Transcript Nuclear Reactor Disasters - Czech Technical University in

Nuclear Reactor Disasters
Chernobyl
1986
Three Mile Island
1979
Andrew Cornwall
Chernobyl
• Worst accident ever in the history of Nuclear power
• Released more than 100 times the radiation produced by the atom
bombs of Hiroshima and Nagasaki
• Affected Western Soviet Union, Eastern, Central and Northern
Europe, and Eastern and Northern America
• 336,000 people evacuated and resettled elsewhere
Where is the Chernobyl Plant?
• Ukraine
• 18km northwest of Chernobyl town
• 110km north of Kiev
Power Plant
• 4 reactors of type – RBMK-1: now obsolete class of graphite
moderated nuclear reactor
• Reactor capacity – 1Gigawatt (total plant capacity: 4 Gigawatt)
• Provided 10% of Ukraine’s electricity at time of accident
• 2 additional reactors under construction at time of accident
What Happened?
Series of events in Reactor 4 resulting in:
• Catastrophic “Steam Explosion”
• Nuclear meltdown
• Graphite fire
Series of Events - April 25th 1986
•
11pm: Control rods were lowered to reduce reactor output for planned
turbine test) BUT too quickly - almost complete shut down
•
1am: control rods raised to increase reactor activity for the test (12%)
•
1:23am: Reactor overheats; water coolant turns to steam
•
6 control rods left; minimum safe number = 30
•
Emergency shut down button pressed
•
Control rods re-inserted BUT fault causes power surge in reactor; Out
put:100 times normal
•
Fuel pellets explode; roof blown off; air sucked in causing fire
Immediate Aftermath
• Area evacuated, but quite slowly - “exclusion zone”
• Tragedy made worse by poor preparation, equipment and
assessments
• Radiation estimated at 20,000 Rontgen/hr (lethal dose = 100 R/hr)
• True radiation unknown
• Fire burned until helicopters
extinguished it by dropping
water, sand, lead and boron
• Radioactive cloud observed
Clean-up
• Liquidators sent in to open sluice gates to vent reactor water
• Worst radioactive debris collected in remains of reactor core
• Covered with bags of sand,
lead and boric acid
(5000 tonnes in first
week after explosion)
• Giant concrete sarcophagus
erected to seal off reactor
and its contents
Ecological Effects
•
Radioactive cloud floated in easterly direction
•
Radiation travelled as far as Sweden (1100km)
•
Initial Soviet Union reports: 60% contamination in Belarus
•
River Pripyat and Dnieper river-reservoir system
contaminated (reduced after initial period)
•
Fresh water fish contaminated
to several times the safe
limits (reduced after
initial period)
•
Pine forest within 4km
radius turned
ginger brown
and died: Red Forest
•
“exclusion zone”
became wildlife haven
Human Effects
•
336,000 people evacuated and resettled
•
237 suffered
from acute
radiation sickness
•
31 deaths within
3 months
•
9,000 cancer deaths expected as direct
result of radiation exposure
•
4,000 thyroid cancer
cases among children by 2002
Causes?
• 1986 IAEA Report: Plant Operators to blame
• 1991 Valeri Legasov:
Reactor design flaws to blame
• 1993 IAEA Report:
revoked original report and placed
blame with flawed reactor design
Flawed Reactor Design
• High void coefficient: weaken convection currents
• Graphite tipped control rods: increase activity for short period
• Vertical water channels in core: temperature gradient in core
• Partial containment measurements to save costs
• Operational for 1 year – stored fission by products
• Reactor vessel warped under intense heat, preventing insertion of
control rods
Long Term Aftermath
• Construction of reactor 5 and 6 terminated
• 200m of concrete built to isolate contaminated reactor from
operational buildings
• Reactors 1,2 and 3 continued to operate due to energy shortage in
Ukraine
• 1991: Fire in reactor 2 – damaged beyond repair and taken offline
• 1996: IAEA recommended the termination of operations at plant –
reactor 3 decommissioned
• 2000: Reactor 3 and entire plant shut down
Current Situation
•
Sarcophagus not effective permanent containment method – strong wind could
dislodge roof, and water leaks in through gaping holes
•
Rising humidity levels inside sarcophagus cause erosion of concrete and steel
•
Chernobyl Shelter Fund
started in 1997 for
Shelter Implementation Plan
•
Planned construction of
“New Safe Confinement” (NSC)
•
Large movable arch:
Span: 250m
Height: 100m
Length: 150m
•
Cost: $1.2 Billion
Three Mile Island
• Worst Accident in history of commercial Nuclear power in America
• Accident unfolded over 5 days
• World’s worst civilian disaster until Chernobyl 7 years later
• No injuries or deaths
Where is Three Mile Island Plant?
• United States of America
• Dauphin County,
Pennsylvania
• Three miles down
river from near
by town, Harrisburg
(Hence the name)
Power Plant
• 2 Pressurised Water Reactors: TMI-1 and TMI-2
• TMI-1 : 850 MWe capacity
• Individual containment
buildings per reactor
• Reactors connected by
cooling towers
What Happened?
• A series of malfunctions resulting in:
• Rupturing of quench tank relief diaphragm
• Small explosion in containment
building
• Melting of half of the core
Series of malfunctions March 27th
1979
• Plants main feed water pumps fail
• Turbine and reactor shut down
• Extra heat causes rise in steam production and increase in pressure
• Pilot operated pressuriser relief valve was opened and jammed –
cooling water escaped
• Pressuriser indicator gave false reading and water was cut off from
reactor
• Reactor core became uncovered causing reaction between fuel rods
and steam – producing explosion
Immediate Aftermath
• 7am: Site area emergency was declared
• 7:24am: Upgraded to “general emergency”
• 8pm: primary loop pumps turned back on and reactor core found to
have melted
• Steam and Hydrogen removed using recombiner
• Controversial vent used to expel radioactive hydrogen and steam
straight into atmosphere
• 13 million curies of radioactive noble gases released
Clean up
• Started in 1979 and officially ended in 1993
• Cost: $975 million
• Removal of 100 tonnes of radioactive fuel between 1985 and 1990
Ecological and Human Effects
• Possible link between lung cancer and offsite exposures, but not
conclusive
• No member of public was injured by the accident
• Average radiation dose to people within 10km radius: 8 millirem;
equal to single X-ray
• Radiation dose no more than 100 millirem; equal to 1/3 background
radiation
Decommissioning
• Reactor gradually dismantled and mothballed by 1993
• De-fuelling completed in 1988
• Damaged reactor safely removed and disposed in 1993
• Unit 1 permitted to resume operations in 1985 following licence
suspension
• Unit 2 maintained and monitored since by various companies:
currently Exelon nuclear
Long Term Aftermath
• Public approval of nuclear power in the U.S fell from 70% to 50%
• Only 53 of 123 newly approved plants were ever completed: demise
in nuclear industry
• Federal requirements became more stringent
• Local opposition became more stringent
• Construction time severely lengthened