Some radioactive wastes are produced in all parts of the nuclear cycle. Unlike other wastes, radioactive wastes diminishes with time. Each radionucleotide contained in the radioactive waste has half-life (which is the time taken for half the atom to decay). Radionucleotides with long half-lives tend to be alpha and beta emitters. Eventually, all radioactive elements decay into non-radioactive elements.

The main objective for radioactive waste management is to protect the environment. This means isolating or diluting the wastes so that the wastes do not harm the biosphere. The waste management consists of containing the wastes with some needing permanent burial.

There are three types of nuclear wastes. These are:-

1. Low level Wastes
2. Intermediate Level Wastes
3. High Level Wastes

Low-level Wastes

These wastes are generated from Hospitals, industry and nuclear fuel cycle. These comprise of mostly paper, rags, cloth etc. which contain low level of radioactivity. These wastes do not require shielding during handling and transport and can be safely disposed off.

Intermediate-level Wastes

These wastes contain higher amount of radioactivity and require some shielding. These wastes comprise of resins, chemical sludge and metal fuel cladding, as well as contaminated wastes from nuclear decommissioning. These wastes may be solidified in concrete or bitumen for disposal.

High-level Wastes

These kind of wastes arise from burning of uranium in nuclear reactors. These wastes contain transuranic wastes and the fission products generated from the reactor. These are hot due to decay heat and highly radioactive, so these require cooling and shielding.

Processing of Used Nuclear Fuel

The used fuel may be reprocessed to recover fissile and fertile materials in order to provide fresh fuel for nuclear power plants.

The main reason for reprocessing used fuel is to recover unused plutonium and to a lesser extent uranium, in the used fuel fuel elements and thereby completing the fuel cycle. This helps in gaining 25% to 30% more energy from the original uranium in the process. This contributes to national energy security. Another reason is to reduce the amount of wastes to be disposed too one-fifth. The reprocessing is done mainly by using PUREX (Plutonium Uranium Extraction) process. Here, the irradiated fuel is dissolved in 7M Nitric Acid. An organic solvent composed of tributyl phosphate in a hydrocarbon solvent such as kerosene is used to extract uranium in complexes. Plutonium is then separated from Uranium by treating kerosene solution with aqueous ferroussulphamate.

Reprocessing used fuel avoids the wastage of valuable resource and saves upto 30% of natural uranium otherwise required.

Reprocessing is not without problems. The main problems associated with it are:

1. The used fuel contains a wide array of nuclides with varying valence states. The varying valence states of nuclides make it difficult for reprocessing.
2. The reprocessed fuel may be contaminated with transuranics and traces of fission products which will affect its suitability for recycling either as blend material or via enrichment.

Treatment and Conditioning of Nuclear Wastes

Treatment and conditioning are used to convert a wide variety of radioactive wastes into forms that are suitable for further management such as transportation, storage and disposal.

There are three types of treatment and conditioning processes. These are

1. Incineration and Compaction
2. Cementation
3. Vitrification

Incineration and Compaction

It has been used for the treatment of low level wastes. The combustible and the non-combustible constituents are first separated out and the combustible constituents are incinerated in a specially engineered kiln to about 1000 degrees Celsius. The gases and fumes produced during the incineration are treated and filtered prior to emission. The resulting ash which contains is then cemented for disposal. Overall volume reduction factor of up to 100 can be achieved depending upon the density of wastes.

Compaction is the straight forward reduction of volume of wastes. Compactors can range from low force compaction systems (5 tonnes or more) to presses with compaction force of over 1000 tonnes. Overall volume reduction factors of up to 10 can be achieved depending upon the type of wastes.

Cementation

Cementation uses specifically formulated grouts which provides the means to immobilise radioactive wastes that is in various forms of sludge, precipitates, or activated materials as well as fragmented solids. The solid wastes are placed into containers. The grout is then added to the container and allowed to set. Sludge is placed into the container and the grouting powder are mixed and allowed to set. The container with monolithic block of concreted waste is then suitable for storage and disposal.

Vitrification

This process is used for high-level wastes. This process deals with the immobilisation of high-level wastes. This immobilisation requires the formation of an insoluble solid waste form that will remain stable for hundreds of years. In general borosilicate glass has been used as a medium for dealing with high-level wastes. The wastes are initially calcined to granular powder and then incorporated into the molten glass. These are then poured into stainless steel canisters of about 1.3 m high and allowed to cool, giving a solid matrix. The canisters are then welded closed then then taken for disposal.

Storage and Disposal

Storage

Interim Waste Storage

Specifically designed interim storage facilities are used to ensure safe storage of hazardous wastes that are pending for long-term disposal option. These are used for storage of intermediate-level wastes and high-level wastes.

Storage Ponds

Storage ponds are generally 7 to 12 metres deep, to allow several metres of water over the used fuel in racked fuel assemblies typically about 4 m long. The multiple racks are made of metal with neutron absorbers incorporated in it. The circulating water both cools and shields the fuel. These ponds are made up of thick reinforced concrete with steel liners.

Disposal

Most low-level wastes are typically sent to land-based disposal.

There are two commonly accepted disposal options. these are:-

1. Near Surface Disposal
2. Deep Geological Disposal

Near Surface Disposal

The International Atomic Energy Agency (IAEA) definition of disposal of wastes with or without engineering barriers, as in:-

• Near-surface disposal at ground level. These facilities are located on or below the ground level where protective covering is of few metres thick. Waste containers are placed in constructed vaults and when full, these vaults are back-filled. Then they are covered with impermeable membrane and topsoil.
• Near-surface disposal in caverns below the ground. This facility is at a depth of several tens of metres below the Earth’s surface and accessed through the drift.

Deep Geological Disposal

Some radioactive wastes are placed in underground repositories in stable geological formations. Isolation is provided by combination of engineered and natural barriers like rock, salt and clay. This is often termed as multi-barrier concept, with the waste packaging, the engineered repository and the geology all providing barriers to prevent radionuclides from reaching the environment.

Radioactive waste management from IshaneeSharma

References:

1. http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-wastes/radioactive-waste-management.aspx
2. http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/processing-of-used-nuclear-fuel.aspx
3. https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-wastes/treatment-and-conditioning-of-nuclear-wastes.aspx
4. https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-waste/storage-and-disposal-of-radioactive-waste.aspx