Center for Reactor Information

Disposal of High Level Nuclear Waste

Eugene E. Voiland
May 2002

High Level Nuclear Waste (HLW) is the intensely radioactive residue from nuclear fission. The fission process provides heat used to generate electricity in nuclear fueled power plants. For disposal, the waste is enclosed in very strong, corrosion-resistant, metallic containers and put in underground chambers (the repository). The whole system is engineered to contain the waste intact and undisturbed for thousands of years.

According to federal regulation, used or “spent” reactor fuel from power plants is considered to be high level nuclear waste.

The US federal government is required to take title to the HLW and provide the physical facilities for its disposition. The cost of such facilities and the cost of the research and engineering studies required for licensing, construction, operation and closure are borne by all users of nuclear electricity. They pay a tenth of one cent per kilowatt hour used.

The Origin of High Level Nuclear Wastes

Following are principal steps in the formation and disposal of high level nuclear waste:

1. Manufacture of nuclear fuel from uranium slightly enriched in the fissionable isotope U-235
2. Use of the fuel in a nuclear reactor
3. Discharge of the fuel when no longer useful in the reactor
4. Temporary storage in a water pool while the heat output of the spent fuel declines
5. Transfer of the spent fuel to a reprocessing facility. If no reprocessing is done, step 8 comes next, after packaging the fuel in sturdy containers.
6. Chemical separation and isolation of the fission products and other elements from the rest of the spent fuel
7. Conversion and encapsulation of the isolated fission products to the form required for disposal
8. Emplacement of the packaged spent fuel/high level waste in a qualified and licensed underground repository.

Early in the development of commercial nuclear power production, it was an accepted belief that the spent nuclear fuel -- fuel no longer useful in the nuclear reactor -- would be chemically processed to recover plutonium and uranium for possible recycle, and to prepare the highly radioactive fission product wastes for disposal.

Such a “reprocessing” plant was built in New York state and in the 1960's operated for a number of years. The separated fission products were the first commercial HLW and they were stored (as dissolved solids and sludge in water) in underground tanks at the site of the reprocessing plant, pending development of a disposal procedure. They remained there until recent years when incorporation of the fission products into a stable glass was completed.

Later, when a policy not to reprocess spent fuel was adopted by the United States, the definition of HLW was extended to the spent fuel itself.

Today the HLW designation is applied to the spent nuclear fuel (fuel no longer useful for powering the reactor), or the fission products after separation from the spent fuel by a chemical process.

Descriptive Information

Radiological Characteristics of High Level Waste. The nuclear process that produces the heat necessary for the production of electricity involves the fission or splitting of an atom of uranium or plutonium. The latter is produced as a by-product of the nuclear process and, like the uranium in the fuel, undergoes fission to contribute to the heat production.

The fission process produces an enormous amount of heat and about 35 “fission products.” These are chemical elements that exist in nature -- iodine, cesium, molybdenum, technetium, palladium, etc. -- except the forms of the elements produced may be radioactive. Being radioactive means that the atoms are unstable and will break down to produce another atom (stable or unstable) and emit radiation. Some of this radiation is called gamma rays and is like X-rays. The radiation from fission products is energetic and penetrating, and in high enough concentration can be harmful to all living creatures, including people.

The radioactive fission products gradually lose their radioactivity -- they “decay” -- with a wide variety of rates, some losing half of their radioactivity in seconds, and some taking more than hundreds of thousands of years. The radiation emitted from the short-lived fission products can be very intense and hazardous. The radiation from the very long-lived ones is hardly detectable and in some cases their hazard may be from their chemistry rather than from radiation.

In addition to the fission products, uranium, and plutonium, spent fuel contains long-lived radioactive elements called “transuranic actinides” -- elements of atomic number 93 (neptunium) and greater. It is these and other long-lived radioactive materials that require the HLW disposal site to be able to isolate and contain the waste for very long times, such as 10,000 years. If they were removed from the waste, the containment requirement could be less than 500 years.

The 10,000-year regulatory criterion is based on the ability to make reasonably accurate predictions of the behavior of materials such as the waste packages and the geology in which waste has been disposed. This time will allow the decay of most of the radioactivity. After this long time, the amount of radioactive material remaining would be so small that, should it reach the surface, it would not add a significant contribution to the natural background radiation.

The radiation from HLW is intense and therefore HLW is very dangerous to people. It must be manipulated and stored behind materials that will absorb the radiation. Thus, HLW may be stored in a deep pool where water absorbs the radiation, or in an engineered surface storage unit where the earth provides shielding, or behind massive barriers of concrete, steel or lead. All materials absorb radiation, but dense materials, such as lead, are more efficient absorbers and thinner shields can be used for the same effect.

The technology for dealing with high levels of radioactivity is well advanced and chemical and physical treatment is routinely and safely accomplished.

Spent Nuclear Fuel. Spent nuclear fuel looks just like it did when it was loaded into the reactor. Within the fuel rods there is a change in the chemical species due to the nuclear processes at work. There is a minuscule loss of weight because in the fission process some mass is converted into energy (heat).

Nuclear fuel consists of 1/2 inch diameter by 1/2 inch long ceramic uranium dioxide pellets stacked in 10 to 12-foot long tubes of corrosion-resistant zirconium alloy. From 50 to 300 of these fuel rods, depending on the particular fuel design, are mounted in metal fixtures in a square array. These fuel rod assemblies are called fuel elements or fuel assemblies.

A large number of fuel elements -- containing a total of 100 to 150 tons of uranium -- are grouped in the “core” of the reactor. When the reactor is operating, the nuclear reaction produces heat, mostly inside the fuel rods. This heat is transferred to water flowing through the fuel assemblies and the water is directly or indirectly converted to steam. The steam powers turbine-generators which produce electricity.

This is how electricity is typically generated except that the heat is provided by a nuclear process rather than by a chemical combustion process, such as the burning of coal, gas, oil, or other combustibles. A significant difference is that in the nuclear-powered system there are no gases -- greenhouse or otherwise -- emitted.

The fuel rods are very resistant to corrosion. They remain in the reactor typically for three or four years, in 600+ degrees Fahrenheit water, and at a pressure of 2,000 pounds per square inch steam or water. In some instances, reactors have run continuously for over a year at essentially full power -- a quite remarkable technical feat.

About every 15 to 18 months, a fourth to one-third of the fuel is removed and replaced by fresh fuel. The fuel rods survive this harsh environment very well and only rarely does inconsequential leakage from within a fuel rod occur.

Upon removal from the reactor, the spent fuel is stored in a pool of water to allow the short-lived radionuclides to decay to more-stable isotopes. When a nuclear power plant is shut down, the rate of heat production immediately drops by a factor of 16, to about 6% of what it was when the fission process was operating. The residual heat is generated by the collective decay of all of the radioactive fission products. An hour later, heat evolution is down by a factor of 100, to about 1%. After a month, the reduction factor is about 1,000 (0.1%). After a year, it is about 3,300 (0.3%), and after 5 years, about 20,000 (0.005%).

Fresh spent fuel is stored in water pools, which both provides shielding from the radiation and carries away the heat from the radioactive decay.

After about five years of water storage, the heat output of the spent fuel is reduced to the extent that it can be safely stored in massive concrete containers that are air-cooled by convection.

According to federal regulation, the used or “spent” fuel is considered to be High Level Nuclear Waste.

Fission Product HLW. Fission product HLW is obtained from spent nuclear fuel by a chemical process that separates the three major components of the spent fuel: (1) uranium, which can be recycled, (2) plutonium, which can also be used as a nuclear fuel, and (3) the fission products, which are the waste products of the nuclear production of electricity. The fission product fraction usually comes out of the process in an aqueous solution, and is stored in underground tanks as a mixture of solution and sludge until the fission products are converted to a more stable form.

Before disposal, the fission products are incorporated in a chemically very inert glass in which they may constitute 20 percent of the weight.

In other countries, such as France and England, where reprocessing of spent fuel is routinely done, glass (vitrified) waste forms are produced and stored in surface facilities awaiting future disposal.

In the United States, HLW from a formerly-operated reprocessing plant has been converted to a vitrified form.

In general, most, if not all nations producing electricity by nuclear fission, anticipate ultimate disposal of HLW deep in the ground in an engineered facility designed for safe containment and isolation of the material for thousands of years.

Amounts of High Level Nuclear Waste Produced. One of the most startling facts about HLW is the incredibly small amount produced for the quantity of electricity produced. This is readily apparent from comparison of the relative yearly amounts of wastes produced by power plants of the same size (1,000 MWe), one burning coal and the other fueled by uranium.

All values in the table below are approximate, especially those for coal.

The HLW requires expensive technology for its disposal. However, because there is so little of it and so much electricity produced by its formation, there is adequate money available for its treatment and disposal.

Consumers who use nuclear-generated electricity pay a tenth-of-one-cent per kilowatt hour to cover waste management costs. To date, a fund of $18 billion has been collected, of which $6 billion has been spent on a waste repository at Yucca Mountain in Nevada. The fund accummulates another $0.75 billion each year.

The carbon dioxide from a coal-burning plant is discharged from tall stacks to the atmosphere (“the disperse and dilute” approach). There is growing concern about the possible adverse effects of increasing the environmental load of carbon dioxide, and there is yet no economic way of sequestering this product gas from coal combustion.

High Level Waste Management. The usual approach to disposing of solid waste is to package the waste and isolate it from the environment. Methods considered for HLW disposal have included disposing of the appropriately contained HLW in the deep seabed, in the Greenland ice fields, rocketing it into the sun, or burying it deep in the earth's crust. At present, the latter option, geological disposal, is preferred and the United States and other nations have undertaken programs to do just that.

The concept is simple: (1) provide strong containment for the waste by the use of multiple barriers designed to resist intrusion into the waste by the disposal environment, and (2) since water is the most likely transport medium for waste to reach the surface, isolate the packaged waste in a benign remote underground location where contact with water is highly unlikely.

• Spent nuclear fuel by itself is relatively inert.
• Fission product waste can be incorporated in a matrix that is itself very stable. Some forms of glass are good candidates. Glasses in nature have survived for eons.
• Spent fuel or vitrified waste can be contained in a corrosion-resistant metal sheathing.
• The contained waste can be deposited in a deep underground site that has been characterized as suitable by extensive scientific and engineering studies.
• The isolation of the waste can also be improved by surrounding it with material, such as clay, to inhibit its contact with water that might make its way to the disposal location.

It is the combination of highly protective containment and a stable, arid site that fosters the belief that the 10,000 year criterion can be met at Yucca Mountain.

Status of High Level Waste Management in Foreign Countries. Eight nations (Belgium, Canada, Germany, Japan, Sweden, Switzerland, the United Kingdom, and the United States) have installed and are operating subter­ranean laboratories or are conducting research programs in existing mines. The foreign nations are committed to geological disposal, but see no reason to hurry the process. Currently, retrievable storage of the spent fuel at the reactor site, or the fission product wastes (in glass form) in shielded surface structures, are accepted interim measures.

Status of High Level Waste Management in the United States. Before HLW can be brought to a site for disposal, a lot of information about the waste, the disposal package, the geology and geochemistry of the proposed site, details of ground water sources and flows, and about many other topics must be obtained and analyzed. In this regard, the US Department of Energy has conducted research studies at a number of academic, governmental and other institutions throughout the country. Much of this research has been experimental, but computer models have also been developed to simulate how the disposal system would behave in the future -- to answer the “what if” questions that arise in a comprehensive study, such as the site characterization study, that has been going on for many years and continues at this time.

The US Department of Energy has also developed an extensive study facility at the designated disposal site at Yucca Mountain, Nevada to test the site's suitability for disposal of HLW. Summarizing this study effort, the Department of Energy has recently issued a report, Yucca Mountain Science and Engineering Report, which is in the process of review at this time.

Some key items from this report are the following:

• The Yucca Mountain site is in an arid part of Nevada adjacent to the nuclear weapons test site. The site has an average of 7.5 inches of precipitation per year. HLW would be stored 600 feet to 1,200 feet below grade and 1,000 feet above the water table. Characteristics of the underground geology have been extensively studied.
• Operating systems have been designed and analyzed to receive and unload the transportation containers (casks), package the waste for burial, transport the waste to the underground site, put the waste in the disposal site, monitor the disposal site for a time, and eventually, close the site.
• Response of the site to the heat load imposed by the emplaced HLW under a variety of operating conditions has been analyzed and estimated.
• Needed surface and sub-surface facilities have been defined, including features for required standard and off-standard conditions.
• Detailed study has been made of the role of the geologic and hydrologic setting on the migration of radioactivity that might possibly be released from the contained HLW.
• Design features that provide additional assurance of containment of the HLW have been defined: encasement in stainless steel and special corrosion-resistant alloys, use of titanium “drip shields” to protect against water intrusion, identification of potential use of special material to surround and further protect the waste packages.
• The design of the entire engineered disposal system is intended to provide substantial package integrity for at least 10,000 years ( a regulatory requirement), even assuming a variety of possible environmental changes, such as climate, water intrusions, flow patterns, and migration mechanism changes.

Transportation of High Level Waste. The transportation of spent fuel by railcar-mounted shipping casks and by tractor trailer-mounted casks has been demonstrated by 3,000 shipments with no release of contained material in the relatively few accidents that have occurred.

The integrity of the shipping casks is not surprising because of the conservative design to stringent licensing specifications, required quality assurance during manufacture, and physical testing of models and actual casks to confirm design validity.

Casks containing unirradiated fuel elements have been propelled into massive concrete walls at high speed, have been impacted by locomotives at high speed, and have been dropped onto concrete surfaces without compromising their ability to physically contain their contents.

A cask containing simulated fuel elements was penetrated by a shaped charge to learn the hazard posed by the particle size distribution of the damaged fuel. The potential hazard turned out to be much less than expected, since the damaged fuel remained in particle sizes not readily dispersed.

Legal Aspects of HLW Management. The legal aspects of HLW management are embodied in the Nuclear Waste Policy Act of 1982 (NWPA) which assigned responsibilities in the following way:

• The Department of Energy is to accept title to the HLW
• The Department of Energy is to site, construct, operate and close a repository for the disposal of spent nuclear fuel and high level radioactive waste
• The US Environmental Protection Agency is to establish safety and public health standards for the repository
• The US Nuclear Regulatory Commission is to promulgate regulations pertaining to the construction, operation, and closure of the repository
• The generators and owners of spent fuel and high level radioactive waste are to pay the cost of disposal of such materials by imposition of a tenth-of-one-cent per kilowatt hour used, on purchasers of nuclear-produced electricity.

In 1987, Congress amended NWPA to require the Department of Energy to investigate the Yucca Mountain site for suitability as the first disposal facility. The NWPA also called out in detail the process by which the Secretary of the Department of Energy will develop a recommendation for acceptance of the site to the President.

Early in 2002, the Secretary of the Department of Energy recommended to President Bush that development of the Yucca Mountain site should proceed, and the President presented the proposal to Congress.

References

1. Executive Summary, Yucca Mountain Science and Engineering Report: Technical Information Supporting Site Recommendation Consideration, US Department of Energy, Office of Civilian Radioactive Waste Management, May, 2001.

For additional information:

• Yucca Mountain Information Center 1-800-967-3477, or
• Yucca Mountain Site Characterization Project Web site: <http://www.ymp.gov>

2. Radioactive Waste Management in Perspective, Nuclear Energy Agency, Organisation for Eco­nomic Co-operation and Development, 2, rue Andre Pascal, 75775 PARIS CEDEX 16, France, 1996.

3. Waste values in the Reactor/Coal plant table are derived from the following Internet sites:

• Uranium Information Center
• Electric Consumption Nationally, Electric Services Power Plant, Ames, Iowa

Additional sources of data will be provided on request.

Acknowledgments

The author gratefully acknowledges the reviews and contributions of Richard Beatty, Les Burris, George Stanford, and Martin Steindler, all scientists or engineers who have retired from life-long careers in nuclear science, technology, or commercial activity.

For additional information, please contact CFRI, an independent organization of retirees with extensive nuclear experience.