The paper presents experimentally evaluated parameters of 14C and 36Cl leaching from irradiated RBMK-1000 graphite in aqueous media of various compositions, including distilled water and those stimulating the groundwater. To predict further radionuclide migration and to demonstrate the disposal safety, an array of initial data was formed presenting the characteristics of RBMK-1000 graphite stacks. The paper proposes an optimal approach describing numerically the function of the leaching dynamics. It also compares the calculated dynamics of 14C and 36Cl leaching from GR‑220 (PUGR) and GR-280 (RBMK-1000) graphite.