Criticality is the state of a nuclear reactor when there is a self-sustaining chain reaction of fissionable material. Used or spent fuel criticality is associated with the scientific issues around assessing criticality safety for systems and operations involving spent nuclear fuel.

The NEA Expert Group on Used Nuclear Fuel criticality (EGUNF) was formed in 2015-2017 as a follow-up activity of the NEA Expert Group on Burn-up Credit Criticality Safety (EGBUC), to assess the accuracy and validity of burn-up credit methodologies of used nuclear fuel (UNF). EGUNF mainly covered storage and transport of UNF and many NEA member countries were interested in the direct disposal of UNF.

EGUNF performed the two benchmarks as below;

• SiO2 Reflector Effect (published in July 2020),
• Burnup calculations of gadolinium-bearing fuel rods in boiling water reactor assemblies for storage and transportation (to be published soon).