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WASHINGTON -- A senior U.S. defense official last week voiced confidence in a newly defined "reuse" approach to modernizing nuclear warheads that some scientists have called into doubt (see GSN, April 7).
(Jun. 16) -
The tail section of a U.S. B-61 nuclear gravity bomb undergoes testing at Sandia National Laboratories in California. The United States might seek to modernize warheads in its aging nuclear arsenal by switching out certain key components, a high-level U.S. defense official said last week (U.S. Sandia National Laboratories/Natural Resources Defense Council).
As the U.S. arsenal ages, atomic arms in some cases could made more reliable by swapping out selected major components for new ones, according to John Harvey, principal deputy to the assistant defense secretary for nuclear and chemical and biological defense programs.
Under such a reuse approach, previously tested warhead parts might for the first time be combined in new configurations that have never been subjected to full experimentation together. The mix-and-match method might involve, for example, joining the first stage of one warhead, called the "primary," with the second stage or "secondary" of another warhead, if doing so is thought to address aging problems.
"Twelve years ago, I would have said that would have been too high a risk to take, in connection with our ability to assure safety and reliability without testing," Harvey said at a June 11 breakfast event on Capitol Hill. Until last July, he headed the policy planning staff at the National Nuclear Security Administration, a semi-autonomous arm of the Energy Department.
"After 12 years of Stockpile Stewardship, I think we've learned quite a bit and I am much more comfortable with the idea that we might take a pit from one warhead that we have in the stockpile or that has been previously tested, and combine it with the secondary on another warhead," Harvey said. "And even though that agglomerated system hasn't been tested, [I think] that we would still have confidence that such a system would be reliable and safe."
A "pit" is the plutonium core of a nuclear weapon's primary stage. Harvey did not offer examples of specific weapons that might undergo modernization employing a reuse approach, but the next aging warhead being evaluated for overhaul is the Air Force's B-61 gravity bomb (see GSN, May 18).
Under the Stockpile Stewardship effort, the U.S. nuclear weapons complex has inspected weapons in the arsenal to monitor any effects of aging, and performed non-nuclear tests and computer simulation to anticipate problems and devise fixes. Physicists and engineers could then repair or remanufacture aging components without changing their precise design details.
The reuse approach is one of two preferred options for updating the stockpile laid out by the Obama administration's Nuclear Posture Review, a yearlong appraisal of strategy, force posture and readiness completed in April. Under the plan, aging warheads would be modernized as they come up for periodic overhaul, and the methods employed could vary on a case-by-case basis, officials said.
The posture review's other favored approach would be the more traditional process of service life extension, in keeping with the Stockpile Stewardship effort.
Only if those two methods appeared infeasible would a third option -- warhead replacement -- potentially be exercised, but only with presidential authorization and congressional approval, according to the Nuclear Posture Review. The policy is to adhere to a Stockpile Management Program laid out by Congress last year, which aims to improve arsenal safety, security and reliability against malfunctions without explosive testing.
President Barack Obama opposed the former Bush administration's advocacy of a Reliable Replacement Warhead that, in pursuit of similar goals, would have eventually supplanted all the weapons in the U.S. arsenal. Critics of the RRW effort asserted that the stockpile could be kept viable for years to come without introducing new weapons, and that a more modest approach could help Washington take the high road in discouraging nuclear proliferation around the globe.
Congress twice refused to fund the RRW program and, once Obama took office, he canceled the effort.
Even so, some aspects of Obama's own plan for warhead modernization have taken a bit of heat. Swapping out newly remanufactured versions of old designs, in itself, might not prove controversial. However, critics have said the Obama administration has defined component reuse in such a broad way -- allowing for the combination of central warhead features that have never been tested together -- that confidence in the reliability of the stockpile could be imperiled in the absence of new underground testing.
The United States has honored an informal moratorium on nuclear explosive tests since 1992, and the Obama administration said in the posture review that no new tests would be conducted. The president has also said he would seek Senate ratification for the Comprehensive Nuclear Test Ban Treaty, a pact signed by President Bill Clinton in 1996 but not approved on Capitol Hill.
Without the ability to test nuclear warheads to ensure their viability, it makes sense to hew closely to design configurations that underwent tests before the moratorium was put in place, according to Roger Logan, the former head of Directed Stockpile Work at the Lawrence Livermore National Laboratory in California.
"Mixing primaries -- pits and related parts that make the primary work -- and secondaries that were not tested together is a crap shoot," he told Global Security Newswire last week. "We don't have to do it, so why do it?"
A carefully bounded approach of mixing and matching warhead components that can be tested without new nuclear explosive experiments should be fine, Logan noted.
"Those [parts] that were tested together as a system or those that do not affect the process of the nuclear event" could be traded out for different designs without necessarily jeopardizing warhead reliability, he said. "You want to make those changes as far as you can away from the nuclear package itself."
Under debate, though, is the advisability of changes to a weapon's crucial "physics package" -- the components directly involved in setting off a nuclear chain reaction.
At last week's breakfast event, Harvey said that knowledge gained from limited tests conducted at the national laboratory facilities over the past decade or so would minimize any risk associated with swapping out physics-package apparatus.
"Subcritical experiments we've done at the Nevada Test Site, non-nuclear experiments but involving plutonium and high-explosive, help us understand the equation of state of plutonium," Harvey said, referring to plutonium's pressure, temperature and volume during the explosive event. "Some of the work that computers have done would allow us to understand better the boost process, of what happens when plutonium sort of mixes up in that process."
In fact, he said, "we have a much better understanding [of] how nuclear weapons work now than we ever did when we were testing. We didn't need to know how they worked; we could prove that they worked by testing them. Today we have to understand the basic physics in such a way that our folks, who are very conservative, are comfortable with these variations and are comfortable enough to certify them without testing."
"I think John Harvey has it exactly right," Linton Brooks, who headed the Energy Department's nuclear security agency from 2003 to 2007, told GSN yesterday.
"The increase in understanding of nuclear weapons from first principles over the past two decades has been remarkable," he said. "I think the science today makes it perfectly plausible to employ a reuse strategy."
Logan agreed that Stockpile Stewardship has helped the scientific community further develop its comprehension of how U.S. nuclear weapons function.
However, as soon as new physics-package features or combinations are introduced into weapons, they would become wild cards in the quest to certify reliability, which should be based on scientifically known performance rather than best guesses, he said.
He took issue with a number of Harvey's statements, opining that the defense official was overly sanguine and simplistic in asserting last Friday, for instance, that if a "secondary gets enough energy, it'll go." Logan also differed with Harvey's contention that, prior to the testing moratorium, lab officials had little need to grasp exactly how atomic weapons operate.
The nuclear complex management "tends to use ego as a substitute for the diligence of quantifying confidence," said Logan, who holds a Ph.D. in engineering. "There are professional standards for doing this."
If nuclear-weapon program officials "make a step change, like adding a feature or widget, confidence cannot be quantified any more," he insisted. "You can then only declare confidence based on a deep, confident-sounding voice."
Kathleen Bailey, a senior associate at the National Institute for Public Policy, said Harvey may be right that certain changes to existing warheads would do little harm, but she worries that it could set a bad precedent.
"What he says is accurate, as far as it goes," she told GSN in an e-mail response to questions this week. "The problem is that we can make some limited changes without testing and have acceptably high confidence -- or, better-said, limited doubt -- about its success. But that makes it easier the next time to make a political decision that change without testing is acceptable."
Bailey called the approach a "slippery slope."
"When do you know that the changes do not warrant confidence?" she asked.