Reduced-pressure electron beam (RPEB) welding is an advanced welding technique being considered for Gen III and Gen IV nuclear piping and pressure vessel applications. RPEB welding allows for high welding throughput, with weldments further benefiting from minimal distortion, both of which are key considerations for future reactors. While residual stress measurements have been made on RPEB welded SA508 steel during the NNUMAN programme, it is important that we characterise the evolution of these stresses, in-situ, during the post weld heat treatment (PWHT). These measurements will inform our time-dependent heat treatment model and, crucially, allow us to optimise the PWHT, which does not fully relieve weld residual stresses. This is a particular concern in safety-critical nuclear components, which experience a shift in ductile-to-brittle transition temperature during long term operation.