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Vegetative Uptake of Technetium-99 from Buried, Solidified, Low-Level Radioactive Waste

Westinghouse Savannah River Co., Savannah River Site, Aiken, SC 29808;

Trudy L. Johnson

formerly with E.I. du Pont de Nemours, Savannah River Lab., Aiken, SC 29808.

^* Corresponding author.

ABSTRACT

At the Savannah River Site, it is planned to dispose of liquid, lowlevel nuclear waste by incorporating the waste into a solid, concrete waste form called saltstone. Saltstone monoliths will be buried in claycapped, subterranean vaults. Although the vault/cap burial system is designed to severely limit the access of vegetative roots to the saltstone, a study was begun in 1984 to determine the potential for root uptake of radionuclides, in case an unforeseen accident should cause exposure of the buried saltstone. Thirty-two lysimeters were designed, constructed, and filled with soil. Saltstone samples, containing the liquid, low-level supernate from a waste disposal demonstration program, were buried in some of the lysimeters. The blocks were placed at varying depths to investigate the relationship between source depth and plant uptake. Other lysimeters, not containing saltstone, were used as controls. Crops, grass, and trees were planted in the lysimeters and sampled periodically to determine radionuclide concentrations. Water samples also were collected from the lysimeter sumps and analyzed for radionuclide content. In the 5 yr following establishment of the lysimeters, only technetium-99 (Tc-99) and cesium-137 (Cs-137) could be detected at elevated concentrations in the vegetation. The Cs-137 concentration difference between the plants grown in control and saltstone lysimeters was so small that the difference was only barely detectible against background. The Tc-99 concentration was significantly higher in the vegetation and the sump water of the lysimeters containing saltstone, when compared to the control lysimeters. The Tc-99 uptake by the vegetation appears to be related to release of the Tc-99 from the saltstone into the soil water. The Tc-99 concentration in vegetation and sump waters shows a similar pattern, including an initial period of very low concentrations followed by a coincident build-up period and a period of relatively uniform concentration. Vegetative uptake factors were calculated for uptake of Tc-99 from soil water. The results were very similar for the vegetative parts of all the species grown, 111 to 376 (nBq/kg_vegetation)/(nBq/kg_water). The uptake factor calculated for the seeds of crops was less by a factor of 100 than the uptake factor for vegetative parts.

The information contained in this article was developed during the course of work under contract to the U.S. Dep. of Energy (DE-AC09-89SR18035). By acceptance of this paper, the publisher and/or recipient acknowledges the U.S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper, along with the right to reproduce and authorize others to reproduce all or part of the copyrighted paper.

Received for publication December 3, 1991.