Construction of the Fukushima nuclear power plants

The plants at Fukushima are so called Boiling Water Reactors, or BWR
for short. Boiling Water Reactors are similar to a pressure cooker.
The nuclear fuel heats water, the water boils and creates steam, the
steam then drives turbines that create the electricity, and the steam
is then cooled and condensed back to water, and the water send back to
be heated by the nuclear fuel. The pressure cooker operates at about
250 °C.

The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a
very high melting point of about 3000 °C. The fuel is manufactured in
pellets (think little cylinders the size of Lego bricks). Those pieces
are then put into a long tube made of Zircaloy with a melting point of
2200 °C, and sealed tight. The assembly is called a fuel rod. These
fuel rods are then put together to form larger packages, and a number
of these packages are then put into the reactor. All these packages
together are referred to as “the core”.

The Zircaloy casing is the first containment. It separates the
radioactive fuel from the rest of the world.

The core is then placed in the “pressure vessels”. That is the
pressure cooker we talked about before. The pressure vessels is the
second containment. This is one sturdy piece of a pot, designed to
safely contain the core for temperatures several hundred °C. That
covers the scenarios where cooling can be restored at some point.

The entire “hardware” of the nuclear reactor – the pressure vessel and
all pipes, pumps, coolant (water) reserves, are then encased in the
third containment. The third containment is a hermetically (air tight)
sealed, very thick bubble of the strongest steel. The third
containment is designed, built and tested for one single purpose: To
contain, indefinitely, a complete core meltdown. For that purpose, a
large and thick concrete basin is cast under the pressure vessel (the
second containment), which is filled with graphite, all inside the
third containment. This is the so-called “core catcher”. If the core
melts and the pressure vessel bursts (and eventually melts), it will
catch the molten fuel and everything else. It is built in such a way
that the nuclear fuel will be spread out, so it can cool down.

This third containment is then surrounded by the reactor building. The
reactor building is an outer shell that is supposed to keep the
weather out, but nothing in. (this is the part that was damaged in the
explosion, but more to that later).

Fundamentals of nuclear reactions

The uranium fuel generates heat by nuclear fission. Big uranium atoms
are split into smaller atoms. That generates heat plus neutrons (one
of the particles that forms an atom). When the neutron hits another
uranium atom, that splits, generating more neutrons and so on. That is
called the nuclear chain reaction.

Now, just packing a lot of fuel rods next to each other would quickly
lead to overheating and after about 45 minutes to a melting of the
fuel rods. It is worth mentioning at this point that the nuclear fuel
in a reactor can *never* cause a nuclear explosion the type of a
nuclear bomb. Building a nuclear bomb is actually quite difficult (ask
Iran). In Chernobyl, the explosion was caused by excessive pressure
buildup, hydrogen explosion and rupture of all containments,
propelling molten core material into the environment (a “dirty bomb”).
Why that did not and will not happen in Japan, further below.

In order to control the nuclear chain reaction, the reactor operators
use so-called “moderator rods”. The moderator rods absorb the neutrons
and kill the chain reaction instantaneously. A nuclear reactor is
built in such a way, that when operating normally, you take out all
the moderator rods. The coolant water then takes away the heat (and
converts it into steam and electricity) at the same rate as the core
produces it. And you have a lot of leeway around the standard
operating point of 250°C.

The challenge is that after inserting the rods and stopping the chain
reaction, the core still keeps producing heat. The uranium “stopped”
the chain reaction. But a number of intermediate radioactive elements
are created by the uranium during its fission process, most notably
Cesium and Iodine isotopes, i.e. radioactive versions of these
elements that will eventually split up into smaller atoms and not be
radioactive anymore. Those elements keep decaying and producing heat.
Because they are not regenerated any longer from the uranium (the
uranium stopped decaying after the moderator rods were put in), they
get less and less, and so the core cools down over a matter of days,
until those intermediate radioactive elements are used up.

This residual heat is causing the headaches right now.

So the first “type” of radioactive material is the uranium in the fuel
rods, plus the intermediate radioactive elements that the uranium
splits into, also inside the fuel rod (Cesium and Iodine).

There is a second type of radioactive material created, outside the
fuel rods. The big main difference up front: Those radioactive
materials have a very short half-life, that means that they decay very
fast and split into non-radioactive materials. By fast I mean seconds.
So if these radioactive materials are released into the environment,
yes, radioactivity was released, but no, it is not dangerous, at all.
Why? By the time you spelled “R-A-D-I-O-N-U-C-L-I-D-E”, they will be
harmless, because they will have split up into non radioactive
elements. Those radioactive elements are N-16, the radioactive isotope
(or version) of nitrogen (air). The others are noble gases such as
Xenon. But where do they come from? When the uranium splits, it
generates a neutron (see above). Most of these neutrons will hit other
uranium atoms and keep the nuclear chain reaction going. But some will
leave the fuel rod and hit the water molecules, or the air that is in
the water. Then, a non-radioactive element can “capture” the neutron.
It becomes radioactive. As described above, it will quickly (seconds)
get rid again of the neutron to return to its former beautiful self.

This second “type” of radiation is very important when we talk about
the radioactivity being released into the environment later on.

What happened at Fukushima

I will try to summarize the main facts. The earthquake that hit Japan
was 7 times more powerful than the worst earthquake the nuclear power
plant was built for (the Richter scale works logarithmically; the
difference between the 8.2 that the plants were built for and the 8.9
that happened is 7 times, not 0.7). So the first hooray for Japanese
engineering, everything held up.

When the earthquake hit with 8.9, the nuclear reactors all went into
automatic shutdown. Within seconds after the earthquake started, the
moderator rods had been inserted into the core and nuclear chain
reaction of the uranium stopped. Now, the cooling system has to carry
away the residual heat. The residual heat load is about 3% of the heat
load under normal operating conditions.

The earthquake destroyed the external power supply of the nuclear
reactor. That is one of the most serious accidents for a nuclear power
plant, and accordingly, a “plant black out” receives a lot of
attention when designing backup systems. The power is needed to keep
the coolant pumps working. Since the power plant had been shut down,
it cannot produce any electricity by itself any more.

Things were going well for an hour. One set of multiple sets of
emergency Diesel power generators kicked in and provided the
electricity that was needed. Then the Tsunami came, much bigger than
people had expected when building the power plant (see above, factor
7). The tsunami took out all multiple sets of backup Diesel
generators.

When designing a nuclear power plant, engineers follow a philosophy
called “Defense of Depth”. That means that you first build everything
to withstand the worst catastrophe you can imagine, and then design
the plant in such a way that it can still handle one system failure
(that you thought could never happen) after the other. A tsunami
taking out all backup power in one swift strike is such a scenario.
The last line of defense is putting everything into the third
containment (see above), that will keep everything, whatever the mess,
moderator rods in our out, core molten or not, inside the reactor.

When the diesel generators were gone, the reactor operators switched
to emergency battery power. The batteries were designed as one of the
backups to the backups, to provide power for cooling the core for 8
hours. And they did.

Within the 8 hours, another power source had to be found and connected
to the power plant. The power grid was down due to the earthquake. The
diesel generators were destroyed by the tsunami. So mobile diesel
generators were trucked in.

This is where things started to go seriously wrong. The external power
generators could not be connected to the power plant (the plugs did
not fit). So after the batteries ran out, the residual heat could not
be carried away any more.

At this point the plant operators begin to follow emergency procedures
that are in place for a “loss of cooling event”. It is again a step
along the “Depth of Defense” lines. The power to the cooling systems
should never have failed completely, but it did, so they “retreat” to
the next line of defense. All of this, however shocking it seems to
us, is part of the day-to-day training you go through as an operator,
right through to managing a core meltdown.

It was at this stage that people started to talk about core meltdown.
Because at the end of the day, if cooling cannot be restored, the core
will eventually melt (after hours or days), and the last line of
defense, the core catcher and third containment, would come into play.

But the goal at this stage was to manage the core while it was heating
up, and ensure that the first containment (the Zircaloy tubes that
contains the nuclear fuel), as well as the second containment (our
pressure cooker) remain intact and operational for as long as
possible, to give the engineers time to fix the cooling systems.

Because cooling the core is such a big deal, the reactor has a number
of cooling systems, each in multiple versions (the reactor water
cleanup system, the decay heat removal, the reactor core isolating
cooling, the standby liquid cooling system, and the emergency core
cooling system). Which one failed when or did not fail is not clear at
this point in time.

So imagine our pressure cooker on the stove, heat on low, but on. The
operators use whatever cooling system capacity they have to get rid of
as much heat as possible, but the pressure starts building up. The
priority now is to maintain integrity of the first containment (keep
temperature of the fuel rods below 2200°C), as well as the second
containment, the pressure cooker.  In order to maintain integrity of
the pressure cooker (the second containment), the pressure has to be
released from time to time. Because the ability to do that in an
emergency is so important, the reactor has 11 pressure release valves.
The operators now started venting steam from time to time to control
the pressure. The temperature at this stage was about 550°C.

This is when the reports about “radiation leakage” starting coming in.
I believe I explained above why venting the steam is theoretically the
same as releasing radiation into the environment, but why it was and
is not dangerous. The radioactive nitrogen as well as the noble gases
do not pose a threat to human health.

At some stage during this venting, the explosion occurred. The
explosion took place outside of the third containment (our “last line
of defense”), and the reactor building. Remember that the reactor
building has no function in keeping the radioactivity contained. It is
not entirely clear yet what has happened, but this is the likely
scenario: The operators decided to vent the steam from the pressure
vessel not directly into the environment, but into the space between
the third containment and the reactor building (to give the
radioactivity in the steam more time to subside). The problem is that
at the high temperatures that the core had reached at this stage,
water molecules can “disassociate” into oxygen and hydrogen – an
explosive mixture. And it did explode, outside the third containment,
damaging the reactor building around. It was that sort of explosion,
but inside the pressure vessel (because it was badly designed and not
managed properly by the operators) that lead to the explosion of
Chernobyl. This was never a risk at Fukushima. The problem of
hydrogen-oxygen formation is one of the biggies when you design a
power plant (if you are not Soviet, that is), so the reactor is build
and operated in a way it cannot happen inside the containment. It
happened outside, which was not intended but a possible scenario and
OK, because it did not pose a risk for the containment.

So the pressure was under control, as steam was vented. Now, if you
keep boiling your pot, the problem is that the water level will keep
falling and falling. The core is covered by several meters of water in
order to allow for some time to pass (hours, days) before it gets
exposed. Once the rods start to be exposed at the top, the exposed
parts will reach the critical temperature of 2200 °C after about 45
minutes. This is when the first containment, the Zircaloy tube, would
fail.

And this started to happen. The cooling could not be restored before
there was some (very limited, but still) damage to the casing of some
of the fuel. The nuclear material itself was still intact, but the
surrounding Zircaloy shell had started melting. What happened now is
that some of the byproducts of the uranium decay – radioactive Cesium
and Iodine – started to mix with the steam. The big problem, uranium,
was still under control, because the uranium oxide rods were good
until 3000 °C. It is confirmed that a very small amount of Cesium and
Iodine was measured in the steam that was released into the
atmosphere.

It seems this was the “go signal” for a major plan B. The small
amounts of Cesium that were measured told the operators that the first
containment on one of the rods somewhere was about to give. The Plan A
had been to restore one of the regular cooling systems to the core.
Why that failed is unclear. One plausible explanation is that the
tsunami also took away / polluted all the clean water needed for the
regular cooling systems.

The water used in the cooling system is very clean, demineralized
(like distilled) water. The reason to use pure water is the above
mentioned activation by the neutrons from the Uranium: Pure water does
not get activated much, so stays practically radioactive-free. Dirt or
salt in the water will absorb the neutrons quicker, becoming more
radioactive. This has no effect whatsoever on the core – it does not
care what it is cooled by. But it makes life more difficult for the
operators and mechanics when they have to deal with activated (i.e.
slightly radioactive) water.

But Plan A had failed – cooling systems down or additional clean water
unavailable – so Plan B came into effect. This is what it looks like
happened:

In order to prevent a core meltdown, the operators started to use sea
water to cool the core. I am not quite sure if they flooded our
pressure cooker with it (the second containment), or if they flooded
the third containment, immersing the pressure cooker. But that is not
relevant for us.

The point is that the nuclear fuel has now been cooled down. Because
the chain reaction has been stopped a long time ago, there is only
very little residual heat being produced now. The large amount of
cooling water that has been used is sufficient to take up that heat.
Because it is a lot of water, the core does not produce sufficient
heat any more to produce any significant pressure. Also, boric acid
has been added to the seawater. Boric acid is “liquid control rod”.
Whatever decay is still going on, the Boron will capture the neutrons
and further speed up the cooling down of the core.

The plant came close to a core meltdown. Here is the worst-case
scenario that was avoided: If the seawater could not have been used
for treatment, the operators would have continued to vent the water
steam to avoid pressure buildup. The third containment would then have
been completely sealed to allow the core meltdown to happen without
releasing radioactive material. After the meltdown, there would have
been a waiting period for the intermediate radioactive materials to
decay inside the reactor, and all radioactive particles to settle on a
surface inside the containment. The cooling system would have been
restored eventually, and the molten core cooled to a manageable
temperature. The containment would have been cleaned up on the inside.
Then a messy job of removing the molten core from the containment
would have begun, packing the (now solid again) fuel bit by bit into
transportation containers to be shipped to processing plants.
Depending on the damage, the block of the plant would then either be
repaired or dismantled.

Now, where does that leave us?

The plant is safe now and will stay safe.

Japan is looking at an INES Level 4 Accident: Nuclear accident with
local consequences. That is bad for the company that owns the plant,
but not for anyone else.

Some radiation was released when the pressure vessel was vented. All
radioactive isotopes from the activated steam have gone (decayed). A
very small amount of Cesium was released, as well as Iodine. If you
were sitting on top of the plants’ chimney when they were venting, you
should probably give up smoking to return to your former life
expectancy. The Cesium and Iodine isotopes were carried out to the sea
and will never be seen again.

There was some limited damage to the first containment. That means
that some amounts of radioactive Cesium and Iodine will also be
released into the cooling water, but no Uranium or other nasty stuff
(the Uranium oxide does not “dissolve” in the water). There are
facilities for treating the cooling water inside the third
containment. The radioactive Cesium and Iodine will be removed there
and eventually stored as radioactive waste in terminal storage.

The seawater used as cooling water will be activated to some degree.
Because the control rods are fully inserted, the Uranium chain
reaction is not happening. That means the “main” nuclear reaction is
not happening, thus not contributing to the activation. The
intermediate radioactive materials (Cesium and Iodine) are also almost
gone at this stage, because the Uranium decay was stopped a long time
ago. This further reduces the activation. The bottom line is that
there will be some low level of activation of the seawater, which will
also be removed by the treatment facilities.

The seawater will then be replaced over time with the “normal” cooling water

The reactor core will then be dismantled and transported to a
processing facility, just like during a regular fuel change. Fuel rods
and the entire plant will be checked for potential damage. This will
take about 4-5 years.

The safety systems on all Japanese plants will be upgraded to
withstand a 9.0 earthquake and tsunami (or worse) I believe the most
significant problem will be a prolonged power shortage. About half of
Japan’s nuclear reactors will probably have to be inspected, reducing
the nation’s power generating capacity by 15%. This will probably be
covered by running gas power plants that are usually only used for
peak loads to cover some of the base load as well. That will increase
your electricity bill, as well as lead to potential power shortages
during peak demand, in Japan.

If you want to stay informed, please forget the usual media outlets
and consult the following websites: