Two limiting regimes of gas release from particles are studied: diffusion and activation. The conditions under which either the diffusion or the activation desorption regime occurs are determined. A model is proposed for the activation release of helium atoms from centers of accumulation and diffusion of these atoms toward the surface of a spherical particle of boron carbide powder. A quantitative estimate is obtained for the release of helium from a material during annealing, and the influence of the size of the powder grains and the rate of change of the temperature of the material on the thermodesorption of helium from irradiated boron carbide is estimated.Information reported on the destruction of the cladding of absorbing elements caused by excess gas pressure [1, 2] dictates the need for developing a computational program, based on this information, for simulating the behavior of (n, α) absorbers and rods in the control and protection system under irradiation, specifically, the development of mathematical models of helium release from boron carbide powder. The amount of helium released from irradiated boron carbide by heating has been determined experimentally many times [3][4][5], but there is still no satisfactory model for describing this process adequately [6]. A method for estimating the amount of helium released from irradiated boron carbide during annealing under nonstationary conditions has been proposed [7]. The model was constructed on the basis of a theory of annealing of radiation defects [8] under the assumption that the limiting stage of helium release is overcoming an activation barrier when gas atoms detach from accumulation centers. However, this mechanism meglects the influence of helium diffusion toward the surface of a powder particle after the atoms detach from the accumulation center. It would be useful to develop a mathematical model that would make it possible to generalize the activation mechanism of helium release [7] so as to include diffusion transfer.Boron carbide powder is now used in absorbing kernels in the VVÉR-1000 control and protection system. The particle size is about 10 µm, which is close to the travel distance of a thermal neutron in boron carbide. Consequently, it can be assumed that accumulation centers capture helium uniformly over the entire volume of a particle.A mathematical model of helium release from a powder particle in the process of annealing was constructed under the following assumptions:• all of the helium in the irradiated boron carbide is located in stationary accumulation centers, and the number of helium atoms occupying interstices is negligibly small compared with the total number in a particle; • when the powder is being heated helium atoms are released from all capture centers by an activation mechanism with a low probability of helium being captured by an accumulation center;