Radiological Characterization of the Philippine Research Reactor- 1 Neutron Sources via Monte Carlo Simulation

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Introduction

The Philippine Research Reactor-1 (PRR-1) facility is currently in possession of three isotopic neutron sources (INS) that have been stored together with slightly irradiated TRIGA fuel in a storage tank for ~35 years. These INS include ²³⁹PuBe (R1), ²⁵²Cf (R3) and ²⁵²Cf (R4), which are legacy sources from the former operation of PRR-1. In order to safely handle, store, transfer, dispose, and/or reuse these INS, Monte Carlo N-Particle (MCNP) [1] simulation was performed to characterize the radiological hazard associated with each source.

Model and Method

Monte Carlo N-Transport Particle (MCNP) [1] models were prepared, which includes volumetric neutron sources that are incorporated in a detailed model of the PRR-1 storage tank. A rectangular mesh centered at the tank with mesh element dimensions of 1cm × 1cm × 50.8 cm was used to calculate the x-y plane distribution of the dose rate. A 1 cm × 1 cm mesh size along the x and y axes was selected to obtain a good resolution, while the 50.8 cm mesh size along the z-axis was chosen to coincide with the active length of the fuel. The F4 mesh tally in MCNP was utilized to map the neutron and gamma doses. The flux tally was modified to dose by means of flux-to-dose conversion factors (DE and DF cards) in MCNP. The ICRP- 74 [1] conversion factors were used for neutrons, while for gamma, the ANSI/ANS-6.1.1 [2] factors were used. Although the actual location of the neutron sources in the tank varies in terms of vertical positioning, they were declared in the MCNP model at a vertical position that coincides with the midpoint of the fuel length. This is to estimate the neutron doses from the legacy INS relative to the gamma dose from the slightly irradiated PRR-1 TRIGA fuel.

Results and Discussion

The dose distribution from photons emitted by the slightly irradiated TRIGA fuel rods and neutrons from the three legacy INS are presented in Fig. 1. For relative comparison of the doses, the combined plots are also presented in a 3D plot in Fig. 2, where the dose values for R3 and R4 sources have been multiplied 100 to be visible. The plots demonstrate that, in the current storage condition, the storage tank provides sufficient shielding against neutrons from the legacy INS such that the neutron doses are insignificant compared to the gamma dose from the storage tank.

Conclusion

MCNP simulation was performed to characterize the radiation hazard associated with the irradiated fuel and three legacy neutron sources. Relevant quantities such as photon and neutron dose distributions of these materials were determined, which will serve as basis for the formulation of procedures for safe handling, storage, transfer, disposal, and use of isotopic neutron sources upon commissioning of the PRR-1 Subcritical Assembly for Training, Education, and Research (SATER). Also, the results of this study will provide a reference for an updated dose map of the facility that includes neutron radiation, which will be incorporated in the existing radiation area monitoring procedure.