Chemical speciation analysis of fission products in the PWR fuel pellet during accidents

During the operation of a pressurized water reactor, large amounts of fission products (FP) are generated in the grains of fuel pellets. The transport behavior of different chemical species of fission products varies under nuclear power plant accidents. In this paper, the diffusion and release characteristics of fission products in UO 2 grains are obtained through a comprehensive chemical thermodynamic analysis of all phase states of the fission products within the fuel pellets. The diffusion of fission gases follows Fick's law and is calculated using the Booth model, with the release rate related to the diffusion coefficient. The release mechanism of volatile fission products is similar to that of fission gases, sharing the same diffusion coefficient. The release of semi-volatile fission products requires consideration of the proportion of gaseous products, with the gaseous forms of these fission products within the pellets determined by their chemical thermodynamic properties. Therefore, the diffusion coefficients of fission products (excluding krypton and xenon) can be obtained by multiplying the diffusion coefficient of fission gases by an appropriate scaling factor Sk for each radionuclide. This paper determines the Sk values for various elements through chemical thermodynamic analysis, which can be used for the analysis of fission product release under accident conditions.