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An explosive study

Drop a lighted match onto a high explosive and you get a blast - everyone knows that. But on a molecular level, who knows exactly how that explosion starts and propagates? For most ignitable substances, nobody knows.

If scientists can pry out nature's secrets about ignitions and explosions, they may improve safety and advance the technology of everything from rocket engines to firefighting.Finding answers to those fundamental questions about explosions is the goal of research in the University of Utah's new Center for the Simulation of Accidental Fires and Explosions.

C-SAFE is funded by the U.S. Department of Energy to the heft of about $20 million over five years. It is a progrm that combines calculations with experimental data. As explained by science dean Peter J. Stang, "The ultimate goal is to develop sophisticated computer software that will simulate the behavior of accidental fires and explosions based on the fundamental behavior of molecules."

"Basically, we're going to model the macroscopic behavior (that is, large-scale explosions) based on microscopic events," said chemistry professor Charles A. Wight, who heads much of the research.

Eventually the DOE hopes to use the resulting computer codes as a jumping-off point for programs to simulate nuclear explosions. The idea is that nuclear testing and safety issues can be settled in a computer rather than inside the Earth.

But C-SAFE - one of five research centers the department is funding around the country - focuses on studies more mundane than nuclear detonations. Its 50 researchers want to uncover the basic nature of fires and explosions.

In a lab in the Eyring Chemistry Building, Wight and Pete Lofy, a master's degree candidate, display sections of a Dewar vessel, an instrument that looks something like a shiny steel periscope.

The lab is a somehow-orderly jumble of racks and work benches, all of them bristling with instruments. There are stainless steel cylinders connected by tube to much large vacuum pumps, a massive laser mounted on a table, reflectors, computers, compressors and a spectrographic analyzer.

The Dewar vessel is used to cool and position a layer of explosive only one micron deep. A micron is just under half a billionth of an inch. To get a layer that thin, the scientists paint a liquid explosive on a small piece of material that looks like glass, and let it dry.

Actually, the circular window isn't glass, Wight explains. It is crystalline calcium fluoride, which is stronger than ordinary glass. The first window is attached to a second so the explosive emulsion is sandwiched between. The sandwich is then mounted inside the Dewar vessel, which is sealed tight with larger windows allowing one to look in and see the layer of explosive.

The next step is to pump all the air from the vessel, creating a high vacuum inside.

"We then cool the sample by pouring liquid nitrogen into this Dewar vessel," Wight said. If it were not a vacuum, when the interior cooled the windows would be fogged with condensing air.

The liquid nitrogen keeps the sample at 77 Kelvin, or 77 degrees above absolute zero. That means the sample is brought down to minus 321 degrees Fahrenheit. (In fact, Wight said, sometimes the sample is cooled to only 20 Kelvin.)

Carefully the tube and its target sample are mounted on a work bench, lined up in the focus of a series of reflectors that will aim a powerful carbon dioxide laser right onto the sandwich. The laser will deliver 60 kilowatts of energy instantaneously.

"We zap the thin film just once with the laser," Wight said. This blast of concentrated laser light shoots up the film's temperature to 1,000 degrees Fahrenheit - a sudden, shocking, blistering increase from its frigid state.

The intense cold of liquid nitrogen quickly reasserts its grip.

"We get very, very fast cooling," Wight said. How fast? "It depends on the thickness of the film but . . . less than a thousandth of a second."

For an instant an explosion begins and just as quickly it ends as the temperature plunges. The sample's reaction halts in its very first stage.

"Think of it as a stop-frame movie," Lofy said. "You freeze the motion."

The sample then goes into an infrared spectrometer, whose sophisticated readings will analyze the material and determine what compounds are present.

Figuring how those compounds formed helps the scientists understand how a particular explosive happens on a molecular basis.

To see what happens next in the process of exploding, the sample is returned to the laser's focus and given another blast. "We can incrementally advance the reaction," Lofy said.

Using the stop-action technique, Wight discovered the explosive NTO detonates by a strange mechanism. "We were able to show that one NTO molecule attacks another as the first step in the reduction sequence," he said.

An explosion begins when an "insult" as unstable material breaks down chemical bonds. The insult can be by impact, heat, friction or a spark.

In this case the laser's heat breaks apart a chemical bond, freeing an oxygen atom that then attacks an adjacent molecule of the same material. The oxygen cracks a carbon atom out of its molecule and bonds with it, forming carbon dioxide.

The rapid formation of gas like CO2 is what gives an explosion its impetus. The reaction accelerates through the material, creating a blast.

Lofy's research is focused on material called HMX, used as a high explosive and as an additive to solid rocket fuel. He has made microphotographs of crystals of HMX that were stopped in the act of blowing up. The crystals are only one-thousandth of an inch across, but high magnification shows intricate detail of their structure.

"HMX seems to burn from the inside out," Wight said.

"The gases are making their way to the surface," Lofy added.

When no reaction has taken place, a microscopic crystal of HMX looks something like a quartz structure. When it has progressed half-way, the surface is curdled and bubbly, showing that gases have tunneled out from the inside.

Lofy thinks of it something like lava that is bubbling, releasing gas.

"You've got a solid that's got a gas inside of it and the surface is somewhat liquefying," he said.

"It's a real solid but it's very spongy," Wight added.

At a later stage, the crystal's original shape is almost unrecognizable, hidden within a froth of material. At that point, Wight said, "it's about the same size of the original crystal, but it's actually about 80 percent gone."

When it progresses a little further, the crystal foam becomes insubstantial and vanishes in a puff of gas. The explosion is releasing the gas that can send a solid-fuel rocket into orbit or demolish a building.

C-SAFE is part of a $40 billion program announced last year by DOE to use advanced computations to simulate such complex reactions as nuclear blasts, the ignition of jet fuel in an airliner crash, the shelling of an explosives storage depot by terrorists, and the burning of a building that contains high-energy fuels.

Computers will generate a view of the environment to be tested. The computer will break pieces of information into billions of "bricks" of matter like air, fuel, aircraft seat fabric, metal superstructure. The data base will predict how all of this will behave in the presence of heat, pressure and flying objects.

An analysis code will show how the billions of elements interact during an explosion, and the computer will translate it all into graphics that a person can watch.

Scientists will be predicting "the flames and the spread of the fire, the toxic gases, the whole bit," Wight said.

Of the five centers that DOE is funding around the country, only one - the University of Utah's - has a corporate partner. The partner is rocket-maker Thiokol, the Utah company whose solid fuel boosters have been lifting the space shuttle into orbit for decades.

"They realized the capability to validate their computer code was going to be vital to their operation," said Jerry Hinshaw, manager of the Energetic Materials Research Department at Thiokol's main plant west of Brigham City. "If they have a computer code that doesn't predict the real world it wouldn't be good for much."

Scientists will test reactions at Thiokol's test facility in a remote canyon on its property, part of the research and development lab. The company already carries out rocket motor tests there and sets off experimental explosions and fires.

"Of course, we instrument all these things," Hinshaw said. Many recording devices operate during each experiment, including high-speed cameras and information collectors that can gather 60 or 70 channels of data. "We can collect data at a millionth of a second," he said.

Both Thiokol and C-SAFE think the partnership will help both. The U. gets validation of computerized models while Thiokol gets computer code simulations that can "save a lot of money, save a lot of time and might even add some accuracy," Hinshaw said.

Besides, he said, "it's a lot of fun" for scientists from industry and academia to interact.