NUCLEAR WASTE

KINDS OF NUCLEAR WASTE

Nuclear waste is radioactive material that has no immediate use. In the United States nuclear waste generally is divided into two main categories: high-level nuclear waste (HLW) and low-level radioactive waste (LLW). Two less common categories are tailings from uranium mining and milling, and a special category derived from particular aspects of nuclear weapons production and defense-related activities. This latter, less commonly discussed, category is called defense wastes and, because of its makeup, is sometimes called transuranic wastes; it makes up about half of the HLW in the United States. This article focuses on HLW and LLW from nuclear power reactors.

HLW comprises most of the radioactivity associated with nuclear waste. Because that designation can cover radioactive waste from more than one source, the term spent nuclear fuel (SNF) will be used to discuss HLW originating from commercial nuclear reactors. LLW comprises nearly 90 percent of the volume of nuclear waste but little of the radioactivity. Nuclear power reactors produce SNF and most of the nation's LLW, although there are approximately 20,000 different sources of LLW. The name SNF is a bit of a misnomer because it implies that there is no useful material left in the fuel, when in fact some fissionable material is left in it.

CONCERN FOR RADIOACTIVE WASTES

Nuclear waste is a concern due to the levels of radioactivity in it, especially in HLW. Relative to the wastes associated with other major methods of power production, the volumes are small. The concern is for the amount of radioactivity and the time of its duration. The time of radioactivity duration is measured in half-lives (the time required for half of the atoms of a given substance to disintegrate, at which time it becomes the new starting amount with which to begin the count again). Generally LLW not only has lower levels of radioactivity, it also has very low concentrations of long-lived radioactive substances. There are short-lived and long-lived substances in nuclear waste, with half-lives varying from seconds to thousands of years.

In the United States the federal government has taken responsibility for HLW, for both SNF and defense wastes, and for mill and mine tailings. Remediation of the effects of such tailings is well under way. Each state or groups of states (compacts) are responsible for the safe isolation of their own LLW. Three departments of the federal government have prime responsibility for matters related to nuclear waste: Department of Energy (DOE) and its predecessor agencies, which created most of the defense wastes; the Nuclear Regulatory Commission (NRC); and the Environmental Protection Agency (EPA). Regulations regarding care of nuclear wastes are also set by individual state agencies, especially for the LLW.

The federal government not only has responsibility for the safe isolation of SNF but also taxes on the generation of electric power to cover the cost of waste isolation at the rate of one mil/kWh ($0.001/kWh). Although the nuclear waste fund has been folded into the general federal budget, it comes from a special levy that can be changed to accommodate the needs of SNF handling. The office that manages this fund and that is responsible for SNF is in the U.S. DOE and is called the Office of Civilian Waste Management (OCRWM).

ORIGINS OF THE RADIOACTIVITY IN SPENT NUCLEAR FUEL

The nuclear waste generated from production arises when uranium atoms in the fuel split. Nuclear power reactors are fueled with assemblies of rods containing pellets, each about the size of a pencil eraser. About 3 percent of the uranium in these pellets is the fissionable isotope, or form, of uranium, U-235. This is the rarer of the two naturally occurring isotopes of uranium. In nature, the U-235 isotope is found in less than 1 percent of uranium ore—usually 0.5 to 0.7 percent of that ore. After uranium ore is mined and processed, the isotopes are separated, in a process called uranium enrichment. This is not a simple task and is mostly done by a gaseous diffusion plant. Usually the process is stopped when the mixture is about 3 to 3.5 percent of the fissionable form in the mix. (The majority uranium, U-238, is radioactive in its own right, emitting a short-range alpha particle, but it is not fissionable. When the fissionable isotope is extracted from uranium, the remaining material is called depleted uranium and has its own other uses, which will be addressed below.) The exact makeup of the reactor fuel varies some what but is mostly in the form of uranium dioxide, with the uranium being about 3 percent fissionable U-235.

There are three main products in the fissioning of uranium-235: tremendous amounts of energy, neutrons that perpetuate the chain reaction, and two new atoms. The splitting is such that eventually every element found on the periodic chart of the elements is made, especially in the flux of speeding neutrons that induced and perpetuated the fission. Every isotope of every element can be found in the waste. Many of the new isotopes produced are highly unstable—that is, they are radioactive. Many decay in seconds, but some exist in their unstable form much longer, eventually decaying into stable isotopes, though often with intermediates that are also radioactive. This results in radioactive decay chains. These chains of radioactive decay mean that SNF will be radioactive for a long time. It has been estimated that it will be approximately 7,000 years before the level of radioactivity in SNF drops to that of natural radioactivity, that of Earth itself. Since some parts of Earth will be radioactive forever, it can be said that SNF will be radioactive forever. In practice, one must decide what levels of radioactivity to call dangerous. That debate influences decisions about nuclear waste isolation. The debate about what is "safe" is not part of this particular topic. The amended version of the Nuclear Waste Policy Act (NWPA) calls for the safe isolation of HLW from the human environment for 10,000 years.

Besides fission products, the various forms of known but newly formed elements in the spent nuclear fuel, there is a small but significant amount of fissionable, or fissile, material in the SNF. This is quite important. There is some unused, unfissioned U-235 that has become too dilute to use. Like natural uranium ores in which chain reactions do not occur, the fuel will not sustain a chain reaction. But new fissile material also has been made in the intense neutron irradiation. Elements are made that are heavier than naturally occurring uranium, and because of their location on the Periodic Chart are referred to as transuranics. There are isotopes of plutonium and uranium made that can fission. None is present in concentrations that are great enough for a self-sustained chain reaction, but they do represent new fuel. The concentrations of U-239, Pu-239, Pu-240, Pu-241, and some short-lived intermediates are 1–2 percent of the SNF. Some of these are fissile material. Breeder nuclear reactors can be designed and run to produce significant amounts of these isotopes. "The SNF from a breeder reactor is rich in newly produced fissionable isotopes but it must undergo extensive reprocessing to become new reactor fuel. That reprocessing will adjust the concentration of fissionable isotopes and eliminate some of the fission products that tend to quench fission process."

HANDLING OF SNF

Workers wearing gloves can handle nuclear fuel that is being freshly installed into a nuclear reactor. This fuel assembly becomes dangerously and highly radioactive SNF after a short time in an operating nuclear reactor. Upon removal from a reactor (after about 18 months), a spent fuel assembly is now HLW and is far too "hot" thermally and radiologically to handle directly. It is removed remotely with a crane and stored in a bay of cooling water beside the reactor.

HLW still looks like a fuel assembly, a collection of long, skinny rods, each filled with fuel pellets, held in a rack that allows water to pass through and pick up thermal energy. The assembly is kept underwater to cool and also to shield the workers from the longer-range gamma radiation. The water also contains corrosion-inhibiting chemicals as well as chemicals to absorb the few neutrons emitted from lingering atoms of U-235 and other fissile material. Generally a ten-year cooling-off period allows that much of the heat to be emitted and the radioisotopes with short half-lives to have undergone decay. For power plants with limited pool storage there are two choices when the cooling pools are filled: rod consolidation and dry cask storage.

Rod consolidation can permit space saving by dismantling the fuel assembly racks. The rods are still maintained underwater. Rod consolidation is not a routine practice due to issues of heat dissipation and criticality, the possibility of a chain reaction continuing. Dry cask storage is the more common approach for the oldest fuel assemblies and calls for removal of the fuel assemblies from the cooling pool. Designs of the casks vary in detail, but they usually accommodate several spent fuel assemblies in a sealed steel container that is enclosed in a concrete box and/or metal canister. Usually these containers are stored and monitored at the plant site. As with all major operations at nuclear power plants in the United States, hearings before the NRC and the public are conducted on temporary SNF storage. Almost all dry cask storage occurs at the power plant site, where monitoring of the containers can be routine.

ULTIMATE ISOLATION OR REPROCESSING OF SNF

The ultimate disposition of HLW or SNF is a matter of significant importance and is controversial for some. In the United States all HLW, including SNF, has always been a federal responsibility, beginning with the Atoms for Peace Program in the 1950s. A study done by the National Academy of Sciences in 1957 indicated that nuclear waste disposal would not provide insurmountable technical problems and recommended deep geological isolation. Every country that has nuclear power plants has considered the safe isolation or disposal of HLW and has concluded that deep geologic isolation is the safest method. In the United States eight options, including deep-ocean-trench burial, space launches, Antarctica burial, and transformation into materials with shorter half-lives have all been studied. The media into which the HLW is to be isolated varies widely from country to country and include granite, salt beds, and tuffaceous material, welded volcanic ash, most commonly called tuff. United States attention is focused on Yucca Mountain, at the western edge of the Nevada Test Site, the location of above-and below-ground nuclear weapons testing in the past. It is a mountain of tuff.

YUCCA MOUNTAIN, NEVADA

The amended version of the NWPA has determined that only Yucca Mountain is to be characterized as a possible geologic repository for SNF and possibly some defense waste. It is approximately 100 miles northwest of Las Vegas, Nevada. The planned location of the SNF is approximately 1,000 feet above any groundwater and about the same distance below the surface. Characterization studies of the viability of Yucca Mountain are under way and have been since the early 1990s. The NWPA also prohibits studies of granite as a host rock. (Canada is considering SNF isolation in granite. Most of the eastern half of the United States is on granite.) Both private and governmental leaders—most concerned about tourism—in the state of Nevada are resisting the Yucca Mountain characterization studies. Furthermore, although the majority of scientists studying the mountain find no clear problems, a few find fault with the studies ad the site as a possible repository. The most difficult part of the studies involves predictions about the behavior of the tuff environment for 10,000 years and beyond. The fact that Yucca Mountain is in a region of ancient volcanic activity complicates the assessments. Most attention is devoted to water movement through the tuff and the vulnerability of the underlying earth to undergo any significant "stretching" in the foreseeable future. The Yucca Mountain project has a website listed in the bibliography for this article, with photos and links to scientific data being accumulated as well as links to opposition points of view.

Yucca Mountain, if it becomes the site for the isolation of SNF, will be laced with tunnels, waste in storage casks and monitoring equipment. A waiting period is planned while better isolation alternatives are sought. If Yucca Mountain is not used, it is to be refilled with the tuff material removed earlier. In the United States the SNF that would be isolated in Yucca Mountain would be waste that has not been reprocessed; it would be material that has come out of nuclear reactors and has been cooled at the plant site.

WASTE REPROCESSING

Since the amount of fissile material in the fuel assemblies is only about 3 percent of the uranium present, it is obvious that there cannot be a large amount of radioactive material in the SNF after fission. The neutron flux produces some newly radioactive material in the form of uranium and plutonium isotopes. The amount of this other newly radioactive material is small compared to the volume of the fuel assembly. These facts prompt some to argue that SNF should be chemically processed and the various components separated into nonradioactive material, material that will be radioactive for a long time, and material that could be refabricated into new reactor fuel. Reprocessing the fuel to isolate the plutonium is seen as a reason not to proceed with this technology in the United States.

Congress has decided that reprocessing will not be practiced in this country so that we will not be in the plutonium production business. This seems like a safe thing to do since this action will minimize terrorism threats. Reprocessing generates chemi cal wastes but greatly reduces the volume of the highly radioactive waste. It also isolates plutonium and unused fuel for possible use as new fuel. Reprocessing means that the volume of material calling for long-term isolation is reduced by one-third what it would have been before reprocessing.

THE VOLUME OF SNF

If one just concentrates on the radioactive material in SNF, the volume is very small, especially compared to waste from other power production practices. However, one can only discuss the separated radioactive material if it has undergone extensive reprocessing. If SNF is to be isolated, as in a place such as Yucca Mountain, with perhaps 70 miles of tunnels, the volume is that of the interior of this minor mountain. Isolation of up to 100,000 metric tons of SNF in Yucca Mountain means that for the United States, approximately all the SNF made to date and that expected in the operating lifetime of all current reactors can be put there. Approximately 2,000 metric tons of SNF are produced each year in the United States. Waste volume and placement depend on the amount of compaction and consolidation at the sites. The plans for the Yucca Mountain present a realistic and understandable picture of the volume of SNF.

A useful perspective may be seen about the amount of SNF by comparing it to the volume of other waste streams associated with power production. More than half of the electricity made in the United States comes from coal burning, where each large power plant generates bottom and fly ash in volumes measured in acre-feet annually. Each of these plants generates its own small mountain of ash. The gaseous wastes from coal burning and from methane and oil burning result in tons of carbon dioxide daily. This carbon dioxide is an infrared active gas and is thought by many to be contributing to global warming and climate change phenomena. The wastes associated with nuclear power are small in comparison, this is not surprising, considering the tremendous power in the nucleus of an atom.

LOW-LEVEL RADIOACTIVE WASTE

The other major category of nuclear waste, LLW, is generally that which is neither HLW nor the transuranic part of defense waste. Generally, LLW is generated in private and public labs, hospitals, and commercial enterprises and can involve lab clothing, paper, packing material and radioactive isotopes used in medical procedures, both diagnostic and therapeutic. Significant amounts of LLW are associated with defense wastes, but mostly are isolated at U.S. DOE facilities. Many pharmaceutical products require extensive use of radioactive tracers. Academic sources are minor. The major sources of LLW in volume and amount of radioactivity are nuclear power plants. Aside from the decommissioning of nuclear power plants, the volume of LLW is decreasing every year. This is because new technologies have become available to separate the radioactive material from that which is not radioactive. Also, generators of LLW are being more selective in the work that they do so that they generate less LLW each year. These changes have become about partly because the cost of isolating LLW has increased.

The LLW from nuclear power plants contains ion exchange resins as well as clothing, tools, and chemicals. Ion exchange resins, which comprise the majority of this LLW, are used to filter the water circulated in nuclear power plants. The ion exchange resins isolate and trap dissolved materials, much of which can be radioactive. Approximately three-fourths of commercially or privately generated LLW is in the form of the contaminated plastic beads that make up ion exchange resins.

The NRC categorizes LLW into Class A, B, or C. The wastes in Classes A and B contain materials that decay to safe levels in about 100 years. Class C wastes will require about 500 years to reach safe levels and contain mostly ion-exchange resins and filters. (There is a minor amount of LLW referred to as beyond class C material.) The majority of LLW is dry and is stored in cardboard or wooden crates. Some are in metal drums. LLW is most likely given shallow burial, compared to the deep geological isolation required for HLW. A difficult issues in LLW disposed involves a small amount of "mixed waste," where the radiological material is mixed with chemically or biologically hazardous substances. The contaminated chemicals may require special handling, since hazardous chemical wastes do not decay as do radioactive materials. In addition, a small amount of LLW consists of animal carcasses or waste and also needs special handling. Currently there are three commercial sites handling LLW for most of the states and compacts. The largest volume of LLW will come with the decommissioning and dismantling of nuclear power plants. In 1995 nearly 700,000 cubic feet of LLW were handled at commercial sites in the United States.

LLW is placed in sturdy, sealed containers. At the isolation site these containers are first put in concrete and/or metal vaults or bunkers and then buried in shallow trenches before being covered with backfill. Some of these are then paved over to prevent rainwater from entering the waste.

The LLW Forum coordinates information and regulations among and between the various state compacts and states. It maintains a website that is overseen by the Idaho Operations Office, which is part of the National Engineering Lab in Idaho. Although LLW is a responsibility of each compact, or of state for the LLW generated in that state, the regulations governing it are those of the U.S. DOE, the NRC, and the EPA. The regulations governing its isolation allow individual states to set additional requirements for handling the material. After being established, any particular compact can refuse to receive the LLW from other states.

OTHER RADIOACTIVE-WASTE SITES

The Waste Isolation Pilot Plant (WIPP) is in an excavated salt cavern in southern New Mexico, twenty-seven miles from Carlsbad. The WIPP site is 2,000 yards underground, and defense waste is being placed. There are plans to place there about 6 million cubic feet of material there containing fewer than five million curies of radio activity.

The most technically difficult category of HLW is that belonging to the U.S. DOE at its major plutonium production facilities in Hanford, Washington, and at the Savannah River facility near Aiken, South Carolina. Just sampling the material for characterization presents problems since much of it consists of radioactive acidic liquids. The rest of it in sludge form, and some is solid. A plant for the vitrification of that part of the material that can be made into glass bars is in operation at the Savannah River facility. The vitrification process encases the waste in glass "logs" to immobilize it and make it easier to handle. A conservative estimate is that it will take decades to clean up these facilities.

NUCLEAR WASTE IN OTHER COUNTRIES

The United States has the most radioactive nuclear waste and the most complicated array of waste types. Reprocessing of SNF is also practiced in some countries. Although costly, this practice reduces the volume of HLW requiring deep geologic disposal. (In the wide variety of elements in the fission products making up SNF, some are toxic heavy metals.) Reprocessing and the general lack of economy of scale in many other countries help to explain why there is less activity or progress related to HLW isolation elsewhere. Interim monitored retrievable storage is generally the focus of activity abroad. Different philosophies and cultures also mean that a wide variety of approaches to the problem are found. Some feel that the generation deriving the benefits of nuclear power is the one responsible for a solution. Others say that succeeding generations may have better technology or ideas about this problem than current generations: Current generations should not do anything permanent, committing future generations to the solutions seen by people living today. Among the technology that wold drastically reduce the cost of cleanup of waste sites robotics.

All the countries that produce nuclear waste have chosen the same alternative for the ultimate disposition of HLW, deep geological isolation, and they did so independently of one another. The United States has the most radioactive nuclear waste and the most complicated array of waste types of any "nuclear" country. Only in the United States can one find the same economy of scale for waste handling. Thus, it leads the world in most activities aimed at safe isolation. In France, Japan, and Great Britain, however, reprocessing is routinely practiced. Those countries reprocess HLW for many other countries. As mentioned above, reprocessing is not currently allowed in the United States.

France and Germany, local protests have dramatically slowed the choice of a final isolation site. As a result, interim storage is widely practiced in other countries. In Sweden, the waste is mixed in molten copper. The radioactive waste is immobilized in copper logs that are easily handled and stored in large underground caverns until a more permanent isolation is chosen. While most feel that those making the waste have the primary responsibility for its isolation, in Sweden they do not wish to commit future generations to solutions that might be vitrified; the glass "logs" are in concrete bunkers.

In the former U.S.S.R. vast areas of the country are contaminated by poor handling of nuclear waste, especially from that associated with the manufacturer of weapons. Some radioactive waste, especially from nuclear submarines has been isolated at the bottom of the Baltic Sea by the former Russian Navy.

CONCERNS

Transportation of HLW is among the most immediate concerns for the general public. Most experts agree that nuclear wastes, especially SNF, are being kept in places not intended for long-term storage and they acknowledge that the problem must be addressed. There is general agreement that SNF should be kept far from population centers (where the electricity is generated and used) and should be kept in a dry place, since water the most likely medium for its movement into the human environment. Movement of SNF is necessary, most likely by rail or roadway. Groups of U.S. DOE and state regulators are working together, continually revising transportation plans and designing and testing transportation casks. Some wish to see a combined transportation and burial cask, while others want very different qualities in each sort of cask. Planners build on past experience handling and transporting radiological materials. Extensive experience was gained when the first commercial nuclear power plant, at Shippingport, Pennsylvania, was decommissioned in 1989 and the reactor vessel was moved to Hanford, Washington, where it is buried.

Despite the challenges, many see nuclear waste issues as being mostly political and social. There is a growing awareness that technical answers alone will not solve the political and social concerns. Some consider nuclear waste issues small in comparison to the volume and challenges associated with other kinds of wastes, whether generated by power plants or other human activities.

Donald H. Williams

See also: Environmental Problems and Energy Use; Government Agencies; Nuclear Energy; Nuclear Energy, Historical Evolution of the Use of; Nuclear Fission; Nuclear Fission Fuel; Nuclear Fission.

BIBLIOGRAPHY

American Nuclear Society. <http://ans.org>.

American Nuclear Society. (annual). High Level Radioactive Waste Management: Proceedings for the International Topical Meeting of the American Nuclear Society and the American Society of Civil Engineers. Chicago: American Nuclear Society.

Murray, R. L. (1994). Understanding Radioactive Waste, 4th ed. Columbus, OH: Battelle Press.

Physics Today. (1998). 50(6).

Radwaste Magazine: Progress in Radioactive Waste Management and Facility Remediation. (monthly).

Savage, D., ed. (1995). The Scientific and Regulatory Basis for the Geological Disposal of Radioactive Waste. New York: Wiley & Sons.

Study Committee Home Page. <http://www.studycommittee.org>.

U.S. Department of Energy. (1996) "Integrated Data Base Report 1995: U.S. Spent Nuclear Fuel and Radioactive Waste Inventories, Projections and Characteristics, DOE/RW-0006, Revision 12." Washington, DC: Author.

U.S. Department of Energy: Office of Civilian Radioactive Waste Management. The Yucca Mountain Project. <http://www.ymp.gov>.

U.S. Environmental Protection Agency. Yucca Mountain Home Page. <http://www.epa.gov.radiation/yucca>.