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PLUTONIUM has sprung a nasty surprise. Chemists have discovered that the
compound of the metal used most by the nuclear industry, plutonium dioxide, is
not as inert as everyone thought.

This could force a rethink of plans for the long-term storage of plutonium
from nuclear power stations and weapons. And accident scenarios for
plutonium-burning reactors may have to be rewritten.

PuO₂ is a yellow crystalline solid that鈥檚 made when spent fuel from
conventional nuclear reactors is reprocessed. It is also the key component of
the mixed oxide (MOX) fuel burnt in some European and Japanese power stations.
Nuclear chemists have always regarded it as insoluble and relatively easy to
contain.

But now a team led by John Haschke at the Los Alamos National Laboratory in
New Mexico has found that plutonium dioxide reacts slowly with water and oxygen
to form a previously unknown green crystalline compound. 鈥淭he new oxide has not
been noticed before because it forms very slowly and is very difficult to
detect,鈥� says Haschke.

In the new oxide, more than a quarter of the plutonium atoms adopt a higher
oxidation state, which allows its crystals to include more oxygen than normal.
The exact composition of the crystals is not yet known, but the compound has
been dubbed PuO_2+x.

Unlike PuO₂, the new compound is soluble in water, which could
explain why contamination has spread so fast in groundwater at the US鈥檚 nuclear
test site in Nevada, says Haschke. Another team from Los Alamos and the Lawrence
Livermore National Laboratory in California reported last year that plutonium
from one blast had migrated 1.3 kilometres in 30 years.

What鈥檚 more, the reaction that forms PuO_2+x also produces hydrogen.
This could mean that, over time, dangerous levels of this explosive gas might
build up in containers of plutonium.

It鈥檚 too early to tell whether the new oxide will pose serious problems for
the nuclear industry. But Haschke鈥檚 findings have already convinced Charles
Madic, research director at the fuel cycle division of the French Atomic Energy
Commission, that plans for storing plutonium will have to be re-evaluated. These
include a French scheme for long-term storage of spent fuel. Constructing stores
that exclude water and prevent the formation of PuO_2+x would have
鈥渕ajor consequences for price and design鈥�, says Madic.

British Nuclear Fuels, which produces PuO₂ at its Sellafield plant
in Cumbria, maintains that Haschke鈥檚 findings don鈥檛 have major implications for
the manufacture and use of MOX fuel, as there are already strict controls on
moisture levels around plutonium. However, Madic says that the new oxide could
increase the likelihood of plutonium being released into the environment in the
event of an accident.