4. Long.Term Reactivity Change and Control: On.Power Refuelling

There are 2 concepts related to the “age” of fuel: irradiation (fluence) and fuel burnup.

The fuel irradiation in a given fuel bundle, denoted ω, is defined as the time integral of the thermal flux in the fuel during its residence time in the core. Another term for irradiation is fluence. Irradiation is also known as the thermal-neutron exposure of the fuel. The units of irradiation are neutrons/cm², or more conveniently, neutrons per kilobarn, n/kb. Since the cut-off of the thermal-energy range may be defined differently in different computer codes, the fuel irradiation may vary from computer code to computer code, and caution must therefore be exercised when comparing irradiation values using different codes. In documents, it has been more and more usual to report values of fuel burnup rather than fuel irradiation, as burnup does not suffer from differences in definition between codes.