A comprehensive study on source terms in irradiated graphite spheres of HTR-10

Highlights: • Four irradiated graphite spheres from the core of HTR-10 were studied experimentally. • The key nuclides in the spheres were identified as H-3, C-14, Co-60, Cs-137, Eu-152, and Eu-154. • The specific activities show a rather symmetric distribution relative to the center. • A sensitive analysis of source terms in the spheres has been provided. - Abstract: With previously developed experimental methods which include the preparation and measurement process for the graphite sample, two new irradiated graphite spheres with surface γ dose rates of 51.00 μSv/h and 0.14 μSv/h from the reactor core of the 10 MW high temperature gas-cooled reactor (HTR-10) have been investigated experimentally. The total β counting rate, the β spectra and the γ spectra for each graphite sample of irradiated graphite spheres were recorded with a total α/β counting measuring apparatus, a liquid scintillation counter and a high-purity germanium detector connected to a multichannel analyzer, respectively. Combined with previous experimental data of two irradiated graphite spheres with surface γ dose rates of 25.10 μSv/h and 1.17 μSv/h, the types of key nuclides in the irradiated graphite sphere of HTR-10 were determined, which were H-3, C-14, Co-60, Cs-137, Eu-152 and Eu-154. The distributions for each nuclide in four irradiated graphite spheres were compared. The generation mechanisms of H-3, C-14, Co-60, Cs-137, Eu-152 and Eu-154 in the irradiated graphite sphere of HTR-10 were discussed and analyzed. Based on all the experimental data regarding impurities and uranium contamination in the matrix graphite of HTR-10 available, a sensitivity analysis was performed to explain the effect of impurities and uranium contamination on the specific activity of key nuclides in the graphite sphere. The influence of the neutron flux and the dwell time in the core on the specific activity of key nuclides was also considered. The differences of experimental specific activities among these irradiated graphite spheres were compared and explained. Current comprehensive studies on irradiated graphite spheres of HTR-10 can provide valuable information for the source term analysis, waste minimization and radiation protection of high temperature gas-cooled reactors (HTGRs).