module 08 fukushima dai-ichi accident 11.3 - oektg.at 08 fukushima dai-ichi accident 11.3.2011 ......
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Module 08 Fukushima
Dai-ichi Accident 11.3.2011 1.10.2016
Prof.Dr. Böck Technical University Vienna Atominstitut Stadionallee 2, 1020 Vienna, Austria ph: ++43-1-58801 141368 [email protected]
Contribution of Dai-ichi NPPs to electric power production in Japan
• ~30% of Japanese electrical grid supplied by 54 reactor units: 30 BWRs / 24 Pressurized Water Reactors, (PWRs)
• ~17% of electrical grid supplied by the 30 BWRs • ~2% of electrical grid was supplied by the 4 damaged BWRs at Fukushima Dai-ichi
Fukushima Dai-ichi npp Status prior to Earthquake / Tsunami Event
Six Boiling Water Reactors (BWRs) at Fukushima Dai-ichi NPP
• Unit 1: 439 MWe, 1971 (in operation prior to event)
• Unit 2: 760 MWe, 1974 (in operation prior to event)
• Unit 3: 760 MWe, 1976 (in operation prior to event)
• Unit 4: 760 MWe, 1978 (shutdown prior to event)
• Unit 5: 760 MWe, 1978 (shutdown prior to event)
• Unit 6: 1067 MWe, 1979 (shutdown prior to event)
Scram Levels
• Normal earthquake scram level: 0,15 g
• Design basis acceleration level: 0,45g, in this case all safety functions must stay operational
• Actual ground acceleration at site: 0,56 g - 20% above design basis
Status upon Earthquake: March 11, 2011 11 operating reactors automatically shut‐down due to ground acceleration exceeding the reactor seismic trip settings • Onagawa Units 1,2,3 • Fukushima (I) Dai-ichi Units 1, 2, 3 • Fukushima (II) Dai-ni Units 1, 2, 3, 4 • Tokai (II) (Tokai (I) is decommissioned)
3 reactors were shutdown prior to earthquake for periodic inspection • Fukushima Dai‐ichi Unit 4 (reactor was defuelled), Unit 5
and Unit 6
Simplified view of BWR reactor vessel and containment design
DW: Drywell
WW: Wetwell
SFP: Spent Fuel Pool
SCSW: Secondary Concrete Sheild Wall
RPV: Reactor Pressure Vessel
Drywell: Bulb-shaped : 30 mm steel,
during operation filled with N2
Wetwell: Torus with about 3000 m3 cold
water
Fuel elements partially uncovered and start to heat up
Containement designed for 4 bars pressure exceeds 8 bars, pressure relief through relief valves
Fuel storage pool damaged, water level decreases, fuel elements overheat are damaged and release fisson products
H2 explosion below reactor roof, release of fission products to the
environment
Accident progression 1
• Emergency battery power provided core cooling for about 8 hours (Unit 1), after which the batteries were discharged. Decay heat in the reactors and spent fuel ponds could no longer be removed
• Offsite power could not be restored, delays were encountered in obtaining and connecting portable generators
• Reactor temperatures increased, water levels in the reactor vessel decreased, eventually uncovering and overheating the Unit 1 core.
• Hydrogen was produced by oxidation of zirconium metal fuel cladding, zirconium-water reactions in core start above 1000ºC
Accident progression 2
• Spent fuel pool temperature rises in Unit 4 (containing all the reactor fuel), as water is lost from possible pool wall damage, 150 tons seawater pumped in, then fresh water pumped in.
• Unit 4 fire breaks out, April 11, that later extinguishes, with subsequent significant fire damage.
• April 2011 core damage status (subject to revision): • Unit 1: 55% Unit 2: 35% Unit 3: 30%.
Accident progression 3
• Operators vented the reactor pressure vessel to relieve pressure, steam and hydrogen were discharged to primary containment (drywell), causing primary containment temperatures and pressures to rise
• Hydrogen explosions occurred (March 12, Unit 1: March 13, Unit 3: and March 15, Unit 2) while venting secondary containment, likely ignited by a sparks and hydrogen encountering free oxygen
• Hydrogen re-combiners, designed to burn vented hydrogen, did not operate probably because of power requirements
Unit 1 Unit 2 Unit 3 Loss of AC power + 51 min + 54 min + 52 min Loss of cooling + 1 hour + 70 hours + 36 hours Water level down to top of fuel* + 3 hours + 74 hours + 42 hours
Core damage starts* + 4 hours + 77 hours + 44 hours
Reactor pressure vessel damage* +11 hours uncertain uncertain
Fire pumps with fresh water + 15 hours + 43 hours
Hydrogen explosion (not confirmed for unit 2)
+ 25 hours service floor
+ 87 hours suppression chamber
+ 68 hours service floor
Fire pumps with seawater + 28 hours + 77 hours + 46 hours
Off-site electrical supply + 11-15 days
Fresh water cooling + 14-15 days
Event sequence following earthquake (timing from it: 14:46, 11 March)
Fuel assemblies: 4m long, About 60 rods per assembly Block 1: ca 24 000 fuel rods (70 t Uran) Block 2-4: ca 33 000 Fuel rods (95 t Uran)
Spent Fuel Pools Dimensions in m
Num,ber of Fuel Assemblies
Heat input if not cooled
Depth in m
Unit 1 12 x 7 900
Unit 2 12 x 10 1240
Unit 3 12 x 10 1220
Unit 4 12 x 10 1590 appr. 3 MW
12*
* Above fuel top: 7m, after accident : - 5m ** Evaporation: 100 m3 per day Normal temperature: 30 C, max: 85 C
Reactor decay heat problem
Decay heat after shutdown (Dai-ichi Units 2 and 3)
Unit 4 with over 1000 fuel rods in the spent fuel pool, the decay heat
could boil off about 100m3 of water per day, if the water was at
boiling point
Time Percentage of Full power Decay Heat MW_thermal 1 sec 7 % 167 1 day 1 – 2 % 47 1 year 0.2 % 5
Reactor decay heat problem
• Radioactive isotopes (fission products) in the fuel produce radiation as they decay (gamma, beta, and alpha radiation)
• This decay radiation deposits most of its energy in the fuel (decay heat), about 7% of thermal power, immediately after a reactor is shutdown
• The decay heat must be removed at the same rate it is produced
or the reactor fuel core will heat up and in the absence of any cooling the fuel may melt
• The removal of decay heat is performed by various cooling systems that provide water flow through the reactor core with the heat being transferred to heat exchangers and the ultimate heat sink of the sea.
• At Fukushima the integrity of the cooling systems was compromised by the tsunami and made it difficult for the operators to sustain decay heat removal
Emergency Actions 1
• Operating and emergency support staff used portable
generators and portable pumps to inject seawater into the reactor vessels of and primary containments of Units 1, 2, and 3 via mobile fire trucks
• This had the effect of flooding the primary containment
to cool the reactor vessel and any core debris that may have been released into primary containment.
• The seawater was the only cooling possibility but
seawater effectively would render reactors unusable due to corrosion of fuel, coolant piping and reactor vessel, as well as leading to salt accumulation and clogging
Emergency Actions 2 • A general emergency was declared in response to the initial
events at Unit 1, with evacuation of public within 20-30 km of the plant: about 200,000 people evacuated
• Electrical power and cooling functions eventually restored,
but much equipment is damaged and status unknown • Offsite radiation and contamination, as a result of venting
and the uncontrolled releases from hydrogen explosions and fires
• Potassium iodide pills distributed but not administered to public to minimize I-131 uptake in the thyroid
• Measures implemented to control affected contaminated food
Summary of a Complex Accident 1
• All 13 effected Japanese NPP units withstood a massive earthquake, with likely minimal damage, but 4 units were overwhelmed by the tsunami
• Seawall was 5.7 m but the tsunami was 14 m (the design basis was underestimated)
• Significant reactor core fuel damage to 3 units (full meltdown predicted at Unit 1), damage to fuel in some fuel ponds (Unit 4 undamaged, Unit 3 damage due to explosions)
• Loss of site emergency diesel generators on site was key.
• 3 staff fatalities (explosion and heart attack), 15 injured by hydrogen explosions and tsunami
Summary of a Complex Accident 2
• 1 indirect death, reportedly from suicide, of a local farmer after farming restrictions
• 4 hydrogen explosions in reactor buildings caused very significant structural and reactor equipment damage, fires in Unit 4
• At plant site: About 20 000 involved during accident 135 workers 100-150 mSv 23 workers 150-200 mSv 3 workers 200-250 mSv 6 workers up to max 678 mSv (500 mSv is the internationa allowable short term dose for emergency workers taking life- saving actions)
• Beyond plant site: UNSCEAR Report April 2014: People of the
Fukushima area are expected to receive less than 10 mSv due to the accident over their whole lifetime compared with 170 mSv lifetime does from natural background radiation
Summary of a Complex Accident 3
• Over 200,000 people evacuated from the area, ground contamination up to 50 km from site
• Huge Japanese economic and financial consequences
• Nuclear industry worldwide: future increased safety vigilance and costs; international scrutiny (IAEA)
• Nuclear ‘renaissance’ projects worldwide now on hold or being reviewed
• TEPCO has said it may take the rest of the year to bring the plant back under control
• The world’s worst industrial accident (Bhopal) far exceeded Fukushima in human toll, but likely not in economic consequences
Major Accident Contributors
• Underestimation of maximum earthquake
• Tsunami height • Location of safety related systems in
buildings (Diesel) • No backfitting of pressure relief pipes • No filters in pressure relief pipe • No H2 recombinators
Stress Tests: Participation
All 14 EU Member States that operate nuclear power plants, plus Lithuania, Switzerland, and Ukraine
Stresstests for NPPs initiated in Europe
• Total loss of all power supply (extern, emergency diesel, batteries)
• Total loss of cooling capacity including spent fuel ponds
• Effects of earthquake on water environment (ocean, river, dams, hydrostations upstreams, mud slides)
• Human deficiencies • Aircraft crash and terrorist attacks considered
through other national security actions
Stresstest Time Schedule
• Until Sept. 2011 all operators had to check their NPP according to the stresstest conditions
• NPP operators reported to the National Regulatory Body which evaluated the results and reported to the EU commission by December 2011
• The EU Commission then recommended further steps to improve the safety of NPP‘s
• Web pages dedicated to public engagement: www.ensreg.eu/EU-Stress-Tests/Public-
engagement
Stress Tests: Follow-up • On October 4th 2012 the EC published its position on the stress
tests • Totally 145 NPPs have been analysed • The risk analysis for earthquake and flooding should be based for
all NPPs on the 10 000 year time frame • On-site seismic instruments • Containment filtered venting systems • Equipment to fight severe accidents • Backup emergency control room • Nuclear insurance and liability should be harmonized in Europe
Conclusion • Acivity release to the environment especially during
explosions • Depending on meteorology local short term increase of
activity without reaching international emergency radiation limits outside NPP site
• Iodine pills distributed but not administered • Activity continuously measured by radiation network
both in air and seawater • No radiological short- or long term effects exspected • Cs-137 and I-131 were measured also in Austria with
high precision equipment (€ 150 000)
What you should remember
• All operating NPPs were shut down by earthquake sensors without any damage
• The tsunami destroyed all possible power supply systems
• In this case core temperature increases rapidly and fuel elements were destroyed both in the core and in the spent fuel pool as no cooling was possible
• Mainly Iodine-131 and Cs-137 were released • Hydrogen explosion took place due to Zr-H2O reaction • No excessive overexposure of public • Four staff members were killed by mechanical accidents