One of the factors of the reactor dynamics is the axial expansion of fuel pellets. In early works, a column of pellets in a fuel element was considered as a homogeneous cylinder with a uniform temperature distribution. The issue of the IBR-2M Reactor instability requires obtaining the dependence of the change in reactivity on time, taking into account the exact design of the fuel element and the temperature distribution in time and space. In this work, non-stationary thermophysical processes in the fuel pellets of the IBR-2M Reactor and the effect of changes in reactivity caused by the deformation of the pellets are studied. Numerical and analytical solutions of the non-stationary heat conduction problem, analysis of heat conduction and thermoelasticity equations, and results of neutronic Monte Carlo calculations are presented. Various methods for calculating reactivity and future research directions are discussed.