Evaluation of Neutron Detection Efficiency of the Unified Non-Destructive Assay Using MCNPX Code

In this study, neutron detection efficiency of the UNDA system, which has been developed for study on nuclear material accountancy in a future pyro-process facility, was evaluated by using the MCNPX code. The detection efficiency was evaluated as a function of (1) positions of $^{252}Cf$ neutron source in the axial and radial directions, and (2) thicknesses and locations of the container filled with the depleted uranium materials for two different designs of the UNDA. In the case of $^{252}Cf$ source positions, detection efficiency was distributed from 6.83% to 13.35%. As $^{252}Cf$ source was positioned at upper part in the axial direction, detection efficiency was decreased after a slight increase. On the other hands, as $^{252}Cf$ source was positioned at outer part in the radial direction, detection efficiency was increased. In the case of container thickness, there was a slight decline when the thickness was increased. As the container was located at upper part, detection efficiency was decreased and as the container was located at outer part, detection efficiency was increased. Detection efficiency was varied from 10.31% to 13.61%. These values were higher than that of $^{252}Cf$ source case. The UNDA with polyethylene cover has about 2% higher detection efficiency than the UNDA without the cover.

MCNPX 코드를 이용한 통합비파괴측정장치의 중성자 검출 효율 평가

본 연구에서는 미래 파이로 시설에서의 핵물질 계량 연구를 위하여 개발하고 있는 통합비파괴측정장치(Unified Non-Destructive Assay, UNDA)의 중성자 검출 효율을 MCNPX 코드를 이용하여 평가하였다. 검출 효율 평가는 두 개의 다른 설계안의 UNDA에 대하여 수행되었으며, $^{252}Cf$ 중성자 발생 선원 위치에 따른 검출 효율 평가와 감손우라늄의 용기 두께 및 위치에 따른 검출 효율 평가를 수행하였다. $^{252}Cf$ 중성자 선원의 위치에 따른 UNDA의 검출 효율 결과는 6.83%부터 13.35%까지 분포로 나타났으며, $^{252}Cf$ 선원이 장치 내부의 상단에 위치할수록 검출 효율은 증가 후 감소하는 경향을 나타냈고, 선원이 외각에 위치될수록 효율이 증가하는 경향을 보였다. 감손우라늄 용기의 두께 및 위치에 따른 검출 효율 평가에서는 용기 두께가 증가할수록 검출 효율은 낮아지는 경향을 보이며, 용기 위치가 장치 상부에 위치될수록 효율은 감소하고, 외각에 위치할수록 효율은 증가하였다. 검출 효율은 $^{252}Cf$ 선원의 경우보다 약간 높게 나타났다(10.31~13.61%). 또한, 장치 상단에 고밀도 폴리에틸렌 덮개가 있는 설계안이 덮개가 없는 설계안 보다 평균적으로 약 2% 정도 중성자 검출 효율이 높은 것으로 평가되었다.