HT-9 is a ferritic/martensitic steel alloy that is a candidate material for use in cladding in various advanced nuclear reactor designs including the Versatile Test Reactor. Although, there have been many successful experiments and studies surrounding the mechanical behavior of HT-9 under irradiation conditions with results from such studies being successfully incorporated into the BISON framework to model creep and simulate strain, there still exists significant inaccuracies to those models in scenarios of elevated temperatures (>600°C) and extended irradiation durations. Such inaccuracies are largely due to parameters from fresh HT-9 being used in the model which do not take into account the remarkable microstructural changes that occur under irradiation in the harsh in-core environment. Thus, it is the objective of this work to provide experimental data and measure such mechanical properties as elasticity from an HT-9 sample that has been previously irradiated in the Fast Flux Test Facility to 25 DPA at 635°C from which such experimental data can be incorporated into the BISON framework to more accurately model creep and simulate strain. Such work will be accomplished through the use of nanoindentation based experiments both in terms of constant strain rate loading as well as constant load creep experiments. Those experiments will be performed at multiple temperatures up to 500°C to obtain clear information of mechanical behavior at such elevated temperatures. The microstructure within the indented regions will be further characterized in TEM to further identify further phenomena contributing to mechanical behavior of heavily irradiated HT-9 cladding. It is expected that such work can be performed over the course of ten days. With such experimental measurements and knowledge, not only will detailed insight be gained into mechanical behavior, but the models can be updated to improve accuracy and provide a further, confident means of material qualification for advanced nuclear reactors.

The proposed work aims to contribute towards the continued development and refinement of the BISON framework to accurately and reliably model creep and simulate strain within materials that have excellent candidacy for use in advanced reactor designs. This objective will be accomplished by addressing knowledge gaps when it comes to mechanical behavior of HT-9 cladding that has been heavily irradiated at elevated temperatures. Incorporating many significant, successful experiments, the BISON framework has been able to accurately model creep and simulate strain within an impressive range of conditions. However, in scenarios of extreme burn up and elevated temperatures (>600°C) it has been found that such models contain significant inaccuracies largely due to gaps in experimental data leading to the use of parameters that are not completely representative of such systems. Thus, in order to gain detailed insight into the mechanical behavior of heavily irradiated HT-9 cladding, a sample previously irradiated in the Fast Flux Testing Facility to 25 DPA at 635°C has been selected for nanoindentation based experiments including creep experiments followed by detailed TEM characterization. Such will further the understanding of the mechanical behavior of HT-9 which is a strong candidate as a cladding material for use in advanced nuclear reactors as well as provide critical data towards the enhancement of the capabilities of the BISON framework to accurately model such mechanical behavior. Such modeling capabilities will be crucial in qualifying fuel systems for advanced reactors including those in the Versatile Test Reactor.