Energy-higher order mode analyses in Feynman-α method

► This paper focuses on the energy-higher order mode effect in Feynman-α method. ► An analytical formula of Feynman method has been derived for a two-group problem. ► A general higher order Feynman formula has been simplified to a multi-group problem. ► The energy-higher order mode effects are very minor in a thermal system. ► In a fast system, however, the effects are notable except for near-criticality.