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Simulating Nuclear Explosions under the Comprehensive Test Ban Treaty

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Sandia National Laboratory vice presidents Roger Hagengruber and Heinz Schmitt have recently confirmed that Sandia and Los Alamos National Laboratory are engaged in the full-scale development of a nuclear weapon design.[6] Information contained in a May 1997 Department of Defense report ("Nuclear Weapon Systems Sustainment Programs") and the declassified February 1996 Department of Energy 'Green Book' ("Stockpile Stewardship and Management Plan") clarifies the nature of this nuclear design work. Additional documentation regarding the Sandia/Los Alamos design effort was recently obtained through the FOIA by the Los Alamos Study Group in Santa Fe, N.M., and these Sandia vu-graphs are attached as Appendix II (print report only).

This nuclear weapons design effort is conducted within the SLBM Warhead Protection Program (SWPP), a Navy/Department of Energy(DOE) collaboration. The SWPP was established to "maintain the capability to jointly develop replacement nuclear warheads for the W76/Mk4 and W88/Mk5 should new warheads be needed in the future." [7] The program currently focuses on two nuclear weapon designs: a design which re-uses plutonium pits ("Pit Re-Use Project"), for which Lawrence Livermore National Laboratory has nuclear explosive package (NEP) design responsibility, and a LANL "high-margin" NEP design incorporating a new pit ("Replacement Warhead Project"). Both warhead designs are intended for the Mk5 re-entry body. The SWPP does not now encompass production, but does include "prototyping" and flight testing.

The "Pit Re-Use Project:"

  • is scheduled for Experimental and Computational Assessment through 2000, and Certification/Prototyping of the design through 2002; [8]

  • contains "major elements of a traditional design program, with the exception of nuclear testing," [9]

  • design and certification process includes advancing computational models, hydrodynamic testing, and may include flight testing of design elements;

  • warhead may incorporate a new Arming, Fuzing, and Firing (AF&F) system based on the system now under development for the W76/Mk4 SLBM warhead;

  • warhead design is likely to be based on the W89-Alt recycled pit warhead design (intended for the Short-Range Attack Missile-2), which was explosively tested at least once before underground testing ceased in September 1992. [10]

The "Warhead Replacement Project:"

  • is scheduled for Conceptual Design through 1997, Experimental and Computational Assessment through 2002, and Certification/ Prototyping through 2004; [11]

  • will "include major elements of a traditional Phase 3 design program, with the exception of nuclear testing.." [12]

  • ("traditional Phase 3" refers to the process of full-scale engineering development of a complete warhead system);

  • according to the Sandia vu-graphs, the "Replacement Warhead is a new design that will not have UGTs [i.e. underground tests] for certification;

  • requires the new experimental capabilities of the Stockpile Stewardship Program to "assess and evaluate the expected nuclear performance…" [13]

The general design criteria for the Pit Re-Use/Warhead Replacement Projects are indicated by the following statement:

Replacement warheads reflect no new weapon requirements but the desirable replacement characteristics include decreased sensitivity to aging, increased design margins, increased ability for surveillance by above-ground testing, and the ability to be certified without an underground test (italics added). [14]

In the argot of nuclear weapon designers, a "high-margin design" is one with an increased design margin for achieving its intended nuclear explosive performance. An increased primary performance design margin is usually achieved by increasing both the amount of plutonium in the pit and the energy delivered by the chemical high explosive. However, such changes usually decrease the "safety margin," i.e., the margin against producing nuclear yield in the event of an accidental one-point detonation of the high explosive (HE).

A high-margin design for a secondary would presumably be one that generates its nominal (or higher) design yield over the widest possible range of primary yields. It is not known whether a new secondary design effort is part of the Los Alamos/Sandia high margin Replacement Warhead project, or whether an existing secondary will be used. Given that existing warhead designs are usually optimized to use all available space on board the re-entry vehicle (RV), at least consideration of a new secondary design is strongly implied by recent vu-graphs presented at a "SWPP Program Review Meeting" in May 1997. These vu-graphs (Appendix II, print report only) indicate that Sandia has been participating in a "trade off study with LANL on physics package orientation [within the re-entry body (RB)] to support orientation decision of 5/14 [i.e. May 14, 1997]."

At issue in such discussions is the orientation of the nuclear explosive package with respect to the tapered end of the RB -- in other words, which component -- primary or secondary -- will face the tighter volume constraints on its design? [15] Another Sandia vu-graph, entitled "Replacement Warhead Safety Features," indicates that the primary for this warhead will have Insensitive High Explosive (IHE) -- a major design change, as this explosive is considerably less energetic per unit volume than the HE it replaces. A "robust" IHE primary with an increased performance margin would therefore likely require more space within the RV -- hence the need to review -- and possibly reverse -- the orientation of the NEP and consider a different secondary design from that of the existing W88.

The Stockpile Stewardship and Management Plan states that the new SLBM warhead design effort will stress increased safety margins. As noted above, given the offsetting design criterion for an increased performance margin, it will likely involve a considerable nuclear design effort to achieve both objectives. The same Sandia vu-graph referenced in the preceding paragraph indicates that in addition to IHE, the new warhead will also have a Fire Resistant Pit (FRP), and that new AF&F system and use control features "may be implemented." Dr. Ray Kidder of LLNL estimated in December 1991 that production of a safer new warhead design incorporating IHE to replace the W88 warhead would require four nuclear explosive tests -- three development tests and a production verification test. [16]

Some background on the Navy's strategic forces may be useful in understanding the SLBM Warhead Protection Program. Under the second Strategic Arms Reduction Treaty (START II), the submarine leg of the U.S. strategic triad will account for about half of all accountable, deployed warheads. Most of the Navy's strategic warheads are of the W76 design, about 3500 of which were produced between 1978 and 1987. About 400 W88 warheads were produced from 1988 to 1991; W88 production was terminated prematurely due to the closure of the Rocky Flats plant near Denver, Colorado. Both the W88 and W76 warheads were designed at Los Alamos National Laboratory. The W88 is heavier and has a much higher yield (475 kilotons) than the W76 (100 kilotons). [17]

In 1990 the W88 warhead, its associated re-entry body the Mk5, and the Trident II D5 submarine-launched ballistic missile (SLBM) were identified as a system with the potential for accident scenarios involving chemical explosion, the release of plutonium, and even nuclear explosion. [18] At issue were the Reagan-era (and earlier) decisions to forgo Insensitive High Explosives, Fire-Resistant Pits, and Class 1.3 solid rocket fuel in the Trident II design in order to maximize its military capability.

Several options were considered to improve the safety characteristics of the Trident II system. However, none of the billion-dollar plus options were judged to provide cost-effective accident mitigation. The dominant accidental detonation scenario, the dropping of a fully loaded missile during missile loading operations, was alleviated by a simple change in loading procedures that provided for installation of warheads after a missile was placed in its launch tube on board the submarine.

The W76 warhead was the first to be subject to the DOE Dual Revalidation Process. In March 1997 testimony before the Senate Appropriations Committee, Livermore National Laboratory director Bruce Tarter described Dual Revalidation:

Dual Revalidation is a formalized peer review process, developed in consultation with the DoD, to assess the condition of U. S. stockpiled weapons. Two teams perform the evaluation, one with personnel from the laboratory that originally designed the weapon and the other with experts from the second nuclear design laboratory. Sandia participates on both teams. Each Dual Revalidation is managed by a DoD/DOE Project Officers Group and is expected to take two to three years to complete. [19]

Before or during the Dual Revalidation process for the W76, an 'issue' with this warhead was encountered. The specific nature of this problem is classified but it has been cited as evidence that the Stockpile Stewardship and Management Plan is developing the appropriate computational and experimental capabilities:

It has been over four years since the last nuclear test. During that time, we have successfully addressed an issue with the Trident I (W76) warhead by using a combination of analysis, new experimental data, archived test and manufacturing data, and most importantly the collective judgment of the two weapon design laboratories.

This success, using the experimental and testing tools available today, provides confidence that the even more powerful computing and testing tools to be developed will allow us to solve future stockpile problems without nuclear testing. [20]

The Stockpile Stewardship and Management Plan contains little unclassified information on the W76 'issue.' It states that "traditional weapons design codes were not developed to handle this phenomenon," and that "designers were able to resolve a real surveillance issue through calculations which were validated by laboratory experiments and by using archived NTS (Nevada Test Site) data." [21]

The Department of Energy has decided to implement War Reserve plutonium pit manufacturing at Los Alamos National Laboratory. Such capabilities are scheduled to be in place by approximately 2003. [22] The DOE plans to manufacture a W88 pit in fiscal year 1998 as a demonstration of this capability. Los Alamos will employ different manufacturing techniques and materials than were used originally at Rocky Flats:

Most of these changes in materials and processing are not expected to have a noticeable impact on the nuclear performance of the W88 primary based on past nuclear test experience. However, two of the proposed changes warranted further evaluation. The use of cast plutonium material instead of wrought plutonium and the equator welding technique. [23]

The DOE states that certification of the nuclear performance of the Los Alamos-produced W88 pits will require the computational and experimental capabilities of the Stockpile Stewardship Program, in particular experiments to be performed at the Nevada Test Site. The total pit manufacturing capacity at Los Alamos will probably range from 50-80 pits per year.

The broad Stockpile Stewardship and Management strategy regarding the Navy's strategic warheads involves:

(1) surveillance and annual certification of the existing W76 and W88 warheads, and replacement of the W76 neutron generator, (tritium) gas transfer, and AF&F systems with new-design components;

(2) the development through 2003 of three options for the production of new Naval strategic warheads -- manufacture by a new net shape casting method of a small number of new W88 surveillance pit replacements to make up for those lost in destructive testing; a Re-Used Pit option; and a New Pit option; and

(3) possible Full-Scale Engineering Development (FSED) of a W88/W76 replacement warhead or warheads beginning around 2004.

There are several issues regarding these systems that appear to be instigating the Navy to examine future SLBM warhead options: the prospect of W76 aging and the possibility of a crippling "common-mode" failure in that weapon; lingering W88 (and W76) safety concerns; and issues of remanufacture and weapon certification without recourse to underground testing. Of more than passing significance is the fact that one of the Navy strategic warhead replacement options includes not only all new non-nuclear components but a new-pit primary (and possibly secondary) with "increased design margins" -- hence this option constitutes a new nuclear weapon design -- albeit for the apparent primary purpose of achieving increased confidence in nuclear weapons performance rather than increased military effectiveness or novel military applications. But the latter attributes can also be achieved by non-nuclear modifications to other aspects of the nuclear weapon system, such as missile guidance upgrades and "repackaging" for new, more demanding weapon delivery configurations.

Hence any "new" nuclear weapon design -- despite disclaimers of any radical new design objectives -- will have to meet the military characteristics (MCs) and Stockpile-to-Target (STS) performance requirements needed to realize new or improved military capabilities for the overall system. In this sense, increased confidence in the performance of the nuclear explosive package represents a significant contribution to the overall improvement in military capability, and thus warheads developed for this purpose are not only "new" in the literal sense, but also represent a potential for new or improved nuclear military capability, contrary to the "vertical" nonproliferation objective of the CTB.


6. "U.S. Labs Redesigning Nuclear Warheads," The Washington Times, July 23, 1997, p. A9. "Labs Craft Warhead Backup," Albuquerque Journal, July 23, 1997, p. A1. "National Labs Working on Warhead Replacement," Albuquerque Tribune, July 23, 1997, p. A10.

7. "Nuclear Weapon Systems Sustainment Programs," Office of the Secretary of Defense, May 1997, p. 18 (this document can be accessed on the World Wide Web at http://www.dtic.mil/defenselink/pubs/dswa/index.html#cover').

8. "Stockpile Stewardship and Management Plan," U.S. Department of Energy, Office of Defense Programs, February 29, 1996, p. IV-3. Note that this schedule is identified in Figure IV-1 as for the Proposed Budget for FY97.

9. "Stockpile Stewardship and Management Plan," p. IV-15.

10. The possible use of the W-89 recycled pit design as a safer replacement for theW88 was noted in 1991 by Dr. Ray Kidder of Livermore, although he also noted that a total of four tests would be required to replace the W88 with the recycle pit version of the W-89, of which only 1-2 were conducted prior to the moratorium. See Assessment of the Safety of U.S. Nuclear Weapons and Related Nuclear Test Requirements: A Post-Bush Initiative Update, UCRL-LR-109503, Dec. 10, 1991, p.4-6.

11. "Stockpile Stewardship and Management Plan," p. IV-3. Note that this schedule is identified in Figure IV-1 as for the Proposed Budget for FY97. The "Gantt Chart for Replacement Warhead Project (19 June 1997)" is given in Appendix II (print report only). There it is indicated that Design, Certification, and Testing of the Replacement Warhead will continue through 2003.

12. "Stockpile Stewardship and Management Plan," p. IV-16.

13. "Stockpile Stewardship and Management Plan," p. IV-16.

14. "Nuclear Weapon Systems Sustainment Programs," p. 18. This statement is given also in the "Stockpile Stewardship and Management Plan," p. II-10.

15. For example, according to Roger Baleras, the location of the secondary toward the front of a French SLBM reentry body, with the primary at the rear, left a small space around the secondary that posed "a special problem for the design." See "A Report on Discussions Regarding the Need For Nuclear Test Explosions to Maintain French Weapons Under a Comprehensive Test Ban, Paris France, Nov. 2-7, 1994, FAS/NRDC, Washington D.C., January 1995, page 14.

16. R. Kidder, Assessment of the Safety of U.S. Nuclear Weapons, p. 4.

17. "U.S. Nuclear Stockpile, July 1997," NRDC Nuclear Notebook, The Bulletin of the Atomic Scientists, July/August 1997, pp. 62-63.

18. "Nuclear Weapons Safety," Report of the Panel on Nuclear Weapons Safety of the Committee on Armed Services, House of Representatives, December 1990.

19. Prepared Statement of C. Bruce Tarter, Director, University of California, Lawrence Livermore National Laboratory, The Department of Energy's Budget Request for FY 1998 before the House Committee on National Security, Subcommittee on Military Procurement, April 10, 1997.

20. Testimony of Victor Reis, Assistant Secretary for Defense Programs, U.S. Department of Energy, House National Security Committee, Military Personnel, FY98 Budget, Nuclear Weapons, April 10, 1997.

21. "Stockpile Stewardship and Management Plan," pp. C-1 to C-3.

22. "Stockpile Stewardship and Management Programmatic Environmental Impact Statement," U.S. Department of Energy, September 1996, Volume I, p. 3-105.

23. "Stockpile Stewardship and Management Plan," p. II-8.

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