Dissimilar metal welds (DMW) are used in light water reactor power plants to join ferritic and austenitic steel piping components. High residual stresses remaining in this type of welded joint can significantly increase its susceptibility to stress corrosion cracking (SCC) under water reactor chemistry conditions. This degradation mechanism has compromised the integrity of many nuclear power plants throughout the world over the past 10 years. The Open University (OU) is undertaking a programme of research aimed at improving the reliability of residual stress measurements in DMWs using neutron diffraction. AREVA, the French nuclear power plant constructor, has developed an improved narrow gap DMW weld using a nickel-based corrosion resistant filler (alloy-52) to eliminate the risk of SCC in next generation nuclear plant. Through-wall neutron measurements have been carried out on a full-size DMW mock-up (352 mm OD, 40 mm thick) using the ENGIN-X instrument at the ISIS Facility in the UK. The results have shown that the largest tensile stress components in the welded component lie in the hoop direction, and have values of around 250 MPa and 225 MPa in the austenitic and alloy-52 materials respectively. These measured stresses were in reasonable agreement with those obtained from deep-hole drilling and numerical simulations. A notable finding in the experimental work was a wide scatter in the measured unstressed lattice parameters within a range equivalent to micro-strains of ≈ 430, 400 and 600 for austenitic stainless steel, ferritic steel and alloy-52 respectively. This scatter was reduced by two orders of magnitude through making additional measurements whilst slowly rotating the stress-free reference cubes. The OU is undertaking systematic studies to identify the origins of the observed measurement scatter in order to improve the reliability of measurements
