Application of transient fuel bundle analysis code FRETA-B to LOCA simulation experiments

Fuel behavior during loss-of-coolant accidents (LOCA) involves two-dimensional phenomena in each elevation of a bundle, arising from the unsymmetrical structure and also such stochastic factors as fuel relocation or uneven coolant flow. The FRETA-B code can calculate heat conduction, deformation and oxidation of the cladding, and internal gas pressure and flow in multiple fuel rods of a bundle, using two-dimensional models. Ex-phases are placed on the treatments of external (rod-to-rod, or rod-to-coolant) radiant heat transfer and ballooning of the cladding. Calculation can proceed beyond the time of rod-to-rod contact. To assess the code, analyses were performed on the ex-pile multi-rod ballooning experiment in JAERI and on the in-pile LOCA simulation experiment in the NRU reactor. The simple and well-defined boundary condition of the ex-pile experiment has produced good agreement between calculation and measurement of the cladding temperatures. The calculation on the in-pile experiment has shown that stochastic factors are dominant over structural ones in determining the distribution of the cladding temperature and deformation. (author)