A study of burnup credit in criticality safety analysis for PBR spent fuel pebbles.

• The burnup credit calculations for PBR spent fuel pebbles were performed using MCNP6. • The impact on the k eff as a function of water density in the storage cask was investigated. • Three single effects and four compound effects were studied. • The mechanisms of all the single and compound effects were researched in depth. This study attempts to investigate the influence due to the use of burnup credit in the criticality safety analysis for pebble-bed reactor (PBR) spent fuel pebbles. Recently, the development of PBR is very quick in China, thus the storage of spent fuel pebbles will become a significant issue in the foreseeable future. For the safe storage, the criticality safety analysis is of vital important. Furthermore, the utilization of burnup credit in the criticality safety analysis is essential due to the fact that it can result in a more compact design of storage system. In this study, all the calculations were performed using MCNP6 associated with the continuous energy neutron data library ENDF/B-VII. In addition, two geometrical models which represent the HTR-10 core and a proposed storage cask were adopted. The proposed storage cask model was established based on the real storage cask for HTR-10 spent fuel pebbles, but the capacity was increased in order to evaluate the impact of burnup credit. Moreover, the most conservative condition for this proposed cask occurs when the cask is full of water with a water density around 0.35 g/cm3. For burnup credit calculations, three operating parameters related to HTR-10 were investigated, including the usage of control rod, the fuel temperature, and the volume fraction of fuel pebbles in total pebbles. Additionally, these operating parameters were combined and classified as three single effects and four compound effects. The single and compound effects were defined as the influence on the effective multiplication factor (k eff) due to simultaneous variations of one and multiple operating parameters, respectively. It is worth noting that in most of the compound effects, the reactivity deviation (or the change of k eff , Δ k) resulting from the compound effect was not a summation of Δ k 's resulting from the associated single effects. This phenomena may affect the precise assessment to some extent. Finally, the mechanisms of both the single and compound effects were explored in depth from the analysis of the spectral distribution of fission. From this analysis, a harder spectral distribution of fission resulting from the single and compound effects corresponds a larger value of Δ k. [ABSTRACT FROM AUTHOR]