[en] The results of the experimental investigation of thermal shock mechanical effects in IBR-2 fuel elements are presented. The height of upper fuel pellet jumping due to the fast (about 100 Ms) increase of fuel temperature (up to 25 K) and thermomechanical stresses in the fuel pin cladding are measured. There is a great disagreement between experimental and theoretical results. The height on which the upper pellet jumps is 1.4 mm, if the temperature rise is 25 K. This result corresponds to the requirement for longitudinal wave propagation to be of the order of 1000 m/s. For qualitative explanation of it a model of the thermal shock in the fuel pin, consisting of pellets, is proposed. The model was tested experimentally. The velocity of mechanical signal propagation along the column of pellets was measured directly. The results agree with thermal shock experimental data