Managing an Arsenal Without Nuclear Testing: An Interview With Jill Hruby of the U.S. National Nuclear Security Administration

December 2023

For decades, the U.S. National Nuclear Security Administration (NNSA) and its predecessor agencies at the Department of Energy have been at the center of the technical and political issues relating to nuclear weapons: warhead design and development, explosive testing, and non-explosive techniques to maintain the nuclear warheads in the U.S. arsenal.

The last U.S. nuclear test explosion was conducted in September 1992, and since then, the United States has observed a test moratorium and supported the 1996 Comprehensive Test Ban Treaty (CTBT). Although the treaty has established a norm against nuclear explosive tests, it has not entered into force because eight specific states, including the United States, have not ratified it.

Meanwhile, Russia, China, and the United States are engaging in activities at their former test sites at Novaya Zemlya, Lop Nur, and the Nevada National Security Site, respectively, prompting accusations of CTBT noncompliance and concerns about the possible resumption of full-scale nuclear testing. Recently, Russia took the unusual step of withdrawing its CTBT ratification in order to “mirror” the U.S. status vis-à-vis the CTBT. Nevertheless, Russian President Vladimir Putin has said that Russia will maintain its nuclear test moratorium as long as the United States does.

Daryl G. Kimball, executive director of the Arms Control Association, and Carol Giacomo, editor of Arms Control Today, explored these issues in an interview with NNSA Administrator Jill Hruby. The transcript has been edited for clarity and length.

ARMS CONTROL TODAY: Can you say why, in your technical judgment, the United States does not need to resume explosive testing to maintain the U.S. arsenal or to build new design warheads?

NNSA Administrator Jill Hruby: When the United States signed the CTBT and made the decision to stop doing full-scale nuclear explosive testing, we simultaneously put in place, during the Clinton administration, this process that we refer to as the annual assessment process, by which we evaluate how the stockpile is aging. The three NNSA lab directors do an evaluation every year on the technical health of our weapons, and a major part of the determination is to say whether there is a technical reason to resume nuclear explosive testing. That evaluation has been done for about 27 years and has resulted in a finding every year that there is no technical reason to conduct nuclear explosive testing.

The process is larger than just the three lab directors. The [U.S. Strategic Command] commander also determines whether he or she believes that the stockpile is effective. So, that’s a separate process. I can’t say as much about that because that’s not the process in the NNSA, but from a technical perspective, there has not been a reason to resume testing.

It’s a very considered judgment. It’s a process by which we spend a lot of time making sure we do enough examination of old weapons. There are flight tests, laboratory tests, smaller subcomponent tests, and component testing of elements of our stockpile. We’re confident that the stockpile has the performance, reliability, safety, and security that it needs.

ACT: What is your response to the Russian suggestion that the United States is making preparations for nuclear testing at the Nevada National Security Site?

Hruby: This is the primary reason why we really stepped up talking about what we were doing at the Nevada National Security Site. Everybody makes allegations about everybody else’s activity at test sites, and it makes sense. We have a treaty that says we’re not going to test, so of course, everybody watches everybody else.

The truth is, we have activity going on at our former test site, the Nevada site. We’ve been using it all along for three reasons. One is to do subcritical experiments for our science-based Stockpile Stewardship Program [SSP]. This is part of what we need to do to make sure that our stockpile is behaving and aging the way that we think it is so that we don’t have to do a full-scale test. Another thing that we’ve done consistently at [the Nevada site] is conduct experiments for the nonproliferation program that helps us improve our ability to detect testing. We do this, as many other countries do, to improve our capability to monitor. Those tests are chemically explosive tests. They use conventional explosives; they don’t use nuclear explosives. But they use enough chemical explosives that we can get the seismic activity that’s sort of equivalent to a low-yield test so we know whether or not we could monitor that.

The third thing is that the Nevada site people have done other national security missions not associated with the NNSA but associated with larger national security missions, in particular for the Department of Homeland Security. When the Department of Homeland Security’s Domestic Nuclear Detection Office was active, they wanted to test the monitors that they were putting at ports in the United States and around the world.

Honestly, we have in the last three or four years gotten sophisticated enough with this other set of experiments that we do now, concentrated on the NNSA subcritical experiments, that we are actually investing in significant new diagnostics. We call those projects “enhanced capability for subcritical experiments,” where we’re preparing to be able to do radiography, for example, which we haven’t been able to do before. To set those experiments up in the U1a tunnel [at the Nevada site], which is where we’ve done all our subcritical experiments, meant that we actually had to mine some new tunnels. These are fairly large pieces of diagnostic equipment, so we’ve had to do some new mining. The same thing is true with our nonproliferation-associated experiments. We’ve done some new mining to do some work that has better diagnostics associated with it.

Not to pick on the Russians, but if you’re the Russians or anybody else that is looking at the activity going on at the Nevada site, you’re going to see activity associated with mining. So, as soon as the Russians started saying these things, our sense was, oh well, we understand why they might interpret it that way. We need to be clearer about what we’re doing because we have nothing to hide and we’re not preparing for an underground [nuclear explosive] test. But it’s not a completely unreasonable thing, when you see mining at a former test site, to believe that something could be going on. That’s really why we wanted to just put everything out there and be very straightforward about what we are doing.

There’s one additional reason why we have actually been upgrading the infrastructure. Because of this increased amount of work associated with preparing for these new diagnostic capabilities, we have actually replaced some of the office buildings [at the site].

We’re very happy to be honest and straightforward and transparent about what we’re doing. Then we thought, well hey, if we’re going to be honest and straightforward, let’s just go the whole step and say maybe there’s more we can do in terms of transparency.

ACT: Let me ask one clarifying question about what a subcritical experiment is and what a supercritical experiment is. According to the Department of State, the United States and other governments participating in CTBT negotiations agreed that the treaty “prohibits all nuclear explosions that produce a self-sustaining supercritical fission chain reaction of any kind.” Can you provide any more clarification for the nontechnical expert about how your scientists distinguish between a subcritical and a supercritical experiment?

Hruby: We use a definition of subcritical that adheres to the strictest standard of zero yield and the international standard that we’ve proposed and hope is adopted by everybody that signed up to the CTBT. For the subcritical testing, we do not produce a sustained fission reaction. It’s hard to describe that in non-physics terms, but the difference is that there is not only not a large explosion, but there is also no sustained reaction.

ACT: You said that the United States wants to be as transparent as possible because it is not planning to or is not engaged in supercritical nuclear explosions. How are you seeking to do that? You proposed back in June at the CTBT: Science and Technology Conference that the NNSA is “open to working with others to develop a regime that would allow reciprocal observation with radiation detection equipment at each other’s subcritical experiments to allow confirmation that the experiment was consistent with the CTBT.” Could you describe what methods, technical or otherwise, you are pursuing to demonstrate that the U.S. activities are consistent with the CTBT and to address concerns about these subcritical experiments?

Hruby: We’ve been trying to be transparent. We announce, and we let the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) know, for example, when we do these chemical explosive tests; we let everybody with monitoring stations know. We publish all the results of the experiment. So, we’ve already been doing a lot of the nonreciprocal transparency. We didn’t agree to do reciprocal, but we’ve done lots of things to try to make the work that we do transparent. We also take people on tours of [the Nevada site]. We’ve invited members of the U.S. nongovernmental community to [the site] at the end of November.

What we’ve put on the table is, if other countries that formerly tested were interested in more transparency about the experiments they are conducting—because we know everybody’s doing some activities at their former test sites—we would be willing to do more intrusive things as opposed to just putting out the information. This includes ideas that would enable you to make sure that they didn’t produce a signature associated with a sustained nuclear chain reaction.

I know Arms Control Today just published an article about a verification approach that [Princeton University physicist] Frank von Hippel was involved in, and he had talked to me beforehand about his ideas. We had a team of people also looking at technical ideas and other ideas that would enhance confidence building. So, we could, for example, do video feeds that might build more confidence. Then we could do more intrusive things like radiation detection monitors within the chamber where other people, other countries, could probe. That would be a reciprocity thing that we could both do to allow people to in fact make sure that there wasn’t a supercritical reaction.

We have not provided all the details. Before I announced the potential for greater transparency, we did enough work on it to say, hey, we think this is technically feasible in a way that everybody should be willing to share, that isn’t going to reveal information about the design of their weapons or anything sensitive like that because these are not weapons that we’re working with, they’re just material samples. We think that this could be a great scientific interchange and good confidence-building measure. We’re trying to determine whether there’s enough interest to go further, to put more detailed approaches on the table.

As you know, all monitoring and verification of other people’s work requires both sides to be comfortable with the approach. So, before we go do a lot of work to put a detailed proposal on the technical approach on the table, we are trying to judge the interest. That’s the stage we’re at now. We believe there are multiple ways you could do this that we would be comfortable with. We’re trying to judge if there’s enough interest to put these ideas on the table and begin a dialogue with our counterparts in other countries and have reciprocity.

ACT: In terms of the dialogue, would the Biden administration be open to consulting with the CTBTO about some of these techniques because, ultimately, it is going to be responsible for verifying compliance with this treaty?

Hruby: Absolutely. [CTBTO Executive Secretary] Rob Floyd has been out to the Nevada site. I think he was our last international visitor. We’re willing to have other CTBTO ambassadors come visit. Again, we really have nothing to hide, but we also feel like the benefit of this is if we all do it, not just if one of us does it. That’s where we’d like to go. Rob’s been out, I’m sure we’ll have him out again. We bring the public to [the Nevada site]. We’ll do a special tour for people that are more interested in the subject, hopefully the ambassadors in Vienna. We’ll see if we can work up some momentum and some interest in transparency and reciprocity.

ACT: You say you’re trying to “judge the interest,” but what has been the interest so far?

Hruby: The interest so far is hard to judge. There are obviously people listening because there’s more chatter about it, including comments by the Russians. That being said, it doesn’t seem to be moving in exactly the direction that we had hoped, where people are saying this seems like a good idea and something that is relatively easy to do from a confidence-building measure or technical measures [perspective].

We know the arms control regime is not in a good place. We know that strategic stability isn’t where we need it to be. We would like to get back to real arms control discussions. We would like to get back to strategic stability discussions. That’s not in a good place, but let’s choose something easy, and we consider this quite easy. But so far, I would say we don’t have a positive vibe. What we have is a vibe of, well, okay, put more on the table. So, that’s going to have to be a whole-government decision whether we put more on the table. I can’t decide just to do that by myself, that would be presumptuous. Congress has a role to play in that, the White House has a role to play in that. It’s not just my decision alone. So, what we’ve gotten is,  
it feels a bit more like a challenge than like a discussion.

ACT: Not only has Russia withdrawn its ratification of the CTBT, but there are reports that Russia is making improvements around Novaya Zemlya. Do you interpret these moves as political signaling, an indication that Russia is going to resume testing, or both?

Hruby: I’m not in the intelligence community. I’m not making assessments. My job is to be aware and prepared for actions that the Russians or anybody else may make. That’s why we’re doing the nonproliferation experiments, to get better at detecting seismic activities at former test sites or anywhere in the world. So, I don’t know. I don’t know whether it’s political signaling or they’re getting prepared to test. But I sure would like to have an agreement that we’re going to abide by the CTBT and that we’ll do this together in a cooperative way. I’m trying to nudge it in that direction because I think the CTBT has been a stabilizing treaty and I’d like to see us all continue to uphold that treaty. If there is anything that we can do to help with that, we would like to do that.

ACT: It’s been about a quarter century since the SSP was established and nuclear explosive testing in the United States was halted. Would you say, in your experience previously as a lab director and now as NNSA administrator, that the United States has a better or diminishing level of confidence in the reliability and performance of the warheads and the arsenal? Are we learning more from  
the SSP as it has evolved than we did during the days of frequent full-scale explosive testing?

Hruby: I feel like we know more fundamentally about weapon performance today as a result of our SSP than we knew during the era of large-scale nuclear explosive tests.

ACT: One of your agency’s responsibilities is maintaining the safety and reliability of the warheads, and the NNSA has a very ambitious schedule and plan for modernizing and upgrading existing types of warheads. But questions come up from time to time about whether this refurbishment program is introducing new variables and new components that veer from previously tested designs and concepts. How are you trying to ensure that the warhead refurbishments now planned do not introduce those kinds of variables that raise questions about reliability that could in turn lead to calls for resuming nuclear explosive testing?

Hruby: We have a robust surveillance program, and that starts as we deploy weapons. We don’t wait for the systems to be in for 10 to 20 years and then surveil them. We begin surveillance immediately, and if we uncover any issues with any components, we address those immediately. There is this thing that we fondly refer to as the bathtub curve, where most problems happen very early from manufacturing defects, then things are pretty stable for a while, and then there’s an increase in issues over time as weapons age. So, we try to find all the problems. Again, we do flight tests. We do lab tests. We have a very active surveillance program. Can I guarantee there won’t be an issue that doesn’t require testing? No, that’s why we have the active surveillance program. But so far, when we find things, we can address them in a way that we don’t need testing. Our models and these experimental programs that we do, including the subcritical programs, help us make sure we don’t need to do nuclear explosive testing again.

ACT: The final report of the Congressional Commission on the Strategic Posture of the United States recommends that your agency plan to increase its production capacity beyond the current program of record to meet the two-peer threat from Russia and China. Is that even practical, given that the NNSA is having trouble fulfilling its plan to produce at least 30 plutonium pits for nuclear warheads per year at Los Alamos and at least 50 per year at Savannah River?

Hruby: We’re going through the recommendations of the strategic posture commission report carefully. The administration will be reviewing, as we already do, our nuclear deterrence posture. At the NNSA, we are trying to design for flexibility as we build these new facilities, including the pit facilities that you referred to. The requirement for us was a minimum of 80 pits per year. We have tried to build these facilities so that there’s some room so that if we have to expand capability in the future that we would have the capacity to do that. We don’t want to overbuild, and we don’t want to underbuild, but we need to build flexibility into the way we think about the facilities that we’re constructing now.

We always talk about how we’re trying to build a resilient and flexible enterprise. Flexible means the ability to scale up as suggested in the strategic posture commission report or the ability to scale down without closing things the way we did at the end of the Cold War, which has now caused us to be in a position where we have to start from scratch on some things. Resilience means that we don’t want single-point failures. So, for example, that’s why we’re building a facility at Los Alamos and another one at Savannah River. If anything were to go wrong at either one of those, we would have resilience.