Radiological Dose Assessment for the Ghana Research Reactor-1 at Shutdown Using Dispersion Model: Conversion from High-Enriched Uranium to Low-Enriched Uranium Fuel

Ghana Research Reactor-1 is a miniature neutron source reactor (MNSR) which is currently fuelled with highly-enriched uranium (HEU) aluminium alloy fuel. Efforts are underway to convert the research reactor fuel to low-enriched uranium (LEU) oxide fuel. The project is coordinated research work funded by the International Atomic Energy Agency (IAEA) through its Coordinated Research Project (CRP) on Core Conversion. The research project was started with thermal hydraulic and neutronic calculation on both fuels. Radiological dose assessment as part of a safety assessment requirement needs to be carried out before the commencement of the core conversion project As such, dose assessment was credibly estimated by employing computer software (Health Physics Code HotSpot Version 3.0) developed by Lawrence Livermore National Laboratory. The code uses the Gaussian plume model for atmospheric dispersion and deposition of radionuclides based on meteorological and demographic site information. The latest IAEA guidelines for radiological dose assessment were considered in the estimation of released radionuclide doses. The anticipated estimated radionuclides released provided a comprehensive theoretical and real basis for estimating the committed equivalent dose (CED) covering the emergency and the low populated zone as expected in most severe accident scenarios. An isotope depletion analysis code ORIGEN-S coupled with MCNP5 code for neutron flux generation was used to study possible available radionuclides present in the reactor core at shutdown. Some few released radionuclides were selected from the inventory generated from the HEU core. The selected radionuclides were used in the dispersion code for dose estimation. The total activity value of two selected radionuclides (iodine and cesium) from the inventory in curies were 2.067E-03 and 6.20E-4, respectively. The values are based on the release fraction of the selected nuclides. The CED values estimated were found to be in agreement with the IAEA and US-NRC regulatory acceptable limit of 1 mSv received as public exposure and 50 mSv for radiation worker exposure in a year. The study results can be recommended when establishing the required emergency planning zones around the Ghana Research Reactor-1 facility in future