The regulatory documents governing and guiding the safety case work are the following:
- STUK’s regulation STUK Y/4/2016 (STUK 2015c), replacing the Government Decree (GD) 736/2008 on the safety of nuclear waste disposal,
- YVL Guide D.5 on the geologic disposal of radioactive waste (STUK 2014a) (possibly to be updated in 2016),
- additional YVL guides (e.g. D.3, D.4, B.1) concerning nuclear facilities in general,
- the STUK’s requirements about the safety case to be fulfilled before or at the time of submitting the operating licence application (STUK 2015a, Decision),
- STUK’s assessment of the safety case TURVA-2012 (STUK 2015b, Review report)
- STUK’s Consultants’ Consensus Reports - Safety Analysis (Sagar 2015)
- STUK’s Consultants’ Consensus Reports - EBS (Apted 2015)
- STUK’s Consultants’ Consensus Reports - Site (Chapman et al. 2015).
Furthermore, the to-be-published YVL Guide D.7 on implementation of the repository (“Ydinjätteiden loppusijoituslaitoksen vapautumisesteet ja kalliorakentaminen”) will also be taken into account once the contents will be available.
The requirements for the safety of the disposal are set out in the above-mentioned STUK regulation, and Guide D.5 presents more detailed requirements. It should be noted that the present YVL Guide D.5 of November 2013 will be updated in the near future based on the above-mentioned STUK regulation, which was issued in December 2015. The present safety case plan is based on the YVL guides that were available at the time of writing (2015).
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 for at least several thousands of years (STUK Y/4/2016 (STUK 2015c), 32 §). In Guide YVL D.5, paragraph 301, it is stated that the disposal of nuclear waste shall be so designed that the radiation impacts arising as a consequence of the expected evolution are as follows:
- the annual dose to the most exposed individuals remains below the value of 0.1 mSv; and
- the average annual doses to other individuals remain insignificantly low.
Paragraph 318 of Guide YVL D.5 gives the following guidance to meet the requirements in STUK’s regulation for the protection of other living species, which is a matter that has to be addressed in the safety assessment:
The disposal shall not have detrimental radiation effects on species of flora and fauna. This shall be demonstrated by assessing the typical radiation exposures of terrestrial and aquatic populations in the disposal site environment, assuming the present kind of living populations. The assessed radiation exposures shall remain clearly below the doses that, on the basis of the best available knowledge, would cause significant detriment to any living population.
The safety case is mentioned in 35 § of STUK Y/4/2016:
Compliance with the nuclear and radiation safety requirements concerning long-term safety, and the suitability of the disposal method, engineered barriers and disposal site, shall be proven through a safety case that must analyse possible evolution scenarios of the disposal system, including the evolution scenarios arisen from unlikely events impairing long-term safety. The safety case comprises a numerical safety analysis based on e.g. the evolution scenarios and complementary considerations.
36 § of STUK Y/4/2016 comments on the reliability of the safety case:
The safety case and the methods, input data and models utilised in the safety case shall be based on high-quality research data and expert judgement and they must be documented in a traceable way. Data and models shall be appropriate and correspond to the conditions likely to prevail at the disposal site during the assessment period.
The basis for selecting the computational methods used shall be that the actual radiation exposure and quantities of radioactive materials released remain below the results of safety analyses, with a high degree of certainty. The uncertainties involved in the input data, models and analyses, and their significance, shall be separately assessed.
The presentation of, and updates to, the safety case are addressed in 37 §:
The safety case shall be presented in connection with the construction licence application and the operating licence application of the nuclear waste facility, and when significant facility changes are made. The safety case shall be updated at regular intervals unless otherwise provided in the licence conditions. The need for updating the safety case must be assessed before making changes that apply to the disposal system. Furthermore, the safety case shall be updated prior to the permanent closure of the facility.
There are more specific regulations for the safety case in the YVL Guide D.5 (paragraphs 704 and 708 and Annex A). The expected contents of the safety case are outlined in paragraph 704 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 proven through a safety case that shall at least include:
- a description of the disposal system and the definition of barriers and safety functions;
- the specification of performance targets for the safety functions;
- a definition of the scenarios (scenario analysis);
- a functional description of the disposal system and a description of the conditions prevailing at the disposal site by means of conceptual and mathematical modelling, and the determination of necessary model parameters;
- 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;
- whenever possible, an estimation of the probabilities for activity releases and radiation doses arising from unlikely events impairing long-term safety;
- uncertainty and sensitivity analyses and complementary qualitative considerations; and
- a comparison of the outcome of the analyses against the safety requirements.
Detailed requirements for the content of the safety case are provided in Annex A.
The more detailed requirements in Annex A (STUK 2014a) are addressed later in this report in the context of the safety case reports addressing them.
STUK also formulated requirements concerning the safety case to fulfil before or at the time of submitting the operating licence application (STUK 2015a), these are reported in Appendix 1 along with their relevance for the present safety case plan.
Posiva’s internal requirement system VAHA is used to store the requirements for the disposal. Of these, long-term safety related requirements are discussed in the Design Basis report (INTERNAL LINK).