In situ TEM Ion Irradiation and Atmospheric Heating of Cladding Materials
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In situTEM Ion Irradiation and Atmospheric Heating of Cladding Materials K. Hattar1, S. Rajasekhara1, B.G. Clark1 Sandia National Laboratories, Physical, Chemical, & Nano Sciences Center, PO Box 5800 Albuquerque, NM 87185, U.S.A. 1
ABSTRACT Over the course of use, both in-service and during storage, fuel claddings for nuclear reactors undergo complex changes that can drastically change their material properties. Exposures to irradiation, temperature changes, and stresses, as well as contact with coolant, storage pool, and dry storage environments, may induce microstructural changes, such as formation of radiation defects, precipitate dissolution, and chemical segregation, that can ultimately result in failure of the cladding if pushed beyond its limit. In order to predict the performance of cladding in-service and during storage, understanding of the dominant processes related to these changes and their consequences is essential. In situ transmission electron microscopy (TEM) allows dynamic observation, at the nanoscale, of microstructural changes under a range of stimuli, making it an excellent tool for deepening our understanding of microstructural evolution in claddings. This proceeding presents details of the new in situ ion irradiation TEM and in situ gas cell TEM capabilities developed at Sandia National Laboratories. In addition, it will present the initial results from both systems investigating radiation tolerance of potential Generation IV cladding materials and understanding degradation mechanisms in Zrbased claddings of importance for dry storage. INTRODUCTION In order to identify suitable materials for next generation claddings and to certify performance of current cladding materials in-service and during subsequent storage, microstructural changes within the cladding and their impact on material properties must be well understood. In the case of Generation IV claddings, new reactor designs are directed at consuming harmful fission gasses for ease of reprocessing and lowering the safety risk in storage. However, to achieve this, fuel assemblies must endure much higher temperatures and irradiation damage than currently required in Generation II reactors. This has led to a push to evaluate new candidate materials for cladding, in particular searching for materials that can withstand greater than 100 dpa (displacements per atom), while still maintaining structural integrity. Another material degradation concern arises as Generation II fuel assemblies reach the end of their in-service specifications and are transferred to storage pools and subsequently dry storage casks. During this time, the cladding provides a barrier to the release of radioactive fuel material and fission gasses. To ensure that fuel assemblies are safe to be moved or transported in the future, the cladding must maintain its strength and ductility over potentially hundreds of years. Currently, fundamental details of microstructural changes under these extreme environments in both Generation IV cladding material candidates and Generation
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