New Frontiers for Nuclear Power Plants Safety

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  • 1. NEW FRONTIERS FOR NUCLEAR POWER PLANTS SAFETY dr. Roberto Adinolfi Chief Executive Officer, Ansaldo Nucleare SpAGenova, 30 ottobre 2013

2. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013NEW FRONTIERS FOR NUCLEAR POWER PLANTS SAFETYHow is it possible to protect community against man-induced risks e.g. from Nuclear Power Plants? The key principles of nuclear safetyLessons learned from the Fukushima accidentWhat is acceptable as man-induced risk?New plants: where are we goingIs it possible to do better? 3. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013THE KEY PRINCIPLES OF NUCLEAR SAFETY The development of a civil nuclear industry introduced a new dimension in industrial safety: Not only protection of the investment Not only protection of the operators but also protection of the general public A nuclear power plant shall not release radioactivity to the general public, even in accident condition, for more than a fraction of natural sources Proper barriers against the release of radioactivity shall be deployed, eg: The fuel pin The pressure boundary of the primary cooling system The containment, which envelopes all the radioactive systems of the plant The defence in depth principle: the barriers shall be designed in such a way not to be impaired at the same time following an accident 4. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013THE KEY PRINCIPLES OF NUCLEAR SAFETY (contd)According to the above principles, emergency plans for evacuation are required by licensing authorities to limit the residual risk for the general public.After Three Miles Island, the deterministic analysis is integrated and supported by probabilistic risk assessmentProtection against external events: Definition of worst case (Design Basis) event, on the basis of statistics and /or probabilistic analysis, with frequency of occurence in the range 10-3 10-4 ev/yr Design of structures and components to withstand, on deterministic bases, the worst case event Limited recourse to probabilistic analyses for external events 5. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013LESSONS LEARNED FROM FUKUSHIMA ACCIDENT (A thorough description of the Fukushima sequence of events is presented in the memory of Mr. Alessandroni in a separate session) The actual Tsunami was largely over the Design Basis event (the actual Eartquake* was only marginally higher that the Design Basis one)The deterministic defenses against Tsunami (e.g.a seaside wall of 6 m) were therefore totally inadequate towards waves of 13-15 m (according to present understanding, seismic input didnt create problems to structures/systems integrity, but to the offsite power supply)The flooding caused by the tsunami was a common mode failure of (almost) all the electric power onsite supply systemsSlipping area 500x200 km(*) magnitude 9, the fourth largest ever recorded worldwide 6. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013LESSONS LEARNED FROM FUKUSHIMA ACCIDENT (cont.d) Recovery actions to reinstate core and spent fuel cooling were made very difficult from the extensive damages and the psycologycal situationContainment systems in Units 1, 2, 3 and 4 were subsequently impaired by explosions caused by hydrogen generated from core melting processesDefense in depth didnt work properly Evacuation of people (113.000 persons) from the 20 km radius area was effective in limiting the early exposure of the publicOn-site testimony: As tremendous aftershocks occurred, with our full face masks still on, we frantically headed off to the upper ground. While laying down cables at night, entailing the search of penetrations and terminal treatment work, we were terrified that we might be electrocuted due to the outside water puddles. 7. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013A FEW CONSIDERATIONS Fukushima has been a very dramatic natural disaster, affecting a large number of infrastructures (civil buildings, roads and bridges, distribution networks, industry plants, dams etc.). All this resulted in almost 20.000 immediate deaths:it is a fact that, out of them, none was related to the nuclear accidents This was largely due to the defense in depth approach, which offered protection at different levels, even if there were flaws on how it had been implementedThese flaws were mitigated by the evacuation of a significant number of people, which after one year are still living out of their home, even if large part of the restricted area is no longer at risk from a purely scientific point of view. 8. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013WHAT IS ACCEPTABLE AS MAN-INDUCED RISK? Preventing accidents is the first requirement for any risky industry or infrastructureProtecting against the consequences of any credible accident, either internally generated or due to an external event, has been generally considered an adequate measure for plant safetyNuclear industry , with its defence-in-depth approach, went already further: at least for internal events, the goal becomes to mitigate the consequences (i.e. the releases to the environment) even in the case of beyond design bases events. (Note that this is not so common in other risky industries)Fukushima shows the need to consistently extend this approach also to severe natural events with very low probability of occurence This was the goal of the stress tests conducted in Europe 9. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013WHAT IS ACCEPTABLE AS MAN-INDUCED RISK? (contd) Is it enough to limit the direct consequences even of very unlikely events? Fukushima persisting evacuation (as it is also at Chernobyl) creates a strong sentiment of inadequate protectionThe new demand seems to be to limit, or to avoid, socio-economic consequences which add on the direct consequences of the initiating eventThis is true also for other risks than nuclear. In Emilia, the structural failure of industrial buildings was perceived as unacceptable not only in terms of human losses but even in terms of loss of jobs.Since the recovery time to eliminate those consequences becomes a crucial factor for acceptability, nuclear is particularly weak (no threeshold approach to long term effects) 10. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013NEW PLANTS: WHERE ARE WE GOING? Generation III reactors The design of these advanced plants was based on the lessons learned from Three Miles Island and Chernobyl accidents and is largely based on a probabilistic approach to safetyThey are a consistent step forward in the technology, extensively tested and reviewed by safety authorities through a ten years long processTwo main types of plant design approach: Evolutionary approach: improvement of redundancy and independence of standard safety features (EPR from AREVA) Alternate approach: extensive use of passive safety features to prevent common mode failures and/or operator errors (AP1000 from Westinghouse) Both types are currently under construction (4 EPR and 8 AP1000) 11. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013NEW PLANTS: WHERE ARE WE GOING? Generation III reactors: some significant steps forward Improved Defense in Depth the containment system is designed to withstand core melt scenarios through: retention & cooling of corium (in vessel or through core catcher) Hydrogen control (igniters and/or recombiners) Independent cooling systems (totally passive in the case of AP1000) so ensuring independence towards other barriers Lower risk of Total station Blackout The Ap1000 has no need of diesels to withstand an accident: only batteries are required, with a capability of at least. Lower dependance on operator action The Ap1000 doesnt require any operator action for 72 hours after an accident. 12. CPEXPO COMMUNITY PROTECTION Genova, 30 0ttobre 2013IS IT POSSIBLE TO DO BETTER? Generation III reactors are addressing many of the safety issues raIsed by the Fukushima accident. Acceptability requires reduced socio-economic consequences There are at least two areas of improvement for the future: 1. Better protection against natural hazards Probalistic approach to be extended to external events cliff edge effects to be avoided, i.e. no sharp increase in consequences for limited variations in the intensity of the phenomenon 2. Limited impact outside the fence Improved containment performances in terms of radiological releases Improved post accident monitoring/emergency management Recovery strategies for extreme situations Clear criteria for evacuation/sheltering measures 13. Thank you for your attention!