Study on Radionuclide Desorption in Matrix Graphite Under Accident Conditions

Abstract High temperature gas cooled reactor (HTGR) is one of the promising advanced nuclear reactor for future electricity generation. The spherical fuel element with thousands of TRISO-coated particles dispersed in the graphite matrix are adopted by the HTGR, which is an important guarantee for the inherent safety of this type of reactor. The TRISO fuel particle typically consists of a very small fuel kernel in the central region, and surrounded by four coating layers. These coating layers constitute the principal containment barrier for radioactivity release under normal and accident conditions. In the spherical fuel element, the radionuclide release is dominated by multiple physical and chemical processes, such as fission product transport, recoil, adsorption, desorption, and etc. The matrix graphite would be oxidized by water or air under water ingress or air ingress accident, which causing an increase of the porosity of graphite. The increase of graphite porosity affects the diffusion and adsorption of radionuclides in the matrix graphite, thereby impacting the release of radioactive nuclides. This paper studies the adsorption process of radionuclides and analyzes the influence of graphite oxidation on the adsorption rate under water ingress and air ingress accident. The adsorption rate of several safety related radionuclides (such as 131I, 110Ag, 137Cs, 90Sr) in the matrix graphite were analyzed in detail. The influence of temperature, adsorption energy and density of radionuclide in the matrix graphite on the adsorption rate is discussed. Among these relevant factors, temperature has the most significant influence. The density of radionuclide in the matrix graphite increases with the increase of graphite porosity and surface area due to graphite oxidation. Although the adsorption rate of nuclides in the matrix graphite will increase due to oxidation, the adsorption rate of nuclides is almost zero under water ingress and air ingress accident, and the increase of the adsorption rate caused by oxidation can be negligible.