Modified Conversion Ratio Measurement in Light Water-Moderated UO₂ Lattices

To investigate the accuracy of the neutronic calculations in various neutron spectra, the modified conversion ratios [(MCR): ratio of ²³⁸U capture rate-to-total fission rate] of four kinds of light water-moderated UO₂ fuel lattices were measured. In the measurements, the relative reaction rates of ²³⁸U capture and total fission were obtained from the nondestructive gamma-ray spectrometry of ²³⁹Np and ¹⁴³ Ce, respectively, which accumulated in the fuel rod irradiated at the Tank-Type Critical Assembly. The moderator-to-fuel volume ratios V_m/V_f of the measured lattices were 1.50 (undermoderate) to 3.00 (overmoderate). The measured MCRs were 0.477 ± 0.014(V_m/V_f = 1.50), 0.434 ± 0.013(1.83), 0.383 ± 0.011(2.48), and 0.356 ± 0.011(3.00), respectively. The Monte Carlo calculation employing the JENDL-3 library showed good agreement with the experiments for all the cores, although they showed a tendency of overestimation for larger values of MCR, as shown in the case of UO₂ tight lattices. Therefore, it was concluded that, for the cores investigated, the accuracy of the neutronic calculation method currently used was very good.