Probability

1
Probability

• To determine the likelyhood that such an accident
  would occur something called an event tree is
  constructed.
• This determines the consequences of a particular
  event occurring
• Each component (pump, valves etc) has a failure
  probability assigned to it
• Bottom line-most recent studies indicate that for
  all 104 reactors operating the US, over their 30
  year operating lifetime, there is a 1
  probability of a large release of radioactivity

2
Core meltdown
3
Nuclear accidents

• 4 (that we know of.)
• Enrico Fermi reactor in Illinois
• Three Mile Island-PA-Worst US accident
• Chernobyl-Ukraine-Worst accident ever
• Fukushima Nuclear plant accident in Japan
• May surpass Chernobyl

4
Enrico Fermi Reactor

• Enrico Fermi demonstration nuclear breeder
  reactor, in Monroe, MI
• Two fuel rod assemblies reached 700F(normal
  operating temp is about 580 F).
• Reactor was shut down, and it appeared fuel was
  melting.
• It was determined a piece of zirconium from the
  meltdown cone (a structure designed to direct the
  flow of fuels should a meltdown occur) came loose
  and was blocking the coolant nozzles.
• The fuel in a breeder reactor is highly enriched,
  thus in molten form it could reassemble into
  critical assembly that could result in a runaway
  fission reaction which would generate enough heat
  to melt the material into the Earth. Some
  engineer remarked it could go all the way to
  China, and the phrase China Syndrome was born.
• Publically used by a Nuclear Physicist Richard
  Lapp in 1971
• No one was injured and no radiation was released.

5
Three Mile Island

• Perhaps the most famous nuclear accident in the
  US
• On March 16, 1979, the movie China Syndrome,
  based on the effect described in the last slide,
  was released.
• 12 days later, March 28, 1979, the worst civilian
  nuclear accident in the US occurred at the Three
  Mile Island Nuclear Power Plant on the
  Susquehanna River, south of Harrisburg, PA
  occurred.

6
Location
7
The accident

• Partial core meltdown as the result of a LOCA
• Main feedwater pumps failed, triggered a
  controlled shutdown (scram).
• But the decay heat (heat generated by the decay
  of radioactive material in the fuel) continued,
  with nothing to remove it
• Auxiliary systems could not pump water, as their
  valves had been closed for maintenance (which was
  a violation of NRC regulations)
• Pressure built up, which was released by a PORV
  valve (Pilot-operated relief valve) which opened
  automatically, but failed to close. This allowed
  coolant water to escape.

8
The accident

• Plant operators had a control light that only
  indicated if power was applied to the valve, not
  if it were open or closed. The light went out
  when the power was cut, the operators did not
  know this did not mean the valve was closed. Bad
  design.
• However, there were other instruments that told
  the operators something was wrong, in fact that
  suggested the valve was still open, but the light
  was out! Bad operators.
• As pressure was lost, some of the coolant turned
  to steam and formed steam pockets (remember the
  high pressure is used to keep the coolant liquid
  at high T). This caused the coolant level in the
  pressurizer to look higher than it was, and the
  operators turned off the emergency core pumps
  which came on after the initial pressure loss.
• The tank that collected the discharge from the
  PORV overfilled and the sump pump in the
  containment building filled and sounded an alarm.
  This, plus abnormally high PORV T and higher than
  normal containment building Temperature and
  Pressure readings were ignored by the operators.
• A failure in the quench tank caused radioactive
  coolant to be pumped into a building outside the
  containment building.

9
Accident cont

• Steam bubbles in the cooling pumps caused them to
  cavitate and need to be shut down, with the
  operators believing the coolant would circulate
  naturally. It did not (they did not know there
  were steam cavities that blocked the water flow).
• The top of the reactor became exposed and the
  steam reacted with the zirconium cladding on the
  fuel rods and damaged the fuel pellets, releasing
  more radioactivity into the coolant water.
• Plant had become seriously contaminated, but it
  wasnt until 165 minutes after it all began that
  contaminated water reached radioactivity
  detectors and the alarms went off.
• At this point a new shift of operators came on,
  who noticed a problem and shut off the coolant
  venting via the faulty PORV valve.
• Several hours later, new water was pumped into
  the primary cooling loop, and a backup valve was
  opened to relieve the pressure so the loop would
  fill up. Around 2pm an explosion rocked the
  containment building. This explosion was the
  result of H released when the zirconium cladding
  was burned off of the fuel rods. 16 hours after
  the start, the primary coolant loop was operating
  the core T began to fall.

10
Another illustration of the accident site
11
Effects of Three Mile Island

• Amount of radiation released is debated, the
  containment building held.
• Official figures indicate a small amount of
  radioactivity was released.
• Independent measures claim radiation of 3-5
  times higher than normal were detected hundreds
  of miles downwind of the plant.
• Long term health effects on residents are hotly
  debated, pick your favorite interpretation.
• The valve had failed in the open position 9
  previous times, and 2 other times in the closed
  position.
• It had also previously failed at another plant,
  but those operators diagnosed the problem in 2
  minutes in a plant only operating at 9 ( as
  opposed to the 97) output at Three Mile Island.
  The valve company never notified its customers of
  the previous failure.
• Often blamed for the demise of nuclear power in
  the US
• Probably an overstatement, but it certainly
  soured public opinion

12
Chernobyl

• Accident at the Chernobyl Nuclear Power plant in
  the Ukraine in 1986
• At the time it was part of the Soviet Union
• Worst Nuclear power plant accident in history
• 2 died in initial steam explosion
• Deaths from radiation exposure cannot be counted,
  Soviet Union covered up the numbers
• Best estimates are 56 direct deaths and 4000
  additional cancer related deaths (2005 report of
  the Chernobyl Forum)

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14
Chernobyl

• Plant experienced power excursion (chain
  reaction went out of control)
• Resulted in a steam explosion and a secondary
  hydrogen explosion which tore the top off of the
  reactor and its building and exposed the core.
• NO containment building!
• Released large amount of radioactive particles
  into the air

15
Chernobyl

• Began with a test of a backup cooling system
• In the event of an external power failure, the
  reactor would shut down, but there would be no
  power to run the plant cooling pumps.
• Backup diesel generators took 1 minute to reach
  full capacity,
• This one minute cooling gap was not acceptable
• It was proposed to use the rotational energy of
  the turbine as it was spinning down to generate
  electricity in this gap. Since the turbine was
  spinning down, a voltage regulator was needed to
  provide stable power to the to the cooling pumps
• The test was to take place as the fuel rods were
  to be replacedthe worst possible time as the
  decay heat and radioactive nuclei present would
  be at its maximum at the end of a fuel cycle.
• Test had already failed once
• Plan was to run the reactor at low power, but the
  turbine at full speed. The steam supply would be
  cut off and the turbines would be allowed to spin
  down, and see if the voltage were regulated.

16
Chernobyl

• Test was delayed many hours by an unexpected shut
  down of another power station.
• This resulted in an untrained night shift taking
  over the experiment
• When power was reduced, the control rods were
  inserted too far, resulting in the an almost
  complete rector shutdown
• Resulted in xenon poisoning, where high levels of
  Xenon 135 absorb neutrons an inhibit the fission
  process.
• Operators saw the power drop too low, but were
  not aware of the Xenon poisoning, assuming
  instead a power regulator failed
• To compensate, they pulled the control rods out
  of the reactor core, beyond the limits of safe
  operation. This would have had to be done via
  manual overrides.
• Extra water was pumped into the core to cool it
  and reduce steam voids, but it exceeded safe
  water level limits. Water acts as a moderator,
  so it further reduced the power output. So the
  control rods were pulled all the way out.
• Reactor was set up for a runaway reaction, but
  the extra water and xenon were acting as a
  moderator. Excess steam and other changes in
  nominal operation were occurring and the
  automatic control system should have shut the
  reactor down, but the operators had disabled this
  system.

17
Chernobyl

• Operators were not aware of the unstable
  condition, and proceeded to shut off the steam to
  the turbines
• As they spun down, the water flow decreased and
  steam voids formed
• Control rods were not completely removed, they
  blocked the heat from reaching the cooling water.
• A massive steam build up occurred, an the reactor
  power and neutron generation increased overcoming
  the xenon poisoning. A runaway situation was in
  progress

18
Chernobyl

• A SCRAM was ordered.
• But, the insertion of the control rods displaced
  coolant (design flaw), increasing the reaction
  rate.
• Core overheated, fracturing fuel rods and
  blocking further control rod insertion
• Cooling pipes ruptured, and fuel rods melted
• Steam explosion occurs, which rips the 2000 ton
  lid off of the reactor
• 2-3 seconds later a second hydrogen explosion
  occurred, either from the reaction of the steam
  with the zirconium fuel rods or by the reaction
  of hot graphite and steam

19
Chernobyl

• Hot debris started fires on the roofs of other
  reactors
• Steam and smoke were highly radioactive
• No public notice was made until radiation alarms
  at a nuclear plant in Sweden went off!
• The cloud spread over Russia, Belarus, Ukraine
  and Moldova, but also Turkish Thrace, the
  Southern coast of the Black Sea, Macedonia,
  Serbia, Croatia, Bosnia-Herzegovina, Bulgaria,
  Greece, Romania, Lithuania, Estonia, Latvia,
  Finland, Denmark, Norway, Sweden, Austria,
  Hungary, the Czech Republic and the Slovak
  Republic, The Netherlands, Belgium, Slovenia,
  Poland, Switzerland, Germany, Luxembourg, Italy,
  Ireland, France (including Corsica) the United
  Kingdom and the Isle of Man.
• Reactor was contained in a concrete sarcophagus
  which has 200 tons of highly radioactive material
  inside
• Entire plant shut down in 2000

20
Effect

• Short term effects on rivers and groundwater
• 4 square KM of pine forest in the vicinity of the
  reactor died
• Some animals died or stopped reproducing
• Since the abandonment by humans, many wildlife
  species have returned to the area, with reports
  of higher incidences of deformities etc compared
  to non contaminated areas
• A black melanin rich fungi is growing on the
  reactors walls
• It is difficult to assess the human impact

21
Extent of the radioactive cloud