Evaluation of the Embrittlement of Nuclear Claddings Following Multiple Transportations

The integrity of spent-nuclear-fuel (SNF) rod claddings must be ensured throughout all phases of the reprocessing or final disposal processes: handling, storage and transportation. In this regard, the potential degradation mechanisms specific to each step must be investigated, taking into account the cladding properties change due to in-reactor operation and subsequent thermo-mechanical transients. EDF is considering the building of a centralized interim SNF storage-facility as a buffer step between SNF cooling-down in fuel pools and fuel reprocessing at La Hague facility. Under this scenario, some SNF assemblies might be subjected up to three dry transportations. During drying and transportation, residual heat of the SNF causes cladding heat-up and generates a high hoop stress because of the internal rod pressure elevation. All or part of the zirconium hydrides, stemming from in-core corrosion and initially oriented along the circumferential direction, are dissolved and could then precipitate as radial hydrides upon cooling. This phenomenon is known to cause additional cladding embrittlement and can be an issue for fuel retrieval. The objective of this study was to evaluate the impact of several transportation-storage cycles on the cladding embrittlement by hydrides reorientation (HR). To address this issue, an experimental program was undertaken on unirradiated pre-hydrided (350 and 700 ppm) Zircaloy-4 cladding. One to three HR treatments were applied on cladding tubes (400 °C, 80-120 MPa under internal pressure) and subsequent ductility was assessed by performing ring tensile tests at room temperature. A decrease of ductility was observed after the first treatment but no additional embrittlement was noticed after two or three treatments. Metallographic examinations were carried out to highlight the hydrides distribution after HR treatments and to explain the mechanical tests results. The findings support the idea that the multiple transportations are not more detrimental than a single one when cladding embrittlement by HR is assessed.