Observations of a steady state effect limiting radiation damage in reactor vessel steels

Reactor vessel material surveillance capsules which contain specimens of actual material used in the construction of a vessel are contained in nearly all operating reactors. These specimens monitor the changes in properties of the reactor vessel and assure that predicted changes based on trend curves which are used to set operating limits for the plant are conservative. Recently, data has been obtained from the Point Beach Unit No. 1 and Connecticut Yankee reactor vessel surveillance capsules exposed to neutron radiation for much longer periods of time, than those irradiated in test reactors and surveillance capsules which were removed at the first refueling and other early refueling outages. The data from these long time surveillance capsule exposures when compared to data from capsules removed from the same reactors earlier in life indicated that a limiting or steady state condition has resulted rather than a continuous embrittlement as predicted by trend curves. It is believed that the limited embrittlement or steady state condition which occurred from the surveillance capsule tests is due to a combination of relatively low neutron flux compared to that existing in test reactors which were the primary source of data used to establish trend curves and the longer exposure periods in the capsules that led to significant “annealing” during irradiation.