Nuclear Reactors and Nuclear Disasters
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boiling water reactors

Nuclear Reactors and

Nuclear Disasters


Nuclear reactors and nuclear safety

• Chernobyl accident

• Three Mile Island accident

• Fukushima Daiichi accident (ongoing)

Pressurized water reactor

Boiling water reactor

Safety challenges

• Need to prevent runaway nuclear reactions at

all costs: control rods!

• Even when nuclear reactions cease, reactor

keeps generator heat by decay of byproducts:

must ensure that adequate cooling continues

even after reactor shutdown!

• If both of the above fail (catastrophic failure),

need to ensure that meltdown is contained

Chernobyl power plant

• Chernobyl nuclear power plant: 4 BWR

reactors (3.2 Gw thermal power/1 GW

electrical power apiece)

• First reactor completed 1977, last in 1983 (2

more reactors under construction at time of


Reactors 1 & 2 were 1st generation, 3 and 4

were 2nd generation (better safety)

• None of the reactors were surrounded by a

hard containment vessel!

Chernobyl design

Chernobyl accident

• April 26, 1986: plant managers attempted to run an

experiment to test a safety feature (emergency generators)

• Prior to accident, plant operators made mistakes with

control rods, reducing margin of safety in reactor. Reactor

control rods almost fully withdrawn b/c of Xenon-135

poisoning in reactor core.

During experiment, reduced water flow caused steam

bubbles in core, reducing cooling capacity: positive feedback

loop - runaway heating!

Attempts to scram result in attempts to insert control

rods: accidentally increased reaction rate in core by

removing coolant first: massive power spike, core explosion.

Chernobyl Reactor

4, post explosion

Chernobyl reactor 4 today

Accident after-effects

• within 30 km of plant:

• inhalation dose: 3-150 mSv for adults, 10-700 mSv for young


• Thyroid dose: 20-1000 mSv for adults, 20-6000 mSv for young

children (mainly iodine-131, cesium-137)

• Ingestion dose: 3-180 mSv adult, 20-1300 mSv children

• longer-range:

• reactor plume deposited radioactive material (fallout) all over

Russia/Belarus/Ukraine, western europe, UK, Canada, with very

uneven contamination

• excess cesium-137 still seen in animals to this day in UK,


• Deaths: ~50 workers from acute radiation poisoning; 4000 est. by

cancer in long term (also ecosystem issues)

Three Mile Island

• Pressurized water reactor near Harrisburg, PA

• Loss-of-coolant accident on March 28, 1979: large

amounts of reactor coolant escaped, ~half of core


• Most of radiation was in noble gases (relatively


• ~20 curies of Iodine-131, Cesium-137 released

(relatively tiny amount)

• Reactor containment vessel completely contained


Fukushima 1 Nuclear Power Plant

• First commissioned in 1971, in the town of Okuma

• 6 boiling water reactors w/combined electrical

output of 4.7 GW (thermal output of ~16 GW).

• Second nuclear power plant, Fukushima II, 11.5 km to


• Both plants used General Electric “Mark 1” design from

1960s (safety issues seen starting shortly after they

entered production)

• 23 of these reactors at 16 locations in US, 32 around


• Spent fuel rods kept in pools onsite until cool enough

to store

Fukushima accident

• 3 reactors shut down prior to accident (#s 4-6); 3

running (#s 1-3)

• At time of earthquake (11 Mar. 2011, 14:46 JST),

the remaining three reactors were automatically

shut down (‘scram’)

• Subsequent tsunami flooded the plant (despite sea

wall): emergency generators used for cooling

reactors #1-3, also spent fuel rod pools knocked


• Flooding, earthquake damage have hampered

efforts to fix reactors!

Fukushima accident

• Evidence of partial core meltdown in reactors #1-3

• Falling water levels in all operating reactors (and in

spent fuel rod pools) caused production of hydrogen

gas: caused explosions, fires in some reactor buildings

• Sea water used to replace distilled water in reactors:

salts in sea water contaminated cores w/corrosive

salts (mainly NaCl): reactors now unusable

• Venting of containment vessel to release pressure has

taken place; some radioactivity released

Present status

• Pressure vessels for reactors #1-3 unknown.

Fuel rods definitely damaged (some radiation


• Containment vessel for reactors 1, 4-6

undamaged. Damage suspected for #2, also

possible for #3.

• Seawater injection continuing for reactors 1-3;

containment vessel venting temporarily stopped.

Radiation released

• Radiation doses at edge of plant property

measured at b/w 0.3-2.6 mSv/hour

• Some radionuclides detected in milk and spinach

from nearby prefectures (Ce, I)

• Reports of radioactive materials (iodine-131,

cesium-137) at levels ~40x normal in Tokyo

(radiation level ~0.8 microSv/hour)

• Radioactivity levels measured near plant are at

least 10x background (which is 2.4 microSv/year)

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