MOX FUEL DEVELOPMENT PROGRAM

The operational performance of the MOX fuel rods and assemblies has been assessed by many number of inspection and measurement campaigns performed in reactor spent fuel pools, in hot cell laboratories and but also in analytical programs [2]. The highlights of the important MOX European experience feedback are:

- Any technical problems resulting from the MOX fuel have been encountered;

- The reliability of MOX assemblies is as good as UO₂ fuel.

Since the use of MOX fuel in French PWRs has now reached its industrial maturity, the main challenge remains to achieve the same standards as UO₂ fuel performance in terms of discharge burn-up and fuel management. Initially, the objective is to achieve parity between UO₂ and MOX fuels at 52 GWd/tM in 2004 by means of a management strategy of ¼ core refuelling for the annual cycles of both fuel types in EDF 900 MWe units. The next step should be the development of a MOX fuel capable of a discharge burn-up of ~60 GWd/tM or

more. In support of this high burn-up target, it is desirable to improve the current fuel product technology.

In addition to the design improvements of the fuel rod and assembly structure, R&D activities are similar to those developed for UO₂, with particular emphasis on the minimization of fission gas release that today appears to be the main limitation to achieving very high burn-ups. At the same time, checks must be carried out to ensure that the other MOX fuel properties (mechanical creep properties, etc) are not unfavourably changed with regard to overall behaviour, and are satisfactory for high burn-up applications (pellet-clad interaction, physical-chemical and thermal properties). The R&D is focussed as a matter of priority, on the current MOX-MIMAS product, but in the longer term perspective, also aims at developing fuels with improved microstructures. The existence of a thermal threshold beyond which FGR is accelerated, is confirmed for MOX fuel as it has been for UO₂ but with a higher level of release. Studies are now under way to examine thoroughly the links between the heterogeneity of the MOX microstructure (size and Pu content of (U,Pu)O₂ particles, microstructure of the UO₂ matrix) and the mechanisms and kinetics of FGR in normal and transient conditions.

To this end, two types of improvements are being studied:

- A reduction in the size of the PuO₂ rich-particles. Today, it is thought that this parameter may influence the retention and/or the release of fission gases, although this effect has not yet been precisely quantified.

For this phase, the current MIMAS process, used in particular in the COGEMA MELOX factory at Marcoule, has not been significantly modified. The desired objective was reached by means of an optimisation of the sieving operation of the primary blend.

As illustrated in Figure 7, in the current MOX MIMAS fuel, about 25% of the total plutonium content of the pellet is contained in the Pu-rich particles above 30 µm size.

However, it was reduced to less than 10 % in the optimised microstructure. Lead fuel rods, using M5™ cladding, are currently being irradiated in an EDF reactor. The improvement in fission gas retention will be evaluated by PIE in 2005.

Standard MIMAS MOX fuel Optimised MIMAS MOX fuel

160 µm 160 µm

FIG. 7. Optimization of the MIMAS MOX fuel microstructure.

- An increase in the fuel matrix grain size, while keeping a smaller agglomerate size.

Instrumented fuel rodlets are being irradiated in experimental reactors with different potential solutions in order to investigate:

• The role played by the microstructure in the behaviour of fission gases with the objective of determining whether the agglomerate size and its Pu content is a more or less important parameter in comparison to microstructures with homogeneous and heterogeneous plutonium distribution;

• The use of additives in the form of different oxides, added in small quantities (in the range of 500 – 2000 ppm) to oxide (U,Pu)O₂. In the case of UO₂, it has been proved that such additions, and in particular chromium oxide, can reduce the release of fission gases, both by enlarging the grain and by causing the gases to diffuse more slowly.

Improved microstructures have already been obtained at a laboratory scale. Transposition of the technology and tests to an industrial line is now contemplated as well as the launching of experimental irradiations.

4. CONCLUSION

The need for enhanced in-reactor performance has led to the development of advanced UO₂ and MOX fuels aiming to eliminate the pellet cladding interaction problem and to reduce the fission gas release at high burn-up. As observed with MOX fuel, reduction of PCI consequences can be achieved by an enhancement of the fuel viscoplastic deformation under power transient situations. For fission gas release, the fuel microstructure characterised by its grain size and level of homogeneity has a direct influence on the phenomenon.

The CONCERTO program, which is the outcome of a development process initiated between the CEA research laboratories and the French industrial operators, has led to the development of an advanced microstructure for UO₂ fuel. This fuel is characterised by the addition of chromium oxide which gives rise to a large grain microstructure with better viscoplastic properties. After two annual irradiation cycles in a PWR reactor, the large grain microstructure shows significant improvement with respect to PCI performance. The extensive creep of the doped fuel into the pellet dishes and the cracking at the pellet periphery are assumed to be mechanisms responsible for the enhanced PCI resistance observed in power ramp testing. Moreover, rod punctures after ramps indicated a higher fission gas retention capability of the Cr₂O₃ doped fuel. This trend has to be confirmed at high burn-up with post-irradiation examinations of CONCERTO fuel rods after four and five post-irradiation cycles. The program achieved fuel rod burn-ups of ~60 GWd/tU in 2003. In the meantime, the completion of the destructive examinations after the ramp tests will help to identify all the mechanisms involved in improved PCI behaviour.

The very promising results of the CONCERTO program led Framatome-ANP, in cooperation with EDF and the CEA, to launch in mid-2001 the irradiation of lead fuel rods with Cr₂O₃ doped UO₂ pellets loaded in M5™ cladding tubes. This irradiation campaign, performed in an EDF 1300 MWe reactor, aims to acquire all the data required to demonstrate the ability of this fuel to be a high burn-up PCI remedy in order to satisfy future customer needs for improved fuel management and greater power plant operating flexibility.

With an increasing experience feedback, the use of MOX fuel in the European LWRs has now reached industrial maturity. In addition to the constant accumulation of data from surveillance programs, an extensive R&D program is continuing to better understand, model and validate

high burn-up fuel behaviour under nominal and transients conditions. In France, beyond the objective of achieving parity between UO₂ and MOX at burn-up of 52 GWd/tM, programs are performed with the aim to further improve the performance of the MOX fuel product. The development is essentially focused on the reduction of fission gas release, which constitutes the major obstacle in reaching very high burn-ups. To this end, optimisation of the current MIMAS microstructure has been already achieved. For the next ten years, the development of advanced microstructures is viewed for a MOX fuel capable of reaching assembly burn-ups of at least 60 GWd/tM.

ACKNOWLEDGEMENTS

The French R&D work is carried out with the cooperation of Electricité de France (EdF). The authors wish to acknowledge the support and the aid of the many persons who are participating in these programs.

REFERENCES

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