Walid Mohamed

The Effect of Thermal Cycling on Mechanical Behavior of U-10Mo Monolithic Mini-Plate

Mini fuel plates used in Advanced Test Reactor (ATR) undergo five steps during the reator operation: startup, ATR Cycle 146A, transition from Cycle 146A to 146B, ATR Cycle 146B, and shutdown. Although the overall irradiation behavior of U-10Mo fuel is expected to be similar to the reactors operating at comparable power levels, there is a concern regarding how variation in operation schedules (that is, how many start-ups and shut-downs could be inserted during the Cycle A and Cycle B) may affect the mechanical behavior of the fuel plate during operation. To investigate any potential effect of number of start-stop activities, we simulated the thermo-mechanical behavior of L1P756 mini-plate, one of fuel plates inserted in ATR under RERTR-12 irradiation conditions with various numbers of start-stop activities artificially added at different points of time in irradiation cycle.This paper reviews four cases by varying the number of thermal cycles. Finite Element (FE) analyses were performed on the cases to investigate the effect of thermal cycling on the mechanical performance of L1P756 mini-plate. As a result, we observed that U-10Mo is in low stress level during the irradiation Cycle A and B due to creep behavior and that the maximum stress of aluminum cladding increases as the irradiation Cycle A and Cycle B proceeds. However, the number of thermal cycles did not affect the maximum stresses of U-10Mo, liner, and aluminum cladding.Copyright

Impact of Mechanical Constraints on the Irradiation Performance of U10Mo Monolithic Mini-Plates

For the conversion of high performance research reactors to low enrichment Uranium fuel, U-Mo alloy based fuels in monolithic form were proposed. These plate-type fuels consist of a high uranium density, low enrichment uranium (LEU) foil contained within a diffusion barrier, and encapsulated within a cladding. To benchmark this new design, effects of various geometrical and operational variables on irradiation performance have been evaluated. In this work, the effects of mechanical constraints on the thermo-mechanical behavior of a plate were studied. To evaluate these effects, a selected plate from RERTR-12 experiments (Plate L1P756) was simulated. Four distinct cases which represent four distinct welding conditions were considered. Evaluation of the stress-strain fields in the fuel elements revealed that mechanical constraints may impact the plate’s performance. These constraints include (a) inlet side, (b) outlet side, (c) both inlet and outlet sides; and finally, (d) entire long edges. Results of these cases were then compared with the ideal case. The peak stress-strain magnitudes, displacement, stress and strain profiles on the fuel elements are evaluated to make a comparative assessment. The results indicated that the cases with constraints on “inlet side only” and “outlet side only” yielded lower cladding strains compared with other cases. The difference on the displacement profiles on the fuel foil was not significant. Peak stresses on the foil did not change considerably. These results imply that the mechanical constraints effects peak cladding strains, while it does not cause significant effects on the fuel behavior.Copyright

Influence of Materials Properties on the Irradiation Behavior of U-10Mo Monolithic Mini-Plates

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.Copyright