Application of Niobium to Determine Fast Neutron Exposure for Reactor Pressure Vessel

Vanadium-encapsulated 238U and 237Np fission monitors are used in EVND sensor sets for fast neutron. These fission monitors fulfilled two roles. First, by counting the ¹³⁷Cs fission product (30.07 year half-life), the fission monitors serve as a long term fluence integrator. Second, ²³⁷Np is useful for measuring neutrons with energies from approximately 0.68 to 5.61 MeV, which is important for confirming the fast neutron response spectrum. However, these fission monitors would be constrained and limited on use because ²³⁸U is fissionable material and ²³⁷Np does not exist in nature. Niobium comprises one stable isotope (93Nb) in nature and when 93Nb get a neutron response, niobium undergo following reaction, ⁹³Nb (n, n') ⁹³mNb. The objective of this paper is to introduce neutron dosimeter measurement including 93Nb and neutron transport calculation to determine fast neutron exposure of the RPV. The fast neutron is defined as energy above 1.0 mega electric volt in irradiation embrittlement evaluation field. In order to provide both the fast neutron energy response and the long term integration, two fission monitors are used traditionally. However these fission monitor has several disadvantages which are availability, correction of fission yield, impurity, and so on. Niobium is in nature and has proper energy response range for fast neutron and half life of daughter nuclide. Through evaluation results with niobium for neutron flux spectrum, measurement to calculation ratio, final bias and uncertainty, niobium could be an alternative to fission monitors.