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PostPosted: May 20, 2013 5:03 am 
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Sodium coolant with vented fuel has similar contamination and radiotoxicity hazards as molten salt fuel. The gamma activity of Na-24 is huge, and with fission products in the coolant, and systems for its radioactive removal, this is basically a molten salt reactor already!

In fact, cesium will be present as metallic cesium, which is volatile at the reactor operating temperature, and above boiling point during certain loss of cooling transients/accidents.

In many ways this could even be more complicated and a higher radiotoxic contamination hazard than a molten salt reactor.


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PostPosted: May 20, 2013 7:58 am 
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PostPosted: May 20, 2013 9:34 am 
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Cyril R wrote:
cesium will be present as metallic cesium, which is volatile at the reactor operating temperature, and above boiling point during certain loss of cooling transients/accidents.
So, does the cesium being volatile make its "continuous removal" easier, compared to CsF in molten salt ?

Cyril R wrote:
In many ways this could even be more complicated and a higher radiotoxic contamination hazard than a molten salt reactor.
Can you be more specific, please ?
If the volatile fission products are "continuously removed", how is that worse ? ...by how much ?


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PostPosted: May 20, 2013 2:27 pm 
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jaro wrote:
Cyril R wrote:
cesium will be present as metallic cesium, which is volatile at the reactor operating temperature, and above boiling point during certain loss of cooling transients/accidents.
So, does the cesium being volatile make its "continuous removal" easier, compared to CsF in molten salt ?


Possibly. But sodium isn't nonvolatile, boiling point below 900 Celsius. Reticulated vitreous carbon traps sounds like absorption trapping, though.

Quote:
If the volatile fission products are "continuously removed", how is that worse ? ...by how much ?


It's not much worse at all, that's my point. If you have loads of Na24 pumping about the entire primary coolant loop, which is massive in pool type reactor design, then having some cesium on top isn't much of an added hazard. But it means that even zero venting fuel with zero fuel failures will have a large activity problem in the primary coolant. So even nonvented perfect retentive fuel would already be close to a molten salt reactor in terms of contamination issues, especially considering sodium's lower boiling point, opacity, and chemical reactivity with air, water and concrete. As the TWR people say, vented fuel is actually much safer for deep burn reactors, as the fuel can operate at coolant pressure, so at near zero pressure differential. This makes it much safer fuel in both normal operation and in accidents (no fuel ballooning blocking coolant flow in severe accidents).

Interestingly, vented fuel will release large amounts of iodine fission product to the sodium coolant, which will then bind to sodium iodide. Which is a highly radioactive molten halide salt; it's a molten salt reactor already! 8)


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PostPosted: May 20, 2013 3:37 pm 
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Cyril R wrote:
Interestingly, vented fuel will release large amounts of iodine fission product to the sodium coolant, which will then bind to sodium iodide. Which is a highly radioactive molten halide salt; it's a molten salt reactor already! 8)

Excellent point !

I'm just not sure that the amounts released to the sodium coolant are meant to be "large": The TWR illustration shows an FP trap on each rod....

Of course the "sodium bond" inside each fuel pin, to maintain good heat conduction between the metal fuel and the tube wall -- so definitely NaI & CsI there (along with lots of other species...)


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PostPosted: May 21, 2013 12:37 am 
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If you can forego the benefits of a fast reactor, you could have a long life thorium thermal reactor.
Have a three dimensional metallic thorium structure enriched with reactor grade PuO2 ceramic fissile feed. At the right level of enrichment, it will become critical if and only if immersed in water. Have one 10m high and immerse partially, 3-4m in water.The bottom 3-4m or so will become a boiling water reactor. On exhaustion of water it will become subcritical and stop. Keep on adding water like a BWR to keep it running.
As the bottom part is depleted, add more water to achieve criticality. The decay heat will continue to be extracted by water/steam. the core area will be getting higher.
As some of lower fuel becomes a net neutron drain, stop the reactor and fill the burned length with lead every year or two. replace the complete fuel grill only when the entire length is burnt out in a few decades.


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PostPosted: May 21, 2013 2:08 pm 
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jaro wrote:
Cyril R wrote:
Interestingly, vented fuel will release large amounts of iodine fission product to the sodium coolant, which will then bind to sodium iodide. Which is a highly radioactive molten halide salt; it's a molten salt reactor already! 8)

Excellent point !

I'm just not sure that the amounts released to the sodium coolant are meant to be "large": The TWR illustration shows an FP trap on each rod....

Of course the "sodium bond" inside each fuel pin, to maintain good heat conduction between the metal fuel and the tube wall -- so definitely NaI & CsI there (along with lots of other species...)



Thanks for the links Jaro,

Anyone care to try a little reverse engineering? I am wondering their starting fissile load (anyone hear a quoted value?). From their graphic it looks like their "fueled" zone is about 3 m wide and 2.5 m high (about 18 m3) and they say this is with under 20% LEU. The S-PRISM has a core power density of 400 w/cc or about 2.1 m3 of fueled core and needing about 1.24 tonnes (for 311 MWe, based on FBR often used quote of 12 tonnes fissile per GWe but only 4 tonnes on day one). That would seem to indicate about 10.5 tonnes fissile Pu to start the TWR 500 MWe demonstration and with U235 you need more (about a 50% more than Pu239) so perhaps pushing 15 tonnes U235 for their 500 MWe unit (roughly 600 million for fuel alone). 1.2$/watt for fuel alone (not including fabrication) is pretty steep. Yes of course they don't need to buy more but a lot to pay on day one. Any comments if my math is off? Hyperion with their tiny 10 MWe unit with a 25 year core is very upfront that the largest part of their cost is paying for day one fuel and only aiming to hit 15cents/kwh.

David L.


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PostPosted: May 21, 2013 4:49 pm 
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According to the paper by Ellis & co.,
Quote:
The comparable TWR requires an initial core load that in the early TWRs may contain on the order of two times as much fissile material as an LWR first core.
However, because the TWR core lifetime can be achieved using only the initial fuel load, no reloads would be needed. Even based on the present value of
the avoided reloads, the TWR would enjoy a fuel cost advantage of several hundred million dollars.


From another document,

Fuel Cycle Analysis of Once-Through Nuclear Systems
Prepared for
U.S. Department of Energy
Systems Analysis Campaign
T. K. Kim and T. A. Taiwo
August 10, 2010

Quote:
3.6 TerraPower Traveling Wave Reactor Concept

~14 % enriched uranium is used for the igniter fuel

Also, the metallic fuel is an alloy with ~8% Zr.


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PostPosted: May 21, 2013 8:36 pm 
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Quote:
According to the paper by Ellis & co.,
Quote:
The comparable TWR requires an initial core load that in the early TWRs may contain on the order of two times as much fissile material as an LWR first core.
However, because the TWR core lifetime can be achieved using only the initial fuel load, no reloads would be needed. Even based on the present value of
the avoided reloads, the TWR would enjoy a fuel cost advantage of several hundred million dollars.


Thanks Jaro, but might that only be a quote for the original traveling wave reactor that started with only a small fraction of the big core with fuel and the majority DU? This "shuffling" reactor concept has mostly fissile fuel elements with just some DU on the outside that they shuffle into the central positions to even out burnup. 18 m3 of core with 14% U235 sounds like an awful lot of fissile to me for 500 MWe.

David


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PostPosted: May 22, 2013 11:32 am 
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My understanding of this seminar given by the TerraPower people
http://www.youtube.com/watch?v=U0oqZX6LXrA
is that TerraPower's differentiating strategy is to forego the benefits of molten salts, and solve the metal-casing-degradation challenge -- with heavy reliance on computer simulations.
This appears to me to be a huge gamble, not a gamble on which I would put my money. Do you guys agree?

Independently, I don't think that it makes sense to disparage Myhrvold and Intellectual Ventures. They are playing by the rules, as written. It's like Apple's taxes. Given the rules, which include the "constructive reduction to practice", guys exactly like participants in this blog should be applying for patents covering ideas (as opposed to demonstrated technologies).

Art Williams


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PostPosted: Jun 24, 2013 7:29 am 
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Cyril R wrote:
Interestingly, vented fuel will release large amounts of iodine fission product to the sodium coolant, which will then bind to sodium iodide. Which is a highly radioactive molten halide salt; it's a molten salt reactor already! 8)


Perhaps, this should not be in the Liquid-Metal-Cooled Reactor section, but if I recall correctly, General Atomics follows the same strategy with their EM2 (Energy Multiplier Module) helium-cooled fast neutron reactor. Conceptually, it appears to be competing with the TerraPower reactor. General Atomics also intends to use fuel venting for the breed-burn cycle of the reactor core, which will have a life-cycle of 30 years.


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PostPosted: Jun 28, 2013 5:00 am 
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The figure shows a large, presumably fission gas plenum at the top. Might they have sodium added in the fuel pin for heat transfer, but not fill the sodium much into the plenum? Might they also only vent the gasses to the cover gas system? Is there any advantage in minimizing release to plenum retention and/or not getting as much FP in the separate coolant Na?

The plenum/trap is a cooler area where condensibles can be trapped, and may have porous carbon to trap or at least hold up some of the FP gasses. Maybe they periodically heat this region up and off gas the trapped materials in a batch clean up mode so it is not in continuous clean up which has more risks for release than batch, and continuous clean up would have to handle very low levels making it harder to operate.


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PostPosted: Jun 29, 2013 12:06 pm 
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Ed P wrote:
The figure shows a large, presumably fission gas plenum at the top. Might they have sodium added in the fuel pin for heat transfer, but not fill the sodium much into the plenum? Might they also only vent the gasses to the cover gas system? Is there any advantage in minimizing release to plenum retention and/or not getting as much FP in the separate coolant Na?

The plenum/trap is a cooler area where condensibles can be trapped, and may have porous carbon to trap or at least hold up some of the FP gasses. Maybe they periodically heat this region up and off gas the trapped materials in a batch clean up mode so it is not in continuous clean up which has more risks for release than batch, and continuous clean up would have to handle very low levels making it harder to operate.


Liquid metal cooled reactors operate at low coolant pressures, so not venting fuel with high burnup (needed for good economics) results in high positive pressure in the fuel cladding. Either a large plenum or a very thick cladding will solve this, but it comes at a cost. Large plenum means bigger fuel elements which is not attractive, thick cladding means more losses to cladding (plus high internal pressure means more potential for leaks).

Vented fuel comes in a bunch of different designs. One can use a diving bell type top plenum, essentially a pressure equalized design with the sodium bond never getting out of the fuel rod and sodium coolant never getting to the bond. Or one can use a filtered vent, with a filter plug at the end.

Vented fuel will release most of the noble gasses to the coolant, and then to the cover gas (or directly to the cover gas if a gas vent line to cover gas is provided). A proper design will not release meaningful amounts of the non-volatile (rare earths, strontium etc.) and chemically reactive stuff (iodine, bromine).

Too much FP in the coolant means the reactor is more difficult to service. But it's probably not such a big deal, considering you have Na-24 all over the place already. Huge activity from that source.


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PostPosted: Sep 29, 2013 4:50 am 
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Here's more recent information on TerraPower from the NY Times:

http://www.nytimes.com/2013/09/25/busin ... d=all&_r=0

The research facilities of TerraPower are shared with other projects of Intellectual Ventures. I wonder, if they really want to get this off the ground, why they don't enter into a CRADA with Idaho National Laboratory to develop a prototype, which could be backed by Bill Gates' billions.


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PostPosted: Oct 02, 2013 3:17 am 
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A travelling wave implies a move of fission space with time. That will require a shift of heat extraction too. It could possibly be done but the complications may make it uneconomical. It has to be a fast reactor in any case.
One interesting variation could be an expanding dome. A 3D lattice can be devised so that the fission space is expanding out from center of a hemisphere. The entire lattice could transfer the heat to a gas coolant.
Any MSR with monitoring of reaction and corrective reprocessing, though initially designed to be balanced, is a less complicated idea.


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