Answer to Question #434 Submitted to "Ask the Experts"

Q

Is it possible to (1) survive a nuclear attack if one is underwater at the time of detonation and/or (2) be protected from radiation if underwater?

A

Certainly if one is far enough away, one might consider that protection from radiation would be a relatively simple matter. For simplification, one could use some of the rules-of-thumb for shielding to understand a degree of protection: HVL for 1 MeV Gamma: 24" Water. So using these numbers approximately 2 feet of water would reduce the dose rate by 1/2. The same for neutrons: TVL for neutrons: 10" water. Again, this implying the dose rate would be reduced to 1/10 by that application. Herein lies the problem with a nuclear attack.

Radiation constitutes only approximately 15 percent of the threat or insult from a nuclear weapon. 50 percent is from the blast itself and 35 percent from thermal energy. Nuclear weapons are typically described in terms of the energy produced by TNT. For example, the weapons used in Hiroshima/Nagasaki were 22kT weapons, or produced the same amount of energy as 22,000 tons of TNT. Just 1 kT of equivalent TNT requires the complete fission of approximately 57 grams (~2 ounces) of fissionable material. This releases on the order 2.6E+25 MeV. So when we begin to speak of the magnitude of modern nuclear weapons, we are talking about megatons (>1E+06 tons equivalent TNT) and a tremendous liberation of energy, even if the efficiency of the weapon were poor. With so much energy released, temperature and pressure are increased. Temperature on the orders of tens of million degrees, virtually causing materials making up the weapon to be vaporized, and gasses expand rapidly resulting in a pressure wave or 'shock' wave. Just how fast is this wave moving? Within 10 seconds the shockfront is 3 miles from ground zero. Within 50 seconds, the blast has traveled 12 miles and is moving at approximately 1150 ft/second. This shockwave results in tremendous overpressures, i.e., pressure in excess of the normal atmospheric pressure (14.7 psi). These overpressures cause a tremendous amount of damage. At 4.5 seconds after the detonation of a 1 megaton (MT) weapon, the blast front has moved out from ground zero approximately 1.3 miles. The overpressure at the blast front is roughly 20 pounds per square inch (psi). Some results of overpressures and associated wind velocity:

 

People as well as structures quickly have a problem dealing with the overpressures. In regards to the associated wind velocity, people have a problem being accelerated rapidly also, with the LD/50 for us being approximately 55mph. So again, I'm led to the first thing I mentioned, that if one were far enough away from the event, water could be protective, but if one were in close, things could be worse. The shock front exerts even more pressure on water than against air. Where a 100 kT weapon exerts roughly 10 psi in air, it exerts over 2500 psi in water. The Baker test (20kT) in the Bikini atoll was a shallow water test. Within 4 milliseconds following detonation, water began rising at approximately 2,500 ft/second finally to a height of about 10,000 feet. The first (at about 1,000 feet from 'water' zero) was measured to be 94 ft. tall and surged away for over four miles, the point here being water will present an entire other set of problems if too close. I didn't address the hazards associated with the thermal component, but I think the questions are addressed. Water could be protective if one were far enough away, but then many other things could as well.

T.D. Naquin CDR MSC USN Chief
Operational Dosimetry Armed Forces Radiobiology Research Institute
8901 Wisconsin Avenue
Bethesda, MD 20889-5603
301-295-9135
fax: 301-295-5673