An embrittlement rate effect deduced from HFIR that may impact LWR vessel support life expectancy

Recent results from the High-Flux Isotope Reactor (HFIR) pressure-vessel-material surveillance program indicate that embrittlement rates of the several ferritic carbon steels are substantially greater than were anticipated. The apparent reason is that the fast neutron flux in the HFIR vessel is much less than that in materials testing reactors (factor of ≅ 10-4); that is, there is a fluence-rate effect. It was soon realized that these results may significantly impact the life expectancy for supports of some light-water reactor (LWR) vessels because the temperature and fast-neutron fluxes associated with supports are about the same as for the HFIR vessel. A preliminary evaluation of LWR supports was conducted by estimating the increases in the nil-ductility transition temperatures (NDTT) of supports in several LWR plants. An analysis based on the HFIR data predicts much higher ΔNDTTs than one based on materials-testing-reactor data. This result, combined with assumptions regarding loading conditions and flaws, provides a basis for concerns about support integrity. (orig.)