Photointerference Corrections in Neutron Dosimetry for Reactor Pressure Vessel Lifetime Studies

The importance of gamma-ray reactions [(γ,f), (γ,γ′), and (γ,n)] that can interfere with the detection of certain threshold neutron reactions [(n,f), (n,n′), and (n,2n)] used in reactor pressure vessel dosimetry was studied via a combined experimental and calculational program. First, an experiment-design calculation of such photocontamination was carried out in a pool-type reactor, indicating ∼0.1% photointerference at the reactor surface and ∼10 000% at 1-m penetration of water (∼1% neutron attenuation/mm). Next, a complete set of threshold activation foils was irradiated fore and aft of a “photofraction gauge,” a tungsten disk that attenuated the important 5- to 10-MeV gamma rays by a factor of ∼30 and the >0.5-MeV neutrons by a factor of ∼3. The photofraction gauge was calibrated for photofraction f_γ, by comparing the large fore to aft activation ratios [R(F/A)] for photocontamination foils with R(F/A) ≃ 3 for noncontamination foils [such as ⁵⁸Ni(n,p) and ²⁷Al(n,α)]. The values of f_γ were calculated and were found to agree reasonably well with those measured, except that the calculated values were a bit too high. The one-dimensional calculation needs to be replaced with an accurate three-dimensional calculation with measured power distribution before accurate (γ,f) and (γ,γ′) cross-section adjustments can be made for the activation foils and/or the gamma-ray production cross sections (from n,γ reactions near the reactor) properly modified. Some one-dimensional cylindrical calculations for pressurized and boiling water reactors are presented that predict up to 55% photocontamination at the pressure vessel wall when determined by the ²³²Th(n,f) reaction.