Recent studies associated with light water reactors (LWR) in both the USA and Russia have raised the question of void swelling in austenitic components of core internals. One question of particular interest is the range of temperatures over which voids can develop, especially the lowest temperature. To address this question a flow restrictor component manufactured from annealed X18H9T was removed from the reflector region of the BN-350 fast reactor, located in Kazakhstan. During operation this component spanned temperatures and dpa rates of direct interest to pressurized water reactors (PWRs) in the West and VVERs in Russia. This steel is analogous to AISI 321 and is used in Russian reactors for applications where AISI 304 would be used in the West and in Japan.
This component was sectioned on a very fine scale to determine in what range of conditions voids existed. Microstructural data were obtained for 157 separate locations, with 111 specimens showing voids over the relevant range of temperatures and displacement rates, allowing construction of a parametric map of swelling with temperature, dpa and dpa rate. These data show that swelling at doses as high as ∼50 dpa persists down to ∼306°C for dose rates in the range 0.11 × 10-7 to 1.6 × 10-7dpa/sec. Since the helium generation rate is rather low in the spectral environment of the flow restrictor, the early onset of swelling is attributed primarily to the lower displacement rate, a conclusion supported by a number of other experimental studies.