FUEL DESIGN

A good review of current and future fuel cycle options for LWRs and HWRs (heavy water reactors) is given in IAEA-TECDOC-1122 (1998) (Table 5.3).

5.6.1 Light Water Reactors

5.6.1.1 Present. LWRs are the most widely operating type of reactor in the world and LWR fuel optimisation is of international interest. There is intense competition between fuel vendors and there are many different designs offering different performance advantages. However, there has been extensive experience amassed on fuel performance, and fuel designs based on a conventional uranium cycle are well optimised. Thus the

Table 5.3. Advanced fuels options

Type of fuel

Plant

MOX

LWR, HWR

Thoria fuel

LWR, HWR

Inert matrix/uranium free fuel

LWR

Slightly enriched uranium

HWR

Recycled uranium

HWR, LWR (with enrichment)

Fuel for direct recycle

HWR

Ceramic fuel

HTR

Fuel cycles for plutonium and minor actinide destruction

FR

Data from IAEA-TECDOC-1122 (1998).

differences in such designs are relatively small. There has been even some measure of collaboration between fuel vendors arising from the need to share costs associated with expensive research programmes. The common drivers in fuel design are to achieve greater reliability, to reduce fuel failures, to move towards higher burn-up and to reduce fuel cycle costs. These fuel performance issues are considered in the Section 5.6.1.2.

In addition to the conventional uranium fuel cycle for LWRs, MOX fuel has also been used and is well established. In the MOX fuel cycle, plutonium oxide is mixed with uranium dioxide for use as fuel in LWRs. MOX fuel is used in France, Germany and Japan. It was first used in Europe and the US in the mid-1960s and since then hundreds of tonnes of MOX fuel have been burnt in commercial LWRs. The success of burning plutonium in MOX fuel demonstrates that plutonium is an asset that can be used for civil nuclear power generation. Further this has been realised by the development and safe operation of large-scale plutonium recycling facilities in France and most recently in the UK, now that the BNFL Sellafield MOX plant has become operational. The IAEA have put in place controls to ensure adequate safeguarding of materials.

5.6.1.2 Future

MOX. MOX fuels represent the most significant developments in LWR fuel technology, particularly in Europe (IAEA-TECDOC-1122, 1998). MOX fuel up to 30% loading can be used in LWRs within current operating and safety margins; higher percentage loadings would require control rod changes to maintain current margins. MOX fuel costs are higher than UO2 fuel costs but this largely reflects reduced production at the present time. The potential of advanced MOX fuel is being studied in France and Japan.

CEA are investigating advanced plutonium fuel assemblies to overcome the problems of multiple plutonium recycling in PWR MOX assemblies. As MOX assemblies are irradiated, the isotonic quality of the plutonium is reduced (Groullier, 2001). CEA are working on high moderation plutonium fuels (Youniou et al., 1998). In conventional MOX assemblies, the moderator/fuel volume of MOX is the same as in UO2 assemblies and new designs are being investigated to increase this ratio which gives a more complete thermal flux and reduces the initial plutonium content. Conversely, the Japanese are looking to lower moderation fuels to achieve plutonium breeding, see for example Tochihara et al. (1998).

Thoria Fuel. The development of thoria fuel has been overshadowed by the emphasis and investment in uranium-based fuel. Nevertheless, thorium is about three to four times more abundant than uranium and represents a good long-term nuclear fuel supply. The cycle produces fissile U-233, thereby enabling breeding potential in a thermal reactor, good in-core behaviour and lower excess reactivity requirements. A disadvantage is that thorium ore does not contain a fissile isotope and so U-235 or Pu must initially be used in conjunction. Thorium fuel is attractive for various reasons. There is very little production of plutonium or transuranics, which reduces the radiotoxicity burden and, therefore, there is a benefit from the point of view of proliferation (Hesketh, 2003). Thoria fuel has been successfully demonstrated in power reactors.

Uranium Free Fuels. The incineration of plutonium from weapons programmes and from reprocessed LWR fuel is under consideration in many countries. Another pressing issue to the nuclear countries is how to burn actinides as part of a waste management strategy. Research programmes are underway in Switzerland, Japan, France and Canada. The idea is to burn the plutonium (or actinides) in a non-fissile inert carrier matrix. Various fuel matrices are being examined, e. g. zirconium oxide in Switzerland, fluorite and spinel in Japan, ceramic (spinel, magnesia) or metallic matrices in France and silicon carbide (SiC) in Canada. Other materials may also be required, burnable poisons (e. g. erbium) for control of reactivity and addition of thorium or uranium to enhance negative temperature coefficient. To date, fuels have largely been irradiated with accelerators; initial results are good for SiC and zirconia. Some in-reactor irradiations have taken place. The main issues relate to materials performance that are not yet resolved and inert matrix fuels are unlikely to enter LWR fuel cycles in the near future.

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