Neutron embrittlement of the reactor vessel in Borssele as determined from Charpy specimens

Two sets of Charpy specimens have been retrieved from the reactor in the nuclear power plant at Borssele after two and four cycles of operation, respectively. The neutron fluxes at the sample positions and at the vessel wall have been calculated with a point-kernel method and S2 calculations. The calculated fluxes at the two specimen positions are in fair agreement with fluences measured by threshold detectors. The Reference Temperature of Nil Ductility has been determined from the Charpy tests by a tan-h fit procedure. An extrapolation to a 40-year vessel life has been made on the basis of a square-root dependence of the change in the reference temperature with effective full-power years. Under these assumptions the heat-affected zone material will reach 296 K. The other materials will remain below 280 K. The vessel life therefore is not limited by embrittlement. (orig.)