The influence of change in the 3He concentration in the beryllium core masonry on the reactivity margin during various campaigns has been described using the MIR reactor as an example. An algorithm is presented for the on-line calculation of  3He concentrations in beryllium blocks of the core masonry, taking into account the actual distribution of the energy release over the cells of the working channels with fuel assemblies, the operating time at power, and the time of shutdowns between campaigns for refueling the reactor. The analysis of the actual performance of the reactor in the campaigns, which are close in terms of the duration of operation at power and in the loading of the loop channels but differing in times of the reactor stopping between the campaigns, is carried out. Through the study and analysis of experimental data on the operation of the reactor in campaigns of various durations, their characteristic properties have been revealed. The effect of the accumulated helium on the reactivity parameters of the reactor has been estimated. The nature of changes in the reactivity margin when operating at power, depending on the duration of the stay has been predicted using this estimate. In addition, the negative impact of poisoning nuclei on the state of beryllium blocks in a series of short-term campaigns, a feature of which is the excess of the preparation time for the campaign over the duration of the campaign itself, has been described. It has been shown that  3He accumulates from campaign to campaign in a series of short-term campaigns, which inevitably leads to a decrease in the reactivity margin. During long-term operation at power, it has been found that this absorbing nuclide acts as a burnable absorber of neutrons, reducing the rate of loss of the reactivity margin.