Declaring more U.S. weapon-grade uranium excess could delay the need to build a new national enrichment plant

Frank von Hippel

The U.S. Department of Energy (DOE) has launched an initiative to build and begin operating by 2038 a new small-capacity national military uranium enrichment plant. Its initial mission would be to produce low-enriched uranium (LEU) to fuel two Tennessee Valley Authority nuclear power reactors that produce tritium for U.S. warheads as a byproduct. DOE's cost range estimate for the new facility is $3.1-11.3 billion.

US nuclear weapons rely on several grams of tritium each to boost more than ten-fold the yields of their primary fission "triggers". The boosted primary explosion drives the fission-fusion secondary stage that releases most of the total yield of one hundred kilotons or more of these weapons. The tritium must be replenished since it decays with a half-life of 12.3 years.

The Tennessee Valley Authority operates a power reactor (soon two) to make tritium in lithium-6 containing rods inserted into the reactor core. The reactors together use about 50 tons a year of low-enriched uranium containing 4.5% U-235. The United States no longer has a government-owned uranium enrichment plant that can produce the LEU for these reactors.

The last LEU for U.S. tritium production was enriched by DOE's Paducah Gaseous Diffusion Plant prior to its shutdown in June 2013. Paducah, the last operating U.S.-government-owned enrichment plant could not compete with newer, more energy-efficient commercial civilian gas-centrifuge plants that have been built by other countries, including a plant built in New Mexico by the British-Dutch-German consortium, URENCO.

DOE argues that the U.S. cannot fuel U.S. tritium-production reactors with LEU enriched in foreign-owned plants since the tritium is for nuclear weapons. The LEU from foreign-owned enrichment plants is "obligated" to be used only for peaceful purposes. It is this interpretation, which is not shared by others, including URENCO, that DOE uses to justify the current drive to build its own enrichment plant.

DOE's 2015 report to Congress, Tritium and Enriched Uranium Management Plan Through 2060 identified sources of enough LEU to fuel the Tennessee Valley Authority reactors until about 2040. If additional HEU could be released from the U.S. weapons stockpile for blend-down, however, the "unobligated" LEU supply for these reactors could be extended.

It is possible to make an independent estimate from public data of the amount of HEU the U.S. still has available for weapons and how much might be declared excess to be blended down to provide LEU fuel for the tritium-production reactors. About 1000 tons of 4.5% LEU would be required to fuel the tritium-production reactors for an additional 20 years. This could be derived by blending down 41 tons of weapon-grade HEU (93.5% U-235) uranium with natural uranium (0.72% U-235).

The United States still has more than 200 tons of HEU available for weapons, including in about 2,600 retired warheads awaiting dismantlement. Assuming, based on public information, that U.S. warheads contain an average of about 25 kilograms of HEU each, there are about 100 tons of HEU in the U.S. current nuclear stockpile of about 3,800 warheads.

This leaves more than 100 tons of HEU that is surplus to arsenal needs. If about 41 tons of HEU were to be declared excess and blended down to LEU by 2035-2055, that would be sufficient to provide for U.S. tritium production until about 2060. In that case, DOE's proposal to have a new national enrichment plant on line to produce LEU by 2038 would be premature by at least two decades. This conclusion should be checked by analysts with access to the classified information on how much HEU is in operational U.S. nuclear warheads.