Reactivity Effect of Iron Reflector in LWR Cores

Abstract

One of the ways of improving fuel cycle cost is to reduce neutron leakage from a core using a reflector. For this purpose, experiments were carried out to investigate the reactivity effect of an iron reflector in a light-water-moderated core using the critical assembly, TCA. The experiment showed that iron reflectors of 15 cm thickness made the core more reactive than water and that the increasing the thickness from 2.2 cm to 15 cm produced 1.8%Δk/k core reactivity gain. The experiment was analyzed with the two-dimensional transport code PHOENIX-P and the continuous energy Monte Carlo code MVP. From the analyses, it has been found that, if the ENDF/B-VI data for iron isotopes are used, the calculated reactivity effect gives good agreement with the experiment and that the epi-thermal and thermal capture reaction rate distributions measured with gold wires and the resultant spectral index distribution are also well reproduced. The experiment and calculations have revealed that increasing a baffle plate thickness in PWRs above 2.2 cm can increase the core reactivity and contribute to fuel cycle cost saving. Based on this result, a stainless steel radial reflector has been employed in the APWR, and it has been shown that a reduction of about 0.07 wt% 235U enrichment can be achieved with the use of this radial reflector. Lastly, through the experimental analyses, the reactivity effect of the iron reflector has been found to give a benchmark useful for the evaluation of iron nuclear data. We expect that the experimental data described in this paper will be used to verify new iron data.