Plutonium can be used to make nuclear bombs. But it can also be recycled and used as a fuel in nuclear reactors. According to the authors of a Nature editorial, the UK—which has the largest civilian stockpile of plutonium, around 90 tons—should skip recycling and stockpiling plutonium and simply bury it. They argue that recycling is simply too expensive and risky.

World stockpiles hold about 500 tons of plutonium, which is an enough to make 100,000 nuclear weapons. That number goes up quite a bit when you account for the amount locked in nuclear waste—around 620 tons in the United States alone, a figure that increases by 23 tons every year.

Now, at first glance, the idea of just burying hundreds of tons of useful plutonium seems ridiculous. With all the world’s energy concerns, why wouldn’t we use it? As we covered in our recent feature on the future of nuclear energy in the US, plutonium can supplement traditional uranium fuel to power existing nuclear reactors. This combined fuel, made up of plutonium and depleted uranium, is called mixed oxide (MOX). Plutonium is even more effective in fast breeder reactors, but these haven’t been commercially successful, despite development work dating back to the 1950s.

Radioactive recycling

France has been separating and recycling plutonium for use in MOX fuel for almost 20 years (although the program was originally for nuclear weapons). However, since reprocessing is so expensive, using recycled plutonium actually adds nearly $750 million a year to electricity generation costs compared to burying it and burning uranium alone.

Both Japan and Great Britain pursued similar plutonium recycling schemes, but neither were functional, much less successful. Japan’s plutonium reprocessing plant only operated for two years, separating just four tons before malfunctioning and shutting down in 2008. The same malfunction killed an attempted restart in January and, since the Fukushima disaster, the future of Japan’s entire nuclear program is up in the air. As of Saturday, none of the country’s nuclear reactors are running.

Britain built a MOX fabrication plant in 2001 that operated at one percent capacity until it was closed last year. That experiment cost $2.3 billion.

Why is plutonium reprocessing and MOX fuel fabrication so expensive? For one thing, the plutonium has to be separated from radioactive nuclear waste. Most approaches are variations on the Plutonium-URanium Extraction (PUREX) technique, which was developed during the Manhattan Project. This process involves dissolving spent fuel in nitric acid, then extracting plutonium and uranium using an organic solvent. This may sound simple enough, but handling and reprocessing extremely radioactive used fuel is expensive.

Creating MOX fuel pellets, on the other hand, requires precise machining of fuel pellets to fit in long zirconium tubes. Add these costs to the already expensive reprocessing stage, and you have plutonium-based fuel that costs around five times that of new uranium fuel.

Just bury it already!

We’ve established that recycling plutonium is more expensive than it’s worth. What else can we do with it? According to the authors of the commentary, the safest and most cost-effective route is to simply stop reprocessing and dispose of our existing stockpiles of plutonium. First, the plutonium would be “immobilized” by encasing it in ceramic. Then, in order to prevent would-be nuclear terrorists from stealing it to make a nuclear bomb, this immobilized plutonium could be buried with radioactive spent fuel or nuclear waste in 500-meter deep repositories.

Another option would be to dump this mixture into 5000-meter-deep boreholes where it could never be retrieved, although this seems like the beginning of a story involving mole people.

Although a number of countries are moving forward with plans for storing nuclear waste, the prospect of storing purified plutonium hasn’t been explored extensively since most countries have been pursuing the recycling option.

Nature, 2012. DOI: 10.1038/485167a (About DOIs)