It is sometimes assumed that thermalized neutron spectra can produce more effectively surviving point defects since gamma-recoil events do not pro­duce cascades and therefore there is less in-cascade annihilation. Thus, a larger fraction of thermally pro­duced defects are postulated to survive to contribute to creep or embrittlement.180,181

Foster and coworkers have published three papers over the past several decades where it appeared that irradiation creep indeed occurred at a higher rate in thermal reactors than in fast reactors.182-184 In the last of these papers it was noted that, as proposed by Garner 34 the previously unsuspected 59Ni con­tributions to dpa might account for the apparent but possibly misleading increase in creep rate. The T/F ratio in the experimental test reactors cited by Foster was rather high compared to that in PWRs.

An additional reason for such enhancement of creep probably lies in the large amounts of trans­muted helium and stored hydrogen in thermalized spectra that results from the 59Ni sequence and the stored hydrogen concept, producing bubbles and voids that accelerate the creep rate. Therefore, it does not appear necessary to invoke an enhanced
survivability or displacement effectiveness of gamma recoil events to explain the apparently higher creep rates in thermal reactors.