Safe, Secure, Reliable Nuclear Weapons: A Discussion with Former Sandia Labs Director Jill Hruby

The United States maintains a robust nuclear weapon stockpile to uphold its strategic deterrent and defense capabilities, but in the words of Spiderman’s Uncle Ben Parker: “With great power comes great responsibility.”

Headlining the latest in the Nuclear Threat Initiative’s Seminar Series, Jill Hruby, former Sandia National Laboratories director and current Sam Nunn Distinguished Fellow at NTI,  explained how national labs complete required annual assessments of the U.S. nuclear weapons stockpile and exactly what it means to keep the most powerful weapons on the planet safe, secure, and reliable.

Why do we assess?

During the Manhattan Project and the Cold War that followed, nuclear testing was the primary method for assessing the reliability and performance of nuclear weapons. Safety and security systems were improved with new systems and military requirements, and these systems were tested both in the lab and in the nuclear tests. However, with the signing of the Comprehensive Nuclear Test Ban Treaty (CTBT), President Clinton recognized the need for a different method to ensure safety, reliability, and performance.

Since then, it has been the responsibility of the national labs to annually assess the safety, reliability, and performance of the nuclear weapons stockpile. Each year, lab directors submit a “letter,” or report, to the Secretary of Energy detailing the outcomes of the assessment. The Secretary then submits his or her own letter to the President along with each lab director’s unrevised reports. Separately, it is the responsibility of US Strategic Command (STRATCOM) to annually assess the stockpile’s military effectiveness.

Writing the Assessment

The assessment process is continuous and goes on year-round. As lab director, Hruby received about 100 hours of briefings on assessment activities and then wrote the letter – a process that could take four to six weeks. The letter is an opportunity for the director to report the results of the assessment and discuss important or concerning trends in the data, lab activities, or elements of lab culture.

It’s easy to understand why the President and the scientists and weapons complex workers should be care about these assessments – but you should care, too. They don’t always get much publicity, but past close calls have hit, quite literally, very close to home.

What is “Safe, Secure, & Reliable?”

Safety refers to how likely it is that a nuclear weapon will produce yield (i.e. create an explosion) when it’s not supposed to. To date, no nuclear weapon has ever fully detonated when it wasn’t intended, but some incidents have scattered nuclear materials. These are called “broken arrows” and they refer to accidents like The Damascus Incident, where a maintenance worker dropped a heavy ratchet down a nuclear silo, rupturing the missile’s fuel tank, and ultimately causing an explosion that launched the missile and nuclear warhead into the surrounding Arkansas countryside. Luckily, in that instance, a safety mechanism within the weapon prevented the nine-megaton warhead from producing a nuclear yield. But this was not an isolated event. Nuclear weapons have been dropped accidentally on the Carolinas in the United States, as well as Spain and Greenland.

When the national labs address the safety of nuclear weapons, they work to minimize the risk of a weapon exploding by accident in both routine situations (i.e. sitting in a silo) or in abnormal situations (i.e. being accidentally dropped from a plane). They use the Walske Criteria, named for nuclear expert Carl Walske, which stipulates that each weapon in the stockpile must have a one-in-a-billion chance of producing an accidental yield under routine conditions, and a one-in-a-million chance in emergency situations. This requirement led to the safety theme known as Enhanced Nuclear Detonation Safety (ENDS), which implements additional, layered components into nuclear weapons that prevent them from accidentally exploding. Compliance with these standards is part of the labs’ annual assessment.

Also on the assessment to-do list is examining reliability. Reliability refers to how likely it is that a nuclear weapon will produce yield when it IS supposed to. It also refers to the likelihood that a weapon will work as it is designed and that it will detonate at the intended time, with the intended yield, on the intended target. The reliability of a weapon is scored on a scale from 0 to 1 and the assessment requirement is often set at a 0.9 or above. The higher the reliability, in theory, the fewer nuclear weapons are needed to accomplish a military objective.

To determine the safety and reliability of the weapons in the nuclear stockpile, Sandia will take about 13 weapons from each weapon family, remove the physics package (i.e. remove the part of the bomb that makes it capable of a nuclear explosion) from each weapon and then subject it to a variety of tests, modeling, and extremely environments. They crash weapons into walls, drop them out of the air, submerge them in water, spin them around in centrifuges, set them on fire, and even strike them with lightning. Don’t believe it? Check out Sandia’s media library for proof.

Though some may call 13 an unlucky number, it’s the magic number of tests needed each year to meet Sandia’s “90-10-2” rule. The rule goes like this: executing 13 random system-level tests annually will statistically provide a 90 percent confidence level that within two years they will find any defect that has affected 10 percent of the stockpile. Any more would be wasteful, and any less would not be effective enough to detect possible defects.

Finally, there is the security of the stockpile. Security can refer to both the internal and external integrity of the weapon at any given point. In even simpler terms, it means ensuring that the people who have access to the weapons are only the people who are intended to have access to the weapons. (Hruby noted the stockpile security breach of a nun who broke in to a nuclear site and graffitied a secure building containing weapons-grade uranium.) Security is not part of the assessment completed by the national labs, but is part of a separate Department of Defense assessment. Typically, the security of a weapon is the responsibility of whoever is in custody of the weapon at any given time, so all branches take measures to ensure their security, whether or not they include it in an annual assessment.

Fixing Defects

Hruby noted that defects are most often detected at the beginning and end of a weapon’s life, either due to errors made during production or wear and tear over time, requiring constant vigilance related to stockpile quality, but especially as the weapons age. The guaranteed life of a weapon is between 20 and 25 years, and the average age of a weapon in the U.S. stockpile today is over 25 years making the assessment necessary for the safety of those handling the weapons and for the integrity of US national defense. To remedy defects found during an annual assessment, the laboratories engage in life-extension programs that involve taking an old weapon apart, assessing each component for usability and effectiveness, and replacing only the parts that need replacing before putting it all back together. This can help extend the life of a weapon for many years without having to spend the time and resources (not to mention the risk implications) of creating more nuclear weapons to replace aging ones.

How long will life extensions continue to work?

Hruby posed this question to the audience at the end of her lecture. She explained that she’s not sure how long the life-extension programs will continue to serve as the best solution for the aging stockpile. As more and more weapon components need to be replaced, she said, the labs are running out of spare parts that are compatible with those weapons. Additionally, each year, the stockpile loses about 13 weapons from each family in order to complete the assessment. Eventually, the infrastructural demands of preserving the stockpile’s integrity will outweigh the resources available for necessary maintenance.

For more information on the US nuclear stockpile, visit the United States Country Profile on the NTI website. Country profiles are created and updated in partnership with the James Martin Center for Nonproliferation Studies (CNS).

February 19, 2019
Caitlyn Collett
Caitlyn Collett

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