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Nuclear Testing in Tanks:
Subcritical Nuclear Tests Resume at Los Alamos

Tests May Help Certify New Trident Warhead

Greg Mello & Andy Lichterman
June 1999

Above Ground Contained Plutonium Explosive Tests Resume at U.S. Weapons Lab

Sometime in the last few months, Los Alamos National Laboratory in New Mexico restarted a clandestine program of above ground subcritical implosions of nuclear weapons primaries, code-named "Appaloosa."  These plutonium implosions are conducted above ground at Los Alamos in special steel tanks six feet in diameter.  The $40,000, two-inch-thick tanks, manufactured from a special steel developed by the Navy for submarines and ship armor, are deformed by the explosion and are used just once.  The tests use high-speed flash radiography to accurately measure the size, shape, and density of the imploding pit.

Traditional implosion hydrotests -- so called because the imploding metal is liquefied by explosive-driven shock -- use a mock pit made from depleted uranium or other metal.  These tests, by contrast, use real plutonium, and so they are able to test not just the exact shape and size, but the precise surface finish, the same welding and joining processes, and all the exact metallurgical properties, both static and dynamic, of the pit in a real weapon.  This exact simulation is possible because pits for the Appaloosa program can be manufactured using a specially-prepared isotope, Pu-242 (code-name: "Cider") which has a bare alpha-phase critical mass of 100 kilograms, ten times that of Pu-239.  After the shot, the Pu-242 can be recovered and purified.  It is virtually identical to Pu-239 in every other way.

This means that an exact copy of a nuclear weapon plutonium pit or "trigger" can be fabricated from Pu-242 and imploded without a nuclear explosion, allowing both extensive testing of actual pit designs and collection of data for use in further nuclear weapons simulations.  Laboratory spokespersons have said that the test program now beginning will use only Pu-242, and not Pu-239.  Sub-scale Pu-239 vessel implosions have also, however, been done at Los Alamos.  Once an accurate physical history of the implosion is measured, the attendant nuclear processes can be modeled and the yield of the primary predicted.

U.S. Weapons Laboratories Continue to Expand Their Capability for Nuclear Weapons Hydrodynamic Testing

The size of the U.S. stockpile of Pu-242 is not publically known with any accuracy, but it is growing.  The processing of irradiated Mark 42 targets at the Savannah River Department of Energy plant to prepare Pu-242 is funded at $4.2 million this year.  Pu-242, together with flash radiography that is capable of producing precision high-speed images of the inner contours of an imploding pit -- called "core-punching" -- gives weapons designers the ability to precisely test full-scale implosions of exact copies of nuclear weapons.

The United States has two powerful flash X-ray facilities operational at the present time, one at Los Alamos and one at Livermore.  A third, the Dual Axis Radiographic Hydrodynamic Test facility, (DARHT) is scheduled to come begin experiments with its first axis this summer at Los Alamos.  Both Los Alamos facilities can perform contained Pu-242 tests.  All three facilities can take multiple exposures of a single implosion.

DARHT is expected to have a completed second axis by the end of 2002, barring further schedule delays, enabling precision radiography from two directions.  Tens of millions of dollars have already been spent on design of a fourth facility, the so-called Advanced Hydrotest Facility (AHF), which is expected to have several diagnostic axes when it comes on line near the end of the first decade of the next century, at an expected cost of anywhere from $400 million to $1 billion.

Secrecy Still the Rule for Key U.S. Nuclear Weapons Testing Activities

Overall, the history of the U.S. Appaloosa program is shrouded in secrecy.  It is not known how many explosions have been conducted, what the size of the explosions has been, how many of those explosions involved the special Pu-242 isotope, or what the purposes of the experiments have been.  We believe the program was idle, but for one test, during the 1982-1992 period, if not until last year.

Even so, the restart of these subcritical tests has been expected.  When lab watchdog groups (including the Los Alamos Study Group) sued for an environmental review of the DARHT in 1994, we received an admission that DARHT hydrotesting would include plutonium hydrotests in vessels.  The impact analysis subsequently produced for these tests was classified.  Any mention of the Pu-242 isotope was itself classified until January of this year.

According to the Albuquerque Journal, the classification barrier was maintained by the DOE Office of Nonproliferation.  DOE Defense Programs saw no need to keep "Cider" a secret, since the properties of Pu-242 and the fact that it was being produced were already well known.  The classification thus did not arise from technical proliferation concerns, but from politics.  DOE may have feared that any announcement that the U.S. could conduct very useful full-scale nuclear weapons pit tests without technically violating the CTBT would cause international political fallout, adding to the serious repercussions that its underground subcritical tests (called "UGTs" at Los Alamos) have already occasioned.

News of the tests began to leak out on November 17, 1998, when the Defense Nuclear Facilities Safety Board (DNFSB) raised safety questions regarding the start-up of an unspecified "classified activity" at Los Alamos National Laboratory (LANL).  The December 17, 1998 response from Dr. Victor Reis, Assistant Secretary of Energy for Defense Programs, was likewise classified except for its cover letter, which was provided to the Los Alamos Study Group in response to a Freedom of Information Act (FOIA) request.  Reis' letter made it clear that the "classified activity" was in fact plutonium explosions in vessels, as subsequent admissions from DOE and LANL officials confirmed, together with the fact that Pu-242 was being used in the tests.


The Appaloosa tests, especially in the context of other ongoing experimental efforts, raise several questions. First, what exactly are they for?  One plausible purpose is to help certify new pit production process at Los Alamos.  It is doubtful, however, that these tests are strictly needed for this purpose.  A certain second purpose is to certify new warheads.

According to documents obtained by the Los Alamos Study Group using the FOIA process, LANL has a 2003 deadline for certification of a new warhead for the Navy's Trident D-5 (Trident II) ballistic missile.  It is the first admittedly "new" warhead expected to be certified in the U.S. without nuclear explosive testing.  The Lawrence Livermore National Laboratory in California also is working on a warhead design for this missile, which will use existing plutonium pits.  By contrast, the Los Alamos design, if selected, would require a manufacturing campaign that could involve up to nearly 3,000 plutonium pits, assuming no changes in the current floor imposed by the U.S. Congress on nuclear weapons stockpile size.  The Los Alamos design is to be certified over a three-year campaign of hydrodynamic testing, which reportedly began last year.

Los Alamos has also recently begun two other series of experiments on dynamically shocked plutonium.  In one series, the so-called plutonium "mini-flyer" experiments, a laser is used to accelerate a metal plate that strikes plutonium at speeds of circa 350 meters/second.  These experiments can be conducted on a one-day turnaround basis, giving very rapid data acquisition.  A second series of tests are being done at Los Alamos' high-speed plutonium "gas gun," apparently using coupons cut from newly-manufactured pits.

In addition to these three series of tests, the underground subcritical tests at Nevada -- the most recent of which reportedly involved two halves of a newly-manufactured pit for the modern W-88 missile warhead made at Los Alamos -- are slated to continue for the foreseeable future.

These four categories of dynamic experiments - underground subcriticals using weapons-grade plutonium; above-ground subcritical Pu-242 tests; miniature, rapidly repeatable mini-flyer experiments, and gas-gun experiments - along with a wide battery of static laboratory tests, add to a more than a half-century of experimental data on pit performance and plutonium metallurgy.  Together, they comprise a highly advanced and redundant suite of techniques that appears to go far beyond what is required to maintain existing nuclear weapons.

These activities raise a number of questions:

  • What is the best way to track contained vessel tests?

  • What countries are conducting tests of this kind?

  • How can it be assured that tests deemed subcritical have no nuclear yield?

  • Are these tests necessary for any purpose consistent with effective nonproliferation and test ban regimes?

  • What is the significance of an increasingly sophisticated array of nuclear weapons component testing and simulation facilities and techniques for the Nonproliferation Treaty Article VI commitment by the nuclear weapons states "to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament...?"

In a climate where tensions among the nuclear weapons states are rising, it is time to scrutinize closely the activities the nuclear weapons states claim they need to maintain their arsenals, before today's virtual arms race once more turns real.


  • The United States is resuming a program of above ground plutonium explosive testing in containment vessels at the Los Alamos National Laboratory in New Mexico.  These tests will use Pu-242, a plutonium isotope with a higher critical mass than weapons grade plutonium.  This will enable nuclear warhead designers to test devices which closely resemble full scale nuclear weapon primaries, the first stage of a thermonuclear warheads.

  • Although the U.S. Government is no longer conducting underground nuclear tests, it is pressing forward with a vigorous program of nuclear weapons component testing and simulation.  This program, known as the "Stockpile Stewardship and Management" Program, will cost more than $45 billion over the next decade, and will include the construction of several new nuclear weapons component testing and simulation facilities.

  • For additional information go to:  Stockpile Stewardship & Management.

  • A key part of this program is so-called "subcritical" tests in which fissile materials, usually plutonium, are explosively tested.  This allows further knowledge about the functioning of nuclear weapons components to be gained.  Subcritical tests, because their energy yield is small and because they are conducted at sites where a variety of other nuclear tests can occur, make verification of a comprehensive test ban more difficult.

  • For additional information go to:  Subcritical Testing and Test Ban Verification.

  • Together with an extensive array of other methods for conducting plutonium tests, these above-ground plutonium 242 tests give the United States an ability to test nuclear weapons components and designs far beyond that required to maintain the existing arsenal in a safe and reliable condition, and may increase the ability of the U.S. to deploy new nuclear weapons designs.

Los Alamos hopes to be able to certify a new design for submarine-launched warheads by 2003, using these and other testing programs.

Sources on Pu-242 Testing at Los Alamos

  1. Letter, John T. Conway, Chairman, Defense Nuclear Facilities Safety Board, to Victor Reis, Assistant Secretary for Defense Programs, U.S. Department of Energy, November 17, 1998.

  2. Letter, Victor Reis, Assistant Secretary for Defense Programs, U.S. Department of Energy to John T. Conway, Chairman, Defense Nuclear Facilities Safety Board, December 17, 1998.

  3. Los Alamos National Laboratory, Memorandum, N.R. Borch to J. Boettner, "History of Selection of HSLA-100 for Confinement Vessels," July 23, 1996.

  4. Los Alamos National Laboratory, Diagram, "Single Axis HSLA-100 Vessel Assembly."

  5. Los Alamos National Laboratory, "Conceptual Design Plan: Contained Explosives Test Complex," 1995.

  6. Nuclear Weapons and Materials Monitor, "Los Alamos to Use Pu-242 in Explosives Tests at DARHT," February 1, 1999, pp. 3-4.

  7. Ian Hoffman, "Lab Critics: Tests will Mock Nuke Blasts," Albuquerque Journal, January 14, 1999, p. 1.

  8. John Fleck, "LANL Mum on Plan to Detonate Plutonium,"Albuquerque Journal, May 28, 1996, p. 1.

  9. Seymour Sack, Laboratory Associate, Lawrence Livermore National Laboratory, "DAHRT Considerations," 1992.

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