Specifics of calculation of heat rate in oxide nuclear fuel during tests in reactor BOR-60
Under domestic and foreign contracts in the BOR-60 reactor various fuel composition types are tested including the oxide ones. The enrichment of the tested nuclear fuel in the fissile nuclides is often considerably lower than the enrichment of the standard nuclear reactor fuel. Thus, the number of fissions inside unit volume and specific heat release in tested nuclear fuel are significantly less than in standard fuel. The standard and experimental fuel pins are the sources of prompt and delayed gammas, and the strength of these sources is proportional to fission rate. Obviously, the fission rate of nuclear fuel in experimental fuel pins is less than in standard ones. Therefore, the heat in the experimental fuel pins is released not only as a result of the tested fuel nuclei fission but also due to additional radiation heating by gamma-ray of the standard FAs of the core. The widespread computation codes based on the Monte Carlo method do not simulate directly the changes in the isotopic composition of nuclear fuel under irradiation and, therefore, the generation and transport of delayed gammas. This results in underestimation of the heat release calculated values for the nuclear fuel tested.
Using the approach developed by the authors, calculations were performed of the contribution of a component of delayed gammas to the heat release in the tested oxide fuel compositions of different enrichment. The heat release calculations were performed using the MCU-RR code. The dependence of delayed gamma-ray contribution to the heat release on the experimental FA location in the core was obtained. It is shown that disregard of delayed gamma-ray can lead to considerable underestimation of the heat release and can achieve 8%. Thus, the method developed by the authors allows increasing the precision of irradiation conditions determining for the experimental fuel pins containing oxide uranium and uranium-plutonium fuel.
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