Safety Case for the Disposal of Spent Nuclear Fuel at Olkiluoto

Transcription

1 POSIVA Safety Case for the Disposal of Spent Nuclear Fuel at Olkiluoto FEP Screening and Processing Posiva Oy January 2014 POSIVA OY Olkiluoto FI EURAJOKI, FINLAND Phone (02) (nat.), ( ) (int.) Fax (02) (nat.), ( ) (int.)

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3 Posiva-raportti Posiva Report Posiva Oy Olkiluoto FI EURAJOKI, FINLAND Puh (31) Int. Tel (31) Raportin tunnus Report code POSIVA Julkaisuaika Date January 2014 Tekijä(t) Author(s) Posiva Oy Toimeksiantaja(t) Commissioned by Posiva Oy Nimeke Title SAFETY CASE FOR THE DISPOSAL OF SPENT NUCLEAR FUEL AT OLKILUOTO FEP SCREENING AND PROCESSING Tiivistelmä Abstract TURVA-2012 is Posiva s safety case in support of the Preliminary Safety Analysis Report (PSAR) and application for a construction licence for a repository for disposal of spent nuclear fuel at the Olkiluoto site in south-western Finland. This report presents and applies the methodology by which Posiva s TURVA-2012 FEP list as described in Features, Events and Processes and used in Performance Assessment, Formulation of Radionuclide Release Scenarios, Assessment of Radionuclide Release Scenarios for the Repository System, Biosphere Assessment and Models and Data for the Repository System is shown to be as comprehensive as necessary at the current stage of the spent nuclear fuel management programme. The main part of the work, i.e. screening methodology and processing of the selected FEPs, has been conducted within earlier safety assessments, and has been applied to TURVA-2012 and to previous safety assessments, but never formally presented before this report. A full screening of Nuclear Energy Agency 2.1 FEP database is carried out again for this report and reported in detail to confirm the comprehensiveness of the TURVA-2012 FEP list. Except for the first stages of the screening process, surface environment FEPs are, however, not considered in this report. Avainsanat - Keywords FEPs, screening, disposal system, regulatory framework, assessment, scenario driver. ISBN Sivumäärä Number of pages 148 ISBN ISSN Kieli Language ISSN English

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5 Posiva-raportti Posiva Report Posiva Oy Olkiluoto FI EURAJOKI, FINLAND Puh (31) Int. Tel (31) Raportin tunnus Report code POSIVA Julkaisuaika Date Tammikuu 2014 Tekijä(t) Author(s) Posiva Oy Nimeke Title Toimeksiantaja(t) Commissioned by Posiva Oy TURVALLISUUSPERUSTELU KÄYTETYN YDINPOLTTOAINEEN LOPPUSIJOITUKSELLE OLKILUODOSSA - FEPIEN VALINTA JA PROSESSOINTI Tiivistelmä Abstract Posivan rakentamislupahakemukseen liittyen tuotettu PSAR (Preliminary Safety Analysis Report) -raportti perustuu osaltaan TURVA-2012-pitkäaikaisturvallisuusperusteluun, joka on tehty käytetyn ydinpolttoaineen loppusijoitusta varten Olkiluodon kallioperään rakennettavalle loppusijoituslaitokselle. Tässä raportissa esitetään metodologia, joka on ollut käytössä FEP-listan (ilmiöt, tapahtumat ja prosessit) muodostamisessa TURVA-2012-turvallisuusperustelua varten. TURVA-2012:n FEP-lista on kuvattu tarkemmin raportissa Features, Events and Processes. FEPlistan käyttö turvallisuusperustelussa on kuvattu raporteissa Performance Assessment, Formulation of Radionuclide Release Scenarios, Assessment of Radionuclide Release Scenarios for the Repository System, Biosphere Assessment sekä Models and Data for the Repository System. Tämän raportin tarkoitus on kuvata FEPien valinta ja prosessointi turvallisuusperustelua varten ja osoittaa että TURVA-2012:n FEP-lista on riittävän kattava ja vastaa käytetyn polttoaineen loppusijoitusohjelmassa meneillään olevan vaiheen tarpeita. Tässä raportissa esitetyn työn pääosa koskee FEP-valintaprosessin metodologiaa, joka on ollut käytössä myös turvallisuusarvioiden aiemmissa vaiheissa ja jota on käytetty myös muodostettaessa TURVA-2012:n FEP-listaa. Tätä metodologiaa ja prosessointia ei ole kuitenkaan aiemmin järjestelmällisesti kuvattu. FEPien valinnan pohjana on käytetty NEAn (Nuclear Energy Agency) 2.1 FEP-tietokantaa. Tässä raportissa esitetään FEPien valinta ja prosessointi, minkä perusteella on tarkasteltu TURVA-2012:n FEP-listan kattavuutta. Lukuun ottamatta FEPien valintaprosessin alkuvaihetta, pintaympäristön FEPit eivät ole mukana lopullisessa tarkastelussa. Avainsanat - Keywords FEPit, loppusijoitusjärjestelmä, viranomaisvaatimukset, turvallisuusarvio. ISBN Sivumäärä Number of pages 148 ISBN ISSN Kieli Language ISSN Englanti

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7 1 TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ TERMS AND ABBREVIATIONS... 3 FOREWORD INTRODUCTION Background The KBS-3 method Posiva s programme for developing a KBS-3 repository at Olkiluoto Regulatory context for the management of spent nuclear fuel Safety concept and safety functions TURVA-2012 Safety Case portfolio Quality assurance Scope and objectives of the present report Structure METHODOLOGY FOR FEP SCREENING AND PROCESSING Overall methodology FEP screening and processing steps Identification and screening of FEPs for potential significance The initial FEP list Relevance screening Initial aggregation and component-wise classification of the FEPs Significance screening Mapping remaining Project FEPs to the TURVA-2012 FEP list Cross-checking SCREENING AND PROCESSING OF THE FEPS FEP lists included in the work Relevance screening results Preliminary aggregation of FEPs and classification under disposal system components Significance screening results MAPPING REMAINING PROJECT FEPS TO THE TURVA-2012 FEP LIST CROSS-CHECKING AGAINST OTHER RELEVANT FEP SOURCES Cross-checking against the Swedish SR-Site Cross-checking against the Canadian Fourth Case Study Comparison with FEPs in Process Report CONCLUSIONS AND THE WAY FORWARD Comprehensiveness of TURVA-2012 FEP list The way forward REFERENCES APPENDIX A. FULL TURVA-2012 FEP LIST APPENDIX B. MAPPING OF THE TURVA-2012 FEPS TO NEA FEPS AFTER SCREENING PROCESS APPENDIX C. CROSS-CHECK AGAINST SR-SITE APPENDIX D. CROSS-CHECK AGAINST THE FOURTH CASE STUDY

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9 3 TERMS AND ABBREVIATIONS CLA DiP EBS EPM FEPs GD IAEA ISAM KBS-3 KBS-3H KBS-3V KTM NDT NEA NWMO OL3 OL4 ONKALO OSD POTTI PSAR QA QC R1 R2 R3 Construction Licence Application. (Government) Decision-in-Principle. Engineered Barrier System, which includes canister, buffer, backfill and closure. Equivalent Porous Medium. Features, Events and Processes. Government Decree. International Atomic Energy Agency. Integrated Safety Assessment Methodology. An abbreviation of kärnbränslesäkerhet (nuclear fuel safety) version 3. The KBS-3 method for implementing the spent nuclear fuel disposal concept based on multiple barriers. (Kärnbränslesäkerhet 3-Horisontell). Design alternative of the KBS-3 method in which several spent nuclear fuel canisters are emplaced horizontally in each deposition drift. (Kärnbränslesäkerhet 3-Vertikal). The reference design alternative of the KBS-3 method, in which the spent nuclear fuel canisters are emplaced in individual vertical deposition holes. Finnish Ministry of Trade and Industry. Non-Destructive Testing. Nuclear Energy Agency. Nuclear Waste Management Organization (in Canada). Olkiluoto 3 reactor. Fourth reactor to be constructed at Olkiluoto. Expected to be similar to OL3 in TURVA-2012 safety case. Underground research facility constructed at Olkiluoto. Olkiluoto Site Descriptive model. Database at Posiva with site-investigation data. Preliminary Safety Analysis Report a part of the construction licence application. Quality Assurance. Quality Coordinator; Quality Control. Relevance screening criterion 1. The Project FEP is defined by a heading without any description of what is meant by the heading. Relevance screening criterion 2. The Project FEP is related to assessment methodology. These FEPs are handled elsewhere in the TURVA-2012 safety case. Relevance screening criterion 3. The Project FEP is not relevant for the context of the TURVA-2012 safety case, especially in respect to the national regulatory requirements and guidelines.

10 4 R4 R5 Repository system RSC RTD S1 S2 S3 SAFCA SKB SR-Site STUK TEM TKS-2009 TURVA-2012 VAHA VVER-440 YJH YVL Relevance screening criterion 4. The Project FEP is not relevant for the KBS-3V type repository design for spent nuclear fuel disposal. Relevance screening criterion 5. The Project FEP is not relevant for the present-day Olkiluoto site characteristics and likely future site characteristics evolving in response to climatic changes and other external factors. Spent nuclear fuel, canister, buffer, backfill (deposition tunnel backfill + deposition tunnel end plug), closure components and host rock. Excludes the surface environment. Rock Suitability Classification. The aim of the RSC is to define suitable rock volumes for the repository, deposition tunnels and deposition holes. Research, Technical development and Design. Significance screening criterion 1. The FEP has insignificant impact on safety functions and radiation protection criteria. Significance screening criterion 2. The FEP has low probability to occur and low impact on safety functions and radiation protection criteria. Significance screening criterion 3. The FEP itself has considerably more serious consequences than any potential radiological consequences from the spent nuclear fuel. Safety Case project. Swedish Nuclear Fuel and Waste Management Company. SR-Site safety assessment for a repository in Forsmark. Finnish Radiation and Nuclear Safety Authority. Finnish Ministry of Employment and the Economy, previously Ministry of Trade and Industry (KTM). Finnish equivalent for RTD (see RTD). RTD programme for Posiva s safety case supporting the construction licence application submitted in 2012 for the Olkiluoto spent nuclear fuel disposal facility. TURVA means safety. Requirements management system at Posiva. Pressurised water reactor type at Loviisa. Finnish abbreviation for Nuclear Waste Management. STUK s (see STUK) regulatory guide series for nuclear facilities.

11 5 FOREWORD The participants in the project group for screening in/out discussions in this report are listed below: Hagros, Annika (Saanio & Riekkola, SROY) Hellä, Pirjo (SROY) Hjerpe, Thomas (Facilia AB) Pitkänen, Petteri (Posiva Oy) Koskinen, Lasse (Posiva Oy) Laine, Heini (SROY) Marcos, Nuria (SROY) Pastina, Barbara (SROY) Snellman, Margit (SROY) The review comments by Kristina Skagius (SKB) are gratefully acknowledged.

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13 7 1 INTRODUCTION 1.1 Background On assignment by its owners, Fortum Oyj and Teollisuuden Voima Oyj, Posiva Oy will manage the disposal of spent nuclear fuel from the Loviisa and Olkiluoto nuclear power plants. At Loviisa, two pressurised water reactors (VVER-440) are in operation; at Olkiluoto, two boiling water reactors are operating and one pressurised water reactor is under construction. Plans exist also for a fourth nuclear power unit at Olkiluoto. At both sites there are facilities available for intermediate storage of the spent nuclear fuel before disposal. In 2001, the Parliament of Finland endorsed a Decision-in-Principle (DiP) whereby the spent nuclear fuel generated during the operational lives of the operating Loviisa and Olkiluoto reactors will be disposed of in a geological repository at Olkiluoto. This first DiP allowed for the disposal of a maximum amount of spent nuclear fuel corresponding to 6500 tonnes of uranium (tu) initially loaded into the reactors. Subsequently, additional DiPs were issued in 2002 and 2010 allowing extension of the repository (up to 9000 tu) to also accommodate spent nuclear fuel from the operations of the OL3 reactor and the planned OL4 reactor. OL4 spent nuclear fuel is handled in the TURVA-2012 safety case assuming it to be similar to OL3 spent nuclear fuel. 1.2 The KBS-3 method The 2001 DiP states that disposal of spent nuclear fuel shall take place in a geological repository at the Olkiluoto site, developed according to the KBS-3 method. In the KBS-3 method, spent nuclear fuel encapsulated in water-tight and gas-tight sealed metallic canisters with a mechanical load-bearing insert is emplaced deep underground in a geological repository constructed in the bedrock. According to the DiP, the repository shall be located at minimum depth of 400 m. In Posiva s current repository design, the repository is constructed on a single level and the floor of the deposition tunnels is at a depth of m in the Olkiluoto bedrock. Posiva s reference design in the construction licence application is based on vertical emplacement of the spent nuclear fuel canisters (KBS-3V; Figure 1-1). Currently, an alternative horizontal emplacement design (KBS-3H) is being jointly developed by the Swedish Nuclear Fuel and Waste Management Company (SKB) and Posiva. The KBS-3V design is based on a multi-barrier principle in which copper-iron canisters containing spent nuclear fuel are emplaced vertically in individual deposition holes bored in the floors of the deposition tunnels (see inset in Figure 1-1). The canisters are to be surrounded by a swelling clay buffer material that separates them from the bedrock. The deposition tunnels and the central tunnels and the other underground openings are to be backfilled with materials of low permeability.

14 8 Figure 1-1. Schematic presentation of the KBS-3V design. 1.3 Posiva s programme for developing a KBS-3 repository at Olkiluoto The Olkiluoto site, located on the coast of south-western Finland (Figure 1-2), has been investigated for over 20 years. During the past few years, key activities in the programme have been related to: completion of the investigations for site confirmation at Olkiluoto both through analyses of data from surface-drilled characterisation holes and surveys and studies carried out in the ONKALO underground research facility, the design of required surface and disposal facilities, the development of the selected disposal technology to the level required for the construction licence application, and demonstration of the long-term safety of the disposal of spent nuclear fuel including the preparation of a safety case (Section 1.6) presented as a portfolio of reports, including the present report. Posiva s ongoing RTD (research, development and technical design) phase has been introduced in the TKS-2009 report (Posiva 2009) for the years , which also provides insight into developments from previous RTD phases. In 2012 a new RTD programme (YJH-2012) for was published (Posiva 2013). In Figure 1-3, a general timeline of Posiva s programme is presented.

15 9 Figure 1-2. Olkiluoto Island is situated on the coast of the Baltic Sea in south-western Finland. Photograph by Helifoto Oy. Figure 1-3. Overall schedule for nuclear waste management relating to the Loviisa and Olkiluoto reactors until The target is to begin disposal of spent nuclear fuel around 2020.

16 10 The repository will be located in the bedrock of Olkiluoto Island taking into account the host rock properties as well as the restrictions set by urban planning in the Eurajoki Municipality. In Figure 1-4 the current reference layout is presented. Figure 1-4. The current reference layout (green). Dark grey areas are not suitable for deposition tunnels based on Rock Suitability Classification (RSC), which has been developed by Posiva. Red ovals denote respect distances to drillholes. The red line surrounding the repository shows the area reserved for the repository in urban planning.

17 Regulatory context for the management of spent nuclear fuel According to the law, the Finnish Ministry of Employment and the Economy (TEM; previously the Ministry of Trade and Industry, KTM) decides on the principles to be followed in waste management of spent fuel and other nuclear waste. The schedule for the disposal of spent nuclear fuel was established in the KTM s Decision 9/815/2003. According to this Decision, the parties under the nuclear waste management obligation shall, either separately, together or through Posiva Oy, prepare to present all reports and plans required to obtain a construction licence for a disposal facility for spent nuclear fuel as stated in the Nuclear Energy Decree by the end of The disposal facility is expected to become operational around the year The legislation concerning nuclear energy was updated in As part of the legislative reform, a number of the relevant Government Decisions were replaced with Government Decrees. The Decrees entered into force on 1 st December The Government Decision (478/1999) regarding the safety of disposal of spent nuclear fuel, which particularly applied to the disposal facility, was replaced with Government Decree 736/2008, issued 27 November Currently, the valid Regulatory Guides pertaining to nuclear waste management are Guides YVL D.1 D.5 issued by the Radiation and Nuclear Safety Authority (STUK) in Guide YVL D.1 deals with nuclear non-proliferation control, D.2 with the transport of nuclear material and nuclear waste, D.3 with the processing, storage and encapsulation of spent nuclear fuel, D.4 with nuclear waste management and decommissioning activities and D.5 with the disposal of nuclear waste. As these Guides were published in late 2013, the version of YVL D.5 used in the preparation of the TURVA-2012 safety case (see Section 1.6) was Draft 4 ( , in Finnish only). 1.5 Safety concept and safety functions The long-term safety principles of Posiva s planned repository system are described at Level 2 of the VAHA (VAHA is Posiva s requirement management system) as follows: 1. The spent nuclear fuel elements are disposed of in a repository located deep in the Olkiluoto bedrock. The release of radionuclides is prevented with a multi-barrier disposal system consisting of a system of engineered barriers (EBS) and host rock such that the system effectively isolates the radionuclides from the living environment. 2. The engineered barrier system consists of a) canisters to contain the radionuclides for as long as they could cause significant harm to the environment, b) buffer between the canisters and the host rock to protect the canisters as long as containment of radionuclides is needed, c) deposition tunnel backfill and plugs to keep the buffer in place and help restore the natural conditions in the host rock, d) the closure, i.e. the backfill and sealing structures to decouple the repository from the surface environment.

18 12 3. The host rock and depth of the repository are selected in such a way as to make it possible for the EBS to fulfil the functions of containment and isolation described above. 4. Should any of the canisters start to leak, the repository system as a whole will hinder or retard releases of radionuclides to the biosphere to the level required by the longterm safety criteria. The safety concept, as depicted in Figure 1-5, is a conceptual description of how these principles are applied together to achieve safe disposal of spent nuclear fuel in the conditions of the Olkiluoto site. Due to the long-term hazard of the spent nuclear fuel, it has to be isolated from the surface environment over a long period of time. The KBS-3 method provides long-term isolation and containment of spent nuclear fuel by a system of multiple barriers, both engineered and natural, and by ensuring a sufficient depth of disposal (the key safety features of the system in Figure 1-5). All of these barriers have their roles in establishing the required long-term safety of the repository system. These roles constitute the safety functions of the barriers (see Table 1-1). The surface environment is not given any safety functions; instead it is considered as the object of the protection provided by the repository system. Most radionuclides in the spent nuclear fuel are embedded in a ceramic matrix (UO 2 ) that itself is resistant to dissolution in the expected repository conditions. The slow release of radionuclides from the spent nuclear fuel matrix is part of Posiva s safety concept. Moreover, the near-field conditions should contribute to maintain the low solubility of the matrix. SAFE DISPOSAL LONG-TERM ISOLATION AND CONTAINMENT FAVOURABLE NEAR-FIELD CONDITIONS FOR THE CANISTER Retention and retardation of radionuclides Slow transport in the geosphere Slow release from the spent fuel matrix Slow diffusive transport in the buffer PROVEN TECHNICAL QUALITY OF THE EBS FAVOURABLE, PREDICTABLE BEDROCK AND GROUNDWATER CONDITIONS SUFFICIENT DEPTH WELL-CHARACTERISED MATERIAL PROPERTIES ROBUST SYSTEM DESIGN Figure 1-5. Outline of the safety concept for a KBS-3 type repository for spent nuclear fuel in a crystalline bedrock (adapted from Posiva 2003). Orange pillars and blocks indicate the primary safety features and properties of the disposal system. Green pillars and blocks indicate the secondary safety features that may become important in the event of a radionuclide release from a canister.

19 13 Implementation of the KBS-3 method entails the introduction of a number of additional barriers because of engineering, operational safety or long-term safety needs. Long-term safety needs arise, for example, because implementation involves the construction of a system of underground openings, including access tunnels and shafts, that would significantly perturb the safety functions of the host rock unless backfilled and sealed at closure of the disposal facility. These closure components with long-term safety functions include: backfill of underground openings, including the central tunnels, access tunnels, shafts, and other excavations, and drillhole plugs, mechanical plugs, long-term hydraulic plugs at different depths and plugs near the surface. The safety functions of the engineered barrier system (EBS) components and host rock are summarised in Table 1-1. In the TURVA-2012 safety case documentation, the spent nuclear fuel, EBS and the host rock are jointly termed the repository system, whereas the term disposal system is used when the repository system and the surface environment are both considered (see Figure 1-6).

20 14 Table 1-1. Summary of safety functions assigned to the barriers (EBS components and host rock) in Posiva s KBS-3V repository. Barrier Canister Buffer Deposition tunnel backfill Host rock Safety functions Ensure a prolonged period of containment of the spent nuclear fuel. This safety function rests first and foremost on the mechanical strength of the canister s cast iron insert and the corrosion resistance of the copper surrounding it. Contribute to mechanical, geochemical and hydrogeological conditions that are predictable and favourable to the canister, Protect canisters from external processes that could compromise the safety function of complete containment of the spent nuclear fuel and associated radionuclides, Limit and retard radionuclide releases in the event of canister failure. Contribute to favourable and predictable mechanical, geochemical and hydrogeological conditions for the buffer and canisters, Limit and retard radionuclide releases in the possible event of canister failure, Contribute to the mechanical stability of the rock adjacent to the deposition tunnels. Isolate the spent nuclear fuel repository from the surface environment and normal habitats for humans, plants and animals and limit the possibility of human intrusion, and isolate the repository from changing conditions at the ground surface, Provide favourable and predictable mechanical, geochemical and hydrogeological conditions for the engineered barriers, Limit the transport and retard the migration of harmful substances that could be released from the repository. Closure Prevent the underground openings from compromising the long-term isolation of the repository from the surface environment and normal habitats for humans, plants and animals. Contribute to favourable and predictable geochemical and hydrogeological conditions for the other engineered barriers by preventing the formation of significant water conductive flow paths through the openings, Limit and retard inflow of water to and release of harmful substances from the repository.

21 15 Figure 1-6. The components of the disposal system. 1.6 TURVA-2012 Safety Case portfolio A safety case for a geological disposal facility documents the scientific and technical understanding of the disposal system, including the safety barriers and safety functions that these are expected to provide, results of a quantitative safety assessment, the process of systematically analysing the ability of the repository system to maintain its safety functions and to meet long-term safety requirements, and provides a compilation of evidence and arguments that complement and support the reliability of the results of the quantitative analyses. As stated in Guide YVL D.5, A01: Compliance with the requirements concerning longterm radiation safety, and the suitability of the disposal method and disposal site, shall be proven through a safety case that must analyze both expected evolution scenarios and unlikely events impairing long-term safety. The safety case comprises a numerical analysis based on experimental studies and complementary considerations insofar as quantitative analyses are not feasible or involve considerable uncertainties (Government Decree GD 736/2008). The TURVA-2012 safety case portfolio is based on the safety case plan published in 2008 (Posiva 2008), which updates an earlier plan published in 2005 (Vieno & Ikonen 2005). In the updated safety case plan, further details are provided on quality assurance and control procedures and their documentation, as well as on the consistent handling of different types of uncertainties. Since 2008, the safety case plan has been iterated based on the feedback received from the authorities, and the contents of the safety case portfolio TURVA-2012 are now as presented in Figure 1-7. In this report, all TURVA-2012 portfolio reports are referenced using the report title in italics. The full titles and report numbers are listed at the beginning of the reference list.

22 16 Site Description Understanding of the present state and past evolution of the host rock Main reports Biosphere Description Understanding of the present state and evolution of the surface environment Description of climate evolution and definition of release scenarios Models and Data for the Repository System Models and data used in the performance assessment and in the analysis of the radionuclide release scenarios TURVA-2012 Synthesis Description of the overall methodology of analysis, bringing together all the lines of arguments for safety, and the statement of confidence and the evaluation of compliance with long-term safety constraints Design Basis Performance targets and target properties for the repository system Production Lines Design, production and initial state of the EBS and the underground openings Description of the Disposal System Summary of the initial state of the repository system and present state of the surface environment Features, Events and Processes General description of features, events and processes affecting the disposal system Biosphere Data Basis Data used in the biosphere assessment and summary of models Biosphere Assessment: Modelling reports Description of the models and detailed modelling of surface environment Assessment of Radionuclide Release Scenarios for the Repository System Performance Assessment Analysis of the performance of the repository system and evaluation of the fulfillment of performance targets and target properties Formulation of Radionuclide Release Scenarios Biosphere Assessment Analysis of releases and calculation of doses and activity fluxes. Complementary Considerations Supporting evidence incl. natural and anthropogenic analogues Main supporting documents Figure 1-7. TURVA-2012 safety case portfolio including report names (coloured boxes) and brief descriptions of the contents (white boxes). Disposal system = repository system + surface environment.

23 17 The main reports and supporting documents of the TURVA-2012 portfolio have been described in the introduction to other main reports and will not be repeated here. This FEPs screening and processing report is not in the portfolio. The FEP screening and processing methodology, although applied in TURVA-2012, has not been formally reported until now, though an auditing of the FEPs taken into account in previous safety assessment was reported by Vieno & Nordman (1997) and discussions on what FEPs should be taken into account in a safety case took place during the compilation of Rasilainen (2004) and of Miller & Marcos (2007). This report presents also an audit against the NEA Version 2.1 Project Databases. The reasoning on why the FEPs are screened out of the final list is also presented. In the review of the pre-licensing documentation, STUK emphasises the need for aggregating/disaggregating FEPs, and this has been taken thoroughly into account. 1.7 Quality assurance The quality assurance (QA) procedures for the safety case (SAFCA) portfolio (see Figure 1-7) have been carried out following Posiva s quality management system, which complies with the ISO 9001:2008 standard and considers relevant regulatory requirements. Even though the quality assurance is based throughout on management according to the standard ISO 9001:2008, a graded approach proposed for nuclear facilities is adopted, i.e. the primary emphasis is on the quality control of the safety case, particularly for those activities that have a direct bearing on long-term safety, whereas standard quality measures are applied in the supporting work. This means, in practice, that the main safety case reports are subjected to stricter quality demands than general research activities. The input from Posiva s own RTD activities and other research also fulfil the ISO 9001 quality standards. The general quality guidelines of Posiva are also applied; the composition and quality management of portfolio reports and the recruitment of expert reviewers are carried out according to the respective guidelines. In addition, special attention is paid to the management of the processes that are applied to produce the safety case and its foundations, which is the basis for the whole safety case process and organisation of the work. The purpose of this enhanced process control is to provide full traceability and transparency of the data, assumptions, models, calculations and results. The safety case production process is a part of Posiva s RTD process and is linked to Posiva s Production lines, Facility design and other main processes. The main customer is the Strategy process and the Licensing sub-process. The aim of the safety case production process is to produce the long-term safety documentation for the construction licence application. The safety case production process is owned by the research manager of Posiva s Long-term Safety Unit in Posiva s Research Department. The overall plan, main goals and constraints for the safety case production process are presented in the Safety case plan (Posiva 2008). The details of how the Safety case plan 2008 is being implemented are described in the SAFCA project plan. The work is managed and coordinated by a SAFCA core group and supervised by a steering group.

24 18 The safety case production process is divided into four main sub-processes: Conceptualisation and Methodology, Data Handling and Modelling, Safety Assessment, and Evaluation of Compliance and Confidence. The Data Handling and Modelling sub-process constitutes the central linkage between Posiva s main technical and scientific activities and the production of the safety case. It is a clearinghouse activity between the supply of, and demand for, quality-assured data for the safety case. Data are produced by Posiva s planning, design and development processes for the EBS (Engineered Barrier System), by the site characterisation process for the geoscientific data and through the biosphere description of the Olkiluoto area. A SAFCA quality co-ordinator (QC) has been designated for the activities related to the quality assurance measures applied to the production of the safety case contents. The QC is responsible for checking that the instructions and guidelines are followed, and improvements are made in the process as deemed useful or necessary. The QC is also responsible for the coordination of the external expert reviews, maintenance of schedules, review and approval of the products, and the management of the expert elicitation process. The QC also leads the quality review of models and data used in the Data Handling and Modelling sub-process. Regular auditing of the safety case production process is done as part of Posiva's internal audit programme. Data sources and quality aspects of the sources are documented according to a specific guideline. Individual data and databases are approved through a clearance procedure supervised by the SAFCA Quality Co-ordinator. The process owner checks and approves the data and the QC checks and approves the procedure. Data used may also be approved using other Posiva databases such as VAHA or POTTI and the respective approval processes. A clearance procedure has been applied to all key data used in the performance assessment (i.e. showing compliance with performance targets and target properties), and in the safety assessment (i.e. radionuclide transport and dose calculations). The control and supervision of the safety case products (i.e. main portfolio reports) has been done in two steps, first an internal review by safety case experts and subject-matter experts within Posiva s RTD programme and then the second step by external expert reviewers. A group of external experts covering the essential areas of knowledge and expertise needed in the safety case production has been set up. The review process is based on review templates, which record each review comment and how it has been addressed. Upon completion, this template is checked and approved according to the quality guidelines of Posiva. An expert elicitation process has been applied to specific cases when the understanding or data basis is conflicting and consensus is needed for the selection of key data (e.g. future climate scenarios, solubility and sorption data). This expert elicitation process has been initiated, recruited, documented and managed by the SAFCA Quality Coordinator. QA issues are discussed further in the Synthesis. Quality assurance and quality control measures related to the production and operation of the repository are discussed in detail

25 19 in the Production Line reports (Canister, Buffer, Backfill, Closure and Underground Openings). 1.8 Scope and objectives of the present report This report documents the screening and processing of features, events and processes (FEPs) starting from NEA Version 2.1 Project Databases that has been carried out within TURVA The main objective is the FEP screening process that has been used to show that Posiva s FEP list, as described in Features, Events and Processes, is adequate for the current stage of the spent nuclear fuel management programme. The disposal system, as defined in Figure 1-6 includes the spent nuclear fuel, the engineered barriers surrounding it, the host rock, and the surface environment. The disposal system will evolve over time. The evolution of the disposal system will depend on: - the initial state of the system (relevant features are summarised in Description of the Disposal System), - a number of processes acting within the disposal system, and - external influences (events and processes) acting on the system. The focus of the present report is on the FEPs related to the repository system and external FEPs. The processing for the FEPs related to the surface environment is here limited to the initial step, the relevance screening (Chapter 3). This is due, on the one hand, to the lack of resources and, on the other hand, to the emphasis given by the Finnish regulator STUK, to the FEPs related to the repository system. 1.9 Structure The structure of the present report is as summarised below. Chapter 2 presents the overall FEP screening and processing methodology. Chapter 3 presents the outcome of the relevance and significance screening steps performed on the NEA Version 2.1 Project Databases. Chapter 4 presents the further processing of the screened in FEPs and the mapping of the TURVA-2012 FEP list to the screened in FEPs. In Chapter 5 a cross-checking of the TURVA-2012 FEP list is performed against FEP lists developed in other relevant projects and FEPs in Posiva s 2007 Process Report (Miller & Marcos 2007). Chapter 6 concludes on the advantages and challenges of the TURVA-2012 FEPs classification and provides a discussion on comprehensiveness.

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27 21 2 METHODOLOGY FOR FEP SCREENING AND PROCESSING 2.1 Overall methodology A central feature of Posiva s TURVA-2012 safety case is the formulation and analysis of a set of scenarios that, collectively, represent the envelope of future evolutions for a KBS-3V type disposal facility at the Olkiluoto site. The definition and formulation of scenarios is supported by the identification and consideration of all potentially relevant features, events and processes (FEPs) that, on the one hand, characterise the system, and on the other hand might occur during system evolution, and that have the potential to affect long-term safety. These FEPs are discussed in detail in Features, Events and Processes. Focusing on FEPs when developing the scenarios is in line with international recommendations regarding the development, operation and closure of disposal facilities and international best practice applied. For example, the Specific Safety Requirements on Disposal of Radioactive Waste (IAEA 2011) states in Requirement 15 on Site characterization for a disposal facility that: The site for a disposal facility shall be characterized 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. This shall include its present condition, its probable natural evolution and possible natural events, and also human plans and actions in the vicinity that may affect the safety of the facility over the period of interest. It shall also include a specific understanding of the impact on safety of features, events and processes associated with the site and the facility. More specifically, the Specific Safety Guide on The Safety Case and Safety Assessment for the Disposal of Radioactive Waste (IAEA 2012) states (5.42): Regardless of the method used for developing the scenarios, all features, events and processes that could significantly influence the performance of the disposal system should be addressed in the assessment. Posiva s methodology for scenario formulation, related to the repository system, follows a top-down approach in first identifying the safety functions that are required of the repository system, then considering the effects of single FEPs or combinations of FEPs on those functions to check that the scenarios are comprehensive (Formulation of Radionuclide Release Scenarios, Section 2.5). However, Posiva s methodology for the FEP screening component in the overall FEP processing is more closely related to the bottom-up approach for scenario development described, for example, in the ISAM project (IAEA 2004). When using this method, a comprehensive list of features, events and processes is developed as a starting point. This may involve the use of generic lists of features, events and processes (internationally agreed lists, regulations, etc.) and the determining of site- and system-specific features, events and processes (as discussed above). This is followed by a screening process to exclude features, events and processes from further considerations that are either irrelevant for the KBS-3 concept or considered to have insignificant impact on the long-term safety of the disposal system. Criteria for screening features, events and processes may include rules relating to regulations and/or to the probability of occurrence or consequences of events and processes. The sections below describe the step-wise FEP screening method, and further process-

28 22 ing, applied in demonstrating that the TURVA-2012 FEP list for the repository system and external events and processes is comprehensive enough. 2.2 FEP screening and processing steps Features, events and processes that might occur, and that have the potential to affect long-term safety under certain conditions are accounted for in the safety assessment in the TURVA-2012 safety case. These FEPs are presented and discussed in Features, Events and Processes. This section describes the identification and screening of FEPs and their further processing and cross-checking that was conducted in order to assess the comprehensiveness of the TURVA-2012 FEP list. The work has been conducted in a structured step-wise manner mainly relying on expert judgement. Each step in the process is elaborated in the sub-sections below and they are summarised as follows: 1. Selecting an initial comprehensive FEP list to use as a starting point. 2. Performing a relevance screening based on a set of screening criteria aiming at screening out FEPs not relevant for the KBS-3 concept or the TURVA-2012 safety case. 3. Performing an initial aggregation of FEPs with an identical heading or meaning, and the classification of the FEPs under the components of the disposal system (or as external if they are external to the system). 4. Performing a significance screening based on a set of screening criteria aiming at screening out FEPs clearly insignificant for long-term safety. 5. Mapping the FEPs not screened out in previous steps to the TURVA-2012 FEP list. This step contains, for example, the aggregation and disaggregation of FEPs to follow Posiva s FEP nomenclature. 6. Cross-check the final TURVA-2012 FEP list against FEP lists developed in other relevant projects. 2.3 Identification and screening of FEPs for potential significance The initial FEP list The FEP screening is based on the FEP list from NEA 2.1 Project Databases (NEA 2006), which was the latest NEA FEP database available in 2011 when the FEPs in Features, Events and Processes were compiled Relevance screening A relevance screening, aiming to screen out FEPs not relevant for a KBS-3V type repository or the TURVA-2012 safety case, is performed on the FEPs in the NEA Version 2.1 Project Databases, below denoted as Project FEPs. A set of criteria has been set out to facilitate this screening. A Project FEP is screened out if at least one of the five following screening criteria is fulfilled: R1 The Project FEP is defined by a heading without any description of what is meant by the heading.

29 23 R2 R3 R4 R5 The Project FEP is related to assessment methodology. These FEPs are handled elsewhere in the TURVA-2012 safety case. The Project FEP is not relevant for the context of the TURVA-2012 safety case, especially in respect to the national regulatory requirements and guidelines. The Project FEP is not relevant for the KBS-3V type repository design for spent nuclear fuel disposal. The Project FEP is not relevant for the present-day Olkiluoto site characteristics and likely future site characteristics evolving in response to climatic changes and other external factors Initial aggregation and component-wise classification of the FEPs The second step in the process includes: the initial aggregation of the Project FEPs screened in that have identical heading or meaning. At this stage, the individual project-specific FEP descriptions are, however, kept in the documentation for the benefit of the experts carrying out the next screening step (Section 2.3.4). the classification of the aggregated FEPs under the component(s) of the disposal system, i.e. spent fuel, canister, buffer, backfill, auxiliary components, geosphere or surface environment or they are classified as external if they are related to conditions external to the disposal system. A FEP is classified under several components if it is not obvious that it only relates to one component (e.g. soil moisture and evaporation is only mapped to surface environment, whereas diffusion is classified to all components of the disposal system) Significance screening The third step in the process is to conduct a screening evaluation of the Project FEPs not screened out in the relevance screening, aiming at screening out FEPs clearly insignificant for long-term safety. A set of criteria regarding a FEP s impact on safety functions and radiation protection criteria (these are doses and activity fluxes; there are FEPs affecting e.g. exposure pathways that have an impact on the end points regarding radiation protection criteria, but these FEPs have no impact on safety functions) has been defined to facilitate this screening. A Project FEP is screened out if at least one of the following significance screening criteria is fulfilled: S1 S2 S3 The FEP has insignificant impact on safety functions and radiation protection criteria. The FEP has low probability to occur and low impact on safety functions and radiation protection criteria. The FEP itself has considerably more serious consequences than any potential radiological consequences from the spent nuclear fuel.

30 Mapping remaining Project FEPs to the TURVA-2012 FEP list The outcome of the steps in the FEP screening and initial processing described above is a list of NEA Project FEPs potentially safety relevant for a KBS-3V repository at the Olkiluoto site. These FEPs have been aggregated and classified under relevant components of the disposal system or as external FEPs. This list is mapped to TURVA-2012 FEPs to check that they include all FEPs relevant for the KBS-3V system at Olkiluoto. The full TURVA-2012 FEP list is presented in Appendix A. An explanation is given, when necessary, in case a remaining NEA Project FEP has not a clear corresponding TURVA-2012 FEP. 2.5 Cross-checking The final step is to cross-check the TURVA-2012 FEP list against relevant sources to increase the confidence in that no important FEPs have been omitted. Of importance is to cross-check against FEP lists developed in other relevant projects, especially the lists underpinning safety assessments performed by SKB for a KBS-3V type repository for spent nuclear fuel disposal. A cross-check against the FEP list in Miller & Marcos (2007) is also included.

31 25 3 SCREENING AND PROCESSING OF THE FEPS 3.1 FEP lists included in the work The screening process was based on the NEA FEP list (NEA Version 2.1 Project Database), which is a compilation of FEPs included in several spent nuclear fuel disposal programmes around the world (i.e. Goodwin et al. 1994, NAGRA 1994, DOE 1996, Chapman et al. 1995, Miller et al. 2002, Miller & Chapman 1993, Bronders et al. 1994). Other FEP lists or sources of information used are discussed in Chapter 5 where the final list is cross-checked against relevant literature. Cross-checking includes data from SKB s SR-Site (SKB 2010a) (Section 5.1), NWMO s Fourth Case Study (Garisto 2012) (Section 5.2) and comparison to earlier work done by Posiva (Section 5.3). 3.2 Relevance screening results Relevance screening was performed following the protocol presented in Section The number of Project FEPs in version 2.1 of the NEA FEP database is large, in total there are 1671 FEPs (Electronic Appendix 1, NEA version 2.1 Project Database). In order to make the relevance screening more efficient, a workshop exercise was carried out by a group of safety assessment experts. This screening process is documented in Electronic Appendix 1 (NEA version 2.1 Project Databases Relevance screening_final), where screening decisions based on R- criteria (see Section 2.3.2) are listed. The final table shows the result of the initial workshop as well as including additional screening decisions that were revised during further steps, i.e. cross-check, of the work. The relevance screening resulted in 1154 FEPs being screened in and 517 FEPs being screened out; the latter were divided among the individual screening criteria: R1: 119 FEPs screened out R2: 88 FEPs screened out R3: 89 FEPs screened out R4: 111 FEPs screened out R5: 110 FEPs screened out. 3.3 Preliminary aggregation of FEPs and classification under disposal system components A preliminary aggregation of the FEPs and their classification under components of the disposal system (or as external to the system) was done before moving to the Significance screening phase. The protocol for the process is described in Section The aggregation was done to collate FEPs that were duplicates in the NEA FEP list in order to simplify the following steps. FEPs were then classified under the following components (reflecting those discussed in Features, Events and Processes): spent fuel

32 26 canister buffer backfill auxiliary components geosphere surface environment external FEPs. The aggregation was documented in an Excel file which was used as a basis for the Significance screening. Hence the, aggregation is shown also in the documentation of the Significance screening results and is not duplicated here (see Section 3.4 and Electronic Appendix 2, NEA version 2.1 Project Databases Significance screening and justifications). After the preliminary aggregation, the number of FEPs in the list was 735. It should be noted that some of these FEPs (168 in total) were exclusively related to the surface environment, and these FEPs were not further processed. 3.4 Significance screening results The Significance screening was performed in accordance to the protocol presented in Section The significance screening requires a deeper process understanding compared with the relevance screening. Hence, the screening involved various subject matter experts for the different components of the disposal system and external conditions. Initial screening was carried out by a group of experts in a significance screening workshop, and the justifications for screening out FEPs were written later by individual experts. The components included in the significance screening were all those listed above (Section 3.3), except for the surface environment. The Significance screening was documented in an Excel file on top of the initial aggregation of the FEPs (Section 2.3.3). As noted above FEPs that relate only to the surface environment (168 FEPs in total) are shown in the Excel sheet but they were not further processed. Thus, 567 FEPs remained to be screened in this phase. The screening decisions based on the Significance criteria and justification for OUT screening decisions are presented in Electronic Appendix 2 (NEA version 2.1 Project Databases Significance screening and justifications). The significance screening resulted in 491 FEPs being screened in and 76 FEPs being screened out. The latter were divided among the individual screening criteria: S1: 55 FEPs screened out S2: 16 FEPs screened out S3: 5 FEPs screened out. See Electronic Appendix 2 for justifications why these FEPs were screened out in this phase (column Justification ).