Nuclear Reactor Disasters

Chernobyl1986

Three Mile Island1979

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 systemcontaminated (reduced after initial period)

• Fresh water fish contaminatedto several times the safelimits (reduced afterinitial period)

• Pine forest within 4kmradius turnedginger brownand 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 Fundstarted in 1997 for Shelter Implementation Plan

• Planned construction of “New Safe Confinement” (NSC)

• Large movable arch:Span: 250mHeight: 100mLength: 150m

• Cost: $1.2 Billion

Three Mile IslandThree Mile Island

• Worst Accident in history of commercial Nuclear power in AmericaWorst Accident in history of commercial Nuclear power in America

• Accident unfolded over 5 daysAccident unfolded over 5 days

• World’s worst civilian disaster until Chernobyl 7 years laterWorld’s worst civilian disaster until Chernobyl 7 years later

• No injuries or deathsNo injuries or deaths

Where is Three Mile Island Plant?Where is Three Mile Island Plant?

• United States of AmericaUnited States of America

• Dauphin County,Dauphin County,

PennsylvaniaPennsylvania

• Three miles downThree miles down

river from near river from near

by town, Harrisburgby town, Harrisburg

(Hence the name)(Hence the name)

Power PlantPower Plant

• 2 Pressurised Water Reactors: TMI-1 and TMI-22 Pressurised Water Reactors: TMI-1 and TMI-2

• TMI-1 : 850 MWe capacityTMI-1 : 850 MWe capacity

• Individual containmentIndividual containment

buildings per reactorbuildings per reactor

• Reactors connected byReactors connected by

cooling towerscooling towers

What Happened?What Happened?

• A series of malfunctions resulting in:A series of malfunctions resulting in:

• Rupturing of quench tank relief diaphragmRupturing of quench tank relief diaphragm

• Small explosion in containmentSmall explosion in containment

buildingbuilding

• Melting of half of the coreMelting of half of the core

Series of malfunctions March 27Series of malfunctions March 27thth 19791979

• Plants main feed water pumps failPlants main feed water pumps fail

• Turbine and reactor shut downTurbine and reactor shut down

• Extra heat causes rise in steam production and increase in pressureExtra heat causes rise in steam production and increase in pressure

• Pilot operated pressuriser relief valve was opened and jammed – cooling Pilot operated pressuriser relief valve was opened and jammed – cooling water escapedwater escaped

• Pressuriser indicator gave false reading and water was cut off from Pressuriser indicator gave false reading and water was cut off from reactorreactor

• Reactor core became uncovered causing reaction between fuel rods and Reactor core became uncovered causing reaction between fuel rods and steam – producing explosionsteam – producing explosion

Immediate AftermathImmediate Aftermath

• 7am: Site area emergency was declared7am: Site area emergency was declared

• 7:24am: Upgraded to “general emergency”7:24am: Upgraded to “general emergency”

• 8pm: primary loop pumps turned back on and reactor core found to 8pm: primary loop pumps turned back on and reactor core found to have meltedhave melted

• Steam and Hydrogen removed using recombinerSteam and Hydrogen removed using recombiner

• Controversial vent used to expel radioactive hydrogen and steam Controversial vent used to expel radioactive hydrogen and steam straight into atmospherestraight into atmosphere

• 13 million curies of radioactive noble gases released13 million curies of radioactive noble gases released

Clean upClean up

• Started in 1979 and officially ended in 1993Started in 1979 and officially ended in 1993

• Cost: $975 millionCost: $975 million

• Removal of 100 tonnes of radioactive fuel between 1985 and 1990Removal of 100 tonnes of radioactive fuel between 1985 and 1990

Ecological and Human EffectsEcological and Human Effects

• Possible link between lung cancer and offsite exposures, but not Possible link between lung cancer and offsite exposures, but not conclusiveconclusive

• No member of public was injured by the accidentNo member of public was injured by the accident

• Average radiation dose to people within 10km radius: 8 millirem; Average radiation dose to people within 10km radius: 8 millirem; equal to single X-rayequal to single X-ray

• Radiation dose no more than 100 millirem; equal to 1/3 background Radiation dose no more than 100 millirem; equal to 1/3 background radiationradiation

DecommissioningDecommissioning

• Reactor gradually dismantled and mothballed by 1993Reactor gradually dismantled and mothballed by 1993

• De-fuelling completed in 1988De-fuelling completed in 1988

• Damaged reactor safely removed and disposed in 1993Damaged reactor safely removed and disposed in 1993

• Unit 1 permitted to resume operations in 1985 following licence Unit 1 permitted to resume operations in 1985 following licence suspensionsuspension

• Unit 2 maintained and monitored since by various companies: Unit 2 maintained and monitored since by various companies: currently Exelon nuclearcurrently Exelon nuclear

Long Term AftermathLong Term Aftermath

• Public approval of nuclear power in the U.S fell from 70% to 50%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 Only 53 of 123 newly approved plants were ever completed: demise in nuclear industryin nuclear industry

• Federal requirements became more stringentFederal requirements became more stringent

• Local opposition became more stringentLocal opposition became more stringent

• Construction time severely lengthenedConstruction time severely lengthened