Steels aging under irradiation - Risk of nuclear reactor vessels breakage

The aim of this study is to take stock of the most recent and in-depth global research on the thermal ageing of steels under irradiation, based on the most advanced measurement methods (such as atomic probe tomography and scanning electron microscopies) and the most accurate numerical simulations from nanoscale up to the vessel scale. This work reveals and confirms an accelerated ageing of these steels when highly irradiated by neutron bombardment beyond a fluence of 6x10¹⁹ neutrons/cm², resulting in changes in the atomic and crystalline structures which accelerate their brittleness (increase in hardness, sharp drop in fracture toughness - i.e. shock resistance - and increase in the ductile-fragile transition temperature). They concern more particularly the 16MND5-type (A508-3-type equivalent) steels used in the nuclear reactors that have been built in France. The major and new result of this work is the following: the former ageing models systematically and significantly underestimate the increase of the ductile-fragile transition temperature at high fluences higher than 6x10¹⁹ neutron/cm² (the one already probably reached by the 900 MW nuclear vessels of Fessenheim and Tricastin) because they do not take sufficiently into account the contribution of the slow but massive 'late blooming MnNiSiP precipitates' which follows the rapid Cu precipitation in the bainitic matrix of the vessel steel induced by the atomic displacements due to the bombardment of neutrons at high fluences, creating brittle segregations at the joints of the internal grains of these steels, which lead to increased intergranular fracture mechanisms, particularly during cold stress shock. Important observations confirm this phenomenon, the main consequence of which lies in the fact that this increase in the ductile-fragile transition temperature ΔRT_NDT increases linearly with the neutron fluence at high irradiations above 6x10¹⁹ neutrons/cm² instead of the square root power law used in ageing prediction formulas based on Charpy V-notch impact tests. This means that the thermal ageing of steels under irradiation of nuclear vessels, covers, bolts, nozzles and internals accelerates rapidly beyond 40 years for all 900 MW reactors, which could deeply compromise their resistance to a possible pressurized thermal shock, their steel becoming suddenly brittle if abruptly cooled down below 60 to 80 deg. C (even 120 deg. C for defects of strong Cu-Mn-P impurities segregation called 'dark veins'). Consequently, the lifetime extension beyond 40 years of the 900 MW reactors presents serious safety problems, with a risk of a sudden vessel fracture under pressurized thermal shock, which would lead to loss of reactor control and major Fukushima-like accident. The dismantling of the oldest PWR nuclear power plant built in France in Fessenheim could be the opportunity for a major European experimental research project on the real thermal ageing under irradiation of nuclear reactor steels, in order to verify the results of worldwide research and to test the hypotheses of extending the operation lifetime of nuclear reactors beyond 40 years. (author)