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“Aging” Weapons

The most direct link between NIF and stewardship is the laser facility’s presumed role in assessing problems that might arise in aging secondaries. The hope is that the facility will be able to reproduce the conditions found in an exploding thermonuclear weapon, but on a much smaller scale.

NIF is an “inertial confinement fusion” facility. It would use the largest and most powerful laser in the world, made up of 192 separate beams, to deliver a laser pulse of 1.8 megajoules of energy (far more than the 40 kilojoules now available on Livermore’s NOVA laser). This energy would be used to implode deuterium-tritium pellets to produce billionth-of-a-second bursts of fusion energy for study. By improving our knowledge of weapons physics, the argument goes, NIF will help scientists gauge the seriousness of defects that might occur as the stockpile grows older.

The aging of the stockpile is related to the end of nuclear testing. Over the last 40 years, the United States continually upgraded and replaced older warheads by developing new designs with the help of nuclear tests. Without such tests, the Pentagon does not currently plan to replace existing warhead types with new ones in the future. According to Harold Smith, assistant to the secretary of defense for nuclear, chemical, and biological weapons, quoted in the May 9, 1996, issue of Inside the Pentagon: “There are no new [designs for] warheads. There cannot be. Because if you cannot test, you cannot develop new warheads. That is almost the eleventh commandment as given to Moses on Mount Sinai.”

As long as this sentiment is so strongly held in the Pentagon, the average age of the stockpile will grow steadily from 13 years today to 20 years by 2005, taking into account the retirement of some of the older weapons. (If the United States and Russia agree to further cuts in their arsenals, this average would fall, since older weapons would likely be eliminated first.) Although 20 years is often characterized as the maximum life of a weapon, it is actually the shortest lifespan contemplated. The Department of Energy’s stockpile management program-which is responsible for manufacturing new warhead parts as the stewardship program deems necessary- states in its February 1996 “Draft Analysis of Stockpile Management Alternatives” that nuclear components “are expected to have service lives significantly in excess of their minimum design life of 20 to 25 years.” According to the report, “Experience indicates that weapons can remain in the stockpile well beyond their minimum design lifetime.”

Since 1958, an Energy Department effort known as the Stockpile Evaluation Program (SEP) has compiled a detailed record of the condition of nuclear weapons.

Significantly, SEP has yet to turn up any evidence that age-related defects appear with greater frequency over time. Nor is there any sign that warhead secondaries, the components most relevant to NIF, are prone to age-related defects at all. If anything, secondaries appear to be the least vulnerable nuclear components of the weapon.

Under SEP, 11 sample warheads of each weapon type are taken out of the stockpile every year, according to “Stockpile Surveillance: Past and Future,” a September 1995 report by the three weapons labs. The samples are dis-assembled and inspected, and the non-nuclear components are subjected to laboratory and flight tests. As a rule, the nuclear explosive package from one sample per year per weapon type is destructively examined (for example, the plutonium components are cut up for metallurgical analysis) by whichever weapons lab produced the warhead. This sample is then retired from the stockpile and must be replaced with components that are either held in reserve or, if spares are not available, newly produced. The other 10 samples per warhead type are returned to the stockpile with original nuclear components and replacement non-nuclear parts as needed. This process begins and ends at the Pantex plant near Ama-rillo, Texas.

Of the 70,000 or so U.S. nuclear weapons produced since 1958, the Stockpile Evaluation Program has examined more than 13,800 weapons of 45 different types. About 800 distinct sorts of findings have warranted further investigation. Of these, about 400 were deemed “actionable,” meaning that the finding resulted in corrective measures (to the weapon itself or to the production process) or in a downgrading of the weapon’s assumed reliability or yield. Most such findings have occurred in the first few years of a weapon’s life as a result of problems in design, fabrication, or production-problems that tend to get worked out early on. As the weapons age, fewer actionable findings appear. Using past experience to project the future health of the stockpile, the weapons labs estimate in their joint report that over the next 10 years there will be an average of one to two actionable findings per year, one of which will result in a change to a warhead.

These numbers, however, do not distinguish between production problems and age-related defects, such as cracks, corrosion, and the like. Production problems are unlikely to reappear, and aging problems serious enough to correct have been restricted almost entirely to non-nuclear components, such as detonators, cables, and neutron generators. If found to be defective, all these parts can be newly fabricated and fully tested.

The challenge today for the laboratories is to assess the nuclear parts (primary “pits” and secondaries) of the warhead that can no longer be tested in actual detonations. So far, the nuclear heart of the primary-the pit, made of plutonium, uranium, and beryllium-has received a clean bill of health. While acknowledging that few data are available for pits older than 25 years, the stockpile management program states in its February 1996 draft analysis that “no age related problem has been observed in pits up to 30 years in age.” Which is not to suggest these components are immortal; at some point, the plutonium’s radioactive decay could lead to performance problems. According to a senior scientist in the Energy Department’s stockpile management program, pits may last “40, 60, 100 years, but not 1,000.”

But what about secondaries, the supposed deterioration of which serves as the raison d’tre for NIF? Here the record is similarly encouraging. Secondaries consist of uranium, lithium deuteride, and other subcomponents isolated from the external environment in a sealed can. Although the materials can still react with each other, this has not been a significant problem, according to DOE documents obtained by the Institute for Energy and Environmental Research in Tacoma Park, Md. Examinations of secondaries since 1958 have uncovered only two types of age-related defect, neither of them serious enough to correct. In fact, the stockpile management program acknowledges that “there has been no degradation or concern for performance for any of the weapons in the stockpile of 2004 and beyond.”

Even if aging problems with secondaries appear, these warhead stages have the advantage of simplicity and reliability. Says the DOE senior scientist, “Once the primary [detonates], the secondary will also, even if it has some defects.” Unlike the primary stage, which drives the nuclear explosion, the performance of the secondary appears to be relatively insensitive to age-related changes.

If the historical record is any indication of future performance, aging of nuclear components seems likely to remain a rare problem for the foreseeable future. With an average stockpile age of 13 years (the oldest deployed warheads are now 18 years old), and the knowledge that nuclear components can last well beyond the design life of the overall warhead, we are possibly decades away from encountering any significant age-related problems with nuclear components. Hence there is no rush to build new facilities to address aging problems, especially with secondaries.

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