An assessment of potential gamma ray enhancement of embrittlement in ABWR pressure vessel walls

It has recently been proposed that the unequal attenuation of γ-rays and fast neutrons in the water gaps of boiling water reactors may lead to a significant underprediction of the atomic displacement rate at the pressure vessel wall. Since a larger water gap has been incorporated in the design of the Advanced Boiling Water Reactor (ABWR), it was suggested that atomic displacements by γ-rays are 100 times more survivable and therefore more damaging than those created by fast neutrons. An independent assessment of the relative displacements caused by γ-rays and neutrons in the ABWR pressure vessel wall has been performed in this study. Based on recent works of others it does not appear that the use of 100-fold enhancement factors is justified for this application. When the full details of the neutron and gamma transport are addressed, it is found that no significant γ-enhancement of damage will occur in the ABWR pressure vessel wall. While the relative amount of damage caused by neutrons and gammas changes with increasing water gap, the overall advantage of low total displacement damage is not significantly changed by the gamma-induced component