Multi-scale Research On The Fission Products Of High Temperature Gas-cooled Reactor

Pebble-bed High-Temperature Gas-cooled Reactor(HTGR) is known as an advanced reactor with the characteristics of fourth generation. As an advanced nuclear reactor with significant international influence, its release behavior of radioactive fission products and the resulting radiation safety features become significantly important. Combined the experimental design with multiple theoretical methods, the research focuses on the HTGR source term of radiation and makes a deep analysis on the release, adsorption and deposition behavior of the radioactive fission products from three different scales which is macroscopic, microcosmic and mesoscopic.On the macroscopic level, based on the self-design HTR-10 High Temperature Helium Experimental Loop(HTHEL) and the deposition sampling device(DSD), the complete process of the experiment and the safety analysis of HTHEL is described. The exact three-sleeve structure and innovation points of DSD are shown. In order to realize the better understanding of DSD, Some fluid dynamics analyses concerning temperature distribution, fluid motion and particle trajectories are studied with simulating calculations. The study demonstrates that DSD can effectively and steadily deposit particles of fission products and graphite dust in a certain temperature range. The HTHEL can obtain the real data of the fission products depositing in the tube. The sutdy verifies the applicability of the theoretical calculation and the source term model, and provides a important reference to the future reactor design.On the microcosmic level, the first principles of quantum mechanics is used to explore the real adsorption mechanism of four important fission products Cs, Sr, Ag, I on the surface of the high temperature alloy and the graphite. This method can avoid unnecessary assumption and parameter estimation. Based on the adsorption model, the best adsorption place, adsorption energy, difference charge density and density of states(DOS) are studied with simulating calculations. The real adsorption conditions of four nuclides are obtained to accurately predict their existential states in HTGR. The study provides a new approach to the source term analysis of radioactive nuclides in the first loop of HTGR.On the mesoscopic level, using the fick’s law of diffusion, the release behavior of HTGR coated fuel particles is studied. The cumulative release fraction of the classic method and the improved method according to reactor physics are given. By comparing the physical meaning, theory assumption and calculation, the physical intension and applied range of the corresponding breakthrough time are clarified. Then combined with the design parameters of HTR-10 and other gas-cooled reactors, the cumulative release fraction and the breakthrough time of three important fission products Cs, Sr, Ag passing through the Si C layer of fuel particles are calculated. The diffusion property of each nuclide is analyzed and the safety property of HTGR is verified. The study provides a new method to better analyze and evaluate the release of radioactive nuclides in the fuel element.