CRITICALITY SAFETY ASSESSMENT OF A TRIGA REACTOR SPENT-FUEL POOL UNDER ACCIDENT CONDITIONS

Additional criticality safety analysis of a pool-type storage for TRIG A spent fuel at the Jozef Stefan Institute in Ljubljana, Slovenia, is presented. Previous results have shown that subcriticality is not guar anteed for some postulated accidents (earthquake with subsequent fuel rack disintegration resulting in contact fuel pitch) under the assumpt ion that the fuel rack is loaded with fresh 12 wt% standard fuel. To m itigate this deficiency, a study was done on replacing a certain numbe r of fuel elements in the rack with cadmium-loaded absorber rods. The Monte Carlo computer code MCNP4A with an ENDF/B-V library and detailed three-dimensional geometrical model of the spent-fuel rack was used f or this purpose. First, a minimum critical number of fuel elements was determined for contact pitch, and two possible geometries of rack dis integration were considered. Next, it was shown that subcriticality ca n be ensured when pitch is decreased from a rack design pitch of 8 cm to contact, ifa certain number of fuel elements (8 to 20 our of 70) ar e replaced by absorber rods, which are uniformly mixed into the lattic e. To account for the possibility that random mixing of fuel elements and absorber rods can occur during rack disintegration and result in a supercritical configuration, a probabilistic study was made to sample the probability density functions for random absorber rod lattice loa dings. Results of the calculations show that reasonably low probabilit ies for supercriticality can be achieved (down to 10(-6) persevere ear thquake, which would result in rack disintegration and subsequent maxi mum possible pitch decrease) even in the case where fresh 12 wt% stand ard TRIGA fuel would be stored in the spent-fuel pool.