Subcritical Experiments After the Test Ban Treaty

Dear friends,

Attached is an update (based on primary sources) on what is known about DOE activities on subcriticals along with some background information.

Issue Brief: Subcritical Experiments After the Test Ban Treaty
Daryl Kimball, Director of Security Programs, Physicians for Social Responsibility

Updated -- February 3, 1996

Recent statements from DOE and other Clinton Administration officials indicate that the United States will likely conduct the first two of a series of "subcritical high-explosive experiments with nuclear materials," which have been delayed for nearly one year.

At his January 30, 1997 confirmation hearing before the Senate Energy and Natural Resources Committee, Secretary of Energy- designate Federico Pena stated that "we believe that the subcritical tests will give us very helpful information to assist us in making a determination that the weapons stockpile is safe, reliable and viable. Obviously we believe that it is consistent with the Comprehensive Test Ban Treaty, our other international obligations, and we will proceed with those experiments this year."

Other sources indicate that the DOE will conduct the first subcritical experiment during the summer in the 2nd quarter of 1997.


On October 27, 1995, the U.S. Department of Energy (DOE) announced its original plan to conduct two subcriticals underground at the Nevada Test Site in 1996 and four subcritical experiments in 1997. The experiments would be conducted 980 feet underground in the LYNER (Low-Yield Nuclear Explosion Research) facility.

On June 18, the Clinton Administration announced the postponement of subcritical experiments at the Nevada Test Site that was originally scheduled for June and September of this year. This avoided a major controversy at the Comprehensive Test Ban Treaty (CTBT) talks in Geneva.

The stated reason for the delay was that additional time was necessary to complete further environmental impact analysis of the subcritical experiments. That environmental review, the final site-wide environmental impact statement (EIS) for the Nevada Test Site was issued on October 9. The additional time was needed to respond to comments submitted by U.S. environmental groups that charge that the EIS for the Test Site does not sufficiently address the environmental impacts of the subcritical experiments and that the experiments represent a new activity at NTS because they have not been conducted within the last five years.

Another reason for the delay was the potentially disastrous impact of such experiments just days before the possible conclusion of the Comprehensive Test Ban Treaty (CTBT) talks in Geneva. In the weeks leading up to the conclusion of CTBT talks in August, the U.S. received numerous private messages and several public messages from governments that the subcritical experiments could seriously upset the CTBT talks and that they should be postponed or canceled. For instance, on May 23, Pakistani Ambassador Munir Akram told the Conference on Disarmament that U.S. plans to conduct the subcritical experiments at the Nevada Test Site would create verification problems for the future treaty because they will be hard to distinguish from low-yield nuclear tests. Akram asked for technical and political assurances that the tests "will not contribute to the qualitative improvement of nuclear weapons."

However, on December 9, the Department of Energy said it will continue its program of subcritical nuclear experiments when it announced its final record of decision for the Nevada Test Site EIS, which includes among the list of "preferred alternative" activities the use of NTS for conducting subcritical nuclear experiments at the Low-Yield Nuclear Explosions Research (LYNER) facility.

What Are "Subcritical" Experiments?

According to DOE weapons scientists, the planned subcritical experiments would use 50 to 500 pounds of high-explosive charge and would involve special nuclear materials, including plutonium- 239. The DOE says that the experiments would not produce a self- sustaining nuclear reaction -- thus the term "subcritical." However, it will be very difficult for independent observers and/or other nations to verify that the experiments actually are subcritical. The DOE states that these experiments are needed to:
(1) improve the knowledge of the dynamic properties of aged nuclear materials [i.e. plutonium] in order to assess the effects of new manufacturing techniques on weapons performance; and
(2)help maintain the capabilities of the Nevada Test Site and support nuclear test "readiness."

The DOE says that the these experiments are a necessary part of its "stockpile stewardship" program, which is designed to maintain a safe and reliable nuclear arsenal without underground nuclear test explosions. The first two tests would ostensibly provide the DOE with additional data on the behavior of plutonium in a "strongly-shocked" state -- data that is said to be needed for improving supercomputer modeling of nuclear weapon performance and assessing changes in weapon remanufacture techniques and materials. Each experiment would cost an estimated $20 million in direct costs.

The first two planned subcritical experiments would not involve nuclear warheads, warhead prototypes, or "weapons configurations." However, DOE officials have not ruled out the possibility of conducting subcritical tests involving "weapons configurations," which could result in "hydronuclear" explosions. In the past, such "hydronuclear" tests resulted in barely critical configurations that released less than half a pound of fission energy.

It is generally acknowledged that the recently-signed CTBT, which prohibits "any nuclear weapon test explosion or any other nuclear explosion," would allow subcritical experiments because they are not designed to produce a release of energy (i.e. explosion) from a self-sustaining nuclear chain reaction.

Rationale for Experiments Unjustified

The DOE claims that the experiments should be conducted underground at the Nevada Test Site in order to fulfill President Clinton's policy to maintain "the basic capability to resume nuclear test activities prohibited by the CTBT should the United States cease to be bound to adhere to this treaty." But, maintaining such a "basic capability" to resume testing does not require ongoing test operations at the Nevada Test Site. The DOE is extending the President's policy beyond what is technically required.

The DOE has yet to make the case that the proposed experiments are required to maintain a reliable and safe nuclear stockpile. There is no evidence to date to suggest that plutonium aging has degraded the expected performance of the modern weapons designs used in the current U.S. nuclear arsenal.

Further, the DOE has not established that there is a need to conduct the experiments. There has been no independent technical review in support of their conduct, timing or site selection. Nor has the U.S. government thoroughly evaluated the nuclear arms control and non-proliferation impacts of conducting such activities. Rather, the experiments appear to be a make-work project for the nuclear weapons labs and test site personnel to exercise capabilities that might be needed to breakout of the treaty at some point in the future.

Negative Impacts on the Comprehensive Test Ban Treaty

Conducting underground subcritical experiments at the Nevada Test Site would have a severe negative impact on U.S. nuclear non- proliferation goals, including the recently-signed Comprehensive Test Ban Treaty (CTBT). Conducting these experiments would set back U.S. goals for the CTBT's formal entry-into-force, which requires the ratification of the Treaty by 44 named, nuclear weapons-capable states. If the U.S. were to conduct the subcritical experiments, it would likely harden the position of several states who are concerned that the United States (and other declared weapons states) will continue nuclear weapons development through their laboratory-based, "stockpile stewardship" programs and activities such as subcritical experiments. This would undermine efforts to encourage these states to ratify the Treaty.

Though the DOE claims that the subcritical experiments would be in conformance with the recently-signed "zero-yield" CTBT, these experiments would severely complicate the ability of the U.S. and other states to verify that the terms of the CTBT are not violated by any nation. Because the CTBT has not yet entered into force, its verification and on-site inspection system is not fully in place. As a result, the United States should, if it chooses to conduct the subcritical experiments, develop a verification plan that is capable of demonstrating to the international community that the U.S. is not violating its legal obligations as a CTBT signatory. Because U.S. subcritical experiments would set a new and destabilizing nuclear proliferation precedent that other nations might choose to emulate, it is in the United States national security interest to ensure that any such verification plan can be applied to other states that might be capable of conducting low-yield nuclear weapons experiments.

Continuing test operations at underground test sites under a CTBT would also contravene the spirit of the of the treaty and the hope of a vast majority of nations that the nuclear test sites will be closed.


As a first step, the U.S. and other governments should issue a clear, "no new nuclear weapons" policy. This would mean that the non-explosive weapons research activities that will continue under the "zero-yield" CTBT will not contribute to the qualitative improvement of their nuclear arsenals.(*)

If the U.S. does not immediately cancel the subcritical experiments, it should: a) conduct a thorough, independent, public review to determine whether the subcritical experiments are technically necessary to address warhead safety and reliability problems known to exist in the current arsenal; b) conduct a thorough, public evaluation of the non-proliferation and treaty implications of conducting such activities, particularly CTBT entry-into-force; and c) it should allow international technical observers sufficient access to the subcritical experiments to verify that they do not produce small nuclear yields in violation of the recently-signed CTBT.

* See PSR's "Issue Brief: Unresolved New Nuclear Weapons Development Issues," Daryl Kimball, February 3, 1997 for more details.

D. Kimball
PSR Physicians for Social Responsibility
1101 Fourteenth Street, N.W., Suite 700, Washington, D.C. 20005
Telephone (202) 898-0150 Fax (202) 898-0172

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