DISASTER struck as James Gubash, a flight engineer on the nuclear-powered
aircraft carrier USS Nimitz, came off duty one night in May 1981. A
reconnaissance aircraft coming in from a routine flight crashed onto the deck
and caught fire. The heat detonated bombs on other aircraft, starting an inferno
that spread like wildfire. “There was a chain reaction. Planes kept exploding.
Ammo stores were detonating and ejection seats were going off. It was like being
in the middle of a war zone,” Gubash said, reporting the incident on US
television’s Military Memoirs. The firestorm raged for three days, and by the
time it was over, 14 crew were dead and 45 were injured. It was only a matter of
luck that the ship’s reactors and the nuclear weapons on board survived
unscathed.
Military explosives are complicated mixtures of chemicals, and they are
designed to cause death and destruction. If you want to use them without
endangering yourself, you need to understand how and why they detonate. But no
one does—and it’s looking as though those who thought they understood them
may have been on the wrong track all along.
A new age of explosives research is dawning. Having spent decades using
experiments to find out what makes a bomb, researchers are now changing tack and
detonating explosions inside computers. And this trick is beginning to work. For
the first time, researchers have a window on one of the most destructive events
known to science. With it comes the hope that devastating accidents with
explosives, both military and civilian, could be a thing of the past.
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The quest began in earnest after the Nimitz disaster. It was not the
first—nor the last—devastating accident involving military weapons
(see “Own goal”), but its severity forced the US Navy to an uncomfortable
conclusion. The accidental triggering of its bombs was capable of causing as
much death and damage as its enemies could ever hope to inflict.
So in 1984 it began a programme aimed at finding an entirely new kind of
explosive—something that would be just as lethal as conventional
explosives on the battlefield, but which would somehow be immune to accidental
detonation. In short, the Navy wanted the ideal bomb.
It’s a tough brief, because it calls for an intimate understanding of
detonation. Scientists know the basics: when an explosive detonates, a
self-sustaining supersonic shock wave rips through it, creating temperatures of
up to 5000 °C and pressures of 500,000 atmospheres. For a few millionths of
a second, it releases energy at a stupendous rate—100 million
kilowatts—and then it’s all over. These factors combine to make
detonations exceedingly difficult to watch. “They literally blow up your
experiment,” says Randy Simpson, an expert in explosives at the US Department of
Energy’s Lawrence Livermore National Laboratory near San Francisco.
As a result, we are still in the dark about what’s going on. “There is a lot
of disagreement over what is the initiating step in a detonation,” says Betsy
Rice, an expert on the computer modelling of explosives at the US Army Research
Laboratory at the Aberdeen Proving Ground in Maryland.
Nuclear threat
But that doesn’t mean there hasn’t been any progress. Thanks largely to
trial, error and plain good fortune, the military do have a few “insensitive”
explosives, of which 1,3,5-triamino-2,4,6-trinotrobenzene or TATB is probably
the best on offer. “TATB is really amazing stuff,” says Simpson. “If you had a
fire in your office, you could pour a bag of TATB onto it and snuff it out.”
In the 1970s, the US put TATB to work in its thermonuclear warheads.
Conventional weapons are used to trigger nuclear bombs and, although a nuclear
explosion can’t be triggered by accident, accidental detonation of the
conventional explosives can blow the bomb apart, spreading radioactive material
over a wide area. A number of accidents with US bombs had showered populated
areas with uranium and plutonium oxides; TATB, with its insensitivity, seemed a
perfect solution.
But the very property that makes TATB useful also causes problems: it can
sometimes fail to detonate when you need it to—not ideal for a nuclear
deterrent. So researchers at Livermore have spent the best part of three decades
doing experiments to work out what goes on when TATB and other insensitive
explosives detonate.
The team runs its experiments in a set of stainless steel chambers, some of
them as big as a caravan. Probes placed around the explosive measure pressure
and temperature in the microseconds before they are destroyed by the blast, and
send their data to instruments kept out of harm’s way outside the chamber. X-ray
flash photography and movie cameras capable of taking 25 million frames per
second allow the team to watch what is going on. “The chambers ring like a bell
when an explosion goes off,” says Simpson. Over the years, this set-up has
allowed them to test explosives of various kinds. But they still cannot see into
the heart of the explosion, and in particular to the instant that detonation
takes place.
“To design an experiment to try to probe what’s happening on the timescale of
picoseconds is practically impossible,” says US Army explosives researcher Brad
Forch. So instead, the Livermore scientists are taking their limited knowledge
gleaned from these explosive experiments and turning it into simulations of
detonation. This means programming in detailed chemical knowledge of the way
each molecule or fragment of a molecule reacts with the others in the mix for a
given temperature, pressure and density.
It sounds straightforward enough, but in fact it quickly becomes fiendishly
complicated. The reaction changes these parameters from moment to moment, and
these changes must be fed back into the equations. In addition, details of how
the by-products of the reactants interact with each other and everything else in
the mix also have to be considered. And that’s just the chemistry: the
simulation also has to model the way the gases mix and expand. Small amounts of
mixing can lead to large amounts of turbulence, and the result is extreme
unpredictability. All in all the task is as complex as modelling the weather,
and the Livermore scientists are still trying to find ways of linking all these
factors efficiently.
To help grapple with the avalanche of data needed for the simulation, the
Department of Energy has set up the Accelerated Strategic Computing Initiative,
which will build the most powerful computers in the world. But while this
gargantuan effort is slowly making progress, Rice has taken a much more modest
approach—with remarkable success.
She has abandoned conventional modelling and the thermodynamics and chemical
equations that have been the mainstay of chemistry for over a century. Instead
she is using the principles of quantum mechanics to generate computer
simulations of individual molecules, and then follow their behaviour as the
explosion reshapes them. Her results are triggering excitement among explosives
researchers.
Rice’s model is based on the laws that govern the way molecules are made from
atoms, and the way their properties arise. The molecules that live inside her
computer can contain up to 50 atoms, and the simulation has to include every
electron in each atom—something modern supercomputers can just about cope
with. Examining the way the electrons are distributed around the most sensitive
explosives molecules has enabled Rice to uncover the first clue to explosive
detonation.
In her simulations, Rice can see that certain regions of the most easily
detonated explosive molecules have almost no electrons. Surprisingly, this
happens just where there should be overlapping clouds of electrons, forming a
covalent bond that links two atoms together. “There can sometimes be a huge
electron deficiency over a covalent bond,” says Rice. “It’s very, very
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Chain reaction
And it seems to be a recipe for explosive disaster. Of course, nothing
happens if the explosive is just sitting there. There needs to be an initiating
step for detonation—a typical detonator might consist of a capacitor
designed to discharge a huge current through a wire, causing the wire to
explode. But a blow from a bullet or dropping the explosives from a shelf can
also do the trick. Whatever the mechanism, it sends a supersonic shock wave into
the explosive, triggering the detonation.
But no one has ever been sure how the shock wave starts a chemical reaction.
Explosives researchers’ first, rather vague explanation was that the mechanical
energy of a shock wave might set molecular vibrations going throughout the
explosive. This energy would break down the molecules, releasing enough energy
to start a reaction. But there was never any experimental evidence to back this
up.
Rice’s simulations point towards a different explanation. She suggests that
the shock wave passing through the solid compresses it to such an extent that
the electrons in each molecule become delocalised, like the electrons in a
metal. “The molecule essentially atomises,” says Rice. Once the pressure wave
passes, the assortment of atoms it leaves behind are free to recombine. But the
old bonds—the ones characterised by a scarcity of electrons—are
unlikely to form again. Instead, the atoms recombine into new molecules that are
energetically more favourable. The result is a huge energy release which carries
on the process.
Although her simulations support this idea, Rice admits that she still can’t
rule out the conventional hypothesis that molecules essentially just shake
themselves apart. “Up till now, no one has been able to design an experiment to
confirm one model or the other,” she says.
Meanwhile Rice has put her quantum mechanical model to work to help design
better explosives. The Army is keen to know everything it can about the
chemistry of possible new compounds to use in its bombs and shells. In the past,
finding chemicals that pack more punch per kilo, or which have just the right
degree of sensitivity has taken years of trial and error.
Rice’s simulations allow her to predict a candidate molecule’s key
characteristics, such as its heat of detonation—the amount of energy
needed to trigger an explosion. This promises to make the process much faster.
“It will allow us to dramatically reduce the time it takes to develop new
explosives,” says Forch. “With computer simulations we can screen candidate
molecules without ever having to make them.”
As Rice’s research leads to further understanding of what happens in an
explosion, it should become possible to control the onset of detonation.
Trial-and-error chemistry has led the Army to develop a substance known as JA2,
a propellant that is specifically designed to burn rather than detonate even
when exposed to fires or the impact of other bullets. But it is one of the few
insensitive explosives available to the military. “The policy is for all weapons
systems to be designated insensitive,” says Richard Bowen, director of the
Navy’s Insensitive Munitions office in Maryland. But he admits that US
forces—the most technically advanced in the world—have only a tiny
small proportion of their weapons certified as insensitive.
Even with Rice’s help in speeding up the process, it could be a long time
before peacetime is truly peaceful.
February 1958, Greenham Common airbase, Berkshire
A US Air Force B-47 experiences engine trouble on take-off and jettisons two
full fuel tanks from an altitude of 5 kilometres. They are meant to drop in a
safe impact area within the base, but miss. One of the falling tanks explodes a
few metres behind a parked B-47 loaded with nuclear weapons, setting it on fire.
The inferno detonates the high explosives in at least one weapon, destroying the
parked bomber, killing two people, and spreading finely powdered uranium and
plutonium oxides into the atmosphere around Newbury.
January 1966, southern Spain
An American B-52 bomber carrying four H-bombs collides with a KC-135 tanker
aircraft while refuelling over the village of Palomares. The tanker explodes,
ripping the bomber apart. The explosives in two of the bombs detonate when they
hit the ground, scattering plutonium dust over the village.
July 1967, South China Sea
The aircraft carrier USS Forrestal is conducting combat flights off the coast
of Vietnam. An F-4 Phantom fighter aircraft preparing for take-off accidentally
fires a rocket into a fully armed and fuelled A-4 Skyhawk, tearing open its fuel
tanks and spilling blazing jet fuel onto the flight deck. The heat detonates one
of the Skyhawk’s 1000-pound bombs, blowing a hole in the deck and allowing
burning fuel to pour into the levels below where other aircraft are being armed
and fuelled. 134 people die in the ensuing firestorm.
January 1968, northern Greenland
A US B-52 bomber carrying four nuclear weapons crashes near Thule airbase.
The bomber explodes, detonating the high explosives in all four bombs,
scattering plutonium and tritium over a large area.
June 1982, Falkland Islands
The British landing ship Sir Galahad is offloading troops and supplies when
Argentinian Skyhawk A-4 aircraft mount an attack. Two bombs make direct hits,
the ship catches fire and the munitions begin to explode. 48 people die.
July 1991, Camp Doha, Kuwait
With the Gulf War over, American and British troops remain to protect Kuwait
from further Iraqi attack. A wiring defect in one of the ammunition supply
vehicles in the motor pool causes a fire, detonating the vehicle’s munitions.
The fire spreads rapidly to other vehicles, detonating more munitions. 4000
troops have to be evacuated while the firestorm destroys 14 of the 11th US
Cavalry Regiment’s Abrams tanks, more than the US forces lost in the whole of
the Gulf War.
