A pellet of plutonium-238 used for power generation glows under its own light.

Credit: U.S. Department of Energy

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SYDNEY: Plutonium from one of the world's most contaminated nuclear waste sites has travelled kilometres by hitching a ride on particles in the groundwater, U.S. researchers say.

"Based on several studies [plutonium] transport rates in the order of a few kilometres in tens of years appear to be possible," said Rodney Ewing, Professor of geological sciences at the University of Michigan in the USA.

In a study published today in the U.S. journal Science, Ewing and colleagues report that plutonium, a hazardous radionuclide found in nuclear waste, travels in groundwater by hitching a ride on small particles called colloids.

According to Ewing, "transport on colloids provides a mechanism that allows higher than expected concentrations of plutonium to show up in groundwaters some distance from a source."

The researchers studied a nuclear waste repository site in Mayak, Russia, where plutonium salts were responsible for contaminating a lake connected to a groundwater system.

"There was a definite source for the plutonium that provided concentrations that were high enough to give us the chance to find the plutonium on the colloids," said Ewing.

The researchers found that through binding to iron-oxide colloids, plutonium had travelled about four kilometres in the groudwater over a period of 55 years.

However the authors note that this study cannot be applied to all nuclear waste repository sites: each has different physical and chemical conditions, so specific studies of how these polluted particles travel will be necessary for each site.

"This is an important mechanism for the transport of [plutonium] that must be considered when developing a remediation strategy for a contaminated site, or for the assessment of the safety of a nuclear waste repository," said Ewing.

Plutonium, a metallic element, is used in some nuclear power generators, and is also a key component in most modern nuclear weapons.

All isotopes and compounds of plutonium are radioactive and toxic. The most important isotope is Pu-239, which has a half-life of 24,110 years. The most stable is Pu-244, with a half-life of about 80 million years.