On the decision to move towards a PWR programme in the UK, it was realised from previous experience that structural and cladding welds in the pressure vessels could be subject to various forms of embrittlement. The nozzle-ring and belt-line areas were seen to be particularly critical.
A joint working-group involving the utility, the construction consortium, government institutions and the statutory body was established to address this.
A significant task was to identify achievable residual-element limits for base metal and weld consumables, so as to reduce the susceptibility to stress-relief, temper and irradiation embrittlement to tolerable levels.
The work involved extensive mechanical testing of controlled-purity alloys and commercial steels from a range of manufacturers, welded to various procedures. Detailed metallography – including Auger analysis – was performed on the test-pieces.
Mechanistic models of the different degradation processes were developed, allowing quantification of the effects of each impurity element on each embrittling mechanism. Thence the kinetics of in-service embrittlement could be determined.
The work resulted in an agreed residual element specification for the selected steels, recommendations as to construction and fabrication routes, and a proposed monitoring programme using surveillance specimens.