Title: Helium production and behaviour in LWR Oxide Nuclear Fuels
Publisher: Politecnico di Milano
Publication Year: 2011
JRC N°: JRC64408
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC64408
Type: PhD Theses
Abstract: The present thesis gives a contribution to the study and the modelling of the helium behaviour (from its production to the release) in oxide nuclear fuels. In particular, a model for the production of helium in LWR oxide fuels has been developed and implemented in TUBRNP. The model takes into account the helium produced by alpha decays, (n,alpha) reactions and ternary fissions. At first, it has been verified by means of detailed neutron-transport depletion calculations, performed by means of the VESTA Monte Carlo-depletion code. A good agreement has been found in terms of helium produced and actinides concentrations for both the UO2 and MOX cases. Finally, the model has been validated against experimental data. In particular, a good agreement has been found in terms of average concentrations and radial profiles of the main alpha- emitters produced in UO2 fuels. As a second step, the microstructure of oxide fuels has been studied experimentally and theoretically, since it is relevant for the behaviour of the gases. More precisely, grain growth of UO2 and MOX fuels has been investigated. In fact, grain boundaries strongly influence the diffusion process. As far as the UO2 is concerned, experimental data of unirradiated fuel pellets have been analysed and interpreted by means of different models available in literature. As far as the MOX fuel is concerned, an experimental campaign has been carried out at JRC/ITU in order to study for the first time the influence of plutonium content (0, 3.7, 9, 25 %wt) and fabrication technique (SOLGEL, SBR, MIMAS) on the grain growth of stoichiometric MOX fuels annealed at different temperatures (1350-1750°C). It has been found that normalized size distributions did not change significantly during the annealing and were similar among the different samples. On the other hand, large differences have been noticed in terms of growth kinetics. The slowest kinetics has been experienced by the SBR sample, followed by MIMAS and UO2 samples, while the two SOLGEL samples had the largest growth. No dependence with the plutonium content has been found and the predominant effect has been attributed to the pore size. In fact, SBR fuel is characterized by the largest and then slowest pores, while SOLGEL is characterized by the smallest and then fastest pores. All things considered, grain growth kinetics of the analyzed samples lies within the large scatter of the values available in the open literature for UO2 fuels, and the correlation proposed by Sari for stoichiometric fuel is in agreement with the growth experienced by the samples manufactured by means of the SOLGEL technique, which is faster with respect to commercial fuels. This means that the adoption of Sari correlation for commercial MOX fuels could lead to an overestimation of the grain growth, with consequencies for the estimation of the fission gas and helium releases. For the fuel performance, on the basis of the large scatter of the experimental data, the same correlation developed for UO2 can be used also for MOX fuels. However, a quantification of the effect of porosity and fission gas bubble on the grain boundary movement should be more deeply investigated. As a final step, the transport of helium in the fuel has been investigated. A model for the release of helium in the gap between the fuel and the cladding has been developed, implemented in TRANSURANUS and preliminarily validated on the basis of pressurized and unpressurized fuel rods. The agreement is satisfactory although some discrepancies have been noticed. More experimental data are needed for a better assesment of the model parameters (diffusion coefficients, solubilities) and for a proper validation of the model.
JRC Directorate:Nuclear Safety and Security

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