April 1999

Sellafield plutonium discharges unaccounted for "Now you see it..."

New Scientist, 24 April 1999
By Rob Edwards

[Posted 25/04/1999]

MORE THAN A THIRD of the plutonium discharged into the sea from the Sellafield nuclear plant in Cumbria has gone astray, according to British government scientists. Some of it may be buried in sand, and more than previously thought could have been carried by currents out of the Irish Sea.

Between 1952 and 1995, Sellafield dumped 182 kilograms of plutonium down a pipeline into the Irish Sea. This amounts to 717 terabecquerels (TBq) of radioactivity--about half the fallout of plutonium in the entire North Atlantic from 520 atmospheric bomb tests in the 1960s.

But Peter Kershaw and his colleagues at the Ministry of Agriculture, Fisheries and Food's Centre for Environment, Fisheries and Aquaculture Science at Lowestoft say there is a "significant shortfall" between this and the amount detected by monitoring. They carried out a detailed audit and conclude that 36 per cent (257 TBq) of the plutonium is unaccounted for, along with up to 40 per cent (387 TBq) of plutonium's daughter element, americium.

These discrepancies are described as "disappointing and surprising" by Murdoch Baxter, the recently retired director of the International Atomic Energy Agency's Marine Environment Laboratory in Monaco. "Having made these excessive releases into the nearshore environment of a populated area and having monitored it extensively, they claimed for decades that almost all of the plutonium had remained in the Irish Sea sediments," he says. "Now, after all that effort and time, they say they don't know where 40 per cent of it is."

But British Nuclear Fuels, the state-owned company that runs Sellafield, says: "Given the period of time over which discharges have occurred that sort of correlation is not bad."

In a study published in a special Irish Sea issue of the Journal of Environmental Radioactivity (Vol 44, p.191), edited by Baxter, Kershaw suggests the missing radioactivity might be concealed in coarse-grained sand deeper than the 25 centimetres sampled in most surveys, such as the intertidal stretch on the Sellafield shore. He also says that the amount carried north out of the Irish Sea to Scotland and Scandinavia could have been underestimated (New Scientist, 27 February, p. 13).

Either way, Kershaw argues that the contamination is becoming less hazardous as it is being diluted. The bulk of the plutonium and americium that has been found is in a large mud bank at the bottom of the Irish Sea, with the rest spread along the coast or dissolved in water.

But Dudley Goodhead, director of the Medical Research Council's radiation and genome stability unit at Harwell in Oxfordshire, is not reassured. Plutonium and americium are dangerous in very small doses, he says. "By diluting them, all one is doing is spreading the risk more widely."

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