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PostPosted: Apr 17, 2013 4:11 am 
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Some of the most recent work of the US national labs working on fluoride salt cooled solid fuelled high temperature reactors has apparently settled on using the open air Brayton cycle with Rankine steam as bottoming cycle.

http://www.jaif.or.jp/ja/wnu_si_intro/d ... 20Heat.pdf

The rationale is to make use of this well developed, commercially available combined cycle (used widely in CCGTs around the world).

That is all fine and agreeable. Yet some of the other rationale appears rather questionable to me. The idea that this cycle is well suited to molten salt cooled reactors appears only partially true. The compressor first compresses outside (filtered/conditioned) air to around 450 degrees C. But the primary salt only operates at 700 C, with a temp drop across the second salt loop and even greater temp drop across the air heater, a realistic design probably wouldn't get beyond 625 degree C heated air. So first the compressor adds maybe 425 C then the actual heat source adds 200 C above that. And then the compressor isn't 100% efficient. It means that most of the power needs to be converted to run the compressor. I don't see this being an economical and efficient option at all. If the air is only 625 C and an efficient steam bottoming cycle must be coupled, then it's clear that there's not much net work to be extracted. The heat recovery steam generator (HRSG) has a considerable temp drop, and efficient steam cycles would need >500 C steam. Leaving less than 100 degree C of actual delta T for work in the Brayton turbine! A compressor that needs to invest 425 C and then only 100 C to work it back, how can this be attractive?

The reason why combined cycles with natural gas are so attractive is the high temperatures available by combusting natural gas. Over 1400 degrees C is possible which makes this cycle so efficient. These temperatures are way beyond what's possible with fluoride cooled reactors. The drop available for work in the turbine can exceed 700 degree C.

Then there is a tritium management issue. FHRs use FLiBe as primary coolant, and actually produce more tritium than MSRs with FLiBe because there's much more FLiBe in the solid fuel option. This tritium will be present in elemental form and will diffuse through hot heat exchangers rapidly. It is claimed that the the bottoming cycle will be protected because it isn't connected to a closed loop on the other side. But this implies that the tritium is all going up the stack into the outside air!! No way that you can get below the 10 Ci/d limit with this for a large reactor.

It is also claimed that the capital cost is low. I don't get this at all. Combined cycles need a gas turbine, a steam turbine, and a heat exchanger in between the two cycles that is inefficient and costly (bulky).

There are clearly some advantages of the combined cycle, but there appear important showstoppers for adoption for FHRs as the base-case power cycle.


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PostPosted: Apr 17, 2013 1:38 pm 
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Turbine development confuses me.

I can imagine that it costs hundreds of millions of dollars to wring the last couple of % efficiency out of gas or steam turbines, because all the work must be done on full size equipment running at the maximum possible temperature with very small clearances.

On the other hand, it doesn't seem to me that the business case for a new kind of reactor can possibly rely on a few % of efficiency. If the turbine goes from 40% to 30% efficient, the entire system scales up by 33% and so $/watt goes up by 33%. That's significant but if it kills the case for the reactor, then the business case for the reactor is not very strong in the first place.

It seems a bigger problem is just deciding on a turbine back end that is known to be low risk, even if low efficiency. The majority of the risk would appear to come from component development, things like bearings that start/stop poorly, heat exchangers that spall or leak tritium or scavengers that fail to collect tritium. How big do these components need to be in order to retire the risk of the turbine development? Would a 30% efficient 1 MW(e) Brayton cycle turbine see most of the same early failures as a 100 MW(e) unit? How much would a megawatt prototype turbine cost to build and demo, fired by a natural gas salt heater?

For comparison, a PT6 turboshaft engine which puts out around a megawatt costs something less than $1/watt and has turbomachinery perhaps a foot in diameter. A high-pressure 1 MW(e) Brayton cycle turbine would have much smaller turbomachinery, maybe three or four inches diameter, which means entire turbine disks can be 3D printed as a unit. I'd guess $20k would cover a single unit. Seems like five iterations of the turbomachinery might get printed for around $1m cost.

It would cost about $10m for the natural gas to run the thing continuously for five years, which seems like a reasonable number. If you can get a grid hookup and dump the output electricity onto the grid (granted, this requires synchronization, but that's something that you need to develop anyway), then the test operating cost comes down by $7m or so.

These numbers don't seem so large. How much risk is involved in scaling a turbine two orders of magnitude if the pressures, temperatures, and materials stay the same? Presumably turbine stresses scale up, but they shouldn't scale up tremendously if tip speeds remain similar.

-Iain


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PostPosted: Apr 17, 2013 1:58 pm 
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For me, the issue isn't just development cost. If we can build a 45% efficient off the shelf supercritical steam turbine with supercritical water at just 550 C, and peak primary salt at 700 C, then why settle on some combined cycle that is much more complex, more costly, more likely to leak tritium to the air, and requires much higher salt temperatures to get that 45% efficiency?

A salt to air heater using pressurized air wouldn't be easier to develop than a molten nitrate third loop steam generator. In fact the latter have been built recently in concentrated solar power plants, whereas the former are entirely theoretical.


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PostPosted: Apr 17, 2013 2:10 pm 
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So how much would it cost to prototype a molten nitrate loop with a tritium getter system? Presumably this scales even better than a turbine, and could be prototyped in the multi-kilowatt scale.

-Iain


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PostPosted: Apr 17, 2013 2:24 pm 
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iain wrote:
So how much would it cost to prototype a molten nitrate loop with a tritium getter system? Presumably this scales even better than a turbine, and could be prototyped in the multi-kilowatt scale.

-Iain


The nitrate loop is the getter. It's one of the strongest oxidizers available, producing hydrogen oxide that is dissolved and sequestered in the molten nitrate.

I don't know about costs. ORNL figured it wouldn't cost much, they did a preliminary cost study:

http://www.energyfromthorium.com/pdf/ORNL-TM-3428.pdf

These days the equipment is all available from CSP suppliers. Probably we can skip the R&D, go straight to engineering study, except for the molten fluoride coolant loop - nitrate salt HX. Heatric from the UK could probably do it.


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PostPosted: Apr 19, 2013 5:18 am 
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Looking at this graph of specific power output:

http://upload.wikimedia.org/wikipedia/c ... GFImg8.png

It's clear that the molten salt heated open air Brayton would achieve less than a third the specific power output of a modern gas fired turbine.

How can this possibly be attractive? Even if the efficiency is good, there's too little net work for this cycle to be attractive for a 700 C salt temp.


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PostPosted: Apr 19, 2013 10:55 am 
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It seems to me that the only scenario where open-air gas turbine applications would be preferred is where one doesn't want to deal with steam or has no access to water, especially for cooling.

Isolated areas in e.g. mountainous regions where water supply is sporadic at best or in arid regions, might make all the "messing about" with steam too complicated and expensive. Construction and mining sites could use such plant for "temporary" (2 to 10 year) operating cycles. The investment in providing a "cold sink" for steam may rule out building the plant. The price of the lower capital cost is less output from the same amount of fuel.


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PostPosted: Apr 19, 2013 1:06 pm 
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berfel wrote:
It seems to me that the only scenario where open-air gas turbine applications would be preferred is where one doesn't want to deal with steam or has no access to water, especially for cooling.

Isolated areas in e.g. mountainous regions where water supply is sporadic at best or in arid regions, might make all the "messing about" with steam too complicated and expensive. Construction and mining sites could use such plant for "temporary" (2 to 10 year) operating cycles. The investment in providing a "cold sink" for steam may rule out building the plant. The price of the lower capital cost is less output from the same amount of fuel.


We recently discussed the indirect dry cooling scheme, commercially marketed as the Heller dry cooling system. Seems to work fine at a reasonable cost. The added equipment cost is paid for by reduced water costs.

One nuclear plant in Russia uses this dry cooling system - the only dry cooled nuclear plant in the world - so it's not theoretical at all.

I think people are missing the point about Brayton cycles. They are only attractive when you have access to really high temperatures such as those by combusting natural gas. Or when weight is important, as in aircraft (which must also burn hot to be efficient, by the way). That compressor is really not nice at all, it diverts all the power.

I don't see these things making sense for any first gen MSR or FHR. This might be swearing in the church.


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PostPosted: Apr 26, 2013 1:47 am 
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Cyril R wrote:
berfel wrote:
It seems to me that the only scenario where open-air gas turbine applications would be preferred is where one doesn't want to deal with steam or has no access to water, especially for cooling.

Isolated areas in e.g. mountainous regions where water supply is sporadic at best or in arid regions, might make all the "messing about" with steam too complicated and expensive. Construction and mining sites could use such plant for "temporary" (2 to 10 year) operating cycles. The investment in providing a "cold sink" for steam may rule out building the plant. The price of the lower capital cost is less output from the same amount of fuel.


We recently discussed the indirect dry cooling scheme, commercially marketed as the Heller dry cooling system. Seems to work fine at a reasonable cost. The added equipment cost is paid for by reduced water costs.

One nuclear plant in Russia uses this dry cooling system - the only dry cooled nuclear plant in the world - so it's not theoretical at all.

I think people are missing the point about Brayton cycles. They are only attractive when you have access to really high temperatures such as those by combusting natural gas. Or when weight is important, as in aircraft (which must also burn hot to be efficient, by the way). That compressor is really not nice at all, it diverts all the power.

I don't see these things making sense for any first gen MSR or FHR. This might be swearing in the church.


Saying no to conventional aeroderivative brayton cycles is not heresy when making a point regarding gross/net power. I would make a quip about a supercritical CO2 loop as an alternative (such as a RamGen style unconventional supersonic compressor/turbine for reduced compressor power consumption), but the point about aeroderivatives is cost reduction through the existing body of engineering knowledge, which limits you to air or pure nitrogen.


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PostPosted: Apr 29, 2013 2:56 am 
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Quote:
the point about aeroderivatives is cost reduction through the existing body of engineering knowledge, which limits you to air or pure nitrogen.


I realise this, but still don't see the application to a first gen MSR or FHR making sense. Cost reduction with a dual cycle and added heat exchanger is not plausible, in my opinion, compared to just superheated or supercritical steam single cycle. The Brayton doesn't add anything substantial and causes a lot of issues with tritium. The net work available in the Brayton would be so low (ie almost all power diverted to run the compressor), it's better to just skip it altogether and go for a single cycle steam turbine.


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PostPosted: May 06, 2013 7:07 pm 
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I think that when you see more detailed arrangements it will make more sense.

As an aside, one of the key benefits of any power conversion setup that includes a Brayton is that as soon as you can increase the working temperature you see an immediate uplift in power and efficiency, so with an eye on the longer term one can make an argument for having a Brayton machine in there.


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PostPosted: May 07, 2013 4:55 am 
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Lindsay wrote:
I think that when you see more detailed arrangements it will make more sense.

As an aside, one of the key benefits of any power conversion setup that includes a Brayton is that as soon as you can increase the working temperature you see an immediate uplift in power and efficiency, so with an eye on the longer term one can make an argument for having a Brayton machine in there.


Still don't see it.

If Braytons make sense in the future, use them in future designs. Don't try to incorporate a 2013 Brayton design in a 2030 molten salt cooled reactor. By 2030, it'd be archaic and stupid.

We need to plan for near term design. Everything else is crystal ball science, which is dangerous as it risks developmental dead end for a large number of reasons (funding environment changes, politicians change their mind, corporate interest shifts, etc. etc.).

I could warm my house all winter by burning all the tomes of paper reactor designs with 1000 C Brayton cycles.


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PostPosted: Dec 19, 2013 1:45 am 
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UC Berkeley has been studying design options for nuclear air combined cycles (NACC), coupled to high temperature reactors using fluoride salts. To provide a reasonable reference case, we've focused on designs based upon GE's 7FB gas turbine, which has a compression ratio of 18.2 and a nominal compressor outlet temperature of 420°C. This is a reasonable inlet temperature for a salt-to-air heat exchanger, and with salt coolant heated to 700°C, it is possible to heat the air to 670°C.

The key modifications are this external heating, and the use of a single stage of reheat, where the air is expanded to approximately 5 atm, where it is again around 420°C, and then is reheated to 670°C.

In this configuration, with a conventional heat recovery steam generation, the THERMFLEX code predicts that this NACC system can produce a net of 100 MWe of base load electricity, with a nuclear thermal heat input of 236 MWe. Co-firing with natural gas after the second nuclear heating stage can increase the power output to 240 MWe peak power output, with the efficiency for converting the natural gas into electricity being 66%.

The major benefit of the single stage of reheat is that it doubles the amount of nuclear heat that can be put into the cycle, and it increases significantly the efficiency of the cycle under nuclear base load operation.

A schematic for the system, coupled to a modular PB-FHR, is attached.

-Per


Attachments:
FHR_Mark 1 B PDF.pdf [63.77 KiB]
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PostPosted: Dec 19, 2013 3:55 am 
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Voltage Source Converter based HVDC has now reached 500kV.
That makes it possible to transport electricity well over a thousand kilometres before the ~10% loss of efficiency from going to dry cooling is overwhelmed by the resistive and other losses in the HVDC link.
VSC voltages will continue to increase and will probably eventually reach the megavolt levels that conventional HVDC is capable of.
VSC lines are capable of easy multi-terminal network forming and can black start connecting grids if power can be provided to another point on the connection.

It is hard to find places with significant population more than a thousand kilometres either from the sea or from a massive lake that should have no difficulty providing the required water, and if they can get 800kV VSC to work the distance will increase to something like 4000km which would enable power transport from the sea to any point on the surface of the earth.


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PostPosted: Dec 19, 2013 6:01 am 
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Thanks for your responses Dr. Peterson.

Could you respond to my concern about tritium emissions with this arrangement?


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