Nuclear Safety Regulation: Before and after Fukushima*

Transcription

1 Nuclear Safety Regulation: Before and after Fukushima* Shridhar Chande, India International Conference on Effective Nuclear Regulatory Systems: Sustaining Improvements Globally, Vienna April 2016 *Accident at the Fukushima Daiichi NPP

2 The main lesson from the Accident at Fukushima Daiichi NPP Changes in Defence in Depth and other requirements for Nuclear Safety Impact of the revised requirements on the safety of new plants Can good regulations alone ensure safety? Robust National Nuclear Safety System -Institutional Strength in Depth (A proposal from INSAG) 2

3 The main lesson from the Accident at Fukushima Daiichi NPP 3

4 More than people were evacuated from their homes. Consequently, nobody received any significant amount of radiation Many of them, still continue to stay in provisional housing, away from home. They may not be able to return for several more years Many are suffering from health issues emanating from uncertainty, anxiety, depression and discrimination It is reported that about 1600 have died during this period due to non-radiological health effects The accident has caused enormous economical, societal and psychological impact on Japanese people Though nobody has received any significant radiation dose, the displacement of large number of people for prolonged period is not acceptable to the society 4

5 Protecting people from radiation alone is not the only focus of safety of NPPs It is essential to ensure that under any circumstances there is no need to relocate the public for prolonged periods Hence the focus of safety must shift from Protect people and the environment from the harmful effects of radiation The necessity for off-site protective actions to mitigate radiological consequences should be limited or even eliminated& 5

6 As a consequence, the safety objectives of the nuclear power plant can be restated as In order of priority A nuclear accident should be prevented If an accident takes place it should be prevented from progressing to a Severe Accident (SA) In case it develops in to a SA, its consequences should be mitigated so as to Prevent/minimise and delay the radioactive releases, and Ensure that no urgent or long term protective actions are needed in the public domain An accident involving early or large releases should be practically eliminated 6

7 1.New nuclear power plants are to be designed, sited, and constructed, consistent with the objective of preventing accidentsin the commissioning and operation and, should an accident occur, mitigating possible releases of radionuclides causing long-term off site contamination and avoiding early radioactive releases or radioactive releases large enough to require long-term protective measures and actions. 2.Comprehensive and systematic safety assessments are to be carried out periodically and regularly for existing installations throughout their lifetime in order to identify safety improvements that are oriented to meet the above objective. Reasonably practicable or achievable safety improvements are to be implemented in a timely manner. 7

8 The most important safety principle used in design and operation of NPPs is the Defence in Depth philosophy to guard against human and equipment failure. Defence in Depth involves building of multiple levels of safety,such that if one level fails the subsequent level prevents progression of the event, limiting consequences. Each level has redundant and diverse safety systems to make it robust and reliable. The levels are made independent to the extent possible, to avoid common cause failures With proper defence in depth in place, an accident with such consequences as in Fukushima shouldn t have happened 8

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10 All offsite and onsite power supplies were lost due to tsunami flooding. This disabled all normal and back up systems needed for cooling the reactor. A common cause failure of all the three levels of defence in depth for prevention due to flooding Accident management guidelines could not be implemented as no instruments were working and there was no power supply. Total darkness and debris in the plant areas made access difficult. A failure of the fourth level of defence again due to flooding and its consequences The essential lesson here is to strengthen the defence in depth, both at Level 3, for the prevention of accident; and at Level 4, controlling the progression and mitigating the consequences of the accident 10

11 Change in Defence in Depth and other requirements for Nuclear safety 11

12 This level deals with the postulated set of design basis accidents (DBA) for which the plant is designed The essential means to deal with the DBA are inherent or conservatively designed engineered safety features Improve the robustness of preventive measures and avoid their common cause failures by use of redundancy, diversity, physical separation and functional independence Change in the radiological acceptance criteria to be consistent with the revised safety objectives Earlier Criteria Prevent significant off-site releases to keep public dose within the specified limit ( msv, allowing due credit for protective measures) Revised Criteria Only minor radiological impact in the public domain that does not necessitate any off-site protective measures. (dose limited to only a few msv, without any credit for off site protective measures) 12

13 Level 4 (earlier called BDBA conditions)addresses the accident conditions that exceed the design basis either due to multiple failures or severity of the PIE and are called Design Extension Conditions (DEC), implying that they are to be considered in the design process This level requires provision of additional safety systems/features, which are different from those used for more frequent accidents and capable of functioning under the expected severe accident conditions DECs are categorised as follows DEC without core melt DEC with core melt Accident conditions leading to early or large radioactive releases are to be practically eliminated 13

14 Level 4 earlier called BDBAhad the objective to limit off-site releases as low as reasonable achievable Defencein depth level Level 4A DEC without core melt Level 4B DEC with core melt SAinvolving early or large releases Objective Radiological Criteria Arrest progression of accident Mitigate the consequences of severe accident To be practically eliminated (Same as Level3, except use of best estimate methods permitted) Protective measureslimited in time and area to be sufficient, with adequate time being available to implement. No significant offsite contamination 14

15 Strengthening the different levels of defence in depth and their independence Conservative consideration of external hazards and sufficient margins Strengthening the reliability of heat sink Strengthening the emergency power supply Ensuring the safety of spent fuel storage under accident conditions Enabling the use of non-permanent equipment Provision of emergency response facilities 15

16 Rigorous implementation of Periodic Safety Reviews* Improving the Emergency preparedness Strengthening the Accident Management provisions and plans *Vienna Declaration on Nuclear Safety 16

17 Impact of Revised Requirements on the safety of new plants 17

18 Dose Criteria for Public (Indian example) Pre-Fukushima Post-Fukushima* Plant State Dose Limits at EZ boundary Normal operation 1 msv/year Anticipated Operational occurrences Design Basis Accidents Beyond design basis accidents 1 msv/year 100mSv No limit *AERB safety code on Siting of Nuclear Facilities AERB/NF/SC/S (2014) Plant State Normal operation 1 msv/year Anticipated Operational occurrences Design Basis Accidents Design Extension Conditions without Core melt Design Extension Conditions with Core melt Dose targets at exclusion zone boundary 1 msv/year 20 msv/year following the event, no need for offsite protective measures No need for offsite protective measures except control on food or agriculture limited to small area & time. Dose Target same as for DBA No permanent relocation of population. The offsite Interventions to be limited in area and time Conditions giving rise to large or early releases have to be practically eliminated 18

19 Pre Fukushima CDF For new NPPs < 10E-5/RY LERF For new NPPs < 10E-6/RY For new NPPs Post Fukushima* <10E-6/RY for internal events, power and shut down states <10E-5/RY for internal events and external hazards together For new NPPs LERto be practically eliminated (LERF <10E-7) *As per new AERB safety code on Design of NPPs based on LWR technology AERB/NPP-LWR/SC/D (2015) 19

20 Design CDF LERF EPR 7.1x10E-07/RY 7.7x10E-08/RY AP1000 5x10E-07/RY 6x10E-08/RY VVER ~10 E-07/RY ~10E-08/RY 20

21 These CDF values are typically two orders of magnitude better than most currently operating NPPs Even if severe accident stage is reached, the radiological consequences will be limited in area and time (No urgent need for protective actions in the public domain and no long-term impact on the society) With LERF values well below 10E-7/RY, large or early releases can be considered as practically eliminated 21

22 Can good regulations alone ensure safety? 22

23 External hazards, in particular Tsunami, had not been reassessed in a systematic and comprehensive manner Operating experience (in plant as well as around the world) indicated vulnerability of safety systems to flooding. This did not lead to any significant safety upgrades Periodic Safety Reviews also did not lead to any safety upgrades based on current safety requirements Accident management guidelines were neither comprehensive nor up to date. Operators were not trained and hence were not prepared for implementing the guidelines Regulatory inspections were not adequately structured to verify safety or identify new safety issues 23

24 The IAEA Fukushima report concludes All stake holders, namely, the NPP people, Regulators and the concerned Government Agencies believed (and even mutually reinforced the belief) that the nuclear plants in Japan are adequately safe and a severe accident is unlikely. Consequently, essential safety upgrades were not implemented None of the organisations or their staff challenged the level of safety. There was an air of complacency all round which was detrimental to safety culture 24

25 The regulations need to be implemented with rigour and sincerity by the Industry The Regulatory Body has the responsibility to verify and ensure proper implementation of all safety regulations by the Industry Individuals and organisations need to continuously challenge and re-examine the prevailing assumptions about nuclear safety and the implications of their decisions and actions that could affect nuclear safety It is essential to promote and continuously strengthen safety culture. 25

26 Robust National Nuclear Safety System: Institutional Strength in Depth (A proposal from INSAG) 26

27 The National Nuclear Safety System (NNSS) comprises of all those players who can/should influence nuclear safety. These include the nuclear industry, the regulators, the national/international nuclear safety bodies, the national Governments and other stake holders such as public and media Its main function is to develop, assess, review and continuously improve nuclear safety, which is also an important element of nuclear safety culture 27

28 The main principle of the NNSS proposed by INSAG is the Strength in Depth approach, a concept derived from Defence in Depth for nuclear safety itself. The main features of this approach are Multiple layered approach Independence of layers Diversity, redundancy and separation of functions These features should prevent failure of the entire system either due to single failure or common mode multiple failures 28

29 Level 1 : A strong nuclear industry, who has the primary responsibility for safety Level 2 : A strong nuclear regulator, with a responsibility to oversee and ensure that the nuclear industry maintains the desired level of safety Level 3 : Strong set of stakeholders, includes those who can get affected and also those who can influence the safety at policy level Transparency and Effective Communication are the primary requirements for these levels to function effectively 29

30 Sub Levels Essential elements/mechanisms 1.1 Licensee/Operator Maintain and improve Knowledge, Competencyand Resources Internal review and audit Worker involvement Vibrant Safety Culture 1.2 National/Regional Nuclear Industry Peer pressure and guidance 1.3 International Industry Peer reviews / guidance ( such as WANO, Owners Groups e.g. CANDU, BWR etc.) 1.4 International Organisations Peer reviews (such as IAEA OSART) 30

31 Sub Levels 2.1 Regulatory Authority 2.2 External Technical Experts/Expert Groups/standing panels 2.3 International standards 2.4 International Organisations/ Conventions Essential elements/mechanisms Maintain/improve Technical and Regulatory competence Independence and legal authority Organisation, structure and procedures Accountability to the Governing Body Consultation and advice Policy issues, high level principles Special topics (e.g.seismic hazards, PSA, Digital C&I) Guidance and bench marking IAEA safety standards WENRA reference levels etc. Peer Reviews Convention on Nuclear Safety IAEA IRRS 31

32 Stakeholders Local Government National Government/Parliament Public and Media NGOsand Special Interest Groups Neighbouring countries (if likely to be affected) World community Means of communication (by Industry and Regulator) Routine information and reports Specialissues and events reports Response to request for information Routine/Special meetings Essential Elements Openness and transparency Effective communication Industry/Regulator Leadership and initiative Safety Culture 32

33 For Industry and the Regulator Competence and resources Leadership and initiative Vibrant Safety Culture Questioning Attitude Willingness to listen, explain, adopt/change where necessary Desire to learn, improve and excel Safety : topmost priority under all circumstances Across all layers Openness and transparency Effective communication 33

34 In the post Fukushima era, with the revised regulations, the nuclear plants are expected to be significantly safer However, rigorous implementation of the regulations, continuous vigilance together with high level of safety culture can alone ensure accident free operation of nuclear installations 34

35 Thank you for your attention Contact : chandes@gmail.com 35