[laffs] Fw: Japan Nuclear Reactors
- From: "Gene Hatfield" <hatter@xxxxxxxxxxx>
- To: <"Undisclosed-Recipient:;"@freelists.org>
- Date: Tue, 15 Mar 2011 09:42:46 -0500
----- Original Message -----
From:Gene
To:Sent: Tuesday, March 15, 2011 8:54 AM
Subject: Fw: Japan Nuclear Reactors
----- Original Message -----
From: Gene, A long read but very interesting.
You Can Stop Worrying About A Radiation Disaster In Japan -- Here's Why
(Link to this article:
http://www.businessinsider.com/japan-reactors-pose-no-risk-2011-3)
This was originally posted as a comment on Japan Death Toll Climbs
Astronomically As Nuclear Crisis Spreads.
UPDATE: Since posting this, we have learned that it was written by Dr. Josef
Oehmen, a research scientist at MIT. It was originally posted here.
I repeat, there was and will *not* be any significant release of radioactivity
from the damaged Japanese reactors.
By "significant" I mean a level of radiation of more than what you would
receive on - say - a long distance flight, or drinking a glass of beer that
comes from certain areas with high levels of natural background radiation.
I have been reading every news release on the incident since the earthquake.
There has not been one single report that was accurate and free of errors (and
part of that problem is also a weakness in the Japanese crisis communication).
By “not free of errors” I do not refer to tendentious anti-nuclear journalism –
that is quite normal these days. By “not free of errors” I mean blatant errors
regarding physics and natural law, as well as gross misinterpretation of facts,
due to an obvious lack of fundamental and basic understanding of the way
nuclear reactors are build and operated. I have read a 3 page report on CNN
where every single paragraph contained an error.
We will have to cover some fundamentals, before we get into what is going on.
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.
Read more:
http://www.businessinsider.com/japan-reactors-pose-no-risk-2011-3#ixzz1GcDmRcKD
Other related posts:
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