Mechanical property and neutron spectral analyses of the big rock point reactor pressure vessel

Abstract

The mechanical property changes in Charpy-V and tensile speciments have been determined as a result of irradiation at 585°F in accelerated, vessel wall and thermal control exposure locations of the Big Rock Point Reactor. The weld metal displayed a higher rate of increase than either the base or heat affected zone material; nevertheless, because of its −70°F initial transition temperature, the maximum final transition temperature that could be projected for the weld metal was only 180°F, and this included 15°F derived from measurements of thermal aging effects. Both the yield and tensile strengths increased as a result of the irradiation, but did not approach coincidence; the retention of tensile ductility thus suggested was reinforced by the reduction in area values, none of which fell below 44%. The 0.2% yield strength increase measured from a three year exposure was significantly higher than the trend exhibited by shorter irradiations at 16–24 times higher flux rates. This suggests an enhanced effect of lower flux rates over long time peroids; this salient effect on the yield strength was not, however also displayed by the Charpy-V results.Theoretical neutron spectra for the Big Rock Point surveillance locations showed the neutron energy distributions to be significantly different from a fission spectrum. The calculation permitted adjustments in the flux values to reflect these differences and provided the means to extrapolate a fluence for the 40 year full power service limit of the reactor. This was projected to be 8.1 × 1019n/cm2 > 0.5 MeV. The service-limit fluence was also projected by establishing a ratio of a specific fluence with its corresponding MWH and the MWH in 40 years at full power. The fluence thus derived was also 8.1 × 1019n/cm2 > 0.5 MeV. The favorable pressure vessel analysis suggested by these data should be valid if the power level of the reactor is not substantially increased and if the neutron spectrum conditions are not detrimentally altered.