[en] High-level radioactive waste has been stored in large cylindrical underground tanks at the US Department of Energy's (DOE's) Hanford Site in southeastern Washington State since 1944. Approximately 285 million liters of caustic waste are contained in 177 carbon-steel tanks. Radiolytic, thermolytic, and chemical reactions in the radioactive waste generate flammable gases such as hydrogen and ammonia. These gas mixtures also include an oxidizer (nitrous oxide) and an inert component (nitrogen). Some wastes retain the gas mixture until a large amount builds up, and then a short-term gas release occurs in the tank headspace. Such a gas release can result in a flammable concentration in the tank headspace. Following a 1990 investigation into flammable gas generation, retention, and release mechanisms within the Hanford Site high-level waste tanks, personnel concluded that the existing authorization basis documentation did not adequately evaluate flammable gas hazards. The existing authorization basis to establish effective controls for the flammable gas accident analysis is based on operational experience and qualitative judgment. Therefore, the flammable gas safety issue remains open because of considerable uncertainty regarding essential technical parameters and associated risk. The Project Hanford Management Contract team has formulated a refined safety analysis approach to resolve the flammable gas safety issue