T91 steel was irradiated in BOR60 reactor at five different temperature/dose conditions. 2 The number density, volume fraction, size and composition of the early stage Mn, Ni, Si -rich clusters have been characterised by atom probe tomography. After irradiation, Ni, Mn, Si and P segregation to carbide/matrix interface has been observed. Fe clustering inside a carbide was observed after irradiation at 524 °C:15.4dpa, which has not been reported before.