State of metal alloy fuel after irradiation in a fast reactor

Abstract

The main domestic and foreign results obtained in the course of reactor tests and post-reactor studies of alloyed fuel are reviewed. The regularities of its radiation swelling and mechanical and chemical interaction with fuel-element cladding are determined. The knowledge accumulated about the properties and behavior under irradiation of alloyed metal fuel can be helpful in designing a fuel element that is serviceable to burnup ≥20% h.a.Main Results of Foreign Research. The most attractive type of fuel for fast reactors has always been metal fuel because of the simplicity of its fabrication using readily available metallurgical techniques, high density of the fissile isotopes, high thermal conductivity and compatibility with sodium coolant. Studies of metal fuel as part of design work on the IFR integrated fast reactor in the USA included a study of the physical properties and radiation characteristic of uraniumbased alloys and uranium with plutonium, choice of the optimal composition of the fuel and fuel-element design, serviceability validation with service-life tests and study of the behavior under transitional and anomalous conditions. More than 78000 experimental fuel elements with U-5Fs fuel [Fs -fissium (equilibrium mixture of the fission products in nuclear fuel, which can secure high radiation resistance of uranium-based alloys and uranium-plutonium)], U-10Zr and U-Pu-Zr were irradiated in EBRII and more than 1000 fuel elements with U-10Zr and U-Pu-Zr in FFTF. Metal fuel is compatible with liquid sodium, which made it possible to use sodium as a heat-conducting contact layer between the fuel and cladding. The results of the studies, specifically, the characteristics of metal fuel according to foreign publications at the end of the1980s, and analysis of its advantages are reviewed in [1]. The main factors affecting the serviceability of fuel elements with metal fuel, which are also important for making design decisions for fuel elements and fast-reactor cores, are radiation swelling and the mechanical and chemical fuel-cladding interaction.At the time the first reactor experiments were planned, there were no data on the behavior of metal fuel under irradiation, so that the particulars of the radiation swelling of such fuel were not incorporated in the design of the Mark-I, IA fuel elements. The fuel elements had no gas collectors, and in the experiments the effective fuel density (U-5Fs) varied from 85 to 100% of the theoretical value. It turned out that metal fuel is subject to considerable swelling at the initial burnup stage, so that in the first experiments the fuel element cladding deformed and failed at burnup less than 3% h.a. In subsequent experiments, the effective density of the fuel was decreased to 75% of the theoretical value by increasing the fuel-cladding gap width, the gas-collector volume was increased with respect to the active part of the fuel element 1.4:1, and structural components impeding free elongation of a fuel kernel were eliminated.