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 Post subject: Air open cycle turbine
PostPosted: Sep 22, 2012 1:12 pm 
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I mentioned the open cycle air turbine in my book, getting a diagram from Forsberg's ORNL presentation last year. Alan Rice wrote this comment to me.

Quote:
However, any open cycle power conversion scheme, I think, will give detractors too much ammo. "Open" implies intimate contact with the environment (through the atmosphere). There is a concrete issue here which they can grasp. There will be some, even if minute, neutron irradiation of the working fluid... atmospheric air. Therefore there will be secondary activation of O16 yielding N16 which gives off formidable gamma radiation. Most of that is gone in about a minute but there's not likely to be a minute holdup in an open cycle system. This in turns brings up the issue of tertiary activation of the atmosphere (and the cooling loop), etc. Sheilding against the neutrons is likely going to impede heat transfer to the working fluid. What I am posing here is an argument that will be made... rightly or wrongly... by opponents. And, rightly or wrongly, they will win the day.


We wouldn't plan to circulate the radioactive fuel salt through the turbine, would we? Wouldn't we use a heat exchanger to transfer heat to a non-radioactive secondary salt loop, perhaps using a salt cheaper than Flibe? What is the amount of neutron activation that would take place? From the primary to secondary salt? From the secondary salt to air?


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PostPosted: Sep 22, 2012 2:50 pm 
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I have never figured out why the molten salt community
wants to compound all its problems by messing with the Brayton cycle
open or closed.
The closed cycle requires a turbine that doesnt exist.
The open cycle requires at a mininium massive modification
of a turbine that (for good reason) was not designed for our temperatures.

At our temperatures, Rankine gives at least as good efficiency (better if super-critical).

Much more importantly, there are scores of already built coal-fired Rankine plants
whose turbines are just waiting for a molten salt reactor to replace the boiler
and all its pollution. . These plants should be our target.

Jack


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PostPosted: Sep 22, 2012 3:05 pm 
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I suspect we just have a different set of assumptions wrt plant size. You don't find many 50MW coal plants these days but that is the size range I am interested in. Even the US Navy is converting from HP steam plants to Gas Turbine plants.

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PostPosted: Sep 22, 2012 3:33 pm 
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robert.hargraves wrote:
I mentioned the open cycle air turbine in my book, getting a diagram from Forsberg's ORNL presentation last year. Alan Rice wrote this comment to me.

Quote:
However, any open cycle power conversion scheme, I think, will give detractors too much ammo. "Open" implies intimate contact with the environment (through the atmosphere). There is a concrete issue here which they can grasp. There will be some, even if minute, neutron irradiation of the working fluid... atmospheric air. Therefore there will be secondary activation of O16 yielding N16 which gives off formidable gamma radiation. Most of that is gone in about a minute but there's not likely to be a minute holdup in an open cycle system. This in turns brings up the issue of tertiary activation of the atmosphere (and the cooling loop), etc. Sheilding against the neutrons is likely going to impede heat transfer to the working fluid. What I am posing here is an argument that will be made... rightly or wrongly... by opponents. And, rightly or wrongly, they will win the day.


We wouldn't plan to circulate the radioactive fuel salt through the turbine, would we? Wouldn't we use a heat exchanger to transfer heat to a non-radioactive secondary salt loop, perhaps using a salt cheaper than Flibe? What is the amount of neutron activation that would take place? From the primary to secondary salt? From the secondary salt to air?


No we wouldn't, yes we would, primary to secondary activation large enough to be really bad for your health to stand next to it for more than a few minutes, secondary to air is negligible, unless there's a leak then it looks bad.

And also there may be tritium if Li and/or Be primary salt is used. So in that sense closed cycles would be on the safer side. It would allow you another opportunity or two to take out the tritium.


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PostPosted: Sep 22, 2012 3:37 pm 
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KitemanSA wrote:
I suspect we just have a different set of assumptions wrt plant size. You don't find many 50MW coal plants these days but that is the size range I am interested in. Even the US Navy is converting from HP steam plants to Gas Turbine plants.


If you can burn some fuel gas turbines are great. We can't burn fuel with nuclear so we can't get 1000+ degrees Celsius. Brayton cycles are actually not very efficient at all, those compressors suck, it's just that if you can burn fuel real hot you can more than make up for that efficiency penalty.

Superheated steam seems just fine in the 50 MWe class, though truth be told, steam turbines only become really nice beyond 100 MWe.


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PostPosted: Sep 23, 2012 5:35 am 
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If you want to retrofit old coal plants with small modular reactors, a simple solution could be to feed a single turbine with the heat of multiple reactors.


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PostPosted: Sep 23, 2012 7:35 am 
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Burghard wrote:
If you want to retrofit old coal plants with small modular reactors, a simple solution could be to feed a single turbine with the heat of multiple reactors.
I don't, I want to REPLACE old coal plants with SMLFTRs. Well, ... I also want to retro, but first let us get the SMLFTRS up and running.

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PostPosted: Sep 23, 2012 9:03 am 
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Thinking about the air open cycle some more. Seems to me that to be really safe and practical, there would have to be 3 salt loops for this to work. That is because the secondary salt is quite radioactive and also has tritium. If the secondary salt is FLiBe then it has low gamma activity (and very short lived) but then there's a chemical problem with BeF2 getting out into the air in the event of the air HX failure.

So use 3 loops. First loop is fuel salt. Second loop is FLiBe. Third loop a carbonate such as CLiNaK. Fourth is the air.


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PostPosted: Sep 23, 2012 10:29 am 
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The 50 MW market is a sideshow and in my view a distraction.

But if that's what you are into, Rankine is perfectly viable and better than Brayton at our temp.

In the 1970's hundred of Rankine steam plants were built.
They were called super-tankers.
I operated 7 of them. The good ones ran like tops
which is more than I can say for the diesels that came after.
60 bar, 510C, 4-5 feedheaters, no reheat.

Of course, they lost out on fuel cost to a much higher temp Otto cycle
when oil prices took off, but fuel cost is not a big deal for a nuke.


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PostPosted: Sep 23, 2012 11:07 am 
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djw1 wrote:
The 50 MW market is a sideshow and in my view a distraction.


Agree, though it could prove a useful entry market for a salt cooled or salt fuelled reactor.

Quote:
Of course, they lost out on fuel cost to a much higher temp Otto cycle
when oil prices took off, but fuel cost is not a big deal for a nuke.


Fuel cost is not, but thermal power related capital cost is.

The 3400 MWt AHTR recent economic analysis for example, showed that the 3400 MWt AHTR has higher fuelling cost than a 3400 MWt LWR, and higher capital costs. But due to the improved efficiency the capital cost per MWe are lower.

If we can build a reactor that has lower capital cost AND higher efficiency, it will be awesome.


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PostPosted: Sep 23, 2012 4:14 pm 
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Nothing new in this post, but just affirming some of the earlier comments 10 MWe, 50 MWe, 100 MWe, they perhaps have their uses and a place as a demonstration plant on the road to a utility class installation, but to think of them as a main-line product is distracting and possibly counterproductive.

Secondary salt will be radioactive (activated sodium at least) and tritium management is a genuine concern even in the secondary loop, so special measures have to be employed to ensure that tritium is managed and that no activated salt can be released to the environment if there is a leak somewhere.

Last of all, the temperatures that are available to us in the early stages of MSR development do not allow the Brayton cycle to do its best work, to see that we need to up around 900C or higher if possible.


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PostPosted: Sep 23, 2012 7:55 pm 
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Cyril R wrote:
Thinking about the air open cycle some more. Seems to me that to be really safe and practical, there would have to be 3 salt loops for this to work. That is because the secondary salt is quite radioactive and also has tritium. If the secondary salt is FLiBe then it has low gamma activity (and very short lived) but then there's a chemical problem with BeF2 getting out into the air in the event of the air HX failure.

So use 3 loops. First loop is fuel salt. Second loop is FLiBe. Third loop a carbonate such as CLiNaK. Fourth is the air.

If you give up the obsession with FLiBe and use NaF-ZrF4 instead as solvent for fuel, you could have it in a pool in the reactor vessel and use a clean salt loop to transport the heat. After that you can have air or any gas or steam to run the turbines.
You also have a choice of thermal or fast spectrum (if short life graphite moderator feels problematic).


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PostPosted: Sep 24, 2012 4:01 am 
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jagdish wrote:
Cyril R wrote:
Thinking about the air open cycle some more. Seems to me that to be really safe and practical, there would have to be 3 salt loops for this to work. That is because the secondary salt is quite radioactive and also has tritium. If the secondary salt is FLiBe then it has low gamma activity (and very short lived) but then there's a chemical problem with BeF2 getting out into the air in the event of the air HX failure.

So use 3 loops. First loop is fuel salt. Second loop is FLiBe. Third loop a carbonate such as CLiNaK. Fourth is the air.

If you give up the obsession with FLiBe and use NaF-ZrF4 instead as solvent for fuel, you could have it in a pool in the reactor vessel and use a clean salt loop to transport the heat. After that you can have air or any gas or steam to run the turbines.
You also have a choice of thermal or fast spectrum (if short life graphite moderator feels problematic).


For the fuel that's ok. But not for the secondary salt since Na and Zr activate with high gamma activity due to the delayed neutrons in the primary heat exchanger. It's possible to use Na and Zr in the second loop, but then you'd need a third loop of another Na-Zr salt, to prevent that activity from mixing with your power cycle.

So you'd have FNaZrUTh in loop 1. FNaZr in loop 2. Another FNaZr in loop 3. Then the open cycle gas turbine in "loop" 4.


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PostPosted: Sep 24, 2012 1:38 pm 
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Is there a particular reason why the secondary loop needs to be a salt? Why not a liquid metal like gallium (large liquid range, low vapor pressure, low thermal expansion, no worry of freezing)?


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PostPosted: Sep 24, 2012 3:04 pm 
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Cthorm wrote:
Is there a particular reason why the secondary loop needs to be a salt? Why not a liquid metal like gallium (large liquid range, low vapor pressure, low thermal expansion, no worry of freezing)?

Wikipedia wrote:
Gallium attacks most other metals by diffusing into their metal lattice. Gallium, for example, diffuses into the grain boundaries of Al/Zn alloys[1] or steel,[2] making them very brittle. Also, gallium metal easily alloys with many metals, and was used in small quantities as a plutonium-gallium alloy in the plutonium cores of the first and third nuclear bombs, to help stabilize the plutonium crystal structure.[3]

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