On Saturday 09 March 2019 23:43:26 Gregg Eshelman via Emc-users wrote:
The one problem I see as being really troublesome with the design of
Fukushima is that it apparently was incapable of being fully self
powering of all its systems at any time.
No, not true. They had at least SEVEN Diesel generators.
Crazily, many of them were in the basement of the buildings,
where they could get flooded. Also, a number of them were
sea-water cooled, and when the tsunami hit, the first thing
it did was submerge the giant sea water pump motor and short
it out. So, they lost all their sea water cooling.
Who designs a power plant
able to run for 25 years or so, producing electricity, without needing
to be refueled, that does not tap its own power generation to run all
of its electronics, pumps etc?
The problem is the turbines, alternators and their exciters
are not designed to run over a 1000:1 power range. This was
what caused the big mess at Chernobyl. In the US, we
require a UPS on the most critical items such as primary
coolant circulating pumps. Then, we back that up with
fast-start Diesel generators that can be on line at full
power in under six seconds. This stuff is REALLY expensive,
especially a UPS that will run FOUR 1000 Hp pumps. The
Russians didn;t want to pay for that, so they came up with
the idea they could run the critical loads off the inertia
of the turbine-alternator set for a minute while the Diesels
came on line. This did require the alternator exciter to be
able to regulate the alternator's output voltage basically
down to zero current. Well, after making the mods to the
exciter, they had to test it. Doing a test like this on an
actual, operating reactor requires extremely careful
planning, and training of all personnel in exactly what to
do, when, and what to do if anything diverges from the plan.
So, they scheduled this test to be done right before a
refueling shutdown. But, somebody didn't get the word, and
shut down the reactor. Uranium reactors build up a huge
amount of radioactive iodine as a fission daughter product,
and it is a strong neutron absorber. So, right after you
shut down the reactor, the iodine builds up, and poisons the
reaction, It can take a whole DAY for the reaction to build
back up and "burn off" the iodine. Well, they needed the
reactor to be above some power level to run the test, so
they pulled all the control rods ALL the way out, to try to
get some reaction going again. Now, when ready for
refueling, the reactor is full of Plutonium, which is more
reactive than uranium. (Note, the Chernobyl RBMK-1000
reactor is identical to a graphite pile Plutonium production
reactor they used for their weapons program, so it is
ENGINEERED to make Plutonium, it isn't an unwanted byproduct
like commercial power reactors.) So, when refueling is
needed, the reactor is quite unstable with all that
Plutonium, and pulling the control rods all the way out
makes it worse. So, they cut over from grid power to their
own alternator, and as the station ran off the inertia of
the alternator, the alternator SLOWED DOWN! This meant the
frequency decreased, and any pumps running off that power
began to slow down. Now, the next horror of the RBMK-1000
is that it has a "positive void coefficient". That means
that if the cooling water boils, it INCREASES the nuclear
reaction rate!
This is due to the graphite being the neutron moderator, and
the water being a neutron absorber. It isn't clear exactly
what the next chain of events and actions were, but at some
point they canceled the test and scrammed the reactor.
Another odd feature of the control rods of that reactor is
the bottom half meter of them has a big graphite plug. The
control rods sit in the middle of the cooling water pipes of
the reactor (the RBMK-1000 is not immersed in water like
most other reactors). So, driving dozens of control rods
inward opposed the flow of the cooling water, the water
started to boil, the slugs of graphite entering the core
increased the reactivity, and the reaction ran away very
quickly.
This insane test, of course, should have NEVER been done on
a fueled reactor. But, that's Russia for you.
No nuclear power plant should require
an external electricity supply for anything as long as at least one
reactor is 'hot' and one turbine is running.
Well, that's why they have BANKS of Diesel generators!
IMHO, an ideal
multi-reactor power plant should have one small
reactor/turbine/generator set for powering everything in the facility.
No, you have to have massive amounts of electrical power to
even begin to start a small reactor.
The Callaway County plant is a pressurized water reactor,
and has FOUR 1000 Hp circulating pumps. In fact, the
bring-up procedure after a shutdown is to use the pumps to
warm the reactor to near operating temperature, as the
reactor itself would raise the temperature too quickly.
One the size of what's used in a nuclear submarine. Under normal
conditions its output would be added to what the big reactors and
generators produce, but in an emergency where the big reactors are
shut down, the little one would stay up and running, in its well
armored and isolated, flood proof, building, supported on isolating
springs.
Diesel generators can be started in seconds when needed, a
reactor takes hours or DAYS to be started safely.
When the earthquake hit, Fukushima went to automatic shutdown.
Apparently the external power supply also went down so the diesel
generators kicked in. That's where the trouble began. The tsunami took
out the generators, which shut down the cooling pumps. Since there
wasn't any other way to get power to the pumps to cool down the
reactor cores, they heated up to the point where the water in the
vessels split into hydrogen and oxygen, which then caused explosions.
Well, not exactly right. The various items in the reactor
core must not absorb neutrons or the reactor won't work.
With a limited choice of material, it is practice to make
the fuel rod cladding from Zirconium. Hot Zirconium in
water is kind of like pouring water on burning Magnesium, it
splits the water to get the Oxygen. This can start as soon
as the reactor core is uncovered, and the Zirconium burns
fine in pressurized steam. From a neutron perspective,
Zirconium is ideal, from all other perspectives, it is one
of the worst things to use.
Better protected generators, and more of them, plus modern control
systems with the ability to quickly self test for damage and get back
online after a SCRAM initiated by the earthquake sensors - might have
gotten one core back online and the facility back under self power in
the time between the quake and the tsunami hitting. But that couldn't
happen because Fukushima was forced to be frozen in the technological
past.
Is Japan still keeping all their nuclear plants idle?
No, they have restarted a few of their better-engineered plants.
Of course, the concept of placing the oldest plants RIGHT on
the ocean in the country that has known about the tsunami
phenomenon for thousands of years was just insane. They DO
have a problem that the government is under the thumb of
industry there.
(My apologies for a very long answer to an off-topic thread.)
Jon
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