SCIENCE

The Hanford Site in Washington -- which produced fuel slugs for nuclear weapons, acted as a waste storage facility for nearly five decades, and was one of three primary locations for the Manhattan Project -- is among the most contaminated nuclear waste grounds in the country. A research team led by Peter C. Lichtner of Los Alamos National Laboratory (LANL) is using the Oak Ridge Leadership Computing Facility's (OLCF's) Jaguar supercomputer, located at Oak Ridge National Laboratory (ORNL), to build a three-dimensional model of an underground uranium waste plume at the Hanford Site's 300 Area. A better understanding of the underground migration properties of uranium, which has infiltrated the Columbia River, may aid stakeholders in weighing options for contaminant remediation.

"The project's results could certainly help one decide how to go about remediating the site, if it's even feasible," said Lichtner, whose project receives funding from the Department of Energy (DOE) Offices of Biological and Environmental Research and Advanced Scientific Computing Research. "The results could apply to other sites along the Columbia River that are contaminated too. And what we learn from this site we should be able to apply to other sites as well, not only at Hanford, but also around the country -- at Oak Ridge and other areas dealing with contamination."

The Hanford plume has been leaking contaminants into groundwater and the nearby Columbia River for decades. Waste from nuclear weapons production has been stored at Hanford since the early 1940s, mostly in underground tanks. But the uranium now penetrating the groundwater and river had simply been discharged to ponds and trenches, Lichtner said.

This research, among the latest in cleanup efforts of the Hanford Site, stems from a 1989 Tri-Party Agreement involving the Washington Department of Ecology, the Environmental Protection Agency (EPA), and DOE.

Lichtner's collaborators include Glenn Hammond of Pacific Northwest National Laboratory, Bobby Philip and Richard Mills of ORNL; Barry Smith of Argonne National Laboratory, Dave Moulton and Daniil Svyatskiy of LANL, and Al Valocchi of the University of Illinois, Urbana-Champaign.

Uncovering the unseen

The Hanford Site covers 586 square miles of land. Contaminants of several types and quantities are spread throughout the site, including uranium, copper, and sodium aluminate. The uranium plume is in Hanford's 300 Area, a roughly 1.5 square mile site approximately 109 yards west of the Columbia River.

As uranium decays it emits alpha particles. Because skin blocks alpha particles, external exposure is not deemed a risk. In fact, uranium is classified as a heavy-metal hazard rather than a radiation one. Ingestion in high doses can cause bone or liver cancer or kidney damage. The EPA has set a contaminant limit of 30 micrograms per liter. The uranium contaminating Hanford's 300 Area exceeds this limit by four times, according to field tests.

A challenge for Lichtner's team is to predict the loss of uranium from the plume into the river. Initial simulation results coupled with field tests indicate that from 55 to 110 pounds of uranium leech into the Columbia River each year from the estimated 55 to 83 tons of source uranium. Yet until further research is conducted, these numbers remain very uncertain, said Lichtner, whose goal is to decrease this uncertainty.

The team performed massively parallel simulations of depleted uranium flow through soil using PFLOTRAN, a code developed under a project called SciDAC-2, which aims to advance computing at the petascale -- or a quadrillion calculations per second. The code has been run on more than 130,000 processors of ORNL's Jaguar XT5 supercomputer to describe the flow of fluid through porous media, in this case the movement of soluble depleted uranium through a soil mixture of sand, gravel, and fine-grained silts. The plume measures 984 × 1,422 × 22 yards and was simulated using nearly 2 million control volumes, or grid cells, of 5.5 × 5.5 × 0.5 cubic meters each. The team calculated the uranium loss from the plume and the flux into the Columbia River at 1 hour intervals, which allowed construction of realistic models of the river's interaction with the migrating plume.

The chemical properties of uranium and additional compounds composing the plume require the model to account for more than 28 million degrees of freedom -- the number of actions these compounds might take as the plume migrates. The team simulated one year in only 11 hours by using more than 4,000 processors. Such speed is crucial to Hanford's timely remediation.