Energy From Thorium Discussion Forum

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PostPosted: Jan 25, 2014 2:10 pm 
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Many of the molten salt reactors discussions here focus on engineering solutions to the problems of running a reactor at high power. Things like high volume salt pumps and high capacity heat exchangers, finding durable core materials, and minimizing neutron capture in 233Pa and loss of delayed neutrons while fuel salt is out of the core. These issues mostly disappear if the reactor is run at low power with natural convective circulation. Pumps are no longer necessary for normal operation. Circulation timescales of minutes instead of seconds would limit delayed neutron loss, even with relatively large out of core salt fractions. Simple countercurrent shell and tube heat exchangers could provide adequate heat transfer at low delta-T. Graphite in the core would last many decades with the lower neutron flux, and the equilibrium concentration of 233Pa would be very low, with almost no neutron capture before decay to 233U and limited reactivity increase following shutdown. Passive management of decay heat becomes easier.

The cost is a much higher volume of fuel salt to achieve a given power output. This is a one time cost though - the isotopically separated lithium can be recycled indefinitely, as can the startup fissile, unlike limited lifetime pumps and fancy heat exchangers and graphite that needs replaced on a regular basis. The availability of fissile would restrict the rate at which molten salt reactors could be constructed, but expansion of nuclear power could still be achieved by stretching out the transition from LWRs to MSRs.


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PostPosted: Jan 25, 2014 2:54 pm 
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would it be simpler to get a license?
Would the ongoing regulations be any simpler?


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PostPosted: Jan 25, 2014 4:14 pm 
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To some extent won't this also be governed to the use the reactor is for? It seems like a small reactor used for district heating would have an advantage in being passive as it may be difficult to get the specialized help needed for maintenance and repair of reactor auxiliaries.

As Charles F Kettering said, "Parts left out cost nothing and never need repair."


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PostPosted: Jan 25, 2014 5:45 pm 
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Please calculate how much mass of fuel salt you can move using passive pumping, then figure out how large the resulting fuel salt volume is. Then translate to your startup capital.
Remember in a nuclear power plant the biggest cost is the money spent before you turn the reactor on so we need to be careful about increasing the capital costs.

ORNL's DMSR was low power enough so that the graphite would last for 24 years at 100% capacity. But it had active pumping.


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PostPosted: Jan 25, 2014 7:03 pm 
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Low pressure low temperature reactors are where the LWR will be even more a king than it is now.

Although they won't let me pursue the idea of an open cycle BWR for district heating...


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PostPosted: Jan 25, 2014 8:11 pm 
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?low pressure???
LWR = light water reactor. If you don't have high pressure then the temperature can only be modestly north of 100C and that doesn't work for electricity production.


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PostPosted: Jan 25, 2014 8:18 pm 
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Lars wrote:
?low pressure???
LWR = light water reactor. If you don't have high pressure then the temperature can only be modestly north of 100C and that doesn't work for electricity production.


Yes, note how I was proposing it for district heating.

Meanwhile, by deoptimising your core you will push your capital costs sky high, removing any of the advantages you thought you had over a once through LWR cycle.
Especially with late model reactors.


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PostPosted: Jan 25, 2014 8:34 pm 
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I would expect the reactor to run for 40 years. I would also expect that pumps that pump high temperature molten radioactive salt just may have a few problems in that time frame. I would also think it may be of a higher pressure than a passive system causing wear and tear in fittings, pipe elbows, etc. The pump would also need an electrical motor. If not, it may be turbine driven. More things to maintain in a 40 year life. The motor would need switchgear and more controls. The KIS principle (Keep It Simple) seems to help with the design of many things.

If a reactor was built right couldn't it be seen as a large thermal battery that just goes and goes like the Eveready Rabbit? Sub reactors go for 20 years.


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PostPosted: Jan 25, 2014 10:14 pm 
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E Ireland wrote:
Low pressure low temperature reactors are where the LWR will be even more a king than it is now.


And indeed they are, in university research reactors. They don't generate any electricity, but you can stand at the top of the tank and look down and see the core, even while the reactor is operating at full power.


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PostPosted: Jan 25, 2014 10:37 pm 
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If your objective is to produce steam for desaliantion or low grade industrial use (evaporating solvents, district heating or what not) then I would think some sort of open cycle BWR would be best if you can link it to a steam accumulator so that there is no significant radioactivity in the product.

With fuel costs being something like 0.3 cents/kWht and bubble formation in even ceramic fuels suppressed by low operating temperatures then you can probably get away with a very low steam cost, lower perhaps than a full power PWR with cogeneration.
You might be able to reach $3/t of steam since your capital costs will be very very low as you probably wouldn't even need a significant pressure vessel.


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PostPosted: Jan 26, 2014 4:19 am 
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Lower power densities indeed have many advantages. Though, up to the point of full power natural circulation, then you're talking really low power densities where salt inventory starts to become unacceptable.

The problem is that salts aren't very good in natural circulation compared to water. And just look at how large the water inventories are for natural circulation water cooled reactors... the NuScale PWR for example. It has something like 80-100 m3 of reactor vessel water for 160 MWt module. With salts you'd need a multiple of that due to lower natural circulation figure of merit. I'm guessing it'd be over 1-2 m3 fuel salt/MWt for natural circulation MSR. This compares to the low power density ORNL DMSR of 0.05 m3 fuel salt/MWt, and ORNL MSBR (medium power density) of below 0.03 m3/MWt.

That's a pity, because avoiding highly radioactive high powered high temperature fuel salt pumps would be a major advantage in my mind.


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