A method for calculating the accumulation time of a column for cryogenic rectification of multicomponent mixtures of hydrogen isotopes is presented. Relations which do not use the theoretical separation step approximation and which are exclusively based on information about the physicochemical properties of the components of the mixture to be separated are used to simulate the process. A system of differential equations for nonstationary mass transfer is solved numerically to obtain the concentration profile of the components along the height of the column at a fixed time. The calculation of the startup period is illustrated by an example in which the dependence of the accumulation time on the feed-material load is obtained.