Measurement of Neutron Fluence by Neptunium-237 and Uranium-238 Fission Dosimeters

Experimental irradiations of pressure vessel materials were conducted at the Saxton reactor and Babcock & Wilcox Test Reactor to provide radiation effects data for fast neutron fluences up to 1 × 10²⁰ n/cm² (E > 1 MeV). Included in the capsule assemblies were ²³⁷Np and ²³⁸U dosimeters and ⁵⁴Fe correlation monitors. The activities of the dosimeters were combined with the neutron spectrum calculations of a multigroup diffusion code to establish the fast-neutron (E > 1 MeV) fluences experienced by the dosimeters. For both reactors the fluences derived from the different dosimeters were in good agreement indicating both the adequaey of the spectral predictions and the successful application of the fission dosimeters. It is shown that the ²³⁷Np and ²³⁸ U dosimeters are responsive to a wider range of neutron energies than the more commonly used threshold detectors and are therefore better able to ensure the aptness of calculated neutron spectra. This effort has indicated that the use of ²³⁷Np and ²³⁸U dosimeters in power reactor vessel surveillance programs leads to more meaningful correlations between neutron fluence and induced radiation effects.