Safety of Waterjets for Demilitarization
Waterjets have been used in demilitarization since early 1950s. However impact initialization of high explosive materials is always a safety concern. Miller’s article* on this subject provides a good source of information. Here is a review of his article.
There are two types of explosives: primary and secondary. Primary explosives (e.g. PETN) are used in small quantities to initiate an explosion. Secondary explosives (e.g. TNT) are used in much larger quantities and are more stable. Primary explosives are extremely sensitive to impact shock. For example, PETN can be ignited by a 2 kg weight dropping from a height of only 0.0125 meter. Explosives are also sensitive to projectile impact. The projectile velocity required to initiate 50% of the explosives is 310 m/s for PETN and 780 m/s for TNT. Two other factors of projectile impacts are projectile diameter and shock pressure.
At 50,000 psi water pressure, the velocity of waterjet is estimated to be 830 m/s, which is higher than the above initiation velocity for most explosives. However, the shock pressure from a slug of water is much less than that of metals impacting at the same velocity. A study by Mader in 1981 determined that the initiation of explosives by a liquid jet depends on the product of ρV2d (ρ = density of liquid, V = jet velocity, d = impact diameter).
For an abrasive waterjet with 0.014” orifice, 0.043” mixing tube, 50,000 psi water pressure, and 0.57 kg/min of 80 mesh garnet abrasive, the product of ρV2d is calculated to be 7.524x105 kg/s2 --- To be on the safe side, the density and velocity of the three phase mixture (air, water, abrasive) are assumed to be equal to those of water, knowing they are much less. This value is way below the published value of 1.50x108 kg/s2 for PBX-9404, a very sensitive high explosive. It is also below the value of 8.6x107 kg/s2 --- the “one chance in a million” accident criterion.
To verify the safety of waterjet demilitarization operations, 50 waterjet impact tests at 1,000 MPa (145,000 psi) were done on TNT and PETN explosives. No denotation occurred. About 500,000 high explosive projectiles were also successfully cut with abrasive waterjets. An independent study by US National Research Council, on behalf of the US Army, reached the following conclusion:
“The use of high pressure water or ammonia to cut explosive-loaded ordnance and/or to wash out energetic materials from ordnance casings is a proven technology.”
* Miller, Paul L. (2001) Impact initialization mechanisms of high explosive materials during waterjet demilitarization, in Mohamed Hashish (Ed.), Proceedings of the 2001 WJTA American Waterjet Conference, Minneapolis, Minnesota, USA, August 18-21.
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