Characterisation of radioactive waste involves determining its physical, chemical, and radiological properties.
It may be carried out in association with several of the other basic steps, such as during segregation. It may be required for record keeping, moving waste between steps and also to determine the best method for managing waste.
It is the assessment of key physical, chemical, radiological and biological properties of waste, allowing the extent and nature of contamination to be determined. It provides evidence to support facility dismantling, the management of material and waste arising, as well as radiological protection and cost estimates.
Characterisation considers and provides underpinning information for all of the subsequent lifecycle stages. Characterisation objectives are defined at the beginning of the lifecycle for waste, which ensures that the right information is obtained at the right time. Boundaries for the characterisation will be used from research, methods of evaluation will be determined and analysis and implementation will ensue.
Case study
First generation Magnox storage pond (FGMSP) waste improvement
Fuel, sludge and solid waste is in the process of being retrieved from the First-Generation Magnox Storage Pond (FGMSP) at Sellafield. Retrievals of solid waste (non-fuel/sludge) are primarily in the wet bay areas, which typically have small operational areas for monitoring, packaging and lifting, therefore making the process more efficient can have a huge effect especially on dose uptake.
Baseline
The baseline for this solid waste is treatment/packaging via the Box Encapsulation Plant (BEP), as it was anticipated that the waste would be ILW. Significant work has led to a portion of this waste being consigned as LLW, which achieves cost savings and dose reduction, demonstrates industry best practice and follows the waste hierarchy to ensure the process is best available technique/as low as reasonably practical (BAT/ ALARP).
Waste characterisation
To understand how this has been achieved, it’s important to understand waste categories and how waste is consigned. Currently the UK uses activity to define waste classification; LLW must have activity less than 4 GBq/te alpha and 12 GBq/te non-alpha. Therefore, wastes containing activity greater than this are classified as ILW. Activity is calculated using an average contact gamma dose measurement and a generic Activity Conversion Factor (ACF) which is derived by Facilities Characterisation in Remediation Value Stream.
Improved approach
The assumption that metallic wastes from FGMSP would be ILW was based on the conditions of the facility, with highly active water and sludge contaminated materials present. Additionally, as health physics monitors are trained in radiological protection to keep people safe, the dose measurements were based on those assumptions; however, monitoring for waste is different.
A cross enterprise team set out to improve the waste characterisation process as part of a campaign of retrieval from the wet bays. This involved dismantling fuel magazines, which are large cylindrical metal items that can be broken down into three pieces. Each item is pulled from the water, monitored, wrapped, placed in a PacTec bag, and then taken to a buffer storage location until the waste’s activity is assessed and the item is routed or sentenced to a disposal location or interim storage.
The team started by creating bespoke models and monitoring sheets for the magazine components, rather than using the generic ACF. In addition, the team trialled best practice monitoring methods utilised at NRS Magnox. This also involved engaging with the health physics teams and getting feedback from operations teams who work in a high dose environment to retrieve these items. Once a way forward was agreed, training and briefing of the teams took place to make sure that all the different shifts were aware of the changes, understood why they were required and the benefit to both themselves and the task.
Benefits
Now that the improvements have been implemented, these components are monitored as waste rather than for radiological protection and a bespoke model is used to generate an ACF. This has led to approximately 30% of the magazine components retrieved to date being routed to the LLW Repository site as LLW. The teams spend less time monitoring, and no re-work is required following the item being placed in buffer storage. The cost saving alone for these items routed as LLW rather than ILW is approximately £500,000.