Modeling the diffusion yield of radioactive fission products from uranium dioxide fuel

Abstract

The numerical aspects of a systematic solution of the problem of diffusion and yield of radioactive fission products from a homogeneous sphere, simulating a uranium dioxide fuel kernel, with realistic boundary conditions are discussed. The numerical scheme is based on a one-group method of calculating the production and radioactive mutual transmutations of fission products in combination with the standard expansion of the radial dependence of the concentration in terms of the eigenfunctions of the Laplace operator. It is demonstrated on illustrative examples that the approach is highly economical.The study of the behavior of fission products, specifically, their escape from nuclear fuel, in standard and emergency regimes remains a very important problem for safe operation of nuclear facilities. In the first theoretical works, it was supposed that the escape of fission products is associated with, first and foremost, near-surface mechanisms [1] in which fission products participate directly. However, as experimental data accumulated it became clear that in a wide temperature range diffusion processes control the escape of fission products [2][3][4]. The diffusion mechanism is now an essential component of the mechanistic models of the escape of fission products.To solve the problems associated with the operational safety of nuclear facilities, it is necessary to make a systematic assessment of the production and escape of the most significant radionuclides in regular and emergency regimes. The urgency of this problem becomes even more obvious considering the growing, at the present time, interest in new types of fuel and service life extension of reactors, currently operating and still in the design stages, with increased burnup, since the admissible limits for increasing the yield of fission products must be determined to substantiate the radiation safety of innovative solutions.The present article discusses the numerical aspects of the systematic solution of the problem of diffusion and escape of radioactive fission products from an individual grain of nuclear fuel. The numerical scheme and the corresponding program module were developed from the perspective of incorporation into the MFPR code [5,6], simulating the formation and evolution of point and extended in-grain defects, in-and intergrain porosity, formation of chemical compounds and solid phases (precipitates) inside the fuel, diffusion of stable fission products and their emergence into open porosity. For illustrative purposes, the proposed numerical scheme is implemented in the form of an autonomous module, so that the interaction of diffusing fission products with lattice defects, the behavior of the fission products in the intergrain space, and the formation of chemical compounds are not considered. At the same time, the model of the boundary conditions, which is adopted in the MFPR code and whose parameters are assumed to be set externally, is used. The standard expansion of the radial dependence of the concentration of fission products in terms of...

Fission Product Release From High and Low-Enriched Uranium Fuels of the IVG.1M Research Reactor

“…For any nuclear reactor, the allowable release of fission products (FP) is determined with the objective of ensuring the necessary level of operational safety. This determination hinges upon the effectiveness of protective barriers against the propagation of radionuclides, as well as the reliability and accuracy of methods used to monitor FP concentrations in the reactor coolant [11][12][13][14][15][16].…”