Attempting to design for an eternity – almost |

Attempting to design for an eternity – almost

Fifty-plus miles south of Grass Valley, the Rancho Seco Nuclear Generating Station was closed by voters in 1989. For more than a decade prior to the shutdown, the station’s reactor had generated electricity … and tons of high-level radioactive waste (“spent fuel”).

Lying in temporary storage on site, the intensely radioactive material awaits a final resting place where it can wait out the 250,000 years needed for its radioactivity to fall below that of the uranium from which it came (or at least, the 10,000 years Congress has deemed sufficient to keep it out of the biosphere).

Secretary of Energy Spencer Abraham announced last month that he would recommend to the president that Yucca Mountain, 75 miles north of Las Vegas, be the permanent repository for high-level nuclear waste.

Yucca Mountain is dry, as befits a mountain that straddles the transition between the Mojave (think Joshua trees and creosote bush) and Great Basin (think sagebrush) deserts.

Is it, however, dry enough?

Deep within a mountain, one can examine the rock of which it’s made before it’s been exposed at the surface, attacked by air and water – before it’s been weathered.

Within the mountain’s seemingly solid, seemingly dry rock, there can be moisture. The volcanic tuff of which Yucca Mountain is built is porous. Within the rock’s pores, water is tightly held, moving very slowly from one pore to the next … unless it gets hot. (I can recall campfires along the South Yuba River, ringed by river rock; as their pore water turned to steam, the rocks shattered.)

Intense radiation makes spent fuel hot to the touch. Placing such fuel below ground in Yucca Mountain will force water from the rock.

But as the fuel cools, the water will return. And if years of heat have weakened the rock, water may return faster and in greater quantities than when it left.

In an attempt to determine how the rock will respond to the heat emanating from 77,000 tons of spent fuel, the Department of Energy has run a test for several years below ground. Giant resistance coils (huge “toasters”), sucking up electricity from Hoover Dam, have been glowing day and night. The “heater test” room is sealed, but as I looked through a window, I could feel the heat on my face. The coils had raised the air temperature close to 400 degrees Fahrenheit.

Power to the coils was shut down last week; the room will now be allowed to cool for several years. The response of the room’s rocks will be monitored in an attempt to predict how the repository’s rocks will behave through the centuries.

Integral to the integrity of the fuel casks (as now planned) is Alloy 22, a type of stainless steel that would represent the last manmade barrier between the fuel and the environment.

Steve Frishman of the Nevada Agency for Nuclear Projects, a state agency set up as a watchdog for DOE, has for years been following technical developments at Yucca Mountain.

Translating DOE’s calculations into English, he points out that once water can freely reach the storage casks, and relative to the length of time the waste must be sequestered, the casks – stainless and all – will dissolve “instantaneously.”

The 10 millennia for which Congress has mandated the waste be kept out of the environment is about how long human civilization has been in existence.

Trained as a biologist, Alan Stahler is also an amateur astronomer. He teaches biology and geology at Bitney Springs Charter High School. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).

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