In order to investigate the failure behavior of fuel cladding under a reactivity-initiated accident (RIA) condition, biaxial stress tests on unirradiated Zircaloy-4 cladding tube with an outer surface pre-crack were carried out under room temperature conditions by using an improved Expansion-Due-to-Compression (improved-EDC) test method which was developed by Japan Atomic Energy Agency. The specimens with an outer surface pre-crack were prepared by using Rolling-After-Grooving (RAG) method. In each test, a constant longitudinal tensile load of 0, 5.0 or 10.0 kN was applied along the axial direction of specimen, respectively. All specimens failed during the tests, and the morphology at the failure opening of the specimens was similar to that observed in the result of post-irradiation examinations of high burnup fuel which failed during a pulse irradiation experiment. The longitudinal strain (ε tz ) at failure clearly increased with increasing longitudinal tensile loads and the circumferential strain (ε tϑ ) at failure significantly decreased in the case of 5.0 and 10.0 kN tests, compared with the case of 0 kN tests. From these tests, the data of cladding failure were obtained in the range of strain ratio (ε tz /ε tϑ ) between about −0.6 and 0.7: this range of strain ratio covers the range between about 0.0 and 0.7 which is estimated in the case of RIAsimulated test. It is considered that the data obtained in this study can be used as a fundamental basis for quantifying the failure criteria of fuel cladding under a biaxial stress state.
A continuum damage mechanics model using FEM calculations was proposed to be applied to an analysis of the fuel failure due to pellet cladding mechanical interaction (PCMI) under reactivity-initiated accident (RIA) conditions. The model expressed ductile fracture via two processes: damage nucleation related to void nucleation and damage evolution related to void growth and linkage. The boundary conditions for the simulations were input from the fuel performance codes FEMAXI-7 and RANNS. The simulation made reasonable predictions for the cladding hoop strain at failure and reproduced the typical fracture behavior of the fuel cladding under the PCMI loading, characterized by a ductile shear zone in the inner region of the cladding wall. It was shown that occurrence of a through-wall crack is determined at an early stage of crack propagation, and the rest of the through-wall penetration process is achieved with a negligible increment in strain. The effect of a local temperature rise in the cladding inner region on the failure strain was found to be less than 5% for the conditions investigated. Failure strains predicted under a plane strain loading were smaller by 20-30% than those predicted under equi-biaxial tensions between the hoop and the axial directions.
Formation of subdivided grains and coarsened bubbles in uranium dioxides under high burn up conditions in nuclear power plants deteriorate the performance of nuclear fuels. To clarify its mechanism, heavy ion irradiations in cerium dioxide followed by Raman spectroscopy, X ray diffractometry and electron microscopy (SEM) were performed. At the early stage of irradiations, increase in the lattice constant and shift in the F 2g peak position were observed presumably indicating accumulation of oxygen vacancies. Further irradiation enhanced recombination or clustering of vacancies depending on the irradiation temperature. The behaviors affect the surface morphology of the sample as well. Especially above 1000K, characteristic changes in Raman spectra and surface features were detected, presumably attributable to remarkable diffusion of vacancies. The relation between displacement of F 2g position and electronic energy deposition of incident ions was observed, suggesting the role of electronic excitations on the formation of oxygen vacancies in cerium dioxide.
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