Radioactive (or nuclear) waste is a byproduct from nuclear reactors, fuel processing plants, and institutions such as hospitals and research facilities. It also results from the decommissioning of nuclear reactors and other nuclear facilities that are permanently shut down.

Types of waste

Uranium mill tailings

Key Lake uranium mill in Canada. Source: CamecoThe solid (sandy) waste from the conventional uranium milling process is called mill tailings. Tailings are placed in huge mounds called tailings piles which are located close to the mills where the ore is processed. This ore residue contains the radioactive decay products from the uranium chains (mainly the ²³⁸U chain) and heavy metals

Uranium mill tailings can adversely affect public health. There are four principal ways (or exposure pathways) that the public can be exposed to the hazards from this waste. The first is the diffusion of radon gas directly into indoor air if tailings are misused as a construction material or for backfill around buildings. When people breathe air containing radon, it increases their risk of developing lung cancer. Second, radon gas can diffuse from the piles into the atmosphere where it can be inhaled and small particles can be blown from the piles where they can be inhaled or ingested. Third, many of the radioactive decay products in tailings produce gamma radiation, which poses a health hazard to people in the immediate vicinity of tailings. Finally, the dispersal of tailings by wind or water, or by leaching, can carry radioactive and other toxic materials to surface or ground water that may be used for drinking water.

Low level waste

Burial of low level waste in a landfill in South Carolina. Source: StatehouseReport.comLow-level waste (LLW) is generated from hospitals, industry and by the nuclear fuel cycle.  LLW includes items that have become contaminated with radioactive material or have become radioactive through exposure to neutron radiation. This waste typically consists of contaminated protective shoe covers and clothing, wiping rags, mops, filters, reactor water treatment residues, equipments and tools, luminous dials, medical tubes, swabs, injection needles, syringes, and laboratory animal carcasses and tissues. The radioactivity can range from just above background levels found in nature to very highly radioactive in certain cases such as parts from inside the reactor vessel in a nuclear power plant. Low-level waste is typically stored on-site by licensees, either until it has decayed away and can be disposed of as ordinary trash, or until amounts are large enough for shipment to a low-level waste disposal site.  Some high activity LLW requires shielding during handling and transport but most LLW is suitable for shallow land burial. To reduce its volume, it is often compacted or incinerated before disposal.

Intermediate level waste

Intermediate level waste (ILW) contains higher amounts of radioactivity and in some cases requires shielding. ILW includes resins, chemical sludge and metal reactor fuel cladding, as well as contaminated materials from reactor decommissioning. It may be solidified in concrete or bitumen for disposal. As a general rule, short-lived waste (mainly non-fuel materials from reactors) is buried in shallow repositories, while long-lived waste (from fuel and fuel-reprocessing) is deposited in deep underground facilities. U.S. regulations do not define this category of waste; the term is used in Europe and elsewhere.

High level waste

Spent nuclear fuel rods at the Department of Energy's Savannah River National Laboratory in Aiken, South Carolina. Credit: SRNLHigh level waste (HLW) takes two forms. Spent (used) reactor fuel is used fuel from a reactor that is no longer efficient in creating electricity, because its fission process has slowed due to a build-up of reaction poisons. However, it is still thermally hot, highly radioactive, and potentially harmful. Waste materials from reprocessing are those materials for nuclear weapons that are acquired by reprocessing spent nuclear fuel from breeder reactors. Reprocessing is a method of chemically treating spent fuel to separate out uranium and plutonium. The byproduct of reprocessing is a highly radioactive sludge residue.

The basic fuel of a nuclear power reactor contains ²³⁵uranium, which is in ceramic pellets inside of metal rods. Before these fuel rods are used, they are only slightly radioactive and may be handled without special shielding. During the nuclear reaction, the fuel “fissions,” which means that an atom of uranium is split, releasing two or three neutrons and a small amount of heat. The released neutrons then strike other atoms, causing them to split, and a chain reaction is formed, which releases large amounts of heat. This heat is used to generate electricity at nuclear power plants.

The splitting of relatively heavy uranium atoms during reactor operation creates radioactive isotopes of several lighter elements, such as ¹³⁷cesium and ⁹⁰strontium, called “fission products,” that account for most of the heat and penetrating radiation in high-level waste. Some uranium atoms also capture neutrons from fissioning uranium atoms nearby to form heavier elements like plutonium. These heavier-than-uranium, or “transuranic,” elements do not produce nearly the amount of heat or penetrating radiation that fission products do, but they take much longer to decay. Transuranic wastes, also called “TRU,” therefore account for most of the radioactive hazard remaining in high-level waste after a thousand years.

Radioactive isotopes will eventually decay, or disintegrate, to harmless materials. However, while they are decaying, they emit radiation. Some isotopes decay in hours or even minutes, but others decay very slowly. ⁹⁰Strontium and ¹³⁷cesium have half-lives of about 30 years (that means that half the radioactivity of a given quantity of ⁹⁰strontium, for example, will decay in 30 years). ²³⁹Plutonium has a half-life of 24,000 years.

High-level wastes are hazardous to humans and other life forms because of their high radiation levels that are capable of producing fatal doses during short periods of direct exposure. For example, ten years after removal from a reactor, the surface dose rate for a typical spent fuel assembly exceeds 10,000 rem/hour, whereas a fatal whole-body dose for humans is about 500 rem (if received all at one time). Furthermore, if constituents of these high-level wastes were to get into ground water or rivers, they could enter into food chains. Although the dose produced through this indirect exposure is much smaller than a direct exposure dose, there is a greater potential for a larger population to be exposed.

Reprocessing separates residual uranium and unfissioned plutonium from the fission products. The uranium and plutonium can be used again as fuel. Most of the high-level waste (other than spent fuel) generated over the last 35 years has come from reprocessing of fuel from government-owned plutonium production reactors and from naval, research and test reactors. A small amount of liquid high-level waste was generated from the reprocessing of commercial power reactor fuel in the 1960's and early 1970's. There is no commercial reprocessing of nuclear power fuel in the United States at present; almost all existing commercial high-level waste is in the form of unreprocessed spent fuel.

Transuranic waste

Transuranic waste is material that is contaminated with ²³³U (and its daughter products), certain isotopes of plutonium, and nuclides with atomic numbers greater than uranium (Elements that have an atomic number greater than uranium are called transuranic i.e., "beyond uranium".  It is produced during the reprocessing of spent fuel to separate plutonium for use in fabrication of nuclear weapons. In the U.S., transuranic waste is defined as waste that is contaminated with alpha-emitting transuranic radionuclides with half-lives greater than 20 years, and concentrations greater than 100 nCi/g (3.7 MBq/kg), excluding High Level Waste.

Sources of waste

Radioactive waste comes from a number of sources. The majority originates from the nuclear fuel cycle and nuclear weapon reprocessing. However, other sources include medical and industrial wastes, as well as naturally occurring radioactive materials (NORM) that can be concentrated as a result of the processing or consumption of coal, oil and gas, and some minerals.

Nuclear fuel cycle

Front end

Waste from the front end of the nuclear fuel cycle is usually alpha emitting waste from the extraction of uranium. It often contains radium and its decay products.  Uranium dioxide (UO₂) concentrate from mining is not very radioactive - only a thousand or so times as radioactive as the granite used in buildings. It is refined from yellowcake (U₃O₈), then converted to uranium hexafluoride gas (UF₆). As a gas, it undergoes enrichment to increase the 235U content from 0.7% to about 4.4% (LEU). It is then turned into a hard ceramic oxide (UO₂) for assembly as reactor fuel elements.

The main by-product of enrichment is depleted uranium (DU), principally the ²³⁸U isotope, with a ²³⁵U content of ~0.3%. It is stored, either as UF₆ or as U₃O₈. Some is used in applications where its extremely high density makes it valuable, such as the keels of yachts, and anti-tank shells. It is also used (with recycled plutonium) for making mixed oxide fuel (MOX) and to dilute highly enriched uranium from weapons stockpiles which is now being redirected to become reactor fuel. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the (very expensive and complex) enrichment process before assembling a weapon.

Back end

The back end of the nuclear fuel cycle, mostly spent fuel rods, contains fission products that emit beta and gamma radiation, and actinides that emit alpha particles, such as ²³⁴uranium, ²³⁷neptunium, ²³⁸plutonium and ²⁴¹americium, and even sometimes some neutron emitters such as californium (Cf). These isotopes are formed in nuclear reactors.

It is important to distinguish the processing of uranium to make fuel from the reprocessing of used fuel. Used fuel contains the highly radioactive products of fission (see high level waste below). Many of these are neutron absorbers, called neutron poisons in this context. These eventually build up to a level where they absorb so many neutrons that the chain reaction stops, even with the control rods completely removed. At that point the fuel has to be replaced in the reactor with fresh fuel, even though there is still a substantial quantity of ²³⁵uranium and plutonium present. In the United States, this used fuel is stored, while in countries such as the United Kingdom, France, and Japan, the fuel is reprocessed to remove the fission products, and the fuel can then be re-used. This reprocessing involves handling highly radioactive materials, and the fission products removed from the fuel are a concentrated form of high-level waste as are the chemicals used in the process.

Proliferation concerns

When dealing with uranium and plutonium, the possibility that they may be used to build nuclear weapons is often a concern. Active nuclear reactors and nuclear weapons stockpiles are very carefully safeguarded and controlled. However, high-level waste from nuclear reactors may contain plutonium. Ordinarily, this plutonium is reactor-grade plutonium, containing a mixture of ²³⁹Pu (highly suitable for building nuclear weapons), ²⁴⁰Pu (an undesirable contaminant and highly radioactive), ²⁴¹Pu, and ²³⁸Pu; these isotopes are difficult to separate. Moreover, high-level waste is full of highly radioactive fission products. However, most fission products are relatively short-lived. This is a concern since if the waste is stored, perhaps in deep geological storage, over many years the fission products decay, decreasing the radioactivity of the waste and making the plutonium easier to access. Moreover, the undesirable contaminant ²⁴⁰Pu decays faster than the ²³⁹Pu, and thus the quality of the bomb material increases with time (although its quantity decreases during that time as well). Thus, some have argued, as time passes, these deep storage areas have the potential to become "plutonium mines", from which material for nuclear weapons can be acquired with relatively little difficulty. Critics of the latter idea point out that the half-life of ²⁴⁰Pu is 6,560 years and ²³⁹Pu is 24,110 years, and thus the relative enrichment of one isotope to the other with time occurs with a half-life of 9,000 years (that is, it takes 9000 years for the fraction of ²⁴⁰Pu in a sample of mixed plutonium isotopes, to spontaneously decrease by half-- a typical enrichment needed to turn reactor-grade into weapons-grade Pu). Thus "weapons grade plutonium mines" would be a problem for the very far future (>9,000 years from now), so that there remains a great deal of time for technology to advance to solve this problem, before it becomes acute.

²³⁹Pu decays to ²³⁵U which is suitable for weapons and which has a very long half life (roughly 109 years). Thus plutonium may decay and leave ²³⁵U. However, modern reactors are only moderately enriched with ²³⁵U relative to ²³⁸U, so the  ²³⁸U continues to serve as denaturation agent for any ²³⁵U produced by plutonium decay.

One solution to this problem is to recycle the plutonium and use it as a fuel e.g. in fast reactors. But in the minds of some, the very existence of the nuclear fuel reprocessing plant needed to separate the plutonium from the other elements represents a proliferation concern. In pyrometallurgical fast reactors, the waste generated is an actinide compound that cannot be used for nuclear weapons.

Nuclear weapons reprocessing

Waste from nuclear weapons reprocessing (as opposed to production, which requires primary processing from reactor fuel) is unlikely to contain much beta or gamma activity other than tritium and americium. It is more likely to contain alpha emitting actinides such as ²³⁹Pu which is a fissile material used in bombs, plus some material with much higher specific activities, such as ²³⁸Pu or Po.

In the past the neutron trigger for a bomb tended to be beryllium and a high activity alpha emitter such as polonium; an alternative to polonium is ²³⁸Pu For reasons of national security, details of the design of modern bombs are normally not released to the open literature. It is likely however that a D-T fusion reaction in either an electrically driven device or a D-T fusion reaction driven by the chemical explosives would be used to start up a modern device.

Some designs might well contain a radioisotope thermoelectric generator using ²³⁸Pu to provide a longlasting source of electrical power for the electronics in the device.

It is likely that the fissile material of an old bomb which is due for refitting will contain decay products of the plutonium isotopes used in it, these are likely to include U-236 from Pu-240 impurities, plus some U-235 from decay of the ²³⁹Pu; however, due to the relatively long half-life of these Pu isotopes, these wastes from radioactive decay of bomb core material would be very small, and in any case, far less dangerous (even in terms of simple radioactivity) than the ²³⁹Pu itself.

The beta decay of ²⁴¹Pu forms ²⁴¹Am; the in-growth of americium is likely to be a greater problem than the decay of ²³⁹Pu and ²⁴⁰Pu as the americium is a gamma emitter (increasing external-exposure to workers) and is an alpha emitter which can cause the generation of heat. The plutonium could be separated from the americium by several different processes; these would include pyrochemical processes and aqueous/organic solvent extraction. A truncated PUREX type extraction process would be one possible method of making the separation.

Medical

Radioactive medical waste tends to contain beta particle and gamma ray emitters. It can be divided into two main classes. In diagnostic nuclear medicine a number of short-lived gamma emitters such as technetium-99m are used. Many of these can be disposed of by leaving it to decay for a short time before disposal as normal trash. Other isotopes used in medicine, with half-lives in parentheses:
 

• ⁹⁰Y, used for treating lymphoma (2.7 days)
• ¹³¹I, used for thyroid function tests and for treating thyroid cancer (8.0 days)
• ⁸⁹Sr, used for treating bone cancer, intravenous injection (52 days)
• ¹⁹²Ir, used for brachytherapy (74 days)
• ⁶⁰Co, used for brachytherapy and external radiotherapy (5.3 years)
• ¹³⁷Cs, used for brachytherapy, external radiotherapy (30 years)

Industrial

Industrial source waste can contain alpha, beta, neutron or gamma emitters. Gamma emitters are used in radiography while neutron emitting sources are used in a range of applications, such as oil well logging.

Naturally occurring radioactive material (NORM)

Processing of substances containing natural radioactivity; this is often known as NORM. A lot of this waste is alpha particle-emitting matter from the decay chains of uranium and thorium. The main source of radiation in the human body is potassium-40 (⁴⁰K). There is a natural background radioactivity that life systems are built to resist. Most rocks, due to their components, have a certain, but low level, of radioactivity.

Coal

Coal contains a small amount of radioactive uranium, barium, thorium and potassium, but, in the case of pure coal, this is significantly less than the average concentration of those elements in the Earth's crust. However, the surrounding strata, if shale or mudstone, often contains slightly more than average and this may also be reflected in the ash content of 'dirty' coals. The more active ash minerals become concentrated in the fly ash precisely because they do not burn well. However, the radioactivity of fly ash is still very low. It is about the same as black shale and is less than phosphate rocks, but is more of a concern because a small amount of the fly ash ends up in the atmosphere where it can be inhaled

Oil and gas

Residues from the oil and gas industry often contain radium and its daughters. The sulphate scale from an oil well can be very radium rich, while the water, oil and gas from a well often contains radon. The radon decays to form solid radioisotopes which form coatings on the inside of pipework. In an oil processing plant the area of the plant where propane is processed is often one of the more contaminated areas of the plant as radon has a similar boiling point as propane.

Sources

• International Atomic Energy Agency, The Management System for the Processing, Handling and Storage of Radioactive Waste Safety Guide, Safety Standards, Series No. GS-G-3.3, June, 2008.
• International Atomic Energy Agency,  The Management System for the Disposal of Radioactive Waste Safety Guide, Safety Standards, Series No. GS-G-3.4, June, 2008.
• International Atomic Energy Agency, Storage of Radioactive Waste Safety Guide, Safety Standards Series No. WS-G-6.1, December, 2006.
• Nuclear Energy Agency, Radioactive Waste Management Committee, Moving forward with geological waste disposal: an NEA RWMC collective statement, NEA/RWM(2008)5/REV2.
• OECD Nuclear Energy Agency, Ad hoc Expert Group on the Timing of High-level Radioactive Waste Geological Disposal, Accessed 25 November 2008.
• United States Nuclear Regulatory Agency, Radioactive waste, Accessed 25 November 2008.
• University of Massachusetts at Lowell nuclear waste page, Transuranic waste, Accessed 25 November 2008.
• Wikipedia Contributors, Radioactive waste, Wikipedia The Free Encyclopedia, Accessed 25 November 2008.
• World Nuclear Association, Radioactive wastes, Accessed 25 November 2008.