Investigation on the pebble bed flow model in VSOP

In pebble bed high temperature gas cooled reactor (HTR), the fuel pebbles will pass the reactor core several times, in order to flatten the power distribution, and to unify the discharging burnup. But the VSOP code approximates the pebble bed flow because of current computer capability. In order to simulate the pebble bed flow in HTR more realistically and to validate the approximation in pebble bed flow model in VSOP, a new code based on Monte Carlo method has been developed to analysis the characteristic of the pebble bed flow, particularly the average burnup distribution in the core, discharging burnup, and the number of pebble passes. Statistics analysis shows that several millions of Monte Carlo simulation can provide reasonable result and the VSOP treatment of the pebble bed flow model is reasonable. The Monte Carlo simulation can provide more information for the pebble flow and reactor operation.