VOLUME 19 (Supplement)

Computational dose assessment of a subcritical assembly driven by a deuterium-deuterium portable neutron generator

This study presents the computational dose assessment of a subcritical assembly driven by a deuterium-deuterium portable neutron generator (PNG). The work was carried out to provide an evaluation of the neutron and photon dose distribution for a proposed PNG-driven reactor configuration and to assess the adequacy of shielding under both routine and abnormal conditions. Monte Carlo simulations were performed using MCNP5 version 1.6 for a model based on the PNG-driven Philippine Research Reactor – 1 Subcritical Assembly for Training, Education, and Research. Two scenarios were considered: normal operation and a loss of shielding. Ambient dose distributions were assessed using mesh tallies in the xy and the xz planes, together with dose estimates from six air-filled detectors placed at selected locations around the assembly. Under normal operation, the highest total dose rate was obtained at detector 103, which is closest to the PNG, with a value of 1.20 x 101 µSv/hr, while the other detector locations were substantially lower. Assuming a total neutron generator operating time of 500 hours, continuous occupancy at detector 103 would correspond to an accumulated dose of about 6.0 mSv, which remains below the occupational dose limit of 20 mSv per year averaged over five consecutive years. Under the loss of shielding scenario, dose rates increased substantially across all detector locations, with detector 103 reaching 2.05 x 103 µSv/hr. The radiation field also changed from being secondary photon dominated to neutron dominated, demonstrating the importance of water as a moderating and shielding material. Since loss of shielding represents an emergency scenario, the results were interpreted in terms of response time and exposure management. At the highest dose rate location, a response window of 2 to 4 hours would correspond to a potential exposure of approximately 4.1 to 8.2 mSv. The results show that the water shielding provides sufficient dose reduction under the conservative exposure assumption for normal operation, while complete water loss would require immediate access restriction, rapid system shutdown, and optimized personnel deployment to minimize exposure. The study provides an initial computational basis for radiation safety, shielding assessment, and emergency planning for portable neutron generator operation in subcritical assemblies.