Cadmium Cutoff Energy for MCNP Modeling of Dosimetry Reactions

The MCNP code modeling of reactor cavity dosimetry reactions to validate the computed neutron flux hitting the pressure vessel of a pressurized water reactor (PWR) requires the choice of an appropriate cadmium cutoff energy. Since the neutron spectrum is harder in the cavity than in the core, the same cadmium cutoff energy may not be universally applied. The correct cutoff energy for the cavity of a PWR was computed by using MCNP4a and a representative cavity spectrum. Four cadmium-covered foil reactions were analyzed, and the appropriate energy cutoff was determined to be ∼0.6 eV for ∼0.5-mm-thick cadmium covers.