It may sound strange to say that nuclear weapons must survive radiation. But as part of Sandia’s role in ensuring the nation’s stockpile is safe, secure, and effective as a deterrent, it must make sure crucial parts can function if they’re hit by radiation, especially a type called fast neutrons.

Sandia is responsible for non-nuclear components in all US weapons systems and for overall system engineering and integration: pulling together thousands of components into a weapon. It qualifies systems — ensuring their safety and effectiveness — through computer simulations and testing at unique facilities that mimic radiation environments a weapon could face during deployment or an accident.

Sandia developed a new way to do that after the Energy Department shut down its facility for creating fast neutrons, the Sandia Pulsed Reactor (SPR), when security concerns over its highly enriched uranium increased after 9/11.

The Labs created a science-based project called QASPR, Qualification Alternative to Sandia Pulsed Reactor. QASPR combines computing modeling and simulation, experiments, and technology development, and draws on expertise throughout Sandia, from materials science to transistor fabrication to sophisticated computer science. The idea is to create better radiation-hardened microelectronics for high-voltage transistors, part of a nuclear weapon’s safety electronics, and to offer a way to qualify the electronics without SPR.

Sandia does more modeling and experimental work than ever before to qualify components to survive fast neutrons produced by a nuclear burst, either from an enemy weapon or one of our own exploding nearby, says QASPR project manager Len Lorence (1341).

Both modeling, experimental work vital

“It’s very important both in the modeling and the experimental worlds that you not only get the right result but you get it for the right reason,” Len says. “It’s very important to understand the physics of what’s going on.”

Experiments don’t simply validate computer models. They are key to developing models in the first place. QASPR didn’t have the models it needed when it began in 2005. But researchers had time to work on them because the next reentry system that needed the tools and expertise for qualification was still years away.

QASPR focuses on how transistors that provide gain, which are crucial in some circuits, react to fast neutron radiation and what happens to its gain in less than a second — an eternity in nuclear weapons work. Transistor gain is the amplification of current passing through the device.

Neutron damage can cause gain to plummet. Designers can compensate for that in their circuit designs, but used SPR to check whether their designs operated correctly.

QASPR uses unique facilities for studies

QASPR does similar studies at Sandia’s Annular Core Research Reactor (ACRR), its Ion Beam Laboratory and two non-Sandia facilities. Each provides unique tests and complementary data that improve computer models.

One of the outside facilities is a fast-burst reactor similar to SPR and the second facility tests response to gamma radiation. ACRR, a long-pulse reactor, creates high levels of damage, although its long pulse makes it less ideal. Still, it provides a calibration point, which simplifies modeling and lets researchers concentrate on phenomena associated with rapid changes in transistor gain. The Ion Beam Laboratory acts as a surrogate for neutron radiation because ions can impart the same kind of neutron displacement damage as neutrons. It combines high damage levels like ACRR with short pulses in one facility. However, it only can irradiate a transistor or a few transistors together, rather than a circuit or component like the larger ACRR can.

QASPR also is creating better radiation-hardened microelectronics in Sandia’s Microsystems & Engineering Science Applications (MESA) fabrication complex. Some of those transistors are based on compound semiconductors, known as III-V for combining elements from the periodic table’s columns III and V. Such compound semiconductor transistors are much more resistant to neutron radiation.

QASPR turns in success story even in early years

Researchers spent QASPR’s early years combining modeling and experiments to understand the basic mechanisms of the silicon commercial-off-the-shelf components then in use and studying III-V devices. The III-V technology has matured to the point it has been chosen for current and future reentry system lifetime extension and alteration programs, Len says. The improved technology, along with more robust modeling and experiments, mitigates risk from the loss of SPR.

“It was a success story for QASPR,” Len says. “We are able to provide information that ended up affecting the design for the future stockpile modernization effort.”

Researchers are interested in the design phase because “we can catch things earlier, we can help guide the design, and ultimately do better qualification,” he says.

QASPR’s computer modeling is hierarchal, beginning with studies of materials inside transistors, using fundamental physics modeling and quantum mechanical tools to understand how radiation damage occurs and evolves. Then researchers create a model of how transistor gain changes during and after radiation exposure, using a Sandia-created transistor model code, Charon. Radiation exposure is modeled with a Sandia code, NuGET. Next, the analog circuit level aggregates transistors and devices such as resistors and capacitors as well as ever-changing voltages — a complex world where some devices respond to gamma radiation but not neutrons. Researchers use another Sandia code, Xyce, to model circuit behavior under radiation.

“The hierarchical approach is very powerful, since it allows traceability from a high-level circuit response all the way down to the most fundamental atomistic material level,” Len says.

Thus, QASPR offers important information. “At the circuit level we can be very impactful, so much so that we can help the system qualification process, which was our goal,” Len says.

Three programs fund QASPR. Advanced Simulation and Computing funds modeling, the Nuclear Survivability Engineering Campaign supports much of the experimental work, and the Readiness in Technical Base and Facilities program provides support through MESA, focusing on new radiation-hardened technologies.

QASPR and similar efforts to blend experiment and modeling will be needed as long as nuclear weapon electronics continue to evolve, Len says.

“It’s hard to put in the stockpile the exact same thing that was originally put in the stockpile. At some point it’s not possible, not cost-effective,” he says.