Management of Nuclear Plant Operation by Optimising Weld Repairs: ENPOWER Project Overview

ENPOWER is a EURATOM collaborative research project sponsored by the European Union. The aim is to produce advanced weld repair techniques and residual stress mitigation procedures for nuclear components in order to eliminate the need for expensive global Post-Weld Heat Treatment (PWHT). These procedures are based on weld repair optimisation combined with novel Alternative Post Weld Treatments (APWT). Repair procedures have been developed by carrying out parametric numerical studies for various repair and component configurations. APWTs are based on local heating at moderate temperatures (300–600°C) to produce local plasticity that redistributes the internal stresses and results in compression in the areas of interest. Two techniques have been identified as prime candidates: namely “hot compression” and “thermal shock” techniques. In combination with internal pressure the hot compression technique reveals itself to be very efficient and of most general use. The thermal shock method is effective for repairs in sections up to 25mm thick. The APWTs were established using numerical models applied to mock-up nuclear components, and validated by measurements. Numerical modelling methods have also been used to study the interactions between residual stresses, post weld treatments, operational loads, crack growth and fracture. Integrity assessment procedures using a modified J-integral definition are applied to confirm that optimised repair and APWT procedures can be developed to reduce welding residual stresses without any secondary detrimental effect, even with the presence of pre-existing cracks.