The Hydrogen Isotope Separation System (HISS) is a general-purpose tritium recovery and enrichment processor that uses low-temperature distillation as the separation process. HISS processes feed mixtures containing all three isotopes of hydrogen (H,D,T) and yields an enriched tritium product up to 99.95% tritium, while producing a discardable raffinate. The three-column system operates continuously with unattended overnight operation and limited operation during weekends. Production runs with a full still inventory were started in October 1987, with individual runs lasting up to seven weeks.