To assess long-term environmental safety of a tritium facility, prediction of consequences of potential tritium releases in the environment is needed both during routine operations and in case of accidents. Modeling is the only method to obtain such assessment without any environmental contamination. The current paper describes the TRIEF model designed to assess consequences of long-term atmospheric tritium emission for such environmental compartments as atmosphere, soil, plants; tritium contamination of ground and underground water is also included. The model takes into account tritium transport among all of the compartments. The model has been successfully validated in model-experiment intercomparison study in framework of the IAEA co-ordinated research programme “BIOMASS” on assessment of environmental contamination from the continuous source of atmospheric tritium release. The experimental data included tritium concentrations in the atmospheric moisture, vegetation, soil and the overlying snow cover. The modelling period was 20 years. Most of the predicted values agreed with observations within experimental uncertainties, which were a factor of 2. The TRIEF model is based on both HTO equilibrium and material balance approach in all the compartments. Average concentrations in atmosphere are calculated by using the Gauss-type model for primary and secondary source. HT and HTO behavior are modeled separately. Both wet and dry deposition of HTO is taken into account in case of HTO emission. HTO concentration in soil moisture is determined by the moisture balance equation. HTO concentration in plant tissue free water and organically bound tritium are estimated as a combination of HTO content in soil moisture and atmospheric humidity. HTO contamination of aquifer is modeled using available finite-differences codes within 12 hydro-geological strata.