The dilatometric investigation of the swelling of irradiated boron stainless steel has been carried out at a constant temperature and constant heating rate. A quantitative swelling model is proposed based on the mechanism of the capture of migrating equilibrium vacancies by immobile bubbles. The differential equation describing the process of establishing gas bubble equilibrium with time has been derived. The application of this equation made it possible to describe quantitatively all the stages of the material thermal annealing and determine the physical parameters characterizing the processes occurring in the material swelling.