TRIGA REACTOR SPENT FUEL POOL UNDER SEVERE EARTHQUAKE CONDITIONS

Supplemental criticality safety analysis of a pool type storage for TR IGA spent fuel at 'Jozef Stefan' Institute in Ljubljana, Slovenia, is presented. Previous results (Ravnik, M, Glumac, B., 1996. Nucl. Techno l. 114, 365-371) have shown that subcriticality is not guaranteed for some postulated accidents. To mitigate this deficiency, a study was ma de about replacing a certain number of fuel elements in the rack with absorber rods (Glumac, B., Ravnik, M., Logar, M., 1997. Nucl. Technol. 117, 248-254.) to lower the supercriticality probability, when the pi tch is decreased to contact las a consequence of a severe earthquake) in a square arrangement. The criticality analysis for the hexagonal co ntact pitch is presented in this paper, following the same scenario as outlined above. The Monte Carlo computer code MCNP4B with ENDF-B/VI l ibrary and detailed three dimensional geometry was used. First, the an alysis about the influence of the number of triangular fuel piles on t he bottom that could appear, if the fuel rack, made of three segments, disintegrates, is presented. Next, the number of uniformly mixed abso rber rods in the lattice needed to sustain the subcriticality of the s torage for hexagonal contact pitch is studied. Because of supercritica lity possibility due to random mixing of the absorber rods in the case of lattice compaction, a probabilistic study was made in order to sam ple the probability density functions for random lattice loadings of t he absorber rods. The results show that reasonably low probabilities f or supercriticality can be achieved even when fresh 12 wt.% standard T RIGA fuel is stored in the spent fuel pool. (C) 1998 Elsevier Science S.A. AU rights reserved.