Thorium Accelerator Driven Nuclear Power - Why not ?

[Darryl]

I had heard about a nulcear reactor, that did not require a critical mass, and therefore did not have the possibility of Loss of coolant or meltdown.

a reactor that required power to run, so if you want you can "Switch it off".

seems the present day reactors create a critcal mass, and all the control systems, (control rods, whatever) are inplace to stop meltdown from occurring. if the control system fail the "reactor will react more" and meltdown.
a standard nuclear power generator design is not "failsafe" but rather, its "FAILUNSAFE".

and accelerator driven power generation system will stop, if you turn off the accelerator.

when i googled ADR nuclear power, the first site i found was by none other Green Peace.

from there i was directed to a paper giving real values of the availabilty of thorium, and how thorium is much safer to mine, and more energy than uranium. and the reactor design produces a very small amount of waste.

low waste and a fairly short half life, the reactore itself can actually treat the waste and with low volume waste, ( and accountable waste), its possible to easily manage it. safely.

Thorum is very common, and from what I've read it is plentiful enough to supple 10 times the worlds present electricity requirements, for the next 1200 centuries !.

so:

are there problems i have not read about.

why are these system not promoted, (is it because they don't make weapons grade stuff ?

why is this technology being heavily promoted as a viable option. ??

i guess they split the atom all the time at CERN, but NO ONE thinks that there will be a meltdown, there is just not enough stuff.

this looks like the same thing,, mabey its got a name problem, and take away the words "Nuclear reactor".

http://members.greenpeace.org/phpBB2/viewtopic.php?t=187&start=0&postdays=0&postorder=asc&highlight=

http://www.uic.com.au/nip67.htm


comments ?

 

[Azael]

I asked a similar question in this thread and got very good replies :)
https://www.physicsforums.com/showthread.php?t=132739

 

[Azael]

Thorium fuel cycle is beeing developed in India btw, they have plenty of thorium but not much uranium. I guess the rest of the world hasnt really bothered with it yet because we have plenty of uranium aswell

http://www.world-nuclear.org/info/inf62.htm

 

[ZapperZ]

are there problems i have not read about.

why are these system not promoted, (is it because they don't make weapons grade stuff ?

why is this technology being heavily promoted as a viable option. ??

i guess they split the atom all the time at CERN, but NO ONE thinks that there will be a meltdown, there is just not enough stuff.

No, they don't "split the atom" at CERN. Particle colliders don't do that.

this looks like the same thing,, mabey its got a name problem, and take away the words "Nuclear reactor".

http://members.greenpeace.org/phpBB2/viewtopic.php?t=187&start=0&postdays=0&postorder=asc&highlight=

http://www.uic.com.au/nip67.htmcomments ?

The one thing you haven't accounted for is the "wall plug efficiency". If you are using CERN as your reference point, then maybe you also need to figure out how much power is being used up JUST to make those few meager collisions, i.e. what is known as the "luminosity" of the interaction. Think of how much it is required to run such an accelerator, and then factor that out on how much power you get suck out of any nuclear reaction that is produced from such a collision.

This is, of course, way before we even consider the feasibility of generating any kind of nuclear fusion in terms of the physics from such a scenario. I mean, just look at RHIC (which would have been a closer comparision than CERN). Do you see them creating fusion interaction and able to extract any useable power out of such a nuclear-nuclear (such as Au-Au) collision?

On a separate note, I am utterly puzzled why people think that an accelerator is a viable alternative to generate nuclear power. This issue seems to be coming up rather often on here. I mean, I know what goes into an accelerator (I work at one), and the amount of effort and energy that is needed just to accelerate electrons alone, much less heavier nuclei (see RHIC). Did people simply not look at all those giant Klystrons that gobbled up megawatts and megawatts of power just to be able to deliver sufficent energies needed by these accelerators? I mean, forget about breakeven, I don't think you can even get 1% efficiency, assuming that you CAN in fact generate fusion reaction in the first place (something I highly doubt from a particle collider).

Zz.

 

[Darryl]

i certainly don't think an accelerator is a vaible alternative to nuclear power, and i was mistaken about CERN, my point was calling something a "Nuclear Reator" creates a fear about meltdowns etc.

and accelerator driven nuclear reaction does not (i believe) use very high energy particles from the accelerator, but low energy neutrons, (or something).

"In an ADS system, high-energy neutrons are produced through the spallation reaction of high-energy protons from an accelerator striking heavy target nuclei (lead, lead-bismuth or other material). These neutrons can be directed to a subcritical reactor containing thorium, where the neutrons breed U-233 and promote the fission of it. There is therefore the possibility of sustaining a fission reaction which can readily be turned off, and used either for power generation or destruction of actinides resulting from the U/Pu fuel cycle. The use of thorium instead of uranium means that less actinides are produced in the ADS itself."

whereas supercolliders type accelerators accelate particles to almost light speed, requireing vast amounts of energy.

ADS requires a stream of protons to liberate neutrons, and energy. as i read it. I am from Australia we apparently also have a lot of thorium. and uranium.

 

[Darryl]

Thanks Azael for pointing me to your other thread, i missed it in my wanderings.

THe main point of that thread seems to be that it required a very high energy neutron steam to get any energy out.

but my quick seach tells me, its a 1 GeV proton stream that is required.

I did not know if that is a lot or not, but for comparisong CERN is a 300GeV
ADS system only require 1/300th of that value, 1 GeV, medical image proton accelerators are being developed or allready in use, that generate 250 MeV, the paper also talked about building in think 1500 MW "modules", each powered by 1 accelerator.

i think the "Fail Unsafe" aspect of modern nuclear power generators is that you require a critical mass, and and self sustaining reaction, and your control systems have to function and work to control the reaction, if the control system fails the reaction still occures.

ADS if the control system fails, the proton beam fails, and the system shuts down.

 

[ZapperZ]

i certainly don't think an accelerator is a vaible alternative to nuclear power, and i was mistaken about CERN, my point was calling something a "Nuclear Reator" creates a fear about meltdowns etc.

and accelerator driven nuclear reaction does not (i believe) use very high energy particles from the accelerator, but low energy neutrons, (or something).

"In an ADS system, high-energy neutrons are produced through the spallation reaction of high-energy protons from an accelerator striking heavy target nuclei (lead, lead-bismuth or other material). These neutrons can be directed to a subcritical reactor containing thorium, where the neutrons breed U-233 and promote the fission of it. There is therefore the possibility of sustaining a fission reaction which can readily be turned off, and used either for power generation or destruction of actinides resulting from the U/Pu fuel cycle. The use of thorium instead of uranium means that less actinides are produced in the ADS itself."

whereas supercolliders type accelerators accelate particles to almost light speed, requireing vast amounts of energy.

ADS requires a stream of protons to liberate neutrons, and energy. as i read it. I am from Australia we apparently also have a lot of thorium. and uranium.

I work right next to a spallation source called the Intense Pulse Neutron Source. The ONLY reason why this is needed here (and at the Spallation Neutron Source being built at Oak Ridge) is the high flux and "monochromatic" energy&momentum distribution of neutrons. It takes a lot of effort and energy for that to occur and they are still struggling with the neutron intensity.

Again, the issue of "wall plug efficiency" is being neglected here in every discussion I have seen on such a proposal for a fusion source. Ask anyone working at such a facility about using it to generate fusion energy and they'll look at you funny.

Zz.

 

[theCandyman]

Again, the issue of "wall plug efficiency" is being neglected here in every discussion I have seen on such a proposal for a fusion source. Ask anyone working at such a facility about using it to generate fusion energy and they'll look at you funny.

Do you mean that it is not possible to off set the energy used in the accelerators by using the resulting neutrons to cause fission?

 

[Morbius]

i think the "Fail Unsafe" aspect of modern nuclear power generators is that you require a critical mass, and and self sustaining reaction, and your control systems have to function and work to control the reaction, if the control system fails the reaction still occures.

ADS if the control system fails, the proton beam fails, and the system shuts down.

Darrl,

You are VERY mistaken about what you characterize as the "Fail Unsafe"
nature of present nuclear reactors. Nuclear reactors ARE designed to be
"Fail Safe". However, in the Three Mile Island Unit 2 accident, the operators
OVERRODE the fail safe systems. The ability of the operators to override
the fail safe systems was mandated by law.

Also, please read my reply to Azael's thread.

Accelerator systems have EXACTLY the same problem with meltdowns
that a critical system has. When a critical system overheats; it naturally
shutsdown the fission power. That's NOT what melts the core. It's the
"decay power" of the fission products that will melt the core, if not cooled.

An accelerator-driven sub-critical system has the EXACT SAME PROBLEM!
When you shut off the accelerator, you don't shutoff the decay power of
the radioactive fission products.

An accelerator-driven sub-critical system is every bit as susceptible to
meltdown as is a critical system.

If it produces power; you have a meltdown problem unless you provide for
its mitigation. Sub-critical systems are no panacea.

Dr. Gregory Greenman
Physicist

 

[Morbius]

when i googled ADR nuclear power, the first site i found was by none other Green Peace.

from there i was directed to a paper giving real values of the availabilty of thorium, and how thorium is much safer to mine, and more energy than uranium. and the reactor design produces a very small amount of waste.

Darryll,

Greepeace is just plain 100% WRONG!

Dr. Gregory Greenman
Physicist

 

[theCandyman]

I agree, Greenpeace is the last source for nuclear related information I would use. Those people have an agenda, so they would likely make the information support their cause. They had something about the IAEA planning to stop promoting nuclear power becuae it was too dangerous (that might have been for April Fool's Day though, it was near that time that I lasted ran across anything from them).

 

[ZapperZ]

Do you mean that it is not possible to off set the energy used in the accelerators by using the resulting neutrons to cause fission?

Remember that in conventional fission reactor, the neutron has to be "thermalized", i.e. slowed down by water before it can cause fission in the fuel rods. So using an accelerator to produce neutrons that generally have a higher energy than the thermal neutrons defeats the whole purpose. A breeder reactor can operate at a larger neutron energy, but is this really in the range that you'd get out of a spallation source? I don't think so.

Still, the issue isn't just creating energy out of such a thing. The issue here is how much energy did you put in, and how much did you get out. If we don't care about that, then this isn't an engineering problem and so why are we talking about it? After all, there are no "physics issues" here. The efficiency of something like this is the whole reason why one would even want to consider building something like this in the first place. I haven't seen anything that come close to justifying using an accelerator to generate nuclear energy. I seriouly doubt that one would even get 1% efficiency. You'll end up sucking in MORE energy than you can produce. What kind of a power plant is that?

Zz.

 

[Morbius]

I agree, Greenpeace is the last source for nuclear related information I would use. Those people have an agenda, so they would likely make the information support their cause. They had something about the IAEA planning to stop promoting nuclear power becuae it was too dangerous (that might have been for April Fool's Day though, it was near that time that I lasted ran across anything from them).

Candyman,

Yes - another example of the self-serving LIES told by Greenpeace.

The IAEA doesn't promote nuclear power and it has nothing to say about reactor
safety. The charter of the IAEA is to police the Non-Proliferation Treaty, the NPT.

That's the IAEA's ONLY mission!

Greenpeace, as you say, has an agenda; and they have no scrupples about LYING
in support of that agenda.

Dr. Gregory Greenman
Physicist

 

[Morbius]

A breeder reactor can operate at a larger neutron energy, but is this really in the range that you'd get out of a spallation source? I don't think so.

ZapperZ,

Correct! Even in a breeder reactor, the median neutron energy is about 200-250 keV.

That puts the median energy right in the middle of the resolved resonance region.

When I was at Argonne National Laboratory, I worked on the design of an "inherently-
safe" or "passively-safe" breeder reactor called the Integral Fast Reactor or IFR.

The main prompt feedback mechanism for shutting down the reactor in the event of
overheating was Doppler broadening of absorption resonances. The fact that the
median energy is in the resonance region makes Doppler broadening of resonances
particularly strong.

If one wants to read about the characteristics of an "inherently safe" reactor, check
out this interview by PBS's Frontline of my former boss, Dr. Charles Till:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

Dr. Gregory Greenman
Physicist

 

[Morbius]

i think the "Fail Unsafe" aspect of modern nuclear power generators is that you require a critical mass, and and self sustaining reaction, and your control systems have to function and work to control the reaction, if the control system fails the reaction still occures.

Darryl,

WRONG!

Modern reactors have feedbacks to shutdown the reactor INDEPENDENT of the
control system!

For light water reactors, the moderator temperature feedback is the main feedback.
As ZapperZ explained, the neutrons in an LWR have to be slowed down or
"thermalized" in order to maintain the fission process. The neutrons are slowed by
collisions with the water. If the water heats up and becomes less dense - then the
collision rate decreases, there is less "slowing down" or "moderation" of the neutrons.

A critical reactor means you have an exact balance of neutron production and
destruction. If there is less moderation due to the heating of the water coolant /
moderator - that reduces the fission production; and the reactivity of the reactor
decreases. This happens INDEPENDENT of the control rods.

In fact, contrary to popular belief, a Pressurized Water Reactor or PWR; naturally
"load follows". You control the reactor with the temperature of the water coolant;
NOT the control rods. The control rods are basically a backup shutdown mechanism
in a PWR!

Doppler broadening of absorption resonances also works as an independent prompt
feedback mechanism in Light Water Reactors too.

Dr. Gregory Greenman
Physicist

 

[entropiax]

New high efficiency energy conversion based NASA HYTEC

Sorry guys, my english is poor.

In Hungary there is a new web site. Unfortunately it is hungarian. It contains brand new information about energy conversion. It is not publicated, because the theory bypasses the second law of thermodinamics. As I know, the guys afraid scientific scandal, this is the reason they has not been publicated it yet.

The GE has a US Patent, converting heat energy to electricity directly.
It is used by satellite nuclear power.
Patent number: 5,139,895, date of patent aug 18, 1992

This patent thermally regenerates lithiumhidride about 850 Celsius, cools back it and gets electric energy from electric cell.

regenaration:LiH = Li + 1/2 H2 and the cell: Li|LiH|1/2H2

Theoretically impossible the regeneration of a cell the same temperature the cell works, because in this way not necessary cooling, so possible converting heat energy to electricity 100 percent.
This bypasses the second law of thermodinamics.

The pure Li|LiH|1/2H2 cell can works 688 celsius because it is the lithiumhidride melting point.
You can regenerate this cell on 850 celsius.
But they recognized potassium can react lithiumhidride.

K + LiH = KH + Li, but the KH is not stable and potassium and lithium do not compose alloy.
So the can regenerate LiH cell with potassium on the same temperature the cell works!
In this way possible high effiency conversion heat energy to electricity.
80-90 percent.
If you convert 1m3 LiH/sec you can convert 1.5 GW heat energy to electricity!
You can cool a nuclear reaktor with potassium.
The main problem of accelator driven nuclear reactor is, you have to put lot of electric energy to accelerator, you get heat energy and during the wrong conversion efficiency of steam turbine (30%) the energy balance is not so good.
Example japanese Omega project.
140 MW for the accelarator, the reactor produces 820 MW thermic energy, and the steam turbine generates only 240 MW electricity.
We get only 100 MW at all.
But we use this energy conversion, and calculate with 80 percent of efficiency, we get 500 MW.

The website is http://secondlaw.acs.hu

 

[entropiax]

Sorry for lot of error

 

[entropiax]

sematic figure of this conversion

http://secondlaw.acs.hu/sematika.gif

hőenergia= heat energy
villamosenergia= electric energy
galváncella= cell
hidrogénáteresztő membrános gázelválasztó= hidrogen separation with membrane (ex. hidrogen can diffuse through Ni, but the potassium steam can't.)

 

[Andrew Mason]

Sorry guys, my english is poor.

In Hungary there is a new web site. Unfortunately it is hungarian. It contains brand new information about energy conversion. It is not publicated, because the theory bypasses the second law of thermodinamics. As I know, the guys afraid scientific scandal, this is the reason they has not been publicated it yet.
...

Theoretically impossible the regeneration of a cell the same temperature the cell works, because in this way not necessary cooling, so possible converting heat energy to electricity 100 percent.
This bypasses the second law of thermodinamics.

I am not sure what this has to do with nuclear physics but if you are telling us that it violates the second law of thermodynamics, you are telling us that it doesn't work. The actual mechanism is irrelevant. This is a perpetual motion machine (of the second kind). It can't work.

AM

 

[Darryl]

thanks a lot everyone, i certainly agree that Greenpeace is NOT a good source for this information. I know that what i read WILL be factural and accurate.

just one thing about "wall plug efficiency" is from my (limited) research, i believe coal powered generation on average worldwide is about 31% efficient.
with a lot of waste products.

I would assume "clean coal" power generation would be even less efficient bacause of the scrubbers/filters etc.

that wall plug efficiency, might just have to be accepted, for the sake of green house gas generation.

and yes, i do understand that its the latent heat, that causes the damage or meltdown, i believe there is a device that poisons the reaction if the temperature gets too high. basically a thermal fuse.

Im guessing this would be Boron in a container with a specific melting temperature.

thanks a lot for your input,

 

[Morbius]

i believe there is a device that poisons the reaction if the temperature gets too high. basically a thermal fuse.

Im guessing this would be Boron in a container with a specific melting temperature.

Darryl,

No - there are no "thermal fuses" with Boron in them in a nuclear reactor.

For a light water reactor [LWR]; the water coolant is also the moderator, the material
that slows down the neutrons. Neutrons have a higher probability of causing a fission
if they are slowed down than if they are faster.

As the reactor heats up, the water gets hotter, and therefore less dense. If the density
of the water goes down, then it is a poorer moderator. That's because the rate at which
neutrons will scatter from the water, which is the mechanism by which they slow down;
is proportional to the density of the water.

The neutron balance in a reactor is "delicate". That's why it's said to be "critical" -
critical in the sense that a small change can have a big result. In this case; since the
neutron production and destruction are exactly balanced so the reactor is at steady state,
a reduction in the moderation ability of the water coolant leads to a reduction in the
fission production because the neutrons are not slowed down as well. The reduction
of the fission production will mean it is no longer equal to the neutron destruction
mechanisms - i.e. leakage and capture - and the loss rate exceeds the production
rate and the reactor loses power.

There are other temperature dependent feedback mechanisms; like "Doppler" broadening
of absorption resonances. This one is more complex to explain. "Doppler" is the
dominant prompt feedback in the inherently safe IFR reactor. Although Doppler, to a
lesser extent is a feedback mechanism in LWRs, their dominant feedback is the
moderator / coolant termperature feedback described above.

Dr. Gregory Greenman
Physicist

 

[Andrew Mason]

just one thing about "wall plug efficiency" is from my (limited) research, i believe coal powered generation on average worldwide is about 31% efficient.
with a lot of waste products.

I would assume "clean coal" power generation would be even less efficient bacause of the scrubbers/filters etc.

The efficiency of thermal generation of electricity is, of course, limited by the laws of thermodynamics whether it is coal, gas, or nuclear. I understand that techiques which cause coal to be burned more efficiently and at higher temperatures have increased that efficiency to as high as 45%. See this paper on http://www.worldbank.org/html/fpd/em/supercritical/supercritical.htm". Nuclear plants do not use these techniques for safety reasons.

Scrubbers do not take much energy to run. Most use electrostatic precipitators to clean the exhaust. There is a little more energy in pushing the exhaust gasses around but not much. But "clean coal" is not simply a matter of minimizing the exhaust pollutants. It is a matter of eliminating CO2 emissions by removing the CO2 from the exhaust and placing it in the earth. This introduces serious inefficiencies because the CO2 has to be compressed and moved. I am not sure how much it would decrease efficiency, however.

and yes, i do understand that its the latent heat, that causes the damage or meltdown, i believe there is a device that poisons the reaction if the temperature gets too high. basically a thermal fuse.

Im guessing this would be Boron in a container with a specific melting temperature.

The CANDU reactor has a sort of thermal fuse. The CANDU uses natural uranium as fuel and achieves criticality only upon heavy water moderator being introduced between fuel rods. The fuel rods are set up in such a way that in order to cause fission, neutrons must pass between fuel rods through the heavy water moderator. As soon as the fuel rods are removed from the reactor, or if the moderator (D2O) is lost, fission stops. So the CANDU cannot have a meltdown.

AM

 

[Morbius]

So the CANDU cannot have a meltdown.

Andrew,

Of course a CANDU can have a meltdown!

The prototype CANDU reactor, the NRX at Chalk River; sustained a partial meltdown
on Dec 12, 1952 [ 54 years ago, yesterday] after the operators dumped the D2O to
shut it down.

I thought we discussed this earlier in this thread!

It's not the fission power that causes a meltdown; it's decay heat!

The reactor at Three Mile Island Unit 2 experienced a meltdown; and the reactor
had been shutdown for over an hour and a half! In fact, it was the shutdown of the
reactor that triggered the meltdown scenario.

If you lose D2O moderator, of course the fission power will turn off, as the reactor goes
sub-critical. Although there's no significant energy due to fission, the decay power - the
energy produced by the radioactivity of all the radioisotopes in the core immediately after
shutdown is aproximately 7% of the steady-state power the reactor was at prior to
shutdown.

So if you have a typical 1000 Mw(e) power plant, it will have a reactor with a thermal
power of about 3000 Mw(t); so when it shuts down there's 210 Mw(t) that needs to be
cooled. If that 210 Mw(t) can not be extracted - then it melts the fuel.

That's why all these claims about accelerator-driven systems being immune to meltdown
are BOGUS! If it produces power via fission, then there are fission products whose
decay heat can melt the reactor if the cooling is compromised.

Dr. Gregory Greenman
Physicist

 

[entropiax]

I am not sure what this has to do with nuclear physics but if you are telling us that it violates the second law of thermodynamics, you are telling us that it doesn't work. The actual mechanism is irrelevant. This is a perpetual motion machine (of the second kind). It can't work.

AM

Yes, you are right. This is the big problem of them.
If you can regenerate an thermally regenerated cell on the same tempereture it works, you need not cooling, so this bypass second law of thermodinamics.
If you say the second law of thermodinamics right, this regeneration must be impossible.
But they have tentative evidence, because they can regenerate pure lithiumhidride with potassium and get potassium, lithium and hidrogen using heat energy.
Every chemists know potassium reacts with lithium salts.
In this case K + LiH = KH + Li. In Hungary an chemist stundent doesn't know this, the professor kicks off him.
But the KH decomposits on 200 C, so at the end we get lithium separated potassium and hidrogen. Lithium and potassium don't compose alloy.
If we create a cycle we use heat energy and get electricity without cooling.
This basically bypass second law of termodinamics.
As I know lot if physics tried to find error on this theory, but nobody was successfull.
The other think, if we can draw a theoretics machine bypassing second law of thermodinamics, the theory of entropy also damaged.


Here is control experiment of them.
They can regenerate of 44 percent of lithiumhidride on 691 Celsius. The pure Li|LiH|H2 cell can work at 688!

http://secondlaw.acs.hu/kiserlet.gif

 

[Andrew Mason]

Of course a CANDU can have a meltdown!

The prototype CANDU reactor, the NRX at Chalk River; sustained a partial meltdown on Dec 12, 1952 [ 54 years ago, yesterday] after the operators dumped the D2O to shut it down.

I thought we discussed this earlier in this thread!

It's not the fission power that causes a meltdown; it's decay heat!

Ok. Overstated a bit. A meltdown is possible in a Candu if the core loses light water coolant as well (ie. if rods are removed from the heavy water and not placed in light water). Although a Candu is designed so that this will not happen, in a catastrophic situation it conceivably could happen. The Candu has an emergency gravity system that will ensure the core is immersed in light water and this will ensure that fission ends without loss of cooling. Since Candus are built adjacent to some body of water, there is no lack of available coolant.

Since in a light water reactor you cannot immediately shut down the reactor by immersing the core in light water (it already is), you need to have some means of absorbing neutrons while in the water (such as control rods) or immerse it in some other coolant that is not a good moderator. If your control rods aren't working you have a big problem. In the Candu you just pour water on it.

AM

 

[Morbius]

Ok. Overstated a bit. A meltdown is possible in a Candu if the core loses light water coolant as well (ie. if rods are removed from the heavy water and not placed in light water). Although a Candu is designed so that this will not happen, in a catastrophic situation it conceivably could happen. The Candu has an emergency gravity system that will ensure the core is immersed in light water and this will ensure that fission ends without loss of cooling. Since Candus are built adjacent to some body of water, there is no lack of available coolant.

Since in a light water reactor you cannot immediately shut down the reactor by immersing the core in light water (it already is), you need to have some means of absorbing neutrons while in the water (such as control rods) or immerse it in some other coolant that is not a good moderator. If your control rods aren't working you have a big problem. In the Candu you just pour water on it.

Andrew,

Afraid not. Just having the core immersed is not sufficient. You need a method
of getting the heat out.

Actually, the CANDU is NOT safer than a US reactor; if fact a CANDU does NOT
meet the U.S. Nuclear Regulatory Commission standards to be licensed in the USA.

The CANDU NEEDS the extra shutdown mechanism of a D2O dump.

In a US reactor, the light water coolant temperature coefficient is negative. That is, if
the reactor coolant gets hot - it is a negative reactivity insertion.

A CANDU is precisely the opposite - if you lose coolant flow to the CANDU, and the
light water coolant temperature increases, and thus the light water gets less dense;
that is a POSITIVE reactivity insertion in a CANDU because all the moderation that
is needed is provided by the D2O.

U.S. reactors are "under-moderated"; which provides an inherent safety mechanism.
CANDUs are "over-moderated" - thus the D2O dump.

This is one of the reasons why the CANDU fails to meet U.S. safety requirements.

I wouldn't worry too much about losing control rods; they drop by gravity. All that's
required is to cut current to the electromagnets that hold them up and they drop.

Dr. Gregory Greenman
Physicist

 

[Dewey2k]

To clarify Dr. Greenman's statement regarding controlling the reactor with coolant temperature:

The LWRs I worked on generate heat that is transferred via a heat exchanger to a secondary boiler system. As the coolant passes through the heat exchanger, it cools an amount directly proportional to the energy output.

In a steady state (constant load) system, the reactor is generating the same amount of heat that is taken out of the steam generator in the form of steam.

If the power demand increases, more steam (more heat) is taken out of the steam generator, causing the coolant will give up more of it's heat energy to the heat exchanger to compensate.

The colder coolant is more dense, so when it returns to the reactor, the moderator/coolant slows (or thermalizes) more neutrons because it is more dense, causing an increase in fission rate, which causes an increase in heat production, which is then fed to the heat exchanger.

Eventually the reactor will be producing more heat than is being required to compensate for the steam being removed. Because of that excess heat, the coolant will not be giving up all the heat energy in the steam generator, leaving the coolant warmer (less dense). The moderator/coolant thermalizes fewer neutrons, causing the fission rate to slow, reducing heat output.

After several progressively smaller cycles, the system will settle at the new, higher power output with the coolant giving up more heat than it was at the lower power level. The mean temperature of the coolant, however, will have remained virtually unchanged steady-state to steady-state.

I don't know if that helped, now that I read it, but there you are.

 

[Astronuc]

In a US reactor, the light water coolant temperature coefficient is negative. That is, if the reactor coolant gets hot - it is a negative reactivity insertion.

I've actually seen some core designs with positive MTC, which depends in enrichment, burnable absorber distribution and cycle length.

I wouldn't worry too much about losing control rods; they drop by gravity. All that's required is to cut current to the electromagnets that hold them up and they drop.

This is the case for PWRs, but BWR control blades must be hydraulically inserted from the bottom.

The LWRs I worked on generate heat that is transferred via a heat exchanger to a secondary boiler system.

That would be a PWR. BWRs generate steam in the core, which passes through a dryer above core before being sent through the main steam line directly to the high pressure turbine. That can be a major disadvantage when there is one or more fuel failures and the Xe and Kr fission gases are transported to the turbine. Another disadvantage is that N-16 can be transported from the core to the turbine if a plant uses hydrogen water chemistry (HWC) in order to protect the stainless steel from IGSCC/IASCC. N-16 has an energetic gamma.

If the power demand increases, more steam (more heat) is taken out of the steam generator, causing the coolant will give up more of it's heat energy to the heat exchanger to compensate.

The colder coolant is more dense, so when it returns to the reactor, the moderator/coolant slows (or thermalizes) more neutrons because it is more dense, causing an increase in fission rate, which causes an increase in heat production, which is then fed to the heat exchanger.

Actually, Morbius has been discussing decay heat removal which is after reactor shutdown.

During operation, the primary coolant system temperature does not vary all that much. Normal core inlet temperature in a PWR is ~290°C and the exit temperature is around 325-330°C. The reactor coolant pumps (RCPs) for coolant through the core. A significant change in load would like cause insertion of the control rods, but I'd have to confirm that. Some plants do have grey rods for axial power distribution control or load follow, however in the US load following is not a practice, although some plants have done it.

 

[theCandyman]

That would be a PWR. BWRs generate steam in the core, which passes through a dryer above core before being sent through the main steam line directly to the high pressure turbine. That can be a major disadvantage when there is one or more fuel failures and the Xe and Kr fission gases are transported to the turbine. Another disadvantage is that N-16 can be transported from the core to the turbine if a plant uses hydrogen water chemistry (HWC) in order to protect the stainless steel from IGSCC/IASCC. N-16 has an energetic gamma.

I thought Nickel (from the crud) was the main gamma problem with HWC. Where is the Nitrogen from, a fission product?

 

[Dewey2k]

Actually, Morbius has been discussing decay heat removal which is after reactor shutdown.

During operation, the primary coolant system temperature does not vary all that much. Normal core inlet temperature in a PWR is ~290°C and the exit temperature is around 325-330°C. The reactor coolant pumps (RCPs) for coolant through the core. A significant change in load would like cause insertion of the control rods, but I'd have to confirm that. Some plants do have grey rods for axial power distribution control or load follow, however in the US load following is not a practice, although some plants have done it.

The reactor plants I worked on were on an aircraft carrier, which regularly received power transients as high as 80% reactor power (self sustaining power was approximately 20% in that mode of operation for RCPs and other equipment). Extreme transients could drop average coolant temperature by 2C or 3C initially before Reactor Power stabilized.

Average coolant temperature was adjusted by rod height only to meet minimum requirements for steam plant operation, temperature compensation for Xe-135 and other fission product neutron absorbers, or for operational readiness.

Unfortunately (or fortunately), my ship was relatively new so we didn't really have a huge issue with decay heat. Even without RCPs we could establish enough flow to keep the reactor cooled by thermal flow if necessary.

As an aside, the last time I touched a control rod shim switch was in 1995, so I'm a little rusty.

 

[Dewey2k]

I thought Nickel (from the crud) was the main gamma problem with HWC. Where is the Nitrogen from, a fission product?

This is taken out of context, but here you go:

http://www.ans.org/pubs/journals/nt/va-147-2-269-283"

As a water-cooled nuclear system with a direct thermal cycle, the supercritical-water-cooled reactor (SCWR) shares with the boiling water reactor (BWR) the issue of coolant activation and transport of the coolant activation products to the turbine and balance of plant (BOP). Consistent with the BWR experience, the dominant nuclide contributing to the SCWR coolant radioactivity at full power is N-16, which is produced by an (n,p) reaction on O-16.

A neutron is absorbed by an O-16 atom, but it kicks out a proton becoming N-16.

 

[Astronuc]

I thought Nickel (from the crud) was the main gamma problem with HWC. Where is the Nitrogen from, a fission product?

Nickel in crud is an issue when it gets into the condensate in a BWR or in reactor cavity in LWRs during refueling. LWRs have filter/demins to remove crud as much as possible, but then the demins get hot.

N-16 comes from the n,p reaction of O-16 and in a reducing environment volatile nitrogen compounds form (IIRC, amines), which can be carried to the turbine in the steam.

 

[Astronuc]

The reactor plants I worked on were on an aircraft carrier, which regularly received power transients as high as 80% reactor power (self sustaining power was approximately 20% in that mode of operation for RCPs and other equipment). Extreme transients could drop average coolant temperature by 2C or 3C initially before Reactor Power stabilized.

Well yes, naval reactors are quite different animals than commercial power reactors. Enrichments are higher and that certainly leads to differences in reactivity management/control.

 

[Morbius]

After several progressively smaller cycles, the system will settle at the new, higher power output with the coolant giving up more heat than it was at the lower power level. The mean temperature of the coolant, however, will have remained virtually unchanged steady-state to steady-state.

Dewey,

What you are describing is the ability of a Pressurized Water Reator or PWR [ which I guess
is probably what you were working with] to "load follow".

In a PWR, the turbine throttle valve is opened/closed to match turbine power to the plant's
electrical load. The reactor power will adjust automatically via the coolant/moderator
temperature coefficient - to match reactor power to that needed to drive the turbine at
the desired throttle setting.

Dr. Gregory Greenman
Physicist

 

[Morbius]

This is the case for PWRs, but BWR control blades must be hydraulically inserted from the bottom.

Astronuc,

Yes - in the case of a BWR; there is a hydraulic cylinder with a piston connected to the
control rod drive shaft. The bottom side of the piston is connected to the inside of the
pressure vessel so that the bottom side of the piston is at reactor vessel pressure.

The volume of the cylinder above the piston can be vented to ambient pressure.

If a fast shutdown or SCRAM is desired; a valve is opened and the pressure above the
piston drops to essentially normal atmospheric pressure. It is the pressure differential
between the reactor pressure and the ambient pressure that drives the control rod up
and into the core.

So as long as the reactor is at pressure; there is drive pressure to force the control rods
into the core.

Dr. Gregory Greenman
Physicist