What to do with plutonium

A highly strategic material and a hazardous type of waste

What becomes of plutonium is central to the waste issue. Should plutonium be considered as an incredible energy resource or as highly radiotoxic waste?

In the 1960s and ’70s, American misgivings about reprocessing operations performed to extract plutonium from reactors were due to the risk of proliferation.

Plutonium extracted from civil reactors run under normal conditions cannot be used to make atom bombs. Weapons-grade plutonium, however, can be extracted from fuel that has spent a little time in a reactor. This was how North Korea managed to make an atom bomb with very limited resources. To limit the risk of proliferation, the United States has maintained the right to inspect the use of plutonium produced, as most reactors were built under U.S. licences.

Used for civil purposes, plutonium is absolutely packed with energy: a single gram of plutonium is the equivalent of a tonne of oil! Should we leave this resource untapped? On the other hand, plutonium is highly toxic if ingested. Before disposing of it once and for all as ultimate waste, we must ensure that there is no way to tap any more of its precious energy potential.

In the 1990s, it was planned for plutonium from French reactors to be burned as fuel in Superphenix, the first large fast breeder reactor. This was an effective way of consuming plutonium and stabilising its inventory, in other words the amount of plutonium that had accumulated.

Since Superphenix was shut down in 1997, France has opted for a midway solution. Plutonium from civil reactors, which is 70% fissile, is incorporated in uranium oxide to make fresh fuel and thus help generate more energy. This fuel is called MOX (for Mixed OXides). Twenty or so French reactors have been adapted to burn MOX.

This alternative is only moderately effective. When the reactor is loaded, some of the plutonium is replaced by fresh plutonium. In the end, the plutonium becomes polluted with non-fissile isotopes, which are not a very profitable energy source. Although tests have shown that a second recycling run is possible, it has been decided to put the fuel through the reactor only once and to store the spent MOX in pools, where it continues to accumulate. The increase in plutonium inventory has slowed down, but not stopped.

MOX has an unexpected – and little-known – advantage: it can be of use in dismantling atomic bombs. There is nothing to prevent MOX from being made with weapons-grade plutonium, which contains more than 90% fissile plutonium-239. Burning this weapons-grade plutonium in a reactor will pollute it with non-fissile isotopes. Completely altered, it becomes useless for making bombs.

There are long-term prospects for reducing or stabilising the quantity of plutonium – or inventory – built up by reactors currently in operation. In theory, Generation IV reactors, distant descendants of Superphenix, would require the quantity of plutonium accumulated over 40 years’ operation of a PWR before they could be started up – around 2040! Thorium reactors could consume plutonium without producing any! And now for a little science fiction. In the distant future, will our descendants send these not enormous quantities of troublesome materials into the farthest reaches of outer space?