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  • The Bomb and the Nuclear Posture Review

    Posted on April 22nd, 2017 Helian No comments

    A Nuclear Posture Review (NPR) is a legislatively mandated review, typically conducted every five to ten years.  It assesses such things as the role, safety and reliability of the weapons in the U.S. nuclear stockpile, the status of facilities in the nuclear weapons complex, and nuclear weapons policy in areas such as nonproliferation and arms control.  The last one was conducted in 2010.  The Trump Administration directed that another one be conducted this year, and the review is already in its initial stages.  It should be finished by the end of the year.  There is reason for concern about what the final product might look like.

    Trump has made statements to the effect that the U.S. should “expand its nuclear capability,” and that, “We have nuclear arsenals that are in very terrible shape.  They don’t even know if they work.”  Such statements have typically been qualified by his aides.  It’s hard to tell whether they reflect serious policy commitments, or just vague impressions based on a few minutes of conversation with some Pentagon wonk.  In fact, there are deep differences of opinion about these matters within the nuclear establishment.  That’s why the eventual content of the NPR might be problematic.  There have always been people within the nuclear establishment, whether at the National Nuclear Security Administration (NNSA), the agency within the Department of Energy responsible for maintaining the stockpile, or in the military, who are champing at the bit to resume nuclear testing.  Occasionally they will bluntly question the reliability of the weapons in our stockpile, even though by that very act they diminish the credibility of our nuclear deterrent.  If Trump’s comments are to be taken seriously, the next NPR may reflect the fact that they have gained the upper hand.  That would be unfortunate.

    Is it really true that the weapons in our arsenal are “in very terrible shape,” and we “don’t even know if they work?”  I doubt it.  In the first place, the law requires that both the Department of Energy and the Department of Defense sign off on an annual assessment that certifies the safety and reliability of the stockpile.  They have never failed to submit that certification.  Beyond that, the weapons in our stockpile are the final product of more than 1000 nuclear tests.  They are both safe and robust.  Any credible challenge to their safety and reliability must cite some plausible reason why they might fail.  I know of no such reason.

    For the sake of argument, let’s consider what might go wrong.  Modern weapons typically consist of a primary and a secondary.  The primary consists of a hollow “pit” of highly enriched uranium or plutonium surrounded by high explosive.  Often it is filled with a “boost” gas consisting of a mixture of deuterium and tritium, two heavy isotopes of hydrogen.  When the weapon is used, the high explosive implodes the pit, causing it to form a dense mass that is highly supercritical.  At the same time, nuclear fusion takes place in the boost gas, producing highly energetic neutrons that enhance the yield of the primary.  At the right moment an “initiator” sends a burst of neutrons into the imploded pit, setting off a chain reaction that results in a nuclear explosion.  Some of the tremendous energy released in this explosion in the form of x-rays then implodes the secondary, causing it, too, to explode, adding to the yield of the weapon.

    What could go wrong?  Of course, explosives are volatile.  Those used to implode the primary might deteriorate over time.  However, these explosives are carefully monitored to detect any such deterioration.  Other than that, the tritium in the boost gas is radioactive, and has a half life of only a little over 12 years.  It will gradually decay into helium, reducing the effectiveness of boosting.  This, too, however is a well understood process, and one which is carefully monitored and compensated for by timely replacement of the tritium.  Corrosion of key parts might occur, but this too, is carefully checked, and the potential sources are well understood.  All these potential sources of uncertainty affect the primary.  However, much of the uncertainty about their effects can be eliminated experimentally.  Of course, the experiments can’t include actual nuclear explosions, but surrogate materials can be substituted for the uranium and plutonium in the pit with similar properties.  The implosion process can then be observed using powerful x-ray or proton beams.  Unfortunately, our experimental capabilities in this area are limited.  We cannot observe the implosion process all the way from the initial explosion to the point at which maximum density is achieved in three dimensions taking “snapshots” at optimally short intervals.  To do that, we would need what has been referred to as an Advanced Hydrodynamic Facility, or AHF.

    We currently have an unmatched suite of above ground experimental facilities for studying the effects of aging on the weapons in our stockpile, including the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, the Z Machine at Sandia National Laboratories, and the Dual-Axis Radiographic Hydrodynamic Test facility (DARHT) at Los Alamos.  These give us a very significant leg up on the international competition when it comes to maintaining our stockpile.  That is a major reason why it would be foolish for us to resume nuclear testing.  We would be throwing away this advantage.  Unfortunately, while we once seriously considered building an AHF, basically an extremely powerful accelerator, we never got around to doing so.  It was a serious mistake.  If we had such a facility, it would effectively pull the rug out from under the feet of those who want to resume testing.  It would render all arguments to the effect that “we don’t even know if they work” moot.  We could demonstrate with a very high level of confidence that they will indeed work.

    But that’s water under the bridge.  We must hope that cooler heads prevail, and the NPR doesn’t turn out to be a polemic challenging the credibility of the stockpile and advising a resumption of testing.  We’re likely to find out one way or the other before the end of the year.  Keep your fingers crossed.

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