IN SITU CHARACTERISATION OF INTERNAL STRESSES IN A CLAD PRESSURE VESSEL STEEL DURING POST-WELD HEAT TREATMENT

Nuclear reactor pressure vessels are safety critical components of pressurised light water reactors. They enclose the nuclear fuel rods, that contain the nuclear fuel (uranium oxide). To resist corrosion and chemical attack from the water of the primary circuit of the reactor, that is the fluid that removes heat from the uranium, the pressure vessel is clad in stainless steel to protect the underlying low alloy steel, that is not corrosion resistant. The stainless steel is welded on the low alloy steel. Cracks may appear, after the welding process, in the low alloy steel close to the cladding. Residual stresses be created because, when heated, the stainless steel tends to dilate more than the low alloy steel. The experiment will help to measure the residual stress after the welding process and to understand if those stresses can contribute to the underclad cracking phenomenon.