Three engineers lean over a workbench in a lab for Sandia’s Integrated Surety Mechanisms, adjusting levers on a scale model as they check a design. In this case, the plastic model is several times larger than the real thing — tiny weapon parts that are hand-assembled, sometimes using a microscope and tweezers.

NNSA’s Kansas City Plant has built its 1,000th stronglink for the W76-1’s arming, fuzing, and firing (AF&F) system. A stronglink mechanism is aimed at improving the safety of nuclear weapons without compromising their reliability. The first stronglinks were designed at Sandia in the 1960s, based on the premise that weapons must always work when needed, but never work otherwise.

“It’s all about never having a nuclear detonation unless the president has released the weapon for war,” says Integrated Surety Systems manager Marcus Craig (2613).

Weapons in the US stockpile are a deterrent “because our adversaries know they’ll work,” he says. “If they thought we were launching a weapon and there was any chance it wouldn’t work, they’d plan differently.”

The W76-1/Mk4A warhead, deployed on the Trident II D5 Special Weapon System on Ohio-class submarines, uses stronglinks in the AF&F, which provides the arming, fuzing, and firing functions of a nuclear weapon. Stronglinks also will be included in modernization programs for the W88 Alt and B61 Life Extension Program (LEP).

Incorporating safety components

An AF&F consists of an impact fuze, an arming subsystem which includes the radar, a  firing subsystem, and a thermal battery that powers the system. The firing subsystem, which provides energy to detonators, incorporates three safety components: a launch accelerometer and two stronglinks.

The missile’s boost phase causes the launch accelerometer to close multiple circuits, but won’t close them if the missile doesn’t perform as required. Once the circuit closes, the missile sends a signal, which then provides power to wake up and operate the system.

Stronglinks are in place in case something goes awry before or after launch, such as an accident that bypasses the launch accelerometer or other parts of the system and mistakenly supplies power that could set off the detonators, Marcus says.

He likens the firing system to a vault. Each stronglink receives a unique signal that opens the vault doors — shutters inside the weapon, one electrical and the other magnetic. Opening the shutters allows energy from the thermal battery to flow to a capacitor, which stores the energy capable of initiating the detonators. “That does not happen unless we get the right very, very unique signals that allow the shutter to open,” Marcus says.

These safety components use mechanical parts because such parts are highly predictable in an accident. Like any system, the more that goes in, the more opportunity there is for something to go wrong.

“So our challenge is trying to provide this required safety without affecting reliability,” Marcus says.

Effort involves mechanical engineering

The work touches many mechanical engineering disciplines. “We are very broad across disciplines, and because it’s a safety component we have to be deep in all these areas, too,” Marcus says.

It’s hard to imagine a better situation for mechanical design engineers, he says.

“We use fluid mechanics, we use strength of materials, we use materials science,” he says. “We have to understand the effects of manufacturing processes: welding, machining, soldering, encapsulation. We do a lot of complex tolerance analysis, we get into dynamics. As a mechanical engineer, all those classes you take in mechanical design, we practice most of them.”

With one generation of weaponeers retiring, the new guard takes over significant responsibilities, Marcus says.

To illustrate, he picks up a glass jar of jelly beans from a table in his office, describing it as a widget that includes many green, red, white, yellow, and multicolor parts, plus the glass that encases them, a sticker, and the lid. All of it requires a profound understanding of the individual pieces and how they work together as a final product.

“Some of these parts are nuclear safety-critical, and the engineers have to identify those parts and ensure that we rigorously understand their design and the materials and processes used in manufacturing the design. With this knowledge, we are confident the white jelly bean with a bit of yellow is exactly what it’s supposed to be. Then when the final assembly passes the rigorous tests we have in place, our product is ready to be integrated into the weapon that becomes part of the nation’s stockpile,” he says.

As an example, consider the jelly bean jar as a stand-in for the electrically activated stronglink, and think about an accident that crushes part of the weapon and burns away parts of its exterior. “What are we going to do to make sure that our safety components continue to keep the vault doors closed, even when there’s an accident? It takes a lot of analysis, knowledge, and engineering judgment,” Marcus says.

His young staff must design against the threat and ensure that the component’s function is not compromised — even in an accident — while meeting budgets and delivery schedules, he says.

Reliability requirements

Development work to modernize the W76-1 began about 2000. Production began in 2006 and will continue for years. Manufacturing the AF&F is complicated by the fact that reliability is paramount and every part must meet stringent performance and safety requirements.

“The ability to produce a design that’s manufacturable, repeatable, doesn’t affect reliability, etc., it’s pretty challenging,” Marcus says.

“We’re building hundreds and hundreds of these things and they all have to be identical and they all have to work,” he says. “It adds another layer of complexity to the design process.”

Every step is done by humans, not robots, from cleaning and testing parts to welding and assembling parts to encapsulating, soldering, and machining parts, Marcus says.

Improving production, design systems

The W76-1 program office holds weekly videoconferences with its Kansas City Plant counterparts and monthly reviews at Kansas City to address the current production status and issues. Studying the production process provides important insights that allow Sandia designers to improve their new designs for programs like the W88 Alt and the B61 LEP.

“We recently had a special review that asked the question, How is our production going, where are we seeing the challenges? Where are we finding ourselves revisiting manufacturing issues over and over again? Is there a recurring theme?” Marcus says. 

Production experience and reviews have shown the Sandia team where designs can be difficult to manufacture, and they help identify ways to improve those designs in the future, Marcus says. “We’re learning that there are opportunities with development programs now to really flesh out precisely what we’re measuring, what features in the design affect those measurements, and where small changes can make the design easier to manufacture and really provide benefit throughout the production cycle.”

The reviews have concluded the electrical stronglink design, with 180 unique parts and 250 separate pieces, is robust and manufacturable, and that small changes can make the next design even better.