Development of new treatment of fuel isotope vector in the core degradation accident analysis of fast breading reactor

ABSTRACT

In a design of future Sodium-cooled Fast Reactor (SFR), there is a demand for evaluation of event sequences and consequences of core disruptive accidents. Future SFRs include a unique core design with axially or horizontally heterogeneous core arrangement having complex fuel isotope distribution. A new model to flexibly represent fuel isotope distribution, called as the Pu-vector model, has been developed in this study for inclusion in the SIMMER-III and SIMMER-IV codes (simply called as SIMMER). The model calculates movement of individual fuel isotopes, assuming they always accompany the convecting fuel in the fluid-dynamics model. The accuracy of the Pu-vector model was confirmed by comparing with the standard Monte Carlo static neutronics calculation. The new model can improve some of the limitations in the current SIMMER code, in which the fuel isotopes are represented only by two groups, fertile and fissile fuels. Assignment of a number of fuel isotopes to the two groups requires a detailed examination of different combinations of fuel isotopes to determine an optimized combination. The Pu-vector model can eliminate this complicated procedure to be performed prior to a SIMMER analysis, and more importantly provides accurate spatial distribution of nuclide isotopes and thus will improve the applicability of SIMMER to the analyses of future large heterogeneous reactors.

KEYWORDS:

• Sodium cooled fast reactor
• numerical simulation
• safety analysis
• core disruptive accident
• SIMMER code
• code developments

Acknowledgments

The authors express their sincere gratitude to Mr. Masaaki Sugaya and Mr. Masahiro Mizuno of NESI Inc. for their enormous programming and computational supports.