gerry d. Moan)

FIGURE 6. SHADOW CORROSION OF ZIRCALOY

FROM CONTROL ROD HANDLE9.

FIGURE 7. NODULAR CORROSION ON

ZIRCALOY CLADDING9.


10 Hydrogen pick-up | Uppsala University

FIGURE 8. HYDROGEN CONTENT IN ZIRCALOY AS A FUNCTION OF TEMPERATURE9.

HYDROGEN CONTENT AT DIFFERENT BURN-UPS

As time goes, more and more hydrogen will gradually diffuse into the zircaloy. Therefore, it is important to

investigate hydrogen concentrations as a function of burn-up, as it has become desirable with higher and

higher burn-ups. Burn-up is by definition11

the amount of energy, expressed in MegaWatt days, per unit mass

fuel, expressed in kg pure Uranium, and measures the amount of thermal energy generated in the reactor in

that time period. In Sweden, the burn-up is 45 MWd/kgU and progressing towards a burn-up of 50 MWd/kgU.

Germany, Switzerland and USA are the countries with the highest burn-ups in the world at the moment, with

about 50 MWd/kgU and progressing towards 55 MWd/kgU. In the long run, the goal is to reach burn-up values

of 70-75 MWd/kgU. Table 3 below shows the hydrogen content depending on burn-up.8

TABLE 3. HYDROGEN CONTENT AS A FUNCTION OF BURN-UP9.

Low burn-up (20-40 MWd/kgU) High burn-up (50-75 MWd/kgU) Temperature [°C]

BWR 20-100 ppm 100-500 ppm 300-340

PWR 20-200 ppm 100-1000 ppm 360-400

There are elements, mainly in the moderator, that accelerate the hydrogen uptake. The most common

accelerant is nickel, hence the name, the nickel window effect. Nickel acts as a catalyst and separates hydrogen

molecules into hydrogen atoms which are far more readily picked up by the zirconium, hence the enhanced

hydriding.9

Since it is generally cooler on the outside of the cladding, hydrides have a tendency of moving from the inside

of the cladding to the outside. This thermal diffusion causes hydrides to form rims surrounding the claddings.

The rims have a significantly higher hydrogen concentration and are therefore a lot more brittle and thus more

prone to fractures. For that reason, rims are a very undesirable effect of hydriding.

8 (Rönnberg)

11 (About fuel burn-up rates)


Uppsala University | Hydrogen pick-up 11

ZIRCONIUM HYDRIDE PROPERTIES

Depending on hydrogen content and temperature, the hydrides will form in different phases which in theory

might affect what kind of effects the precipitates might cause on the cladding. Figure 9 below shows the

different existing hydride phases, however, only the δ-phase has been observed.9

To understand the hydrogen-zirconium interaction in detail it is helpful to know basic properties such as crystal

structures, density and such. These have been obtained and are shown in Table 4.12

TABLE 4. BASIC PROPERTIES OF ZIRCONIUM HYDRIDES9.

Phase Crystal structure

Latticeconstants [nm] a c

Density [g/cm3] Mean interatomic distance [nm]

α-Zr hcp 0,323 0,515 6,51 0,322

γ (ZrH) fct 0,46 0,497 5,82 0,334

δ (ZrH1,6) fcc 0,478 - 5,64 0,338

ε (ZrH2) fct 0,498 0,445 5,61 0,339

9(Studsvik AB)

FIGURE 9.PHASE DIAGRAM FOR ZIRCONIUM HYDRIDES9.


12 Effects of hydriding | Uppsala University

EFFECTS OF HYDRIDING Effects that can occur are accumulation of hydrides in blisters, delayed hydride cracking and embrittlement.

Hydride blisters are, as the name suggests, accumulations of hydrides in certain areas in which hydrogen

concentrations can be very high. Several types of failures have been discovered in this research. Some of them

are covered below.

HYDROGEN EMBRITTLEMENT

Hydrogen embrittlement is as the name suggests a change in the mechanical properties of the zircaloy due to

hydrogen pick-up. Embrittlement occurs mainly in the temperature range between 100 - 300°C9. At higher

temperatures the hydrides become ductile and are no longer hazardous as the risk for fractures decreases

significantly.

FIGURE 10. A) STRESSDIAGRAM FOR DIFFERENT TEMPERATURES. B) RELATIVE DUCTILITY AS A FUNCTION OF HYDROGEN CONTENT FOR

DIFFERENT TEMPERATURES9.

DELAYED HYDRIDE CRACKING

A well-known property of Zirconium and its alloys is that it has a low solubility for hydrogen. When a sufficient

amount of hydrogen has been solved into the Zirconium matrix, precipitation of the hydrogen commences,

forming platelets consisting of hydrides. If such a platelet is formed sufficiently close to a notch or crack tip in

the material, a diffusion of hydrogen, from the platelet towards the crack tip, occurs (Figure 11). As more and

more hydrogen is gathered at the crack tip, the yield stress there reduces, until it reaches the actual applied

stress, causing material failure. This process is called delayed hydride cracking or DHC.

FIGURE 11. SCHEMATIC ILLUSTRATION OF THE DIFFUSION OF HYDROGEN TOWARDS A CRACK TIP. THE LARGER ARROWS INDICATE THE

DIRECTION OF THE EXTERNAL STRESS. THE CRACK PROPAGATION DIRECTION IS NORMAL TO THE DIRECTION OF APPLIED STRESS9.


Uppsala University | Incidents caused by hydriding in fuel claddings 13

The reason for the migration of hydrogen, from the platelets to the crack tip, is that the chemical potential of

the hydrogen at the crack tip differs from the chemical potential of the hydrogen in the platelets. The chemical

potential depends on the local hydrostatic stress field: an increase in stress causes the chemical potential to

reduce. Hence, since the hydrostatic stress field is reduced in the platelets, the chemical potential is greater

there, inducing a diffusion of hydrogen towards the crack tip. The crack velocity is determined by the chemical

potential as well. The greater the chemical potential in the matrix, the higher the driving force and the larger

velocity is a consequence.

BLISTERING

Sometimes the inside of the fuel cladding is lined with another zirconium alloy or pure zirconium, to prevent

corrosion from the fission products. If the cladding wall is penetrated, perhaps by corrosion, coolant may enter

the fuel chamber, where it vaporizes. The steam will now oxidize the fuel, leaving it very rich in hydrogen. If the

protective layer on the inside of the cladding is defective, in the vicinity of the steam cavity, hydrides will form

in large quantities. These formations are called hydride blisters (Figure 12). From these blisters, hydrogen may

diffuse into the matrix creating radial hydrides. They are called radial because their direction of propagation is

radial. Thus, their crack direction is axial as is the direction of applied stress. This is dire, because it greatly

enhances the risk of the cladding to rupture. 13

INCIDENTS CAUSED BY HYDRIDING IN FUEL CLADDINGS Nowadays, primary damages caused by hydriding are almost completely extinct in BWR and PWR type

reactors14

. However, there have been some occurrences. For example, during the 1990 outage at Ringhals

nuclear power plant, two guiding tubes broke during insertion of control rods in the pool. Investigation showed

high hydrogen contents in the Zircaloy-4 guide tubes. Hot cell examination revealed that the average hydrogen

content was about 3000 ppm, which may be compared to normal hydrogen contents of about 200 – 300 ppm.

This was a clear indication that hydrogen uptake was stemming from a different source than ordinary

corrosion. These particular guide tubes had been grit-blasted on the inner surfaces with a stainless steel lance.

13 (Alvarez, Banerjee och Bickel)

14 (Rönnberg)

FIGURE 12. A PHOTOGRAPH OF A HYDRIDE BLISTER. COURTESY OF STUDSVIK9.


14 Transporting used up fuel | Uppsala University

It was shown that grit-blasting with such lances causes small stainless steel particles to be embedded at the

inner guide tube surface, acting as hydrogen windows. Ni was deposited onto the cladding surface, causing

hydrogen, which was added to the coolant at about 150 °C, to catalytically diffuse into the cladding. It was

shown during the investigation that a necessary condition for hydrogen uptake was that Ni in the coolant was

high during start-up, and that hydrogen was added to the water before the creation of a protective oxide layer

could prevent hydrogen uptake15

.

The problem was solved by altering the production method, i.e. the grit-blasting, the annealing etc. Other

measures were also taken, for example adding hydrogen to the coolant at a later time, allowing the protective

oxide layers to form. No incidents have been reported since16

.

Although primary damages caused by hydriding are no longer problems in neither BWR’s nor PWR’s, secondary

damages are still the root of much trouble. At Oskarshamn Nuclear Power Plant, the fuel claddings in reactor 3

are much more prone to hydriding than the fuel claddings in reactors 1 and 2. Quite much time has been spent

trying to understand the cause for this, and some measures have been taken to reduce a lot of the effects of

hydriding in reactor 3. For example, the coolant purification system in O3 is slightly different from the coolant

purification system used in O1 and O2. Therefore, it is believed that this may be the source of the problem. The

water purification system has been altered, but whether or not this will solve the problem is not yet known,

since these measures have been taken rather recently and some time must pass in order for sufficient statistics

to be built up.16

TRANSPORTING USED UP FUEL When transporting radioactive materials, security is a very important issue. High demands are made on the

package in which the material is being stored. Depending on the potential hazard posed by the transported

material, different packages are utilized. There are three types of packages: type A, type B and type C, where

type C is the most secure one. Nuclear waste is transported in B type packages, which are designed to maintain

shielding from gamma and neutron radiation, even under extreme conditions.17

FIGURE 13. LOADING OF A SHIPMENT OF NUCLEAR WASTE PRODUCTS IN A TYPE B PACKAGE17.

Since1971, some 7000 shipments of used up nuclear fuel has been made over many millions of kilometers with

no personal injury or property damages, no breach of containment and very low dose rate to the personnel

involved.

15 (Pettersson, Bengtson and Andersson)

16 (Rönnberg)

17 (How nuclear works)


Uppsala University | Fission gas release and radiation dosages 15

TRANSPORTING DAMAGED FUEL CLADDINGS

One might think that even though there are very little risks involved in transporting unspoiled fuel claddings,

there might be many risks involved in transporting burnt up fuel, if a fuel cladding has cracked open. But

protocols have been developed to maximize safety. For example, at the Oskarshamn nuclear facility, damaged

fuel claddings are removed from the fuel element and gathered in a bundle. The bundle is then put in a leak

proof container suitable for transport, i.e. a B type package. If a cladding has been completely ruptured, it must

be put inside a security container of its own, before it is put into the bundle.

Thus, transporting damaged fuel claddings does not appear to be a tougher issue than transporting intact

ones18

.

FISSION GAS RELEASE AND RADIATION DOSAGES As mentioned earlier, a negative consequence to cracks in the cladding is fission gas release into the

moderator. This is an undesirable effect since these fission products are often radioactive and thus increase

radiation levels in the moderator and therefore also to the personnel working at the power plants.19

The

figures below show the collective radiation exposure, i.e. the average effective dose multiplied by the number

of people exposed to the radiation20

. Measures are taken to decrease these values further.

18 (Rönnberg)

19 (2008 Performance Indicators)

20 (Glossary of Terms Used in Radiological Protection)

FIGURE 15. COLLECTIVE RADIATION EXPOSURE, BWR19. FIGURE 14. COLLECTIVE RADIATION EXPOSURE, PWR19.

FIGURE 16. COLLECTIVE RADIATION EXPOSURE, CANDU19.


16 Current Research and Development | Uppsala University

CURRENT RESEARCH AND DEVELOPMENT Since the phenomenon of hydrogen uptake in the fuel cladding was discovered several measures to prevent

this have been taken. One of these measures has been changing the material of which the cladding is

constructed. The alloys used have always been based on zirconium due to its beneficial properties such as

resistance to corrosion and its low cross-section values for neutron capture.

Even though changing to these new alloys have proven to be a very effective way to reduce the hydrogen

uptake there is still a rather extensive demand on more research within the area. The reason for this demand is

that many of the reactors currently active today are being upgraded to newer models with extended fuel burn-

ups, increased cycle lengths and elevated energy output. These upgrades lead to higher core temperatures and

an increased strain on the materials.

ONGOING PROJECTS

CLADDING MATERIAL The nuclear fuel companies, Westinghouse in particular, continue to conduct research on ways to improve the

cladding and its material to enhance its resistance to hydrogen uptake. Westinghouse is also trying to minimize

the space found between the fuel and its cladding where the hydrides, which attract to lower temperatures,

tend to gather up. However, this space does serve a purpose when the temperature of the fuel pellets rises and

the fuel itself begins to expand.

SCIP Since 2004 Studsvik has been the operating agent of an OECD project called SCIP, Studsvik cladding integrity

program, which has just been extended to run for another 5 years last summer under the name of “SCIP II”.

This project has addressed most hydrogen-related failure mechanisms within the fuel cladding. Many cladding

materials have been tested and models have been created to simulate how the fuel as well as these materials

is affected during reactor operation. The projects main focus is to increase the overall operation safety of our

nuclear power plants to avoid unplanned maintenance due to fuel failure.

Most failure mechanisms studied have been well-known for many years, but SCIP gives a deeper knowledge of

the mechanisms as fuel conditions change now and in the future in connection with increased power extraction

from the fuel.21

21 (Studsvik Group)


Uppsala University | Bibliography 17

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