Search

• 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]

• A modified simulation model in MCNP 6.1 was developed for the dynamic HTR-10 core. • Applying the composition library can simplify processes with accurate results. • Due to longer residual time, the discharge burnup in the outer channel is higher. • Core power distribution using multi-pass refueling is more uniform compared to OTTO. This study aims to precisely investigate the core characteristics of HTR-10 using an online-refueling fuel loading scenario through an innovative approach that incorporates a pre-generated fuel composition library. HTR-10 is an experimental high-temperaturegas- cooled pebble-bed reactor constructed in China. Applying multi-pass fuel loading scenario, the pebbles in HTR-10 pass through the core several times via onlinerefueling. In this study, we proposed a model that simulates the dynamic operation of HTR-10 by using continuously static calculations. The model was constructed by dividing the core into 3 flow channels with different amounts of horizontal zones according to the residence time of fuel pebbles. However, random distribution of the pebbles with various burnup values complicates the refueling process. In order to simulate online-refueling more conveniently, a comprehensive method based on multipass fuel loading scenarios was thus developed. Firstly, a pre-generated fuel composition library containing the content of both actinides and fission products of fuel pebbles as a function of burnup, was built by implementing full core depletion calculations. During the refueling processes, a specific ratio of discharged fuel pebbles to fresh fuel pebbles was reloaded into the top zones. Thus, the average burnup values in the top zones can be resulted by considering the supplement of fresh fuel pebbles. Eventually, the fuel composition in the top zones were obtained by interpolating the average burnup value from the pre-generated library. In this study, all the calculations were performed using the MCNP6 computer code together with the ENDF/B-VII continuous energy neutron data library. The results revealed that the core reached a state of equilibrium after seven refueling processes with an increased rate of volume fraction. Our findings correspond to those of other studies. Another model constructed by locating the actual distribution of fresh pebbles also confirms the verification of the refueling method. Eventually, the burnup characteristics as well as the power and flux distributions in the HTR-10 core were analyzed using the simulation method. [ABSTRACT FROM AUTHOR]