By Andrew Kishner of NuclearCrimes.org
In mid-September 2012, revelations that the U.S. had conducted two controversial Z-Machine shots, one on August 27th and another 'sometime between April and June' in 2012, provoked condemnation from two Japanese Hiroshima-based organizations. (The Z Machine, which is operated by the U.S. National Nuclear Security Administration (NNSA), the nuclear weapons stockpiling arm of the Energy Department, discharges huge bursts of electromagnetic energy, including incredibly strong X-rays, and has been used six times since 2010 on plutonium fuel). The Hiroshima Peace Memorial Museum, in traditional fashion, reset its 'Peace Watch Tower' to reflect the most recent of the two Z-Machine 'shots.' The tower has two clocks: one marking the days since the bombing of Hiroshima and the other marking the days since the last nuclear test. The curators consider a Z-Machine 'shot' on plutonium a 'nuclear test.'
Another Hiroshima-based nonprofit, the Mayors for Peace, also in traditional fashion, penned a letter of protest for each shot. In his September 19th letter, Matsui Kazumi, Hiroshima's mayor, argued that the Z-Machine x-ray explosive studies on plutonium fuel demonstrate a total lack of U.S. interest in nuclear abolition. Kazumi, who writes protest letters on behalf of mayors for 5,400 cities represented in the nuclear abolition coalition, also said that the NNSA's delayed media communications concerning both Z-machine and sub-critical experiments in 2011 and 2012 raise suspicion about what the U.S. is really up to:
'Though the test did not involve a nuclear explosion, a series of such experiments, one in November two years ago, one each in March, September and November of last year and yet another one this time, can be understood as evidence that the U.S. intends to cling to its nuclear stockpile. Coupled with the fact that you revealed the testing only several months after the fact, in the same fashion as the subcritical nuclear tests you conducted in December 2010 and in February 2011, the test has aroused suspicion regarding your intentions.'
In mid-June 2011, the worldwide community first found out about the 'news' that the NNSA had carried out controversial subcritical nuclear experiments in December 2010 and February 2011 via a tiny chart buried in a PDF file that was published online. Since 1997, the NNSA had pre-announced all subcritical nuclear tests but that 'policy' was abandoned without notice in September 2010 with the 'Bacchus' subcritical test. Each of Kazumi’s letters were addressed to President Obama, who was awarded the Nobel Peace Prize in part for his expressed 'commitment' to a world without nuclear weapons.
First U.S. Subcritical Nuclear Test Involving Warhead Mockup To Happen By Year's End
Sometime this fall, the U.S. Department of Energy's National Nuclear Security Administration (NNSA) will be conducting 'Pollux,' a 'first-of-a-kind demonstration' combining a 'scaled subcritical experiment' with plutonium-239 (the fuel used in nuclear warheads). According to a September 2011 article by Hans Kristensen titled 'Hydrodynamic Tests: Not to Scale,' scaled experiments are 'experiments in an implosion geometry that is essentially identical to an actual warhead design, but reduced in size.' Since the Manhattan Project, the U.S. nuclear weapons program has spent extraordinary amounts of time and money experimenting and studying implosion in nuclear warheads. At the Los Alamos National Laboratory Dual Axis Radiographic Hydrodynamic Test (DARHT) facility, scientists have, since 1999, used this part-electron accelerator to pump powerful x-ray beams through exploding fake 'primaries' to render a series of images that give scientists a better idea of the fluid state of plutonium when it is heated up, compressed and thrown about.
A primary is one of the two main components of a nuclear warhead (1). The primary design was 'patented' during the Manhattan Project as an arrangement of chemical explosives surrounding a spherical 'pit' of plutonium. The non-nuclear chemical explosion causes an inward explosion, or implosion, that causes the 'pit,' or a hollow sphere of plutonium-239, to implode. The implosion of the plutonium-239 causes supercriticality, which is the modern definition of a 'nuclear explosion' (and one used by the Comprehensive Test Ban Treaty).
At the DARHT facility, the implosion experiments aren't nuclear explosions because they involve an isotope of plutonium (plutonium-242) with a lower 'critical mass' than plutonium-239. This low critical mass of plutonium-242 allows nuclear weapons scientists in the U.S. to conduct implosion experiments on full-scale mock-ups of primaries with plutonium without the fear of a nuclear explosion. However, because the 'Pollux' test will involve plutonium-239, a full-scale model couldn't be used since it would obviously result in a nuclear explosion and violate test ban agreements (on nuclear explosions) and so a scaled down model is used. Kristensen writes that regarding 'scaled' subcritical experiments: 'Rather than a full-scale warhead with the plutonium replaced by other material, a one-third to one-half scale model is built that does use plutonium. At a half-scale size, only one-eighth of the plutonium in an actual warhead is required. The smaller amount of plutonium keeps the explosion from beginning a nuclear chain reaction. Eventually, NNSA wants to build scaled experiments to almost three-quarters (0.7) the size of a full primary.'
Pollux is called a subcritical nuclear experiment because it will be held underground and use larger-than-normal (but nevertheless 'small') amounts of plutonium-239 fuel than used in ordinary hydrodynamic tests. Since 1997, the U.S. has conducted 26 subcritical nuclear experiments and these tests have involved the application of extreme compression, high explosives or other shock-methods on plutonium-239 fuel. Pollux will be conducted at the NNSS (previously named the Nevada Test Site) in the U.S. at a newly completed radiographic facility.
The NNSA argues that implosion X-ray studies improve the accuracy of computer models for predicting "weapons performance" - to understand the properties of aging plutonium in nuclear weapons and to learn what would affect reliability when the DOE ever repairs, replaces, retrofits or upgrades existing weapons.
But one needs to look no further than the 1995 DARHT Record of Decision to understand why tests such as Pollux might be worrying to other nation-states. The Record of Decision notes "However, in the event that this nation decides...that new nuclear weapons should be developed...DARHT could be used to assist in the development of weapons or weapons components."(2)
This has been the criticism of subcritical nuclear experiments since their re-introduction in 1997 by the Department of Energy. A declassified document known as the DOE "Green Book," obtained in 1997 through the Freedom of Information Act, indicated that the Stockpile Stewardship program - of which subcritical tests are a part - is not really about stewardship at all, but about new nuclear options(3). The 'Green Book' stated, "In the meantime, future national policies are supported for deterrence by retaining the ability to develop new nuclear options for emergent threats."
If plutonium experiments being conducted on mock primaries across the U.S. nuclear weapons complex could be useful for understanding 'weapons performance' or developing 'new weapons,' how are we to know what our tax dollars are really being used for. While Americans find it easy to trust the government at its word that new nuclear weapons aren't being developed, that might not be so easy for other countries with nuclear weapons or the ambition to develop weapons. Since the Comprehensive Test Ban Treaty is not yet ratified, on-site inspections of U.S. laboratories and nuclear weapons facilities is not an option for those harboring suspicion of U.S. weapons activities. Even when the treaty is ratified, it will not ban such subcritical experiments, which in itself is a problem.
Consider the point of view aired by 42 members of the U.S. House of Representatives in a 1997 letter to President Bill Clinton: 'The fact that these subcritical experiments would be conducted 900 feet underground - a depth sufficient to contain nuclear explosions with large yields - sets a precedent for conducting underground nuclear tests that a test ban treaty violator would find useful...The U.S. is unwisely creating a testing norm under which other nations could justify conducting similar underground nuclear weapons experiments at their test sites. An even more dangerous consequence is that countries with nuclear capability, but lacking the sophisticated testing technology of the declared nuclear weapons states, could be provoked to resume full-scale underground testing.'
So, even though CTBT verification methods would - whenever it gets ratified - easily put out fires of suspicion about U.S. (or other nation's) subcritical testing actions, there is no easy way to prevent countries from going wild with this subcritical 'testing norm.' However you look at it, subcritical nuclear tests, especially those of the 'scaled' variety, need to be banned if we want to curb nuclear proliferation.
1 Most nuclear weapons today are thermonuclear two-stage weapons. The primary creates terrific temperatures and pressures that trigger a reaction in the secondary (in which isotopes of hydrogen are stored) called fusion. This fusion is what makes modern nuclear weapons capable of thousands of times more destruction than the fission weapons used on Japan in 1945.
2 http://www.epa.gov/EPA-IMPACT/1995/October/Day-16/pr-1395.html 
3 http://www.lasg.org/archive/1998/doe-stewardship.htm 
Hans Kristensen article 'Hydrodynamic Tests: Not to Scale' - http://www.fas.org/blog/ssp/2011/09/stockpileplan2011-3.php