FINITE ELEMENT SIMULATIONS OF MONOLITHIC PLATES FOR THE CONVERSION OF HIGH PERFORMANCE RESEARCH AND TEST REACTORS: NBSR, MITR, MURR AND AFIP

摘要

This article presents evaluation of the stress-strain characteristics of U10Mo alloy based monolithic fuel plates for the conversion of high power research and test reactors. Monolithic plate-type fuel is a new fuel form being developed to achieve higher uranium densities within the reactor core to allow the use of low-enriched uranium fuel in high performance reactors. For this work, irradiation behavior of four different reactor plates (NBSR, MURR, MITR and AFIP plates) with different foil and cladding geometries were benchmarked against each other. For each plate, three distinct cases were considered: (1) fabrication induced residual stresses (2) thermal cycling of fabricated plates and finally (3) mechanical behavior under proposed irradiation conditions. Given that the temperatures approach the melting point of the cladding during the fabrication and thermal cycling, high temperature material properties were incorporated to improve accuracy. Residual stress fields due to the fabrication process (Hot Isostatic Pressing) were computed first. Solutions of fabrication simulations were used as initial states for the irradiation and thermal cycling simulations. For the thermal cycling simulation, an elasto-plastic material model with thermal creep was used. The transient irradiation behavior was formulated by a fully coupled thermal-structural interaction. Temperature fields on the plates were used to compute the thermal stresses. Volumetric swelling and irradiation creep of the foil were considered. The irradiation analysis showed that the stresses evolve rapidly in the reactor. It was found that the stress field of the fuel elements is dependent on the plate geometry, especially the foil thickness. Furthermore, the foil-cladding thickness ratio is the determining factor for the mechanical behavior. The compressive stresses of the foil are reduced with an increasing foil-cladding thickness ratio. The cladding deformation becomes severe for the plates with thicker foils.