specific issues to be considered in the safety case

Specific issues to be considered in the Specific issues to be considered in the safety case.safety case.

Learning ObjectivesLearning Objectives

• Specific issues important for the development of the safety case.• Graded approach

• Defence in depth

• Reliability (Robustness)

• Time frame for the assessment

• Expected lifetime of the facility

• Long term storage

• Retrievability of waste

• Waste acceptance criteria and interdependences

• Human intrusion

• Institutional control

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IntroductionIntroduction

In the last presentation we studied the role of the safety case, the development of the safety case and components of the safety case.

The safety case is the basis for the safety considerations in respect of siting and locating facilities, construction, operation, decommissioning or closure of the facility, including the justification for changes

The basis for interaction and dialogue between the operating organization and the regulatory body

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Graded approachGraded approach

• Disposal and predisposal waste management includes a wide range of facilities or activities, and characteristics of waste processed, which may pose different degrees of hazard and risk

• A graded approach to safety assessment should be used, therefore, which recognizes these different levels of hazard and risk.

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Graded approachGraded approach

• The scope and level of detail of the safety case carried out for any particular facility or activity shall be consistent with the magnitude of the possible radiation risks arising from the facility or activity

• Greater effort should be put into developing safety cases and safety assessments for large treatment facilities or for HLW disposal facilities than for a small low-level waste stores or for landfill type disposal facilities

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Graded approach: CriteriaGraded approach: Criteria

• When undertaking a safety assessment and safety case, it is necessary to ensure that: Assessment is based on an appropriate level of understanding of

the system and its potential behavior, All safety relevant issues are considered and addressed.

• The degree of detail required in the safety assessment and safety case should be determined by first undertaking relatively simple safety assessments that provide an indication of the potential levels of risk.

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Graded approach: CriteriaGraded approach: Criteria

• Various criteria may be used to help in determining the level of understanding that should be expected for a particular facility or activity;

• Criteria may be: Safety significance, Complexity, Maturity.

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Safety significanceSafety significance

• Safety significance will usually be the most important criterion

• Use of this criterion will necessitate consideration of facility or activity performance in terms of: Releases from normal operation, Potential consequences of anticipated

operational occurrences and reasonably foreseeable accidents,

Potential significance of low probability events with potentially high consequences.

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ComplexityComplexity

A complex facility or activity might suggest the need for a correspondingly complex representation of the design in safety assessment:

• Development of a safety case for a comparatively simple waste management facility such as a storage facility in a hospital may require only a few weeks of time and may be conducted using a checklist approach.

• Development of a safety case for a large centralized waste processing facility may require a large team with several different specializations and require several years of work.

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MaturityMaturity

The use of proven practices and procedures, proven designs, data on operational performance of

similar facilities or activities, experienced manufacturers and

constructors typically require less consideration than the use of novel approaches

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Defense in depth: ConceptDefense in depth: Concept

• The defense in depth concept is centered on several levels of protection including successive barriers and other safety functions preventing the release of radioactive material to the environment and minimizing exposures

• The concept includes: Maintaining the effectiveness of the barriers by averting damage to

the facility and to the barriers themselves; Further measures to protect the public and the environment in case

of unexpected malfunction or degradation of barriers.

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Defense in depth: ConceptDefense in depth: Concept

• Each safety function should be independent of the others, to the extent possible, to ensure that they are complementary and that barriers are unlikely to fail through a single failure mode.

• In the safety case, the functions provided by each barrier should be explained and justified and the time periods over which they are expected to perform their various safety functions should be identified, together with alternative or additional safety functions that operate if a barrier does not fully perform

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Defense in depth: ApproachDefense in depth: Approach

• The most important safety functions are usually fulfilled by means of passive barriers the physical or chemical property of conditioned waste, the waste

package, or process piping

• Active controls can also provide safety functions but these should not be relied on as the primary component of

defense in depth

• Consideration should be given to combining physical barriers and administrative controls into an effective defense in depth strategy.

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Defense in depth: ApproachDefense in depth: Approach

• For disposal facilities, following closure, actions can only be taken during the period of institutional control. In general, the period for which waste containment and isolation will be necessary is much longer than the period of institutional control.

• Consequently, the focus of implementation of the concept of defence in depth for a disposal facility lies in ensuring that the design and construction of the facility will fulfil multiple complementary safety functions.

• Assessing the defence in depth is becoming normal practice in preparing the safety case for waste disposal facilities.

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Defense in depth: JustificationDefense in depth: Justification

Justification of levels of defense in depth can be made by: Identifying barriers and other safety functions; Explaining the diversity of such barriers and other safety functions; Explaining the resilience of such barriers and other safety functions

under normal and abnormal conditions; If appropriate, making a quantitative estimate of their contribution to

the margin of safety; Showing that if any single safety barrier fails then the safety of the

facility is not unacceptably compromised.

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Defense in depth: Justification (cont.)Defense in depth: Justification (cont.)

• Special attention should be paid to internal and external hazards that have the potential to adversely affect more than one barrier

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3000

mm

3000

mm

5500mm

3000mm3300mm

Interlocking shielded access panels

Waste drums/packages

Access apron forhandling equipment

MULTI BARRIER SYSTEM

Engineered barriers Natural barriers

Stable environment

A system of multiple barriers gives greater assurance of isolation and helps ensure that any release of radionuclides to

the environment will occur at an acceptably low rate. 19

Defense in depth: CriteriaDefense in depth: Criteria

• Criteria for determining whether safety function indicators are met will be an aid in determining whether safety will be achieved

• As there will be a range of safety functions, safety function indicators and criteria, a failure to meet a criterion for a particular safety function indicator does not necessarily mean that the disposal system fails to comply with regulatory limits or targets (e.g. on dose or risk) but, rather, that more elaborate analyses and data are needed in order to evaluate safety

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Reliability (Robustness)Reliability (Robustness)

• When selecting components for use in a facility it is important to know their reliability

• Safety case should justify the level of reliability demanded of component

• Demanded level of reliability will depend upon Safety demands made of the component, Defense offered by other components in the

system.

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• The concept of robustness may be applied to individual components of the disposal system, to the disposal system as a whole and to safety assessment.

• The engineered barriers can be designed for robustness, for example, by expanding the dimensioning of certain components beyond the necessary values to ensure their resilience to disturbances and uncertainties.

• A related term is the robustness of a disposal system, which addresses the robustness of individual components as well as their interactions.

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Consideration needs to be given to the reliability of the component over the lifetime of the facility:

Components should be designed to have a lifetime commensurate with the demands that will be placed upon them;

This should be complemented by an appropriate maintenance regime to ensure the continued reliability of the component: Older components may well have lower levels of reliability,

unless they have been well maintained.

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Time frameTime frame

The assessment time frame should be defined taking into account:• National regulations and regulatory guidance;

• Characteristics of the long-term storage facility or activity;

• Characteristics of the site;

• Characteristics of the waste to be stored.

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Time frameTime frame

Other factors to consider:• Safety assessment calculations should cover a period that is

sufficient to determine the maximum, or peak, dose or risk associated with the facility or activity

• Return period of natural external hazards such as extreme meteorological events or earthquakes

• Factors, that can significantly affect safety assessment results may change with time. Assessments may consider several scenarios to reflect different evolution paths of long-term storage facility

• Habits and characteristics of the receptor group, as well as the conditions in which they are located, may change over time

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Expected lifetimeExpected lifetime

• The expected lifetime of the facility needs to be sufficient for the activity being undertaken For storage facility this lifetime may need to include some

contingency i.e. for unloading of the wastes or for delay in the availability of disposal facilities

• For facilities or activities with long lifetimes it will be necessary to use well-proven and well documented materials so that there is confidence that they will last for the duration of the facility or activity life.

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• For facilities planning for extensions beyond their original planned lifetime expectancy, it is necessary to update the safety case (including the safety assessment) to consider the potential impacts on safety.

• The update should: Consider the degradation of barriers or components, Be performed well in advance of the end of the original license to

facilitate regulatory review.

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• For disposal facilities the assessment time frame should be defined by taking account of national regulations and regulatory guidance, as well as the characteristics of the particular disposal facility, the site and the waste to be disposed of

• Other factors that should be considered when deciding on the time

frame for the assessment include the following:

• Safety assessment calculations should cover a time period that is long enough to determine the maximum, or peak, dose or risk

• Several factors that can significantly affect safety assessment results may change with time

• The decision about the time frame for the assessment has implications for the type and severity of disturbing events that are considered in the safety assessment

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In view of the complexity and variability of these factors, it is not possible to establish a universal timescale:

•For above surface disposal facilities ~ 102 – 103 years

•For engineered near surface disposal facilities ~ n103 years

•For deeper facilities, such as geological disposal ~ n 104 years

•The safety case should also address the evolution of the disposal facility and its potential impacts for times beyond the end of the safety assessment calculations, if at that point in time non-negligible hazards are still expected to exist.

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DefinitionDefinition

Long-term storage (facility or activities), involves a period of time which: Exceeds the normal design life of civil structures, Have implications for the choice of:

• Materials,

• Operating methods,

• Quality assurance,

• Quality control requirements,

• etc.

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Definition (cont.)Definition (cont.)

• Long term storage in the context of predisposal waste management is considered to be storage beyond approximately fifty years;

• Long term storage is not expected to last more than approximately one hundred years. This timeframe is based on

technical experience with civil construction.

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Specific issues of long term storageSpecific issues of long term storage

• Specific issues that require special consideration in the safety case for long term storage include: Time frame of the storage facility or activity Importance of passive safety features Retrievability Management systems

• An ageing management programme should be set up to deal with ageing related degradation The programme should specify the monitoring necessary for early

detection of any deficiency

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Passive safetyPassive safety

• The assessment of long-term safety should account for the degradation of passive barriers over time

• The complementary performance of the different safety functions should be tested over different time periods Each safety function should be as independent as possible from the

others to ensure that they are complementary and cannot fail through a single failure mode;

The safety case should explain and justify the functions provided by each barrier and identify:

• the time periods over which barriers are expected to perform their various safety functions, and also

• the alternative or additional safety functions that operate if a barrier does not fully perform.

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Management systemsManagement systems

• Because of the long time frames, the safety case should: Include provisions for the regular surveillance, inspection and

maintenance of the waste and the storage facility; Consider a plan for adequate record keeping

• Periodically, the safety case should be reviewed to consider: Adequacy of the storage capacity, with account taken of the

predicted waste arising, both for normal operation and for possible incidents,

Expected lifetime of the storage facility and availability of disposal options.

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RetrievabilityRetrievability

• The intention in storing waste is that the waste can be retrieved for clearance, processing, transportation and/or disposal at a later time, or in the case of effluent for authorized discharge;

• The safety case should: Consider a plan for safe handling of the waste following long-term

storage; Assess the potential effects of degradation of containment on the

ability to retrieve and handle the waste.

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• Although the term ‘disposal’ refers to the emplacement of radioactive waste into a facility or a location with no intention of retrieving the waste, there may be situations in which there is an intention to provide the possibility of retrieving

• Clear plans for development of the disposal facility, including its closure, should be prepared even if flexibility is allowed to future decision makers in their implementation of the plans. the waste.

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• If retrievability of waste is a design option, the safety case should address administrative and technical arrangements that ensure that:

• an appropriate level of technical ability to retrieve waste is maintained at each stage following emplacement of the waste

• the methods for retrieval are specified

• periodic evaluations are made of the appropriateness and necessity of proceeding with the next step towards closure of the facility, maintaining the facility at the current step, or reversing a step, including retrieval of the waste if necessary.

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Waste acceptance criteria and interdependences

• It is important to notice that there are interdependences among and between the various steps of radioactive waste management. Decisions made at one step may affect subsequent steps or foreclose viable alternatives

• Such interdependences should be identified in the safety assessments and safety case for each predisposal waste management activity and it should be ensured that no conflicting requirements arise that could compromise safety.

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• Disposal of the waste should also be taken into account when any other upstream radioactive waste management activity is being considered.

• In many countries decisions on waste forms to be produced might have to be made before all radioactive waste management activities are fully established.

• Such circumstances emphasize the importance of preparing adequate specifications on waste forms, for waste to be accepted by a facility (e.g. a storage facility) as well as for waste forms to be produced by a facility (e.g. a waste processing facility).

• Waste form specifications must consider radiological, mechanical, physical, chemical and biological properties of a broad range of different types of waste, or may be established for particular waste types.

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• The specifications for acceptable waste forms are required to be consistent with the safety case for the facility or activity

• When deriving the specifications on waste forms within a safety assessment and safety case, the situation can arise in which a comparison of different waste treatment options is needed to find a balance between possible improvements in safety and higher economic costs

• Another example of the need to balance options against each other relates to decisions on the treatment of waste in situations in which final acceptance criteria for disposal of the waste are not yet available.

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• There may also be a need to take safety cases for other facilities and activities into account

• The basis for the decisions made should be recorded thoroughly and sufficient justification should be provided in the safety case

• The need for a thorough review of the assumptions made and arguments used is greater, the more complex the situation and the interdependences are

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Human intrusionHuman intrusion

• Future human actions may disrupt a waste disposal system giving rise to radiological consequences which are known as human intrusion

• During operation of the facility and for any subsequent period of institutional control, it is assumed that a variety of measures will be in place to ensure that human actions do not adversely impact the safety of the disposal system

• These measures will not only be based on safety considerations, but also will satisfy security related requirements and, if relevant, requirements relating to accounting and control of nuclear material

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Intrusion scenariosIntrusion scenarios

AQUIFER

HUMAN INTRUSION

BIO INTRUSION

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Human intrusionHuman intrusion

• In the safety assessment and safety case for a waste disposal facility, inadvertent (unintentional) human intrusion should be considered but quantification of the potential risks associated with deliberate intrusion need not be carried out

• In developing scenarios for human intrusion, two contrasting approaches can be adopted:• One is that a few generic scenarios are developed for all or most

situations

• An alternative approach is that scenarios are developed on a site specific basis

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Institutional controlInstitutional control

• Institutional control should be seen as a component of the overall system of protection against the hazards of radioactive waste

• This is consistent with the general defence in depth concept, as it adds a layer of protection to the natural and engineered barriers of the facility

• However, the presence of institutional control should not be used to justify a reduction in the level of design performance of the containment and isolation system

• Any facility whose safety case is based on the assumption of effective long term institutional control should be subject to regular review

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Institutional controlInstitutional control

• The long term safety of a radioactive waste disposal facility is not to be dependent on institutional control

• Institutional control is an important safety component for surface disposal facilities or near surface disposal facilities for preventing human intrusion for a certain period of time.

• Any reliance placed on institutional controls in the safety case should be justified

• The provision of passive institutional control should be encouraged. Some credit can be taken for this in the safety case but it should not be assumed that it will remain effective in preventing human intrusion in the long term

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Summary Summary

This presentation has covered specific issues important for the development of the safety case - it should help to understand:

• Graded approach








• Human intrusion


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specific issues to be considered in the safety case

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