A sophisticated electronic sensor buried in hardened metal shells at the tip of a growing number of America’s ballistic missiles reflects a significant achievement in weapons engineering that experts say could help pave the way for reductions in the size of the country’s nuclear arsenal but also might create new security perils.

The wires, sensors, batteries and computing gear now being installed on hundreds of the most powerful U.S. warheads give them an enhanced ability to detonate with what the military considers exquisite timing over some of the world’s most challenging targets, substantially increasing the probability that in the event of a major conflict, those targets would be destroyed in a radioactive rain of fire, heat and unearthly explosive pressures.

The new components — which determine and set the best height for a nuclear blast — are now being paired with other engineering enhancements that collectively increase what military planners refer to as the individual nuclear warheads’ “hard-target kill capability.” This gives them an improved ability to destroy Russian and Chinese nuclear-tipped missiles and command posts in hardened silos or mountain sanctuaries, or to obliterate military command and storage bunkers in North Korea, also considered a potential U.S. nuclear target.

The increased destructiveness of the warheads means that in some cases fewer weapons could be needed to ensure that all the objectives in the nation’s nuclear targeting plans are fully met, opening a path to future shrinkage of the overall arsenal, current and former U.S. officials said in a number of interviews, in which some spoke on the condition of anonymity to discuss sensitive technology.

Production of the first of many high-yield nuclear warheads containing the gear, developed over the past decade at a cost of billions of dollars, was completed in July for installation on missiles aboard Navy submarines, the National Nuclear Security Administration announced. It follows the development and installation of similar fuzes — designed by the same nuclear laboratory — on hundreds of smaller-yield submarine warheads in a program completed in January 2019. After the Air Force installs some of the same technology aboard new land-based missiles slated for deployment by the end of the decade, it will be deployed on more than 1,300 warheads in the U.S. arsenal.

The Defense Department has publicly described the components as a routine engineering improvement to the W88 series of warheads that provides no substantial new military capabilities. Air Force budget documents provided to Congress describe it as a “form, fit, and functionally equivalent replacement” for existing nuclear warhead fuzes. But those familiar with highly sensitive nuclear planning say it will make the warheads significantly more damaging than previous weapons.

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“It’s an astounding piece of technology,” said mechanical engineer Paul J. Hommert, who directed the government-owned Sandia National Laboratories during the initial years of the technology’s development by a team of several hundred people on its New Mexico campus. He said that while existing U.S. weapons are highly accurate, the sensors the lab created are even better at computing the best moment for a blast to be ignited to produce the highest pressures on targets. They accomplish this even while the warheads approach at speeds that other experts have said exceed 15,682 miles per hour.

Hommert said he agreed with others that there are a lot of deeply buried installations, like command posts, that “these will give you a better chance of holding at risk.” He called it an “underappreciated” enhancement.

Georgetown University professor Keir A. Lieber and Dartmouth University associate professor Daryl G. Press, a consultant to the Defense Department, have estimated in an article that the fuzes have roughly doubled the destructive power of the U.S. submarine fleet alone.

This shift in weapons capabilities has both military and political consequences, current and former officials and analysts said. On one hand, the leaders of target countries, knowing that U.S. nuclear strikes are likely to be effective in destroying their weapons, might be more deterred from taking provocative actions that could draw a U.S. nuclear attack, some said.

Others worry, however, that those leaders — knowing that many of their protected, land-based nuclear weapons and associated command posts could not escape destruction — might be more prone to order their early use in a crisis, promoting a hair-trigger launch policy that could escalate into a general cataclysm.

Physicist James Acton, who codirects the nuclear policy program at the Carnegie Endowment for International Peace and has written extensively about the need to avert unnecessary conflicts, said that efforts to modernize the nuclear arsenal should be more focused on ensuring the weapons’ safety, security and reliability, and less on goosing up their accuracy.

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“If China or Russia believe in a conflict or a crisis that we are going to attack or destroy their nuclear forces and command posts, that gives them an incentive to use nuclear weapons first, or to threaten their use,” he said.

The warhead fuze and its accompanying sensors and computers are embedded in a stubby capsule about two feet high and a foot wide, compact enough for Hommert to carry a model with him to a congressional hearing in 2014. There, he said they would be installed on three new types of warheads atop land- and sea-based missiles as well as, in part, a warhead to be carried by U.S. F-16 and F-35 warplanes deployed in Europe.

Hommert and other advocates of the technology emphasized that by deploying a single new component across the warhead force — a rare exception to the long-standing insistence of the Navy and the Air Force on using unique warhead designs — the government would save around $170 million. But in practice, the decision to field a common device backfired, when a $5 capacitor in the fuzing system that stores and generates electrical current tested poorly and had to be abruptly replaced by a more expensive device in hundreds of the modules. The resulting production delays set the entire effort back about a year and led to a roughly $750 million hike in its budget.

Widespread installation of the fuzing system nonetheless has aroused little controversy on Capitol Hill, partly because both Democratic and Republican administrations have depicted it as a slight modernization of a single component that they say doesn’t violate a 2010 promise by President Barack Obama to forswear the development of new nuclear weapons or their modification to support new military missions.

Hans Kristensen, who monitors such technological efforts for the Federation of American Scientists, a nonprofit group in Washington, D.C., says that the warhead improvements in total look uncomfortably like new designs. He says in some ways this is not surprising: As the U.S. arsenal has shrunk by roughly a third because of arms agreements struck in the past two decades, “the engineers and weaponeers began looking for ways to enhance the capabilities of the weapons that would be left.” And the results, he said, “are so far removed from the Obama era’s limitation that [they are] … one step short of a new nuclear weapon.”

Citing the sensitive nature of the technology in the assembly, the Sandia lab declined to make a member of its staff available to discuss it; nor would the National Nuclear Security Administration in Washington, which funded the work. But a Sandia employee overseeing the work, Dolores Sanchez, was quoted in a lab news release in August describing the assembly as “the brains of the warhead … It looks for the correct code and the correct environmental signals that will unlock the system, and it also ensures that it’s an authorized flight. In short, it makes sure it always works when we want it to and never when we don’t.”

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The government is also simultaneously modernizing virtually all the launchers for these warheads, including key elements in the U.S. missile, bomber and submarine force, requiring an investment of $634 billion over the next decade and bringing the complete cost of the nuclear arsenal to roughly $1.2 trillion over the next three decades.

Arms reduction agreements between the United States and Russia have typically measured the relative military might of both nations by the numbers of nuclear weapons they held, not how destructive the weapons were.

But Navy Adm. Charles A. Richard, who commands the U.S. Strategic Command that stewards the nuclear arsenal, told the House Armed Services Committee in April that “the size of a nation’s weapons stockpile is a crude measure of its overall strategic capability … It is necessary to consider the capability, range and accuracy of the associated delivery systems.”

Richard’s words were intended to rebut any claims that China’s nuclear arsenal — which has an estimated 350 warheads, or less than a tenth of those deployed and stored by U.S. forces — poses a comparatively small threat to America. “I have no choice but to view China as a significant strategic nuclear threat,” he said.

But other experts say the same conclusions can be drawn from the improved capabilities of U.S. forces. Their enhanced nuclear killing power justifies taking a look at the plan to spend a trillion dollars on its modernization, operation and maintenance, or at the need to keep so many warheads in the stockpile, they say.

The fuze will be “more reliable, almost certainly,” said Michael Elliott, a former nuclear bomber weapons system officer who was deputy director of strategic stability for the Joint Chiefs of Staff and previously worked on nuclear plans for the U.S. Strategic Command. “When you improve reliability, you improve effectiveness, and that drives up the probability of success,” which could mean that “fewer warheads are needed,” including fewer required in a stockpile reserve that others say currently has 2,000 warheads. But Elliott added that any decision to reduce warheads should also be based on “the projected strategic situation and health of our forces.”

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“Our hard-target kill capability was good before, but it’s great now,” said Jon Wolfsthal, a key White House adviser on arms control and nuclear issues when Joe Biden was vice president. He said the improvements had already helped convince the Pentagon’s military leaders to agree that the nuclear force could be smaller than it was then and is now.

Obama made this verdict public in June 2013 after a comprehensive classified review, through a statement affirming that even after the New START limitations were fully met, “we can ensure the security of the United States and our Allies and partners and maintain a strong and credible strategic deterrent while safely pursuing up to a one-third reduction in deployed strategic nuclear weapons” beyond what that treaty required.

Wolfsthal, who is now a senior adviser to the Global Zero advocacy group, which seeks the eventual elimination of nuclear arms, said the military’s support for this reduction of a third — or about 500 warheads — from the current arsenal was determined in part by a recognition that the U.S. arsenal was becoming more accurate and in part by the fact that the total number of targets that America needed to destroy in Russia had declined.

Wolfsthal said that in his view the reason Obama didn’t implement these approved reductions was political, rather than military. Obama and Biden, he said, opposed acting unilaterally because they hoped to persuade Russia to act similarly, contributing to an overall reduction in global nuclear risks. He said he thought it was “crazy” to be spending so much on new weapons when “we could live with a smaller force.”

So far, the Biden administration has said little about its larger goals for the nuclear arsenal, besides affirming in budget plans that it intends no major change in the modernization programs created by Obama and continued or expanded by President Donald Trump. An internal administration review of the nation’s nuclear posture is just getting underway at the Pentagon, and its leadership recently changed, drawing expressions of concern from arms control advocates that the new review will only continue the status quo.

Andrew Weber, the assistant secretary of defense for nuclear, chemical and biological defense programs from 2009 to 2014 and chair of the Nuclear Weapons Council that approved development of the fuzes, said that in his view their deployment reduces the need to keep developing some smaller nuclear weapons slated for deployment in the next decade.

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New air- and sea-launched cruise missiles in particular, he said, are not necessary, and will undermine deterrence because they are stealthy, surprise attack weapons that will make opponents nervous enough to adopt hair-trigger launch policies. Since they can be deployed with both conventional and nuclear warheads and it is impossible for opponents to tell the difference, their use could cause unintentional escalation from a conventional to a nuclear war.

Those two programs are estimated to cost more than $35 billion. It’s time, Weber said, to stop “replacing everything mindlessly.”

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This article is from the Center for Public Integrity, a nonprofit, nonpartisan investigative media organization in Washington.