Abstract: A rational space distribution model for spent fuel element dissolving at each stage of process in reprocessing plant was formulated. The nuclear criticality on safety issue was studied by calculating numerical model on the process of spent fuel dissolution in consideration of the given plant arrangement. An assessment of the influence of several main critical parameters to the plant safety was given. The calculation results show that the most dangerous status occurs at the initial stage of dissolution when fissile nuclide transforms under ideal conditions. A negative influence to the system is indicated by the increase of temperature and concentration of nitric acid, and the effect is less than 4%. System safety can be improved greatly by the addition of neutron poison or the application of fuel burnup credit, and the effect reaches 30%.