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PostPosted: Sep 29, 2010 8:14 pm 
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Owen T wrote:
Another factor in having no excess reactivity. This is made possible by continuous removal of gaseous fission products.


It is also made possible by having the ability to carefully control the amount of fissile material in the salt either by addition or removal (in the case of a breeder). There simply is no extra fissile material to require significant reactivity control (through control rods, burnable absorbers, or soluble absorbers). There are still undercooling transients that could add reactivity, but these are perhaps limited because of the large heat capacity of the salt.


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PostPosted: Oct 01, 2010 10:42 am 
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I know that it is heresy for anti proliferationists but a good idea is to have excess fissile fuel and control by varying the fertile like control rods. I believe it was tried in the Light Water Breeder. Thorium is eminently suitable in this fertile fuel-cum-consumable poison role. It was taken to its logical conclusion in Thorium-LEU MOX fuel concept.
In an ironic twist of rules, only people authorised to hold fissile matter for weapons could hold the fissile feed for thorium fuel for power. I am glad that India has kept some of power reactors out of IAEA supervision to enable it to continue its fast and thorium powered reactor research.
DOE could do it in the USA.


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PostPosted: Oct 29, 2010 12:51 am 
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the disadvantage of thorium in the early stage of nuclear weapon project become the advantage now for it's nonproliferation capability because the same reason, haha, very interesting! times aren't what they were!you never know


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PostPosted: Oct 31, 2010 11:46 am 
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Owen T wrote:

Another factor in having no excess reactivity. This is made possible by continuous removal of gaseous fission products.

One of the fission products produced is an isotope of Xenon that has an extremely high tendency to absorb neutrons. In a solid-fueled reactor this gas remains trapped inside the fuel pellets until it decays. This would stop the reaction and shut down the reactor unless the reactor was designed with excess reactivity i.e. configured such that each fission can potentially cause much more than one other fission. This excess reactivity is held in check using control rods and unleashed when Xenon accumulates in the reactor to help it continue running in the presence of this "neutron poison". It's easy to see why this is potentially dangerous. If the control rods are lifted for anyreason in the absence of Xenon the reactor will generate a huge spike of energy and heat up beyond the capability of any heat removal system.

In a liquid fueled reactor this Xenon simply bubbles out and is collected for storage until it decays. The reactor need not be designed with any excess reactivity to compensate for its presence. This is inherently safer.



I think this effect is vastly over rated as a certainty. Xenon fluorides are surprisingly stable compounds. The hexafluoride, tetrafluoride and difluoride are all well known characterized compounds. There are many stable complex ions. Bartlett's first xenon compound was, in fact, a platinum perfluoroxenate. (The trioxide is also known, but is very unstable and can spontaneously explode but some oxoanions are more stable.) It is a mistake to assume that xenon will be gaseous and will be completely removed from the system.

This may be apocryphal, but I have heard that after Bartlett discovered xenon chemistry, that the LTFR folks realized that this was probaby the reason that xenon yields seemed to be lower than expected.

We should keep that chemistry in the back of our minds.


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PostPosted: Oct 31, 2010 12:40 pm 
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NNadir wrote:
Owen T wrote:

Another factor in having no excess reactivity. This is made possible by continuous removal of gaseous fission products.

One of the fission products produced is an isotope of Xenon that has an extremely high tendency to absorb neutrons. In a solid-fueled reactor this gas remains trapped inside the fuel pellets until it decays. This would stop the reaction and shut down the reactor unless the reactor was designed with excess reactivity i.e. configured such that each fission can potentially cause much more than one other fission. This excess reactivity is held in check using control rods and unleashed when Xenon accumulates in the reactor to help it continue running in the presence of this "neutron poison". It's easy to see why this is potentially dangerous. If the control rods are lifted for anyreason in the absence of Xenon the reactor will generate a huge spike of energy and heat up beyond the capability of any heat removal system.

In a liquid fueled reactor this Xenon simply bubbles out and is collected for storage until it decays. The reactor need not be designed with any excess reactivity to compensate for its presence. This is inherently safer.


I think this effect is vastly over rated as a certainty. Xenon fluorides are surprisingly stable compounds. The hexafluoride, tetrafluoride and difluoride are all well known characterized compounds. There are many stable complex ions. Bartlett's first xenon compound was, in fact, a platinum perfluoroxenate. (The trioxide is also known, but is very unstable and can spontaneously explode but some oxoanions are more stable.) It is a mistake to assume that xenon will be gaseous and will be completely removed from the system.

This may be apocryphal, but I have heard that after Bartlett discovered xenon chemistry, that the LTFR folks realized that this was probaby the reason that xenon yields seemed to be lower than expected.

We should keep that chemistry in the back of our minds.


ORNL did find that much of the xenon bubbles out when they sparged. They did not specify what percentage was removed but did show low solubilities for pure xenon gas. I'm no chemist but would assume that the xenon fluorides you mention aren't stable at MSRE operating temperatures - almost 1000 K. Recall from some ancient documents that most xenon compounds were only stable at low T.


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PostPosted: Oct 31, 2010 12:44 pm 
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There was no chemistry in Owens post. It is simply that xenon absorbs a lot of neutrons if left in the reactor and is dynamic in a way that requires active controls. I should note that I don't think this is a safety issue really - given that you live by the operational constraints it imposes. But LFTR is nice in that it avoids those operational constraints by removing the xenon rapidly.


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PostPosted: Oct 31, 2010 2:31 pm 
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NNadir wrote:

...
I think this effect is vastly over rated as a certainty. Xenon fluorides are surprisingly stable compounds.


"Surprisingly stable" for a xenon compound or in the absolute sense of the word "stable"? We are still talking about 700+ deg C and periodic additions of beryllium metal that will compete with xenon for that fluorine.


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PostPosted: Oct 31, 2010 3:41 pm 
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Owen T wrote:
NNadir wrote:

...
I think this effect is vastly over rated as a certainty. Xenon fluorides are surprisingly stable compounds.


"Surprisingly stable" for a xenon compound or in the absolute sense of the word "stable"? We are still talking about 700+ deg C and periodic additions of beryllium metal that will compete with xenon for that fluorine.


Well the question has to do with the lifetime of the xenon fluoride species relative to the lifetime in the presence of a neutron flux. There are a host of issues - some of which are chemical and some of which are physical involving issues like surface tension, viscosity,solubility etc.

The physics of bubbles can be quite complex.

In fact if there is any fluorine potential at all - and I think we can all assume that there is one - there will be xenon fluoride species, including some complex ions. The position of the equilibrium may favor the gas, but I'm not quite sure that the absolute phase diagrams in these systems have been determined or even calculated.

In fact, to make Krypton fluorides, one needs to insert considerable energy into the system, discharges and stuff - radiation can substitute, I would guess - although Krypton fluorides are only stable at cyrogenic temperatures.

A discussion of xenon removal with He sparging at the MSRE is found in Nucl.Appl.Tech.Vol.8 FEBRUARY.1970.179-189 by Scott and Eatherly at Oak Ridge. They also reference a paper showing that xenon is not very soluble in FLIBE. (I'll try to remember to pick it up.)

But the bottom line is that to remove xenon from the reactor they felt the need to sparge, and suggested doing so for neutron economy.

Of course, sparging - assuming it's fairly efficient - raises an issue of higher accumulations of Cs-135 as a fission product than we see in solid phase reactors. I personally have no problem with that - I've made peace in my mind with all cesium isotopes and have no problem with their accumulation - but paranoid anti-nuke types might carry on and on about that issue.

As for helium, I'm not sure we're going to have access to that gas for very long. The supply is more tenuous than people want to acknowledge.


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PostPosted: Nov 01, 2010 7:46 am 
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Fluorine has its good points like low neutron absorption and creating a liquid nuclear fuel, but its extreme chemical reactivity is also a problem. I had not realized that it extends to the so-called inert gases.
I think I should stick to lower melting Chlorides and a fast spectrum. It will also save on the graphite waste problem. However isotope separation of Chlorine(Cl37) in place of Li7 may be necessary.
More fast fission of U238 is a plus point of uranium over thorium. But then U233 created from thorium is a better fissile fuel than Pu239.


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PostPosted: Nov 01, 2010 2:07 pm 
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One: I'm not sure you will see any xenon fluorides or kypton fluorides at 700C. Will these compounds survive as compounds at this temperature or will they decompose?

Two (and more important): These are gases at 700C - so won't they go out with the sparge gases and not hang around in the fuel salt to absorb neutrons? Or are these highly soluble in the fuel salt and not tend to cling to the He bubbles?


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PostPosted: Nov 01, 2010 5:23 pm 
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Lars wrote:
One: I'm not sure you will see any xenon fluorides or kypton fluorides at 700C. Will these compounds survive as compounds at this temperature or will they decompose?

Two (and more important): These are gases at 700C - so won't they go out with the sparge gases and not hang around in the fuel salt to absorb neutrons? Or are these highly soluble in the fuel salt and not tend to cling to the He bubbles?


I went looking for the free energy of formation for XeF6 from which one can calculate the equilibrium constant at any temperature. What I came to was the Wikipedia reference for the compound. Xenon hexafluoride is formed at 300C from the elements but has a boiling point of 75.6C.

However it is a Lewis acid, and thus - as I expected - forms heptafluoro and octafluorocomplex anions, suggesting that there are fairly stable salts of this compound.

According to the wikipedia entry - for what it's worth - the cesium and rubidium salts are the most stable salts.

http://en.wikipedia.org/wiki/Xenon_hexafluoride


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PostPosted: Nov 01, 2010 8:11 pm 
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XeF6 is also a gas so the same question applies. If these exist at all won't they be extracted together the helium in the sparging? If so, does it matter whether they are Xe, XeF2, XeF4, or XeF6?


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PostPosted: Nov 04, 2010 9:52 am 
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Lars wrote:
XeF6 is also a gas so the same question applies. If these exist at all won't they be extracted together the helium in the sparging? If so, does it matter whether they are Xe, XeF2, XeF4, or XeF6?

To answer your first question. Probably to some extent, although frankly this creates a materials science question since XeF6 is a very powerful fluorinating agent. Water in particular must be rigorously excluded since XeF6 reacts quantitatively to give HF and XeO3 which is explosive.

I suppose that one could control this by putting reduced uranium or other reduced compound, possibly even a metal into the system to control this.

To answer your second question, in the absense of a reducing agent, fluoroxenate salts might form, with no gas released. Cesium perfluoroxenate and rubidium perfluoroxenate, as mentioned above, a particularly stable and prone to form. I believe only the higher fluorxenates form such compounds.


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PostPosted: Nov 04, 2010 10:00 am 
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All forms of oxygen are exclude for a myriad of reasons. Do I understand right that a powerful fluorinating agent is one that readily gives up its fluorine atoms and that the concern is that these fluorine atoms will tend to attack the containing vessel? If so, this is a concern that is already addressed. The fuel salt contains 1-2% UF3 which should absorb any fluorine atoms that something like XeF6 would like to give up. Continuously maintaining the UF4/UF3 balance is the main chemical balance that needs be done at the reactor.


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PostPosted: Nov 04, 2010 8:41 pm 
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Lars wrote:
All forms of oxygen are exclude for a myriad of reasons. Do I understand right that a powerful fluorinating agent is one that readily gives up its fluorine atoms and that the concern is that these fluorine atoms will tend to attack the containing vessel? If so, this is a concern that is already addressed. The fuel salt contains 1-2% UF3 which should absorb any fluorine atoms that something like XeF6 would like to give up. Continuously maintaining the UF4/UF3 balance is the main chemical balance that needs be done at the reactor.


Well, yes, one would like to have reducing agents, and UF3 in this context is probably pretty good. Nevertheless, with increasing burnup, the chemistry of the salt system becomes quite complex. I have always been intrigued to understand how many phases might eventually evolve, even small localized phases that are effectively emulsions.

Some very strange things happen in solid fuels. For instance, I have always been very surprised to read about the presence of metallic cesium and elemental iodine in the same fuel elements.

One might suppose that similar things are possible - if not probable - at very high burn-ups. The idea of course is to cycle the salts out and continuously repurify them, and with experience this may prove trivial.


But the case with xenon is an intersting one and I hope that because we will have fluid phase reactors in the future, we will develop considerable experience with this interesting business.


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