Abstract: Irradiation ability was suggested as a technical index and performance parameter. Irradiation ability could be used not only as an assessment index of core facility arrangement design, but also as a pa-rameter for the calculation of in-core irradiation costs. Relevant parameters of irradiation ability are: irradia-tion space volume, averaged total neutron flux, reactor power and operative time. Expressions for several definitions of irradiation ability were given, and their features and roles were compared. The High Flux En-gineering Test Reactor was selected as a main example to give the values of irradiation ability and their applications.