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PostPosted: Feb 05, 2016 1:10 am 
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I've been looking at the Moltex website and watching their videos and trying to understand their design. I'm not expert at nuclear reactors but I do try to get my head around the engineering. They put a positive spin on their design but my natural inclination is to seek out any obvious weaknesses in the design. The only one I'm aware of is the cost of testing Plutonium based salts as suggested by Kirk Sorensen in another discussion about TerraPower. Is anybody able to point out any other development issues or fundamental design problems?

http://www.moltexenergy.com


Last edited by TerjeP on Feb 05, 2016 1:20 am, edited 1 time in total.

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PostPosted: Feb 05, 2016 1:12 am 
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Their air cooling system seems to put the atmosphere very close to the core. Unless it's some sort of enclosed air loop. But perhaps I've misunderstood the design.


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PostPosted: Feb 05, 2016 9:00 am 
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TerjeP wrote:
I've been looking at the Moltex website and watching their videos and trying to understand their design. I'm not expert at nuclear reactors but I do try to get my head around the engineering. They put a positive spin on their design but my natural inclination is to seek out any obvious weaknesses in the design. The only one I'm aware of is the cost of testing Plutonium based salts as suggested by Kirk Sorensen in another discussion about TerraPower. Is anybody able to point out any other development issues or fundamental design problems?

http://www.moltexenergy.com


I'm fairly new to the technology but was at a Moltex presentation and have looked at the design. So, others may disagree, but:

Firstly, they a Fast Spectrum design, which makes it quite different for comparison purposes to most other MSR concepts. They've done this partly as it can then burn up the UK's stockpile of Plutonium, but agree that it may not be appropriate for export around the world, as it's less proliferation resistant than thermal designs.

A Fast design is in some ways easier, as you don't have to worry about a graphite moderator and the life time and disposal costs of this moderator. On the other hand, to get the required flux, there is a minimum size needed, which I think is about 500MW or so, making it hard to prototype. Then again, the design looks quite easy to scale - costs rising slower than volume, which could make for lower cost 2500MW reactors.

I believe control is harder in a Fast Reactor - see this recent post;
viewtopic.php?f=5&t=4631

That means that there is less margin for errors causing the reactor to go super-critical. The regulators will therefore look very closely at whether an external event can change the geometry (push the fuel tubes closer together) and cause the reactor to go super-critical.

The design is nice and simple, but they are running water through heat exchangers in the secondary salt. What would the consequences be of a HX failure, with water mixing with the salt, and turning to steam. Could that risk super-criticality?

The core salt runs very hot - over 1000C. That leaves less margin before it boils. On the other hand, with some material tweaks, they could get the tertiary coolant up to 850C. Currently that coolant is water/steam, which only needs to be 650C for the turbines. But it could be Sulfuric Acid and hence a path way to Hydrogen synthesis (Sulfur-Iodine thermochemical cycle). I would think that the other designs (Thorcon, Terrestrial etc) can't get so hot so easily. (Though a 850C Sulfuric acid / salt heat exchanger is a subject for another thread).

The design and cost estimates have been validated by Atkins. Whilst not a guarantee that it can built for the price, it's rather a step ahead of the other designs in this respect. However, that cost was about $2,000 / KW, so this "estimate" is a bit higher than the "guestimates" of some other companies.

Finally they are based in a jurisdiction that appreciates and wants molten salt (or at least, non PWR) reactors, even if it will make them go through a gold plated accreditation process to get there. I would also guess that they are behind Hualong, Prism, and NuScale in the queue for assessment.


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PostPosted: Feb 05, 2016 11:44 am 
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Well A-USC gear is pushing 700/730C or similar in development now.

So we can certainly use higher steam temperatures than 650C.


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PostPosted: Feb 06, 2016 5:21 am 
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alexterrell wrote:
Firstly, they a Fast Spectrum design, which makes it quite different for comparison purposes to most other MSR concepts. They've done this partly as it can then burn up the UK's stockpile of Plutonium, but agree that it may not be appropriate for export around the world, as it's less proliferation resistant than thermal designs.

A Fast design is in some ways easier, as you don't have to worry about a graphite moderator and the life time and disposal costs of this moderator. On the other hand, to get the required flux, there is a minimum size needed, which I think is about 500MW or so, making it hard to prototype. Then again, the design looks quite easy to scale - costs rising slower than volume, which could make for lower cost 2500MW reactors.


Correct me if I am wrong, but Moltex has changed its offering in the last couple of months, and offers two different designs. They propose a fast spectrum design as well as a thermal spectrum design. See:

http://www.moltexenergy.com/stablesaltreactors/


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PostPosted: Feb 06, 2016 6:51 am 
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camiel wrote:
alexterrell wrote:
Firstly, they a Fast Spectrum design, which makes it quite different for comparison purposes to most other MSR concepts. They've done this partly as it can then burn up the UK's stockpile of Plutonium, but agree that it may not be appropriate for export around the world, as it's less proliferation resistant than thermal designs.

A Fast design is in some ways easier, as you don't have to worry about a graphite moderator and the life time and disposal costs of this moderator. On the other hand, to get the required flux, there is a minimum size needed, which I think is about 500MW or so, making it hard to prototype. Then again, the design looks quite easy to scale - costs rising slower than volume, which could make for lower cost 2500MW reactors.


Correct me if I am wrong, but Moltex has changed its offering in the last couple of months, and offers two different designs. They propose a fast spectrum design as well as a thermal spectrum design. See:

http://www.moltexenergy.com/stablesaltreactors/


Thanks for that. I hadn't seen that.

I think a thermal version makes the product a much more attractive proposition. As their webside implies, the market is 10 times bigger. It should also be easier to accredit.

What I can't see - and critically important - is their method for replacing the graphite moderator. It could be that as they originally designed a fast reactor, and then added the moderator, it's not optimised for this. Though this can't be too difficult - I assume the crane will pull out the moderator blocks and replace them.

The thermal design puts them up in my view of the UK "GDA entry queue", still behind two PWR designs and Prism.


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PostPosted: Feb 06, 2016 8:51 pm 
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The fact that the steam generators are situated in the molten salt pool, close to the core, can be problematic in case of steam generator tube rupture. Having an intermediate molten salt loop and placing the steam generators outside of the containment will be better for safety.

But I also understand that we need to reduce the capital cost of nuclear power plants. A lot of Lead cooled reactor designs don't have an intermediate loop and have their steam generators (with double wall tubes) in the pool of lead like the moltex design.

The designers of ASTRID chose to keep the intermediate sodium loop even with the nitrogen gas turbine which eliminates the sodium-water reaction. I guess that they didn't like the possibility of reactivity insertion due to bubbles of gas passing into the core. The designs which eliminate the intermediate sodium circuit are not pool designs but loop designs (like the Japanese Sodium Fast Reactor, but without the intermediate sodium loops, a nitrogen gas turbine instead of steam turbine and a degasser between the sodium-nitrogen heat exchanger and the core).

Some lead cooled reactors designs use a loop configuration like the BREST reactor and don't use an intermediate loop.

The problem of the intermediate loop seems a very difficult choice.

Quote:
Their air cooling system seems to put the atmosphere very close to the core. Unless it's some sort of enclosed air loop. But perhaps I've misunderstood the design.


Yes, that bothers me as well, but I think it is still very safe if they can guarantee that the vessels will not be broken.


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PostPosted: Feb 07, 2016 5:00 am 
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The Moltex design is very careful to ensure that there are no welds in the heat exchanger parts exposed to the molten salts, and they have the space to ensure a decent wall thickness, so it shouldn't cause a problem.

However, they will need to model and test what does happen if there is a problem. Ideally it should not result in loss of plant. There needs to be ~zero chance (ALARP to annoy some people here) that it can lead to radioactive release. That's quite easy for the thermal design, perhaps not for the fast design.

To avoid loss of plant, they'd want a solenoid valve to quickly shut off water flow into the heat exchanger, and good instrumentation to warn of a leak within milliseconds.


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PostPosted: Feb 07, 2016 2:33 pm 
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In his presentation http://www.moltexenergy.com/learnmore/
"Dr Ian Scott speaking at a joint meeting of the Institutes of Mechanical and Chemical Engineering in Nov 2015"
from 11:00 on Dr. Scott says that he can use inexpensive stainless steel SS316 as structural material and avoid corrosion by simply coating the steel with zirconium. He further says that this is new to molten salt reactors. Sounds too good to be true?


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PostPosted: Feb 07, 2016 3:29 pm 
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Burghard wrote:
In his presentation http://www.moltexenergy.com/learnmore/
"Dr Ian Scott speaking at a joint meeting of the Institutes of Mechanical and Chemical Engineering in Nov 2015"
from 11:00 on Dr. Scott says that he can use inexpensive stainless steel SS316 as structural material and avoid corrosion by simply coating the steel with zirconium. He further says that this is new to molten salt reactors. Sounds too good to be true?


There's been quite a lot of work on steel corrosion in molten salts and as long as the salt is maintained as electro-positive then corrosion is slow enough to allow SS316. This is some of the work Manchester University have been doing for Moltex.

SS316 also in the ThorCon presentations, though to be checked after 4 years.

That said, the Moltex design has the steel tubes, which receive a very high flux of fast neutrons.


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PostPosted: Mar 02, 2016 7:08 pm 
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What about using this reactor configuration breed 233U? The topic was barelly mentioned in the ThEC15 conference.


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PostPosted: Mar 05, 2016 11:36 am 
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Strengths:

- modularity
- replaceable core
- use common alloys

Weaknesses:

- totally unproven fuel salt (don't be fooled by the sales talk - no one has ever used this fuel, not even the class of fuel, in any reactor)
- severe high temperature embrittlement plus chlorides corrosion at super high temperatures. No alloy will last in this environment. Severe environmental qualification issues.
- venting Xe137, Kr89, etc. is not "harmless". This is a lie. Cesium and Iodine bind to salts, but the Xe and Kr parents don't, and some of these will leave the salt so won't be converted to stable halides. Xe137->Cs137. Kr89->Rb89->Sr89. These are nasty boys that make cyanide look like chocolate. This is well known. The MSRE had to have months of holdup and charcoal beds before they could release these gasses, and that was in time with much less stringent regulations. If Moltex vents constantly, they had better have a holdup system for the gasses or they will severely contaminate their entire containment cell with radiostrontium, radiocesium, and other nasties. It is entirely possible to deal with this by sound and simple engineering, but claiming that simple venting is "harmless" is crazy sales talk.
- the behavior of fission products in chloride reactors is not known well at all (makes venting even more risky).
- super hot steam generator in a tub of salt. Serious materials issues, likely to fail. SG tube leak causes reactivity effects. H2O is moderator. H2O reacts with fuel salt - chemical potential in a high rad environment (bad).
- what to do upon cladding leakage? It makes a nasty tub of salt. How is this cleaned or otherwise recoverable? Millions of tiny thin walled tubes with tiny thin walled welds can spring small leaks, this is the experience with nuclear energy.
- coolant activation is huge. They have coolant salt right in the core, and is made up of materials that activate severely, so this salt becomes nasty very quickly (not a "non radioactive coolant salt").

Truth be told I see many more weaknesses than strengths. Its an interesting design, but lacks realism which makes it, in my opinion, not credible.


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PostPosted: Mar 05, 2016 9:13 pm 
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Good analysis !
Thanks for writing it down & sharing.
My thinking exactly.


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PostPosted: Mar 05, 2016 11:00 pm 
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Quote:
coolant activation is huge


It is maybe not that bad. The workers in Phénix received on average 10 times less radiation than workers in an average PWR (and 20 times less than BWR's workers) despite the sodium's activation. I guess that they just had to wait a week to let 24Na decays before doing work on the primary circuit.

Of course if there are fission products everywhere in the primary system due to the venting of the gasses, that changes the game. We lose one of the main advantages of having all the fission products contained in the tubes.


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PostPosted: Mar 06, 2016 8:59 am 
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fab wrote:
Quote:
coolant activation is huge


It is maybe not that bad. The workers in Phénix received on average 10 times less radiation than workers in an average PWR (and 20 times less than BWR's workers) despite the sodium's activation. I guess that they just had to wait a week to let 24Na decays before doing work on the primary circuit.

Of course if there are fission products everywhere in the primary system due to the venting of the gasses, that changes the game. We lose one of the main advantages of having all the fission products contained in the tubes.


Remote maintenance explains more of that - sodium is combustible so you can't have the kind of operations in a water cooled reactor (no workers looking at the coolant pool for example).

Moltex however are not proposing sodium, but a Na/K/Zr fluoride eutectic as coolant. Loads of zirconium in there making a lot of gamma rays long after shutdown.

Also Phenix was a fast reactor. Thermal spectrum makes activation (even in just sodium coolant) a lot worse.


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