The objective of the Phase II-D Burn-up Credit Criticality Benchmark was to study the impact of control rod (CR) insertion on spent fuel composition and on reactivity for a PWR-UO2 assembly. For this purpose, a range of CR insertion profiles during irradiation were defined, and participants were asked to calculate the spent fuel inventory and the neutron multiplication factor for each case. To assist in the evaluation of the benchmark results, the sensitivity of the neutron multiplication factor to a variation of isotope concentration was performed.
The large effect of CR insertion (9 000 pcm when the CRs are inserted from 0 to 45 GWd/t) is due in part to the fact that the CRs are axially fully inserted in this benchmark. A more “typical” CR insertion profile would not consider CRs fully inserted throughout the irradiation, particularly over three cycles. An additional benchmark has been initiated to study the effect of CR insertion when considering partial axial CR insertion and an axial burn-up profile.