Approach to Practical Elimination in Finland

Transcription

1 Approach to Practical Elimination in Finland M-L. Järvinen, N. Lahtinen and T. Routamo International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants 6 th 9 th June 2017 Vienna, Austria

2 Content Finnish safety requirements stipulating practical elimination concept The implementation of the requirements for NPPs Examples of application: Pressure vessel rupture Severe accident management 2

3 Evolution of the Finnish safety requirements References in the requirements General Design Principles of a Nuclear Power Plant, criteria Based on 10CFR50, Appendix A YVL 1.0 Design Principles of a Nuclear Power Plant, 1982 State of the Art requirements, e.g. severe accident management requirements Consideration of the IAEA Safety Series after midd-1980 Constitution Laws, Decrees STUK Regulations YVL Guides A new level, STUK mandatory requirements, 1 st January 2016 EURATOM regulations, directives WENRA reference levels and safety goals for new reactors Standards 3

4 Safety requirements related to practical elimination concept Nuclear Energy Decree Section 22 b states 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. In STUK regulation Y/1/2016 items with extremely low possibility include for reactor design criticality accidents ( 10, 3. a) iii.); (nuclear reactor's physical feedback characteristics ( 11, 1) ) loss of 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.). Further detailed requirements in YVL Guides, especially such as Guide YVL B.1 (design), Guide YVL A.7 (PSA) and Guide YVL C.3 (radioactive releases) 4

5 Basic features of the YVL requirements YVL B.1 deterministic analyses complemented by probabilistic risk assessments and expert assessments YVL A.7 cannot be based solely on compliance with a cut-off probabilistic value all practicable measures shall be taken to reduce the risk examples of events a) the mean value of the frequency of a release of radioactive substances from the plant during an accident involving a Cs-137 release into the atmosphere in excess of 100 TBq is less than /year; b) the accident sequences, in which the containment function fails or is lost in the early phase of a severe accident, have only a small contribution to the reactor core damage frequency. YVL A.7 YVL B.1 YVL C.3 YVL C.3 any release of radioactive substances in a severe accident shall not warrant the evacuation of the population beyond the protective zone or the need for people beyond the emergency planning zone to seek shelter indoors Reference to Guide YVL A.7 5

6 The Defence-in-Depth levels, event categories, frequencies and relation to the phenomena to be practically eliminated in the Finnish context. *) *) and the accident sequences, in which the containment function fails or is lost in the early phase of a severe accident, have only a small contribution to the reactor core damage frequency. 6

7 Continuous improvement of safety, implementation of YVL Guides issued in 2013 Section 7 a Guiding principles The safety of nuclear energy use shall be maintained at as high a level as practically possible. For the further development of safety, measures shall be implemented that can be considered justified considering operating experience and safety research and advances in science and technology. New YVL guides are applied as such to the new NPPs i.e. those for which Decision in Principle was made in 2010 For the operating NPPs or other nuclear facilities and the NPP under construction an implementation decision is made the nuclear facility is reviewed against new YVL Guides the modifications considered reasonably practical are implemented The application of Section 7a of the Nuclear Energy Act and exemptions are explicitly described in building regulations. Operating NPPs Implementation of updated regulations has been completed in and resulting safety enhancement projects are ongoing. licensees are required to analyse a wider spectrum of initiating events compared to examples given in Guide YVL B.1. Olkiluoto 3 NPP unit in operating license phase reviewed as part of licensing process Hanhikivi 1 in construction license phase reviewed as part of licensing process 7

8 Example Loviisa unit 1 and unit 2 Reactor pressure vessel mainly based on standards that were in force in the Soviet Union sufficient safety factor ISI-inspections, qualified system embrittlement margins Severe accident top level critical safety functions of SAM strategy Containment isolation Primary system depressurisation Absence of energetic events (mitigation of hydrogen combustion) Reactor core cooling (RPV lower head coolability and melt retention) Containment pressure control (long-term containment cooling) in addition Subcriticality of the core Cooling of refuelling pools Severe accident safe state (SASS) in the long-term SAM safety functions and the SAM systems of the Loviisa NPP. 8

9 Example Olkiluoto unit 1 and unit 2 Reactor pressure vessel ASME Boiler and Pressure Vessel Code sufficient safety factor ISI-inspections, qualified system lifetime for 60 year needs to be demonstrated Severe accident Ex-vessel SAM strategy Automatic primary system depressurisation Gravity driven flooding of lower drywell for exvessel core melt cooling penetrations in lower drywell have been shielded steam explosions must not endanger containment integrity Containment water filling from external source Filtered containment venting a venturi scrubber with filter retention of 99.9 % for aerosols and 99 % for elemental iodine Containment ph control NaOH solution can be injected to the containment spray to prevent formation of molecular and organic iodine Containment filled with inert nitrogen gas during power operation SAM safety functions and the SAM systems of the Olkiluoto NPP units 1&2. 9

10 Example Olkiluoto unit 3, (EPR under construction, operating license phase) Reactor pressure vessel ASME Boiler and Pressure Vessel Code sufficient safety factor ISI-inspections, qualified system 60 year design lifetime Severe accident SAM provisions integrated already in the basic design Ex-vessel SAM strategy Primary system depressurised prior to pressure vessel failure Core melt cooling in a separate compartment. Passive flooding Containment heat removal by sprays Hydrogen mixing and removal by passive autocatalytic recombiners Containment filtered venting a controlled way to decrease the containment pressure in the long-term a diverse path to release heat in specific accident scenarios during refuelling outages SAM safety functions and the SAM systems of the Olkiluoto NPP units 3. 10

11 Example Hanhikivi unit 1 ( DiP assessment 2014) Reactor pressure vessel design objectives and design principles of the main nuclear components of the AES-2006 are mainly in compliance with the Finnish safety requirements effects of reactor pressure vessel material (radiation embrittlement ) Severe accident STUK finds that it is possible to implement the systems and strategy for managing severe accidents in compliance with the Finnish safety requirements. The implementation of the functions for severe accidents by independent systems in compliance with the Finnish requirements shall be verified in the construction licence phase. 11

12 Conclusions The practical elimination of early or large releases consist of two aspects Defence-in-Depth effectively in design and operation of a reactor specific accident sequences, provisions ensure no need to be included mitigation in the design The idea of practically eliminated reactor pressure vessel rupture, fast increase of reactivity already in early designs conditions arising during severe accident needs to be assessed as part of demonstration severe accident management systems have been implemented also for operating NPPs The Finnish approach for practical elimination of large or early releases is driven by limiting the overall frequency of accidents addressing sequences for which mitigation is not feasible, extremely low probability addressing specific sequences supports also achieving the probabilistic overall safety goal 12

13 Thank you! 13