Radioactive discharge measurement

Treatment of Magnox fuel at Sellafield produces a liquid waste product known as medium-active concentrate (MAC), which contains radioactive nuclides such as ¹³⁷Cs and actinides. A second process extracts most of this radioactivity from the MAC but is unable to remove ⁹⁹Tc, which is discharged into the Irish Sea under licence.

The current discharge limit set by the Environment Agency is 20 TBq/year but this is due to fall following the introduction of new waste treatment methods in 2004. Nevertheless, there is concern about the effects of⁹⁹Tc - with its long half-life of 213 000 years - on the marine environment.

Measuring seawater concentrations of ⁹⁹Tc at a particular location presents inherent difficulties, especially due to the pulsed nature of the discharge and the conservative characteristics of the pertechnetate ion. In order to overcome this, Jacqueline Pates and colleagues from Lancaster University and the Westlakes Research Institute, UK, have adapted the diffusive gradients in thin-films (DGT) technique to obtain time-integrated measurements.

In DGT, the solute diffuses through a membrane and is then trapped on a separate layer of selective binding agent. For technetium, the researchers employed TEVA resin gel as the binder, with a trialkylmethylammonium ion as the active component.

In laboratory tests on spiked sodium chloride solutions, the DGT response, measured by liquid scintillation spectrometry, was proportional to solution activity and deployment time, and independent of solution chemistry at pH 3-8 and ionic strength 0.01-1.3 M. Seawater has pH 8 and ionic strength about 0.7 M, so fits neatly into these ranges.

Field experiments were conducted in the River Calder, which runs close to Sellafield power station, says Pates. But all results were below the detection limits, she reports, possibly because there were no discharges during the test period.

Although the detection limits were relatively high at 0.05 and 0.025 Bq/L for deployments of either two or four weeks, respectively, they are consistent with concentrations observed recently in the Irish Sea.

Nevertheless, discharge rates could be reduced by using larger resin gel disks, by bulking those disks, or by using inductively coupled plasma mass spectrometry to measure bound technetium.

Steve Down