Cuts to the Fissile Materials Stockpile

December 12, 2016
Eryn MacDonald

The Final Countdown

The United States maintains stockpiles of weapons-usable fissile materials—plutonium and highly enriched uranium (HEU)—that are much larger than needed. This material is a security risk, and is also expensive to store safely. Some of this fissile material has already been declared “excess to military needs” and is awaiting disposition. Even after that excess material is disposed of, however, the United States will still have far more material than it needs for its current or future arsenal. President Obama should declare additional material excess and schedule it for disposition as soon as possible. If done correctly, this would reduce opportunities for nuclear terrorism. It would also be a step toward making nuclear reductions more difficult to reverse.


The United States currently has just over 95 metric tons of plutonium. Of the original 99.5 metric tons of plutonium in the 1994 U.S. inventory, it declared 61.5 metric tons as excess to military needs. About four metric tons of this has already been disposed of,  and the U.S. is examining methods for disposing of the  other 57.5 metric tons of plutonium that was declared excess. Of the remaining 33.5 metric tons that is reserved for weapons use, some is in deployed and hedge weapons, and some is in plutonium pits removed from weapons that have been retired and dismantled.

U.S. nuclear weapons contain less than four kilograms of plutonium in their pits, which means that the current U.S. nuclear arsenal of roughly 4,500 deployed and reserve weapons contains no more than 18 metric tons of plutonium. Thus, President Obama could declare up to an additional 15 metric tons of plutonium excess to military needs while still maintaining the U.S. arsenal and hedge at its current size. Earlier this week the Department of Energy issued a press release saying that the United States “commits to IAEA [International Atomic Energy Agency] monitoring for the verifiable disposition of six metric tons of surplus plutonium.” But lest there be any confusion, this is not a commitment to declare any additional plutonium excess, it simply means that of the plutonium it has already declared excess, six metric tons will be made subject to monitoring and verification by the IAEA. The president should declare as excess the additional 15 metric tons of plutonium mentioned above.

However, disposing of this plutonium will only reduce the terrorism risk if it is done right. Until recently, the United States was planning to convert the plutonium into “MOX” fuel for nuclear power reactors. (MOX is a mixture of plutonium and uranium oxides.) It has been building a large facility at the Savannah River Site in South Carolina to produce the MOX fuel, but this project has run into cost and schedule problems and the Department of Energy is considering other options. At the top of its list is “dilute and dispose,” which involves mixing the plutonium with inert materials and placing small quantities of the mixture in waste drums, which would then be buried in a deep underground geologic repository—the Waste Isolation Pilot Plant in New Mexico. The Waste Isolation Pilot Plant is currently closed due to a 2014 accident, but the Department of Energy says it plans to resume operations by the end of 2016.

You know it’s going through your head now anyway

You know it’s going through your head now anyway

Disposing of the plutonium through MOX fuel, on the other hand, would entail converting it into an oxidized form, then mixing it with low-enriched uranium. The mixture would then be made into fuel rods for use in commercial nuclear reactors, and after the fuel is irradiated, the plutonium would become protected by the high levels of radiation in spent fuel. However, unused MOX fuel is not highly radioactive, and separating plutonium from MOX fuel requires only a straightforward chemical process. In addition, the fuel would be used in commercial reactors that do not have the security levels required for weapons-usable material. This means that the manufacture, transport, and storage of MOX fuel creates a greater security risk than would the dilute and dispose option.

Apart from security concerns, cost estimates for construction of the facility to fabricate the MOX fuel have jumped from $7.7 billion to more than $17 billion, and it is now not predicted to be operational until 2048more than 40 years behind schedule.

For all of these reasons, the United States should abandon the MOX approach to disposing of its excess plutonium and focus on the dilute and dispose option. In fact, the Obama administration is already trying to kill the MOX program because of the enormous increase in its costs. However, powerful supporters—including Senator Lindsey Graham of South Carolina—have prevented its demise. In the meantime, the most important priority is safe and secure storage of this material until the dilute and dispose program, which has already begun at a low level at the Savannah River Site, can be fully implemented.

Highly Enriched Uranium

In addition to its use in nuclear weapons, HEU is also used as a fuel for the nuclear reactors that power all U.S. submarines and aircraft carriers, though there are efforts to end this reliance on weapons-usable material. A 2014 report to Congress from the Office of Naval Reactors states that “recent work has shown that the potential exists to develop an advanced fuel system that…might enable either a higher energy naval core using HEU fuel, or allow using LEU fuel with less impact on reactor lifetime, size, and ship costs.” It is used in some U.S. research reactors as well, but this number is declining as more switch to low-enriched uranium (LEU), which is highly impractical for use directly in nuclear weapons.

The International Panel on Fissile Materials (IPFM) estimates that the U.S. stockpile of HEU is currently about 600 metric tons, with 253 tons of this in weapons or available for use in weapons. U.S. nuclear weapons contain about 15 kilograms of HEU in their secondaries, and some also contain another 10 kilograms of HEU in their primaries. If each weapon in the arsenal contains 15 to 25 kilograms of HEU, the current U.S. arsenal of 4,500 total weapons contains between 68 and 113 metric tons of HEU. This means that the president could declare an at least an additional 140 metric tons of HEU as excess while still maintaining the current arsenal and hedge size.

Over the last two decades, the United States has declared 374 metric tons of HEU to be excess to needs for nuclear weapons. Part of this—152 metric tons—has been set aside for use as fuel for naval nuclear reactors, and another 20 metric tons was reserved for space and research reactors. The remaining excess HEU is scheduled for disposal; most will be down-blended—mixed with natural or depleted uranium to turn it into LEU, which cannot be directly used in weapons but can be used to produce reactor fuel of the sort currently used. Another 22 metric tons that is in spent fuel will be disposed of in a geological repository.

According to the IPFM, by the end of December 2014 the United States had down-blended or shipped for down-blending 146.6 metric tons of HEU. The Department of Energy’s FY 2016 budget request indicates that all 186 metric tons of excess HEU that are scheduled for down-blending will be completed by the end of 2030. This is sooner than the previous target date of 2050, but still only requires down-blending about 2.5 metric tons of HEU per year to complete disposal of the 40 metric tons of HEU that is left, a much slower rate than in the past, when it was about 10 metric tons per year. The Department of Energy has said that this rate depends on “decisions regarding the U.S. nuclear weapons stockpile, the pace of warhead dismantlement and receipt of HEU from research reactors,” among other things. So far, speeding up the down-blending rate further does not seem to be a priority, but until all this excess HEU is down-blended, it remains a security risk.