In October 1996, the Japanese Maritime Self Defence Force (JMSDF) undertook an operation to recover 26 units of WWII Chemical Warfare Ordnance from Lake Kussharo in Hokkaido, the northernmost island of Japan

The weapons were of a conventional bomb configuration originally designed to be dropped from an aircraft. The bombs were approximately 4 feet in length with a fin on one end and contained mustard gas.

The consignment was dumped in Lake Kussharo at the end of the Second World War in an attempt to conceal from the allies, the fact that their manufacture existed

Lake Kussharo is an area of outstanding natural beauty and the lake is a popular tourist resort for boating, fishing and most importantly its natural volcanic hot springs.

Throughout 1995 and 96, chemical contamination of the atmosphere was a high profile issue in Japan following the terrorist sarin attacks on the Tokyo underground This led the local government in Hokkaido to address the problem of the known dump of mustard gas bombs in Lake Kussharo and in turn to JMSDF planning their recovery


An initial video survey of the site located the munitions in 42 metres (around 140 feet) of water in an area approximately 50 metres square. The video showed many of the bombs were corroded and ruptured and their contents exposed. Although the mustard gas chemical is normally inactive in water, the exact composition of the chemical was unknown and so the recovery operation required maximum diver protection be afforded.

The JMSDF contracted Japanese specialist underwater equipment company Nippon Kaiyo Ltd. to provide the DIVEX "Dirty Harry" system to the military to carry out the operation.


"Dirty Harry" works on the same principles as gas reclaim; the Divex Ultrajewel exhaust regulator ensures the divers exhausted breath is ducted away from the helmet into an exhaust hose in the diver's umbilical and then vented at the surface rather than into the water (Fig. 1). This absence of exhaust into the water ensures no potentially contaminated water leaks back into the helmet even as airborne vapour, which distinguishes the helmet from other twin-exhaust or free-flow helmets traditionally used for contaminated water diving.

Fig. I : Dirty Harry Diver showing Divex Ultrajewel Exhaust Regulator(Available in full paper)

The demand valve on the Dirty Harry is a balanced, high flow design which ensures the diver receives adequate breathing air despite line losses in the potentially long umbilicals typically used in contaminated water dives The hydrostatic positioning of the exhaust valve below the demand valve ensures the helmet is always slightly positive in pressure relevant to the surrounding water (as long as the diver is upright) which assures that in the unlikely event any leak occurs in the helmet then the a r will escape out rather than water in

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