Effect of fuel particles' size and position variations on multiplication factor in pebble-bed nuclear reactors

The effect of TRISO particles' size variations and inter-particle distance on infinite multiplication factor (kinf) of a pebble bed reactor (PBR) is studied using MCNP code. It is observed that kinf decreases with increasing TRISO particles' size, mainly due to increased total particle surface leading to stronger resonance absorption. Negative temperature reactivity effect is observed at all particle sizes. The kinf increases strongly when particles form in clusters as the clusters behave as a single bigger particle. It is also observed that the white boundary condition is not an appropriate approximation of the unit-cell flux boundary condition in the calculations as it yields significantly higher values of kinf than periodic and reflective boundary conditions. (orig.)