The DOE/NNSA Conversion [1] Program in the US aims to minimize the use of high enrichment uranium in civilian applications. This initiative is being approached by converting research and test reactors from the use of highly enriched uranium (HEU) to low enrichment uranium (LEU, <20% 235U) with high density of uranium to achieve stable operation of converted reactors. Among variety of fuel materials investigated to serve in the conversion process, U-Mo based alloys have shown stable and acceptable swelling response under typical operation conditions of research and test reactors.
For the conversion of high performance research reactors, a large number of irradiation experiments were conducted to evaluate the mechanical behavior of the U-10Mo monolithic mini-plate; however, it is difficult to investigate all design and operation variables with potential impact on the irradiation behavior of the fuel experimentally.
Thus, this study performed Finite Element Analyses (FEA) on a 3-D monolithic plate by changing material properties of components. The material properties considered in this study included thermal, mechanical, and irradiation specific properties of the fuel, cladding, and liner. Among FEA results, higher Young’s modulus of cladding material caused a significant decrease in all stress values in the three sections of the monolithic mini-plate. On the other hand, variation in the Young’s modulus of Zr-liner showed the minimal effect on the overall mechanical response of the monolithic mini-plate. Results showed that increasing the yield stress of the cladding material directly caused a increase in the maximum stress observed in the cladding section by almost 40 %. Considering the thermal properties of materials in the monolithic plate, maximum and minimum stress in fuel foil were found to either increase or decrease in proportional with the coefficient of thermal expansion of the fuel material. However, variation in the coefficient of thermal expansion in the cladding section caused a remarkable increase in peak stresses in the fuel foil.
While mechanical and thermal properties of the foil, liner, and cladding sections are known, other irradiation-dependent properties such as coefficient of irradiation creep of U-10Mo are not firmly determined to date. The mechanical response of L1P756 is being simulated with different values of the coefficient of irradiation creep and the observed “bulging” in the plate will be compared to available post-irradiation measurements. Thus, it will be possible to determine an accurate value of irradiation creep coefficient of U-10Mo which in turn would allow predicting its mechanical behavior under different irradiation conditions.
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