Excess criticality mitigation for pebble bed reactor fuels by pebble displacement and replacement

Pebble bed reactor fuel has both high enrichment and low fuel density. Previous research by the authors has determined the peak possible keff by examining the effect of water infiltration of all possible densities in a variety of container sizes. The previous research has found that for larger containers, the established sub-critical limit of 0.95 if exceeded. These current research has examined two possible criticality mitigation methods. The examined container is the largest of the previously examined ones, the MPC-37. The first method involves adding 9 hollow stainless steel rods that span the length of the container into the model. 1 in the middle, and 8 at the midpoints between the center cylinder and the wall of the container. By displacing fuel pebbles in the most active regions of neutron multiplication, keff is most effectively reduced. In the second method, dummy pebbles comprised of stainless steel or boron carbide replace at random fuel pebbles in the container with various replacement fractions. The stainless steel displacement rods cut the value of peak keff from roughly 1.33 to 0.94, a difference of ~40,000 pcm at the cost of ~15% of the container's capacity. Dummy pebbles are roughly twice as effective, cutting keff down a similar amount with only a 7% replacement required for the stainless steel pebbles, and 3% replacement required for the boron carbide pebbles.