A storage manual for highly-active waste

Storage, often for a period of decades, is an essential feature in the cradle-to-grave management of radioactive waste. In England and Wales, the long-term solution is an underground Geological Disposal Facility, or GDF, which is still very much in the development phase, as yet without an agreed location and, therefore, many years from becoming operational. In Scotland, the policy is for long-term management in near-surface facilities, also still undeveloped.

In the UK, higher-activity waste includes high level waste, intermediate level waste and some low level waste that is unsuitable for disposal at the Low Level Waste Repository.

The newly-published Industry Guidance on the Interim Storage of Higher Activity Waste (HAW) Packages (available via www.tinyurl.com/ndahawguidance) is intended as a practical support manual for all those organisations that need to store waste for the foreseeable future, and assumes a potential storage period of at least 100 years. Organisations likely to benefit from the guidance include all those involved in nuclear operations and decommissioning, as well as defence, research and healthcare.
The key requirement is to keep the waste safe and secure in engineered packages and inside stores that are resilient while waiting for the GDF and the Scottish HAW strategy to be fully developed. The guidance addresses wide-ranging technical issues associated with both storing the waste packages and the store buildings. The aim is to provide a coordinated, consistent approach that takes account of all the challenges likely to be encountered when storing packaged HAW for many decades. The work programme looked at areas such as package performance, package monitoring and inspection, store longevity, including facility monitoring and inspection, store environmental controls and package reworking.

The focus was on the intermediate level waste (ILW) that arises at most nuclear sites, rather than high level waste (HLW), which exists in much smaller quantities and is stored only at Sellafield, where it is generated by the reprocessing of spent fuel. However, the principles in the guidance apply to all conditioned wastes held in long-term storage.

A substantial amount of HAW is already stored at various locations across the country, with around 60,000 packages on sites owned by the UK government's Nuclear Decommissioning Authority. Much of this is at Sellafield, home to some of the UK's oldest packages and stores, constructed at a time when a requirement for 100 years of storage was not expected. Tens of thousands more packages will be produced as the nuclear clean-up mission continues and operational power stations -- largely the AGR fleet -- eventually close down and move into decommissioning. The guidance is therefore intended to assist with designing future stores and monitoring of packages, as well as improving or maintaining existing storage systems.

The first issue was published in August 2011 for a period of 'road-testing' by industry and other stakeholders. Based on feedback received, the guidance was updated and launched at a seminar on HAW storage towards the end of 2012. Guests at the launch event, organised by the NDA, included representatives from the International Atomic Energy Agency, which is carrying out a peer review of the guidance, as well as the regulators and store operators. Presentations during the day included a summary of the research which focused on generic topics such as waste package corrosion, salt deposition, waste packaging measuring techniques, store humidity controls and peer reviews, which will help to provide technical underpinning for many of the guidance recommendations.

James McKinney, the NDA's head of integrated waste, said: "Many individuals and organisations have contributed to this guidance, and feedback from the grassroots suggests that it is already proving useful." The cross-industry Integrated Project Team (IPT) that drew up the guidance has now become the Store Operations Forum, which continues to meet annually with the intention of updating and refining the contents to take account of new developments (comments are invited through strategy@nda.gov.uk).

Contents

The guidance seeks to cover all the significant technical issues arising from the interim storage of packaged HAW, aims to be practical in implementation of measures, and to be relevant to most designs of UK packages and stores. It recognises that some issues are short-term in nature, while many are longer-term and span several generations. It is intended to be an interface between national legislation and regulatory guidance. There are four main sections (Figure 1).

The UK's inventory of HAW is extremely diverse and non-standard, largely as a consequence of the early post-war research and experimental facilities, together with a power station template, the basic Magnox design, that was tweaked and refined over the decades as each new plant was built. The end result is a huge variation in types of waste, many unique to a single facility and requiring tailor-made treatment and conditioning. To accommodate this diversity, the guidance describes a number of common principles that provide an overarching framework, a list of recommended good practices and a range of approaches covering the lifecycle of interim storage and variation in HAW properties. Potential techniques and solutions, derived from operational experience and R&D, are covered as 'toolkits', while the individual 'tools' are also described.

Robust storage is based on understanding the overall waste storage system, including the processes that may affect the integrity of packages and stores over time, as well as the interactions and relationship between the different components. The guidance recognises that challenges of managing the waste packages and stores differ both between and even within sites, opting to use a common set of principles that describe practicable and proportionate methods (that is, 'approaches') and options for solutions (that is, 'toolkits'). Additionally, 'good practices', based on feedback from operational stores, R&D studies and Regulators, are highlighted. A generic view of waste packages is shown in Figure 2.

This approach takes account of the factors that influence different storage system designs such as the properties of the waste groups. For example, packages that can be handled by contact will typically be more readily managed and maintained than those which must be managed remotely.

As a framework, the following principles are applied:

A. Cradle-to-grave lifecycle.

Packages should be managed to protect their overall longevity, from manufacture of the container through to closure of a disposal facility many years into the future. As interim storage is temporary, packages should be retrievable and exportable to a disposal facility or another store for continued storage.

B. Right Package -- Right Store.

Good package design should be matched by equally good store designs with due consideration of the hazards presented by the waste packages and quality of storage required. The overall storage system, that is, the wasteform and its container, the store environment and the store structure, should have minimal need for active safety systems, monitoring or prompt human intervention. Overall value for money should be demonstrated through both avoiding over- and under-engineering.

C. Minimising waste generation

The waste hierarchy should be deployed across the lifecycle, from design through to decommissioning of the store, thus avoiding generating waste unnecessarily while using resources sustainably.

D. Prevention is better than cure.

The storage system should be managed to minimise the risk that intervention will be required to maintain safety functions. The storage system should be subject to regular and proportionate monitoring and inspection to demonstrate performance and improve understanding of how the system may evolve in the future.

E. Foresight in design.

The storage system design should be flexible to meet likely future needs that take account of uncertainties and incorporate proportionate contingencies. Designs should take account of the need for the store to be part of the UK-wide storage asset.

F. Effective knowledge management.

The experiences and lessons learned from existing store operations should be shared between store operators to inform development of standards and designs. Learning from relevant overseas storage facilities should also be utilised effectively through collaboration. This collective information should be used to further inform development of UK storage standards, with an emphasis on exploiting the insights and experiences gained, that is, continual improvement. Appropriate records must be maintained throughout.

In strategic terms, the guidance aims to:

• Standardise the overall approach to interim storage based on maximising package and store performance, while minimising the need for package reworking
• Promote cross-industry working and establish common approaches for long-term management of waste packages in interim storage
• Inform store planning and design, and monitoring and inspection programmes
• Enhance the recognition of the important relationship between waste packaging, storage, transport and disposal by stakeholders
• Improve the visibility of the wide range of work generated by the IPT to Regulators and other stakeholders.

The four main sections are:

Waste package performance and design
Six good practices, three approaches and five toolkits are described in this section of the guidance including:
(a) A package design approach, outlining steps to establish a robust package design, including toolkits of existing 'proven' container designs, materials, encapsulants and emerging innovations.
(b) Application of the package performance approach across the waste-management lifecycle based on nine fundamental safety functions provided by the package.
(c) An assessment approach to identify how packages may evolve with time based on the latest research findings from RWMD-led work. It includes a toolkit of computer-based models.
(d) A lifetime package care and management approach.

Store performance and design
Six good practices, four approaches and six toolkits are described to promote robust store performance and design, including:
(a) A store design approach
(b) A store longevity approach (Figure 3) to meet a target lifetime of at least 100 years
(c) An environmental control approach
(d) An environmental operational limits and conditions (OLCs) approach to support the development of store-specific OLCs for salt deposition, relative humidity and temperature controls.

Storage system operations
Within the guidance, this section covers seven good practices, eight approaches and four toolkits to promote robust waste storage system operations, including:
(a) Approaches for package movements
(b) An emplacement approach
(c) An approach to maintain package safety functions
(d) An approach to determine when a store's environmental conditions may benefit from being changed to protect package and store life-limiting components
(e) Approaches to maintain store life-limiting features and extend store lifetimes including a toolkit of credible repair techniques.

Storage system assurance
Covering five good practices, eight approaches and seven toolkits to assure the waste storage system across its life-cycle, the guidance highlights the following:
(a) An approach to set a baseline for the waste storage system so that ongoing monitoring and inspection results can be interpreted
(b) A systematic approach to monitor and inspect the storage system
(c) A statistically-based approach to define monitoring and inspection rates to achieve a defined level of confidence over the storage period
(d) An approach to establish strategic archives in storage of materials that could be used to help ensure the longevity of the system
(e) An approach to optimally deploy inactive samples and simulants, including dummy packages.
(f) An approach on the beneficial role of formal audits, such as the UK government's Radioactive Waste Management Directorate periodic letter of compliance (LoC) reviews, is developed as an approach.
(g) The fundamental importance of effective knowledge management is outlined.
(h) An approach to maintain human resources and skills is provided. This includes outlining NDA initiatives and others UK-wide.

Background

Drafting of the Guidance was led by the Nuclear Decommissioning Authority and a team spanning all sectors of the nuclear industry. The main author was Mark Tearle, principal consultant on waste packaging at Magnox (although he has since moved to the UK's Office for Nuclear Regulation). Project management support came from Dave Stretton, Magnox waste programme manager, Bernard Wheeler, strategy team leader at Sellafield, and James McKinney, NDA's head of integrated waste.
The impetus, however, arose from a number of UK and Scottish policy developments together with subsequent recommendations from the Government's Committee on Radioactive Waste Management (CoRWM). In 2006, CoRWM recommended to Government, with regard to higher activity waste:

"A robust programme of interim storage must play an integral part in the long-term management strategy. The uncertainties surrounding the implementation of geological disposal, including social and ethical concerns, led CoRWM to recommend a continued commitment to the safe and secure management of wastes that is robust against the risk of delay or failure in the repository programme."

The requirement in both the UK and Scotland was that storage arrangement should safe and secure for at least 100 years.

A later CoRWM review, in 2009, also recommended a more strategic approach, as the current approach was 'fragmented'.

The NDA reviewed UK arrangements for interim storage of HAW in 2009, addressing the issues and recognising the importance of an integrated, standard approach that could be adopted industry-wide. This led to the formation of the Integrated Project Team (IPT) which included representatives from the Site Licence Companies (those with HAW), the Radioactive Waste Management Directorate that has responsibility for GDF development, EDF Energy, the Ministry of Defence, the Atomic Weapons Establishment, and supply chain organisations. Regulators and CoRWM were observers as the guidance developed, providing input through workshops and commenting on draft versions. In all, 150-200 individuals contributed to the guidance.
Such collaboration has been ground-breaking, promoting shared learning, the exploring of good practice, and enabling areas for further research to be refined and agreed.