[en] LWR fuel reprocessing plants reprocess spent fuels which have been burned to approximately a few tens of GWD/MTU in reactors. Viewing the ''burnup'' process from the standpoint of the criticality safety design of the reprocessing plants, the criticality safety of the equipment treating the fuel is considered to be enhanced or the design requirements for criticality safety is presumed to be reduced, as the reactivity of the nuclear fuel materials is lowered because of the following two reasons; changes in the isotopic composition of the fissile nuclides, and increase of the fission products (neutron poisons). In this paper, 19 nuclides in the fission products (FP) having comparatively large neutron absorbability were selected and the results of criticality evaluations with consideration to their neutron absorption effect at the barrel section of the dissolver were described. As a result, by taking the existence of the FP into account, the effective multiplication factor is lowered considerably. The extent of this reduction is expected to be around 50 % to 100 % of that attributable to changes in the isotopic composition of the fissile nuclides. In addition, it has also been found that, by taking the effect of FP into account in the criticality safety design, the processing capacity of a dissolver can be increased up to 1.1 ∼ 4.4 times (the burnup at 5 ∼ 40 GWD/MTU) in comparison with designs which disregard the existence of FP. (author)