Alternative safety scenarios

Alternative safety scenarios arise from the potential for deviations from the expected evolution and performance of the repository system that have been identified in the performance assessment. They postulate a degraded functionality of one or more of the main components of the repository system compared with that assumed in the reference safety scenario. They include unlikely, or highly unlikely, but scientifically non-excludable evolution of the repository system and its environment in which the assumptions made in the reference scenario are significantly changed.

Analysis of radiological consequences

The use of quantitative models to analyse radionuclide release, retention and transport to the surface environment, which results in an evaluation of radionuclide release rates to the surface environment as a function of time, based on the assumed transport properties of the near field and geosphere. The release rates are then converted into annual individual effective doses, annual individual risks and complementary safety indicators. The outcomes are then compared with regulatory protection criteria (and other suitable measures) and the safety margin for each calculation case is evaluated.

Assessment basis

The methodologies, assessment tools, databases and all other evidence and knowledge developed or acquired in support of the safety assessment.

Key elements of the assessment basis include the general scientific understanding of the waste inventory and engineered barriers, an integrated understanding of the containment-providing rock zone and its geological setting, and an understanding of the phenomenological evolution of the repository. The assessment basis also includes the assessment methodologies and the models, codes and data used in the analyses.

Assessment perspectives

In the context of the qualitative evaluation of the containment-providing rock zone (CRZ) during site comparison, assessment perspectives are the different viewpoints of the siting criteria (classified into four groups), from which the CRZ should be evaluated in terms of safety and engineering feasibility.

Calculation cases

Calculation cases define specific calculations that are carried out to analyse the radiological consequences of a particular safety scenario or “what-if?” case for which a conceptual model is formulated. A calculation case consists of a mathe­matical model and a set of input parameter values, solved using one or more computer codes or analytical solutions. Each safety scenario or “what-if?” case will typically have one or more calculation cases associated with it.

Combined repository

A repository comprising disposal areas for both HLW and L/ILW in the same siting region.

Compartmentalisation

Partitioning of waste, e.g., in different sections of the repository, through a designated layout of the repository and the installation of seals and backfill.

Complementary lines of argument

Argumentation that supports the safety assessment, in addition to those arguments developed in performance assessment, safety scenario development and the analysis of radiological consequences. This includes arguments based on natural and archaeological analogues, performance and safety indicators complementary to dose and risk, and conservatism in the calculations of radiological consequence analysis, including the existence of reserve FEPs.

Conservative / conservatism

Simplifications, assumptions, parameter values, etc., that are confidently expected to lead to consequences that are less favourable than those anticipated under real conditions, taking uncertainty into account. Conservative choices can lie outside the range of what is physically possible (e.g., hypothetical assumptions).

Conservatisms include the deliberate omission from quantitative analysis of some phenomena that are considered likely and beneficial to safety because suitable models, codes or databases are unavailable.

Containment-providing rock zone (CRZ)

The volume of rock that, together with the engineered barriers, ensures the containment and retention of radioactive substances contained in the waste during the time period for assessment considering the expected evolution of the deep geological repository. It comprises the host rock as well as the upper and lower barrier-effective confining geological units.

Defence-in-depth

The principle through which safety is defended at various levels, with layers of overlapping provisions so that, if a deficiency should occur in one layer it would be compensated for or corrected by others. This includes the multi-barrier system and how its safety functions ensure a robust repository system.

Demonstration of post-closure safety

The demonstration of safety brings together the evidence, arguments and analyses of the three main processes of safety assessment (performance assessment, safety scenario development, and analysis of radiological consequences), together with complementary lines of argument, to form the overall post-closure safety case. The demonstration comprises:

• the safety-driven repository siting, design and implementation process,

• the favourable qualities of the siting region, the containment-providing rock zone and the engineered barrier system,

• the quality and comprehensiveness of the safety assessment,

• the favourable results of the safety assessment,

• complementary lines of argument.

Design requirements

Requirements placed on the design of system components such that they are expected to fulfil their respective performance objectives and component-specific functions. These design requirements are developed in association with the repository design and system analysis as part of an iterative design development process. For the safety assessment in support of the general licence application milestone, this process leads to the provisional design and implementation plan, for which safety is assessed.

Design specifications

Solutions to the design requirements, concerning technical, quality and organisational aspects of individual components or the whole repository system.

Deviation from expected performance

Occurs when an individual barrier component, or the repository as a whole, does not provide its intended safety functions or component-specific function. This may be quantitatively assessed using performance indicators evaluated against performance targets. The importance of such a deviation can depend on its relevance to safety and on the likelihood of occurrence.

Disposal area

Underground body of rock that encloses a group of adjacent emplacement rooms, including a minimum distance around these emplacement rooms. In addition to the emplacement rooms, a disposal area also has branch tunnels, unloading areas and sealing structures.

Emplacement rooms

Generic term for HLW emplacement drifts (HLW repository or repository section) and L/ILW emplacement caverns (L/ILW repository or repository section) for the long-term emplacement of radioactive waste.

Envelope scenarios

Scenarios that cover potential lines of evolution.

Expected performance (of the multi-barrier system)

The situation when the multi-barrier system of the repository provides its intended safety functions and component-specific functions, both at the component level and at the total system level, and for which all performance assessment claims are expected to be met. The repository is designed such that, for the expected evolution (see reference safety scenario), it performs as required.

FEPs

Features, events and processes. Internal or external phenomena (or factors) that can either define the repository system or influence its evolution or radiological consequences.

Component-specific functions

A specific set of properties of, or actions to be performed by, one or more system features that contribute directly (or indirectly) to the safety functions.

Future human actions (FHA)

Potential actions, activities, behaviours and lifestyles undertaken by human society in the long term which can impact repository safety, particularly regarding human intrusion into the repository (e.g., by drilling deep borehole).

Future human actions safety scenarios consider actions which appear credible from the viewpoint of today’s society, i.e., considering current human activities in the regional vicinity of the site combined with state-of-the-art technology (e.g., drilling equipment and techniques, etc.).

Matrix (waste)

In the case of L/ILW and RP-HLW, this comprises the solidified waste and the material in which it is embedded. In the case of spent fuel, this comprises the crystalline phases which make up the UO² or mixed oxide fuel pellets.

Multi-barrier system

A system of staged, passive engineered and geological barriers that fulfil the overarching safety functions. In the context of the present safety case, the multi-barrier system includes engineered barriers consisting of the waste matrix, disposal canisters, backfill, and seals, as well as the containment-providing rock zone as a geological barrier.

Optimisation of protection

A principle of radiological protection to keep radiological exposure as low as reasonably achievable (ALARA), which is also applied throughout the stepwise and iterative development of the repository design in view of post-closure safety.

Performance assessment

An analysis of the thermal, hydraulic, mechanical, and chemical evolution of the repository system. Performance assessment verifies whether the expected evolution of the repository system, as provided by the assessment basis, is indeed the expected evolution based on the arguments and evidence available. Performance assessment also identifies deviations from the expected performance that could degrade the functionality of the components of the repository system or of the repository system as a whole, as inputs to safety scenario development.

Performance assessment scenarios

Performance assessment scenarios address features, events and processes that could potentially degrade the repository barriers and their safety functions. They permit a quantitative assessment of the impact of deviations from expected performance on the performance of the repository system as a whole and enable the consideration of uncertainties related to the evolutionary path. This provides information on their relevance to safety, which facilitates the identification of those deviations that require an in-depth analysis of the radiological consequences.

Performance indicators

Quantitative metrics of the performance of individual or multiple system components that permit evaluation against a given performance target.

Performance objectives

Performance objectives describe how the system components, including both engineered and geological barriers, perform their individual, component-specific functions and thus, collectively, the safety functions. They are a part of the current safety and repository concept relating to the pillars of safety, safety functions and component-specific functions, that guide the design development process to achieve the provisional design and implementation plan.

Performance targets

Specified requirements for the performance of individual or multiple system components, or of the total system. A target value is set for a relevant performance indicator to allow quantitative evaluation of whether the expected performance is met, including the assessment of uncertainties.

Pessimism / pessimistic

The use of an assumption or parameter value that is possible but leads to consequences that are less favourable than the most likely consequences. The use of such a value means that not every single parameter variation needs to be carried forward into a calculation case to demonstrate safety. While both pessimism and conservatism lead to calculated consequences that are less favourable than those that are most likely, conservatism includes the use of hypothetical assumptions or parameter values and results in consequences that are less favourable than the most likely consequences, including uncertainty.

Pillars of safety

The main features or characteristics of the current repository concept that contribute to the safety functions of the multi-barrier system which, together, ensure post-closure safety. These are:

• deep underground location of the waste emplacement rooms in a stable geological environment

• Containment-providing rock zone

• Closure system (backfill and seals)

• Spent fuel and RP-HLW matrix

• Spent fuel and RP-HLW canisters

• Bentonite buffer of the HLW emplacement drifts

• Cementitious L/ILW near field.

Each of the pillars of safety makes a direct and significant contribution to at least one of the safety functions, but may also make indirect or more minor contributions to other safety functions.

Provisional design and implementation plan

The provisional layout, material choices and dimensions of all components of the repository, and the sequence of operations for repository construction, operation and closure. It results from the iterative design development process that is informed by the current safety and repository concepts, system analyses and other design considerations, such as engineering practicality and operational safety.

Reference safety scenario

A general description of the initial state of the repository barriers, their expected evolution over time (including the release, retention and transport of radionuclides within the repository system), and the assumptions made regarding the biosphere for the analysis of radiological consequences. Essentially, in the reference safety scenario, the repository system will perform as expected and the pillars of safety are assumed to provide their assigned functions, in accordance with the current safety and repository concept.

Repository section

In the repository, in addition to the main HLW and L/ILW disposal areas, the HLW and L/ILW repository sections each include a pilot facility, in which the behaviour of the wastes, the backfill, the seals and the host rock is monitored until the end of a monitoring period.

Repository system

The repository system comprises the underground structures, the emplaced waste, the installed engineered barriers, and the containment-providing rock zone.

Requirements

Conditions that must be fulfilled or attributes that must be possessed by the repository system or its components to achieve certain objectives. Requirements are organised hierarchically. At the highest level, these are overarching objectives and principles, including those specified in Swiss legislation and regulations. Low-level requirements are more detailed and specific and support the fulfilment of the overarching requirements.

Reserve FEPs

Features, events and processes considered likely to occur and beneficial to safety that are deliberately excluded from quantitative analysis because the level of scientific understanding is insufficient to support quantitative modelling, or because suitable models, codes or databases are unavailable.

Reserve FEPs can be mobilised at a later stage of repository planning if the level of scientific understanding is sufficiently enhanced, and the necessary models, codes and databases are developed. The existence of reserve FEPs constitutes an additional, qualitative argument for reserves of safety beyond those indicated by the quantitative analysis.

Robustness principle

The principle of siting and designing the repository system such that it is insensitive to detrimental processes and events and that post-closure safety can be demonstrated irrespective of the remaining uncertainty.

Safety analysis (site-specific)

A synonym for analysis of radiological consequences in the context of site-specific analyses performed within the framework of site comparison.

As for the more detailed safety assessment in support of the general licence application, this analysis also comprises a systematic and quantitative analysis approach, including a reference scenario and alternative safety scenarios, but does not include future human action safety scenarios or “what-if?” cases.

Safety assessment

The process of gathering the claims, arguments, evidence, and performing associated analyses, regarding the safety of the repository system and its environment during the post-closure phase. It is the means by which the safety case is developed. Safety assessment includes performance assessment, safety scenario development, analysis of radiological consequences and the demonstration of post-closure safety.

Safety assessment methodology

A description of how the four processes of safety assessment (performance assessment, safety scenario development, analysis of radiological consequences and demonstration of post-closure safety) are implemented to provide the safety case. Thus, it explains how the four processes of safety assessment are related to each other and how information flows between them.

The safety assessment methodology is considered part of the assessment basis.

Safety case (post-closure)

The synthesis of claims, arguments and supporting evidence gathered, and analyses carried out, in the course of the safety assessment to make the case for the post-closure safety of a repository system.

Safety and repository concept

The safety and repository concept explains how, through the safety barriers and the safety functions they perform, the deep geological repository ensures the protection of humans and the environment.

It provides information about the basic principles of how the deep geological repository is designed. This includes, for example, the concept of the multi-barrier system or the concept of storing disposal containers in long, layer-parallel emplacement rooms in a suitable host rock.

Additionally, it includes the performance objectives, which describe how the system components, including both engineered and geological barriers, are envisioned to perform their individual, component-specific functions and thus, collectively, the safety functions.

Safety functions

Safety functions are the roles of the multi-barrier system that together ensure post-closure safety. Engineered and geological barriers and other system components contribute to one or more of these functions. They consist of:

• S1: Isolation of radioactive waste from the surface environment.

• S2: Complete containment of radionuclides for a period of time.

• S3: Immobilisation, retention, and slow release of radionuclides.

• S4: Compatibility of the multi-barrier system elements and the radioactive waste types among each other and with other materials.

• S5: Long-term stability of the multi-barrier system with respect to long-term geological and climatic processes.

Safety indicators

Quantitative metrics of post-closure safety used for the analysis of radiological consequences. These include the annual individual effective dose and annual individual risk as the primary safety indicators, and radioactivity fluxes and radiotoxicity as complementary safety indicators.

Safety scenarios

The different possibilities for the initial state of key features, conditions and processes of the repository system and the ways in which they may evolve and perform their safety functions over time. They capture uncertainty in the broad ways the system may evolve, considering all relevant sources of uncertainty, and include:

1. a reference safety scenario, corresponding broadly to the expected evolution,

2. a set of alternative safety scenarios, arising from conceiv­able deviations from the expected performance/evolution

3. a set of future human actions safety scenarios.

Note that "what-if?" cases are not defined as safety scenarios, although they are an output of safety scenario development.

Safety scenario development

The identification and description of a set of safety scenarios that capture uncertainty in the initial state of the repository system and the different ways in which it could evolve over time.

Based on the knowledge provided by the assessment basis and the findings of the performance assessment, safety scenario development enables a systematic assessment of how key features, events and processes can impact post-closure safety.

Safety scenario development also leads to a set of additional "what-if?" cases that are used to demonstrate robustness of the repository system.

Safety scenario variants

Variants of a given safety scenario (reference, alternative or future human action) that share largely the same general description of features, events and processes occurring over time, but may differ, for example, in the extent or timing of key phenomena associated with that safety scenario as well as positioning or properties of a feature. In contrast with alternative safety scenarios.

Scenario analysis

With respect to the regulatory guidance, this refers to a combination of (in Nagra’s terminology) safety scenario development and radiological consequence analysis for the defined calculation cases.

Sensitivity analysis

Technique to investigate how the variance in modelling results is impacted by the variance in input parameters. Thus, it identifies the parameters to which the computational model outcomes are most sensitive.

Stylised approach

An approach that involves imposed, rather than scientifically derived, assumptions for handling poorly quantifiable or unquantifiable uncertainties in scenario development. This approach aims to limit arbitrary speculation on matters that are inherently unpredictable. Thus, no attempt is made to cover the full range of possible scenarios or to assign probabilities to them. Rather, a limited set of illustrative cases are analysed.

In agreement with the regulations and international standards (e.g., SSG-23 IAEA 2012, see also ENSI Guideline G03, ENSI 2023), stylisation is used by Nagra for aspects of biosphere evolution, future human actions and lifestyles and for developing human intrusion scenarios.

Time period for assessment

The time frame over which assessment of post-closure safety is performed. For a repository for both HLW and L/ILW in the NL siting region, a 10⁶-year assessment period is considered.

Consistent with the regulatory requirement that dose and risk calculations should extend up to the time of maximum radiological impact of the repository, dose calculations are generally performed through to 10⁷ years for HLW and 10⁶ years for L/ILW. Analyses for volatile radionuclides that focus on ¹⁴C, with a 5,730-year half-life, are carried out for a 10⁵-year period.

Time period under consideration

The time frame over which safety analysis calculations are performed in the frame of the site comparison in Stage 3 of the Sectoral Plan. This is defined as extending up to 10⁵ years for an L/ILW repository and up to 10⁶ years for an HLW repository.

Uncertainty analysis

Quantification of the influence of uncertainties in data, processes and model assumptions on calculation results.

Waste packages

Generic term for SF/RP-HLW disposal canisters and L/ILW disposal containers.

"What-if?" cases

"What-if?" cases are calculation cases that use a particular set of assumptions and/or parameter values that lie outside the range of possibilities supported by currently available evidence and knowledge. Such cases are primarily aimed at demonstrating the robustness of the repository system or to improve understanding of the system performance (e.g. by hypothetically removing a barrier or favourable process).

Additionally, phenomena that could realistically occur, but whose occurrence is implausible during the one-million-year time period for assessment, can also be considered as a "what-if?" case.