Critically safety in the waste management of spent fuel from NPPs

During irradiation in the reactor core, fissile material from nuclear fuel is depleted for power generation. However, significant amounts of fissile nuclides, mainly 235U and 239Pu, are still present in spent fuel unloaded from the core, with the inherent potential for maintaining an inadvertent nuclear chain reaction under unfavorable conditions. Due to this reason, criticality safety has to be demonstrated for all stages of waste management of spent nuclear fuel. This pertains e.g. to wet storage in cooling ponds, dry storage and transport of the fuel in casks, dissolver stages in reprocessing plants, as well as the direct final disposition of the spent fuel. Usually, for system design the reactivity of the spent fuel has been considered as being equal to fresh fuel. This can lead to large safety margins and thus an over-conservative design of corresponding waste management systems. In recent past, the reduction in reactivity due to irradiation of the fuel is considered to a greater extent in the design of new systems or the modification of existing systems, e.g. due to the increase in initial enrichment. However, this so-called ''burn-up credit'' imposes additional efforts on the validation of calculation systems and nuclear cross-section data bases. (orig.)