Energy From Thorium Discussion Forum

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PostPosted: Mar 02, 2013 6:28 am 
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The challenging thing in this area is the combination of high pressure and high temperature at the same time, if one only has one thing or the other to deal with, life is a lot easier. One saving grace is heat exchangers with small diameter channels for containing high pressure fluid, that is within the grasp of everyday engineering.

Large pressure vessels are the hardest as they require very thick walls to resisting the internal pressure. For example the reactor pressure vessel for the SCWR has a wall thickness that varies between 300 and 630 mm thick. That is a very serious pressure vessel.


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PostPosted: Mar 02, 2013 4:56 pm 
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Lindsay wrote:
The challenging thing in this area is the combination of high pressure and high temperature at the same time, if one only has one thing or the other to deal with, life is a lot easier. One saving grace is heat exchangers with small diameter channels for containing high pressure fluid, that is within the grasp of everyday engineering.

Large pressure vessels are the hardest as they require very thick walls to resisting the internal pressure. For example the reactor pressure vessel for the SCWR has a wall thickness that varies between 300 and 630 mm thick. That is a very serious pressure vessel.


According to Japan Steel Works, the limiting factor for forgings is forging weight, rather than thickness. Apparently a SCWR vessel weighs less per GWe than a BWR vessel.

The first turbine of a supercritical water cycle is also basically contained in a thick pressure vessel. There are many of them around the world, built quite quickly, so it seems reasonable.


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PostPosted: Mar 02, 2013 6:36 pm 
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Cyril R wrote:

According to Japan Steel Works, the limiting factor for forgings is forging weight, rather than thickness. Apparently a SCWR vessel weighs less per GWe than a BWR vessel.


I would think that both weight and overall size would be limiting factors for the forging. Of course that is not a problem for small reactors.

As for the SCWR vs BWR, the SCWR is more efficient, therefore smaller for the same MWe output, plus the SCWR coolant density in the reactor vessel is still high, unlike the BWR, so the reactor can remain smaller.


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PostPosted: Mar 03, 2013 1:12 am 
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Poly Hexafluoro Propyl Ether has been advertised as a commercial lubricant by Du pont.
http://www2.dupont.com/Lubricants/en_US ... _Oils.html
It or similar perfluorocarbons could be used to transfer heat from a CANDU type reactor tubes to the heat exchanger at low pressure.
It probably cannot work with liquid salts but could work with solid thorium fuels.


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PostPosted: Mar 03, 2013 5:55 am 
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How radiation resistant is it?

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PostPosted: Mar 03, 2013 6:28 am 
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KitemanSA wrote:
How radiation resistant is it?


Below is what was in their literature. This is good radiation resistance, probably due to using the fluorine and having no hydrogen), but not very high compared to the doses coming off our primary coolant.

Radiation Stability
Krytox® oils are remarkably stable to radiation when compared
with many materials used as lubricants or power fluids .
Irradiation of Krytox® lubricants causes minor depolymerization,
with a consequent reduction in viscosity and formation of
volatile products but not solids or sludge . In one test exposure
of a Krytox® sample to an electron bombardment of 10e7
rad at ambient temperature in air resulted in a viscosity decrease of
only 8%.The irradiated sample contained no sludge and was
unchanged in appearance.


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PostPosted: Mar 03, 2013 7:52 am 
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Yes, the dose inside a reactor is very high. 10 million rad sounds like a lot, but what is the dose a reactor coolant gets? It has to be way over 10 thousand rad/hour inside the core, right?

Perhaps this Krytox is useful for cooling electric motors in the hot cell. It would beat having to use gasses for component cooling.


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PostPosted: Mar 06, 2013 12:01 pm 
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My basic thinking is perfluorinated hydrocarbons. Poly fluorocarbons are quite safe at higher temperatures but all polymers could break up or get further polymerised to higher ones. Perhaps higher hydrocarbons like grease or bitumen could be perfluorinated to more suitable fluids. However a commercial product would have been tested before making the claims. It could be verified by testing a sample.


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PostPosted: Mar 06, 2013 2:46 pm 
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Cyril R wrote:
Yes, the dose inside a reactor is very high. 10 million rad sounds like a lot, but what is the dose a reactor coolant gets? It has to be way over 10 thousand rad/hour inside the core, right?


Easy to do an order-of-magnitude estimate. Say the coolant's heat capacity is 1500 J/(kg K) and ten percent of the delta 'T' it goes through is due to direct gamma-ray heating and that delta 'T' is from 200°C to 500°C and we have 45 kJ/kg of gamma heating per pass. That's 45 kilograys, 4.5 million rads. Multiply by the number of go-rounds per hour and you're there.

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PostPosted: Mar 06, 2013 2:55 pm 
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jagdish wrote:
My basic thinking is perfluorinated hydrocarbons. Poly fluorocarbons are quite safe at higher temperatures but all polymers could break up or get further polymerised to higher ones. Perhaps higher hydrocarbons like grease or bitumen could be perfluorinated to more suitable fluids. However a commercial product would have been tested before making the claims. It could be verified by testing a sample.


Fluorocarbons are very unsafe if you let them and oxygen meet at high temperatures. People who keep birds know that if you let a PTFE-coated aluminum frying pan melt on the stovetop in a space inhabited by birds, that space becomes bird-free. I've seen this myself. Mammals seem to be tougher.

The very high radiation dose rate above mentioned would catalyse the thermodynamically favoured conversion of middle-weight, low-viscosity, low vapour pressure perfluorocarbons into CF4, C2F6, and perfluorotar. There would have to be an onsite polishing system to distill the desired surviving stuff off the tar, and condense it, and send the light gases to a fluorocarbon destroyer, presumably something that contacts them with an alkali metal.

I'm not sure what one would do with the tar.

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PostPosted: Mar 12, 2013 11:04 am 
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I was an energy geek, so this is a great discussion. I'm wondering if we are missing an important point about LFTR. Yes, we could use a Rankin cycle if we were replacing the boiler on a conventional coal plant with a LFTR reactor. One of the huge advantages of LFTR is that it can be a much smaller reactor, say 100MWth, and retain good nucleaonics. Would we really make a 100 MWth plant and run the six feedwater heat stages you see in a modern large steam plant? I've worked on large plants. Measuring and maintaining water quality required human effort and a lot of equipment for condensate polishing.

I'm late to this discussion, but I wonder what heat cycles and working fluids are suitable for a small closed system. The largest problem I see with Brayton cycles is the size of the gas/air heat exchanger at the cold end and any intermediate gas/gas reheaters or recuperators.

Isn't the reactor salt inlet-outlet temperature difference fairly small and doesn't that small temperature difference require large heat exchangers for recuperation?

Thank you for your forbearance with foolish questions.
Curious Rob

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PostPosted: Mar 12, 2013 2:28 pm 
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100 MWth is very small for a supercritical rankine. The smallest ones off the shelf (on the market) are around 1000 MWt. There was some issue with minimum throughput on the high pressure turbine stage, that required a minimum size. Don't recall exactly what it was but I do recall it could be solved by steam attemperation or something. But superheated steam cycles are available on the market at 100 MWt. So you'd be better off, I think, with superheated steam for a small plant.

Regarding the inlet/outlet temperature difference, it is usually quite large because it reduces the size of the heat exchanger which is important for a fluid fuel reactor. ORNL assumed an inlet of 565 degree Celsius and outlet 704 degree Celsius, around 140 C delta T. The range of 100-200 degrees is probably practical for MSRs. So this is a quite large temperature difference.

The secondary salt (or third salt loop) can have smaller temperature differences with a Brayton cycle. But with steam cycle you have big temperature differences, which is an advantage I think in compactness even though many engineers suggest it's not for some reason.


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PostPosted: Apr 03, 2013 3:59 pm 
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There was a related discussion about small LFTRs in the applications section about ship power plants. (A large ship needs about 100 MW, a small one maybe 20). The goal in these cases is to replace large diesels.

Romawa Ltd. wanted to make an open-cycle nuclear ship's turbine. The idea here would be to avoid the low temperature heat exchanger by exhausting hot air directly from an open-cycle gas turbine using air. The hot section would be heated by a heat transfer fluid, like helium (helium has a terrible heat capacity, but I haven't done the engineering). Adam's atomic engines wanted to make closed-cycle Nitrogen turbines heated by pebble-bed nuclear fuel.

Both designs can be mechanically simple, because the pebble bed reactors can produce more heat when they're cooled (i.e. have a negative thermal coefficient). A simple throttle on the heat transfer gas can open to extract more energy. Well-designed LFTRs also have a negative thermal coefficient, so a throttle should work as well. A LFTR would add a heat exchanger for the working fluid, and a salt pump with a thermostat, but reduce the size of the pressure vessels, so the expense is probably equivalent or less. Refueling a LFTR is simpler.

One very sensible comment from a ship's engineer with 30 years' experience was to use a simple Rankine cycle. He liked them (he'd operated 6 over the years) and they worked great at oil-tanker sizes. The U.S. navy likes them, too. LFTRs have cheap energy to waste, so the lower thermal efficiency (30%) is not an issue. FYI... Navy nuclear guys who are not LFTR enthusiasts tend to describe this forum as a "bunch of nuts", and the LFTR as a "paper reactor." (If they're being kind.)


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PostPosted: Apr 03, 2013 6:41 pm 
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rgvandewalker wrote:
Well-designed LFTRs also have a negative thermal coefficient, so a throttle should work as well. A LFTR would add a heat exchanger for the working fluid, and a salt pump with a thermostat, but reduce the size of the pressure vessels, so the expense is probably equivalent or less. Refueling a LFTR is simpler.

LFTRs have cheap energy to waste, so the lower thermal efficiency (30%) is not an issue. FYI... Navy nuclear guys who are not LFTR enthusiasts tend to describe this forum as a "bunch of nuts", and the LFTR as a "paper reactor." (If they're being kind.)


ORNL originally worked out the LFTR concept because they were working on them for use as a jet engine. The liquid fueled reactor was selected in part BECAUSE it could be throttled without anyone extra along to control the reactor.

I am one of the Navy nukes that IS a LFTR enthusiast, and I like it BECAUSE it is more than just a paper reactor, a.k.a. the ARE, and MSRE experiments at ORNL did a lot to prove the concept. Of course there are many other reasons to like them also.


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PostPosted: Apr 03, 2013 9:12 pm 
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rgvandewalker wrote:
One very sensible comment from a ship's engineer with 30 years' experience was to use a simple Rankine cycle. He liked them (he'd operated 6 over the years) and they worked great at oil-tanker sizes. The U.S. navy likes them, too.


I was under the perception that practically every Navy ship that doesn't use a nuclear reactor used gas turbines (Brayton cycle).


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