Uncertainties about nuclear waste storage

Published 28 November 2007

The waste created in the production of U.S. nuclear weapons is buried in Hanford, Washington; there is a growing uncertainty about the subsurface paths nuclear contaminants take, where they travel, and how fast

There is a growing interest in nuclear power, owing to the rise in oil prices and worries about climate change. As we wrote the other day though, it is not yet clear whether this interest will translate into action, such as building more nuclear reactors. It is true that there is a growing number of applications in the United States, Europe, and Asia for permits to build such reactors — but even if all the pending applications are approved, the number of reactors in the world will actually decline by 2020 because many of the current, and aging, reactors are due to shut down by then. There is no denial, though, about the growing interest in power generation. When we talk about nuclear power generation, we must talk about nuclear waste. So here is an example of dealing with nuclear waste.

The U.S. nuclear weapons program produces millions of gallons of hazardous waste. This waste lies in a remote location in southeastern Washington state called Hanford. Deep beneath — critics say that this is not deep enough — the stark desert landscape about two million curies of radioactivity and hundreds of thousands of tons of chemicals are captured within the stratified vadose zone, which gives rise to complex subsurface flow paths. We do not know much about these subsurface paths, and there are uncertainties about where, exactly, the contaminants go and what happens to them. With the mighty Columbia River bordering much of the site, where these nuclear wastes migrate, their composition, and how fast they are traveling are of critical importance to both people and the environment.

The November issue of Vadose Zone Journal offers a series of papers addressing the mysteries within the vadose zone beneath Hanford. The series outlines scientific work funded by the Department of Energy (DOE) and carried out by scientists at Pacific Northwest National Laboratory and contributing associates with other national laboratories, universities, and contractors. The detailed series outlines how researchers have investigated Hanford’s vadose zone so as better to understand the migration of these contaminants, ultimately reducing or stemming their flow toward the Columbia River in order to protect the river and the people living downstream. By studying the geologic, biologic, geochemical, and hydrologic conditions at the Hanford site, the researchers seek to understand and manipulate the factors that control contaminants’ fate and transport. To date, studies show that fine-grained sediment layers along with rain, snowfall, and other climatic conditions affect contaminant transport. For three decades scientists have studied what happens when water enters and exits the soil, particularly how it affects the movement of the contaminants under various conditions. “Understanding how hydrology and chemistry are interacting below the land surface in the vadose zone and the factors that control those interactions are keys to ultimately dealing with the legacy from nuclear waste production at the Hanford site,” said Glendon Gee, Laboratory Fellow at Pacific Northwest National Laboratory. Gee is lead author on the overview paper of the series.

Chemical studies indicate that a number of contaminants, such as cesium, react strongly with Hanford sediments and move only under extreme conditions. Researchers found that another contaminant, uranium, reacts with the sediments in complex ways and its migration varies under different conditions. Other contaminants, such as tritium and nitrate, are relatively mobile. These contaminants have been transported deep into the vadose zone and reached the groundwater. Carbon tetrachloride and other organic compounds have moved in complex ways, as both vapor and liquid, and reached the groundwater. Additional studies of the fate and transport of contaminants in the vadose zone are ongoing at the Hanford Site. These studies will characterize the extent of contaminant plumes, determine how fast or slow they are migrating and evaluate remediation solutions.