Study on the Characteristics of Effective Delayed Neutron Fraction (β_eff) for Pebble-Bed Reactor with Plutonium Fuel

A current effort focused on mechanisms of maximizing plutonium consumption is one of the solutions to control large stockpile of plutonium produced from nuclear power plants and military programs. The pebble-bed reactor with plutonium fuel is a unique case in the high-temperature reactors intended to reduce the concentration of plutonium that has been accumulated in the world. However, the plutonium core with tendency to produce a lower effective delayed neutron fraction (β_eff), which is a crucial parameter in reactor safety, should be examined. The purpose of this study is to investigate the characteristics of effective delayed neutron fraction (β_eff) for pebble-bed reactor with plutonium fuel. The uranium and thorium contents with a 1:1 and 1:2 ratio of heavy metal mass per pebble were combined into the plutonium core to study their effect on the characteristics of β_eff. A series of calculations with various mass of plutonium per pebble were performed by using the MCNP6 code and ENDF/B-VII library. The β_eff of pebble-bed reactor with plutonium fuel and its combination with uranium and thorium fuels were analyzed in term of core safety aspect because it could affect the transient behavior into a fatal accident situation.