Nuclear fuel and reactor accidents

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This page is devoted to a discussion of how uranium dioxide nuclear fuel behaves during both normal nuclear reactor operation and under reactor accident conditions such as overheating. Work in this area is often very expensive to conduct, and so has often been performed on a collaborative basis between groups of countries, usually under the aegis of the CSNI.

This is a false colour tomography picture of a bundle (FPT1) of 18 irradiated fuel rods (23 GWd/tU mean burn-up) degraded under steam as part of the PHEBUS set of experiments. The black and blue is for areas of low density while red is an area of high density. It can be seen that the fuel has failed mechanically and has formed a pool near the bottom of the bundle, it is interesting to note that the bottom of the bundle did not melt.

[edit] Swelling

[edit] Cladding

It is important to note that both the fuel can swell and the cladding which covers the fuel to form a fuel pin can be deformed. It is normal to fill the gap between the fuel and the cladding with helium gas to permit better thermal contact between the fuel and the cladding. During use the amount of gas inside the fuel pin can increase because of the formation of noble gases (krypton and xenon) by the fission process. If a Loss Of Coolant Accident (LOCA) {eg Three Mile Island or a Reactivity Initiated Accident (RIA) {eg Chernobyl or SL-1} occurs then the temperature of this gas can increase. As the fuel pin is sealed the preasure of the gas will increase (PV = nRT) and it is possible to deform and burst the cladding. It has been noticed that both corrosion and irradiation can alter the properties of the zirconium alloy commonly used as cladding, making it brittle. As a result the experiments using unirradated zirconium alloy tubes can be misleading.

According to T. Nakamura, T. Fuketa, T. Sugiyama and H. Sasajima, Journal of Nuclear Science and Technology, 2004, 41, 37.[1] the following difference between the cladding failure mode of unused and used fuel was seen.

Unirradated fuel rods were pressurized before being placed in a special reactor at the Japanese Nuclear Safety Reasearch Reactor (NSRR) where they were subjected to a simulated RIA transient. These rods failed after ballooning late in the transient when the cladding temperature was high. The failure of the cladding in these tests was ductile, and it was a burst opening.

The used fuel (61 GW days / ton of Uranium) failed early in the transient with a brittle fracture which was a longitundinal crack.

[edit] Fuel

The fuel can swell during use, this is because of effects such as bubble formation in the fuel and the damage which occurs to the lattice of the solid. The swelling of the fuel can impose mechanical stresses upon the cladding which covers the fuel. A document on the subject of the swelling of the fuel can be downloaded from the NASA web site.[2].

[edit] Fission gas release

As the fuel is degraded or heated the more volatile fission products which are trapped within the uranium dioxide may become free. For example see J.Y. Colle, J.P. Hiernaut, D. Papaioannou, C. Ronchi, A. Sasahara, Journal of Nuclear Materials, 2006, 348, 229.

P. Wood and G.H. Bannister published a paper on the release of ⁸⁵Kr, ¹⁰⁶Ru and ¹³⁷Cs from uranium when air is present. It was found that uranium dioxide was converted to U₃O₈ between about 300 and 500 ^oC in air. They report that this process requires some time to start, after the induction time the sample gains mass. The authors report that a layer of U₃O₇ was present on the uranium dioxide surface during this induction time. They report that 3 to 8% of the krypton-85 was released, and that much less of the ruthenium (0.5%) and cesium (2.6 x 10^-3%) occurred during the oxidation of the uranium dioxide.[3]

[edit] Corrosion and other changes to materials in the reactor

[edit] Graphite moderated reactors

In the cases of carbon dioxide cooled graphite moderated reactors such as magnox and AGR power reactors an important corrosion reaction is the reaction of a molecule of carbon dioxide with graphite (carbon) to form two molecules of carbon monoxide. This is one of the processes which limits the working life of this type of reactor.

[edit] Water cooled reactors

In a water cooled reactor the action of radiation on the water forms hydrogen peroxide and oxygen. These can cause stress corrosion cracking of metal parts which include fuel cladding and other pipework. To mitigate this hydrazine and hydrogen are injected into a BWR or PWR primary cooling ciruit to adjust the redox properties of the system. For recent developments on this topic see K. Ishida, Y. Wada, M. Tachibana, M. Aizawa, M. Fuse and E. Kadoi, Journal of Nuclear Science and Technology, 2006, 43, 65-76.[4]

[edit] Aging of steels

Irradiation causes the properties of steels to become poorer, for instance SS316 becomes less ductile and less tough. Also creep and stress corrosion cracking become worse. For a recent paper on the subject see K. Fukuya, K. Fujii, H. Nishioka and Y. Kitsunai, Journal of Nuclear Science and Technology, 2006, 43, 159-173.[5]

[edit] Cracking and overheating of the fuel

This is due to the fact that as the fuel expands on heating, the core of the pellet expands more than the rim. Because of the thermal stress thus formed the fuel cracks, the cracks tend to go from the centre to the edge in a star shaped pattern. A PhD thesis on the subject has been published [6] by a student at the Royal Institute of Technology in Stockholm (Sweden).

The cracking of the fuel has an effect on the release of radioactivity from fuel both under accident conditions and also when the spent fuel is used as the final disposal form. The cracking increases the surface area of the fuel which increases the rate at which fission products can leave the fuel.

The temperature of the fuel varies as a function of the distance from the centre to the rim. At distance x from the centre the temperature (T_x) is described by the equation where ρ is the power desnity (W m^-3) and K_f is the thermal conductivity.

T_x = T_Rim + ρ (r_pellet² - x²) (4 K_f)^-1

To explain this a for a series of fuel pellets being used with a rim temperature of 200 ^oC (typical for a BWR) with different diameters and power densities of 250 Wm^-3 have been modeled using the above equation. Note that these fuel pellets are rather large; it is normal to use oxide pellets which are about 10 mm in dimater.

Temperature profile for a 20 mm diameter fuel pellet with a power density of 250 W per cubic meter. Note the central temperature is very different for the different fuel solids.

Temperature profile for a 26 mm diameter fuel pellet with a power density of 250 W per cubic meter.

Temperature profile for a 32 mm diameter fuel pellet with a power density of 250 W per cubic meter.

To show the effects of different power densitys on the centreline temperatures two graphs for 20 mm pellets at different power levels are shown below. It is clear that for all pellets (and most true of uranium dioxide) that for a given sized pellet that a limit must be set on the power density. It is likely that the maths used for these calculations would be used to explain how electrical fuses function and also it could be used to predict the centreline temperature in any system where heat is released throughout a cylinder shaped object.

Temperature profile for a 20 mm diameter fuel pellet with a power density of 500 W per cubic meter. Because the melting point of uranium dioxide is about 3300 K, it is clear that uranium oxide fuel is overheating at the centre

Temperature profile for a 20 mm diameter fuel pellet with a power density of 1000 W per cubic meter. The fuels other than uranium dioxide are not compromised.

Reference Radiochemistry and Nuclear Chemistry, G. Choppin, J-O Liljenzin and J. Rydberg, 3rd Ed, 2002, Butterworth-Heinemann, ISBN 0-7506-7463-6

[edit] The Chernobyl release

The release of radioactivity from the used fuel is greatly controlled by the volitility of the elements. At Chernobyl much of the xenon and iodine was released while much less of the zirconium was released. The fact that only the more volatile fission products are released with ease will greatly retard the release of radioactivity in the event of an accident which causes serious damage to the core. Using two sources of data it is possible to see that the elements which were in the form of gases, volatile compounds or semi-volatile compounds (such as CsI) were released at Chernobyl while the less volitle elements which form solid solutions with the fuel remianed inside the reactor fuel.

According to the OECD NEA report on Chernobyl (ten years on)[7], the following proportions of the core inventry were released. The physical and chemical forms of the release included gases, aerosols and finely fragmented solid fuel. According to some research the ruthenium is very mobile when the nuclear fuel is heated with air.[8]

Some work has been done on TRISO fuel under similar conditions.[9]

[edit] Table of chemical data

J.Y. Colle, J.-P. Hiernaut, D. Papaioannou, C. Ronchi and A. Sasahara, Journal of Nuclear Materials, 2006, 348, 229-242 reported the releases of fission products and uranium from uranium dioxide (from spent BWR fuel, burn-up was 65 GWd t^-1) which was heated in a Knudsen cell. Fuel was heated in the Knudsen cell both with and without preoxidation in oxygen at c 650 K. It was found even for the noble gases that a high temperature was required to liberate them from the uranium oxide solid. For unoxidized fuel 2300 K was required to release 10% of the uranium while oxidized fuel only requires 1700 K to release 10% of the uranium.

According to the report on Chernobyl used in the above table 3.5% of the following isotopes in the core were released ²³⁹Np, ²³⁸Pu, ²³⁹Pu , ²⁴⁰Pu, ²⁴¹Pu and ²⁴²Cm.

[edit] Degradation of the whole fuel element

It is important to note that water and zirconium can react violently at 1200 ^oC, at the same temperature the zirconium cladding can react with uranium dioxide to form zirconium oxide and a uranium/zirconium alloy melt.[12]

[edit] PHEBUS

In France a facility exists in which a fuel melting incident can be made to happen under strictly controlled conditions.[13][14] In the PHEBUS research program fuels have been allowed to heat up to temperatures in excess of the normal operating temperatures, the fuel in question is in a special channel which is in a toroidal nuclear reactor. The nuclear reactor is used as a driver core to irradate the test fuel. While the reactor is cooled as normal by its own cooling system the test fuel has its own cooling system, which is fitted with filters and equipment to study the release of radioactivity from the damaged fuel. Already the release of radioisotopes from fuel under different conditions has been studied. After the fuel has been used in the experiment it is subject to a detailed examination (PIE), In the 2004 annual report from the ITU some results of the PIE on PHEBUS (FPT2) fuel are reported in section 3.6.[15][16]

[edit] LOFT

The Loss of Fluid Tests (LOFT) were an early attempt to scope the response of real nuclear fuel to conditions under a Loss of Coolant Accident, funded by USNRC. The facility was built at Idaho National Laboratory, and was essentially a scale-model of a commercial PWR. ('Power/volume scaling' was used between the LOFT model, with a 50MWth core, and a commercial plant of 3000MWth).

The original intention (1963-1975) was to study only one or two major (large break) LOCA, since these had been the main concern of US 'rule-making' hearings in the late 1960s and early 1970s. These rules had focussed around a rather stylised large-break accident, and a set of criteria (eg for extent of fuel-clad oxidation) set out in 'Appendix K' of 10CFR50 (Code of Federal Regulations). However, following the accident at Three Mile Island, detailed modelling of much smaller LOCA became of equal concern.

38 LOFT tests were eventually performed and their scope was broadened to study a wide spectrum of breach sizes. These tests were used to help validate a series of computer codes (such as RELAP-4, RELAP-5 and TRAC) then being developed to calculate the thermal-hydraulics of LOCA.

Some details of the tests can be read on-line.

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[edit] See also

• NUREG-1150
• Nuclear power

[edit] Contact of molten fuel with water and concrete

[edit] Water

Extensive work was done from 1970 to 1990 on the possibility of a steam explosion or FCI when molten 'corium' contacted water. Many experiments suggested quite low conversion of thermal to mechanical energy, whereas the theoretical models available appeared to suggest that much higher efficiencies were possible. A NEA/OECD report was written on the subject in 2000 which states that a steam explosion caused by contact of corium corium with water has four stages.[27]

• Premixing

As the jet of corium enters the water, it breaks up into droplets. During this stage the thermal contact between the corium and the water is not good because a vapour film surrounds the droplets of corium and this insulates the two from each other. It is possible for this meta-stable state to quench without an explosion or it can trigger in the next step

• Triggering

A externally or internally generated trigger (such as a pressure wave) causes a collapse of the vapour film between the corium and the water.

• Propagation

The local increase in pressure due to the increased heating of the water can generate enhanced heat transfer (usually due to rapid fragmentation of the hot fluid within the colder more volatile one) and a greater pressure wave, this process can be self-sustained. (The mechanics of this stage are similar to those in a classical ZND detonation wave).

• Expansion

This process leads to the whole of the water being suddenly heated to boiling. This causes an increase in pressure which can result in damage to the plant.

[edit] Recent work

Some work has been done in Japan where uranium dioxide and zirconium dioxide was melted in a crucible before being added to water. The fragmentation of the fuel which results is reported in the paper [28] which is in Journal of Nuclear Science and Technology[29]

[edit] Concrete

A review of the subject can be read at [30] and work on the subject continues to this day; in Germany at the FZK some work has been done on the effect of thermite on concrete, this is a simulation of the effect of the moltern core of a reactor breaking through the bottom of the pressure vessel into the containment.[31][32][33]

[edit] lava flows from corium

It is possible to see in the photo shown below that the corium (molten core) will cool and change to a solid with time. It is thought that the solid is weathering with time. The solid can be described as Fuel Containing Mass, it is a mixture of sand, zirconium and uranium dixoide which had been heated at a very high temperature[34] until it has melted. The chemical nature of this FCM has been the subject of some research.[35] The amount of fuel left in this form within the plant has been considered[36]. A Silicone polymer has been used to fix the contamination.[37]

A photograph of one of the lava-flows formed by corium in the basement of the Chernobyl plant. 1 is the lava flow, 2 is concrete, 3 is a steam pipe and 4 is some electrical equipment

The Chernobyl melt was a silicate melt which did contain inclusions of Zr/U phases, moltern steel and high uranium zircon. The lava flow consists of more than one type of material a brown lava and a porous ceramic material have been found. The uranium to zirconium for different parts of the solid differs a lot, in the brown lava a uranium rich phase with a U:Zr ratio of 19:3 to about 38:10 is found. The uranium poor phase in the brown lava has a U:Zr ratio of about 1:10. S.V. Ushakov, B.E. Burakov, S.I. Shabalev and E.B. Anderson, Mater. Res. Soc. Symp. Proc., 1997, 465, 1313-1318.[38] It is possible from the examination of the Zr/U phases to know the thermal history of the mixture, it can be shown that before the explosion that in part of the core the temperature was higher than 2000 ^oC. While in some areas the temperature was over 2400-2600 ^oC.

The radioactivity levels of different isotopes in the FCM, this has been back calculated by Russian workers to Aprl 1986, note that the levels of radioactivity have decayed a great deal by now

[edit] Spent fuel corrosion

[edit] Uranium dioxide films

Uranium dioxide films can be deposited by reactive spluttering using an argon and oxygen mixture at a low preasure. This has been used to make a layer of the uranum oxide on a gold surface which was then studied with AC impedance spectrscopy.

F. Miserque, T. Gouder, D.H. Wegen and P.D.W. Bottomley, Journal of Nuclear Materials, 2001, 298, 280-290.

[edit] Noble metal nanoparticles and hydrogen

According to the work of the corrosion electrochemist Shoesmith[39][40] the nanoparticles of Mo-Tc-Ru-Pd have a strong effect on the corrosion of uranium dioxide fuel. For instance his work suggests that when the hydrogen (H₂) concentration is high (due to the anaerobic corrosion of the steel waste can) the oxidation of hydrogen at the nanoparticles will exert a protective effect on the uranium dioxide. This effect can be thought of as an example of protection by a sacrificial anode where instead of a metal anode reacting and dissolving it is the hydrogen gas which is consumed.