An explosion caused by a back up oxygen generation system has killed two submariners on board the British HMS Tireless say US authorities.

Electrolysers provide the main supply of oxygen on board Trafalgar-class submarines such as the Tireless, creating sustainable oxygen supplies and a small amount of hydrogen that is then disposed of. The blast came after an $quot;oxygen candle$quot; (also known as a self-contained oxygen generator) used as an emergency fall back exploded in the forward escape section of the boat.

As former First Sea Lord, Admiral Sir Alan West explained about the candles, $quot;These particular oxygen generators, they're a secondary system. There's another, the primary system (the electrolysers), that produces oxygen all the time; and there's one forward and one aft.$quot; The boat would only need one of the primary systems functioning at any one time, so the candles form a third-choice emergency standby.

The candles themselves are cylindrical chemical oxygen generator containing a mix of sodium chlorate and iron powder. When ignited, the mixture smoulders at about 600 °C, producing sodium chloride, iron oxide, and about 6.5 man-hours of oxygen per kilogram of the mixture releasing oxygen at a fixed rate.

The mixture has an indefinite shelf life if stored properly with candles stored for 20 years showing no decrease in oxygen output. The oxygen is released by thermal decomposition and the burning iron supplies the heat. Importantly, the candle has to be wrapped in thermal insulation to maintain the reaction temperature and to protect surrounding equipment. Speculation is now building that a candle being ignited outside of these strict usage conditions caused the damage to the boat. The Ministry of Defence will conduct a full inquiry.