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Nuclear reactors in the news - accurate reporting? - Page 2

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Posted by Gordon on March 23, 2011, 9:20 pm
 


OK, I stand corrected.
My point being (In response to the comment that they were
vertually identical to American BWR plants) that they were
infact American designes and that they were identical.
Since they were designed and built (I presume) by GE,
an American company.

Posted by daestrom on March 23, 2011, 10:52 pm
 
On 3/23/2011 17:20 PM, Gordon wrote:

Yes, that's certainly true.  The US only has two BWR-2 plants (Oyster
Creek and Nine Mile Point Unit 1).  Many of the older BWR are -3 and -4
design, virtually identical to Fukushima Daiichi plants.

But my co-workers and I have been trying to figure out just what
differences there are.  One we found so far is the output connections
for the generator, seem to be in another building instead of an open
switch-yard like US plants.

The US BWR's with a Mark I containment had to do a 'back-fit' some years
ago regarding the containment vent lines and we haven't been able to
figure out if they had done something similar.

A regulation in the US that came out around '88 was the 'station
black-out rule'.  This required US plants to either provide yet another
on-site power source or demonstrate exact procedures that could be used
to preserve battery life for a 4 hours.  Again, we don't know if the
Japanese had similar rules.  Although, obviously in this event 4 hours
of battery life wouldn't have really made much difference (their
batteries actually provided power for something on the order of 8 hours).

daestrom


Posted by daestrom on March 20, 2011, 10:03 pm
 On 3/20/2011 12:56 PM, Morris Dovey wrote:

One issue is that the reactor vessel isn't stainless-steel through and
through.  It is between six and eight inches of low-carbon steel,
high-strength steel.

Another is that the media is not always clear about which component they
are discussing.  In a BWR (or PWR) there are three boundaries between
the fission-products and the public.  (actually, a forth also exists
called the 'secondary containment' but credit isn't taken for it in most
accidents.)

The first barrier is the tubes containing the fuel pellets, called the
cladding.  This is made from zircaloy (an alloy of about 98% zirconium,
1.5% tin and some trace other metals).

The second barrier is the one holding the water coolant, called the
reactor vessel (or 'reactor pressure vessel' RPV).  That's made from
high-strength steel and for a BWR has an internal design pressure of
about 1250 psi and normal operating temperature of about 545 degF.

The third barrier is called the primary containment.  It is a
steel-reinforced concrete structure that has a steel liner.  The liner
is only about 1/2 inch thick and is not intended for structural strength
but merely forms a leak-tight liner on the inside of the concrete
structure.  The Mark I containment is designed for about 60 psi and 350
degF (although under normal operation it is kept at only about 1 psi and
120 degF).  The original design strength was based on a sudden breaking
of the largest pipe holding the reactor coolant and the containment
being able to contain the resulting steam/water that would result.

The BWR reactors use a smaller containment around the reactor than PWR
reactors.  This was a lot cheaper to build but of course still had to
'contain' the energy/mass from that large pipe break.  Because of its
smaller size, the peak pressure would have been much higher than in a
PWR containment.  To remain cost-effective, the BWR containment needs a
way to suppress this pressure spike.  In order to do this, the BWR
containment has an ingenious arrangement to force all the hot steam from
such a break into a large pool of water that will condense the steam,
suppressing the pressure spike from such an accident.  In the 'Mark I'
design, that pool of water is kept in the torus.

But Japan's reactors didn't have a large pipe break that dumped a lot of
steam/water into the containment, so that is pretty much a moot point.



When talking about the 'reactor' and all that, I've noticed that the
media sometimes confuses the reactor vessel and the primary containment.
  And they often confuse the terms 'core damage' and 'meltdown'.

'Core damage' has a very specific definition, related to damage to the
cladding material.  An accident that can directly cause some of the fuel
rods/pins to rupture, releasing fission products into the coolant is
'core damage'.  It can be a relatively small amount of radioactivity
released into the coolant and still be classified as a 'core damaging
event'.  Such events can be just enough to require additional
maintenance all the way up to TMI type of 'core damage'.

'Meltdown' is not as specifically defined.  To some it can mean
overheating the cladding to the point that it melts, spilling the fuel
pellets into the coolant (a severe form of 'core damage').  Others
define it as melting the actual fuel (zircaloy clad has a much lower
melting point than the UO2 fuel, so you can have melt the latter without
melting the latter).

Another definition sometimes feared is having the clad melt, deposit a
lot of fuel into the bottom of the reactor pressure vessel and have the
vessel heat up to the point that it weakens and fails.  This of course
releases the fission products from the first and second barrier into the
containment.

And finally, the third idea of 'meltdown' is that after the fuel fails
out the bottom of the failed reactor vessel, it would burn/melt its way
out the bottom of the containment structure.

So when discussing 'meltdown', you have to listen hard to figure out
which version / stage the speaker is referring to.

daestrom

Posted by Jim Wilkins on March 20, 2011, 10:30 pm
 
This is the only kind of meltdown reporters understand:


jsw

Posted by sno on March 21, 2011, 4:24 am
 On 3/20/2011 11:56 AM, Morris Dovey wrote:

I have often wondered if the poor reporting is caused by lack of
scientific understanding.....by the politicians and the reporters....
If a politician does not really understand something he is going to
leave things out, so as not to appear "dumb"...and a news person will
take what her hears and convert it to what he thinks he understands....

Language is often a poor means of conveying information...unless to
people have a common background and have the same internal definition of
the words used....

I imagine in this case they are going to the physicists for help in
explaining things that are going on....these scientific people are
trying to convey information using the vernacular of their science...

When you are used to thinking about something one way it is hard to
think of it in another way in order to explain it....

Some good teachers develop this skill over time....

I would think that engineers/technicians  would be able to explain
things in a simplified/more understandable manner.....and they are who
they should go to for help in understanding.....

just thinking.....have fun.....sno

--
Correct Scientific Terminology:
Hypothesis - a guess as to why or how something occurs
Theory - a hypothesis that has been checked by enough experiments
  to be generally assumed to be true.
Law - a hypothesis that has been checked by enough experiments
  in enough different ways that it is assumed to be truer then a theory.
Note: nothing is proven in science, things are assumed to be true.


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