Science 1663

• Los Alamos Science and Technology
  Magazine
  JANUARY 2007
• About 1663     |     Past issues

The Reliable Replacement Warhead: Catalyst for Change

by Paul White

The RRW has the potential to significantly reduce the country's nuclear stockpile while transforming the nuclear weapons complex into a leaner, more responsive enterprise.

Realistic 3-D visualizations of computer calculations enable Los Alamos weapons designers to predict weapons performance without nuclear testing. In the complicated flow pattern shown above, the golden mesh and white marker particles are calculational aids. Calculation by Bob Weaver. Photo by Presley Salaz.

Sixty years ago, in the high desert north of Alamogordo, New Mexico, a blinding flash of light suddenly filled the early morning sky. It was "the brightest light I have ever seen...A new thing had just been born," recalled Isador Isaac Rabi in Science: The Center of Culture. The device that spawned the first manmade nuclear explosion was conceived by the brilliant minds Robert Oppenheimer had gathered at Los Alamos during World War II.

Oppenheimer's team, driven by the wartime need and the technical challenge, delivered the two distinct weapon designs whose devastating effects ended the war in the Pacific. Technical uncertainties about one, the Fat Man plutonium implosion design, were great enough that a full-scale nuclear test was needed before military use could be risked. Confidence in the other, the Little Boy uranium gun design, was high enough that the weapon (containing virtually the entire U.S. supply of uranium-235) was deployed without a nuclear test.

Today a new cadre of physicists, engineers, materials scientists, and others is giving birth to a new warhead concept—the Reliable Replacement Warhead (RRW)—with an intensity and sense of urgency that recall the early years at Los Alamos. No longer bound by the imperatives that led to the high-performance designs of the Cold War, the New Mexico RRW team is looking to a conservative design that, like Little Boy, minimizes performance uncertainties and can be deployed without nuclear testing. The RRW program would replace yesterday's high-performance designs with prudent alternatives that can be maintained with the modern tools of Stockpile Stewardship, the program launched in the early 1990s as the best approach for maintaining stockpile reliability without nuclear testing.

According to Joe Martz, the bold and enthusiastic New Mexico project lead, the RRW effort "presages a whole new way of doing business for the weapons complex, closely integrating design, engineering, supporting experiment, tooling, and manufacturing." If Little Boy and Fat Man foreshadowed the long, slow buildup of Cold War nuclear stockpiles, the RRW will be the catalyst for enabling further stockpile reductions and transforming today's nuclear complex into the leaner, more responsive infrastructure of the future.

Joe Martz, project leader of the New Mexico RRW team, has a gift for pulling people together across disciplines and institutions. Team members are from Los Alamos and Sandia national labs.

Today's Aging Stockpile

Today's stockpile systems were designed during the superpower struggle with the Soviet Union. A driving requirement was to package more and more explosive power (yield) in ever smaller and lighter packages, whether for ballistic missile or air carrier delivery. Evolving military needs and delivery vehicle modernization often led to new requirements for warheads with different yields, weights, sizes, and other characteristics. Consequently, each warhead that was added to the nation's stockpile was of a new design, manufactured by unique processes and procedures. The entire composition of the stockpile turned over every 20 years.

John Pedicini, the Los Alamos RRW design team leader, is one of the few active Los Alamos nuclear weapons designers to have been through the whole process of designing and testing a nuclear weapon. For several decades Pedicini championed the concept of a robust weapon that could be deployed without testing.

The United States, however, has not fielded a new warhead design since the early '80s. The average age of the systems now in the stockpile is 22.5 years. At least one system is more than 30 years old. This aging stockpile is being maintained with increasing difficulty using the tools of Stockpile Stewardship. Among those tools is a vital program of stockpile surveillance. Every year, physicists and engineers take representative warheads apart and scrutinize them with the thoroughness of crime scene investigators. They are learning that time is an enemy of the stockpile.

Surveillance is revealing a growing number of age-related problems, such as material failure or corrosion, that individually or cumulatively will eventually affect weapon performance. Most problems identified in stockpile surveillance are resolved without any impact on the safety, security, or reliability of stockpile weapon systems.

When an assessment reveals the possibility of a serious effect on stockpile systems, there are several options, each with its own impact. For some issues it is possible to shorten the time interval for replacing limited-life components in the field, for example, tritium gas reservoirs. For other issues, establishing an exception to the ways in which the military stores or uses a weapons system may be considered. For example, limits might be placed on the temperature environment to which a warhead may be exposed, or restrictions might be placed on the mechanical stresses it may experience. Sometimes it proves necessary to modify the military characteristics originally established for a system, for example, to specify a wider performance band for a militarily significant parameter, such as yield over target. If none of these options is feasible or acceptable, then the National Nuclear Security Administration (NNSA) and the Department of Defense may jointly agree to initiate a system life-extension program.

In response to changing national security needs, the United States has been decreasing its nuclear weapons stockpile through unilateral actions and bilateral agreements. These reductions illustrate the U.S. commitment to Article VI of the Treaty on the Non-Proliferation of Nuclear Weapons. RRW deployment can help continue this trend.

In that event, all affected warheads are returned to the production complex for remanufacture of some of their components. But such a path inevitably leads to more and more uncertainty. Many of yesterday's materials, subcomponents, and manufacturing methods cannot be duplicated today. Consequently, remanufacturing decisions are required during every life-extension program. Alternative materials and manufacturing procedures are chosen to minimize the uncertainty introduced in key performance parameters, but the cumulative uncertainty grows inexorably. In the context of the extremely tight Cold War design constraints, this growing uncertainty will eventually become unacceptable. Maintaining the legacy stockpile in this manner—through life extension but without nuclear testing—cannot indefinitely sustain confidence in our nuclear deterrent.

An RRW Stockpile for Tomorrow

There is an alternative, more sustainable path. In today's post-Cold War world, the United States can afford to relax yesterday's constraints on warhead weight, size, and other military characteristics, including—in some cases—yield. That leeway in constraints can allow a new approach that permits replacement warheads with increased performance margins.

Confidence in these greater margins is based on data from years of nuclear test experience. There is no more diligent student of that testing history than John Pedicini, the intense, highly creative leader of the Los Alamos RRW design team. Poring over the test data, Pedicini and his team of younger physicists have cataloged all of the ways that nuclear explosives can fail—the edges, or "cliffs," past which a nuclear design parameter cannot be pushed without causing the explosion to fail.

A non-nuclear test of the Los Alamos RRW design was performed at DARHT, the Los Alamos Dual-Axis Radiographic Hydrotest facility. X-ray pictures and other diagnostics showed that the implosion system behaved as expected.Photo by HX-3

Hundreds of wire pins extending from the central yellow pin dome are designed to generate signals upon contact with a moving surface. Those signals are used to track the implosion of material as it moves toward the center of an RRW assembly. Photo by Mark Hoverson, HX-3

The RRW design is planted right on the middle ground of design parameters, far away from all failure-mode cliffs, which translates into a large performance margin. Consequently, designers are confident that RRW designs can be certified for deployment using the increasingly powerful computational and experimental tools of Stockpile Stewardship and without any need for nuclear testing. In other words, choosing the RRW path actually reduces the likelihood that the United States would ever have to resume nuclear testing to sustain a reliable nuclear stockpile.

The RRW program will also be a vehicle for transforming the nuclear enterprise—the sum of all the people, operations, and facilities that are needed to produce nuclear weapons. Design engineers used modern computational methods to share details of RRW component designs with production plants in near real time. Production engineers can immediately see how components fit together and develop assembly strategies. Because RRW designs are simpler and use fewer but more-standardized parts and materials, production costs can be reduced. Enhanced intrinsic warhead safety and security simplify operations for production plants. The net result will be a nuclear enterprise that can be responsive to changes in national security requirements.

As RRW designs are developed, they can gradually replace legacy systems. Over time, it is envisioned that the stockpile will be transformed, first into a mixture of legacy and RRW systems and ultimately into an all-RRW stockpile.

With a transformed stockpile comes another important benefit—the total numbers of weapons in the nuclear stockpile can be smaller. Deployed nuclear warheads are supplemented by a reserve that serves as a hedge against two possibilities: a catastrophic technical failure of one of the nuclear systems and/or a sudden change in international circumstances. Even the reserve is larger than it might otherwise be because today the United States cannot produce the plutonium parts used in modern nuclear warheads. As an RRW stockpile becomes a reality and our production infrastructure is restored and transformed, the size of the reserve stockpile can be reduced.

Moving Ahead with RRW

The RRW didn't just emerge overnight as a preferred path for the future of the U.S. nuclear deterrent. The two nuclear design laboratories—Los Alamos and Lawrence Livermore—have worked together with NNSA and the Department of Defense to forge this new approach to sustaining the nuclear stockpile. The U.S. Congress, too, became convinced of the promise in this approach and authorized the investment of funds to make this path a reality.

Right now the two laboratories, working with Sandia National Laboratories, are competing to come up with the best possible design for the RRW. In December the Nuclear Weapons Council endorsed the RRW program as the right approach for sustaining the stockpile; the council will announce its preferred design in the near future. But that will mark just another beginning.

Making the RRW a reality will not be the work of just one laboratory. All parts of the nuclear enterprise will share the responsibility for turning the winning design concept into production reality, and all parts will work together to make the nuclear enterprise leaner and more responsive. Everyone will be involved, and the whole country will win.

Sixty years ago, the conservative, high-margin Little Boy design helped launch the original nuclear production complex and stimulated the succession of warhead designs that provided the nation's deterrent strength through the Cold War. Today, the RRW is paving the way for a transformed and leaner stockpile in support of our nation's defense requirements in an uncertain world. The RRW approach promises a stockpile that can be sustained with high confidence in its safety, reliability, and effectiveness for as long as the nation requires.

Paul White is the director of the Los Alamos National Security Office.