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PostPosted: Dec 01, 2013 4:42 pm 
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KitemanSA wrote:
Could the density on all that CO2 be tapped to moderate this concept into a thermal spectrum reactor? Then if there is a leak, the loss of moderator would shut it down quick.


The density is good but the carbon density per liter is poor compared to graphite. Oxygen does little moderation or in fact anything to neutrons. Still it is good enough to reduce reactivity upon LOCA in a fast(ish) spectrum. An advantage of CO2 is that it captures very few neutrons, so replacing it with air or something in a loss of coolant event increases neutron capture that reduces reactivity.


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PostPosted: Dec 01, 2013 7:41 pm 
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Cyril R wrote:
Oxygen does little moderation or in fact anything to neutrons. Still it is good enough to reduce reactivity upon LOCA in a fast(ish) spectrum. An advantage of CO2 is that it captures very few neutrons, so replacing it with air or something in a loss of coolant event increases neutron capture that reduces reactivity.


For clarification, CO2 at the critical point (0.469 g/cc) Carbon is 6% of graphite carbon density, and at approximately max CO2 density (~1 g/cc) 12.7% of graphite carbon density. These are probably good enough to bring the neutron spectrum down to the intermediate spectrum, especially with the benefit of oxygen added in. I would have to check the benefit of the oxygen since there is twice as much of that. But it is not likely enough to get to the thermal spectrum without having huge CO2 to fuel volume ratios, which are impractical and would make for a very large reactor, which at 3000+ psi would make for extremely thick/heavy reactor vessel forgings.

Actually, for water, oxygen IS very helpful in pulling down the neutron energy to the realm where hydrogen works well. Pure hydrogen at the density it is in water would be a really poor moderator overall. But, in CO2 the carbon is almost as good at the higher energies as O2, but I am sure the O2 is still beneficial relative to Carbon at the same density, just not as beneficial as in water.

As for neutron capture for a LOCA, I would have to check the cross sections on absorption and also check the change in moderation effect on reactivity. CO2 at supercritical density would likely have much better moderating, but definitely would have much higher absorption than air at very low pressure. The air would essentially be nothing for either moderation or absorption at atmospheric density, if that was to be used to guarentee shut down, so moderation and absorption would be more dependent on the fuel and cladding materials at that point, not the air coolant. Not sure if I missed something in the conversation line or something.


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PostPosted: Dec 02, 2013 3:48 am 
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Please keep in mind also the physical quantities involved, or more precisely the amount of carbon relative to the uranium inventory.

This reactor has a lot of fissile and is compact (not much coolant in the core). There would be many more atoms of actinide than there are atoms of carbon in the CO2. Hence the spectrum would be fast. Not as fast as a sodium fast reactor perhaps, but still pretty fast (can breed on U/Pu cycle).


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PostPosted: Dec 03, 2013 5:08 am 
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This reactor, proposed by Sandia, is an interesting SMR concept, but its development will depend on the development of the supercritical CO2 Brayton cycle: advanced heat exchangers and turbo machinery that can withstand the high pressures involved. Perhaps it will take another 10 years before we will see a SCO2 turbine gen-set in the 100 - 200 MWe range.

The pressure of 20 megapascals is very high, but as the paper cites, can be reduced to 13 MPa, if a split cycle is used, which is less than a PWR. As the whole system is relatively compact and DHR using a natural convection flow is possible, I wonder whether a containment/guard vessel would suffice for LOCA incidents.


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PostPosted: Dec 03, 2013 5:50 am 
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20 MPa is not so much, most modern coal plants are in the range of 24-32 MPa, and also supercritical conditions.

Though the flow rates (kg/s) are much higher for SCO2 than for SH2O (like an order of magnitude). I think that is a trouble area especially considering the compact turbomachinery. The stresses on turbine blading and shaft must be enormous.

The HX is not so much a trouble area. Airfoil type PCHE is an amazingly compact, efficient, and reliable technology well suited to SCO2 conditions. You can get over 200 MWth/m3 easily in these conditions, at ultra low pressure drops.


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PostPosted: Dec 06, 2013 10:18 pm 
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Cyril R wrote:
...startup fissile load. This reactor needs about 24 tonnes fissile to get started. It would cost $1500/kWe just for the first core.

Yes, 24.7 tons/GWe of fissile (p. 23 says 20.6 tons of U with 12% enrichment for 100MWe), for the first 20 year core.

But the remarkable thing they say is that the recycled core costs just 20% as much, or $0.3/Watt, including reprocessing and make-up U. It's not clear what they have in mind for reprocessing but I would assume off-site reprocessing at a large plant (the fuel is not metallic as with IFR).

For Pu-cycle reactors, I would think on-site reprocessing would be preferable to the public, to avoid having large Pu loads transported off-site. And I would not expect small (sub-GW) reprocessing plants to be cost competitive with the larger ones? (I think 15 GW is typical for reprocessing LWR fuel).

Nifty idea, but I can't help being drawn to plants with indirect cycles, low-pressure reactors, and the turbines in a non-radioactive part of the building.


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PostPosted: Dec 07, 2013 3:20 pm 
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Reprocessing cost estimates are always too optimistic. $12/gram fissile is just not going to happen with this scheme.

In case of CO2, the coolant won't activate, so won't have any issues in the turbine. In any case the turbine is tiny and simple so it won't be a serious issue even if there is a rad field (in case of some fuel failures).


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PostPosted: Dec 08, 2013 2:45 am 
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Cost of fissile feed is common to the cost of disposal of used fuel. Reprocessing reduces the volume of waste to be disposed off and produces fissile feed for thorium or fast reactors. UK and Japan have a lot of RG plutonium as a result of reprocessing, awaiting disposal. Cost of fissile feed should not be a deterrent to fast reactors or thorium fueled reactors.
Gas is not an efficient or economical cooling medium for heat transport according to British experience. Water needs high pressures. Molten salts and metals are the answer. Sodium carries fire risk. Others carry corrosion challenge which needs to be overcome.
IFR option has to be resumed with an MSR option.
http://info.ornl.gov/sites/publications ... b29596.pdf


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PostPosted: Dec 08, 2013 3:18 am 
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Water might need high pressures but we have centuries of experience in containing high temperature, high pressure water.

It seems foolish not to use it.


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PostPosted: Dec 08, 2013 4:14 am 
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Agreed. Even an indirect cycle with low operating pressure would still need a high pressure power cycle at the back end, at least until we manage a more direct photo-electric conversion at high efficiency. Higher pressure isn't all bad, it means lower pump power, improved efficiency, and more compact equipment.

With that high pressure loop terrorizing the back end in an indirect cycle with a low pressure primary loop. All sorts of fun scenarios can occur with HX ruptures suddenly pressurizing the primary loop. That means a lot of clever engineering required to prevent that. Certainly doable, yet a direct SCO2 cycle would not have that issue.

We shouldn't write of water as a coolant just yet, though. Most people don't realize just how good a coolant water is. SCO2 is a potential improvement in compactness and simplicity over Rankine, though not a great efficiency improvement in fact, and its compactness may be the death of it. Mass flows per MWe are an order of magnitude greater than Rankine, in an order of magnitude smaller space. That's asking for trouble.


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PostPosted: Dec 08, 2013 12:38 pm 
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Why not a SCCO2 cycle on a LFTR? Big pot of hot salt. Graphite bajonetts carrying SCCO2 down into the MS thru its core and out thru its graphite foam filled sheath. No fuel salt outside the core except for a small bit in the circ pump and in the repro units.

Might be able to fit an entire 100MW unit into a single upright shipping container. (PowerPoint design at best, but interesting none-the-less)

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PostPosted: Dec 08, 2013 12:47 pm 
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KitemanSA wrote:
Why not a SCCO2 cycle on a LFTR? Big pot of hot salt. Graphite bajonetts carrying SCCO2 down into the MS thru its core and out thru its graphite foam filled sheath. No fuel salt outside the core except for a small bit in the circ pump and in the repro units.

Might be able to fit an entire 100MW unit into a single upright shipping container. (PowerPoint design at best, but interesting none-the-less)


If the CO2 loop springs a leak, then it will pressurize the pot of fuel salt. And expel nasty radionuclides all over the place.

There's also a problem of putting stuff inside a molten salt reactor core. Neutron capture, embrittlement, tritium management, the whole shebang.

The first step towards realism is to not have the HX stuff inside the core, so it can't be badly embrittled and whatnot. Biggest downside is you need a primary pump, but at least it doesn't have to be inside the core.


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PostPosted: Dec 08, 2013 10:48 pm 
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Does _graphite_ suffer all those issues?

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PostPosted: Dec 09, 2013 6:56 am 
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KitemanSA wrote:
Does _graphite_ suffer all those issues?


Yes, hence the plumbing problem of the two fluid reactor.

With CO2 it would be worse, due to the higher pressure and chemical reaction of CO2 with graphite (C+CO2 = 2CO).


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PostPosted: Dec 09, 2013 9:39 am 
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Nathan2go wrote:
Cyril R wrote:
...startup fissile load. This reactor needs about 24 tonnes fissile to get started. It would cost $1500/kWe just for the first core.

Yes, 24.7 tons/GWe of fissile (p. 23 says 20.6 tons of U with 12% enrichment for 100MWe), for the first 20 year core.

But the remarkable thing they say is that the recycled core costs just 20% as much, or $0.3/Watt, including reprocessing and make-up U. It's not clear what they have in mind for reprocessing but I would assume off-site reprocessing at a large plant (the fuel is not metallic as with IFR).

For Pu-cycle reactors, I would think on-site reprocessing would be preferable to the public, to avoid having large Pu loads transported off-site. And I would not expect small (sub-GW) reprocessing plants to be cost competitive with the larger ones? (I think 15 GW is typical for reprocessing LWR fuel).

Nifty idea, but I can't help being drawn to plants with indirect cycles, low-pressure reactors, and the turbines in a non-radioactive part of the building.


Off-site reprocessing does not have to be problematic, if you take the right safety precautions. It already happens in France (La Hague & Marcoule). I think a centralized reprocessing center is preferable if you would have a large fleet of SMRs (<300 MWe). According to this scoping study by Sandia, the core lifetime can be very long, 20 years, so that would mean fewer trips to and from the reprocessing center. The fuel that is intended for this conceptual reactor is very much like the AGR fuel that is manufactured today at Westinghouse's Springfields facility in Preston, UK. The real downside I see to this reactor is the safety of the reactor during a major depressurization event, although things like a guard vessel and blowdown systems are proposed in the paper. And, of course, the supercritical CO2 turbine technology has really to get off the ground first.


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