Study on optimizing neutron moderation in HTR pebble-bed core

In HTR pebble bed, the neutron moderation is strongly influenced by the ratio of the fuel pebbles (F) and the moderator pebbles (M) contained in the reactor core. Optimum moderation can be achieved by adjusting the ratio F/M, but the combination of the pebble with the ratio of F/M more than 1:1 in pebble bed reactor core raises a problem in the calculation. This paper discusses a method to solve this problem. Optimizing neutron moderation in the HTR pebble bed core was investigated by conducting a series of calculations using Monte Carlo transport code MCNP5. The model of the fuel pebble with UO₂ kernel and the moderator pebble with specific density of graphite was chosen to fit the current design of HTR pebble bed. The continuous energy nuclear data library ENDF/B-VI was used to complete the analysis of moderation optimization. The calculation results show that the values of k_eff tend to increase in line with the rise of ratio of F/M and go down slightly after a particular value. A large ratio of F/M causes a decrease in k_eff value because of the resonance neutron capture (under-moderated). A small ratio of F/M causes a decrease in k_eff value because of parasitic neutron absorption by the moderator (over-moderated). From all calculation results it can be concluded that the neutron moderation optimization of HTR pebble bed core is achieved with a combination of pebbles which is specified by the ratio of F/M 3:2. (author)