Uncertainty analysis of criticality safety for the plate type fuel assembly storage rack

Abstract

To evaluate the criticality safety of the fresh and the spent fuel storage racks in an open pool type research reactor designed by KAERI, the upper subcriticality limit (USL) analysis was carried out. First, the bias and its uncertainty of MCNP code system with ENDF/B-VII library were evaluated using the calculation results of the 183 benchmark experiments. The criticality calculations for the fuel storage rack are carried out under a normal state, an increased water temperature, a fuel assembly drop, and an eccentric insertion which can affect the criticality. Considering biases and uncertainties for the MCNP code system, abnormal conditions, and the manufacturing tolerance of the cell tube thickness, the USL value that can guarantee sufficient subcriticality is determined. It was found that the criticality of the fresh and the spent fuel storage racks currently designed satisfy the USL condition. Additionally, it was concluded that the pitch size of a fresh fuel storage rack can be reduced for efficient space availability, and even under a worst case in which the fresh storage rack is surrounded by a lower water density and the smallest pitch size satisfies the USL conditions.