Canadian CANDU nuclear reactors can't melt down or go critical the way
that these GE reactors are doing in Japan.
It's too bad that they were basically forced into using the GE rectors
in Japan. Now we will have a new generation of people in Japan that can
thank the US for the nuclear "gift" that keeps on giving.
It is the reactor design.
Even when all the control rods are inserted to stop the reaction, the
core still operates at 7% heat output - not zero percent. A constantly
operating coolant system must be available at all times to maintain this
type of reactor in a safe state, even during shut-down. Clearly in an
area prone to earth quakes and tsunami's, such a requirement seems to be
===========Canadian CANDU reactor overview:
The large thermal mass of the moderator provides a significant heat sink
that acts as an additional safety feature. If a fuel assembly were to
overheat and deform within its fuel channel, the resulting change of
geometry permits high heat transfer to the cool moderator, thus
preventing the breach of the fuel channel, and the possibility of a
meltdown. Furthermore, because of the use of natural uranium as fuel,
this reactor cannot sustain a chain reaction if its original fuel
channel geometry is altered in any significant manner.
Today there are 29 CANDU reactors in use around the world, and a further
13 "CANDU-derivatives" in use in India (these reactors were developed
from the CANDU design after India detonated a nuclear bomb in 1974 and
Canada stopped nuclear dealings with India). The countries the reactors
are located in are:
* Canada: 17 (+3 refurbishing, +5 decommissioned)
* South Korea: 4
* China: 2
* India: 2 (+13 in use, +3 under construction)
* Argentina: 1
* Romania: 2 (+3 under construction, currently dormant)
* Pakistan: 1
CANDU fuel bundles, each about 50 cm in length and 10 cm in diameter,
weight approx. 20 kg (44 lb), generate about 1 GWh of electricity during
its time in the reactor.
The Bruce Nuclear Generating Station, the second multi-unit CANDU
station, was constructed in stages between 1970 and 1987 by the
provincial Crown corporation, Ontario Hydro. It consists of eight units
each rated at approximately 800 MWe each, and is currently owned by
Ontario Power Generation (OPG) and run by Bruce Power.
The Bruce station is the largest nuclear facility in North America, and
second largest in the world (after Kashiwazaki-Kariwa in Japan),
comprising eight CANDU nuclear reactors having a total output of 6,232
MW (net) and 7,276 MW (gross) when all units are online. Current output
with six of the eight reactors on line is 4,640 MW. Restart of the
remaining two units is planned by 2012.
(note: The Kashiwazaki-Kariwa reactor mentioned above is NOT a
CANDU-type reactor. It is a Boiling Water varient of a Light Water
Reactor, made by General Electric).
You forgot this:
"The main difference between CANDUs and other water moderated reactors
is that CANDUs use heavy water for neutron moderation. The heavy water
surrounds the fuel assemblies and primary coolant.
The heavy water is unpressurized, and a cooling system is required to
keep it from boiling."
The big problem with ALL nukes is cooling.
Lose cooling and you get disaster.
You see what's going on in Japan?
Think there's 6 reactors on one site.
And cooling pools for depleted rods.
Those also need cooling or you get a disaster.
They cool depleted rods for 6-10 years before they can encase them for
And the cooling pools aren't in a containment vessel like the active
That's what's happening in Japan.
Even the depleted rods in the cooling pools are melting down and
releasing radiation to the atmosphere.
I think nukes are a good energy source, but when it goes wrong, it
goes VERY wrong.
After this Japan disaster, I only see 2 options for nukes going
forward. You need to do both.
1. Radical redesign so cooling loss can't cause disaster.
By "disaster" I mean environmental disaster.
That's what's scary about nukes. Last I saw 140k people have been
evacuated from around the Japan nuke plant.
No wonder nukes are subject to NIMBY.
If things go bad you can abandon the site, and no harm is done except
loss of investment and real estate.
This means cooling pools must also be in containment.
2. NEVER have "too much" fissionable material at one site.
Go smaller, not bigger.
That way when one breaks, there's no way it can be a huge disaster
like what might happen in Japan.
And they will break. Nobody believes that won't happen.
Building more and smaller nuke plants would be more expensive, but
that's how it is.
Anyway, that's my cracker barrel view as a newsgroup physicist.
That's when it's operating. A Candu core can be shut down without
needing a cooling system to remain functioning after shutdown.
This is the key point:
Criticality of CANDU fuel bundles in light water is impossible, avoiding
one concern of severe accident analyses that light-water reactors must
contend with. Furthermore, since the geometry of the CANDU core is near
optimal from a reactivity standpoint, any rearrangement under severe
accident conditions ensures shutdown.
The CANDU system is a strong example of safety through both engineered
redundancy and passive design. The core has numerous triple-redundant
detectors that feed to two logically, conceptually and physically
separate shutdown systems (shut-off rods and high-pressure poison
injection). Each system is capable of shutting down the core within 2
seconds following a LOCA ("Loss-of-Coolant Accident" -- the design-basis
accident for CANDU reactors), without credit given to operator
In addition to engineered safety systems, CANDU reactors have a number
of inherent safety features that distinguish it from other reactor
designs (e.g. PWRs, BWRs):
* The subdivision of the core into two thermalhydraulic loops (in
most CANDU designs), and hundreds of individual pressure tubes within
each loop, localizes a LOCA (Loss-of-Coolant Accident) to one small
region of the core, and reduces the reactivity effect of a LOCA
accordingly. Furthermore, the two core-passes per loop mean that only a
quarter of the core would likely suffer a mismatch between heat
generation and removal under such conditions (and only the highest-power
fuel elements within this one-quarter-core region).
* The large-volume, low-pressure, low-temperature moderator
surrounding the pressure tubes acts as a heat sink in large LOCA
scenarios, rendering negligible the risk of "fuel meltdown". The
moderator, in turn, is surrounded by a thick light-water shield tank
(used for biological and thermal shielding) which can also act as a heat
sink in severe accident scenarios.
* The moderator also provides a low-pressure environment for the
control rods, eliminating the "rod-ejection" scenarios considered in PWR
safety analyses. In addition, the location of neutronics measurement
devices in the moderator avoids subjecting this equipment to a hot,
* Heavy-water neutron kinetics is slower by several orders of
magnitude than light-water kinetics, reducing the discontinuity between
prompt and delayed kinetic behaviour, and making control easier.
* Criticality of CANDU fuel bundles in light water is impossible,
avoiding one concern of severe accident analyses that light-water
reactors must contend with. Furthermore, since the geometry of the CANDU
core is near optimal from a reactivity standpoint, any rearrangement
under severe accident conditions ensures shutdown.
* On-power refuelling means that the power distribution reaches an
equilibrium within a year of start-up, and remains virtually unchanged
for the reactor's operating life. This greatly simplifies the analysis
of core behaviour as a result of postulated accidents.
* On-power refuelling also allows defective fuel to be detected and
removed from the core, reducing the contamination of the reactor coolant
piping and simplifying maintenance.
* The low excess reactivity of the CANDU core leads to relatively
low reactivity worth of the control devices, limiting the potential
severity of postulated loss-of-regulation accidents.
* The positioning of the steam generators well above the core
promotes natural thermosyphoning (i.e. movement due to the coolant's own
density differences), which can remove decay heat if shut-down cooling
is lost. At the same time, the large amount of small-diameter piping in
the feeder network acts as a natural "radiator" under such conditions.
This significant amount of inherent, or "passive", safety in the CANDU
system, in conjunction with fast-acting, robustly engineered safety
systems and backup safety systems, is the reason why a complex
technology like nuclear power can be one of the safest and most reliable
energy options available.
Didn't know about CANDU reactors. But I've read up a bit.
I can't argue about heavy water versus light water reactors.
But they both use uranium.
Saying a CANDU can't melt down when it loses cooling is just wrong.
That's what they all say.
Let's wait until a CANDU loses cooling, then we'll see.
I'll stick with what I said.
Good containment of cores and depleted rod cooling pools, and never
put too much uranium in one place.
Smaller reactors/generating plants, and spread them around.
Then when the shit hits the fan it's not a huge disaster.
Just a small disaster.
"Burning gas, coal and oil kills more people every day than nukes have
done in 50 years."
That's until now. We won't know until the stuff has cooled down and the
extent of contamination is known.
There are very expensive lessons to be learned from this quake and
tsunami. Especially on the West coast. Hopefully the lessons will
indeed be studied and acted upon, both the physical threats directly from
a tsunami, and the nuclear physics threats from misbehaviors of nuclear
Still, nuclear power seems to me to be a very viable alternative energy
source, IF properly executed.
Sarcasm noted ...
Punishing I leave to the justice systems. Wait, that involves lawyers.
Maybe your god can do it?
By lessons executed I meant (you didn't grok that <grin>?) acted upon the
lessons and change whatever needs changing. In ths case, it seems
particularly important to make sure that total loss of power doesn't lead
to damage of the kinds seen in the Japanese reactors, or make sure the
backup power can't be interrupted by earthquakes or tsunamis. The latter
probably evokes: Fat chance!
I, too, hope that some important lessons will be learned here, especially
since we have serious earthquake vulnerability on the West Coast. But then
I think about what I thought we learned in Vietnam and where we are now and
I would say that in 25 years, nearly all lessons learned are forgotten
On Wed, 16 Mar 2011 11:23:08 -0400, "Robert Green"
US wars are always to insure that wealthy Americans can invest abroad
safely. The soldiers are there to protect that investment. What was
the lesson from Vietnam? To continue to have wars overseas so that the
wealthy can make more money, but try to make sure that the outcome is
successful. The lesson was not to stop getting involved in foreign
Sad but true. Vietnam is now supplying us with computer parts for even LESS
than Chinese wage slaves can make them. Conquer and turn into a sweat shop.
Ironically, we didn't even win in 'Nam!
I had hoped we had learned that a war in which we can't tell friend from foe
is a war to avoid. Obviously not. Amazingly in the 80's the various war
colleges were full of "lessons learned" from 'Nam but when the old war wagon
got rolling and fast promotions started coming, all bets were off.
Indeed, that is the situation today. We don't know how many of the heroic
personnel working on site have been sickened, and how badly. That will not
be apparent until later.
Moreover, the disaster is still progressing, and if I have to believe
others with more knowledge of the half-lives of the fission products, that
will be several to many weeks from now.
Also, with all those attempts to use seawater for cooling, where has all
that gone? I expect that what hasn't become steam, did run back to the
ocean, with the radioactive "shit" that dissolved in it or was washed away
by it, all into the sea.
Well, when your local gas-fired plant develops a rupture and explosion
of it's fuel source, or when your nearby oil-rig blows up and spews
crude oil all over the ocean, or when dozens of miners die in a coal
mine digging coal for your local coal-fired electricy plant, or when you
divert entire rivers or flood huge areas to create lakes to generate
hydro power, it all comes back to the same thing: People want
electricity, there are nasty consequences to the ways we generate it,
and there are too many people on this planet in the first place.
Here's a fact. Unless the Canadians have re-invented the laws of
your whole premise and understanding of what's going on is wrong. No
nuclear reactor can go from 100% power to zero power instantly or
even a few hours. That has nothing to do with the reactor design,
has everything to do with physics. The fission of uranimum produces
radioactive byproducts that in turn decay over time. That decay
for hours and days after the control rods are inserted. The control
absorb neutrons and stop the uranium chain reaction, but do nothing
to stop the self decay of the other radioactive elements. Every power
reactor has to have some means of removing that waste heat or the
reactor will start to melt down.
Also, nothing in that cite says anything
close to what you claim it does. It comments on one narrow aspect
of the design. Show us where it says cooling water is not critical
after inserting the control rods.
I'ts also particularly foolish to start claiming some Canadian
which your obvioulsy don't understand, is superior and would have
prevented the accident. Wouldn't it be better to first at least
out the full story and sequence of events from an investiation?
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