Geologists slamm Yucca models

By Kennedy Maize

How bad is the modeling that supports the Department of Energy's assertions about the safety and permanency of the Yucca Mountain nuclear waste dump? Execrable, according to legendary Duke University geologist Orrin Pilkey and his geologist daughter, Linda Pilkey-Jarvis, who works for the Washington state ecology department.

In their new book -- Useless Arithmetic: Why Environmental Scientists Can't Predict the Future -- Pilkey and Pilkey conclude, "If the project is required to go ahead on its scientific merit, the end of the Yucca Mountain saga is in plain sight." In a book that criticizes all attempts at quantitative modeling of complex environmental processes, the Pilkeys single out the Yucca Mountain project as a case of the worst of the worst.

The Yucca Mountain models, they argue, make unwarranted assumptions in order to come to foregone conclusions, a charge my late father, a mining engineer, used to make of economists. Their key example is the issue of "percolation flux," or the rate of downward movement of water in the rock above the storage site. "The higher the flux," they note, "the greater the likelihood that water will seep into areas where the waste material is stored."

Coming up with a percolation flux figure is difficult, particularly in an arid environment such as Yucca Mountain, noted the Pilkeys. Early in the 1980s, energy department scientists variously estimated the rate at 1-4 millimeters per year and changed that to 0.3 mm later in the decade. "One of the fundamental assumptions behind such downward flow rates," they said, "was that groundwater was moving not through fractures and faults but was moving through the tiny interstices between grains within the matrix of the rock." This, they say with considerable understatement, "was a puzzling assumption."

It likely was an assumption necessary to get the models to predict that water wouldn't penetrate the site earlier than the licensing conditions called for.

But starting in 1996, when tunnel boring into the site started, there was the opportunity for empirical data, by sampling levels of chlorine-36, an artifact of bomb tests on Bikini Atoll prior to 1963. "The discoveries from this sampling took project managers by complete surprise," wrote the Pilkeys, "and complete contracdicted the models." Rather than a percolation flux of 0.3mm/y, the data showed a rate more like 3,000 mm/y.

Suddenly, the DOE's models showing that the repository could meet the preposterous guidelines for the performance of the site was invalid. The site requirements call for no groundwater flowing through the site for at least 1,000 years. Noted the Pilkeys, "At Yucca Mountain, this mark was missed by two orders of magnitude! Water from the surface arrived in the tunnel in 50 years, after having traveled through almost 1,000 feet of rock."

This led DOE to abandon its earlier approach to waste isolation and shift to engineered barriers, including titanium "drip shields" to direct repository growundwater drizzle away from the spent fuel packages. DOE's multi-billion dollar modeling effect, the Pilkeys say, "was based on the questionable, even absurd, assumption that water was not flowing down fractures and faults within the rock."