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PostPosted: Sep 10, 2013 3:42 am 
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The online processing is the feature for molten salt reactor, do anyone know the pattern of I-135 and Xe-135 removel in MSRE? Do MSRE has iodine pit? I know the xenon is disposaled by bubbling helium, but how about I-135?
If I-135 still remain in core, it will decay and Xe poison formed. If iodine was processed online, which method it choose?

And how about MSBR?


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PostPosted: Sep 10, 2013 5:57 am 
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We don't yet understand iodine as well as we'd like. It is one of the few exceptions to "not needing any fundamental research to build a molten salt reactor". Some research would be welcome here.

We do know a few things.

1. In redox controlled fluoride salts, iodine is present as the element, the halogen. Not as halide.

2. Iodine has a significant solubility in fluoride salts, orders of magnitude greater than Xenon.

3. Iodine has a large vapor pressure at reactor operating temp.

This allows us to hypothesize that iodine will behave as if it were a volatile soluble species, like BeF2 but much more volatile. It will likely show some cold trapping behaviour, and some go-to-cover-gas behaviour. I think we can cold trap the cover gas to remove iodine purposefully.


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PostPosted: Sep 10, 2013 10:20 am 
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ORNL did work on see if they could grab I135 and eventually decided that this wasn't going to be successful.


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PostPosted: Sep 10, 2013 12:25 pm 
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Why should we care? So it turns into Xe135. That will come out quickly enough, no?

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PostPosted: Sep 10, 2013 1:52 pm 
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In one sense, ORNL settled on He bubbles as the best tradeoff so it is a decent choice.
ORNL did look to see if they could remove the iodine - which indicates that they weren't completely happy with the plan.

We have seen challenges in dealing with the large volume of off-gas. The helium makes up 99+% of the total gas volume but separating the helium from xenon and krypton isn't trivial. ORNL's plan was to use carbon traps - but the gasses decay in the carbon and leave radioactive material inside the carbon so it generates a waste flow. Second, ORNL's cooling of the carbon traps meant a significant volume of water which I would rather avoid. We have tossed around ideas of using pitot tubes to separate the helium from the xenon/krypton to reduce the volume without generating additional waste.

Further, extracting the offgas rapidly means that there are many short lived offgases coming along as well which translates to a pretty significant (20MW) heat load.

Third, some of the delayed neutrons get extracted in the offgas - which we would very much rather leave in the core.

Fourth, the bubbles create concerns about instability - if a region doesn't get the average number of bubbles it will have higher reactivity - does this cause problems?

If one extracted the iodine then the big neutron absorber would not be present and we could be less aggressive about how rapidly we extracted the remaining off-gas and this could allow us to have significantly lower percentage (by volume) helium in the fuel salt and extract things more slowly. That would reduce the magnitude of all the challenges with the offgas process.


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PostPosted: Sep 10, 2013 2:10 pm 
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The carbon traps are absolutely something to avoid in a safety sense. They only absorb when they are cold, and it's easy to see a loss of coolant type of accident where the water is drained away, producing steam somewhere you don't want any (steam + lots of volatile radioactive products like cesium, iodine, krypton, xenon). Then the carbon traps heat up and further lose their absorbed substances. This can get into a nasty big volatile release scenario. ORNL admitted that they hadn't done any safety assessment on this, so consider this a major problem area.


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PostPosted: Sep 11, 2013 7:05 am 
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Lars wrote:
We have tossed around ideas of using pitot tubes to separate the helium from the xenon/krypton to reduce the volume without generating additional waste.
I remember the venturi tube discussion, but pitot tubes? How would that work?

Besides, would vacuum purging actually be better than He sparging?

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PostPosted: Sep 11, 2013 9:19 am 
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KitemanSA wrote:
Lars wrote:
We have tossed around ideas of using pitot tubes to separate the helium from the xenon/krypton to reduce the volume without generating additional waste.
I remember the venturi tube discussion, but pitot tubes? How would that work?

Besides, would vacuum purging actually be better than He sparging?

Sorry wrong name. Vortex tubes is what I meant.

From Wiki "The vortex tube was used to separate gas mixtures, oxygen and nitrogen, carbon dioxide and helium, carbon dioxide and air in 1967 by Linderstrom-Lang".


So, the hope would be that we can separate Xe/Kr from He/H in conditions when the Xe/Kr is a very small percentage of the gas volume and everything is pretty hot and the Xe/Kr generate heat.


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PostPosted: Sep 11, 2013 8:55 pm 
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Would it not be simpler to store the off gas with the helium still inside it at near atmospheric pressure?

The volumes involved still aren't huge and you can then compress the gas after most of the short lived stuff has decayed away.


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PostPosted: Sep 11, 2013 10:24 pm 
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E Ireland wrote:
Would it not be simpler to store the off gas with the helium still inside it at near atmospheric pressure?

The volumes involved still aren't huge and you can then compress the gas after most of the short lived stuff has decayed away.

Actually the volumes are quite substantial. Storing the gas for an hour operating time of the reactor filled the dump tank in the ORNL design. This leads to a couple of questions:
1) what would be the impact of circulating 10% as much helium
2) could we have a simple scheme to achieve modest separation of Xe/Kr from He/H - specifically the vortex tube looks enticing.


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PostPosted: Sep 12, 2013 10:50 am 
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Lars wrote:
E Ireland wrote:
Would it not be simpler to store the off gas with the helium still inside it at near atmospheric pressure?

The volumes involved still aren't huge and you can then compress the gas after most of the short lived stuff has decayed away.

Actually the volumes are quite substantial. Storing the gas for an hour operating time of the reactor filled the dump tank in the ORNL design. This leads to a couple of questions:
1) what would be the impact of circulating 10% as much helium
2) could we have a simple scheme to achieve modest separation of Xe/Kr from He/H - specifically the vortex tube looks enticing.
3) what would be the effect of circulating zero helium?

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PostPosted: Sep 14, 2013 3:17 am 
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Cyril R wrote:
It will likely show some cold trapping behaviour, and some go-to-cover-gas behaviour. I think we can cold trap the cover gas to remove iodine purposefully.


Cyril, could you please explain "cold trapping behavior" more clearly? I'm not very understand its means.

Do you mean we can use off-gas system remove iodine and xenon at the same time?


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PostPosted: Sep 14, 2013 4:45 am 
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Cyril R wrote:
We don't yet understand iodine as well as we'd like. It is one of the few exceptions to "not needing any fundamental research to build a molten salt reactor". Some research would be welcome here.

We do know a few things.

1. In redox controlled fluoride salts, iodine is present as the element, the halogen. Not as halide.

2. Iodine has a significant solubility in fluoride salts, orders of magnitude greater than Xenon.

3. Iodine has a large vapor pressure at reactor operating temp.

This allows us to hypothesize that iodine will behave as if it were a volatile soluble species, like BeF2 but much more volatile. It will likely show some cold trapping behaviour, and some go-to-cover-gas behaviour. I think we can cold trap the cover gas to remove iodine purposefully.


Does it mean the I-135 removal is necessary in MSR? If there is xenon removal but no iodine extracting, whether xenon poison and iodine pit sill exist?


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PostPosted: Sep 14, 2013 6:17 am 
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longwei1221 wrote:
Cyril R wrote:
It will likely show some cold trapping behaviour, and some go-to-cover-gas behaviour. I think we can cold trap the cover gas to remove iodine purposefully.


Cyril, could you please explain "cold trapping behavior" more clearly? I'm not very understand its means.

Do you mean we can use off-gas system remove iodine and xenon at the same time?


Cold trapping, as in, it should be condensable in a cold trap at reasonable temperatures while xenon and krypton remain a gas, but even without a cold trap it might settle somewhere in the coldest part of the system, to a small degree. There are other iodine isotopes, and it might be useful to reduce the source term of the single biggest radioisotope risk, radio-iodine. Some noble metals are also found in the cover gas, especially if helium sparging is used, more of the metal will come out (maybe it attaches to the bubbles as the metal atoms are really tiny particles, sometimes single atoms).

Before cold trapping though, some sort of short holdup period is needed to let the really short lived stuff decay (like Xe-137). Cold traps don't like all that heat generation.


Last edited by Cyril R on Sep 14, 2013 6:22 am, edited 1 time in total.

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PostPosted: Sep 14, 2013 6:20 am 
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longwei1221 wrote:
Cyril R wrote:
We don't yet understand iodine as well as we'd like. It is one of the few exceptions to "not needing any fundamental research to build a molten salt reactor". Some research would be welcome here.

We do know a few things.

1. In redox controlled fluoride salts, iodine is present as the element, the halogen. Not as halide.

2. Iodine has a significant solubility in fluoride salts, orders of magnitude greater than Xenon.

3. Iodine has a large vapor pressure at reactor operating temp.

This allows us to hypothesize that iodine will behave as if it were a volatile soluble species, like BeF2 but much more volatile. It will likely show some cold trapping behaviour, and some go-to-cover-gas behaviour. I think we can cold trap the cover gas to remove iodine purposefully.


Does it mean the I-135 removal is necessary in MSR? If there is xenon removal but no iodine extracting, whether xenon poison and iodine pit sill exist?


It is not necessary. Removing xenon with decent efficiency will reduce poisoning sufficiently. Any iodine that comes along is a bonus, but must be considered in the design provisions (eg, cold trapping it later on to prevent excessive iodine buildup).

Why do you think iodine will cause pitting? Materials of construction are designed for fluoride environment and the chemistry is reducing at all times. Any alloy that can take that environment should not be bothered by iodine.


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