Optimal Moderation in the Pebble-Bed Reactor for Enhanced Passive Safety and Improved Fuel Utilization

The concept of optimal moderation is introduced as a reactor configuration in which any change in the fuel-to-moderator ratio causes a reduction in reactivity. It is shown that such a configuration cannot persist over time in a reactor with stationary fuel, but can do so in a reactor with moving fuel, such as a pebble-bed reactor. It is further shown by the example of pebble-bed versions of the proposed Next Generation Nuclear Plant that such optimal moderation in gas-cooled reactors leads to significant advantages in response to water ingress, fuel utilization, and resistance to nuclear weapons proliferation.