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The most toxic and voluminous nuclear waste in the U.S.—208 million liters —sits in decaying underground tanks at the Hanford Site (a nuclear reservation) in southeastern Washington State. It accumulated there from the middle of World War II, when the Manhattan Project invented the first nuclear weapon, to 1987, when the last reactor shut down. The federal government’s current attempt at a permanent solution for safely storing that waste for centuries—the Waste Treatment and Immobilization Plant here—has hit a major snag in the form of potential chain reactions, hydrogen explosions and leaks from metal corrosion. And the revelation last February that six more of the storage tanks are currently leaking has further ramped up the pressure for resolution.
After decades of research, experimentation and political inertia, the U.S. Department of Energy (DoE) started building the “Vit Plant” at Hanford in 2000. It’s intended to sequester the waste in stainless steel–encased glass logs, a process known as vitrification (hence “Vit”), so it cannot escape into the environment, barring natural disasters like earthquakes or catastrophic fires. But progress on the plant slowed to a crawl last August, when numerous interested parties acknowledged that the plant’s design might present serious safety risks. In response, then-Energy Secretary Steven Chu appointed an expert panel to find a way forward. Because 60 of the 177 underground tanks have already leaked and all are at increasing risk to do so, solving the problem is urgent.
Vitrification prep 101: Some tough homework
The plant’s construction, currently contracted by the DoE to Bechtel National, Inc., may be the most complicated engineering project underway in the U.S. But back in 2000 the DoE and Bechtel decided to save time and money by starting construction before crucial structures and processes had been designed and properly tested at a scale comparable to full operation. This wasn’t such a good idea, says Dirk Dunning, nuclear material specialist with the Oregon Department of Energy. “The worst possible time to save money is at the beginning. You’re better off to be very nearly complete on design before you begin construction.”
The vitrification project calls for the waste to be analyzed chemically and radiologically before it enters a pretreatment facility to be separated into various constituents such as cesium 137, strontium 90 and metals. After that, each separate waste stream is channeled as either high-level or low-activity waste into designated melters. The glass is created by mixing sand with a few additives like boron; the waste is stirred in, and the whole mess is melted, then decanted into the steel canisters. After the glass logs solidify the waste is trapped and should be isolated from the environment for long enough for most of the radioactivity to decay to safe levels.
The low-level waste canisters will be stored permanently at Hanford. Because the planned Yucca Mountain geologic repository project was halted by the Obama administration, the high-level waste canisters will be kept at Hanford in an as-yet unconstructed building. In January the DoE announced it is beginning work on a new “comprehensive management and disposal system” that will make a permanent geologic repository available by 2048. Yet even if all goes perfectly from now on, it will take until 2062 to vitrify all the waste.
The waste presents significant challenges for Vit Plant project engineers and nuclear chemists. For one thing, the waste varies wildly from tank to tank. The former nuclear weapons facility at Savannah River, Ga.—also part of the Manhattan Project—has been successfully vitrifying weapons waste for years, but only one fuel separation process was used there. At Hanford there were nine production reactors making plutonium and uranium fuel using at least six different radiochemical processes whose chemistry, and thus constituents, were very different. This remains true of the waste as well. There are large differences in composition from tank to tank that necessitate chemically profiling the waste in batches before it enters the Vit Plant, which may also require changes to the glass formula at the other end of the process.