Molten salt irradiation loop source term analysis and shielding scheme design

[Background]: Radioisotopes are unstable and emit energetic particles during radioactive decay, making them widely used in medicine, industry, agriculture, and other fields. [Purpose]: To verify the isotopes production in a molten salt reactor, a high-temperature molten salt irradiation loop based on an accelerator-driven neutron source was designed and will be conducted. A shielding scheme corresponding to the source term analysis of the molten salt irradiation loop is designed. [Methods]: A four-step methodology is developed, which is based on the well-known MCNP5 and ORIGEN-S codes. which include neutron energy spectrum calculations, single-group cross-section processing, irradiation calculations and source term analyses, and shielding calculations. [Results]: When the molten salt in the loop is irradiated by the neutron source for 20 days, its total neutron flux in the molten salt reaches 2.75×108 n∙cm-2∙s-1, and its gamma flux is 5.83×1011 p∙cm-2∙s-1. After being cooled for a day, the short-lived fission products almost completely decay out, making its neutron flux decreasing to 4.78×102 n∙cm-2∙s-1, while the gamma flux remains 2.56×1011 p∙cm-2∙s-1. Therefore, gamma rays are the priority for loop shielding. By analyzing the main nuclides that contribute to the gamma flux and their distribution in the molten salt, a shielding is designed. With the application of a lead layer, the dose in the vicinity of the molten salt loop can be reduced to less than 5 μSv∙h-1. [Conclusions]: Various radioactive products are produced after a 20-day irradiation, among which 239Np has the highest activity. Lead can effectively shield the radiation produced by these radioactive products. The simulation indicates that when the lead thickness reaches 10 cm, the radiation dose in the vicinity of the molten salt loop is lower than 5 μSv∙h-1, which satisfies the safety dose limit.