OECD/NEA Burnup Credit Criticality Benchmark, Analysis of Phase II-B Results: Conceptual PWR Spent Fuel Transportation Cask

The OECD/NEA “Burn-up Credit Criticality Benchmark” working group has studied the effect of axial burn-up profile on the criticality of a realistic PWR spent fuel transport cask (Phase II-B). The final results of this benchmark are presented and analysed in this report. Nine basic cases and two additional accident configurations were considered with the following varying parameters: burn-up (0 GWd/t for fresh fuel, 30 and 50 GWd/t), fuel composition (actinides only and actinides with fifteen fission products), axial burn-up discretisation (1 or 9 zones). In all, fourteen participants from seven different countries submitted partial or complete results (multiplication factors, fission reaction rates). Good agreement was found between participants for calculated keff. The dispersion of results, characterised by 2 sr (where sr is the ratio between the standard deviation and the average value) ranged from 0.5% to 1.1% for irradiated fuels and was equal to 1.3% for fresh fuel. The reactivity effect of axial burn-up profile for basic cases was similar to that obtained in Phase II-A: less than 1000 pcm for cases with burn-up less than or equal to 30 GWd/t or for cases without fission products and about -4000 pcm for 50 GWd/t burn-up and composition including fission products. However, two accident cases highlighted that the reactivity effect of axial burn-up discretisation depends on the configuration studied. For the accident conditions defined for this benchmark, the axially averaged flat distribution was found to be a non-conservative approximation even for low burn-ups (10 GWd/t) and without fission products; the reactivity effect of burn-up profile reached -14000 pcm for 50 GWd/t burn-up and composition including fission products. The calculation of fission fractions and densities was also investigated. The analysis identified problems of source convergence when the axial burn-up profile is modelled.