속중성자 다중도 측정 기반 특정핵물질 계량 시스템 전산모사

Abstract

One of the main tasks for implementing nuclear safety measures is nuclear material accounting management. The development of technology for nuclear disarmament verification is the focus of numerous studies, but the study is concentrated on using it to verify the pyroprocessing system in Korea. The new accounting technique combines a thermal neutron multiplication-based accounting method based on the Pu/Cm ratio with a fast neutron energy multiplication method. However, there is a limitation in detecting thermal neutrons using 3He tubes. While alternative detector are being developed internationally due to the 3He depletion problem, fast neutron multiplicity detection method is being developed to replace the function of the current accounting system and improve performance. For 239Pu or 241Pu, the spontaneous fission rate is significantly lower, so neutrons can be detected by irradiation of exogenous sources. However, to develop a system that estimates the effective mass of 240Pu with a high spontaneous fission rate due to low economic feasibility and increased complexity for system construction, an optimized design was carried out through computer simulation. Mathematical modeling previously proposed for application to a fast neutron detection system was verified. In this study, using MCNPX-PoliMi, the characteristics of neutrons caused by spontaneous fission and induced fission of 240Pu were observed by assuming a 4 π type 20 mm thick stilbene detector. Through this, it was verified that the point model to be applied to the fast neutron detection system was valid. For the development of a compact system, an optimized arrangement was selected among system designs using six stilbene. It was confirmed that the decrease the source-to-detector distance in the ring arrangement and the cubic arrangement, the higher the efficiency. The effective mass in the range of 5 g to 100 g of 240Pu samples was derived and compared in each design. Through this, it was confirmed that the maximum value of the relative error of the estimated mass was lower than that of the ring arrangement when the stilbene detector achieved a cubic arrangement. In addition, a method using parameters was additionally performed to compare the accuracy of estimating the effective mass of the sample. When comparing the one-parameter assay and the two-parameter assay, it was confirmed that the accuracy was improved, especially for trace samples, when the two-parameter assay was applied. Therefore it was confirmed that the parameter analysis method using the information of reference samples among various methods was the most accurate in developing a specific nuclear material metering management system. Based on this, a neutron multiplicity accounting system using fast neutron detection is developed to measure specific nuclear materials, and the estimated effective mass of 240Pu can be used as a relative criterion for determining spent nuclear fuel and weapon-grade Pu.