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Influence of thermal neutrons on embrittlement and hardening in weld metals

The influence of thermal neutrons on embrittlement of a number of submerged arc weld metal steels has been studied in a series of experiments undertaken in a reactor which provided a highly thermalized flux spectrum with a ratio of thermal displacements per atom (dpa) (neutron energies <1 keV) to fast dpa (neutron energies >1 keV) of about two at an irradiation temperature of 240 °C. A statistical analysis set in a Bayes framework is presented for Charpy impact energy test data measured on the submerged arc weld metal specimens subjected to accelerated neutron irradiation for two different exposure periods. Prior to the accelerated irradiation, the specimens were subjected to neutron irradiation under surveillance conditions. The analysis presents models for changes of Charpy impact energy properties measured over a range of test temperatures extending from the lower to upper shelf energy levels, as a function of neutron irradiation dose. Markov chain Monte Carlo sampling was applied to obtain estimates of the posterior probability distributions of the model parameters. These parameters were then used to compute the ductile to brittle transition temperature shifts at the 40 J energy level in the Charpy impact energy against temperature curves. Using the results for the ductile to brittle transition temperature shifts, the effectiveness of the thermal neutrons can be estimated for submerged arc weld metal used in the construction of operating Magnox reactor steel pressure vessels.