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 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 fuel loading varied from 0.05 g to 30 g 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. (author)