Understanding dissimilar metal welds

The reactor pressure vessel containing the reactor core and coolant is typically made of a different type of steel than some of the connecting pipes. Obviously, the performance of these dissimilar metal welds is of paramount importance to the safety of nuclear power plants. A lack of standardised tests for these welds combined with their critical importance makes extremely conservative decisions necessary regarding their structural condition.

In response to the need for standardisation of integrity and leak-before-break (LBB) assessments, scientists initiated the EU-funded project 'Structural performance of multi-metal component' (MULTIMETAL).

Scientists collected field data and experience from typical dissimilar metal welds in western European and eastern European light water reactors. They characterised their physical and metallurgical properties and behaviours, leading to the design of two dissimilar metal weld test configurations. A third weld test configuration is being prepared to benchmark fracture resistance.

The intact mock-ups have been carefully characterised using microstructural studies, hardness measurements and tensile tests, and various fracture mock-ups are being prepared for characterisation as well. In parallel with the comprehensive materials testing programme, the team is developing numerical analyses using innovative micromechanical modelling approaches for ductile failure processes. The combination of experimental data collection and model development will enable scientists to develop standardised procedures.

MULTIMETAL's ultimate goal is to deliver recommendations for measuring the fracture toughness of dissimilar metal welds and for assessing LBB behaviour. Through provision of the technical basis for harmonised European standards for multi-metal components, MULTIMETAL will fill a gaping hole and make a major contribution to the future of nuclear energy in the EU.