Radioactive waste management refers to the technology and policies that guide the processing, treatment, transportation and disposal of radioactive wastes such that they do not pose undue risk to to the health of living organisms.

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.

Of particular concern in nuclear waste management are two long-lived fission products, ⁹⁹Tc (half-life 220,000 years) and ¹²⁹I (half-life 17 million years), which dominate spent fuel radioactivity after a few thousand years. The most troublesome transuranic elements in spent fuel are ²³⁷Np (half-life two million years) and ²³⁹Pu (half life 24,000 years).  Nuclear waste requires sophisticated and expensive treatment and management in order to successfully isolate it from interacting with the biosphere. This usually necessitates treatment, followed by a long-term management strategy involving storage, disposal or transformation of the waste into a non-toxic form. Governments around the world are considering a range of waste management and disposal options, though there has been limited progress in the actual implementation of long term waste management solutions.

Types of wastes

Low level radioactive waste stored at the Australian Nuclear Science and Technology Organisation. Credit: Nick CubbinLow-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. 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. High-level radioactive wastes (HLW) are the highly radioactive materials produced as a byproduct of the reactions that occur inside nuclear reactors. High-level wastes take one of two forms: spent (used) reactor fuel when it is accepted for disposal, or waste materials remaining after spent fuel is reprocessed. Spent nuclear fuel is used fuel from a reactor that is no longer efficient in creating electricity, because its fission process has slowed. However, it is still thermally hot, highly radioactive, and potentially harmful. 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.

Storage

Following its generation, untreated radioactive waste may be subject to a number of waste management processes prior to its disposal such as handling, treatment and conditioning. During these processing steps, radioactive waste may be subject to storage at a number of stages. Hence, radioactive waste will be stored in processed and unprocessed forms and for varying periods of time. There are many reasons why it may be appropriate to store radioactive waste for varying periods of time. Examples include the following: (a) To allow for the decay of short lived radionuclides to a level at which the radioactive waste can be released from regulatory control (clearance) or authorized for discharge, or recycling and reuse; (b) To collect and accumulate a sufficient amount of radioactive waste prior to its transfer to another facility for treatment and conditioning; (c) To collect and accumulate a sufficient amount of radioactive waste prior to its disposal.

There are two acceptable storage methods for spent fuel, a high level waste, after it is removed from the reactor core: spent fuel pools and dry cask storage.

Spent fuel pools

Spent fuel pool. Source: U.S. Dept. of EnergySpent fuel pools are storage pools for spent fuel from nuclear reactors that typically 40 or more feet deep, with the bottom 14 feet equipped with storage racks designed to hold fuel assemblies removed from the reactor. These fuel pools are specially designed at the reactor in which the fuel was used and situated at the reactor site. The rods are moved into the water pools from the reactor along the bottom of water canals, so that the spent fuel is always shielded to protect workers. About one-fourth to one-third of the total fuel load from the pools is spent and removed from the reactor every 12 to 18 months and replaced with fresh fuel.

In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years before being sent for reprocessing or dry cask storage. The water cools the fuel and provides shielding from radiation.

Dry cask storage

Dry cask storage. Source: U.S. Nuclear Regulatory CommissionDry cask storage is a method of storing high-level radioactive waste, such as spent nuclear fuel that has already been cooled in the spent fuel pool for at least one year. The fuel is surrounded by inert gas inside a large container. These casks are typically steel cylinders that are either welded or bolted closed. Ideally, the steel cylinder provides leak-tight containment of the spent fuel. Each cylinder is surrounded by additional steel, concrete, or other material to provide radiation shielding to workers and members of the public. Some of the cask designs can be used for both storage and transportation.

There are various dry storage cask system designs. With some designs, the steel cylinders containing the fuel are placed vertically in a concrete vault; other designs orient the cylinders horizontally. The concrete vaults provide the radiation shielding. Other cask designs orient the steel cylinder vertically on a concrete pad at a dry cask storage site and use both metal and concrete outer cylinders for radiation shielding.

Initial treatment of waste

Vitrification

Long-term storage of radioactive waste requires the stabilization of the waste into a form which will not react, nor degrade, for extended periods of time. One way to do this is through vitrification. Vitrification refers to the process of mixing radioactive waste, mixed waste, or materials such as plutonium with molten glass and forming them into glass marbles, blocks, logs, or frit (fragments). The first pilot vitrification plant was built in Marcoule, France in 1967 to vitrify highly radioactive waste. Since that time, vitrification on a commercial scale has been successfully carried out in Russia, the United States, France, and several other countries.

At the Sellafield nuclear processing facility in the UK,  high-level waste is mixed with sugar and then calcined. Calcination involves passing the waste through a heated, rotating tube. The purposes of calcination are to evaporate the water from the waste, and de-nitrate the fission products to assist the stability of the glass produced.

The 'calcine' generated is fed continuously into an induction heated furnace with fragmented glass. The resulting glass is a new substance in which the waste products are bonded into the glass matrix when it solidifies. This product, as a molten fluid, is poured into stainless steel cylindrical containers ("cylinders") in a batch process. When cooled, the fluid solidifies ("vitrifies") into the glass. Such glass, after being formed, is very highly resistant to water.

After filling a cylinder, a seal is welded onto the cylinder. The cylinder is then washed. After being inspected for external contamination, the steel cylinder is stored, usually in an underground repository. In this form, the waste products are expected to be immobilized for a very long period of time (many thousands of years). The glass inside a cylinder is usually a black glossy substance. All this work (in the United Kingdom) is done using hot cell systems. The sugar is added to control the ruthenium chemistry and to stop the formation of the volatile RuO₄ containing radio ruthenium. In the west, the glass is normally a borosilicate glass (similar to Pyrex), while in the former Soviet bloc it is normal to use a phosphate glass. The amount of fission products in the glass must be limited because some (palladium, the other Pt group metals, and tellurium) tend to form metallic phases which separate from the glass.

Ion exchange

It is common for medium active wastes in the nuclear industry to be treated with ion exchange or other means to concentrate the radioactivity into a small volume. The much less radioactive bulk (after treatment) is often then discharged. For instance, it is possible to use a ferric hydroxide floc to remove radioactive metals from aqueous mixtures. After the radioisotopes are absorbed onto the ferric hydroxide, the resulting sludge can be placed in a metal drum before being mixed with cement to form a solid waste form. In order to get better long-term performance (mechanical stability) from such forms, they may be made from a mixture of fly ash, or blast furnace slag, and portland cement, instead of normal concrete (made with portland cement, gravel and sand).

Synroc

Synroc.The Australian Synroc (synthetic rock) is a more sophisticated way to immobilize such waste, and this process may eventually come into commercial use for civil wastes (it is currently being developed for U.S. military wastes). Synroc was invented by the late Prof Ted Ringwood (a geochemist) at the Australian National University. The Synroc contains pyrochlore and cryptomelane type minerals. The original form of Synroc (Synroc C) was designed for the liquid high level waste (PUREX raffinate) from a light water reactor. The main minerals in this Synroc are hollandite (BaAl₂Ti₆O₁₆), zirconolite (CaZrTi₂O₇) and perovskite (CaTiO₃). The zirconolite and perovskite are hosts for the actinides. The strontium and barium will be fixed in the perovskite. The caesium will be fixed in the hollandite.

Long term waste management

The decay products in high level waste have practical lifetimes ranging from 10,000 to 1,000,000 years based on the effect of estimated radiation doses.These wastes must be contained and isolated from humans and the environment for many tens of thousands of years. Disposal of these wastes in engineered facilities, or repositories, located deep underground in suitable geological formations is being developed and further investigated world wide as the reference solution in order to protect humans and the environment both now and in the future. The nuclear industry contends that deep geological disposal after a reasonable cooling time and long-term storage followed by eventual disposal of high-level radioactive waste (HLW), including spent fuel, are both technically feasible and safe options. In most nations, extended interim storage of HLW is considered as a temporary solution pending the implementation of repositories for this type of waste. This poses two principle challenges: (1) the identification of stable geological formations, and (2) the design and implementation of stable human institutions over hundreds of thousands of years.

Geologic disposal

An engineered barrier system for high level waste, which consists of a waste package, drip shield, and supporting structures. Source: U.S. Nuclear Regulatory CommissionThe argument for geologic disposal goes as follows. At depths of hundreds of meters, the rock formation will protect the disposal facility from human interference and from natural processes such as earthquakes and climate changes. Additionally, careful selection of the disposal facility location and waste positioning aim to reduce as far as practicable the risks of perturbations from such processes. The concept of geological disposal takes advantage of the capabilities of both the local geology and the engineered materials to fulfill specific safety functions in complementary fashion providing multiple and diverse barrier roles. Releases from the engineered barriers would occur thousands of years after disposal and would be very small. Additionally, these small releases are diluted and slowed by the geological formation surrounding the repository and are further reduced by radioactive decay. The ensuing potential radiological exposure in the biosphere would not represent, at any time, a significant increment above the natural background. The concept of geological disposal, including its safety and ethical implications, has been debated and approved in many forums, including national legislatures; state, provincial and local discussions; by individuals; in peer-reviewed literature; by international organizations; and by national scientific bodies. This demonstrates a broad consensus on the geological disposal option, achieved through open and participative processes in many nations.

Reversibility and retrievability are considered by some countries as being important parts of the waste management strategy. Reversibility implies a disposal program that is implemented in stages and that keeps the options and choices open at each stage, and provides the capacity to manage the repository with flexibility over time. Retrievability is an example of reversibility that describes the possibility to reverse the step of waste emplacement.

National geologic disposal plans

As of 2008:

• Final isolation of high-level waste, spent fuel and other classes of waste with long-lived components in geological repositories is now the recognized reference solution in Canada, France, and the United Kingdom.
• In the United States, a license application has been developed and was submitted for review to the Nuclear Regulatory Commission in 2008.
• In Finland, a site and a disposal system design have been identified and work is ongoing towards the development of a license application to allow the construction of a deep disposal facility for spent fuel.
• In Sweden, a reference design has been developed and two sites are being characterized. Selection of the Swedish site for final disposal of spent fuel is approaching.
• In Switzerland, a broad, transparent and stepwise site selection process has been initiated as required in the new nuclear energy legislation.
• In Japan, after promulgation of the Final Disposal Law for high-level waste disposal, which has been amended to include other long-lived waste (referred to as “trans-uranium waste”), a stepwise siting process has started and is ongoing.
• In Germany, a license has been granted to operate the deep disposal facility at Konrad for “non-heat emitting wastes”, which include waste with long-lived components.

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.