The Swedish approach to severe accident management from 1979 onwards

Transcription

1 ASME Workshop on A New Nuclear Safety Construct The Swedish approach to severe accident management from 1979 onwards Lars Högberg Director General (retired) 1. The primary lesson learned after TMI, Chernobyl and Fukushima 2. Swedish conclusions after TMI 3. Design objectives for severe accident management backfits after TMI 4. Swedish actions after Fukushima and EU stress tests 5. Conclusions

2 TMI, Chernobyl, Fukushima: the Primary Lesson Learned All three accidents had their root causes in system deficiencies indicative of poor safety management and poor safety culture in both the nuclear industry and government authorities. TMI: Instrumentation, procedures, training, feedback of operating experience Chernobyl: Design features, respect for tech specs, feedback of operating experience Fukushima: Design basis (feedback of tsunami experience), state ofart accident management procedures and training Thus, maintaining high global standards of safety management and safety culture including a questioning mindset must always have first priority, or else

3 Swedish conclusions after TMI Severe accidents, including core melt, can occur. Should a severe accident occur, a containment with preserved integrity can protect against significant releases to the environment. Swedish emergency planning studies indicated that source terms of several thousand TBq of Cs 137 or more (>1 % of core content) might cause ground contamination calling for evacuation and land usage restrictions with significant socio economic impacts at distances well beyond 20 km from the plant. So, severe accident management should aim at limiting source terms to less than ~150 TBq Source: NRC Parliament endorsed in 1981 an overarching, qualitative cost/benefit assessment of measures to be taken based on socio economic considerations

4 Justification: Hypothetical impact of a TBq Cs-137 release (~ Fukushima) from Barsebäck SOARCA 2012: Peach Bottom SBO unmitigated: ~7 500 TBq Cs 137 Barsebäck NPP (closed in 2005) Population within 40 km radius (solid line yellow circle): Denmark: ~2 million Sweden : ~1 million 3 30 MBq/m2 1 3 MBq/m2 Map source: Google Maps

5 Justification: Hypothetical impact of a 100 TBq Cs-137 release ( FILTRA-type ) from Barsebäck SOARCA 2012: Peach Bottom SBO unmitigated: ~7 500 TBq Cs 137 Barsebäck NPP (closed in 2005) Population within 40 km radius (solid line yellow circle): Denmark: ~2 million Sweden : ~1 million 3 30 MBq/m2 1 3 MBq/m2 Map source: Google Maps

6 Severe accident management measures after TMI Key overall objectives in case of a severe accident: reach a stable state without undue delay, with damaged core covered by water and preserved containment integrity; keep releases of radionuclides that cause long term land contamination below the equivalent of ~150 TBq Cs 137. Specific functional requirements include: Design basis includes: 24h total station blackout; single failure in BWR pressure suppression function (based on lessons learned) Pressure relief shall not need operator action if containment integrity is threatened (but manual relief possible) No political or regulatory approval of venting needed under stress Capability to flood containments above core level (requires venting in BWR) combined with protection of vulnerable containment penetrations against molten core. Appropriate procedures and training Government decisions in 1981 and February 1986 on measures to be implemented Barsebäck (pebble bed filter, operational in 1985) Source: Sydkraft/E.ON Source: Vattenfall AB Remaining ten reactors (multi venturi scrubbers, operational in 1988)

7 Lessons learned: Socio-economic impact of Cs-137 deposits in Sweden in 1986 Total amount of Cs 137 deposited over Sweden: ~4000 Tbq Max. 200 kbq/m 2 European Commission km Total amount of Cs 137 released from Chernobyl: ~ Tbq Source: Swedish Radiation Protection Institute Mapping deposits and implementing appropriate food control measures topped agendas of governments, media and food industry in Europe in most of 1986.

8 Swedish actions after Fukushima and EU stress tests Safety upgrades (and power upgrades) continue based on periodic safety reviews: redundancy, diversity, separation Reassessment of capabilty to cope with extreme natural events (seismic, flooding, weather) Reassessment of I&C response to extreme voltage transients Continuation of work started after Forsmark and other incidents Separate last resort feedwater and emergency power train considered Totally independent and possibly bunkered, also for security reasons Upgrading existing severe accident management capabilities: Long time endurance (>>24h). Simultaneous failures at several reactors. Reassessment of fuel pool cooling capabilities Regulatory decisions on new requirements expected in 2013

9 My conclusions: Robust protection against socio-economic impacts intolerable to both society and industry is justified ACCIDENT TYPE RELEASE OF Cs 137 (TBq) EVACUATED ESTIMATED COSTS million US$ Three Mile Island Core melt << 1 Voluntary short term evacuation ~ of nearby communities Tjernobyl 1986 Runaway fission process > relocated destroying the reactor Fukushima 2011 Three cores severely damaged, probably melted ~ evacuated, prospects to return still unclear after 20 months Present Swedish release mitigation objective for station blackout sequence Core melt <150 Precautionary short term evacuation in the vicinity of the plant? < ? A New Nuclear Safety Construct should include a call for severe accident management preparedness based on Best Available Technology (BAT) with the objective to reach a stable state with preserved containment integrity without undue delay, and keep releases of radionuclides that cause long term land contamination below the equivalent of about 100 TBq Cs 137 in case of a core melt.