Neutron tomography of two different internal structures in dry cask storage

Abstract

A: Given the increasing quantity of spent nuclear fuel stored at nuclear plants across South Korea, the demand for dry storage in South Korea has been on the rise over the past few years. It is also necessary to develop a new safeguard technique that can effectively indicate internal configuration changes caused by any undeclared activities, such as diversions, misuse, or any theft of encapsulated spent nuclear fuel assemblies. Therefore, the present study proposes a passive neutron tomography method based on the result from a feasibility test via an MCNP simulation.As the first step of examination, the authors designed a new passive neutron tomography system, employing He-4 gas scintillation detectors (S670E, Arktis Radiation Detectors Ltd., Switzerland) newly developed to measure both thermal and fast neutrons simultaneously without any moderators. Monte Carlo simulations for two different cases (Case I and II) were then carried out in order to investigate the potential of the developed system. Each case was also simulated with five different variables.As for the simulations, Case I was executed under the hypothesis that some of the spent fuel assemblies would be discharged when empty space exists in the casks. Case II, the condition of which was identical to Case I except that the empty spaces in the casks are filled with dummy materials, was executed. The results of the simulations indicate that the system offers the capability to investigate the integrity of spent fuel assemblies in a cask visually. Further studies will help to find solutions to the attenuation by high-Z dummy materials and the degradation of the spatial resolution when only central assemblies are discharged from a cask.