Implications of accident tolerant fuels on thermal-hydraulic research

Abstract

Accident tolerant fuel deployment has been rapidly accelerating in the US with lead test rods and assemblies already inserted in commercial reactors. With the current deployment target of mid 2020s for near term concepts and long term concepts thereafter, the implication of these fuels on thermal-hydraulic (TH) research should be documented to support their development and value proposition. Without economic argument, once the US government ATF program ends in ~2021, the US utilities will not adopt the more expensive ATF in their already NRC licensed safe and robust LWRs. Moreover, only limited severe accident gain can be realized by adopting ATFs alone. Thus economic opportunities with ATF cladding lies within challenging the current conservative technical specifications and safety procedures and pairing with more cost-effective high density fuel forms. Opportunities exits to go higher that current Zircaloy peak clad temperature (PCT) of ~1200 °C and survive a departure from nuclear boiling (DNB) or dryout event for considerable longer time with coated clad or SiC fiber composite ATF concepts. Higher PCTs along with longer durations at high PCTs emphasizes more importance on radiation heat transfer and fuel collapse limits during severe accidents. The tolerance to DNB and dryout for longer periods motivates their replacement by a fuel performance based limit. This path enabled by ATFs requires accurate prediction of post-DNB or dryout heat transfer and rewet characteristics. TH research is also needed in exploring new failure modes especially for SiC cladding where more accurate local temperature, flow and quenching is required to assess its thermomechanical performance. At the same time, adoption of ATF cladding could bring about less emphasis on GTRF or CRUD deposition that is being tackled by the TH community.