Next-century radiography method to be selected by 2003
By Chris Burroughs
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TARGET PRACTICE -- Peter Menge (9515) takes a close-up look at a used X-ray target. Before being hit with the world's most powerful electron beam, this target was a solid cylinder of stainless steel. (Photo by Randy Montoya)
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"This race has turned out to be a lot of fun," John says.
Until about six years ago it was easy to certify nuclear weapon primaries (the primary is the fission device) by detonating them underground at the Nevada Test Site. These full-scale proof tests unambiguously showed how they worked and whether they were reliable and safe.
Full-scale nuclear testing in the United States was stopped in 1992, forcing scientists to develop a new way to verify weapons' viability -- primarily using computer models validated by radiography.
Today's technologies Today, when testing nuclear weapons, the nuclear pit is removed and replaced with a surrogate material. The now non-fissile weapon is detonated, and an X-ray picture of the interior is taken during the implosion. The X-ray image, formed very quickly to freeze the motion of the moving components and features, is then compared to a computer prediction, providing confidence the simulation reflects the reality of the complex detonation.
In X-ray radiography, a pulse of electrons is accelerated and then focused to a millimeter spot on a target, producing a bright flash of X-rays. Within the flash time of 50 nanoseconds, the beam must produce an adequate dose to see through the imploded object. Unfortunately, no such "perfect" radiographic source as yet exists.
"We need a radiographic capability that will provide accurate information about densities and material positions," John says. "To do it right, the radiographic pictures must come from a number of directions so that the interior material densities can be reconstructed accurately. We almost need a moving picture of the explosion area from all angles at once."
A partial answer to the problem is on its way, John says, in Los Alamos' $250 million Dual-Axis Radiographic Hydrotest (DARHT) facility. Instead of taking just one picture, like the current method, this device will view from two angles 90 degrees apart, providing more information about what occurs when the weapon detonates.
However, John says, it is unlikely that DARHT will provide adequate information to certify the enduring stockpile into the indefinite future.
That, he says, is the reason for the DOE-sponsored three-way technical race. Called the Advanced Hydrodynamic Radiography Program, the competition will eventually result in the construction of an Advanced Hydrotest Facility -- possibly in Nevada.
Sandia, Los Alamos, and Lawrence Livermore are exploring different technologies to determine the best radiographic technique.
Pulsed power research "Sandia chose to pursue the use of pulsed power as our radiographic source because we have long been a leader in this field," John says. "Pulsed power was originally developed by the British to drive flash radiography of nuclear weapon systems. We have leveraged their initial discoveries to support Sandia's nuclear weapon effects mission for the past 30 years, becoming the acknowledged world leader. We can now use these new Sandia techniques to dramatically advance the original radiography mission."
Pulsed power is a way of packaging energy. Capacitors, like large batteries, are charged with electricity for more than a minute. The electricity is then switched through a series of pulse-compression stages and released in 50 billionths of a second, resulting in a 12-million-volt, 100,000-amp pulse. The radiographic load converts this electrical pulse into a tightly focused electron beam, achieving some of the highest power densities ever measured. This extremely bright electron beam is converted to X-rays that can penetrate even the densest of substances.
John's goal for the Advanced Hydrodynamic Radiography Program is to build a system in which scientists can reconstruct a near three-dimensional view of a weapon's interior at eight separate times, forming a moving picture of the detonation process. This would require four accelerators (each 100 feet long, 30 feet wide, and 20 feet tall), driving two axes each, producing a total of 64 views of the weapon's interior.
Other approaches While Sandia is taking the pulsed power approach, Lawrence Livermore will be developing an improved conventional accelerator X-ray source where a low current (2,000 to 4,000 amps) at a high voltage (20-40 million volts) generates the same series of pulses needed for the radiographic pictures.
Los Alamos is investigating the use of proton radiography, which illuminates the object being radiographed with a high-energy beam of protons. While the technology is 30 years old, its use in radiography is a new concept.
As part of the technology race, each lab must demonstrate the fundamental feasibility of its approach, measured against common metrics, and work within the same timeline.
Proof of concept For example, Sandia demonstrated proof of concept in FY96 and verified scaling to the program's goals in FY97. In FY99 and 2000 the Labs will build a Radiographic Integrated Test Stand to demonstrate both full-power single-pulse operation and half-voltage multipulse performance. In 2002, when DARHT becomes operational (demonstrating Lawrence Livermore's conventional accelerator approach), each lab must have completed its technical contract and be ready for a DOE selection process.
John says that people may ask why DOE wants to investigate three technologies instead of immediately adopting just one.
"The reason is that each technology has advocates who honestly believe their approach is the best, fastest, cheapest, and smartest solution," he says. "No one has yet demonstrated the required parameters, but neither have we been able to find any fatal flaws. The certification of nuclear weapon primaries is such an important problem that we must mitigate the technology development risk by competing these most likely approaches and find the best one."
Last modified: November 6, 1998
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