Engineering a Fix

In the hot and dry desert of the Middle East there are places where salt is squeezing up to the surface and forming "salt glaciers." Salt reaches the surface because there is not enough rain to dissolve it and wash it away. If you wait until it rains you'll see the salt glacier start to move. A little water mixed in helps the salt flow.

In 1983, after the main waste-handling shaft at WIPP was sunk and completed, a salt-eating monster was lowered down the hole and began chewing its way underground, hollowing out tunnels and rooms. As soon as a tunnel was cut, the cavity began squeezing inward and the floor and roof and walls began closing in three to five times faster than engineers had predicted. What was the reason? The salt beds contained more brine than expected.

If you take the public relations tour and descend into the WIPP tunnels the mine will appear to be dry ­ very dry. You may wonder why geologists were ever concerned that brine would seep into the completed repository. But the tunnels and rooms are dry, not from a lack of brine, but with the help of huge fans that circulate the underground air and evaporate water than seeps in.

Brine reacts with the drums and waste and generates gas. At first, the engineers thought that the gas might be of some advantage because the pressure that built up in the sealed chambers would help prevent brine from reaching the drums. But they also recognized that the pressure in the repository could build up to a point where it became "lithostatic" and rose as high as or higher than the pressure imposed by the weight of the overlying rock. An over-pressured repository was unthinkable when strategies were being developed for nuclear waste disposal. One of the firmest criteria was that a repository should never be allowed to reach lithostatic pressure. Such a condition would permit venting and provide pathways for moving water to get to and from the waste.

The criteria for overpressure was set aside at WIPP in favor of an engineered solution to the problem. Engineers hope to create a delicate balance been the inflow of brine, the creation of gas, and the amount of pressure that builds up after the repository is sealed. They will pack material around the drums to absorb some of the gas and maintain a pressure balance. It is hoped that the drums will remain reasonably dry and that the pressure will stay under control. The problem with this fix comes with realizing that WIPP is a one-shot experiment. When one considers that engineers owe their success to learning from their failures and we can only hope that in this 'best case" scenario of finding a perfect balance they get it right the first time.

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