인공 희토류원소(Re2O3) 기반의 중성자흡수체 설계 및 그 활용에 관한 연구

Abstract

A new concept for neutron absorption material (i.e., an artificial rare earth compound) is introduced for criticality control in spent fuel storage systems. Although extensive use of boron-rich materials (e.g., BoralTM, NeutroSorbTM, and B4C) is necessary and inevitable for maintaining subcriticality in such systems, these materials can cause difficulties with respect to management of spent nuclear fuels such as increased manufacturing costs and space required. In this study, it was confirmed that the criticality of a spent fuel storage system could be efficiently controlled by introducing a new neutron absorber utilizing an artificial rare earth compound. The capability of this material as an absorber for thermal neutrons is the same as boron-rich materials. The main characteristics of this absorber are summarized as follows.

a) Since the new neutron absorber (hollow cylinder type) is designed to be inserted into the guide thimbles of existing nuclear fuel assemblies, it can be uniformly distributed among nuclear fuel rods, regardless of the type of nuclear fuel assembly or storage system. b) The use of an artificial rare earth compound can be easily adjusted according to the facility’s characteristics for spent fuel storage and can be supplemented with additional materials (e.g., cadmium, indium, etc) to complement the rare earth compound employed in the inner cylinder.

In addition, the neutron absorber presented in this study exhibits a lower criticality change (by a factor of ~2) than another boron-rich material (BoralTM), despite the neutron irradiation over a 40 year period (source strength = 6.3×109 n/sec). Application of this absorber to a KSC-4 spent fuel cask revealed that its criticality is well controlled by using a small amount (~94.31 kg) compared with an earlier design employing 147 kg of BoralTM plate. In addition, the radiation dose rate distribution around the cask is influenced primarily by the neutron source in the active fuel region, whereas the neutrons and gamma-rays emitted from the artificial rare earth compound do not contribute to surface and external dose rates. Therefore, this concept of a new neutron absorption material can mitigate criticality control problems that arise in the design of spent fuel storage systems. Likewise, this approach is directly applicable to recycling and volume reduction of High-Level Waste (HLW). Lastly, the proposed method is expected to vastly improve the efficiency of radioactive waste management by simultaneously keeping High-Level Waste (HLW) and spent nuclear fuel in a restricted space.