Due to the good radiation resistance and low activation performance,the reduced activation ferritic/martensitic(RAFM)steel has been selected as the primary candidate structural material for the fusion DEMO and first fusion power plant.The RAFM steel will undergo radiation-induced swelling and other radiation damage behaviors under the harsh fusion environment.Since the displacement damage and transmutation gas production is inhomogeneous under the high energy neutron irradiation,a spatially dependent rate theory(SDRT)model has been developed to model the swelling in RAFM steels.This model considers both the void swelling and gaseous swelling mechanisms.The local swelling predicted by this SDRT model in certain regions is higher than the averaged swelling.It is more effective than the conventional mean field rate theory in predicting the safe service life time of RAFM steels.This research will predict the swelling behavior of RAFM steels under the fusion neutron irradiation,which will be helpful to estimate the safe service performance of RAFM steels in the fusion reactor.