Recent comprehensive dosimetry experiments revealed the presence of an intense gamma field at the High Flux Isotope Reactor (HFIR) surveillance locations near the pressure vessel. Measured responses of fission dosimeters 238U and 237Np, as well as 9Be helium accumulation fluence monitors were placed in accordance with responses of threshold and thermal neutron activation dosimeters only when photofission and photo-neutron reactions, respectively, were taken into account. Gamma-flux (E > 1MeV)-to-neutron-flux (E > 1MeV) ratios as high as 104 were observed.
It has been generally accepted that neutron interactions with materials are the main cause of changes in mechanical properties of ferritic steels in nuclear reactor environments. However, gamma rays can also create displaced atoms that can in turn contribute to material embrittlement. Gamma-induced displacements may be quite negligible when compared with neutron-induced displacements in most irradiations, and need not be taken into account. However, at the HFIR surveillance position the gamma-induced atom displacement rate dominates or at least equals the neutron-induced atom displacement rate. When the sum of neutron- and gamma-induced displacements per atom is used as an irradiation exposure parameter the HFIR surveillance results are in agreement with other test reactor results. The “accelerated embrittlement” phenomenon, therefore, appears to be caused by the previously unaccounted atomic displacements induced by gamma rays. However, independent verification that gamma-induced displacements can cause material embrittlement is necessary.