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PostPosted: Sep 11, 2012 7:03 am 
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We usually assume that a supercritical Rankine (steam) cycle can couple well to a LFTR/DMSR. They are more efficient than the lower pressure, subcritical steam units.

However, it looks like this isn't as simple. For starters, the supercritical units don't come in under 500 MWe size. So that doesn't go well with the modular LFTR idea. Developing one would be more expensive, and would be barely more efficient than a subcritical unit anyway.

http://deq.state.wy.us/eqc/orders/Air%20Closed%20Cases/07-2801%20Dry%20Fork%20Station/Earthjustice.Exhibit%2028%20-%20Subcritical-Supercritical%20Boiler%20Comparison%20by%20S&L%20(6-11-07).pdf

If that's true then we may have to revise the supercritical unit to a subcritical unit. From the document, the subcritical turbine has an efficiency of 42% (without boiler losses). Which is almost as good as the supercritical unit for a small plant (42.5%).


Last edited by Cyril R on Sep 11, 2012 9:36 am, edited 1 time in total.

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PostPosted: Sep 11, 2012 9:02 am 
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Your link doesn't work- how about the whole link in plain text.


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PostPosted: Sep 11, 2012 9:37 am 
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cloa513 wrote:
Your link doesn't work- how about the whole link in plain text.


Fixed it.


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PostPosted: Sep 11, 2012 12:52 pm 
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That was a good read.

I thought it was interesting that an air cooled condenser requires the use of motor driven boiler feed pumps. I didn't quite follow the reasoning... the air cooled condenser somehow leads to variations in turbine backpressure... is this a putative turbine driving the boiler feed pump, or the cooling air fans for the condenser, or the main power turbine?

In any case, I thought it was interesting that there is a choice between a steam turbine driven pump, or a motor driven pump. I would have thought that you could get the efficiency of the steam driven pump with the load variability of the motor driven pump by having a steam turbine drive the "baseload" power for the pump, and let a smaller electric motor on the same shaft drive whatever variable load there was.

I'll say again that a $2B, 1 GW(e) MSR seems like a pretty good module size to me. I'd like to see four to six going into each site presently occupied by a LWR.

-Iain


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PostPosted: Sep 11, 2012 1:15 pm 
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iain wrote:
I thought it was interesting that an air cooled condenser requires the use of motor driven boiler feed pumps. I didn't quite follow the reasoning... the air cooled condenser somehow leads to variations in turbine backpressure... is this a putative turbine driving the boiler feed pump, or the cooling air fans for the condenser, or the main power turbine?


Not sure exactly, but here's what I make of it.

The ACC has a higher pressure drop - more piping, from the lower HTC of air cooling. The feedwater pump then needs more pump power. So you also need more condensing capacity for that feed turbine steam exhaust (which operates off poor quality steam, needing lots of condensing per unit power). Since the condensing capacity of an ACC is more expensive & inefficient than water cooled condensers, that's bad. Further, the larger pressure drop also varies more than a water cooled version, because the ambient air temperature is very important - you're cooling with air. In a steam turbine the efficiency changes rapidly with backpressure. Not a problem with an electric motor driven pump. I think the cooling air fans are all motor driven as well.

Quote:
I'll say again that a $2B, 1 GW(e) MSR seems like a pretty good module size to me. I'd like to see four to six going into each site presently occupied by a LWR.


I agree, and used this thread to strengthen my opinion on larger plants :lol:

Though the early plants would be smaller to reduce development cost and financing risk, I think there would eventually be a move towards larger plants. Economy of scale is very strong in nuclear reactors.


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PostPosted: Sep 11, 2012 4:45 pm 
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iain wrote:
That was a good read.

I thought it was interesting that an air cooled condenser requires the use of motor driven boiler feed pumps. I didn't quite follow the reasoning... the air cooled condenser somehow leads to variations in turbine backpressure... is this a putative turbine driving the boiler feed pump, or the cooling air fans for the condenser, or the main power turbine?

In any case, I thought it was interesting that there is a choice between a steam turbine driven pump, or a motor driven pump. I would have thought that you could get the efficiency of the steam driven pump with the load variability of the motor driven pump by having a steam turbine drive the "baseload" power for the pump, and let a smaller electric motor on the same shaft drive whatever variable load there was.

I'll say again that a $2B, 1 GW(e) MSR seems like a pretty good module size to me. I'd like to see four to six going into each site presently occupied by a LWR.

-Iain

The electric feedpump wording was rather clumsy, I suspect that the changing back pressure conditions associated with the ACC makes the turbine match for the application harder, possibly driving them to fit an oversized unit to cover all variations in operating conditions. This would be compounded by the fact that feedpump turbines are driven by steam bled from the main unit at a relatively low pressure, so variation in back pressure have a much bigger effect than they do on the main turbine.

The bottom line (as you suggest) is if base-loading, go electric because the main turbine is more efficient than a feedpump turbine is, but because feedpump turbines can operate at variable speed they are sometimes chosen where part load or load following operation is expected, especially where the main boiler pressure is permitted to 'slide' up and down with changing load. In terms of industry magazines and new plants, electric drives using variable speed drive are be mentioned more frequently.

For this application the feedpump power would be about 7 MW, so not massive and not trivial either.

Yes to nominal 1 GWe MSR's, for economic bulk generation in most countries 1 - 1.5 GWe would be hard to beat IMO. For smaller grids, smaller countries 400 MW units should fit pretty well most of the time.


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PostPosted: Sep 11, 2012 5:20 pm 
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Cyril R wrote:
We usually assume that a supercritical Rankine (steam) cycle can couple well to a LFTR/DMSR. They are more efficient than the lower pressure, subcritical steam units.

However, it looks like this isn't as simple. For starters, the supercritical units don't come in under 500 MWe size. So that doesn't go well with the modular LFTR idea. Developing one would be more expensive, and would be barely more efficient than a subcritical unit anyway.

A very keen observation Cyril.

For steam turbines, steam pressure, steam temperature and regenerative feedwater heating all contribute to improved cycle efficiency. The last two items contribute more than pressure does, so while it is true that the most efficient unit with be the supercritcal one, a subcritical unit operating at the same temperatures with the same number of feedwater heaters will be very close.

Another good observation is that in the smaller sizes it is not practical or possible to procure supercritical steam turbines operating at high temperatures. As one might reasonably expect the higher steam conditions are only available with the increasing steam turbine size. Personally I don't see any problem staying with subcritical pressures at any size, so long as one is working the highest available temperatures.

The following table shows a breakdown of the Siemens range of STG's. I believe that in practical terms the Min MW values in the table are suspiciously low, so some caution should apply.

Attachment:
Siemens STG Families.pdf [2.28 KiB]
Downloaded 157 times


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PostPosted: Sep 12, 2012 2:49 am 
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Thanks Lindsay. A supcritical unit may be easier to design as well, with lower pressures meaning less stresses on the equipment, and no supercritical phase means less stuff should get corroded as opposed to a supercritical water generator.

Doing more Googling found this IAEA document on steam power cycles for helium cooled fusion reactor power cycle with modest temperatures.

http://www-pub.iaea.org/MTCD/publicatio ... CA-O-1.pdf

Looks like the subcritical cycles can be designed for high efficiency. Also looks like the helium pump power is massive, which should not surprise us, that hurts the efficiency. LFTRs don't have that parastic load. Clearly a very high efficiency (45%) can be achieved with subcritical steam, but it requires a high temperature steam to do it (642C). The coal fired plant study had more reasonable temperatures and gave a 42% efficiency. Sound good for a first prototype, smaller plant.

It's interesting to think about the implications for the LFTR.

Hotter steam means either a bit hotter operating temperature, needing a different material than Hastelloy N, for the vessel.

...or a more compact primary HX, needing a new technology there.

Whereas developing a supercritical water generator with molten salt on the other side, could be more tricky than a designing a subritical unit molten salt-steam generator (these are already operating in CSP plants in Spain).

In any of these cases it means more development cost or settle for lower efficiency.


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PostPosted: Sep 12, 2012 5:54 am 
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Hmm, the improved supercritical cycle is also impressive. It needs reheat and superheat, but only to 556 Celsius. It gets almost 50% gross efficiency. Substracting the feed pump and condensate pump load, gives 48% net efficiency. With standard wet cooling towers and some power to run the pumps of the salt circuits, that would get about 47% net efficiency.

The low superheat temperature is interesting because it can couple to a NaNO3-KNO3 third loop (must operate <600 Celsius).

Though it must be kept in mind, this is for a 2.5 GWe system!


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PostPosted: Sep 12, 2012 6:37 am 
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Cyril R wrote:
Thanks Lindsay. A supcritical unit may be easier to design as well, with lower pressures meaning less stresses on the equipment, and no supercritical phase means less stuff should get corroded as opposed to a supercritical water generator.

Just to help polish your halo a bit more Cyril, for thin walled pipes wall thickness is proportional to pressure, double the pressure, double the wall thickness. So comparing a 250 bar SC vs a 177 bar subcritical setup, 250/177 = 1.41, i.e. 41% more material is required for piping and pressure parts for the higher pressure. This includes heat exchangers of course. When you start thinking about the cumulative benefits across the system of using subcritical steam pressures, that last bit of efficiency is not quite so appealing any more. Regarding a connection between SC conditions and corrosion, even the subcritical conditions that we are talking about here are extremely demanding on feedwater chemistry any significant deviations will be punished severely, but SC will be a little more demanding.


Cyril R wrote:
Hotter steam means either a bit hotter operating temperature, needing a different material than Hastelloy N, for the vessel.

...or a more compact primary HX, needing a new technology there.

Whereas developing a supercritical water generator with molten salt on the other side, could be more tricky than a designing a subritical unit molten salt-steam generator (these are already operating in CSP plants in Spain).

In any of these cases it means more development cost or settle for lower efficiency.

Hastelloy N is hot enough IMO, Siemens can accept 600C main steam and 620C on the reheat for the bigger units (SST-6000), it should be possible to achieve that with a core outlet temperature of 704C (Hastelloy N temp). It would be easier with a bigger temperature difference, but at least you have choices; lower steam temp or slightly bigger HX or higher core outlet temperature. As one contemplates what to develop and what to accept and buy from someone else, it's always nice to have choices.


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PostPosted: Sep 12, 2012 7:32 am 
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Well, I wasn't so much wondering about the small diameter piping, more about the bigger piping, HP turbine casing, seals and welds, and possible booster pumps (for a fluoride salt HX the mp. would be high so we need more feedwater heating).

But, doesn't SC water dissolve stuff, as opposed to superheated steam? If my halo has not suffered from pitting corrosion, isn't it true that in a subcritical steam system, the liquid water is only present in the lower temperature parts of the system?

Of course a subcritical system does need steam seperators and steam dryers. A supercritical unit doesn't need these (but maybe needs a moisture seperator-reheater downstream of the HP turbine to protect the IP and LP turbines?).

As you know I'm not a steam turbine guy at all, it's hard enough for me to get the basics right, so it's tough to see all the pro's and con's in this subcritical/supercritical discussion.


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PostPosted: Sep 12, 2012 3:19 pm 
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Cyril R wrote:
Well, I wasn't so much wondering about the small diameter piping, more about the bigger piping, HP turbine casing, seals and welds, and possible booster pumps (for a fluoride salt HX the mp. would be high so we need more feedwater heating).

But, doesn't SC water dissolve stuff, as opposed to superheated steam? If my halo has not suffered from pitting corrosion, isn't it true that in a subcritical steam system, the liquid water is only present in the lower temperature parts of the system?

Of course a subcritical system does need steam seperators and steam dryers. A supercritical unit doesn't need these (but maybe needs a moisture seperator-reheater downstream of the HP turbine to protect the IP and LP turbines?).

As you know I'm not a steam turbine guy at all, it's hard enough for me to get the basics right, so it's tough to see all the pro's and con's in this subcritical/supercritical discussion.

The thin wall comment was on the basis of being able to make an accurate statement while illustrating the point, it is generally also true for thick wall components, but it's more complicated and no longer linear. Basically anything connected to the water steam systems at those pressures will be at least 41% thicker at that higher pressure than the lower one.

>But, doesn't SC water dissolve stuff
Yes, hot ultrapure water can be very aggressive chemically. For these systems at either pressure the water must be ultrapure (resistance> 10 MOhm/cm), it will happily dissolve base alloys like brass and copper. SC conditions are the most demanding, but 177 bar/600C conditions are sufficiently close, that there is not a lot of difference.

Regarding steam separators, this is an area of significant difference between SC LWR's and a SC steam system attached to a MSR (remembering that MSR allows us to use much higher steam temperatures). In the former, steam starts out at lower temperature and quickly becomes wet steam requiring separation and then reheating, for the latter, steam starts out at a much higher temperature and is then reheated before passing into the IP turbine. The steam remains superheated until it gets to quite a low pressure. It is that reheating process that ensures that the dryness of the steam within the turbine is at an acceptable level.

>so it's tough to see all the pro's and con's in this subcritical/supercritical discussion.
It is definitely a technical area for good reason, but I believe that you've captured the key differences to think about comparing SC to subcritical steam cycles for MSR applications.


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PostPosted: Sep 13, 2012 2:42 am 
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If supercritical carbon dioxide turbines are developed, could you couple them directly to the primary salt circuit, or would you still need an intermediate salt circuit? Or would the high pressure of the S-CO2 make water/steam a better option?


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PostPosted: Sep 13, 2012 5:21 am 
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jon wrote:
If supercritical carbon dioxide turbines are developed, could you couple them directly to the primary salt circuit, or would you still need an intermediate salt circuit? Or would the high pressure of the S-CO2 make water/steam a better option?


CO2 doesn't react with fluoride salts. That's nice.

We would still use an intermediate salt circuit, because that allows us to deal with a pressure wave in the event of the power cycle HX rupture. This would be done with rupture discs mounted on the intermediate circuit piping. Actually it would be even better to deal with this in a third loop, because the second loop is also radioactive from delayed neutron activation. Most people don't seem to realize this, but the second loop is actually quite radioactive, unless it is FLiBe.

For CO2 the third loop could be another fluoride loop or a carbonate loop (CLiNaK, FLiNaK's carbonate cousin).


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PostPosted: Sep 13, 2012 7:13 am 
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The Sandia guys are proposing to run S-CO2 directly through the reactor and into the turbine. Surely if you can make a simpler design, even at the cost of some plant contamination, it would have to be cheaper to build


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