The Three Mile Island nuclear accident occurred on March 28, 1979 at the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania. There was a partial meltdown of the reactor core as the result of a combination of equipment failures and design-related issues. Small amounts of radioactive gases and radioactive iodine were released into the environment, but no deaths or injuries to plant workers or members of nearby communities were reported from radiation exposure. The accident significantly slowed the growth of the nuclear industry in the United States.

1. Three Mile Island
• Perhaps the most famous
nuclear accident in the US
• 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, Pennsylvania
occurred.

2. Location

3. 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.

4. 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.

5. Accident con’t
• 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
wasn’t 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.

6. Another illustration of the accident site

7. 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

8. 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)

9. 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

10. 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 replaced…the 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.

11. 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.

12. 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

13. 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

14. 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

15. 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

16. Extent of the radioactive cloud

17. Other accidents-1980s
• March 13, 1980 –- Orléans, France - Nuclear materials leak
– A brief power excursion in Reactor A2 led to a rupture of fuel bundles and a minor release (8 x 1010 Bq) of nuclear
materials at the Saint-Laurent Nuclear Power Plant. The reactor was repaired and continued operation until its
decommissioning in 1992.
• March, 1981 - Tsuruga, Japan – Overexposure of workers
– Overexposure of workers More than 100 workers were exposed to doses of up to 155 millirem per day radiation
during repairs of a nuclear power plant, violating the company's limit of 100 millirems (1 mSv) per day.
• September 23, 1983 Buenos Aires, Argentina - Accidental criticality
– An operator error during a fuel plate reconfiguration in an experimental test reactor led to an excursion of 3×1017
fissions at the RA-2 facility. The operator absorbed 2000 rad (20 Gy) of gamma and 1700 rad (17 Gy) of neutron
radiation which killed him two days later. Another 17 people outside of the reactor room absorbed doses ranging
from 35 rad (0.35 Gy) to less than 1 rad (0.01 Gy).
• April 26, 1986 — Prypiat, Ukraine (then USSR) - Power excursion, explosion, complete meltdown
– A mishandled reactor safety test led to an uncontrolled power excursion, causing a severe steam explosion, meltdown
and release of radioactive material at the Chernobyl nuclear power plant located approximately 100 kilometers north-
northwest of Kiev.
• May 4, 1986 – Hamm-Uentrop, Germany (then West Germany) - Fuel damage
– A spherical fuel pebble became lodged in the pipe used to deliver fuel elements to the reactor at an experimental
300-megawatt THTR-300 HTGR. Attempts by an operator to dislodge the fuel pebble damaged its cladding, releasing
radiation detectable up to two kilometers from the reactor.
• November 24, 1989 — Greifswald, Germany (then East Germany) - Fuel damaged
– Operators disabled three of six cooling pumps to test emergency shutoffs. Instead of the expected automatic
shutdown a fourth pump failed causing excessive heating which damaged ten fuel rods. The accident was attributed
to sticky relay contacts.

18. Other Accidents 1990s
• April 6, 1993 — Tomsk, Russia – Explosion
– A pressure buildup led to an explosive mechanical failure in a 34 cubic meter stainless steel reaction vessel
buried in a concrete bunker under building 201 of the radiochemical works at the Tomsk-7 Siberian Chemical
Enterprise plutonium reprocessing facility. The vessel contained a mixture of concentrated nitric acid,
uranium (8757 kg), plutonium (449 g) along with a mixture of radioactive and organic waste from a prior
extraction cycle. The explosion dislodged the concrete lid of the bunker and blew a large hole in the roof of
the building, releasing approximately 6 GBq of Pu 239 and 30 TBq of various other radionuclides into the
environment. The contamination plume extended 28 km NE of building 201, 20 km beyond the facility
property. The small village of Georgievka (pop. 200) was at the end of the fallout plume, but no fatalities,
illnesses or injuries were reported. The accident exposed 160 on-site workers and almost two thousand
cleanup workers to total doses of up to 50 mSv (the threshold limit for radiation workers is 100 mSv per 5
years)
• June, 1999 - Ishikawa Prefecture, Japan - Control rod malfunction
– Operators attempting to insert one control rod during an inspection neglected procedure and instead
withdrew three causing a 15 minute uncontrolled sustained reaction at the number 1 reactor of Shika
Nuclear Power Plant. The Hokuriku Electric Company who owned the reactor did not report this incident and
falsified records, covering it up until March, 2007.
• September 30, 1999 — INES Level 4 - Ibaraki Prefecture, Japan - Accidental criticality
– Workers put uranyl nitrate solution containing about 16.6 kg of uranium, which exceeded the critical mass,
into a precipitation tank at a uranium reprocessing facility in Tokai-mura northeast of Tokyo, Japan. The tank
was not designed to dissolve this type of solution and was not configured to prevent eventual criticality.
Three workers were exposed to (neutron) radiation doses in excess of allowable limits. Two of these workers
died. 116 other workers received lesser doses of 1 mSv or greater though not in excess of the allowable
limit..

19. Other accidents -2000s
• April 10, 2003 - Paks, Hungary - Fuel damaged
– Partially spent fuel rods undergoing cleaning in a tank of heavy water ruptured and spilled
fuel pellets at Paks Nuclear Power Plant. It is suspected that inadequate cooling of the rods
during the cleaning process combined with a sudden influx of cold water thermally shocked
fuel rods causing them to split. Boric acid was added to the tank to prevent the loose fuel
pellets from achieving criticality. Ammonia and hydrazine were also added to absorb iodine-
131.
• April 19, 2005 — Sellafield, England, United Kingdom - Nuclear material leak
– Twenty metric tons of uranium and 160 kilograms of plutonium dissolved in 83,000 literes of
nitric acid leaked over several months from a cracked pipe into a stainless steel sump chamber
at the Thorp nuclear fuel reprocessing plant. The partially processed spent fuel was drained
into holding tanks outside the plant.
• November 2005 — Braidwood, Illinois, United States - Nuclear material leak
– Tritium contamination of groundwater was discovered at Exelon's Braidwood station.
Groundwater off site remains within safe drinking standards though the NRC is requiring the
plant to correct any problems related to the release.
• March 6, 2006 — Erwin, Tennessee, United States - Nuclear material leak
– Thirty-five liters of a highly enriched uranium solution leaked during transfer into a lab at
Nuclear Fuel Services Erwin Plant. The incident caused a seven-month shutdown and a
required public hearing on the licensing of the plant.

20. Fusion
• Powers the sun
• In fusion, we combine
two atoms and release
energy
• Easiest to do this with H
or its isotopes
– We already talked about
the proton-proton chain in
the sun
• D-T reaction-takes
deuterium and tritium
and creates He

21. Fusion
• In order for these reactions to occur, one
needs the deuterium and tritium at high
temperatures
• For the DT reaction, T = 40 x 106 K
• For DD reaction, T = 100 x 106 K
• DT is good for bombs, not so good for long
term power, and tritium has a half life of 12
years

22. Thermonuclear devices
• Also called H bombs, these are
fusion bombs
• Require fission to compress
and heat the fusion fuel.
• The fusion releases enormous
amounts of high speed
neutrons which are then
often used to induce fission in
matter that it is normally
difficult to induce fission in
(such as depleted Uranium,
Uranium composed mostly of
238U).
• This adds to the radioactive
fallout of the bomb