What are different types of radioactive waste forms? How are they disposed in geological repository?

Types of Radioactive Waste Forms

Radioactive waste is broadly classified based on its activity level and the period for which it remains radioactive. This classification dictates the appropriate handling and disposal methods.

• High-Level Waste (HLW): This is the most radioactive waste, primarily consisting of spent nuclear fuel and waste from reprocessing spent fuel. It generates significant heat and requires robust shielding.
• Intermediate-Level Waste (ILW): ILW contains higher levels of radioactivity than low-level waste but generates less heat. It includes reactor components, resins, and chemical sludges.
• Low-Level Waste (LLW): LLW comprises items that have become contaminated with radioactivity, such as clothing, tools, and filters. It represents the bulk of the volume of radioactive waste but contains a relatively small amount of radioactivity.
• Transuranic Waste (TRU): This waste contains alpha-emitting transuranic elements (elements heavier than uranium) with half-lives greater than 20 years, primarily originating from nuclear weapons production.

These waste types are further processed into different waste forms to enhance their stability and reduce the risk of radionuclide release:

• Vitrification: HLW is often vitrified – incorporated into a glass matrix – to create a highly durable and leach-resistant waste form.
• Cementation: ILW and LLW are frequently cemented – mixed with cement – to immobilize the radioactive materials.
• Bituminization: Using bitumen (asphalt) as a matrix for encapsulating waste, particularly ILW.
• Compaction & Incineration: LLW is often compacted to reduce volume, and combustible LLW can be incinerated.
• Metal Matrix: Some wastes are encapsulated in stainless steel or other corrosion-resistant metals.

Geological Repository Disposal

Geological repositories are engineered systems designed for the long-term isolation of radioactive waste in stable geological formations. The concept relies on a multi-barrier system to prevent the release of radionuclides into the environment.

1. Site Selection Criteria

Selecting a suitable site for a geological repository is a complex process involving rigorous scientific investigation. Key criteria include:

• Geological Stability: The site must be located in a geologically stable area with minimal seismic activity, volcanic activity, or tectonic movement.
• Hydrogeology: Low groundwater flow rates and minimal permeability are crucial to limit radionuclide transport.
• Rock Properties: The host rock should have properties that inhibit radionuclide migration, such as high sorption capacity (ability to bind radionuclides) and low fracture density. Common host rocks include granite, clay, salt, and shale.
• Depth: Repositories are typically located at depths of several hundred meters to provide isolation from the surface environment.
• Distance from Human Activity: The site should be located in a remote area with minimal current or foreseeable human activity.

2. Engineered Barriers

Engineered barriers complement the natural barriers provided by the host rock. These include:

• Waste Form: As described above, the waste is processed into a durable form to minimize leaching.
• Waste Canisters: The waste form is sealed in robust, corrosion-resistant canisters, typically made of stainless steel or copper.
• Buffer/Backfill Material: The space between the canisters and the host rock is filled with a buffer material, such as bentonite clay, which swells when hydrated, creating a low-permeability barrier and absorbing radionuclides.
• Seals: Shaft seals and plug systems are used to prevent water ingress and radionuclide migration through the access tunnels.

3. Repository Layout & Operation

Repositories are typically designed with a network of tunnels and disposal rooms. Waste canisters are placed in these rooms and surrounded by the buffer material. The repository is then backfilled and sealed.

4. Monitoring & Closure

Long-term monitoring of the repository is essential to verify its performance. This includes monitoring groundwater, gas emissions, and ground deformation. After a period of institutional control (typically several decades to centuries), the repository is closed, and the site is transferred to long-term stewardship.

Example: The Onkalo spent nuclear fuel repository in Finland is a leading example of a geological repository under construction in Olkiluoto. It is being built in crystalline bedrock and utilizes a multi-barrier system including copper canisters and bentonite clay.