CAT Scanner Provides 3D View Inside Nuclear Weapon Components

Thursday, April 01 2010

The Department of Energy’s (DoE) National Nuclear Security Administration (NNSA) is pursuing a smaller, safer, more secure, and less expensive nuclear weapons complex. Meeting that goal means relying on advanced scientific tools and procedures to assure a high level of confidence in the performance of aging weapons in the stockpile.

In response to NNSA’s needs, Lawrence Livermore National Laboratory (LLNL) physicists, engineers, and computer scientists have developed a new computed tomography (CT, also known as computerized axial tomography or CAT) x-ray system to image nuclear weapon components removed temporarily from the stockpile for inspection. The Confined Large Optical Scintillator Screen and Imaging System (CoLOSSIS) consists of a scintillator (a material that emits light when struck by ionizing radiation); a pyramid-shaped central mirror; four turning mirrors; and four high-resolution, low-intensity, visible-light, charge-coupled-device (CCD) cameras. The system’s software assembles the collected digital radiographs into a large three-dimensional (3D) image that scientists can “walk through” to discover any problems or anomalies.

CoLOSSIS was recently installed at NNSA’s Pantex Plant near Amarillo, TX. The overall CT system includes a 9-megaelectronvolt (MeV) linear accelerator and a tungsten target (built by Varian, Inc.) that produce the x-ray beam, three tungsten collimators that shape the x-ray beam and prevent unwanted scatter, and a positioning table that securely holds the test object and rotates it in precise increments for the CoLOSSIS detector.

A typical CoLOSSIS inspection comprises about 1,500 separate radiographic images taken of an object from all sides. The radiographs are then assembled, using LLNL-developed tomographic image algorithms, to provide a 3D reconstructed image with greater resolution than previously achieved using a 9-MeV x-ray system. Nearly all elements of the inspection system — x-ray source, collimators, positioning table, detector, and tomographic algorithms — contribute to its high-resolution capability.

Pantex is the nation’s only nuclear weapons assembly and disassembly facility. To maintain the reliability of the nation’s nuclear weapons stockpile, weapons are randomly selected and transported to Pantex for testing and evaluation. X-ray radiography is used to probe inside the nuclear pit of a weapon in a nondestructive manner; that is, the weapon does not have to be dismantled. Plutonium pits are one of the most important components routinely inspected. Tests on these pits can reveal structural variations arising from so-called birth defects (manufacturing flaws) or from aging. Potential variations include cracks, voids, gaps, and density variations.

LLNL chemist Pat Allen, deputy program manager of the Laboratory’s enhanced surveillance effort, said, “Without x-ray diagnostic tests at Pantex, we would have to resort to destructive evaluation of these very expensive weapon components. With the right diagnostic tools, we can conserve valuable resources by eliminating some destructive procedures and disassembly operations.”

A core competency at LLNL, nondestructive evaluation is a means of examining and identifying flaws and defects in materials without damaging them. Laboratory engineers routinely use x-ray, ultrasonic, acoustic, infrared, microwave, visible-imaging, and other noninvasive techniques to examine defects, measure properties, and accurately determine part thicknesses of materials for a variety of research programs.