Shielding analysis of neutron detector guide tubes of advanced marine reactor MRX

The advanced marine reactor MRX has the out-of-core neutron flux detectors outside of the reactor vessel. To provide the neutron flux detectors, the detector guide tubes are equipped from the outside of the reactor containment vessel to the surrounding area of the reactor vessel. These guide tubes cause radiation streaming as well as the shield defect. In the present paper, shielding analysis of the neutron detector guide tubes of the MRX is carried out to contribute to its optimum shielding design. Calculations are carried out for the problems of the neutron streaming through the guide tubes and of the radial shield defect to gamma rays. Results calculated by a two-dimensional transport code DOT3.5 are used as the source radiation for the calculation of a Monte Carlo code MCNP4B. For the neutron streaming, very near region of the guide tube is treated as shield area in the Monte Carlo calculations to reduce the statistical error. To assess this treatment, analysis of the streaming experiment utilizing the JRR-4 reactor is carried out. As a result, overestimation of a factor of about four for fast neutrons and about two for slow neutrons are found. Streaming analysis of the neutron detector guide tubes of the MRX indicates that the neutron dose equivalent rate is 5.4 μSv/h outside of the reactor vessel where the design limit is 10 μSv/h. Since the allowance is small, it is recommended that the guide tubes are bent so that the source area is not seen from the outlets, and an about 6-cm-thick iron compensational shield is equipped for the radial shield defect to gamma rays. (author)