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Vol. 52, No. 2 January 28, 2000
[Sandia National Laboratories]

Albuquerque, New Mexico 87185-0165    ||   Livermore, California 94550-0969
Tonopah, Nevada; Nevada Test Site; Amarillo, Texas

Researchers model neutron generator in hostile radiation environment
Reentry vehicle radiation transport simulated in 3-D for first time

By Chris Burroughs

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Virtual testing of nuclear weapons is reaching new highs as researchers in Simulation Technology Research Dept. 15341 successfully expand the capabilities of the Integrated-Tiger Series (ITS) Monte Carlo radiation transport computer code to simulate a neutron generator in a hostile environment.

The researchers rewrote the 20-year-old code to illustrate in three-dimensional (3-D) models what would happen if the MC4380 neutron generator was exposed to hostile conditions such as X-ray radiation. The computer modeling of the neutron generator ‹ done on massively parallel computers that are part of DOE¹s Accelerated Strategic Computing Initiative (ASCI) ‹ was one of several requirements to qualify the weapon component as radiation hardened.

³This was the first time radiation transport throughout a reentry vehicle has been simulated in 3-D,² says Len Lorence, Manager of Dept. 15341. ³It¹s a real accomplishment.²

The neutron generator is Sandia¹s first new weapon component to enter the stockpile that must be qualified as radiation hardened in the absence of underground testing. Len says qualification thus relied heavily on computational simulation. Researchers worked on the code development during FY97 and FY98, were in the application phase in FY98 and FY99, and submitted results for the qualification early this fiscal year.

The ITS three-dimensional modeling represented only a portion of the hostile environment qualification project that was led by William Barrett (15344). In addition to X-ray effects, the performance of the neutron generator was assessed against all other radiation environments by the project team. Other calculations, physical experiments, and the judgment of experienced engineers were also involved.

As the part of the nuclear weapon that serves as the ³trigger,² the neutron generator produces a burst of neutrons, fragments of atomic nuclei. Their purpose is to help initiate the fission reaction in the weapon. Because it uses short-lived tritium, the neutron generator is one of the components in a nuclear weapon that must be replaced regularly. The Navy will soon be installing new neutron generators in its W76 nuclear weapon missile systems, replacing older components.

ITS Monte Carlo radiation transport codes have been used since the 1970s for one-, two-, and three-dimensional modeling. However, major enhancements in areas such as geometry modeling and preparing the code to run on parallel computers were required to adapt it for the three-dimensional modeling of the neutron generator.

Working with Len in the development and application of the ITS were Ron Kensek (15341), John Halbleib (15341), Wesley Fan (15332), and Gary Harms (6442).

In the MC4380 qualification effort, Len and his group examined the neutron generator in great detail, focusing on the areas of greatest radiation exposure.

³Simulating the effects of the radiation required us to predict the path X-rays will take as they penetrate the reentry vehicle and the neutron generator, determine how high the radiation doses will be, and look for vulnerable spots in the component,² he says. For calculations and analyses like these, the researchers use TeraFlop computers, which are capable of doing a trillion operations per second.

³The work we¹ve done would take half a decade on a PC. On the TeraFlop it takes one day,² he says. Len and his team used two types of radiation transport calculations ‹ ³adjoint² and ³forward.² They start with the adjoint mode to survey radiation attack angles and, thanks to the power of a TeraFlop computer, can identify more than 8,000 different angles. The information can be used to determine the worst-case exposure angle of the X-rays, giving them insight to potential damage levels that can be caused by the radiation. ³Each of the adjoint-transport calculations determines radiation dose for all the potential attack angles at once,² Len says.

After the researchers do the adjoint calculations, they then use the forward mode to predict the 3-D spatial distribution of the dose. This information is inputted into yet another computer code, PRONTO3D, to determine potential mechanical responses caused by the radiation, such as stresses, strains, and deformations.

Among the biggest challenges the researchers faced was using computer-aided design (CAD) tools with ITS. Because ITS was not compatible with CAD models developed by weapons designers, they had to develop a separate stand-alone geometry model of the weapon. This, Len says, took more than a year. The researchers are currently developing a version of ITS that is compatible with CAD.

In addition to enhancing the ITS, researchers are working on another computer code, called Coupled Electron Photon Transport for Radiation Effects (CEPTRE), which will be used to help determine radiation effects in electrical components, such as arming, fusing, and firing units.

Len says that the team of researchers has come a long way in the three years they¹ve been working on the radiation hardening project.

³Modeling the complex environment inside a weapon required us to go many steps beyond where we were,² Len says. ³We have come up with a tool that is very capable and accurate. And along the way, we have learned a lot, including how we can do it better in the future.²

Last modified: January 26, 2000

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