Reactor vessel fluence evaluation and dosimetry

The methodology currently in use for the estimation of the fast neutron fluence to the pressure vessel (inside surface and reactor cavity) is based on discrete ordinates two-dimensional codes such as DOT or its updated version DORT. This methodology assumes a P3 scattering, an S8 quadrature approximation, and cross sections based on the ENDF/B-IV file. Associated one-dimensional codes are often used for the cross-section collapsing portion of the calculation. The neutron spectrum at the pressure vessel location of interest is estimated assuming a 235U, 239Pu, or 241Pu source spectrum or an appropriate combination thereof. The two-dimensional codes and associated methodologies were benchmarked in the early eighties using the results of the PCA and PSF Oak Ridge National Laboratory reactor experiments. The benchmarking experiments were estimated to provide an uncertainty of ∼10%. The results of the calculations applied to a reactor were estimated to have an uncertainty of ∼20%. This level of uncertainty was assumed in the estimation of the margin term defined in 10CFR50.61