Criticality of a spent fuel pool filled with burned FLIP fuel

It has been shown that supercriticality might occur for some postulated accidents for the TRIGA spent fuel pool at Jozef Stefan Institute in Ljubljana, Slovenia. Until recently, the analyses were mainly done with all types of fresh fuel in the storage. As the effect of burned standard fuel was already done, similar analysis is presented here for burned FLIP fuel in the storage pool. Besides the dependence of criticality on FLIP fuel burnup, the influence of pitch among the elements and the number of uniformly mixed absorber rods for a square arrangement are studied also. The Monte Carlo computer code MCNP4B with ENDF-B/VI library and detailed three dimensional geometry was used. WIMS-D code was used to model the isotopic composition of fresh and 5, 10, 20 and 30 % burned fuel immediately after the removal from the core and for 2 and 4 years cooling time. The results show that out of the three studied effects (pitch from contact (3.75 cm) up to rack design pitch (8 cm), number of absorbers from 0 to 8 and burnup up to 30 %) the pitch has the greatest influence on the multiplication factor keff. In the interval in which the pitch was changed, keff decreased for around 0.3. The number of absorber rods affects the multiplication factor much less. This effect is bigger for more compact arrangements, e.g. for contact of fuel elements with 8 absorber rods among them, keff decreases for more than 0.1 almost regardlessly of the burnup. For 8 cm pitch the same effect is smaller for some 30 %. The effect of burnup is the smallest one. In all cases the keff changes for 0.02 to 0.04, but due to the increased reactivity with burnup, keff increase for compact arrangement and keff decrease for 8 cm pitch could be seen. Cooling of fuel has just a minor effect on the criticality of spent fuel pool and can be neglected in spent fuel pool design.(author)