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---
layout: default
title: The Fukushima Accident
category: issues
description: Learn about the Fukushima nuclear accident
author: nick
date: 2011-03-15
byline: true
---
<div class="row">
<div class="col-md-8">
<div class="float-end">
<img
class="rounded img-fluid"
src="img/fuku-march30-aerial-small.jpg"
title="Fukushima Dai-ichi unit 3"
alt="Fukushima Dai-ichi unit 3"
/>
</div>
<p>
On March 11, 2011, a historically large earthquake struck Japan, which
gets a large percentage of its electricity from nuclear power plants. All
affected plants automatically shut down by inserting control rods into the
cores (an operation called a SCRAM). After each SCRAM, the nuclear chain
reaction in each core stopped and total heat generation went immediately
down to about 6% and decreasing. However, nuclear reactors cannot be
turned completely off immediately. Splitting atoms produces smaller atoms,
many of which are radioactive. These produce quite a bit of heat as they
emit alpha, beta, and gamma
<a href="{% link radioactivity.md %}">radiation</a>, even in the absence
of a chain reaction. As these fission products decay, the heat generation
decays away with time exponentially.
</p>
<p>
The plants were not built to be able to handle an earthquake and tsunami
of the magnitude seen. This error in judgment and knowledge of the
environment at that site led to under-engineering of the backup systems.
This has caused the loss of these plants, a general downturn in public
acceptance of nuclear energy, and radioactive contamination of a large
chunk of Japan. So far, no one has died from radiological reasons (2
workers were lost in the tsunami).
</p>
<div class="card border-success mb-3">
<div class="card-header bg-success text-white">Meltdown</div>
<div class="card-body">
Even if a nuclear reactor is shut down, it still produces quite a bit of
<a href="{% link decay-heat.html %}">decay heat</a> that must be cooled
to prevent the fuel from overheating and melting. If all the fuel melts
(known as a meltdown), there is a higher chance that the radiation that
was contained within the pins could get outside the reactor.
</div>
</div>
<p>
The emergency diesel generators and/or their fuel supplies at the 5 of the
6 reactors at the Fukushima Daiichi site were flooded by the tsunami that
followed the quake. At this time, the plants entered a station blackout
condition. Extra engineered features that do not require electricity to
operate attempted to remove the decay heat and prevent fuel melting, but
these can only work for a certain amount of time, and they eventually
failed in units 1 through 3. Unit 4’s fuel was all in the spent fuel
pool (the reactor had been undergoing an outage at the time) and it is
unclear why there was a hydrogen explosion there.
</p>
<div class="card border-success mb-3">
<div class="card-header bg-success text-white">Earthquakes</div>
<div class="card-body">
Extremely large earthquakes leading to loss of decay heat removal is
considered one of the highest likelihood events to cause core damage in
a nuclear reactor.
</div>
</div>
<p>
After station black-out, unit 1’s engineered safety system (an
isolation condenser, or IC) went dry after 18 minutes (should have lasted
days). Hours later, the fuel melted and produced hydrogen, which soon
exploded. In units 2 and 3, a system called the RCIC (which uses steam
generated by the decay heat to circulate water) cooled the reactors for ~2
days before failing, finally allowing the fuel to melt.
</p>
<p>
Fuel melted in the reactor vessels of units 1 through 3 and seems to have
penetrated through the reactor pressure vessel. In unit 4, the fuel in the
spent fuel pool seems to have melted, probably due to a leak in the pool
(since it should have taken at least 10 days to boil off that much water).
When fuel gets very hot in these reactors, the Zirconium cladding
interacts with water and produces hydrogen. In units 1 through 3, hydrogen
explosions occurred and caused damage to the reactors.
</p>
</div>
</div>
<hr />
<div class="row">
<div class="col-md-8">
<h1>What’s happening?</h1>
<h2>
These old updates are left here for historical perspective. Info is much
more understood at this time
</h2>
<h2>Update, March 16</h2>
<p>
Today’s concerns seem focused on the spent fuel pools, particularly
in units 3 and 4. White smoke was seen coming out of unit 3, indicative of
burning Zirconium and therefore melting rods. With the secondary
containments damaged, these exposed spent fuel pools would have a direct
path to release radiation into the environment if the water level truly
has dried out, leading to local radiation levels that would make it
difficult to continue cooling operations on the site. This development is
extremely surprising because spent fuel pools do not require substantial
cooling to keep cool. If a firetruck could hook up outside the building
and pump in water, this problem seems like it would be avoided!
We’re anxiously awaiting details and hoping off-site power can be
restored immediately. We keep cheering on and appreciating the efforts of
the heroes who are working on this disaster in the high radiation
environment.
</p>
<h2>Update, March 15</h2>
<p>
A third hydrogen explosion occurred yesterday at unit two at Fukushima,
but rather than happening in the not-so-important secondary containment,
this one happened below the core, near an area called the suppression
pool. Pressure dropped and coolant levels did not rise after this,
suggesting that coolant is going from the pressure vessel into the primary
containment. Meanwhile, at unit 4 (which was shut down before the
earthquake), a fire broke out last night and was subsequently
extinguished. The spent fuel pools in that unit (and the others) is in
need of cooling as well, and will probably be doused with cold water.
Decay heat in the spent fuel heats up the water surrounding it. As in the
core, if the rods are exposed to air, they may melt and release their
radiation. Very high doses were measured on the site when the fire
happened (40 rem/hr), but these have since fallen drastically. Readings
off-site are still fairly low.
</p>
<h2>Update, March 14</h2>
<p>
Three of the 6 reactor cores on the Fukushima Daiichi site are
experiencing coolability problems and have experienced partial melting of
the fuel elements. Two barriers of protection are still intact (the
pressure vessels and the reactor containments), but the vessel in at least
one of the reactors has so much pressure in it (from steam buildup) that
attempts to pump in seawater are failing. The valve that is supposed to
relieve this pressure is apparently stuck. Efforts to ensure cooling are
ongoing.
</p>
<h2>Initial posting, March 12</h2>
<p>
In an attempt to piece the details together, it seems that Fukushima shut
down like the other plants and the main pumps failed probably due to the
loss of offsite power (since the whole electricity grid went down at the
same time). At that time, diesel generators kicked in to power the
emergency core cooling systems to cool the decay heat. Then, when the
tsunami hit, the redundant diesel generators failed (probably due to fuel
tank and/or piping problems). This kind of large event is known as a
common mode failure. With no power for the emergency pumps, the water
covering the hot fuel rods started to heat up. At this point, operators
would have been trying to find some way to get the coolant flowing past
the fuel rods again. As the coolant heats up, it produces steam which
pressurizes the containment of the reactor. Hydrogen can also be produced,
which is explosive.
</p>
<p>
At this point, it seems that the operators opened a valve to reduce steam
pressure in the containment. Oxygen and hydrogen mixed between the
containment and the reactor building, resulting in an explosion that
allegedly did not damage the primary containment. Generators and batteries
have arrived on site and seawater impregnated with Boron (a strong neutron
absorber, to keep the reactor well-subcritical) is being pumped into the
containment to help remove the decay heat from the fission products. Some
fuel may have melted already, so we’re hoping that they are able to
provide reliable cooling to the rest of the core until the heat generation
decays down to safe levels.
</p>
<p>
By the way, some later-generation nuclear reactors are capable of doing
decay heat removal with natural circulation, so no pumps or electricity
would be needed for this event. This is called passive safety, and is an
important feature of modern designs.
</p>
<p>
The best live updates so far are at
<a
href="https://en.wikipedia.org/wiki/Timeline_of_the_Fukushima_nuclear_accidents"
>Wikipedia timeline of Fukushima</a
>, with more coverage at
<a
href="http://www.nei.org/Issues-Policy/Safety-Security/Fukushima-Response"
>The NEI</a
>
and at
<a
href="http://www.world-nuclear.org/information-library/safety-and-security/safety-of-plants/fukushima-accident.aspx"
>World nuclear news</a
>
as well.
</p>
<p>
Find a detailed timeline of Fukushima
<a
href="http://www.nei.org/newsandevents/newsreleases/inpo-compiles-timeline-of-fukushima-events-after-japan-earthquake-tsunami"
>from INPO here</a
>
</p>
</div>
</div>