The modular steady states approach (MSSA) has been developed to meet the particular needs of probabilistic risk assessments (PRAs) of facilities at the back end of the nuclear fuel cycle. Applicability to front-end facilities can be easily achieved by adding appropriate modules. The MSSA is not intended for nuclear power plant PRA. Atmospheric dispersion and subsequent dose assessments have been treated in the conventional manner. They are not part of the MSSA code package STAR developed for use on an IBM AT-PC. The application of MSSA is demonstrated for a high-level waste storage tank facility. The two principal release paths via the tank off-gas system and the cell exhaust-air system have been investigated. It has been found that component failures with negligible consequences are relatively frequent and typical for the off-gas system. The release risk due to failure is of the same order of magnitude as the release risk due to normal operation. Release via the cell exhaust-air system is possible only after a tank leakage and failure to pump the liquid into a reserve tank, resulting in a very low probability but relatively high release.