Energy From Thorium Discussion Forum

It is currently Dec 09, 2018 9:27 pm

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 55 posts ]  Go to page 1, 2, 3, 4  Next
Author Message
PostPosted: Jun 12, 2010 9:52 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
I didn't notice any thread on this yet so here's a start.

While molten salt reactors can operate at very high temperatures there are many materials advantages to going to much lower temperatures. This seems important for the first developments.

One candidate salt that might be suitable is LiF-NaF-BeF2 (31-31-38) with a melting point of just under 320 degrees C. So this might allow operation from 600 C hot to 500 C cold with a very conservative safety margin to freezing. The solubility for uranium and/or transuranics might be a little low though the advantage is we can use no chromium alloys such as nearly pure nickel, and high quality non-stainless steels should do well also. Even with the traditional hastelloy N there will be much less corrosion at these lower temperatures.

The secondary coolant could just be solar salt, which is a eutectic of alkali nitrate salts, NaNO3 and KNO3 (roughly around 50-50 compositions). This is very cheap, nontoxic, industrially available and good at oxidising hydrogen (ie tritium control). Better than the fluoroborates crap with their higher vapor pressure and instability in water, the nitrates aren't bothered by water at all (in fact they are often shipped in solution!) It is suggested for operational ranges 265 C (cold) to 565 C (hot) which is ideal for this temperature range. Solar thermal technology has proven this salt's value; it is also an ideal salt for direct thermal storage in large tanks to provide peaking power & an additional safety cooling system in case of emergencies (additional decay heat cooling mode).

A large modern Rankine or Kalina cycle does at least 45 percent net peak efficiency at this temperature (>500 C).

Anyway this is just a little start, David LeBlanc has looked into the issue so maybe he can comment some more...


Top
 Profile  
 
PostPosted: Jun 12, 2010 11:37 am 
Offline

Joined: Dec 01, 2006 7:23 pm
Posts: 143
Anything that lowers risk is a good thing for the first reactor. One problem is that the delayed neutrons will activate the sodium in the solar salt to Na-24, so that part has to be shielded.

_________________
Paul Studier


Top
 Profile  
 
PostPosted: Jun 12, 2010 11:43 am 
Offline

Joined: Feb 17, 2010 10:17 am
Posts: 36
Location: Boston, MA
Also lower temperature LFTRs could used as a drop in replacement for coal boilers in existing coal plants.


Top
 Profile  
 
PostPosted: Jun 12, 2010 1:08 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
pstudier wrote:
One problem is that the delayed neutrons will activate the sodium in the solar salt to Na-24, so that part has to be shielded.


I'm not really worried about that. Sodium 24 has a 15 hour half life so it's gone in a week. It's radiation is dwarfed by the radioactivity of the fission products anyway so there's no extra shielding requirement. When sodium 24 decays it becomes magnesium which will form the highly stable magnesium fluoride in the melt. No problem there, since magnesium is reasonably transparent to neutrons and we have a huge margin to freezing in this temp setup. Just let it build up, if it really gets too bad after many years it can be removed or the carrier salt swapped.


Top
 Profile  
 
PostPosted: Jun 12, 2010 1:42 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
Oh, you're talking about the solar salt: that already has significant natural radioactivity, due to radioactive potassium. Someone else calculated Andasol's solar thermal salt storage tank contained 4 curies of potassium-40!!! The solar folks do not appear to be bothered about running slightly radioactive salt through pipes and heat exchangers and... hey... they're not that far off from an MSR :lol:

Delayed neutrons are a tiny flux anyway, can't imagine much would end up transmuting the Na in the solar salt (which has rather small cross section) and even that will be diluted by the huge volume of solar salt, and then decay away in a week...


Top
 Profile  
 
PostPosted: Jun 12, 2010 1:54 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3060
It would be a bit comical if the solar salt exposed the workers at a nuclear power plant (with their radiation monitor badges) to sufficient radiation that it was deemed inappropriate while it is acceptable at a solar plant (where presumably no one wears a radiation badge).


Top
 Profile  
 
PostPosted: Jun 12, 2010 2:47 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
Nah, potassium-40 has a billion year half life and doesn't have any penetrating gamma rays (just beta decay). Even though it's more than 0.01 percent of all potassium, and solar salt is 50-66% potassium nitrate - so there are many tons of the stuff in Andasol - you probably couldn't detect it even when standing right next to the main storage tanks or heat exchangers... Though I wouldn't mind finding out, I could use a vacation to Spain 8)

Potassium nitrates are widely used in food additives, fertilizers, and industrial chemistry as well as heat transfer loops. There are absurd amounts of it in seawater too (as potassium chloride). Loads of uranium in there too. Maybe the NRC should put nuclear trefoil symbols on our beaches, next to the oil barrel symbol... oh... :|


Top
 Profile  
 
PostPosted: Jun 12, 2010 7:48 pm 
Offline

Joined: Dec 01, 2006 7:23 pm
Posts: 143
Cyril R wrote:
Delayed neutrons are a tiny flux anyway, can't imagine much would end up transmuting the Na in the solar salt (which has rather small cross section) and even that will be diluted by the huge volume of solar salt, and then decay away in a week...


It's not a deal killer, but Na-24 has a hard gamma. It probably means that the steam generator or the salt to gas heat exchanger probably has to be shielded, and waiting a few days after shutdown before servicing it.

_________________
Paul Studier


Top
 Profile  
 
PostPosted: Jun 13, 2010 3:30 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
The heat exchanger, maybe. The steam generator probably not. Though it should be noted BWR have a somwhat similar issue since they use the primary coolant as working fluid and seem to do fine (they do have higher O&M costs for the power block I've heard, it must be a bit more tricky).

It would depend on dose rate, until I see some numbers I'm not very scared yet. The volume of solar salt in my plan would be rather huge, since it is also used for thermal storage (peaking) so radioactivity of any kind would be greatly diluted (including tritium oxide).


Top
 Profile  
 
PostPosted: Jun 13, 2010 5:20 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
I just checked the documents again and the MSBR suggested sodium fluoroborate coolant for tritium control. That has a lot of sodium in it but sodium activation being a problem was never mentioned.

The higher performance industrial FLiNaK is also often suggested for the secondary loop (with good reason, it's a great coolant) and theres loads of sodium in there too. Never heard about sodium activation there either.

I suspect the sodium activation in the secondary loop will be very very tiny. IIUC the delayed neutrons in the primary heat exchanger wouldn't be moderated much, they would be very fast (even with fluorine going at it it will certainly be over 100 keV neutron energy) and sodium has very low absorption cross section in fast spectrum.

Pretty much all of the solar salt will actually be in the storage tank, where the large bulk absorbs virtually all gammas except edge gammas. And the tank has to be quite thick carbon steel and stainless steel to hold that much solar salt.

But I would certainly be interested in the dose/year numbers, also for potassium-40 and tritium oxide.


Top
 Profile  
 
PostPosted: Jun 13, 2010 5:24 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
By the way, I couldn't accurately determine the lowest eutectic for FLiNaBe. Some references suggest it is equal parts (33:33:34) causing just 240 degrees C melting point, that figure is in the ORNL big document on salt eutectic points, but it is not listed in the table (instead it's 320 degrees C or so).

David LeBlanc was also confused by this. Anyone know what's going on with FLiNaBe?


Top
 Profile  
 
PostPosted: Jun 13, 2010 11:11 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3060
The delayed neutrons are emitted at a much lower speed than fission neutrons so they start life more moderated (200-500keV versus the fission neutrons in the 0.5-5 MeV range). But I agree with your conclusion that Na activation due to delayed neutrons in the HX will likely be very small.


Top
 Profile  
 
PostPosted: Jun 14, 2010 2:35 pm 
Offline

Joined: Mar 07, 2007 11:02 am
Posts: 909
Location: Ottawa
Very good points Cyril. Yes I've also been a fan of looking into lower operating temperatures for those same materials considerations. I also agree there are some strange unknowns about the melting points in the Li-Na-Be eutectic.

A quick point that the solar salt nitrates are out of the question for a secondary salt if you have any graphite in your core (even composites for a barrier, not just moderator). The reason is they are such strong oxidizers that any leak of salt into the primary salt it would literally burn up your graphite. ORNL really thought highly of possibly using nitrate salts but only as a third salt loop. Their idea was flibe for fuel salt, a clean flibe as secondary and then nitrate salt as the third loop. This added the cost of a third loop but had great advantages all around from easily catching tritium, to making the connection to steam better and making leaks of secondary flibe into primary flibe trivial. In that respect, if we did have carbon in the core, you could go FliNaBe to clean FliNaBe and then to Nitrates. Also I've considered NaF-BeF2 which also has a pretty low melting point (and cheaper since no enriched lithium)
Attachment:
ORNL-TM-3428ThirdLoop.pdf [1.3 MiB]
Downloaded 200 times


Getting back to lower operating temperature, I think it is an important route to examine closer. It goes against everything holy to your typical engineer though as there is such a powerful allure to going to higher temperatures and higher thermal efficiencies. That allure to high temp was also with the gas cooled folks and the U.S. plan to go to 1000 C was abandoned just recently when they looked into things closely. The French with the TMSR (now MSFR) are calling for unproven temperatures of 800 C now and for the molten salt cooled folks, I also see too strong an urge to raise up the temperature (quickly to 800 and soon after to 1000). Materials issues can become horrendous at higher temperatures so making your R&D life easier with going a little bit the other direction just makes sense to me.

David LeBlanc


Top
 Profile  
 
PostPosted: Jun 14, 2010 3:38 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
Thanks for your response David, I was looking forward to it.

Good that you mention the carbon oxidation risk, it brings up bad images of gunpowder to my mind (even without the sulphur it's bad!)

Dirty FLiNaBe to clean FLiNaBe to NaNO3 KNO3 sounds promising. The NaNO3 KNO3 melts around 220 degrees C, which is nicely close to the FLiNaBe - if the FLiNaBe really melts at 240 degrees C that is. It will mean we could go as low as 340 degrees C on the cold end of the fuel salt! Even with the more conservative 320 degrees C melting point that's an impressive 420 C on the cold end!

The cost of the 7LiF isn't that bad, it seems that the main advantage is order of magnitude reduction in tritium production. The FLiNaBe uses only half the 7LiF which might almost half tritium production which is nothing to laugh at.

Reading your document right now. The cost of the third loop looks very reasonable, as expected, and engineering is straightforward. I'd like to make one modest improvement: no sodium nitrite in the third loop, it is a bit corrosive and doesn't give cost advantages. Molten salt cooled solar power towers had fairly negative experience with it, though it may not be a problem for a quality alloy like Hastelloy with it's great oxidation resistance, especially at lower temperatures. Still there is much more favorable experience with the simple binary NaNO3 KNO3 in solar thermal powerplants. It is cheap and good, and industrially available in huge quantities. Some of the nitrate will be reduced to nitrite since that is how we trap the tritium but this is so tiny (kilograms at equilibrium) that it is a marginal corrosion issue.

One cool thing (actually a hot thing) is that the solar folks will develop large scale NaNO3 KNO3 thermal storage which can be used for peaking & extra active mode of decay heat cooling for easy shutdown and emergencies.

The NaF will hurt the conversion ratio a little bit but as you've suggested it's not as bad as it looks at first. Maximising conversion ratio isn't strictly necessary for a first LFTR or DMSR development anyway, so that's ok.

The DMSR will likely use graphite and graphite lasts longer when it's 500 C peak rather than 700 C peak. This raises interesting options such as running on uranium only and increasing the power density for the same graphite life etc.

Even with higher temperature designs, FLiNaBe might have the advantage of higher temperature drop across the core. Power block efficiency will suffer a bit but in many designs we are quite limited in flow rates even with a modern compact heat exchanger so a higher temp drop will mean more heat transfered for a given flow rate (and hx size). Many people are talking about 50-100 C temp drop but the MSBR planned 140 C. But it seems that with an alternate salt they wanted to go higher because 460 C (FLiBe MP) plus 100 C margin = 560 C which means 140 C to 700 C. Would it be realistic to have a temp drop as large as 200 C? Say 340 - 540 C assuming the FLiNaBe melts at 240 C?

Though I think Kirk really likes his FLiBe, he seems emotionally attached to it or something (Just kidding Kirk! :mrgreen: )


Top
 Profile  
 
PostPosted: Jun 15, 2010 1:56 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1055
It is true that the lower the MSR operating temperature, the more cost effective it is to construct. But the hotter we can keep the molten salt, the less trouble we will have with core melt coagulation.

The most problematic element is plutonium. When plutonium begins to solidify and clump together, we start to run into problems. Lumps of plutonium can burn through reactor walls and clog up and destroy heat exchangers. There can be over 100 different chemical elements dissolved in the core salt and all have the ability to turn solid if their concentrations get high enough. The hotter the core salt, the less likely for core salt coagulation, and the more nuclear wastes we can keep in suspension.

Burning U238 adds greatly to the plutonium and nuclear waste solubility consideration.

When you give something up to get an advantage in another area, this is called a tradeoff. But tradeoffs are subjective.

In terms of tradeoffs on the subject of this thread, there is a balance to be struck between structural material costs and problems with core salt freeze out.

For example, in the PB-AHTR, “clean” fluoride based core salt allows for the use of steel reactor walls coated with nickel alloy. The TRISO fuel format protects structural material from uranium corrosion and other nasty things that dirty core salt can do.

I have considered giving up the use of fluoride to get and advantage in structural material costs, total overall costs, ease of fuel reprocessing, high operating temperature, long reactor life, true thermal neutron spectrum operation, no graphite problems and loads of other goodies. But that means using a slurry fuel format instead of fluoride salt. By doing this tradeoff, you give up a lot, but you also get a lot of advantage.

In a baseball analogy, it’s like trading Babe Ruth for 25 good minor league players. In the beginning, it might break your heart, but in the long run, it might be best for the team.

_________________
The old Zenith slogan: The quality goes in before the name goes on.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 55 posts ]  Go to page 1, 2, 3, 4  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group