Prediction of neutron radiation effects to pressure vessel steels has long been a part of the design and operation of light water reactor power plants. Both the federal regulatory agencies (see 2.2) and national standards groups (see 2.1) have promulgated regulations and standards to ensure safe operation of these vessels. Recently, it has become apparent that the support structures for pressurized water reactor vessels may also be subject to similar neutron radiation effects (1, 2, 3, 4, 5).⁷ The objective of this practice is to provide guidelines for determining the neutron radiation exposures experienced by individual vessel supports.

It is known that high energy photons can also produce displacement damage effects that may be similar to those produced by neutrons. These effects are known to be much less at the belt line of a light water reactor pressure vessel than those induced by neutrons. The same has not been proven for all locations within vessel support structures. Therefore, it may be prudent to apply coupled neutron-photon transport methods and photon induced displacement cross sections to determine whether gamma-induced dpa exceeds the screening level of 3.0 × 10^-4, used in this practice for neutron exposures. See 1.2.