Criticality control and long term storage of spent fuel

Development of adequate facilities for long term storage of spent nuclear fuels is a current concern in the nuclear industry. The lack of chemical reprocessing is forcing reactor operators to increase the capacity of their existing pools by structural modifications or by costly investments in new facilities. One of the main constraints governing the utilization of space in a storage pool is nuclear criticality control. The degree of subcriticality of stored fuel is dependent on the fissile material inventory, the arrangement of fuel, and the presence of materials providing parasitic capture of neutrons. Long term storage of spent fuel requires that nuclear criticality considerations include time dependent variations in: fissile isotopic inventory, dispersion of fissile materials, transmutation of fission products, and the deterioration of special neutron control materials. According to current practices in power reactor pools, subcritical units of fuel are stored in a lattice arrangement with sufficient neutronic decoupling between individual fuel bundles. Such decoupling is achieved by adequate isolation in water or by the insertion of special neutron absorbing materials such as Boral. Pool capacity increases by a factor of two have been realized using Boral inserts. An assumption is made in criticality safety analyses that spent fuel has a fissile isotopic composition of unirradiated fuel. The fact that a large fraction of the fissile isotopes have been converted to fission products is not considered. Criticality control calculations and experiments with spent fuel indicate that use of green fuel isotopic compositions introduce safety margins in fuel storage basins which would be equivalent to the neutron control provided by materials such as Boral inserts. Therefore, the use of spent fuel critical mass parameters can serve as an alternate, or as a supplement to special poison inserts for achieving maximum utilization of basin capacity for long term storage of fuel