Current Arrangements and Requirements for the Conditioning, Packaging and Storage of Intermediate Level Radioactive Waste: Joint RWMAC/NuSAC Report

8. TOWARDS PASSIVELY SAFE STORAGE

8.1 The 1995 White Paper, Cm 2919 ², listed one of the relevant factors to be taken into account in treatment of ILW as the "benefits of placing the waste in a chemically and physically stable form, so that safety may be achieved by passive means" (see chapter 4). Since that time, increasing emphasis has been placed on the desirability of conditioning radioactive wastes to achieve passive safety. In particular NII guidance ¹⁰ states that the "NII requires that, so far as is reasonably practicable, radioactive .. waste should be stored in a passively safe state" (see chapter 5). This emphasis has enjoyed very wide stakeholder support. For example, the BNFL National Stakeholder Dialogue has recommended that "all existing waste and waste arisings must be packed in passively safe, monitorable and retrievable interim storage in the shortest possible time" ¹⁶.

8.2 Although the principle of passive safety has secured widespread support, a number of concerns have been expressed about its practical application. These include what passively safe storage means in practice, and the timescales within which it should be achieved. The purpose of this chapter is to examine these issues. A related question of the length of time for which it is prudent to plan for interim storage is also addressed.

NII Guidance on passive safety

8.3 NII's recent guidance to its inspectors defines passive safety in the following way:

"Passive safety .. requires .. radioactive wastes .. to be immobilised in a form that is physically and chemically stable and stored in a manner which minimises the need for control and safety systems, maintenance, monitoring and human intervention. The wastes .. should be stored in discrete packages which are resistant to degradation and hazards and which can be inspected and retrieved for final disposal."

8.4 This broad definition is then broken down into a checklist of the attributes of passively safe storage, which NII describes as "general principles for passivity". These are as follows:

• the radioactivity should be immobile
• the waste form and its container should be physically and chemically stable
• energy should be removed from the waste form
• a multibarrier approach should be adopted in ensuring containment
• the waste form and its container should be resistant to degradation
• storage environment should optimise waste package life
• the need for active safety systems to ensure safety should be minimised
• the need for monitoring and maintenance to ensure safety should be minimised
• the need for human intervention to ensure safety should be minimised
• the storage building should be resistant to foreseeable hazards
• access should be provided for response to incidents
• there should be no need for prompt remedial action
• the waste packages should be inspectable
• the waste packages should be retrievable for inspection or reworking
• the lifetime of the storage building should be appropriate for storage period prior to disposal
• the storage facility should enable retrieval of wastes for final disposal (or re-storing)
• the waste package should be acceptable for final disposal.

8.5 On implementation of these principles, the NII guidance explains that waste producers should meet a majority of the principles but not every one, and that they should demonstrate that any shortfall does not result in any significant safety detriment or compromise the overall aim of passive safety. The guidance also states that:

"It can be expected that licensees aim to apply the principles for passive safe storage within a framework of reasonable practicability and cost-effectiveness. In the regulatory context, this is interpreted to mean that processing for passive safety should be carried out where it constitutes current best practice and to comply with good engineering principles, unless the costs are grossly disproportionate to the safety benefits."

8.6 On the relative importance of different principles, the guidance states that the "primary consideration" is to ensure that the radioactive waste is immobile and contained in order to minimise the potential for dispersal. It acknowledges that certain raw wastes may be in a form for which the radioactivity is already immobile and therefore meet a key principle of passive safety without the need for processing (e.g., robust metallic components). Where processing is required, the guidance states that it should be possible to do this in a way that is compatible with anticipated requirements for disposal. The guidance describes the storage building as the final physical barrier to the release of radioactivity, but points out that the most significant barriers are "first and foremost" the waste form itself, and secondly the waste container. It adds that the storage building will need to provide sufficient protection to the stored wastes so as to optimise the life of the packages, and that this may necessitate control and monitoring of the building's environment.

8.7 On timescales for achieving passively safe storage, the NII guidance states that: "..... wastes should be placed into a passively safe state as soon as reasonably practicable. The more hazardous the waste ..... and the more mobile its form, the greater the safety benefit from passively safe storage and the sooner this should be achieved". The guidance goes on to clarify that a range of safety-related factors could influence timing: the magnitude of the radioactive hazard; the current form of the waste; the safety of the current facilities for accumulating raw waste; the rate of deterioration of the facilities; the rate of deterioration of the waste; uncertainty in knowledge of the waste; and radioactive decay or ingrowth".

Waste producer views

8.8 When asked whether full passive safe storage was a realistically achievable goal, the main waste producers made the following points:

"The NII definition of passive safety is not presented in absolute terms or values so, provided a reasonably practicable interpretation is taken, passive safety should be achievable for many waste streams .. However, we recognise that there is some subjectivity over the interpretation of passive storage .. It is necessary to take a balanced view of all the criteria, not necessarily to achieve each one absolutely." (BNFL)

"Passive safety cannot be regarded as an absolute. Rather there are degrees of passivity whose adequacy must be assessed against the particular circumstances. The engineering principles that NII ascribes to passive safety are reasonable provided they are realistically and practicably applied." (BE)

"UKAEA believes that the principle of full passive safe storage can be met, as defined by NII's principles ... However, this does not necessarily mean complying with all of NII's principles ... The long-term storage of ILW converted to passively safe form .. will require continued human intervention to control environmental storage conditions and monitor packages for signs of degradation." (UKAEA)

"Yes. Existing legacy wastes can be made passively safe over a 15-20 year period" (AWE/MoD)

8.9 These responses endorse the NII view that passive safety is not an absolute, in the sense that judgement will always be required in implementation of the passivity principles. They also show a broad level of support for NII's approach, with the proviso that a reasonably practicable interpretation is taken. A number of areas of tension about what constitutes "reasonable practicability" were, however, drawn to the attention of the joint study.

8.10 An example concerns the storage of ILW from AGR reactor operations. The waste is primarily stainless steel or graphite debris from AGR fuelling operations. BE wishes to continue to store this waste in Highly Active Debris Vaults (HADV) at the reactor sites until final decommissioning, and considers this arrangement sufficiently passively safe. However, in its recent review of BE's decommissioning strategy ¹⁷, NII took the view that the long-term storage of this ILW within the vaults was not "fully passive", largely because of potentially mobile surface deposition on the debris and the possibility of water ingress into the vaults. Nonetheless, BE had been able to persuade NII of the case for some deferral of retrieval from the HADVs, subject to additional monitoring and inspection of the vaults. NII is continuing to press BE for further justification of its proposals.

RWMAC/NuSAC views

8.11 RWMAC/NuSAC support the view that ILW should be treated so that it can be stored in accordance with the majority of the principles of passive safety, as set out in NII guidance to inspectors. The Committees concur with the regulator's view that licensees should aim to apply these principles within a framework of reasonable practicability and cost-effectiveness. We recommend that a statement of Government policy should endorse this perspective. Such endorsement should help promote a common understanding of the term "passively safe storage".

8.12 Nonetheless, as further ILW management programmes are developed, there are likely to be further tensions around passive safe storage and interpretations of reasonably practicability and cost effectiveness. Within the framework established by the NII guidance, RWMAC/NuSAC consider that it is legitimate for NII to scrutinise and, where necessary, robustly challenge waste producer proposals, as long as conclusions are reached on the basis of transparent, consistent, case by case judgements that take proper account of all relevant factors. The Committees consider that there would also be of benefit in asserting this position in a statement of Government policy. Such a statement should also affirm that processing for passive safety should be carried out where it constitutes current best practice, and to comply with good engineering principles, unless the costs are grossly disproportionate to the safety benefits.

8.13 Given the extended periods of storage now likely, and regulatory concerns about the storage conditions of some ILW waste streams, RWMAC/NuSAC consider that the NII emphasis on the implementation of programmes to achieve passively safe storage "as soon as reasonably practicable" is appropriate. This is as long as due recognition is given to the safeguarding of disposability (see chapter 6). Also, in reaching judgements about what should constitute "as soon as is reasonably practicable", we would expect proper regard to be paid to the safety-related factors that should influence the timing of conditioning programmes, as listed in NII guidance and indicated above in paragraph 8.7. We recommend that this position should be asserted in a statement of Government policy.

Planning horizons for interim storage

8.14 When asked about lifetime objectives for the packaging and storage of ILW, NII informed the joint study that it would be prudent for licensees to assume a period of the order of 100 to 150 years. NII bases this time period on "current views on the likely availability and operation of a future repository". It states that the time period comprises the following:

"a period of at least 50 years prior to the availability of a disposal facility; an operational period of about 50 years during which waste is transported to and emplaced in the repository; and a further period of at least 50 years during which the facility remains open and waste packages can be retrieved if required".

8.15 EA informed the joint study that: "the minimum design lifetime of 150 years set out in NII's guidance .. provides a reasonable starting point". It added, however, that it would be prudent for waste producers to demonstrate that these periods could be extended without encountering rapid degradation of packages. Nirex described the 150 year packaging objective as "adequate as far as it goes", but warned of the need to take longer-term issues into account (chapter 6 suggests improvements in the ways of doing this). In addition, the three main civil waste producers - BNFL, UKAEA and BE - stated that they can accept the 150-year target, and anticipated that it would be achievable. AWE/MoD commented that justification and demonstration that a package can meet a 150-year life could be difficult.

8.16 The waste producers also referred to on-going industry research to demonstrate that packages stored in controlled environmental conditions will be suitable for handling, transport and disposal after a period of 150 years. This research is being co-ordinated through a sub-group of a Nirex Customers' Technical Meeting. Nirex informed RWMAC/NuSAC that the initial output from this sub-group was a guidance note on required environmental conditions during storage ¹⁸.

8.17 In practice, extended periods of interim storage are likely to raise local development control and planning issues. For example, Copeland Borough Council, the local planning authority for Sellafield, informed the joint study that its December 1992 consent for ILW stores includes a condition that no such building should be used for a period in excess of 30 years, unless given planning approval for a further defined period.

8.18 RWMAC/NuSAC agree with the view put to the joint study by NII that it would be useful if policy could provide an indication of the Government's view of the timescale objectives for interim storage of radioactive waste. This would encourage a greater focus on the standards required for the provisions of packages and stores for the timescales envisaged above. It would also help inform local development control and planning decisions.