The relevance of the discussed issue is caused by the need to evaluate the effect of beryllium blocks poisoning on nuclear reactivity, flux density of thermal neutrons and power distribution of IVG.1M reactor. The main aim of the study is to evaluate the effect of beryllium blocks poisoning on neutronic characteristics of the IVG.1M reactor. The methods used in the study. A solution of the problem included two stages: at the first stage the author carried out neutronic calculations of rates of a reaction and the neutron-flux density using MCNP5 program. At the second stage the author developed the program in the programming language Visual Basic in the Excel to solve the systems with known start values of concentrations for beryllium, lithium, tritium and helium. The results. The paper introduces the results of neutron and physical calculations of the IVG.1М. reactor active zone from 1975 to 2013 taking into account the periods of start-up and idle times between start-ups, and the results of determination of beryllium poisoning effect on reactivity. The results show that the reactor shutdown periods influence significantly on helium-3 accumulation in beryllium blocks. This changes neutron spectrum in its turn. The shutdown period duration does not influence lithium-6 concentration. Poisoning substances are mostly accumulated not in reflector perimeter zones but in beryllium interchannel displacers near water-cooling channels. The results of calculations can be used to assess neutron and physical characteristics of the reactor during the long time. The design model with the specified impurity of of beryllium displacer materials and a reflector can be used for receiving more exact results at calculations.