Meeting Nonproliferation Agreements Requires Destroying Thousands of Surplus Plutonium Pits.

On September 1, 2000, the United States and Russia committed to each “permanently dispose” of “no less than or at least” 34 metric tons of weapons-grade plutonium— enough plutonium to make thousands of weapons.

To help meet this commitment, the Department of Energy (DOE) announced a strategy for the permanent disposition of U.S. surplus weapons-grade plutonium. This strategy included burning the plutonium as fuel in existing domestic commercial nuclear reactors. In essence, this meant finding a way to convert some of the energy stored in the nation’s stockpile of surplus plutonium pits into electrical power for homes and businesses.

Nuclear Swords into Nuclear Plowshares

A pit is the spherically shaped nuclear fuel inside a warhead that, when imploded with high explosives, “triggers” (initiates) a thermonuclear explosion. Because pits are made of plutonium they are much heavier than they look. Plutonium is almost two and a half times denser than iron, so a plutonium pit weighs almost two and a half times more than an iron pit of the same shape and size.

Ironically, plutonium can be destroyed in the same way it was created, through modern alchemy.

The ancient search for a process to artificially convert one element into another—alchemy—became successful in the early twentieth century with the discovery that bombarding some elements with subatomic particles could transmute them into different elements. This discovery made it possible, by 1940, to create plutonium by irradiating uranium-238 (U-238) with neutrons. The uranium nuclei capture the neutrons. The additional neutrons transmute U-238 into plutonium-239 (Pu-239).

A solution to meeting U.S. commitments is to convert plutonium used to trigger thermonuclear weapons into fuel suitable for powering civilian nuclear reactors.

By 1945 the process of making plutonium—through neutron irradiation of U-238 inside a nuclear reactor— had been improved and expanded to an industrial level. In the final months of World War II, reactors at Hanford, Washington, met the Manhattan Project’s need for enough plutonium to make the first plutonium-fueled atomic bombs. One was tested successfully (the Trinity experiment) and one was subsequently used on Nagasaki, Japan, to help end World War II.

Before 1945, plutonium was so rare as to be virtually nonexistent on Earth. Today, the estimated 2,000 metric tons of plutonium in use or in storage around the world were created in reactors. (The majority of the world’s plutonium resides inside spent nuclear reactor fuel that is in storage.)

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The B61 nuclear gravity bomb (shown here) is assembled and disassembled at the Pantex Plant in Texas. When older versions of these bombs are retired from the nuclear arsenal, they are disassembled and their plutonium pits, which trigger their nuclear explosion, are removed and stored at Pantex. Plutonium pits are being recovered from thousands of retired nuclear weapons. However, Pantex is rapidly reaching its pit storage capacity. (Photo: courtesy Department of Defense)

So if plutonium is created in reactors, can it be destroyed in reactors? The answer is yes. A solution to meeting U.S. commitments is to convert plutonium used to trigger thermonuclear weapons into fuel suitable for civilian nuclear power reactors. Irradiating plutonium with neutrons makes it fission (split apart), which releases enormous quantities of energy, and the energy is used to generate electricity. Through the use of reactors, the energy from completely fissioning one kilogram (2.2 pounds) of Pu-239 could produce enough heat to generate approximately 10 million kilowatthours of electricity—enough electricity to power almost 1,000 households for a year. When plutonium is burned as fuel, some of the billions of dollars it cost to produce the plutonium is recovered.

The energy from completely fissioning 2.2 pounds of Pu-239 could produce enough heat to generate approximately 10 million kilowatt-hours of electricity—enough electricity to power almost 1,000 households for a year.

In a reactor, the irradiation with neutrons can be controlled and kept at a critical level. This makes it possible to control the release of energy and use it for peaceful purposes. Perhaps best of all, the process of fissioning destroys plutonium by transmuting it into different elements.

And that is exactly what the U.S. government has in mind: a strategy for the destruction of surplus weapons-grade plutonium pits by irradiation with neutrons inside already built domestic commercial reactors.

Where do the surplus plutonium pits come from?

Plenty of Pits

In 1967, at the height of the Cold War, the U.S. stockpile of nuclear weapons was at its maximum of 31,255 weapons. (In contrast, the Soviet Union is reported to have reached its maximum stockpile number—approximately 45,000 nuclear weapons—sometime during the mid-1980s.) The U.S. stockpile included intercontinental ballistic missiles as well as a variety of smaller missiles, gravity bombs, artillery shells, land mines, torpedoes, depth charges, and even miniaturized “backpack” bombs light enough to be carried by a single person.

A combination of factors allowed the United States to begin downsizing its nuclear stockpile after 1967. For example, in the early years of the Cold War, the limited accuracy and range of U.S. missiles and bombers meant more weapons were built, deployed, and aimed at targets to increase the probability of their destruction. But continuing improvements made in the accuracy and range of weapons’ delivery systems meant it was possible to reduce the overall numbers of weapons without reducing U.S. defensive capabilities.

Thus, improving nuclear weapon technology played a significant role in reducing the numbers of U.S. nuclear weapons.

A series of Cold War–era arms control agreements dramatically reduced the nation’s nuclear stockpile even further. These agreements included the United States and Soviet Union’s Strategic Arms Limitation Treaties (SALT I in 1972 and SALT II in 1979) and the Intermediate-Range Nuclear Forces Treaty (1987). By 1991, the stockpile was leaner, but it still contained a substantial 19,000 weapons.

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Pantex Plant, located 17 miles northeast of Amarillo, Texas. Pantex workers assembled thousands of weapons during the Cold War. The last new nuclear weapon was completed in 1991. Since then, Pantex has safely dismantled thousands of weapons retired from the stockpile and placed their plutonium pits in interim storage until a solution is found for their permanent disposal. Los Alamos is the only place in the nation capable of disassembling these pits and transforming them into a proliferation-resistant powder for use as nuclear reactor fuel. (Photo: courtesy Pantex)

Overall, the result of reducing the size of the U.S. stockpile is breathtaking. Today, the stockpile is only about one quarter of what it was in 1991, down to approximately 5,000 weapons—the lowest level since the late 1950s.

But it was the end of the Cold War—and with it the end of the nuclear arms race—that really allowed the U.S. stockpile numbers to shrink. Since the fall of the Soviet Union in 1991 and the end of the Cold War, the reduction of nonstrategic nuclear weapons has been particularly successful. Through the Bush/Gorbachev 1991 Presidential Nuclear Initiative’s political commitments, nearly 90 percent of these so-called “tactical” weapons were removed from the stockpile by 2009. Only a few hundred remain. The Strategic Arms Reduction Treaty (START) in 1991 and the Moscow Treaty in 2001 further reduced the stockpile. In addition, weapons that are retired because of age and changes in military requirements are not replaced with newly built weapons. Today the average age of a nuclear weapon is about 25 years. The United States has not built a new nuclear weapon since 1991.

Overall, the result of reducing the size of the U.S. stockpile is breathtaking. Today the stockpile is only about one quarter of what it was in 1991, down to approximately 5,000 weapons— the lowest level since the late 1950s.

The Pantex Challenge

The Pantex Plant (near Amarillo, Texas) is the DOE facility where all nuclear weapons are assembled. It is also where they are disassembled. The nuclear weapons removed from the stockpile to be dismantled go to Pantex. Although the weapons are disassembled at Pantex, the plutonium pit—one of a weapon’s most critical components—remains intact.

Weapons that are retired because of age and changes in military requirements are not replaced with newly built weapons. Today the average age of a nuclear weapon is about 25 years. The United States has not built a new nuclear weapon since 1991.

Pantex is not equipped to handle the special, complex operations required to dispose of these pits or their weapons-grade plutonium. So, safely and securely, it stores the pits. Consequently, the result of years of nuclear weapon dismantlement is a new Cold War legacy: thousands of stored pits awaiting some method of safe, permanent disposal.

Meanwhile, the nonproliferation agreements with the Russians regarding permanent disposal of 34 metric tons of surplus weapons-grade plutonium remain unmet.

Pantex is faced with a challenge because it does not have unlimited storage space—it is authorized to store up to 20,000 pits—and the facility is nearing capacity. Yet there are more than 1,000 warheads awaiting disassembly. In addition, the ratified 2011 New START with Russia requires that the number of deployed long-range weapons be cut from some 2,200 down to 1,550. This could mean even more nuclear warheads being dismantled and their pits stored at Pantex. Future arms control treaties may reduce the stockpile numbers even further, making more pits surplus.

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Gloveboxes are airtight, allowing radioactive plutonium pits to be safely disassembled from the outside. Working inside a glovebox is challenging, requiring manual dexterity skills that would impress a surgeon. While portions of the process are automated, teams of technicians are still required to manipulate some precision tools, maintain equipment, and move large and small objects back and forth, inside a complex maze of scientific apparatuses. (Photo: LANL)

Destroying surplus pits and permanently disposing of their plutonium would help meet nonproliferation agreements and make room for additional pits to be stored.

Still more pits could come from the National Nuclear Security Administration’s (NNSA’s) nuclear weapons Life Extension Programs (LEPs). These programs aim to extend the life of stockpiled weapons, most of which were produced 30 to 40 years ago. National security depends in large part on the nation’s nuclear deterrence. Refurbishing these aged weapons is required to assure the nation, its allies, and its adversaries that the weapons remain safe, secure, and reliable. The LEPs will refurbish, reuse, or replace weapon components as necessary. This could include, of course, replacing old plutonium pits with new ones. These old pits could become surplus, too. . .

So destroying surplus pits and permanently disposing of their plutonium would help meet nonproliferation agreements with Russia and make room for additional pits to be stored as more nuclear weapons are retired or refurbished.

Center for All Things Plutonium

But destroying pits and preparing their plutonium for permanent disposal, and doing so safely, is not a simple task—the pits must first be dismantled and their plutonium extracted, then the plutonium has to be converted into a form suitable for burning in a reactor. Plutonium pits have many unique characteristics that make them extremely difficult to handle and dismantle, requiring specialized equipment like airtight and pressurized gloveboxes. Glovebox technicians require highly specialized training. In addition, the extracting and converting of plutonium is a complex set of challenges. The entire process demands a unique set of knowledge, skills, technologies, and facilities for safely doing the work.

Los Alamos National Laboratory is the nation’s only “full service” center for understanding and manipulating plutonium. At Los Alamos plutonium science, production, and manufacturing all come together at places like the Chemical and Metallurgy Research facility and the Lab’s plutonium processing facilities at Technical Area 55.

The NNSA came to LANL with questions. Could LANL continue destroying pits and converting them to plutonium oxide? Could it do enough of this to help the U. S. meet its agreements with Russia?

Because Los Alamos is the nation’s center for all things plutonium, it is the only place in the nation where the people, science, technology, and infrastructure exist for safely destroying surplus plutonium pits and preparing the plutonium to become reactor fuel.

Beginning in 1995, DOE began funding the conceptual and planning work at Los Alamos. The result was ARIES: the Advanced Recovery and Integrated Extraction System.

ARIES

At Los Alamos, ARIES was originally designed as a pilot project, a proof-of-concept that would demonstrate a process for safely and securely disassembling pits and converting their plutonium into plutonium oxide. Plutonium oxide (a compound of plutonium and oxygen in powder form) has properties making it suitable for use as a reactor fuel. In addition, it is more proliferation resistant because the powdered oxide would have to be reprocessed back into plutonium metal to make a pit, requiring a significant and technically sophisticated infrastructure.

LANL is the nation’s center for all things plutonium—the only place in the nation to safely destroy plutonium pits and transform them into reactor fuel.

When plutonium oxide is combined with uranium oxide, the resulting mix—mixed-oxide fuel, or MOX—can be used to fuel current U.S. nuclear reactors. MOX fuel has been burned successfully in reactors in Europe, Japan, Russia, and elsewhere.

So the idea was to perfect ARIES at Los Alamos, eventually making it automated, for example, heavily reliant on robotics, to increase cost effectiveness and worker safety. 24 Los Alamos National Laboratory ARIES technology would then be transferred to the Pit Disassembly and Conversion Facility (PDCF) planned for construction at DOE’s Savannah River Site, in South Carolina. At that facility, ARIES would be used to dispose of the nation’s surplus pits on an industrial scale. The proliferation-resistant plutonium oxide would be mixed with uranium oxide to make MOX fuel at the MOX Fuel Fabrication Facility (MFFF), also at DOE’s Savannah River Site.

ARIES has destroyed every surplus pit type and converted the plutonium into MOX-ready plutonium oxide, which was successfully burned in the Catawba nuclear reactor in South Carolina in 2005.

ARIES has been successful. “LANL proved there was a safe way to dispose of the nation’s plutonium pits and convert them into material suitable for MOX fuel,” says Kane Fisher, an ARIES manager. The program has been able to destroy every pit type in the inactive stockpile and convert the plutonium into MOX-ready plutonium oxide. That plutonium oxide was successfully used in making MOX, which in turn was successfully burned in the Catawba nuclear reactor in South Carolina in 2005.

Then, after more than a dozen years of research and development, “In 2011, ARIES destroyed enough pits to produce more than 240 kilograms of plutonium oxide,” says Fisher.

The challenge of how to safely transform plutonium pits into reactor fuel was met. But other challenges loomed.

Overcoming Pitfalls

First and foremost, due in large measure to the nation’s current budgetary challenges, construction of the PDCF, estimated to eventually cost several billion dollars, was cancelled in 2011.

The target for fiscal year 2014 is 300 kilograms: doubling the production target of 2012. At 300 kilograms a year, by 2018, Los Alamos will have destroyed two metric tons of plutonium pits.

As a result. the NNSA came to LANL with questions. Could Los Alamos continue destroying pits and converting them to plutonium oxide? Could it do enough of this to help the United States meet its agreements with Russia?

“Our initial scope was to develop the process for destroying pits to meet our international agreements. With the cancellation of the PDCF, the ARIES process at LANL now becomes a key player for this important nonproliferation activity,” says Alex Enriquez, also an ARIES manager. “If requested by the NNSA to move from a process development to a production mission, we stand ready to serve. We can do it. Not as fast, of course, nor on the scale of a large, dedicated facility, but our process works.”

Hence, another 150 kilograms of plutonium oxide were targeted for production in fiscal year 2012. The ARIES team surpassed that target by producing over 200 kilograms. The target for fiscal year 2014 is 300 kilograms, doubling the production target of 2012. At 300 kilograms a year, Los Alamos will have destroyed two metric tons of plutonium pits by 2018 and shipped the proliferation-resistant plutonium oxide to MFFF.

Like a Chili Roaster

How does Los Alamos destroy a pit? “Very carefully,” says Steven McKee, another ARIES manager. “It’s taken years of research and development to come up with the ARIES process, in part because each type of pit presents its own challenges due to its size, shape, weight, and other characteristics. And the entire process has to be done inside a series of connected gloveboxes that keep pits safely isolated from the disassembly technicians.”

Inside the furnace is a rotating perforated drum containing the pieces of plutonium. It works like a typical New Mexican green chili roaster.

Gloveboxes are airtight steel containers with windows that allow radioactive materials to be safely manipulated from the outside. Highly skilled technicians insert their forearms and hands, covered by lead-lined gloves, into the glovebox. Working inside a glovebox is challenging, requiring manual dexterity skills that would impress a surgeon. While portions of the process are automated, teams of technicians are still required to manipulate some precision tools, maintain equipment, and move large and small objects back and forth, inside a complex maze of scientific apparatuses.

To oversimplify, the pits are cut in two inside the gloveboxes using an automated, custom-made mill and lathe, along with custom cutting tools. A vacuum system located directly below the cutting area collects all the lathe turnings and cutting chips. This part of the process is important because the Laboratory must account for the total plutonium mass of a pit. The mass is determined before the pit enters ARIES, and the total mass of the pit’s separate components, including any turnings and chip waste, is determined again at the end of the process. The two masses have to match exactly. All of the pit’s plutonium is thereby accounted for.

Accounting for all of the pit’s original mass continues throughout the process, including after it has been converted into plutonium oxide and is ready for shipment to the Savannah River Site.

Following dismantling, the pit’s plutonium parts and pieces are made to…well…rust, rapidly oxidize into plutonium oxide by being cooked inside a custom furnace. Plutonium oxidizes spontaneously, but as the temperature increases, the oxidation rate increases exponentially.

“Inside the furnace is a rotating perforated drum containing the pieces of plutonium. It works like a typical New Mexican green chili roaster,” says manager Elizabeth Bluhm.

The surplus pits from the nation’s Cold War deterrence can now be transformed—from being nuclear weapons triggers into a clean energy source for the nation.

What is a chili roaster? Across New Mexico, beginning in mid-July, at farmers’ markets, in grocery store parking lots, at roadside stands, in parks, and at backyard cookouts, the air is regularly filled with the smoky smell of fire-roasting fresh green chilies. The chilies are loaded into a horizontally mounted drum made of heavy wire mesh and the drum then rotated over an intense heat source. This method allows the tumbling chilies to be roasted evenly on all sides, and quickly, too, because it presents the most surface area of the chilies to the greatest amount of heat in the shortest time. The roasting process is thorough and efficient.

There is no proof that ARIES scientists got the idea of oxidizing pit plutonium by watching green chili roasts in the summertime. But the principle is the same.

Clean Energy

After oxidation, the plutonium compound is ground into a powder. The powder is sealed inside a special stainless steel container suitable for long-term storage. To meet the Department of Transportation’s demanding safety and security requirements for shipping plutonium oxide, the first container is then sealed inside a second stainless steel container, which is then sealed inside a third stainless steel container.

After a final decontamination check and the completion of an audit confirming the nature of the containers’ contents, the plutonium oxide is ready for shipment to MFFF, where it will be blended with uranium oxide.

Because of the years of effort by the ARIES team working at Los Alamos’ unique plutonium facilities, the surplus pits from the nation’s Cold War deterrence can now be transformed—from being nuclear weapons triggers to serving as a clean energy source for the nation.

–Clay Dillingham

For more information about ARIES visit arq.lanl.gov/source/orgs/ nmt/nmtdo/AQarchive/1st_2ndQuarter08/

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Plutonium pits are transformed into plutonium oxide powder by roasting them in a way similar to roasting green chili, shown here. (Photo: LANL)

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