The production of electricity by any technology produces waste: coal burning creates ash and air pollution, solar-cell wastes contain hazardous heavy metals such as arsenic and cadmium, and nuclear power generates radioactive wastes. Protecting the environment requires careful control of all of these wastes. Radioactive wastes are also a by-product of defense activities, research, medical applications, and industrial production. ORNL has long engaged in development of new technology to ensure safe and environmentally benign disposal of these wastes.
The basis of radioactive waste management is simple. Because radioactive materials become less radioactive over time, the way to safely dispose of radioactive materials is to store them until they become nonradioactive. Different radioactive materials have different half-lives, so their storage times are different. For example, cobalt-60, a radioactive isotope used to sterilize medical equipment and treat cancer, has a five-year half-life. Thus, a pound of radioactive cobalt-60 five years later would decay to a half pound. Every five years the quantity is reduced in half. A rule of thumb is that, after 10 half-lives have passed, the material is essentially no longer radioactive. In contrast, chemical wastes, such as dioxin, can be destroyed by incineration, and toxic heavy-metal wastes, such as lead and arsenic, remain toxic forever and require other waste management technologies to isolate them from the environment.
Very short-lived radioactive wastes are disposed of by storing them in buildings until they are nonradioactive. For example, hospitals use radioactive molybdenum-99/technetium-99m to diagnose diseases and injuries of the brain, liver, and other organs. Because this material has a half-life of three days, hospitals can store molybdenum-99 radioactive wastes for a month until they become nonradioactive. Longer-lived radioactive wastes containing cobalt-60 and similar radioactive materials must be isolated from the environment for approximately 100 years. Such materials are called low-level waste (LLW) and typically become relatively nonhazardous within a century or less. High-level waste (HLW), including spent fuel from power reactors, contains radioactive materials that must be isolated for thousands of years.
Disposal Technologies
The major goal of radioactive waste disposal is to isolate waste materials from groundwater and air until the radioactivity has decayed away. The basic approach is to convert waste to solids that do not readily dissolve in water, put these insoluble solids in sealed waterproof containers, and store the containers away from moving water. For LLW a well-built structure can store the waste. For HLW, the waste must be buried hundreds of meters underground to ensure its isolation from people for thousands of years. In nature, toxic ore deposits, such as lead, arsenic, and cadmium, which have toxicities similar to HLW in a disposal site, do not poison people because the ores are isolated deep underground. Thus, similar underground isolation is planned for disposal of HLW.
ORNL is involved in developing improved methods of LLW and HLW isolation, including engineered waste storage. Michael Gilliam of ORNL's Chemical Technology Division (CTD) is working to devise better wasteforms to solidify liquid LLW. Better methods to predict and demonstrate the long-term performance of waste disposal sites are being developed by researchers in the Environmental Sciences Division. Larry Shappert of CTD and others are testing waste packages to ensure their ability to both store wastes and survive accidents during waste transportation.
On the Oak Ridge Reservation, the staff of the Waste Management and Remedial Action Division is demonstrating tumulus disposal of LLW--a method of storing wastes for more than 100 years until they become nonhazardous. In a tumulus, the solidified waste is confined in concrete packages that are placed on a special concrete pad and covered with layers of different materials to prevent the flow of rainwater through the wastes.
Finally, research is under way at ORNL on advanced disposal options for spent fuel and HLW. Two options are island repositories and waste destruction.
The island repository is a good example of advanced exploratory research--work I have been involved in--to find environments for hazardous wastes where natural forces isolate the wastes from humans. The idea is to place the waste 500 meters under the ocean floor in manmade tunnels accessible by mineshaft on a temporary manmade island. After the repository is filled, the tunnels are backfilled and sealed, and the temporary island is removed. It has several potential advantages.
* No groundwater flow. Waste disposal sites fail when groundwater runs downhill through a disposal site, dissolves hazardous material, and carries it to the environment by a spring or well water. Under most of the ocean floor, groundwater doe