Application of Waste Package Specifications for Low Heat Generating Waste: Containment
Source: gov.uk (2025-11-13)

This page describes the concept of immobilising and encapsulating hazardous materials within a wasteform and how adequate immobilisation can be proven as part of Disposability Assessment submissions.

A key safety purpose of a waste package (https://www.gov.uk/guidance/supporting-information-on-waste-packages) is to prevent radioactive materials from being released. There are various waste types with different physical, chemical and radiological properties and waste packages may provide the necessary performance in different ways. Some wastes will be naturally fixed whereas others will require further treatment to fix them in a wasteform or the use of a suitably robust container.

Immobilised waste

Waste that requires immobilisation often contains radionuclides in particulate, liquid or gaseous form, which may also be in volatile or soluble compounds. These waste characteristics are associated with saturated solids, suspensions, sludges, dry powders, crumbly solid materials and solid materials that may corrode. In practice, the waste may be incorporated into the wasteform by mixing with a suitable material or encapsulating by introducing a fluid material.

Either approach will result in a wasteform where mobile radionuclides being chemically bound or physically trapped within the matrix. The aim is to create a single block of wasteform that will control and minimise the release of radionuclides and other harmful waste components.

A waste package with adequate immobilisation will usually have the following properties:
- Low and predictable releases of radionuclides and other hazardous materials following an accident, for example on impact and/or a fire
- As the wasteform evolves, changes to immobilisation and waste package performance will be predictable
- Predictable gas releases within the rate limits defined by the packaging specifications
- Reduced solubility of key radionuclides and toxic chemicals
- Wasteform properties, for example, voids and chemistry, compatible with the backfilled GDF (https://www.gov.uk/guidance/gdf-geological-disposal-facility) disposal vault environment
The wasteform must immobilise liquids and fine particulate material. This will eliminate fluidity, dispersal and movement of radionuclides and bulk material within the solid wasteform.

Non-encapsulated waste

Non-encapsulated waste will not have characteristics that cause their radionuclides to become mobile. An example of this would be neutron-irradiated metals where the radionuclides are an integral part of the waste rather than a surface contamination. It is important that the evolution (for example, corrosion) must not generate radionuclides in particulate, liquid or gaseous form throughout the package lifetime, causing immobilisation to be required.

Encapsulation describes how immobilisation of the radionuclides and other hazardous materials within the waste could be achieved.

Unimmobilised waste

Wastes packaged using a robust waste container will not require immobilisation to ensure the sufficient containment by the waste package.

Demonstrating adequate immobilisation

As part of the Disposability Assessment (https://www.gov.uk/guidance/managing-higher-activity-radioactive-waste-disposability-assessment), waste packagers must demonstrate a waste conditioning process that will result in a wasteform with radionuclides that will remain immobilised for a specific period of time. This must include the degree of immobilisation provided by the waste itself and any additional containment, for example by pre-treatment of the waste to improve immobilisation.

The evidence required to demonstrate adequate immobilisation by a wasteform includes:
- Small-scale and large-scale testing of the wasteform
- Testing or modelling of the waste package to determine radionuclide releases under normal and accident conditions
- Data to show that evolution of a wasteform over the specified period will not reduce the immobilisation performance
See next: Application of Waste Package Specifications for Low Heat Generating Waste: Transport and Disposal (https://www.gov.uk/guidance/application-of-waste-package-specifications-for-low-heat-generating-waste-transport-and-disposal)

See previous: Application of Waste Package Specifications for Low Heat Generating Waste: Standardised Waste Container Designs (https://www.gov.uk/guidance/application-of-waste-package-specifications-for-low-heat-generating-waste-standardised-waste-container-designs)