M-L. JÄRVINEN et al.
approach to Practical elimination in Finland
M-L. JÄRVINEN
Radiation and Nuclear Safety Authority (STUK)
Helsinki, Finland
Email:
N. Lahtinen
Radiation and Nuclear Safety Authority (STUK)
Helsinki, Finland
Email:
T. Routamo
Radiation and Nuclear Safety Authority (STUK)
Helsinki, Finland
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Abstract
The concept of practical elimination has been an elemental part of the design of nuclear power plants. However, there has been a need to revisit this concept after the Fukushima Dai-ichi accident in 2011 and the lessons learned have lead to further elaboration of the concept and enhancement of related regulatory requirements in many countries. In Finland the updated requirements on practical elimination were issued in 2013. The paper will present the Finnish requirements related to practical elimination concept and their development as well as practical examples on how the concept is applied in the operating NPPs and NPPs under construction.
According to Nuclear Energy Act, the concept of continuous improvement of safety is applied in Finland and the safety of nuclear energy use shall be maintained at as high a level as practically possible. For the further enhancement of safety, measures shall be implemented that can be considered justified considering operating experience and safety research and advances in science and technology. The practice of implementation of the updated regulatory requirements for the operating nuclear power plants and related safety enhancements will also be presented.
The Finnish approach for practical elimination of large or early releases is driven by limiting the overall frequency of initiating events that could lead to such a situation. Specific approach for practical elimination is addressing sequences, for which mitigation is not feasible due to the nature of the phenomena resulting from some specific initiating events. The possibility of occurrence of such sequences has to remain extremely low, which supports also achieving the probabilistic overall safety goal.
1. INTRODUCTION
The concept of practical elimination has been an elemental part of the design of nuclear power plants. However, as a concept the practical elimination was first introduced in INSAG-12 in 1999 in context of design requirements for new reactors. In the Finnish legislation and subordinate regulations the terminology is not used. This leaves room for interpretation on how the concept is applied. However, there has been a need to evaluate both national and international requirements after the Fukushima Dai-ichi accident and the lessons learned have lead to further enhancement of the regulatory requirements dealing with practical elimination in many countries.
In Finland the concept of continuous improvement of safety is applied and the safety of nuclear energy use shall be maintained at as high a level as practically possible. For the further enhancement of safety, measures shall be implemented that can be considered justified considering operating experience and safety research and advances in science and technology. As the YVL Guides are written for the new nuclear power plant separate decisions are taken on the application of updated YVL Guides to operating plant or plants under construction.
In the new set of YVL Guides issued 2013 the requirements for practical elimination are presented systematically. The implementation of the updated regulatory requirements for the operating nuclear power plants has been completed in 2015 – 2016 and resulting safety enhancement projects are ongoing. Furthermore the licensees are required to analyse a wider spectrum of initiating events compared to examples given in Guide YVL B.1. The analysis with conclusions shall be submitted to STUK before the end of this year. The fulfilment of practical elimination concept and related requirements has been reviewed in the periodic safety review of the Fortum's Loviisa 1 and 2 finished in 2017, and will be reviewed in the renewal of the operating licence of TVO's Olkiluoto 1 and 2.
The Olkiluoto 3 NPP unit is currently in operating license phase and the fulfilment of practical elimination concept and related requirements is being reviewed as part of licensing process.
Fennovoima's Hanhikivi nuclear power plant project is now in construction license phase and the application of practical elimination concept and related requirements are being discussed and reviewed as part of the licensing process.
2. The development of the Finnish requirement on practical elimination
2.1. Overview of development of the concept
The term “practically eliminated” was originally introduced in the IAEA publications in INSAG-12 /1/ in 1999 and introduced into the IAEA Safety Standard Series for the first time in 2004 in Safety Guide NS-G-1.10 /2/ dealing with the containment systems of the nuclear power plants. The NS-G-1.10 explanation of “practically eliminated” state that “In this context, the possibility of certain conditions occurring is considered to have been practically eliminated if it is physically impossible for the conditions to occur or if the conditions can be considered with a high degree of confidence to be extremely unlikely to arise”. After the TEPCO Fukushima Dai-ichi nuclear power plant accident in March 2011 the concept of “practical elimination” of some specific initiating events leading to early or large releases has been dealt in several international publications.
In the IAEA Safety Fundamentals published in 2006 Principle 8 states that measures shall be taken to ensure that the likelihood of an accident having harmful consequences is extremely low. In the IAEA requirements level document the “practical elimination” of early or large releases was introduced in design requirements of the nuclear power plants SSR-2/1 /3/ published in 2012. There has been a need to evaluate these requirements after the TEPCO Fukushima Dai-ichi nuclear power plant accident in March 2011 and the lessons learned have lead to further enhancement of the requirements. The updated IAEA nuclear power plant design requirements SSR-2/1 Rev.1 (2016) /4/ systematically address the requirements for “practical elimination” of early or large releases. The ongoing updating of the IAEA Safety Standards introduces the concept explicitly to the safety guides dealing with design or safety demonstration such as deterministic analysis, containment system design or content of the safety analysis report /5,6,7/. More technical discussion of the concept is presented in IAEA TECDOC 1791 (2016) /8/ on application of SSR-2/1 requirements.
The explanation of IAEA for the “practically eliminated” has been widely adopted. Footnote 4 in SSR-2/1 Rev.1 states “The possibility of certain conditions arising may be considered to have been ‘practically eliminated’ if it would be physically impossible for the conditions to arise or if these conditions could be considered with a high level of confidence to be extremely unlikely to arise. In the safety demonstration the physically impossible is considered to be the preferred method. However, the distinction in between these two ways of demonstration is not always simple.”
Practical elimination of early or large releases deals with the strength of the Defence-in-Depth and demonstration of the effectiveness of the protection, and on the other hand prevention of certain conditions from arising threatening the integrity of the containment. These conditions could be grouped into five categories as shown in the IAEA TECDOC-1791:
1. Events that could lead to prompt reactor core damage and consequent early containment failure:
a. Failure of a large component in the reactor coolant system (RCS);
b. Uncontrolled reactivity accidents.
2. Severe accident phenomena which could lead to early containment failure:
a. Direct containment heating;
b. Large steam explosion;
c. Hydrogen detonation.
3. Severe accident phenomena which could lead to late containment failure:
a. Molten core concrete interaction (MCCI);
b. Loss of containment heat removal.
4. Severe accident with containment bypass;
5. Significant fuel degradation in a storage pool.
The safety goals for new reactors publish by WENRA in 2010 /9/ include safety objective for accident with core melt considering the “practical elimination” of early or large releases and WENRA considers in its reference levels (for the existing reactors) published in 2014 the concept of “extremely unlikely with a high degree of confidence” /10/. The concept of practical elimination is discussed more in the WENRA report on Safety of new NPP designs published in 2013 /11/. This report clarifies the concept of practical elimination as “Accident sequences that are practically eliminated have a very specific position in the Defence-in-Depth approach because provisions ensure that they are extremely unlikely to arise so that the mitigation of their consequences does not need to be included in the design”. In addition, the paper takes a wider perspective: “All accident sequences which may lead to early or large radioactive releases must be practically eliminated.”, which arises directly from WENRA Safety Objective O3 “Accidents with core melt”.
The international discussion on the topic builds up on understanding of the different approaches as the integration of the concept into practice is done. This discussion reveals also the difficulty in building the common understanding around the concept. This is interesting acknowledging that “practical elimination” is crucial part of the safety demonstration of a nuclear power plant. OECD NEA Green Book /12/ published in 2016 discusses the “practical elimination” of the significant releases through Defence-in-Depth.
2.2. Historical development of practical elimination concept in the Finnish nuclear safety regulations
The Finnish requirements were established right from the beginning of the use of nuclear energy in Finland. The fist 55 criteria were presented in the General Design Principles of a Nuclear Power Plant in 1976 /13/. The US requirements 10CFR50, Appendix A /14/ were used as a reference when developing the Finnish requirements. At that time no systematic approach was used in the requirement to quantify the need for safety demonstration. As an example the prevention of occurrence of a certain condition such as a criticality accident or loss of the integrity of the containment was required. The expression of “the possibility of … shall be extremely low” was used for the integrity of primary circuit and simultaneous loss of internal and external electric power. The Guide YVL 1.0 Design Principles of a Nuclear Power Plant published in 1982 /15/ represented the state-of-the-art guidance at that time and good coverage of the safety issues. For instance the severe accident management was required for the design of new nuclear power plants. From the middle of 1980s the IAEA Safety Standards were considered in the development of the YVL Guides.
The renewal of the Atomic Energy Act (356/1957) and publication of the Nuclear Energy Act (990/1987) and subordinate regulations systematized the approach for setting safety requirements at different levels in the Finnish regulatory framework. The safety requirements were issued for new reactors and principle that all the reasonable practicable safety improvements shall also be made for the already licensed nuclear power plants was applied. For the first time in 1991 the binding safety requirements were presented in the Government Decree on safety of a nuclear power plant (395/1991) /16/. In the legislation and subordinate regulations the terminology “practical elimination” was not used. This left room for interpretation in application of the concept. In Finland the terms such as “the possibility of … shall be extremely low” were used in this context.
The updating of the requirements in Government Decree on safety of a nuclear power plant (733/2008) /17/ in 2008 considered the safety enhancements required for the new build and lessons learned form the start of the construction of the Olkiluoto 3 nuclear power plant. The update of the Government Decree on safety of a nuclear power plant (717/2013) took into account also the TEPCO Fukushima Dai-ichi accident in March 2011. These updated requirements are reflected in the total revision of the set of YVL Guides conducted during years 2006 to 2013. Government Decree (717/2013) /18/ considered the updated WENRA safety goals and IAEA SSR2/1 requirements for the design of nuclear power plants. The requirements indicate systematically needed strength of safety demonstration and particularly underline the requirements for “practical elimination”. The Defence-in-Depth principle in the design of nuclear power plants is strengthened by emphasizing the consideration of extreme external conditions. The new criteria take also into account social and economical aspects in addition to the risks based on doses in case of severe accidents with releases. The consideration of the need for large scale protective measures for the public strengthens the earlier requirements related to restrictions on use of land and water. Government Decree(717/2013) Section 10 “Limits for accidents” stated that:
“The release of radioactive substances arising from a severe accident shall not necessitate large scale protective measures for the public nor any long-term restrictions on the use of extensive areas of land and water.
In order to restrict long-term effects the limit for the atmospheric release of cesium-137 is 100 terabecquerel (TBq). The possibility of exceeding the set limit shall be extremely small.
The possibility of a release requiring measures to protect the
public in the early stages of the accident shall be extremely small. ”
At the beginning of year 2016 the structure of the Finnish legal framework was changed and STUK Regulations were issued. Those regulations included technical requirements earlier covered by the Government Decrees. However, some requirements such as dose constrains needed to be presented in higher level legislation and were incorporated into Nuclear Energy Decree (161/1988).As an example the above mentioned Section 10 “Limits for accidents” is now presented in the Nuclear Energy Decree Section 22b. Regulation STUK Y/1/2016 /19/ on the Safety of a Nuclear Power Plant supersedes Government Decree (717/2013). The Regulation uses the phrase “the possibility of … shall be extremely low” in cases that can be considered to be treated under “practical elimination” concept, although the general wording does not necessary imply this to be strictly followed. Cases to be “practically eliminated” are introduced in YVL Guides in more detail.
In Regulations items with extremely low possibility include for reactor design
— criticality accidents (§10, 3. a) iii .); (nuclear reactor's physical feedback characteristics (§ 11, 1) )
— integrity of (major) pressure bearing components (§10, 3. b) i.);
— containment leaks as a consequence of reactor pressure vessel failure (§10, 3. c) iii);
and in connection of fuel handling and storage
— criticality accidents (§12, 4.);
— severe accidents (§12, 5.).