Computing nuclear crash and burn scenarios

A jet aircraft skids along a runway. Its wing slams into a vertical pole, rupturing a fuel tank. The sprayed fuel ignites to create a giant fireball that engulfs the plane. The flames lick at a cruise missile, heating up critical electronic components in the missile's nuclear warhead.

 Thanks to a new computer capable of calculating at a rate of up to 1.8 trillion operations per second, researchers at the Sandia National Laboratories in Albuquerque, N.M., now have a vastly improved tool for simulating various scenarios involving nuclear weapons. Such simulations are designed to reduce the need for full-scale tests of aging nuclear systems.

"It is a very important step in shifting from a test-centered program to a computation-centered program," says Sandia director C. Paul Robinson.

 Built at a cost of $55 million as part of the U.S. Department of Energy's Accelerated Strategic Computing Initiative, the so-called teraflops ultracomputer is the fastest computer in the world. Assembled from 9,072 Pentium Pro microprocessors, originally developed by Intel for use in workstations and servers, the computer covers an area roughly equal to the floor space of a modest home.

 In a December test at Intel's Beaverton, Ore., plant, this computer became the first to calculate at a rate of 1 trillion operations per second, using just 7,264 of its processors. Installation of the full computer at Sandia was completed late last month.

 Even before all the processors had been installed, Sandia researchers were running simulations on the computer. One key effort modeled, in four separate stages, the crash of an airplane carrying a nuclear warhead.

 Preliminary results provide insights into such complex phenomena as wind-fanned fires. The simulations reveal, for example, that air rushing past a column of flames spawns vortices that greatly intensify the fire. The temperatures are highest at the fringes of the inferno.

"The physics involved is really complicated," says Sandia's Carl W. Peterson, who specializes in fluid mechanics. Even the ultracomputer isn't powerful enough to enable researchers to model the full scenario -- from crumpled wing to heated electronics -- in one continuous run.

 At the same time, researchers are discovering they need experimental data to feed into their models and to validate their findings. In the missile simulation, for example, new experiments had to be performed to determine how the special solid foam in which electronic components are packed responds to heat.

"We're finding we have to increase the amount of experimenting, say, on materials," says Robert K. Thomas, who manages the materials and structural mechanics effort at Sandia.

"You must be willing to bet your paycheck on a simulation," says Sandia's Russell Skocypec.