Managing waste from decommissioning17 July 2019
As more nuclear sites begin decommissioning, Charlotta Sanders and Michel Pieraccini examine the challenges of managing the materials and waste from decommissioning activities.
THE GLOBAL NUCLEAR INDUSTRY HAS entered a new era of decommissioning activity. Some nuclear facilities are reaching the end of their operating lifetimes, while others are being closed due to market forces or national policies. A new report by the World Nuclear Association’s Waste Management & Decommissioning Working Group, Methodology to Manage Material and Waste from Nuclear Decommissioning, highlights key principles and stages of efficient waste management processes and good practices resulting from real site experiences.
Although there are differences between countries in terms of regulations and infrastructure, the report recommends a sequence to manage the decommissioning process and decision-making strategies regarding end-states, characterisation and inventories, waste routes and management, as well as financial planning and funding. While there is no single approach to decommissioning, there are a number of common principles.
To best manage material and waste from decommissioning the working group recommends applying core principles to guide behaviours, using a set of pre-determined values. These include defining the end-state of the site at the beginning of the life-cycle of the plant and establishing the radiological, physical and chemical inventories as early as possible.
The radiological inventory is the basis for selecting the most suitable decommissioning strategy and waste management processes, as well as for minimising the quantity of radioactive waste to be sent to disposal in order to reduce the environmental impact. The core principles for managing material and waste from nuclear decommissioning are presented in Figure 1.
Defining the end-state
The selected end-state and associated decommissioning strategy influence what activities should be carried out and when. The strategy also strongly affects the waste volume generated for disposal and final site re-use. There are two end-state scenarios: re-use of land with some restrictions and regulatory control (brownfield or entombment); or re-use with no restriction or regulatory control (greenfield). The choice may depend on the extent of land decontamination required at the site and its envisaged re-use.
There are two main routes: immediate or deferred. Each comes with benefits and with costs, risks and regulatory factors, as outlined in Table 1. A strategy requiring immediate decommissioning will produce more radioactive waste of a higher category than a deferred strategy, as there will be no benefit from radioactive decay.
Characterisation and Inventory
To carry out efficient decommissioning and accurate materials management, an up-to-date record of the inventory and material characteristics (type, nature, quantity, composition, activity) is essential. The objective of an accurate materials inventory is to sort material into categories in order to identify the best decommissioning methodologies. Categorising systems and structures is vital —along with calculating nuclide vectors — for minimising radioactive waste volumes and decommissioning costs, and also to meet waste acceptance criteria for disposal.
There are two principal sources of ionising radiation during decommissioning: equipment and structures that have been activated by neutron irradiation, and radioactive contamination by radioactive isotope- containing material.
Despite the variety of nuclear plant types, there are common patterns in the processes of formation of radiation fields due to residual radioactivity (although the specific quantities can vary greatly). The main source of radioactive contamination for equipment is Co-60 and for structures is Cs-137 and Sr-90 (along with its decay product Y-90). Due to these isotopes’ long half-lives, there will not be any substantial improvement in radiation levels due to radioactive decay. Decontamination is required. Therefore, consideration should be given to determining the proportions of different waste categories affected by the timing of decommissioning and waste reductions accrued through the decay of short-lived radioactive isotopes, to allow the inventories of radioactive material to be assessed and planned for.
The waste routing — the activities and logistics for managing the material — is a key point in a decommissioning project. It determines the routes from the material inventory to the envisaged material end-states.
The materials’ properties, logistical challenges and regulatory and stakeholder considerations, mean a variety of waste routes is required. Given that particular waste routes may be temporarily or permanently unavailable, at least one alternative should be kept open for each category of material wherever it is practicable. Generating waste streams or packages without a disposal route, with significant uncertainties in composition or with properties that are hard to manage, should be avoided.
If waste has to be reconditioned or retrieved it could be very costly. It is therefore important to carry out full planning and waste route analyses to ensure that the waste is effectively and efficiently managed.
In summary, it should be kept in mind that all waste generated from a decommissioning project should have a dedicated and agreed waste route and material end-state. The most suitable waste routes rely on the waste management strategy, which depends on the site end-state and selected decommissioning strategy (immediate or deferred).
Treatment and processes
The different amounts and types of materials, waste routes and waste management strategies require a variety of treatment processes, according to the selected waste route, and in compliance with the decommissioning strategy and end-state.
The waste hierarchy principle provides a basis for reducing waste volume and selecting treatment processes. This principle encourages recycling and therefore minimises the amount of waste for final disposal.
In a non-nuclear environment, waste management follows the waste hierarchy principle (i.e. reduce, re-use, recycle, recover and disposal). In the nuclear environment, decontamination, volume reduction and conditioning are additional measures used to minimise the waste prior to final disposal. Figure 2 shows the concept behind the waste hierarchy, which promotes a preferred end-state of re-using or recycling the waste as material or, preferably, the avoidance of waste generation.
It is essential to keep the material arising from decommissioning separated, to manage it according to the waste hierarchy. Non-contaminated material should be kept clear of contaminated material. Lower activity material should be separated from higher activity material and contaminated material (which can be decontaminated) should be segregated from activated material. Segregation will maximise the amount of non-contaminated or non-activated material to be re-used or recycled and minimise the amount of contaminated material.
Economics and financial planning
The cost of decommissioning has several drivers. An important driver is waste management, which must be carefully done to avoid cost escalation and schedule overruns. Although the costs of waste treatment, conditioning, packaging and transport are not a major part of the overall decommissioning cost, these activities have a strong influence on the schedule. Avoiding schedule overrun reduces the time-related costs (project management and site operation) and volume reduction lowers the overall disposal costs. The approximate shares of the different cost categories are shown in Figure 3.
A new phase of decommissioning
Decommissioning and related material and waste management requirements are becoming ever more a global phenomenon. Despite some differences in national policies, a number of common principles can be identified which will assist States as they take on the challenge of decommissioning.
A decommissioning strategy and site end-state has to be clearly defined early in the planning process. This includes the gathering and maintaining an inventory record management system.
Time management schemes should be well established, along with measures for waste segregation, clearance or recycling and volume-reduction, which are key parameters for a successful decommissioning project with limited environmental impact, duration and costs. Doing this will ensure that waste handling and treatment do not become bottlenecks in site decommissioning activities. Lastly, by focusing on robust and flexible waste treatment solutions, with the early removal of fuel, large components and waste, sites will experience a shorter decommissioning schedule and lower overall cost.
To benefit from this experience, future nuclear reactor designs should consider decommissioning from the conception phase by allowing major internal equipment (including the reactor vessel) to be replaced in order to re-use the buildings. This could help transform the decommissioning process from an expensive activity lasting 20-30 years into a maintenance activity requiring about 5-6 years.
The World Nuclear Association's Waste Management & Decommissioning Working Group promotes the re-use and recycling of materials along with the safe disposal of waste from decommissioned nuclear sites.
Author information: Charlotta Sanders, Senior Project Manager, World Nuclear Association; Michel Pieraccini, International Cooperation Director, EDF, and Chair of the World Nuclear Association’s Waste Management & Decommissioning Working Group