Britain has 139 tons of plutonium. That’s a real problem.

The United Kingdom’s last plutonium reprocessing plant, B205, located in Sellafield in northern England, will shut down by the end of 2020. It will bring an end to the era of plutonium separation in the country, which began 68 years ago. Because the United Kingdom never used any of the material it recouped from reprocessing except in nuclear weapons, today it has amassed a stockpile of almost 139 metric tons of separated plutonium.

This creates lasting problems: Plutonium stored in Sellafield is highly toxic and poses a permanent risk of proliferation. It is enough material to build tens of thousands of nuclear weapons. According to parliamentary estimates, storage will cost the British government about 73 million pounds a year for the next century. But after decades of public and private consultation, there is still no accepted plan for its disposition. In the meantime, the Nuclear Decommissioning Authority is working on the consolidation of the stockpiles in Sellafield and developing the capability to retreat the packages to allow for long-term storage once the government makes a final decision on permanent disposal. The United Kingdom views the material as a resource and is pursuing options that involve burning the plutonium in reactors, even though multiple assessments have shown risks associated with such a choice, namely immature concepts and technology. A better alternative would be to treat it as waste and begin planning for its permanent immobilization and burial.

Where did it come from? In the beginning, the British plutonium separation program was justified by military needs. A few years later, nuclear euphoria led to an increasing number of civil nuclear power plants and to dreams of nuclear-powered cars and planes. It was predicted that uranium resources would not be able to fulfill the need. As a result, the idea of a “closed” fuel cycle was born: instead of using nuclear fuel once and throwing it away, the spent fuel is reprocessed and reused in (mostly fast) reactors. In theory, this would allow greater utilization of uranium. However, the concept has never been demonstrated on an industrial scale, and only a few countries still aim at closing the nuclear fuel cycle.

From 1956 until 2015 the United Kingdom operated 26 Magnox reactors for commercial use. Magnox reactors are fueled with natural uranium, moderated by graphite, cooled by carbon dioxide gas, and designed in a way for efficient plutonium production. A pilot reprocessing plant, B204, started operation in 1952 and was replaced in 1964 by the B205 reprocessing plant. Combined, the two plants have separated more than 85 metric tons of plutonium from spent fuel.

In 1976, the United Kingdom started operating a new reactor class, the Advanced Gas-cooled Reactor, and 15 such reactors still operate today. To reprocess spent fuel from these reactors as well as spent fuel from overseas, the United Kingdom opened the Thermal Oxide Reprocessing Plant in 1995. Its operation record is a disaster: It never reached planned throughputs, had a serious leakage of radioactive material in 2005, and was much more expensive than originally intended. Consequently, it was shut down in 2018 before reaching the end of its planned service life—and after separating only 23 metric tons of plutonium.

But what happened to the fast breeder reactors that were supposed to burn up reprocessed fuel and close the nuclear fuel cycle? There are several reasons why there are only two fast reactors commercially operating, the Russian-designed BN-600 and BN-800. First, nuclear energy did not expand as foreseen in the 1950s and 1960s, while at the same time new uranium resources were discovered, easing worries about a dwindling uranium supply. Second, multi-cycled use of spent fuel has proven to be far more difficult than expected, and there are some risks inherent only to fast reactors. Finally, there is the latent proliferation risk of the technology to separate plutonium and uranium from the spent fuel.

Where will it go? Today, the United Kingdom’s civilian stockpile contains 139 metric tons of plutonium, including 23 metric tons owned by other countries, mostly Japan. The Nuclear Decommissioning Authority has discussed two disposition options. First is the reuse of plutonium in reactors. Reuse is touted as a proliferation-resistant option because the spent fuel would be too radioactive to handle, at least at first. Second is immobilization. Here, the plutonium is mixed with other materials that reduce the risk of leaching and complicate extraction. Potentially, the radioactive waste in the mix could also serve as a toxic obstacle to proliferation. Both options would still ultimately require disposal in a deep geological repository.

The Nuclear Decommissioning Authority’s preferred option still seems to be the reuse of the plutonium in mixed-oxide fuel for light water reactors. However, such an option depends on the availability and willingness of reactor operators to use such fuel. And not all operators are keen on the idea: EDF, the French-owned utility company that operates Hinkley Point C, the first nuclear power plant built in Britain in decades, denied the suggestion to consider the use of mixed-oxide fuel in 2013.

 Alternatively, the plutonium might be used in reactors that, according to their vendors, are better suited to cope with the plutonium stockpile. These could be either a CANDU-EC6 heavy water reactor or the small, fast, sodium-cooled reactor concept PRISM. Using mixed-oxide fuel in CANDU reactors seems viable, but the Nuclear Decommissioning Authority assesses no potential benefit compared to using the same fuel in a light water reactor—at greater implementation risk. In March 2019, the authority officially removed the PRISM reactor from the list of viable options, though even as early as 2011 it was stated internally that the “technology maturity for the fuel, reactor, and recycling plant are considered to be low.”

Nevertheless, the Nuclear Decommissioning Authority confirms that it will continue to monitor fast reactor programs.

Leaving aside the viability of fuel production, costs, and everything related to actual operation of the PRISM reactor, we conducted an analysis of GE Hitachi’s claims that PRISM “could conceivably make the entire UK plutonium stockpile proliferation-resistant in 20 years” by irradiation. Our calculations show that the claim is highly optimistic. Using plutonium as a reactor fuel has two effects: Some plutonium is burned, and the remaining is left in highly radioactive spent fuel. The radioactivity creates a barrier for malicious actors intending to steal and separate the plutonium from that fuel—providing proliferation resistance. However, due to radioactive decay, this barrier continuously decreases, while treatment of other parts of the stockpile is underway. Even though our study’s findings apply specifically to the PRISM reactor, we anticipate similar effects from other irradiation options. In the time it takes to treat the United Kingdom’s massive stockpile in reactors, the already treated material will slowly lose its proliferation resistance.

Why should the public sector continue to pay money for “new” reactor concepts—sometimes under development for decades—when it is not even clear whether these concepts might solve the problem at hand? The United Kingdom has to find a solution for its plutonium stockpile, and quickly. The British government, the Nuclear Decommissioning Authority, and reactor operators in general should accept that separated plutonium is a burden, not a resource, and authority should again take a closer look at immobilization options. These do not have the sheen of new, high-tech solutions like burning the plutonium in specially-tailored reactor concepts. But given that action is urgently needed, established and working concepts should be the way forward.