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'Oxygen candle' caused sub explosion

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Yesterday's accident in which two sailors were killed aboard the Royal Navy submarine Tireless did not involve any of the submarine's machinery or installed systems, it has emerged.

According to US and British naval sources, the explosion instead involved an oxygen candle. Some news sources have said oxygen candles are "equipment" which is "fitted" in submarines, giving a misleading impression of what they are.

Oxygen candles are consumable items not unlike large marine flares in construction and use. When lit or initiated, they give off oxygen rather than regular combustion products. They are carried for use in an emergency, where the normal atmosphere equipment is unserviceable or inaccessible (perhaps flooded) and the submarine cannot surface. In such a situation, sailors may need to replenish local oxygen levels while awaiting rescue or a chance to escape – hence the candles. They are normally stowed in the escape compartments forward and aft.

Escape compartments are pressure-tight airlocks where submariners can retreat in the event of a disaster. From there, depending on the situation, they may be able to escape to the sea surface using specialised breathing sets. But in some circumstances – if the sub is too deep, or if the sailors have been under too much atmospheric pressure for too long – it may be better for them to wait until rescuers can reach the scene.

Like any oxidising material, oxygen candles present a fire and explosion hazard – the more so in a confined space. All submarines, and indeed surface fleet units, are full of things like this. Emergency breathing apparatus, diving sets, smoke markers, flares, munitions, medical gases … the list goes on. Every warship is a highly dangerous industrial facility, full of hazmat. Tireless has suffered a relatively normal industrial accident.

All defence sources so far have stressed that the sub's reactor wasn't involved, with the explosion reportedly taking place in the forward escape compartment. Of course, the forward end of the ship is where the weapons are, but there isn't a good place for an explosion in a warship. Anyway, the energy potential of oxygen candles isn't enough on its own to have any effect through watertight bulkheads.

The Royal Navy's sub fleet has suffered other troubles over recent years. Reactor-safety scares took most attack boats out of service for long periods at the turn of the century; Tireless herself had to tie up at Gibraltar for almost a year while precautionary work went on. With training and operations severely curtailed, morale in the submarine service suffered and many ambitious junior officers left.

Embarrassingly, Tireless' sister ship Triumph inadvertently glanced off the sea bed during this period, when she was the only sub (apart from the Trident boats) cleared for use. Subsequently, HMSM Trafalgar suffered a more serious grounding in 2002, though her pressure hull was unbreached.

In recent years there has also been a lot of internal debate - almost amounting to mutiny at one stage, when the Trafalgar went back to sea - over safety and maintenance issues. Some have alleged that vital precautions have been dropped to save money: others saw the moves as a reasonable attempt to rein in the ever-burgeoning nuclear safety bureaucracy, which has pushed the costs of operating naval nuclear reactors to incredible levels.

Yesterday's incident will be subject to a navy board of inquiry in due course. ®

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