VOLUME 19 (Supplement)

Monte Carlo dose assessment of a neutron irradiation facility with two PuBe sources

The Neutron Irradiation Facility (NIF) utilizes neutron sources and moderating assemblies that require radiation protection assessment under various operating conditions. In this study, neutron and photon ambient dose equivalent distributions were evaluated using the Monte Carlo N-Particle (MCNP) code. Five operational configurations involving different source positions relative to the high-density polyethylene (HDPE) moderator assembly and source handling system (SHS) were simulated to characterize spatial radiation fields and support radiation zoning. Under the default condition (Configuration 1), with both neutron sources positioned within the HDPE moderator assembly, the maximum total ambient dose equivalent rate was 8.56 μSv/h, with radiation fields strongly localized near the moderator perimeter. Elevated source configurations (Configurations 2 and 3) produced higher maximum rates of 1.18 × 10² and 1.46 × 10² μSv/h, respectively. For SHS configurations, Configuration 5 yielded a lower maximum ambient dose equivalent rate than Configuration 4, decreasing from 9.95 to 4.44 μSv/h. The counting room consistently exhibited low ambient dose equivalent rates, ranging from 1.82 × 10⁻³ to 1.81 × 10⁻¹ μSv/h. Annualized ambient dose equivalent values in Controlled Areas remained below the applicable occupational effective-dose limits of 20 mSv/y for adult workers and 6 mSv/y for student trainees aged 16–18, while supervised public-access areas outside the NIF remained below the 1 mSv/y public effective-dose limit specified in PNRI CPR Part 3 Rev. 02. These comparisons were treated as conservative screening assessments. The results demonstrate the strong dependence of radiation fields on source configuration and shielding geometry and provide a quantitative basis for radiation zoning and operational controls. Independent experimental validation is recommended for future work.