Submerged-arc welds in Magnox RPVs are expected to show substantial shifts in ductile-to-brittle transition temperature (DBTT), due to their high copper content, and also because of a contribution from intergranular fracture. For structural integrity arguments, the fracture toughness of irradiated welds is predicted by applying an irradiation shift in DBTT to a start-of-life toughness curve. The shift is obtained from a trend curve derived from Charpy impact data. An uncertainty allowance is obtained by combining uncertainty contributions in start-of-life fracture toughness and shifts, including a contribution from uncertainties in neutron dose.Through-thickness samples were removed from four submerged-arc welds in a decommissioned Magnox RPV at Trawsfynydd. Fracture toughness tests were made on pre-cracked Charpy geometry specimens made from the samples, in order to compare the measured toughnesses with those predicted for irradiated material. Specimens were tested from several positions along the welds and also at four different through-thickness locations with dpa doses varying by a factor of more than 2. The paper presents the results of nearly 400 toughness measurements and demonstrates that the prediction methodology is sound. (C) 2002 Elsevier Science Ltd. All rights reserved.

An important contribution to the life extension of Magnox nuclear power stations comes from the assessment of microstructural changes in reactor pressure vessel (RPV) weld metal after extended in-service exposure to a neutron Aux. A project was undertaken to remove through-thickness weld samples from an RPV at a power station undergoing decommissioning. Sampling was carried out in a region of the reactor where the RPV was exposed to a high fast neutron irradiation dose at a low operating temperature. Each sample was cut into several specimens for mechanical tests to establish fracture toughness, chemical analysis and dosimetry data.This paper describes some of the preliminary results obtained from a microstructural characterisation programme. Samples were cut in various positions along the weld and at different depths through the vessel wall. These samples were examined by a number of techniques to evaluate the macro and microstructure after different amounts of neutron irradiation. In-situ micro-analysis and fractography was carried out on specimens after mechanical testing. (C) 2000 Published by Elsevier Science Ltd.

The microstructures of Magnox submerged-arc welds have been characterised to investigate the effects of surveillance and accelerated irradiation at temperatures in the range 190–290°C. The radiation hardening and embrittlement is influenced by the precipitation of Cu from solid solution. Mn has been found in the Cu-rich precipitates, together with an indication of P. The precipitates have structure coherent with the ferrite matrix and maintain a constant mean diameter during extended irradiation. Evidence has been obtained indicating that dislocation loops contribute to a matrix damage component in these welds.