Introduction

The mission of Posiva Oy is to dispose safely of the spent nuclear fuel generated in the Olkiluoto and Loviisa nuclear power plants. In 2012, Posiva Oy submitted to the Council of State a construction licence application for a geological disposal facility situated at Olkiluoto (Figure 1.1-1). The licence application was supported by a post-closure safety case, named TURVA-2012, summarised in the corresponding Synthesis report (Posiva 2012 / 3325Posiva 2012. Safety case for the disposal of spent nuclear fuel at Olkiluoto – Synthesis 2012. POSIVA 2012-12, Eurajoki, Finland: Posiva Oy 277 p.^[1]).

In November 2015, the Council of State granted the construction licence for Posiva’s disposal facility. The present updated safety case supports the operating licence application (OLA) to be submitted for the same disposal facility. In the present context, a safety case is a synthesis of evidence, analyses and arguments that quantify and substantiate the safety, and the level of expert confidence in the safety, of a geological disposal facility for radioactive waste (NEA 2013 / 3058NEA 2013. The nature and purpose of the post-closure safety cases for geological repositories. NEA/RWM/R20131, Paris, France: OECD/Nuclear Energy Agency (NEA), Radioactive Waste Management 53 p.; IAEA 2011 / 4792IAEA 2011. Disposal of radioactive waste: Specific Safety Requirements. IAEA Safety Standard Series No. SSR-5. Vienna, Austria: International Atomic Energy Agency (IAEA), 62 p., para. 4.6-4.11). The present safety case portfolio is referred to as SC-OLA. 

Figure 1.1-1. Location of Olkiluoto, Finland.

The starting points for this safety case are the Olkiluoto site and the spent nuclear fuel to be disposed of. The site and the spent nuclear fuel present the given constraints upon which the safety concept is constructed (described in Chapter 3 (Design Basis) ). The safety concept results in a given selection of engineered barriers (Section 1.2 (Design Basis) ). The evolution of the disposal system is driven by a selection of FEPs (see Sections 1.5 ((Design Basis)) and 3.2 (Design Basis) ). The handling of FEPs in different reports is described in the Synthesis report (Posiva 2021 / 4291Posiva 2021. Safety Case for the Operating Licence Application - Synthesis (SYN). POSIVA 2021-01. Eurajoki, Finland: Posiva Oy., Appendix C (Design Basis) ).

[1] In the safety case reports, an automatic reference database has been used to link references to text and for developing the reference list. Hence, each reference in text has a short reference and ID number in format of "Reference / number". These ID numbers correspond to those listed in the reference list. In Posiva's safety case content management system (CMS), where all safety case reports are presented, full references can be inspected by clicking the in-text reference while reading (this functionality does not appear in PDF output of the report).

Posiva’s current reference solution for the disposal of spent nuclear fuel in a geological repository is the KBS-3V design, based on the KBS-3 method. The KBS-3 method was first developed in Sweden in the early 1980s. Since then, the method has been developed and its key elements tested by the Swedish Nuclear Fuel and Waste Management Co. (SKB) in Sweden and Teollisuuden Voima Oy and later Posiva Oy in Finland, and in various joint projects. The method envisages disposal of spent nuclear fuel within a system of multiple barriers, which consists of engineered barriers and the natural barrier provided by the host rock. Knowledge of the disposal system is based on over four decades of studies on the Olkiluoto site and the knowledge of the spent nuclear fuel to be disposed of and of the development of the engineered barrier system. The disposal system is based on the safety concept described in Section 3.1 (Design Basis).

The operating licence application as well as the present safety case are based on the reference design, KBS-3V (Figure 1.2-1). An alternative design (KBS-3H), in which multiple canisters are emplaced horizontally in deposition drifts, is considered as a possible alternative to KBS-3V.

 Figure 1.2-1. Schematic illustration of the disposal facility at Olkiluoto, including a KBS-3V-type spent nuclear fuel repository. The disposal facility also includes a repository for low and intermediate level waste (LILW repository). The original ONKALO^® underground rock characterisation facility (URCF) is shown in blue and now forms a part of the disposal facility. Figure by Posiva Oy.

In the KBS-3V design, the spent nuclear fuel assemblies are placed into copper canisters with cast iron load-bearing inserts, and the canisters are emplaced vertically in individual deposition holes bored in the floor of the deposition tunnels. The canisters are surrounded by a swelling clay buffer material that separates them from the bedrock. The deposition tunnels are filled with backfill material, and other underground openings are also backfilled and plugged (closure) with materials that help restore the natural conditions in the bedrock after operations. The natural barrier surrounding the spent nuclear fuel repository (SNF-R) is called host rock.

Posiva Oy is also responsible for the disposal of the low and intermediate level waste (LILW) arising from the operation and decommissioning of the encapsulation plant for the spent fuel. The low and intermediate level waste (LILW) will include dried liquid wastes and solid wastes, typically packaged in drums or metal boxes. These wastes will be disposed of in a LILW repository, which is assumed in this safety case to be excavated from the access tunnel to the spent nuclear fuel repository. The engineered barrier system (EBS) components for the LILW repository (LILW-R) are the concrete basin and the vault backfill, and the natural barrier is the LILW-R host rock. The disposal system is shown in Figure 1.2‑2.

Figure 1.2-2. A schematic presentation of the components of the disposal system (figure not to scale).

There will thus be two repositories, one for spent nuclear fuel (at the depth of about 400−500 m) and one for LILW (at a depth of about 180 m), with shared access (Figure 1.2‑2). The disposal system consists of these two repositories, other underground openings and all non-barrier structures in them, closure structures, bedrock, including the host rock, and the surface environment.

ONKALO® is a registered trademark of Posiva Oy.

National regulations and requirements in Finland (see Section 1.4 (INTRODUCTION) below) are based on international treaties, agreements, requirements and recommendations (e.g. IAEA 1970 / 4787IAEA 1970. Treaty on the Non-Proliferation of Nuclear Weapons. Information Circular INFCIRC/140. Vienna, Austria: International Atomic Energy Agency (IAEA), 5 p., IAEA 1978 / 4788IAEA 1978. Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter. Information Circular INFCIRC/2205/Add.1/Rev.1. Vienna, Austria: International Atomic Energy Agency (IAEA), 26 p., IAEA 1997 / 2222IAEA 1997. The Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management. Vienna, Austria: International Atomic Energy Agency (IAEA)., IAEA 2016 / 4795IAEA 2016. Amendment to the Convention on the Physical Protection of Nuclear Material. Information Circular INFCIRC/274/Rev.1/Mod.1. Vienna, Austria: International Atomic Energy Agency (IAEA), 10+17 p.). Typically, they set out the broad principles to be adopted, but the detailed implementation can vary to accommodate specific national needs (see NEA 2010 / 4997NEA 2010. Regulation and guidance for the geological disposal of radioactive waste: Review of the literature and initiatives of the past decade. NEA Report No. 6405. Paris, France: OECD/Nuclear Energy Agency (NEA), 43 p. for an international overview). In addition to the international developments discussed below, there are also a number of other safety cases and safety assessments that have been or are being developed for the geological disposal of radioactive waste, as well as a wide variety of relevant results of research and development activities in specific countries and in international collaborations.

The purpose of radioactive waste disposal is to protect individuals, society and the environment from harmful effects of ionising radiation, now and in the future, in such a way that the needs and aspirations of the present generation are met without compromising the ability of future generations to meet their needs and aspirations (IAEA 1997 / 2222IAEA 1997. The Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management. Vienna, Austria: International Atomic Energy Agency (IAEA)., Article 1). The international consensus is that only deep geological disposal offers the long-term passive safety required for long-lived high-level radioactive waste (NAS 1957 / 3021NAS 1957. Disposal of radioactive waste on land. Report of the Committee on Waste Disposal of the Division of the Earth Sciences. Publication 519, Washington D.C., USA: National Academy of Sciences – National Research Council 142 p., NRC 2001 / 4799NRC 2001. Disposition of high-level waste and spent nuclear fuel: The continuing societal and technical challenges. Report of the Committee on Disposition of High-Level Radioactive Waste Through Geological Isolation. Washington, DC, USA: National Academy of Sciences – National Research Council (NAS-NRC)., 198 p., NEA 1999 / 3043NEA 1999. Progress toward geologic disposal of radioactive waste: Where do we stand? An international assessment. Paris, France: OECD/Nuclear Energy Agency (NEA)., NEA 1999 / 3044NEA 1999. Confidence in the long-term safety of deep geologic repositories. Its development and communication. Paris, France: OECD/Nuclear Energy Agency (NEA).). Low-level radioactive waste can be disposed in near-surface repositories and intermediate-level waste at intermediate depth. The safety of geological disposal relies on natural and engineered (man-made) barriers, the functions of which are to isolate, limit and/or prevent releases of radionuclides from the repository and to retard radionuclide transport, such that releases to the surface environment do not occur at unacceptable levels.

The International Atomic Energy Agency (IAEA) establishes or adopts “standards of safety for protection of health and minimisation of danger to life and property”, which are to be used in IAEA operations and which the member states can apply, and provides for the application of these standards. The IAEA Safety Standards consist of Safety Fundamentals (IAEA 2006 / 4790IAEA 2006. Fundamental safety principles: Safety fundamentals. IAEA Safety Standards Series No. SF-1. Vienna, Austria: International Atomic Energy Agency (IAEA), 21 p.), which present the fundamental safety objective and principles and provide the basis for the protection and safety; the set of general and specific Safety Requirements, which “must be met to ensure the protection of people and the environment, both now and in the future” (here, especially IAEA 2011 / 4792IAEA 2011. Disposal of radioactive waste: Specific Safety Requirements. IAEA Safety Standard Series No. SSR-5. Vienna, Austria: International Atomic Energy Agency (IAEA), 62 p.); and the collection of general and specific Safety Guides, which provide recommendations and guidance on how to comply with the safety requirements according to international good practices (here, especially IAEA 2010 / 4791IAEA 2010. Seismic hazards in site evaluation for nuclear installations: Specific Safety Guide. Safety Standards Series No. SSG-9. Vienna, Austria: International Atomic Energy Agency (IAEA), 60 p., IAEA 2011 / 2236IAEA 2011. Geological disposal facilities for radioactive waste – Specific safety guide. IAEA Safety Standard Series No. SSG-14, Vienna, Austria: International Atomic Energy Agency (IAEA)., IAEA 2012 / 2237IAEA 2012. The safety case and safety assessment for the disposal of radioactive waste – Safety specific guide. IAEA Safety Standard Series No. SSG-23, Vienna, Austria: International Atomic Energy Agency (IAEA)., IAEA 2014 / 4793IAEA 2014. Monitoring and surveillance of radioactive waste disposal facilities: Specific Safety Guide. IAEA Safety Standard Series No. SSG-31. Vienna, Austria: International Atomic Energy Agency (IAEA), 73 p., IAEA 2015 / 4794IAEA 2015. Site survey and site selection for nuclear installations: Specific Safety Guide. IAEA Safety Standard Series No. SSG-35. Vienna, Austria: International Atomic Energy Agency (IAEA), 61 p.). The Safety Standards are to be understood in terms of the IAEA Safety Glossary ^[1], which provides definitions of selected safety-related terms. In addition, the IAEA has provided and continues to make available a number of reports, technical documents and other publications for further information.

For radiological protection per se, the International Commission on Radiological Protection (ICRP) maintains the International System of Radiological Protection used world-wide as the common basis for radiological protection standards, legislation, guidelines, programmes and practice – adopted in general also by the IAEA. The most recent ICRP overall recommendations (ICRP 2007 / 2243ICRP 2007. The 2007 recommendations of the International Commission on Radiological Protection. In: Annals of the ICRP, Publication 103, 37(2-4). Ottawa, Canada: International Commission on Radiological Protection (ICRP).) maintain the three fundamental principles of radiation protection (justification, optimisation and application of dose limits) and individual dose limits in planned exposure situations and include an approach to demonstrating radiological protection of the environment (ICRP 2008 / 2244ICRP 2008. Environmental protection: the concept and use of reference animals and plants. Annals of the ICRP. Publication 108, 38(4-6), Ottawa, Canada: International Commission on Radiological Protection (ICRP).). The ICRP has also specifically addressed radiological protection in the context of geological disposal of radioactive waste (ICRP 2013 / 4796ICRP 2013. Radiological protection in geological disposal of long-lived solid radioactive waste. ICRP Publication 122. Annals of the ICRP. Vol. 42, no. 3: International Commission on Radiological Protection (ICRP), 57 p.), especially from the perspective of how to apply the overall recommendations for the protection of future generations over the long time scales associated with geological disposal. These considerations also emphasise the basic principle (ICRP 1998 / 2240ICRP 1998. Radiation protection recommendations as applied to the disposal of long-lived solid radioactive waste. In: Annals of the ICRP, Publication 81, 28(4). Ottawa, Canada: International Commission on Radiological Protection (ICRP).) that “individuals and populations in the future should be afforded at least the same level of protection as the current generation” and the need for “watchful care” throughout the decisions and implementation of the waste management and disposal (ICRP 2013 / 4796ICRP 2013. Radiological protection in geological disposal of long-lived solid radioactive waste. ICRP Publication 122. Annals of the ICRP. Vol. 42, no. 3: International Commission on Radiological Protection (ICRP), 57 p., p. 6).

More technically, a disposal facility for radioactive waste “shall be sited, designed and operated to provide features that are aimed at isolation of the radioactive waste from people and from the accessible biosphere. The features shall aim to provide isolation for several hundreds of years for short lived waste and at least several thousand years for intermediate- and high-level waste” (IAEA 2011 / 4792IAEA 2011. Disposal of radioactive waste: Specific Safety Requirements. IAEA Safety Standard Series No. SSR-5. Vienna, Austria: International Atomic Energy Agency (IAEA), 62 p., Req. 9). Furthermore, the “site for a disposal facility shall be characterised at a level of detail sufficient to support a general understanding of both the characteristics of the site and how the site will evolve over time” (IAEA 2011 / 2236IAEA 2011. Geological disposal facilities for radioactive waste – Specific safety guide. IAEA Safety Standard Series No. SSG-14, Vienna, Austria: International Atomic Energy Agency (IAEA)., p. 30). Such a repository shall also be monitored (e.g. IAEA 2011 / 4792IAEA 2011. Disposal of radioactive waste: Specific Safety Requirements. IAEA Safety Standard Series No. SSR-5. Vienna, Austria: International Atomic Energy Agency (IAEA), 62 p.) to provide information to the operator and to the society to assist in safe and environmentally acceptable development and operation and to support in the decision making, including assistance in the confirmation of the key assumptions of the disposal concept (e.g. EC 2004 / 4784EC 2004. Thematic network on the role of monitoring in a phased approach to geological disposal of radioactive waste: Final report. Report EUR 21025 EN, Contract no. FIKW-CT-2001-20130. Luxembourg: Office for Official Publications of the European Communities, 116 p., IAEA 2001 / 4789IAEA 2001. Monitoring of geological repositories for high level radioactive waste. IAEA-TECDOC-1208. Vienna, Austria: International Atomic Energy Agency (IAEA), 25 p., White 2014 / 4802White, M.J. (ed.). 2014. Monitoring during the staged implementation of geological disposal: The MoDeRn project synthesis. Luxembourg: European Commission. MoDeRn deliverable D-6.1, 90 p.).

In the technological development of repositories, Posiva is actively participating together with the Swedish Nuclear Fuel and Waste Management Co. (SKB) in several EU projects, as well as in the ‘European Technology Platform’ IGD-TP (EC 2009 / 4786EC 2009. Implementing Geological Disposal of Radioactive Waste Technology Platform, ‘Vision Document. Special Report KI-NA-24160-EN-C. Luxembourg: Office for Official Publications of the European Communities, 43 p., IGD-TP 2011 / 4797IGD-TP 2011. Implementing Geological Disposal of Radioactive Waste Technology Platform: Strategic Research Agenda 2011. Report IGD-TP SRA 2011. Luxembourg: Office for Official Publications of the European Communities, 65 p.). In addition to the key topics of safety case, waste forms and their behaviour, technical feasibility and long-term performance of repository components, development strategy of the repository, safety of construction and operations, monitoring and governance, and stakeholder involvement, the platform encompasses further cross-cutting and programme-specific activities (IGD-TP 2011 / 4797IGD-TP 2011. Implementing Geological Disposal of Radioactive Waste Technology Platform: Strategic Research Agenda 2011. Report IGD-TP SRA 2011. Luxembourg: Office for Official Publications of the European Communities, 65 p.).

At the global level, Posiva follows the guidelines of the IAEA and the NEA concerning the long-term safety of geologic repositories. According to the IAEA and the NEA, the long-term safety of the repository shall be demonstrated by a safety case that is a synthesis of evidence, analyses and arguments that quantify and substantiate the safety, and the level of expert confidence in the safety, along multiple lines of reasoning (IAEA 2011 / 4792IAEA 2011. Disposal of radioactive waste: Specific Safety Requirements. IAEA Safety Standard Series No. SSR-5. Vienna, Austria: International Atomic Energy Agency (IAEA), 62 p., IAEA 2011 / 2236IAEA 2011. Geological disposal facilities for radioactive waste – Specific safety guide. IAEA Safety Standard Series No. SSG-14, Vienna, Austria: International Atomic Energy Agency (IAEA)., IAEA 2012 / 2237IAEA 2012. The safety case and safety assessment for the disposal of radioactive waste – Safety specific guide. IAEA Safety Standard Series No. SSG-23, Vienna, Austria: International Atomic Energy Agency (IAEA)., NEA 2004 / 3048NEA 2004. Post-closure safety case for geological repositories. Nature and purpose. 3679, Paris, France: OECD/Nuclear Energy Agency (NEA) 56 p., NEA 2009 / 3054NEA 2009. International Experiences in Safety Cases for Geological Repositories (INTESC). Outcomes of the INTESC Project. 6251, Paris, France: OECD/Nuclear Energy Agency (NEA)., NEA 2012 / 3057NEA 2012. Methods for safety assessment of geological disposal facilities for radioactive wastes – Outcomes of the NEA MeSA Initiative. 6923, Paris, France: OECD/Nuclear Energy Agency (NEA).). It shall consider also extremely long time scales with the help of various safety indicators (e.g. RPNSA 1993 / 4801Radiation Protection and Nuclear Safety Authorities in Denmark, Finland, Iceland, Norway and Sweden . 1993. Disposal of high level radioactive waste: Consideration of some basic criteria. NEI-SE-150 . Stockholm, Sweden: Swedish Radiation Protection Inst. , 64 p., Digitally available through the INIS Collection, ref. no. 25029320., NEA 2009 / 3054NEA 2009. International Experiences in Safety Cases for Geological Repositories (INTESC). Outcomes of the INTESC Project. 6251, Paris, France: OECD/Nuclear Energy Agency (NEA)., NEA 2012 / 4798NEA 2012. Indicators in the safety case: A report of the Integrated Group on the Safety Case (IGSC). NEA/RWM/R(2012)7. Paris, France: OECD/Nuclear Energy Agency (NEA), 143 p., IAEA 2003 / 2223IAEA 2003. The long term storage of radioactive waste: safety and sustainability: A position paper of international experts. IAEA-LTS/RW, Vienna, Austria: International Atomic Energy Agency (IAEA)., IAEA 2011 / 2236IAEA 2011. Geological disposal facilities for radioactive waste – Specific safety guide. IAEA Safety Standard Series No. SSG-14, Vienna, Austria: International Atomic Energy Agency (IAEA).). For the computational analyses, an appropriate level of cautiousness (or ‘conservatism’) needs to be chosen (e.g. IAEA 2003 / 2228IAEA 2003. “Reference Biospheres” for solid radioactive waste disposal – Report of BIOMASS Theme 1 of the BIOsphere Modelling and ASSessment (BIOMASS) Programme. IAEA-BIOMASS-6., Vienna, Austria: International Atomic Energy Agency (IAEA).) and the codes, models and data need to be verified, benchmarked and validated as far as possible (e.g. NEA 1999 / 3044NEA 1999. Confidence in the long-term safety of deep geologic repositories. Its development and communication. Paris, France: OECD/Nuclear Energy Agency (NEA)., NEA 2004 / 3048NEA 2004. Post-closure safety case for geological repositories. Nature and purpose. 3679, Paris, France: OECD/Nuclear Energy Agency (NEA) 56 p., IAEA 2011 / 2236IAEA 2011. Geological disposal facilities for radioactive waste – Specific safety guide. IAEA Safety Standard Series No. SSG-14, Vienna, Austria: International Atomic Energy Agency (IAEA)., IAEA 2012 / 2237IAEA 2012. The safety case and safety assessment for the disposal of radioactive waste – Safety specific guide. IAEA Safety Standard Series No. SSG-23, Vienna, Austria: International Atomic Energy Agency (IAEA).).

[1]  IAEA 2007 / 2232IAEA 2007. IAEA Safety Glossary – Terminology used in nuclear safety and radiation protection. 2007 Edition. Vienna, Austria: International Atomic Energy Agency (IAEA). and its revision, IAEA 2018 / 4803IAEA 2018. IAEA Safety Glossary: Terminology used in nuclear safety and radiation protection. 2018 Edition. Vienna, Austria: International Atomic Energy Agency (IAEA), 261 p..

In Finland, the regulatory documents governing and guiding the safety case work are:

• regulation STUK Y/4/2018 on the safety of disposal of nuclear waste (STUK 2018 / 4500STUK 2018. Radiation and Nuclear Safety Authority regulation on the safety of disposal of nuclear waste. Regulation STUK Y/4/2018. Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).) issued by the Finnish Radiation and Nuclear Safety Authority (STUK),
• STUK's YVL Guide D.5 on the geological disposal of radioactive waste (STUK 2018 / 4499STUK 2018. Disposal of nuclear waste. Guide YVL D.5 (13.2.2018). Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).),
• additional YVL Guides (e.g. D.3, D.4, B.1) concerning nuclear facilities in general,
• YVL Guide D.7 on the release barriers of a spent nuclear fuel disposal facility, which was published for the first time in early 2018 (STUK 2018 / 4569STUK 2018. Release barriers of spent nuclear fuel disposal facility, 13.2.2018(in English). Guide YVL D.7 (13.2.2018). Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).).

STUK has also formulated requirements specific to Posiva about the safety case to be fulfilled before or at the time of submitting the operating licence application (STUK 2015 / 3850STUK 2015. Safety case for the disposal of spent nuclear fuel in Olkiluoto. Decision, Presentation memorandum and Review report - post-closure safety case (10 February 2015). 1/H42252/2015, Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK) 7+17+127 p., Decision). These requirements are based on STUK’s assessment of the safety case TURVA-2012 (STUK 2015 / 4573STUK 2015. STUK’s review on the construction license stage post closure safety case of the spent nuclear fuel disposal in Olkiluoto. STUK-B 197. Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK), 146 p., Review Report). The requirements are reported in Synthesis, Appendix B (SYNTHESIS), along with their relevance to the present safety case.

The requirements for the safety of the disposal are set out in the above-mentioned STUK regulation, and YVL Guide D.5 presents more detailed requirements.

The regulations emphasise the goal of long-term containment by stating that safety functions shall effectively prevent releases of disposed radioactive materials into the bedrock in case of long-lived waste for at least several thousands of years, and in case of short-lived waste, for at least several hundreds of years (STUK Y/4/2018 (STUK 2018 / 4500STUK 2018. Radiation and Nuclear Safety Authority regulation on the safety of disposal of nuclear waste. Regulation STUK Y/4/2018. Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).), Section 32). In Guide YVL D.5 (STUK 2018 / 4499STUK 2018. Disposal of nuclear waste. Guide YVL D.5 (13.2.2018). Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK)., paragraph 307), it is stated that the disposal of nuclear waste shall be so designed that the radiation impacts arising as a consequence of expected evolution are as follows:

a. the annual dose to the representative person remains below the value of 0.1 mSv; and

b. the average annual doses to other persons remain insignificantly low.

The safety case is mentioned in 35 § of STUK Y/4/2018 (STUK 2018 / 4500STUK 2018. Radiation and Nuclear Safety Authority regulation on the safety of disposal of nuclear waste. Regulation STUK Y/4/2018. Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).):

Compliance with the requirements concerning nuclear and radiation safety and the suitability of the disposal method, engineered barriers and disposal site shall be demonstrated by means of a safety case that shall study the possible evolutions of the disposal system, including evolutions caused by rare events impairing long-term safety. The safety case includes, for example, calculational safety analysis based on the evolutions and the complementary considerations.

36 § of STUK Y/4/2018 comments on the reliability of the safety case:

The safety case and the methods, data and models used in it shall be based on high-quality research data and expert judgement, and they shall be documented in a traceable manner. The data and models shall be appropriate and correspond to the anticipated conditions at the disposal site and system during each assessment period.

The basis for calculational analyses shall be that the actual amounts of radioactive substances released and the actual radiation exposure shall be, with a high degree of certainty, lower than the results received from the safety analyses. The safety case shall separately assess the uncertainties included in the data, models and analyses and their significance.

The presentation of, and updates to, the safety case are addressed in 37 §:

The safety case shall be presented when applying for a construction licence and operating licence for the disposal facility and when making substantial plant modifications. The safety case shall be updated during the periodic safety assessments of the disposal facility unless otherwise provided in the licence conditions. The need for updating the safety case shall be assessed before making modifications that concern the disposal system. Furthermore, the safety case shall be updated prior to the closure of the facility.

There are more specific regulations for the safety case in YVL Guide D.5 (STUK 2018 / 4499STUK 2018. Disposal of nuclear waste. Guide YVL D.5 (13.2.2018). Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK)., Section 7.2 and Annex A). The expected contents of the safety case are outlined in paragraph 706 of YVL D.5:

Compliance with the requirements concerning long-term radiation safety, and the suitability of the disposal method and disposal site, shall be demonstrated through a safety case that shall at least include:

1. description of the disposal system;
2. definition of the barriers and the long-term safety functions they provide;
3. specification of performance targets for the long-term safety functions;
4. definition of the scenarios (scenario analysis);
5. description of factors affecting the release and migration of radioactive materials and the long-term safety functions by means of conceptual and mathematical models, and the determination of necessary model parameters;
6. an analysis of the quantities of radioactive materials that are released from the disposed waste, penetrate the barriers and enter the biosphere, and an analysis of the resulting radiation doses;
7. assessment of the probability of rare events impairing long-term safety and the activity released and radiation doses arising from the events;
8. uncertainty and sensitivity analyses;
9. complementary considerations; and
10. comparison of the outcome of the analyses against the safety requirements specified in paras. 307 and 313.

Detailed requirements for the content of the safety case are provided in Annex A to this Guide.

Posiva’s safety case for the operating licence application (SC-OLA) consists of a portfolio of eight main reports (Figure 1.5-1), of which Design Basis (DB) is one. The methodology is described in Synthesis (Chapter 3 (Design Basis) ). The Design Basis report corresponds to Step 1 and Step 2 of the methodology.

Figure 1.5-1. Safety case methodology and connection with main reports. The Design Basis report is highlighted in blue.

The main safety case reports in the portfolio are supported by background reports and various databases. They form a hierarchical structure (Figure 1.5-2), in which the Synthesis report provides a summary of the safety case supporting the operating licence application. When moving down the hierarchy, the details increase accordingly. The safety case portfolio reports are described briefly in Table 1.5-1 and in more detail in the Safety Case Plan report (Posiva 2017 / 3373Posiva 2017. Safety case plan for the operating licence application. POSIVA 2017-02, Eurajoki, Finland: Posiva Oy 152 p.).

Figure 1.5-2. Hierarchical structure of the safety case.

The safety case reports are planned to be read in web format, and a content management system, Posiva's CMS, has been used to produce the safety case. Safety case reports on the website (https://cms.posiva.fi/) and safety case models are connected to databases, safety assessment database (SAdb, see the Models and Data report (Posiva 2021 / 4288Posiva 2021. Safety Case for the Operating Licence Application - Models and Data (M&D). POSIVA 2021-04. Eurajoki, Finland: Posiva Oy.), Section 4.3 (Models and Data)) and features, events and processes database (FEP database, see Hjerpe et al. 2021 / 5240Hjerpe, T., Marcos, N., Ikonen, A.T.K., Reijonen, H & Åstrand, P-G. 2021. Safety Case for the Operating Licence Application: FEP database report. Working Report 2020-19. Eurajoki, Finland: Posiva Oy.). Throughout the reports, FEPs are referred to using FEP codes, which are of the form "[AAAXX]", where AAA is the abbreviation of the component and XX is its number. For the list of all FEP codes, see the FEP database. 

The production of the safety case is an iterative process. Figure 1.5-3 presents the data flow within the safety case report portfolio and the starting and ending point of the iterative cycle.

Figure 1.5-3. Data flow and iterative cycle of the present safety case.

Each iteration of the safety case starts by updating the design basis, i.e. the system of requirements used as basis for the design of the EBS and underground openings and the loads and conditions that are considered in the design. The starting points for the present DB report were the new STUK regulation Y/4/2018 (STUK 2018 / 4500STUK 2018. Radiation and Nuclear Safety Authority regulation on the safety of disposal of nuclear waste. Regulation STUK Y/4/2018. Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).) and the updated regulatory guide YVL D.5 (STUK 2018 / 4499STUK 2018. Disposal of nuclear waste. Guide YVL D.5 (13.2.2018). Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK).) regarding the geological disposal of nuclear waste, as well as the feedback from STUK (STUK 2015 / 3850STUK 2015. Safety case for the disposal of spent nuclear fuel in Olkiluoto. Decision, Presentation memorandum and Review report - post-closure safety case (10 February 2015). 1/H42252/2015, Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK) 7+17+127 p., STUK 2015 / 3851STUK 2015. STUK’s statement and safety assessment on the construction of the Olkiluoto encapsulation plant and disposal facility for spent nuclear fuel. STUK-B 196, Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK) 91 p., STUK 2015 / 4573STUK 2015. STUK’s review on the construction license stage post closure safety case of the spent nuclear fuel disposal in Olkiluoto. STUK-B 197. Helsinki, Finland: Radiation and Nuclear Safety Authority (STUK), 146 p.) on the Design Basis report in the previous safety case, TURVA-2012 (Posiva 2012 / 1110Posiva 2012. Safety case for the disposal of spent nuclear fuel at Olkiluoto – Design Basis 2012. POSIVA 2012-03. Eurajoki, Finland: Posiva Oy 173 p.). The requirements harmonisation project carried out jointly with SKB (Posiva SKB 2017 / 3377Posiva SKB 2017. Safety functions, performance targets and technical design requirements for a KBS-3V repository. Conclusions and recommendations from a joint SKB and Posiva working group. Posiva SKB Report 01. Eurajoki, Finland: Posiva Oy 116 p.) and the previous KBS-3H safety assessment (e.g. Posiva 2016 / 3375Posiva 2016. Safety evaluation for a KBS-3H spent nuclear fuel repository at Olkiluoto - Design Basis. POSIVA 2016-05, Eurajoki, Finland: Posiva Oy 294 p.) were also important starting points for the present report. Knowledge has also been gathered as part of characterising the Olkiluoto site and the spent nuclear fuel, which are the constraints to be accommodated as part of the safety concept, as well as an updated understanding of the FEPs acting on the disposal system.

Based on new and existing knowledge on the disposal system, an updated set of design basis loads and conditions is introduced in the present report. The outcome is a set of performance targets needed to withstand the loads and conditions and a set of requirements that aim at reducing the hazard from the loads and conditions that might occur during the long-term evolution of the system. Additional input for the design basis has come from the need for implementability and sustainability of the design (see Section 3.2 (Design Basis) ).

The fulfillment of the design requirements is achieved through the definition of design specifications, work instructions, requirements for production and installation as well as quality assurance and quality control programmes. This is part of design work. The initial state of the disposal facility corresponds to the requirements as implemented taking into account the uncertainties related to their implementation and potential quality non-conformances that may remain unnoticed and are present at the beginning of the long-term evolution.

One of the challenges in the design of the EBS and underground openings is that there are uncertainties in the loads and conditions as well as in the nature, likelihood and magnitude of some of the FEPs that might act on the system. In the case of some loads and conditions and corresponding performance targets in the present report, it is acknowledged that work is still ongoing at the time of writing and, therefore, the loads and conditions as well as the performance targets are to be considered preliminary, working values. The PAFOS report (Posiva 2021 / 4287Posiva 2021. Safety Case for the Operating Licence Application - Performance Assessment and Formulation of Scenarios (PAFOS). POSIVA 2021-06. Eurajoki, Finland: Posiva Oy.) and underlying supporting reports re-evaluate the uncertainties mentioned in the Design Basis and will assess the fulfilment of performance targets in light of new information gathered since. If there are remaining epistemic uncertainties, scenarios are formulated and analysed in the PAFOS report.

If the analysis of scenarios shows that there is potential for canister failure, the AOR report (Posiva 2021 / 4289Posiva 2021. Safety Case for the Operating Licence Application - Analysis of Releases (AOR). POSIVA 2021-03. Eurajoki, Finland: Posiva Oy.) analyses the potential releases and their radiological consequences. The potential radiological consequences of scenarios formulated in PAFOS are compared with the regulatory requirements for the assessment of compliance. The AOR report also includes what-if cases to showcase the role of a particular barrier or to probe the robustness of the system.

The main results are presented in the Synthesis report together with a statement of overall confidence and feedback for the new iteration of the safety case, starting from the feedback to the design basis (in case safety functions, performance targets or design requirements need to be modified), to the design, to the performance assessment and analysis of releases. This marks the end-point of the iterative cycle of the safety case.