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PostPosted: Jan 07, 2014 11:18 pm 
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It is nifty that the NACC nuclear plant can co-fire natural gas, but I'm a little disappointed that (apparently) the more variable renewables that are on the grid, the greater the tendency for the remaining generation to be natural gas fired. NACC helps only a little (by combining nuclear baseload with spinning reserve).

It seems to me that a NACC plant could also accommodate thermal energy storage using solar salt, to allow more efficient following of cyclical loads (e.g. complementing solar PV). Note that solar salt (the cheapest thermal energy storage material) only works between around 280 and 565 C. Unlike steam cycles, in which most of the heat is input near the maximum steam temperature, the Brayton cycle could use tandem heaters to first heat the air to 550 C using solar salt, then to 670 C using the primary coolant. Alternatively, the stored heat could be used only in the steam bottoming cycle (separate boiler or preheater/superheater for HRSG?), presumably with some efficiency loss.

But I'm curious how efficient the NACC plant is when it is used for load following (assuming the unused power is sent to storage)?


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PostPosted: Jan 08, 2014 12:40 am 
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Nathan2go wrote:
It is nifty that the NACC nuclear plant can co-fire natural gas, but I'm a little disappointed that (apparently) the more variable renewables that are on the grid, the greater the tendency for the remaining generation to be natural gas fired. NACC helps only a little (by combining nuclear baseload with spinning reserve).

It seems to me that a NACC plant could also accommodate thermal energy storage using solar salt, to allow more efficient following of cyclical loads (e.g. complementing solar PV). Note that solar salt (the cheapest thermal energy storage material) only works between around 280 and 565 C. Unlike steam cycles, in which most of the heat is input near the maximum steam temperature, the Brayton cycle could use tandem heaters to first heat the air to 550 C using solar salt, then to 670 C using the primary coolant. Alternatively, the stored heat could be used only in the steam bottoming cycle (separate boiler or preheater/superheater for HRSG?), presumably with some efficiency loss.

But I'm curious how efficient the NACC plant is when it is used for load following (assuming the unused power is sent to storage)?


MIT has done work on higher temperature concentrating solar systems with fluoride salts which store thermal energy into graphite. This may be the preferable direction to take for coupling NACC to solar. Coupling nuclear, concentrating solar, and NACC all together would be challenging given the relatively high area requirements for solar contrasting with the relatively small sites one would want to co-locate NACC modules onto.


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PostPosted: Jan 08, 2014 1:15 am 
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Unfortunately the 7Li comes contaminated with neutron poison 6Li.
If you do not insist on a thermal breeder/iso-breeder, you can use FNaBe with more fissile.
If you need a breeder, you can use FNabe in a fast spectrum, use more initial fissile and save on graphite and the size of the core.
If you must have a thermal breeder, just improve on Shippingport reactor with heavy water.
Why delay matters on unobtainium?


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PostPosted: Jan 08, 2014 10:28 am 
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If you do not insist on a thermal breeder/iso-breeder, you can use FNaBe with more fissile.


It seems to me that for a salt cooled thermal reactor you must use Flibe if you want to have a negative void coefficient.

Negative void is required in USA I think.


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PostPosted: Jan 08, 2014 11:42 am 
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fab wrote:
Quote:
If you do not insist on a thermal breeder/iso-breeder, you can use FNaBe with more fissile.


It seems to me that for a salt cooled thermal reactor you must use Flibe if you want to have a negative void coefficient.

Negative void is required in USA I think.


It is possible to achieve quite negative void with FNaBe with the use of burnable poisons.


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PostPosted: Jan 10, 2014 9:39 am 
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Per Peterson wrote:
...Coupling nuclear, concentrating solar, and NACC all together would be challenging ...

Thanks for responding Dr. Peterson, but my intention was not to include concentrating solar power. I was trying to say that the grid could accept a higher nuclear penetration if nuclear was used for load-following in addition to baseload, and I assume that nuclear could load-follow more cost effectively if energy storage were included. In suggesting use of "solar salt" for thermal storage, I meant the blend of NaNO3-KNO3 which is often used for CSP (it is cheaper and longer-lasting in cyclic use than solids like concrete and graphite), but heated with nuclear power rather than CSP.

So if the power block is over-sized, and throttles between say 50 and 150% of nominal (with the reactor at constant power, and storage making up the difference), presumably the power block will be less efficient than if it were to run at constant power. But how does NACC compare to pure steam in this application? I understand that the helium Brayton cycle should have constant efficiency vs load (by varying the gas inventory/pressure), but that the supercritical CO2 does not throttle efficiently.


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PostPosted: Jan 10, 2014 11:04 am 
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Here's an interesting story on NBF about the recent explosion in natural gas price.

http://nextbigfuture.com/2014/01/east-c ... l-gas.html

If gas surges up to $100/mmbtu, presumably the natural gas portion of the NACC would be idled. If such price spikes become more common they could be an economic threat to the whole concept of NACC. It will take a lot of time (decades) for NACC to become widely used in grids, so the future natural gas prices could potentially economically sink the whole concept of NACC.


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PostPosted: Jan 10, 2014 11:26 am 
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Nathan2go wrote:
Per Peterson wrote:
...Coupling nuclear, concentrating solar, and NACC all together would be challenging ...

Thanks for responding Dr. Peterson, but my intention was not to include concentrating solar power. I was trying to say that the grid could accept a higher nuclear penetration if nuclear was used for load-following in addition to baseload, and I assume that nuclear could load-follow more cost effectively if energy storage were included. In suggesting use of "solar salt" for thermal storage, I meant the blend of NaNO3-KNO3 which is often used for CSP (it is cheaper and longer-lasting in cyclic use than solids like concrete and graphite), but heated with nuclear power rather than CSP.

So if the power block is over-sized, and throttles between say 50 and 150% of nominal (with the reactor at constant power, and storage making up the difference), presumably the power block will be less efficient than if it were to run at constant power. But how does NACC compare to pure steam in this application? I understand that the helium Brayton cycle should have constant efficiency vs load (by varying the gas inventory/pressure), but that the supercritical CO2 does not throttle efficiently.


The downstream portion of the NACC is basically a CCGT, which have good part load efficiency. Attached is an interesting study which shows the efficiency at 40% output is still 88% of the efficiency of full throttle.
Attachment:
TAUSCHITZ study on combined cycle Graz.pdf [220.24 KiB]
Downloaded 148 times


The NACC would have rougly similar performance I should think (assuming the nuclear portion keeps running full out). Is that correct Dr. Peterson?


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PostPosted: Jan 16, 2014 7:25 pm 
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Those Ansaldo isolation condensers do indeed look like Candu feeder headers.
Apparently they intend to use them on the Alfred lead-cooled fast reactor, to be built in Romania.
If this type of design does in fact do the job, then good for them.
But it doesn't look like the right kind of technology for much larger power plants (Alfred is only 150MWe/400MWth). Relatively few tubes of large diameter, instead of large numbers of small tubes..... the latter requires different technology.


Attachments:
Ansaldo_isolation_condenser.JPG
Ansaldo_isolation_condenser.JPG [ 337.35 KiB | Viewed 2462 times ]
Ansaldo_isolation_condenser_ALFRED.JPG
Ansaldo_isolation_condenser_ALFRED.JPG [ 173.27 KiB | Viewed 2462 times ]
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PostPosted: Jan 17, 2014 6:07 am 
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The isolation condensers are for a different purpose. They are for removal of decay heat (only a few MWth to a few tens of MWth) in natural circulation. Natural circulation is better with bigger tubes.

For a normal heat transport system at full reactor power, you'd use much tigher pitches and smaller tubing. That's entirely reasonable and a lot of gas burning furnaces use internal drum/tube exchangers to generate steam, often even having more tube banks for superheating. As Dr. Peterson mentioned they avoid thick tubesheets and related quality control and thermal stress issues.

An interesting design issue with these tube-and-drum exchangers in a molten salt heated version is how do you design the shell? To take full advantage of the tube-and-drum system, the drums have to be inside the shell so you only have some nozzles for salt in/out the shell and water/steam/hot gas in/out through the drums. That's likely a rather big shell if those big drums have to fit into it, but if you can hang the tube/drum exchanger in the shell you've got a great design in terms of thermal expansion.


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PostPosted: Jan 17, 2014 2:06 pm 
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Cyril R wrote:
An interesting design issue with these tube-and-drum exchangers in a molten salt heated version is how do you design the shell? To take full advantage of the tube-and-drum system, the drums have to be inside the shell so you only have some nozzles for salt in/out the shell and water/steam/hot gas in/out through the drums. That's likely a rather big shell if those big drums have to fit into it, but if you can hang the tube/drum exchanger in the shell you've got a great design in terms of thermal expansion.
Sounds like the VVER SG design.


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PostPosted: Jan 17, 2014 11:42 pm 
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Cyril R wrote:
An interesting design issue with these tube-and-drum exchangers in a molten salt heated version is how do you design the shell? To take full advantage of the tube-and-drum system, the drums have to be inside the shell so you only have some nozzles for salt in/out the shell and water/steam/hot gas in/out through the drums. That's likely a rather big shell if those big drums have to fit into it, but if you can hang the tube/drum exchanger in the shell you've got a great design in terms of thermal expansion.


There is quite a bit of experience with ducting hot air from gas turbines through heat recovery steam generators, at temperatures up to about 700°C, which is relevant to the heating of air for NACC. HRSG's use internal insulation with a carbon-steel external shell, and the heat exchanger tubes inside the HRSG have a higher internal pressure, rather than lower pressure than the air. But the most important difference for ducting hot air for NACC is that the air is at moderately high pressure, so the ducts must be cylindrical rather than rectangular as in the case of HRSGs.


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PostPosted: Jan 18, 2014 4:19 am 
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So will the molten salt tube and drums all be inside the hot air shell?


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PostPosted: Jan 18, 2014 5:45 am 
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jaro wrote:
Cyril R wrote:
An interesting design issue with these tube-and-drum exchangers in a molten salt heated version is how do you design the shell? To take full advantage of the tube-and-drum system, the drums have to be inside the shell so you only have some nozzles for salt in/out the shell and water/steam/hot gas in/out through the drums. That's likely a rather big shell if those big drums have to fit into it, but if you can hang the tube/drum exchanger in the shell you've got a great design in terms of thermal expansion.
Sounds like the VVER SG design.


Well, the VVER SG is a bit hard to categorize. I'm taking it as a hybrid, with a thick tubesheet at the inlet cone of primary side, unclear what the outlet is (likely thick tubesheet as well), but the secondary side being a shell-header-drum design.

I wonder if this design is applicable to very high temperatures. The inlet cone is fairly rigidly connected to the shell and looks to have a thick tubesheet. On the other hand the cone is only attached on one side which would allow a lot of thermal expansion...

I also wonder about vertical versus horizontal layout for a high temperature design. Horizontal has more sagging issues but vertical has more deadweight tensile loading on the tubing. Perhaps with hot air as the shell side heat transfer fluid, more layout options are available.


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PostPosted: Jul 20, 2014 1:52 pm 
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Cyril R wrote:
Here's an interesting story on NBF about the recent explosion in natural gas price.

http://nextbigfuture.com/2014/01/east-c ... l-gas.html

If gas surges up to $100/mmbtu, presumably the natural gas portion of the NACC would be idled. If such price spikes become more common they could be an economic threat to the whole concept of NACC. It will take a lot of time (decades) for NACC to become widely used in grids, so the future natural gas prices could potentially economically sink the whole concept of NACC.

Clearly I'm very late in responding, but in those conditions NACC would actually float to the top, for two reasons, one is the marginal efficiency of NACC on gas is very high rivalling the most efficient CCGT plants today, secondly because NACC is already hot and spinning there is 0 startup cost from a fuel gas perspective. In addition NACC's ability to easily turn peaking on and off in real time means that it can accommodate variations in gas supply availability very quickly while being very efficient.

We should note that the price spike noted in the linked article is almost certainly due to gas delivery constraints driving up localised prices, not due to the change in gas price at a well connected hub.

For NACC you need to recall this analogy, Cyril and Lindsay walking in the beautiful Canadian wilderness, they stumble across a very mean and hungry Grizzly Bear. Does Cyril have to run faster than a grizzly to survive, no, just faster than Lindsay :shock:

In the same way NACC's highly efficient use of natural gas means that in any system where natural gas is a significant fuel for power generation, NACC should float to the top even if gas prices are high because they are faster/more efficient/better than the competition.


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