In recent years concerns have arisen regarding low temperature irradiation damage processes following the observation of an apparent tenfold increase in embrittlement in the HFIR reactor surveillance programme at Oak Ridge compared with accelerated material irradiation's in MTR's. The possible importance of this phenomenon to the integrity of LWR Support Structures was recognised by USNRC and it was formally designated Generic Safety Issue 15. This topic has also become an important UK regulatory issue in the context of the Sizewell PWR. Here the concern was that the high copper material used in the RPV Supports, when taken in light of the HFIR results of that time, highlighted the possibility that irradiation induced copper precipitation could occur at the low temperatures (50–600°C) and peak end-of-life dose of 2mdpa, ~1×1018 n.cm−2 (E>1MeV), encountered in RPV Supports, which contain significant levels (>0.20%) of copper.
This paper describes the results of an experiment set up to establish whether copper precipitation could occur at low temperatures in the Sizewell “B” RPV Support Structure steel. The irradiation was set up to achieve a target dose consistent with the full precipitation of copper, based on modelling data from conventional tests, at the lowest possible damage rate compatible with an irradiation of less than 1 year. The irradiation achieved a high dose of 49.6mdpa, 3.74×1019 n.cm−2 (E>1MeV), at an average temperature of 71°C.
Mechanical properties measured by conventional hardness and Charpy test were performed. The data interpretation was aided by a combination of micro structural and post irradiation annealing experiments. In addition to the Support Structure material, two series of reference alloys with varying copper and nickel content were also irradiated. These alloys have been used in other programmes and offer the ability to cross check the effect of irradiation temperature.
The series of model steels previously studied after irradiation at 288°C in the IAEA CRP-3, under which conditions they exhibited a dependence of hardening on copper and nickel content, showed no clear evidence of copper precipitation under the low temperature irradiation conditions employed. Post irradiation annealing studies demonstrated that the irradiation embrittlement in the A588 support structure steel at low temperatures is due to matrix damage. Although the exact nature of the matrix defect/defects responsible for the embrittlement was not identified, the copper content is believed to be unimportant. There is, therefore, no reason to believe that the relatively high copper specification of A588 support structure steel will be detrimental to the steels toughness after low temperature/high dose irradiation as expected in the Sizewell B support structure.