Requirements for the documentation of the general licence application and for the final stage of the site selection process are set out in ENSI 33/649 (ENSI 2018). Chapter 5 of that document states that, for the safety case supporting the general licence application, the safety analysis carried out for the proposed site in the site comparison is to be supplemented by a comprehensive scenario analysis and analysis of radiological consequences. Excavation of the repository by erosive processes is specifically mentioned as a scenario to be evaluated.
The protection objective and the criteria by which the post-closure safety of a deep geological repository is to be evaluated are those set out in ENSI Guideline G03 (ENSI 2023). The objective of deep geological disposal is to ensure long-term protection of humans and the environment by means of a system of staged, passively functioning engineered and geological barriers, termed the multi-barrier system. It is acknowledged that absolute containment of all radioactive substances over very long periods of time is impossible, and the multi-barrier system has therefore to be designed in such a way that the release of radionuclides through the engineered and geological barriers to the biosphere remains so low that the protection of humans and the environment is ensured. Specific elements to be addressed in the safety assessment, according to ENSI guideline G03, are presented in Tab. 2‑2, along with the sections in this report where each element is covered. Further requirements for safety assessment and the safety case are also set out in the guideline, and their implementation in the present safety assessment methodology is described in NTB 24‑19 (Nagra 2024t).
Tab. 2‑2:Elements to be addressed in safety assessment, according to (ENSI 2018) and sections in the present report and key supporting reports where these are covered
|
Element |
Section in this report |
|
|
A |
Description of the deep geological repository system. |
Repository concept and design in Sections 3.2 and 3.3; geological setting in Section 5.2.1 |
|
Beschreibung des geologischen Tiefenlagersystems. |
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|
B |
Use of verified data on the geological conditions at the site. |
Data on geological conditions summarised in Section 5.2.1 (see |
|
Verwendung verifizierter Daten zu den geologischen Gegebenheiten am Standort. |
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|
C |
Demonstration of the function, retention capacity and robustness of the engineered and geological barriers. |
Performance assessment in Chapter 6 |
|
Aufzeigen der Wirkungsweise, des Rückhaltevermögens und der Robustheit der technischen und natürlichen Barrieren. |
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|
D |
Description and evaluation of the effects of coupled processes and of gas formation and dispersion on the engineered and geological barriers and on radionuclide transport. |
Assessment of total system, as part of the performance assessment, in Section 6.2 |
|
Darlegung und Bewertung der Auswirkungen gekoppelter Prozesse und der Gasbildung und -ausbreitung auf die technischen und natürlichen Barrieren sowie auf den Radionuklidtransport. |
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|
E |
Description of the expected long-term geological evolution. |
Long-term geological evolution described in Section 5.2.2 |
|
Beschreibung der zu erwartenden geologischen Langzeitentwicklung. |
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|
F |
Description of the expected development of the materials contained in the deep geological repository, including the radioactive waste and the engineered and natural barriers. |
Expected evolution of the repository system described in Section 5.3; fate of radionuclides described in Section 5.4 |
|
Beschreibung der zu erwartende Entwicklung der im geologischen Tiefenlager befindlichen Materialien, einschliesslich der radioaktiven Abfälle und der technischen und natürlichen Barrieren. |
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|
G |
Scenario analysis and definition of the calculation cases used to investigate the evolution of the deep geological repository. |
Safety scenario development and definition of calculation cases described in Chapter 7 |
|
Szenarienanalyse und Festlegung der Rechenfälle, mit denen die zu betrachtenden Entwicklungen des Tiefenlagers untersucht werden. |
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|
H |
Justification of why the assumptions and calculation models used are applicable to the situation under consideration. |
Validation of codes and databases used in performance assessment described briefly in Section 6.3 (see NTB 24‑22 Rav. 1 (Nagra 2024u) for details). See Appendix A of NTB 24-18 (Nagra 2024p) for justifications of assumptions and simplifications used in the analysis of radiological consequences |
|
Begründung, warum die verwendeten Annahmen und Rechenmodelle auf die vorliegende Situation anwendbar sind. |
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|
I |
Systematic sensitivity and uncertainty analysis to determine the influence of uncertainties in the data, processes, and models on the calculation results. |
Described in the context of performance assessment in Sections 6.2.4 and 6.3. Described in the context of the analysis of radiological consequences of the reference safety scenario in Section 8.2 |
|
Systematische Sensitivitäts- und Unsicherheitsanalyse zur Ermittlung des Einflusses von Unsicherheiten in den Daten, Prozessen und Modellen auf die Berechnungsergebnisse. |
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|
J |
Consideration of envelope scenarios3 covering possible lines of evolution, particularly regarding surface morphology and climate, within the framework of biosphere modelling. |
Safety scenario development described in Chapter 7. Noted in Section 7.2.1 that biosphere dose conversion factors are used to convert the radionuclide release rate to dose rates by assuming different enveloping biospheres with respect to climate and geomorphology. |
|
Betrachtung umhüllender Szenarien von möglichen Entwicklungen, insbesondere der Gebietsmorphologie und des Klimas im Rahmen der Biosphärenmodellierung. |
Quantitative protection criteria in the form of individual effective dose and risk limits are specified in the ENSI guideline G03 (ENSI 2023). These are as follows.
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Any future evolution of a deep geological repository must not lead to the release of radionuclides causing an individual dose exceeding 0.1 mSv per year, or not cause the risk value according to criterion (b) in paragraph 2.15 of IAEA Safety Standard SSR-5 (IAEA 2011a) to be exceeded4.
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The IAEA Safety Standard SSR-5 gives a risk constraint of the order of 10–5 per year, where risk is understood as the probability of fatal cancer or serious hereditary effects.
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During the assessment period, the radiological consequences of inadvertent human intrusion into the deep geological repository have to be assessed on the basis of criteria (c) and (e) as set out in paragraph 2.15 of IAEA Safety Standard SSR-55.
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Here, SSR-5 states that, if intrusion leads to a calculated annual dose of less than 1 mSv to those living around the site, “ ... then efforts to reduce the probability of intrusion or to limit its consequences are not warranted” whereas, if the calculated annual dose is in the range 1 – 20 mSv, “... then reasonable efforts are warranted at the stage of development of the facility to reduce the probability of intrusion or to limit its consequences by means of optimisation of the facility’s design”.
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After the end of the time period for assessment (see Section 4.3), the effects on the surface must not be significantly higher than the average current radiation exposure of the Swiss population6.
Compliance with the protection criteria for the post-closure phase has to be shown in a safety case. Protection criteria for the operational phase are also set out in ENSI Guideline G03 (ENSI 2023). These are addressed in the separate report NAB 24‑02 (Nagra 2024f).
Nagra uses the term “safety scenarios“ in the present safety assessment and safety case. As these safety scenarios are broad descriptions of the initial state and subsequent evolution of the repository that each cover a range of different, though similar, lines of evolution, the safety scenarios can be regarded as “envelope scenarios”. ↩
Für keine zukünftige Entwicklung eines Tiefenlagers darf die Freisetzung von Radionukliden zu einer Individualdosis grösser als 0,1 mSv pro Jahr oder zu einer Überschreitung des Risikorichtwerts gemäss Kriterium (b) in Absatz 2.15 des IAEA Safety Standard SSR-5 führen. ↩
Im Nachweiszeitraum sind die radiologischen Folgen eines unbeabsichtigten menschlichen Eindringens ins Tiefenlager anhand der Kriterien (c) und (e) gemäss Absatz 2.15 des IAEA Safety Standard SSR-5 zu beurteilen. ↩
Nach Ende des Nachweiszeitraums dürfen die Auswirkungen an der Oberfläche nicht wesentlich höher sein als die durchschnittliche heutige Strahlenexposition der Schweizer Bevölkerung. ↩