Docstoc

Nuclear Reactor Disasters

Document Sample
Nuclear Reactor Disasters Powered By Docstoc
					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

				
DOCUMENT INFO