Here Dr Charlotta E Sanders and Dr Michel Pieraccini take a closer look at the waste challenges surrounding nuclear plant decomissioning
With an ever-increasing number of nuclear sites beginning the costly process of decomissioning, the industry is now faced with the challenge of managing the resultant material and waste.
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.
Its recently published report, Methodology to Manage Material and Waste from Nuclear Decommissioning, brings together international knowledge and expertise, providing guidance to manage decommissioning challenges.
Here the organisation’s senior project manager Dr Charlotta E Sanders and international cooperation director at EDF Dr Michel Pieraccini take a closer look at the waste problem that accompanies closing down nuclear sites.
Managing waste from nuclear decommissioning
The global nuclear industry has entered into a new era of decommissioning activity as more nuclear facilities reach the end of their operating lifetimes, while others are being prematurely closed due to market forces and/or national policies.
A new report by the World Nuclear Association’s Waste Management & Decommissioning Working Group highlights the key principles and stages of efficient waste management processes and good practices resulting from real worksite experiences.
Although there are differences between countries in terms of regulations and infrastructure, the report recommends a sequence to best manage the decommissioning process and decision-making strategies regarding end states, characterisation and inventories, waste routes and management, as well as financial planning and the provision of funds.
While there is not a “one size fits all” approach to decommissioning, there are a number of common principles shared by international operators/experts.
To implement the best strategy for the management of material and waste from decommissioning activities, the working group recommends applying certain common 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 provides 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 the decommissioning activities that should be carried out and when they should be implemented.
The decommissioning strategy also strongly affects the generated waste volume for disposal, as well as determining final site re-use.
The strategies can be classified according to two main routes: Immediate or deferred.
Each route comes with benefits and associated costs, risks, and regulatory factors, which are outlined in Table 1.
It should be noted that a strategy requiring immediate decommissioning will produce more radioactive waste of a higher category than a deferred strategy, as the benefits of radioactive decay will not be realised.
|Immediate Dismantling||Deferred Dismantling|
|Cost Factors||Risk Factors||Regulatory Factors||Cost Factors||Risk Factors||Regulatory Factors|
|High source term consideration||Higher radiological risks during dismantling and waste processing||Regulator and stakeholder interface and approach are known and can be planned for||Preparation for care and maintenance||Deterioration of plant||Regulator and stakeholder requirements may change|
|Lower lifetime costs and minimal care and maintenance||Funding requirements forecast over a shorter term with increased predictability||Long-term site management||Loss of knowledge and skills|
|Utilisation of existing staff with key knowledge||High confidence in inventory accuracy. Inventory processed to lower risk forms||Lower dismantling cost||Closure of waste routes|
|Potential loss of value associated with the plant footprint
|Difficulty forecasting future economic conditions|
|Uncertainties regarding availability of funds when needed|
Table 1: Costs, risks, and regulatory factors associated with dismantling strategies
Characterisation and inventory
In order to carry out efficient decommissioning and accurate materials management, an up-to-date record of the inventory and material characteristics (i.e., 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 most suitable routes and decommissioning methodologies.
Categorisation of systems and structures is vital – along with calculating nuclide vectors – not only for keeping radioactive waste volumes and decommissioning costs down, but also to meet waste acceptance criteria for disposal.
There are two principal sources of ionising radiation during decommissioning: 1) equipment and structures that have been activated by neutron irradiation, and 2) 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 containment for equipment is determined by Co-60 and for structures by 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, without carrying out decontamination.
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 in order to allow the inventories of radioactive material to be assessed and planned for.
The waste routing – the activities and logistics for managing the material generated – is a key point in a decommissioning project as it determines the routes from the material inventory to the envisaged material end states.
The material properties, logistical challenges, as well as regulatory and stakeholder considerations, require a variety of waste routes.
Given that particular waste routes may be temporarily or permanently unavailable, it is recommended that at least one alternative should be kept open for each category of material wherever it is practically possible.
The generation of waste streams/waste packages without a disposal route, or with significant uncertainties in composition or properties that are hard to manage, should be avoided.
If waste needs to be reconditioned or retrieved, this 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 – waste hierarchy
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 in compliance with the chosen decommissioning strategy and associated end state.
The waste hierarchy principle provides a basis for the reduction of waste volume and the selection of treatment processes, and 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 (reduce, re-use, recycle, recover and landfill 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 in order to fully manage the waste according to the waste hierarchy.
This means that non-contaminated material should be kept clear from contaminated material.
Additionally, lower activity material should be separated from higher activity material, and contaminated material (which can be decontaminated) should be segregated from activated material.
The segregation of material 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 is influenced by several drivers, in particular waste management, which must be carefully handled 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 (e.g., 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
As the nuclear community enters into a new accelerated phase of decommissioning activities with more nuclear facilities reaching the end of their operating lifetimes and/or being prematurely closed due to market forces or national policies, 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 needs to be clearly defined early in the planning process, which includes the gathering and maintenance of an inventory record management system.
Secondly, time management schemes should be well established, as well as measures for the segregation, clearance/recycling and volume reduction of waste, 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/clearance 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 shortened decommissioning schedule, with a reduction in overall costs.
It should also be noted that in order to benefit from this ongoing experience, future nuclear reactor designs might have to consider decommissioning from the conception phase by allowing the replacement of all major internal equipment (including the reactor vessel) in order to re-use the buildings.
This could help transform the decommissioning process from an expensive activity lasting 20 to 30 years into a maintenance activity requiring about five to six years.
A site may be returned to two end state scenarios: The first is the re-use of land with some restrictions and regulatory control (brownfield or entombment), while the second involves the re-use of the land with no restriction or regulatory control (greenfield).
The scenario selected may depend on the extent of land decontamination required at the site and its envisaged re-use.