Continued efforts to improve the robustness of the French Gen II PWRs with respect to the risks of severe accidents.

Transcription

1 E. RAIMOND, J.M. BONNET, G. CENERINO, F. PICHEREAU. F. DUBREUIL, J.P VAN-DORSSELAERE IRSN Continued efforts to improve the robustness of the French Gen II PWRs with respect to the risks of severe accidents. Safety assessment and research activities /35

2 Introduction In the context of post Fukushima accident, the presentation describes the continuous efforts performed in France to upgrade progressively the French Gen II pressurised water reactors safety features in order to face the risks of any severe accident It reminds some decisions taken after the TMI2 and the Chernobyl accidents and describes the situation in France before the Fukushima accident : progress done on severe accident consequences analysis thanks to recent research activities, improvement of Gen II PWRs safety features, in relation with the periodic safety review process, definition of higher safety levels requirement directly linked to the protection of population in the framework of Gen II PWRs long term operation. The last part of the presentation comments carefully how the Fukushima accident will interfere on all these previous efforts to increase the Gen II PWRs robustness 2/35

3 Introduction Impact of the TMI2 accident for the French PWRs Impact of the Chernobyl accident in France Progress on accident consequences analysis Gen II PWRs improvement face to the risk of severe accident, situation in France before the Fukushima accident Impact of the Fukushima accident in France Conclusion 3/35

4 Impact of thetmi2 accident for the French PWRs (1/3) The TMI accident (28 th of March 1979) a single incident : failure of the normal supply of water to steam generator (SG), after SCRAM and activation of SG emergency pumps, water could not reach the SG because the valves located between the SGs and the pumps were closed (human error before the initiating event) these valves were reopened manually, eight minutes later; however, the RCS pressure has already increased until opening of the pressurizer safety valves After RCS cooling by the SG, one discharge valve of the pressurizer received the order to close but stayed in the open position (second failure), following indications of water level ( full ) in the pressurizer and position of pressurizer valves in the control room (wrong close indication), operators have stopped the water safety injection and unconsciously led to core dewatering and fuel melt. After in-vessel water injection, the reactor vessel luckily survived and the containment has efficiently protected the population despite internal hydrogen combustion 4/35

5 Impact of thetmi2 accident for the French PWRs (2/3) Procedures in normal and accidental conditions : It was recommended to provide operators in control room reliable information on the real state of the reactor and operating procedures able to keep the reactor in long term stable situation. Containment of radioactive material : Main releases were due to the leakage in the auxiliary buildings and it was recommended to limit in accidental conditions possibility of any leakage, to improve the efficiency of the auxiliary buildings containment and to improve the monitoring of radioactivity in these buildings. The operating experience feedback : The accident analysis showed that such a situation could have been avoided if some precursor incidents on other NPPs had been taken into account. At this period, incidents without any real significant consequences were not systematically analyzed. They had no impact on existing procedures, operator training It was recommended in France to develop an efficient organization able to identify any design or operation weakness, precursors of severe accident. The TMI accident has also accelerated the establishment of the French emergency organization with crisis teams able to help for the reactor management, some clarification of the role of the actors and mutual information management of the stakeholders. The need for training was also identified. Beginning of 80 s were developed an accident management plan for each nuclear plant and also specific plans for the protection of populations. 5/35

6 Impact of thetmi2 accident for the French PWRs (3/3) From TMI accident and previous on-going analysis, it was concluded that some additional procedures were needed to manage some accidental situations leading to the loss of some redundant safety systems. Five procedures H were established for the management of such situations : H1: loss of heat sink, H2: loss of normal and emergency steam generator feedwater system, H3: station black-out, H4: mutual rescue of containment heat removal system (CHRS) and safety injection systems, H5: site protection in case of river flooding. In addition, five other procedures were established to limit the accident consequences whatever its cause. U1: use of all water injection means into the vessel for the core cooling, U2: management of containment leakage, U3: mobile equipments to cope with the failure of CHRS or safety injection systems, U4: suppression of leakage paths in the basemat, U5: management of slow containment overpressurization by a filtered containment venting. These procedures U were a precursor of the state-oriented procedures. Specific requirements were defined for all materials used in the H and U procedures (but their robustness to external events is now questioned after the Fukushima accident) 6/35

7 Impact of thetmi2 accident for the French PWRs Impact of the Chernobyl accident in France Progress on accident consequences analysis Gen II PWRs improvement face to the risk of severe accident, situation in France before the Fukushima accident Impact of the Fukushima accident in France Conclusion 7/35

8 Impact of the Chernobyl accident in France (1/1) Seven years after the TMI2 accident, the Chernobyl accident highlighted the huge difficulties to manage the short and long term consequences of an accident with massive release of radioactivity into the environment. This accident had conducted to review accident scenarios that may lead to reactivity accident on PWRs, and to several plant modifications in order to limit the possibility of non borated water injection. Today, in France, the Chernobyl accident is also associated to some communication failures of the French public bodies and has motivated many recent efforts to make the nuclear activities more transparent. This concerns the communication of the emergency organizations but also most of the day-to-day activities. 8/35

9 Impact of thetmi2 accident for the French PWRs Impact of the Chernobyl accident in France Progress on accident consequences analysis Gen II PWRs improvement face to the risk of severe accident, situation in France before the Fukushima accident Impact of the Fukushima accident in France Conclusion 9/35

10 Progress on accident consequences analysis (1/5) Research programs initiated after the TMI2 accident In France, after TMI2, research programs have been in several directions: reviewing the plant features able to limit the contamination outside the containment, improving the understanding of the phenomena that can occur during a core meltdown accident through a program of analytical and global experiments, development of a system of computer codes, now known as ASTEC and jointly developed by IRSN and GRS, using validated models with the above experiments. 10/35

11 Progress on accident consequences analysis (2/5) National research programmes (examples) Thermal-hydraulics : CATHARE 2 (best-estimate thermal-hydraulics) code development in relation with a broad (analytical and integral - BETSHY) experimental program; plant simulators that contribute to the definition and validation of emergency operationg procedures (EOPs) and to operators training, the behavior of a damaged core and behavior of fission products released by the damaged fuel and transported in the primary circuit into the containment building (PHEBUS CSD (core severely degraded), and later PHEBUS PF (fission products), detailed study of fission products release from an irradiated fuel rod subject to a temperature rise: HEVA and then IRSN VERCORS experiments made by the CEA-Grenoble (these tests are now supplemented in the frame of ISTP by tests in the installation VERDON operated by the CEA- Cadarache a first VERDON experiment has just been succeed by the end of september 2011 ), detailed study of iodine chemistry in the primary circuit and containment; this study is still ongoing at this time as part of an international program of analytical experiments (International Source Term Program - ISTP), study of the hydrogen stratification and of the effects of an hydrogen explosion in the containment (TONUS CFD code development supported by analytical experiments), study of mitigation safety systems in the frame of H2PAR experiments to assess the efficiency of passive autocatalytic hydrogen recombiners or FUCHIA experiments to validate the efficiency of sand filters used French PWRs containment venting system. study of steam explosion induced by corium water interaction (MC3D CFD (multiphasic) code development supported by analytical experiments), corium-concrete interaction programs (which are continued with the VULCANO experimental program operated by the CEA-Cadarache and supported by IRSN, EDF, SUEZ-TRACTEBEL and EC). 11/35

12 Progress on accident consequences analysis (3/5) IRSN has also joined research programs conducted on severe accidents in other countries (Europe, United States, Russia, Japan), or benefited from results, which have completed the knowledge base for validation of simulation tools. programs on the corium-concrete interaction: BETA (Germany), ACE and MACE in US, programs in pile on the degradation of fuel rods: PBF programs (United States) and FLHT (Canada), programs out-of pile (with simulated non radioactive fuels: CORA (Germany) and, more recently, QUENCH (Germany) for the early degradation of the core and reflooding, various analytical programs financed by the European Union (Framework Programs) like DISCO for the melt dispersal after lower head failure and direct containment heating (DCH), programs performed by ISPRA European Joint Research Center on steam explosion (FARO, KROTOS), OECD programs (MASCA and RASPLAV by the Russians to study the properties of the corium relocated in vessel bottom, OLHF United States for the mechanical failure of the bottom of the vessel and currently SERENA for interaction between the corium and water), BIP for iodine interaction with paint or THAI for iodine and hydrogen behaviour in containment, MCCI in Argonne US to complete MACE results). IRSN has also recently promoted the development of the SARNET Severe Accident European network of excellence in the 6th and 7th FwP of the European Commission. 12/35

13 Progress on accident consequences analysis (4/5) Codes development (2 classes of severe accident codes) integral codes to simulate the overall nuclear power plant (NPP) response (reactor coolant system, the containment, and the source term to the environment) : ASTEC : Accident Source Term Evaluation Code (developed by GRS and IRSN and linked to SARNET) Released to about 40 organizations in 20 countries, most of them in Europe, but also some in Russia and Asia, All types of severe accident scenarios can be simulated for PWRs. Validation, source term modeling dedicated codes to simulate precisely a single phenomenon Structures mechanics (CAST3M, LS-DYNA, ABAQUS), CFD (TONUS : hydrogen, MC3D : steam explosion/dch) 13/35

14 Progress on accident consequences analysis (5/5) Accident consequence analysis : simulation of accidents Existing simulations tools allow today the calculation of many beyond design transients on NPP. It allows an extensive verification of existing EOPs and plant automatisms and the development of detailed realistic PSAs. In particular, the development of L2 PSA at IRSN for 900 MWe, 1300 MWe PWRS and now EPR help IRSN to develop precise knowledge on how the NPPs would behave in beyond design conditions and to orient the safety reviews on topics where safety improvements are possible. This activity completes the analysis performed by the utility and results are periodically discussed during each periodic safety review (PSR). Simulation of severe accident remains a difficult tasks and sharing experience at international level is particularly recommended Some differences were identified (within SARNET) in the development and applications of L2 PSA in European organizations. In response, the European Commission has promoted a collaboration project in the 7th Framework programme to develop bestpractices guidelines on L2 PSA. This is today the ASAMPSA2 project, which gathers 21 Partners and has promoted exchanges with more than 100 organizations in the world. The guidelines will be published beginning of 2012 and proposes many solutions to study numerous severe accident issues. 14/35

15 Impact of thetmi2 accident for the French PWRs Impact of the Chernobyl accident in France Progress on accident consequences analysis Gen II PWRs improvement face to the risk of severe accident, situation in France before the Fukushima accident Impact of the Fukushima accident in France Conclusion 15/35

16 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (1/7) Plant design upgrades in relation with periodic safety review The knowledge gained from the research, code development and accidents consequences analysis activities have been progressively ( slowly ) used to improve the NPPs robustness in the case of a severe accident. Some examples are provided hereafter 16/35

17 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (2/7) First plant design upgrades Hydrogen recombiners (all Gen II PWRs in 2007) U2 procedure (containment isolation) Filtered containment venting (sand filter) (all Gen II PWRs in the 90 s) 17/35

18 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (3/7) Severe accident management guidelines Severe accident management guidelines (SAMG) have been developed by EDF since 1990 s with the objective to define actions based on the containment protection. Whereas in the emergency operating procedures, before SAMG application, the main objective is to ensure the short and long terms core cooling. These SAMGs reflect the state-of-the-art knowledge from accident consequences analysis and are a mirror of all improvements done since the TMI2 accident. The latest versions of French EDF SAMG for Gen II PWRs include some specific recommendations regarding in-vessel water injection to limit the risks on the reactor containment, 18/35

19 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (4/7) Plant design upgrades decided during the 3rd PSR of the 900 MWe PWRs ( ). They are now being installed by EDF. H2 detection Reinforcement of RCS safety valves opening Material access reinforcement Vessel rupture detection 19/35

20 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (5/7) Additional safety improvements for long term operation Since 2010 are examined the conditions needed to allow a safe long term operation of Gen II PWRs in France : ageing effects on NPP systems and components must not affect the plants safety, Reinforced safety objectives are discussed with objective to reduce the difference between safety level of Gen II and Gen III (EPR) PWRs. For safety objectives, IRSN deems necessary to work on both core damage prevention features and limitation of radioactive release (for design and beyond design basis accidents). 20/35

21 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (6/7) Additional safety improvements for long term operation Concerning the mitigation of the consequences of any severe accident, IRSN promotes the practical application of all knowledge gained in the R&D programs since the TMI2 accident. The demonstration that situations leading to large early release are now almost impossible, must be credible (hydrogen combustion, steam explosion, direct containment heating, containment bypass..). In addition, IRSN promotes a drastic reduction of possible late release: by an upgrade of the existing sand filter containment ventingfiltration system, to limit as far as possible iodine release, by a plant back-fit able to make almost impossible any containment failure by basemat penetration (MCCI), as, for example, systems that can provide some cooling water below and above the corium in the vessel pit PWRs. 21/35

22 Gen II PWRs improvement face to the risk of severe accident situation in France before the Fukushima accident (7/7) Remaining issues and on-going R&D activities Highest priority safety issues are identified within SARNET and shared by IRSN. Damaged core reflooding : simulation (ICARE-CATHARE & ASTEC) and experimental (PEARL) programmes, MCCI (corium stratification, coolabilily ) Steam explosion (SERENA, MC3D) Risk of hydrogen combustion (CFD analysis, OECD THAI, SETH programmes ) Radioactive release from fuel, RCS and containment (ISTP, STEM, BIP2, THAI2 ) 22/35

23 Impact of thetmi2 accident for the French PWRs Impact of the Chernobyl accident in France Progress on accident consequences analysis Gen II PWRs improvement face to the risk of severe accident, situation in France before the Fukushima accident Impact of the Fukushima accident in France Conclusion 23/35

24 Impact of the Fukushima accident in France (1/8) The Fukushima accident gravity The consequences of the beyond design Fukushima Daiichi accident are extremely severe in Japan but one should remind that they could also have been really worst : they have been minimized by the meteorological conditions (direction of wind toward sea) during a large part of the atmospheric radioactive releases period, the melt of spent fuel in the desactivation pools would have induced more severe consequences (cliff edge effect). The lessons to be taken from this accident in France have to be defined on the basis of the worst consequences that could have occurred. 24/35

25 Impact of the Fukushima accident in France (2/8) Three lines of defense have failed : the safety margin at design, the accident management and the containment of radioactive materials. This type of initiating of event (earthquake and tsunami) was of course included in the Fukushima Daiichi plant design, but the amplitude of the event was beyond the design and has conducted to a cliff edge effect on the consequences. The accident management procedures, the available equipements and maybe the staff were not sufficient to maintain the long term cores cooling (despite all efforts made by the operator teams) during these extreme conditions Cores melt after station black-out on Fukushima reactors 1,2,3 have led systematically to hydrogen combustion in the secondary containment and large fission products release (limited only by some retention in the suppression pool). In France (for IRSN) : a need for a review of the defense-in-depth concept and the nuclear safety rules (even if PWRS would behave differently) 25/35

26 Impact of the Fukushima accident in France (3/8) Beyond design external events must be considered external events depend on the plant localization ; the choice of a nuclear site should be done to minimize the possibility of external events and plays a key role on plant safety, the range of uncertainties on the amplitude of external hazards has to be carefully taken into account in the design, cliff edge effect on consequences of external events on a NPP must be analysed, combination of external events is crucial (here seismic event along with flooding) and must be considered in the design, induced indirect effect of external events must be taken into consideration (earthquake along with explosion, fire, internal flooding ) 26/35

27 Impact of the Fukushima accident in France (4/8) Beyond design external events and accident management (the following issues may have been underestimated) The management of very long term loss of cooling and electrical power Diversification of safety systems and separation of the localisation of redundant systems Requirements applied on systems used for the ultimate management of the NPPs (e.g. seismic qualification not required for severe accident equipments), Accidents concerning a whole nuclear site In case of a severe accident, all dependencies between reactors and spent fuel pools should be considered to avoid any cliff edge effect. The feasability of accident management after radiological contamination All means to manage an extreme / beyond design event concerning a NPP or a site should be designed based on simplicity (easy to use) and robustness criteria. 27/35

28 Impact of the Fukushima accident in France (5/8) The stress test review will conduct to significant safety improvements for the French NPPs Additional measures to help management of beyond design (extreme) events : water supply, emergency teams and materials, procedures in case of SBO for a whole site, Modification of the defense in depth concept ; requirements on the last lines of defense will be reinforced (e.g seismic qualification) Acceleration of installation of previously decided plant upgrades (e.g. RCS safety valves reinforcement) Precise list of improvement to be provided by EDF in June 2012 MAJOR POINT IS THE DEFINITION OF A LIST OF EQUIPEMENTS THAT COULD BE ROBUST IN CASE OF BEYOND DESIGN ACCIDENT AND AVAILABLE TO GARANTY THE 3 SAFETY FUNCTIONS (Core criticality, core cooling, containment). 28/35

29 Impact of the Fukushima accident in France (6/8) Defense in depth concept / before & after Fukushima (IRSN views) After the Fukushima acc. Higher requirement on level (3 H), 4 and 5 for beyond design accidents Before the Fukushima acc. Lower requirement on level (3 - H), 4 and 5 for external events due to the provision existing at level 1 and 2 to avoid any core damage 29/35

30 Impact of the Fukushima accident in France (7/8) Additional research activities The behavior of the Fukushima Daiichi reactors 1, 2 and 3 (core melt, hydrogen combustion ) in a long term station black-out situation could be predicted with existing simulation tools. The order of magnitude of the release could also be predicted (100 % of volatile fission products (iodine, cesium,..) considering total core melt and some limited retention factors in suppression pool (wetwell) ) Nevertheless, only a precise description of all events by TEPCO, associated to accident modeling with integral severe accident codes will help to understand all details of the accident. The precise status of the corium (in-vessel, ex-vessel) in each of the reactors 1, 2 and 3 will be used to check the relevance of the modeling of codes like ASTEC (corium relocalisation, vessel rupture, MCCI if any). The construction of a common and shared database for studies and codes calculations of the Fukushima accident (plant characteristics, geometry, materials, timing of the accident events, radioactivity measurements, etc.) is a logical need for deepening the involved phenomena. 30/35

31 Impact of the Fukushima accident in France (8/8) Additional research activities Some questions stay also open like the possibility of hydrogen production (water radiolysis) in a boiling spent fuel pool to understand the Fukushima accident but also to assess possible risks for other facilities. Some specific research on air oxidation and ad-hoc 3D modeling of convecting flow (air, steam, water) in a spent fuel pool after partial dewatering may be useful to precise management measures. Results may help to define which design of storage conditions can limit the consequences of any spent fuel pool dewatering. 31/35

32 Impact of thetmi2 accident for the French PWRs Impact of the Chernobyl accident in France Progress on accident consequences analysis Gen II PWRs improvement face to the risk of severe accident, situation in France before the Fukushima accident Impact of the Fukushima accident in France Conclusion 32/35

33 CONCLUSION (1/3) After the TMI2 accident, many efforts on research and accident consequences analysis have been achieved in France. This has conducted to substantial plants safety upgrading. Nevertheless, taking into account the conclusions of recent risk assessment activities, NPP robustness improvement on accident prevention and severe accident mitigation was considered as a major issue in France before any decision on long term operation of Gen II NPPs in France. A specific working program including NPP improvement is in discussion between EDF and the French Safety Authority supported by IRSN. This working program included some NPP safety improvements like additional water make-up systems or double-containment ventilation system reinforcement. 33/35

34 CONCLUSION (2/3) For IRSN, the Fukushima Daiichi event confirms the interest to ask for safety improvement on Gen II reactors in relation to long term operation. The accident also clearly highlights a need of additional efforts to identify possible cliff edge effect in case of beyond design events (especially external events). The definition of additional accident management procedures and means to secure a reactor (or a site) whatever the conditions will be a major consequence for the French NPPs. In a second step, some complements on the existing defensein-depth approach are now expected : additional requirements to define lines of defense against adverse consequences of beyond design situations. 34/35

35 CONCLUSION (3/3) The need for specific additional research activities after the Fukushima accident seems to be limited to some specific issues (for example spent fuel pool behaviour in case of long term loss of cooling). Nevertheless, the development and the validation of severe accident codes need to be continued as far as possible. They help in simulating NPP response in case of accident and in final, to check that the protection of populations is sufficient whatever the situation. In future, the detailed analysis of the Fukushima accident, from common and shared database for (plant characteristics, geometry, materials, timing of the accident events, radioactivity measurements, etc.) may contribute to the validation of existing simulation tools. 35/35