A recent runaway reaction emergency relief system design includes a passive quench tank to collect and contain the two-phase emergency relief discharge. This avoids the release of a large vapor cloud to the atmosphere and the attendant flammability and toxicity hazards. The feasibility of quenching is demonstrated with small scale experiments. A pressure relief simulation model is shown to accurately reproduce the observed temperature and pressure profiles in both the reactor and quench tank during relief. The model is then applied to the design of a full scale system. Quench system design considerations are discussed.