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PostPosted: May 15, 2013 8:38 am 
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On the other hand, maintaining a monolithic array of ten-thousands of rods might be challenging but not that unusual..



At least they admit the enormous amount of surface area this will require. I'm afraid I'd certainly call that an "unusual" challenge.

David L.


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PostPosted: May 16, 2013 10:02 am 
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David wrote:
Quote:
On the other hand, maintaining a monolithic array of ten-thousands of rods might be challenging but not that unusual..



At least they admit the enormous amount of surface area this will require. I'm afraid I'd certainly call that an "unusual" challenge.

David L.

Isn't that exactly what's done in stem generators?


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PostPosted: May 16, 2013 1:32 pm 
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No, a steam generator in MSR design is between virtually non radioactive clean salt (perhaps a little tritium) giving up heat and there are no true safety issues with any sort of tube failures (potential mess for sure but no criticality concerns). In the proposed design they are looking to have tens to hundreds of thousands of small tubes in an intense neutron field and be compatible with both molten salt and molten lead at the same time. In core cooling of MSR tech has been proposed many times, but as always it relies on quite extreme solutions.

David L.


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PostPosted: May 16, 2013 5:30 pm 
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High radiation field with tens of thousands of tubes isn't unusual, this is how almost all nuclear reactors are operating today, PWR, BWR, CANDU, AGR. It's the status quo.

Molten salt and lead is unusual and very difficult. For example, high nickel alloys are easily fabricated and compatible with molten salt, but lead rapidly dissolves nickel. Whereas alloys that are compatible with lead tend to be not compatible with molten salt or can't take high temperatures.

Molybdenum and its alloys are suitable. Niobium is suitable as well, with better fabrication properties at the cost of lower strength compared to molybdenum. Silicon carbide composites are perhaps more promising, with better neutronic performance and oxygen resistance.


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PostPosted: May 16, 2013 6:56 pm 
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I think they just need to drop their power density ambitions a bit: After all, the DFR concept as proposed, is made up largely of cheap materials, so why push the limits ? ...just build more units.

The other helpful change would be to switch from a monolithic fuel HX circuit, to a bunch of serviceable modules.


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PostPosted: May 22, 2013 8:23 am 
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In case anyone is still following the DFR thread, here is an interesting reply I received from the inventors, via their facebook site, https://www.facebook.com/DualFluidReaktor.
The reply was in response to my comments, posted at bottom below.....

I would be interested in your comments, particularly on the fabrication and endurance aspects (Thnx).

Quote:
You hit exactly the problem why we invented the DFR.
What you described, separating the criticality function from the (external) heat exchange function, is the classical MSR.
It is a good concept but has the problem that the molten-salt can not provide sufficient heat capacity to remove the heat fast enough.
In order to manage this problem, the fuel circulation speed needs to be very fast, and the fuel needs to be diluted very strong.
This, in turn, reduces not only the power but also makes a fuel reprocessing very difficult.
Moreover, the neutron economy is reduced by softening the spectrum.
Instead of separating criticality function from the heat exchange function, the DFR separates the fuel function from the cooling function.
The latter separation has more advantages than the first one.
The only disadvantage, as you mentioned, is that we now have to deal with tenthousands of tubes in the reactor core.
We checked this disadvantage very carefully.
In the past, there have been fabrication problems, in particular for the very high-temperature durable materials.
Today, not only the materials improved but especially the fabrication methods made remarkable progress.
For example, MHC (Mo-Hf-C) is used for matrix bar extrusion devices up to 1500 C to form steels.
Even though extremely tensile-tight and high-temperate creep-resistant, it can be worked, e.g. by electron beam welding or laser sintering.
Even better, MHC is by far not the only option.
This is another advantage of the DFR, since the neutron spectrum remains very hard (low neutron capture), the choice of materials opens widely.
Moreover, the material consumption is low (no exchange of fuel rods), so that high-qualitative expensive materials and fabrication costs in the core have only a moderate impact on the overall costs.
There is no closure in the tubes, they are connected all the time with the PPU and the fuel circulates (slowly).
This means that outgasing of volatile fission products, happening all the time in the tubes, needs no additional vent lines or other parts.
They automatically collect in the PPU where they are processed.
Also noble metal flushing, as well as fractional distillation/rectification, all happens in the PPU, outside the core.

Jaro wrote:
Thanks.

So, what if the fuel inside the fuel elements were liquid (like DFR) instead of solid (like TerraPower's TWR) ?
The problem still remains, to collect the volatile fission product emissions from individual fuel elements, to a common header and then to storage.

The DFR solution, to use a rigidly connected matrix of tens of thousands of fuel elements, is perhaps the obvious solution, but not a practical one for fabrication and endurance.

Like many other reactors, TerraPower's TWR contains about 300 fuel assemblies, each with ~170 fuel elements: Connecting all of them would require some 50,000 vent lines -- similar to the interconnected DFR core matrix.

Evidently the designers at TerraPower didn't think that was a practical idea: Individual fuel element vent lines in such large numbers are NOT feasible.

Even if the core were separated into modules equivalent to four assemblies each, that would result in 75 separate modules, each with nearly 700 fuel elements and associated vent lines !
Not feasible.

Why are so many thousands of fuel elements necessary ?
For heat transfer reasons only. Not for reactor criticality.

The advantage of liquid fuel is that we can separate the heat transfer function from the reactor criticality function, by using an external heat exchanger (HX).
When we do that, then we can have far fewer, larger fuel channels to make up the core, while the HX can take a more suitable form, and can be made of materials chosen independently of any concerns about interfering with reactor function.

A rigidly connected matrix of a few hundred fuel channels might actually be practical: It would be roughly the same number of fuel channels as the number of fuel assemblies in an ordinary reactor.
Obviously, one would not be able to get all the heat out of the fuel, while inside the reactor.
But the same lead coolant circulating through the DFR core could also be circulated through the external heat exchanger.
The ratio of external (HX) to internal (reactor) heat transfer might realistically be something like 100-to-1, depending on design details.
Very different from the current DFR concept, but much more likely to be practically feasible.


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PostPosted: May 22, 2013 9:43 am 
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Thanks for the effort Jaro.

Much of the response is sensible, but some strange stuff in it though:

Quote:
It is a good concept but has the problem that the molten-salt can not provide sufficient heat capacity to remove the heat fast enough.
In order to manage this problem, the fuel circulation speed needs to be very fast, and the fuel needs to be diluted very strong.


Molten salt has excellent heat capacity. FLiBe has 2.7x the volumetric heat capacity of molten lead. The only advantages with lead would be that you can have lots of it without excessive fissile inventory, so a less constrained heat exchange design is possible.

To reduce flow rates, increase delta T through the core. This is much easier to do without fuel rods in the core, as you then would have to deal with both high temperature differentials and high neutron flux in fuel rods at the same time. I've read PWR work where uprates were considered by increasing the delta T across the core, but they quickly ran into primary and secondary (Sm) thermal stress limits. This is one of the biggest problems that developers of supercritical water cooled reactors face. If you have just graphite then you're out of trouble, as refractory graphite floating about in salt (almost no stress on it) isn't bothered by high thermal differentials.

One of the issues with lead coolant is it's great weight. Perhaps, with heavy solid fuel, this can be used as an advantage, if the fuel has similar specific gravity as the lead coolant. If the net weight of the fuel is low, its deadweight stresses are low and it doesn't need such rigid connections that cause high secondary thermal stresses. It would just have to be kept in place by some sort of structure, preferably a silicon carbide composite structure, perhaps with hold-down springs or such to accomodate thermal and irradiation growth.

Hot thorium metal has about the same density as molten lead.

I wouldn't use MHC as cladding. The central station AHTR work wants to use this material as control rods!!! The neutron economy will be terrible. I would go for either TZM or triplex SiC.


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 Post subject: Dual Fluid Reactor
PostPosted: Mar 08, 2015 9:21 pm 
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Good evening gentlemen!

I would like to open up a thread to solicit comment on the German "Dual-Fluid Reactor" concept.

In summary, a chloride salt fueled, lead cooled reactor.

It would have one of the hardest neutron spectra possible (apart from liquid metal fueled reactors).

Supposedly martensitic steels have manageable corrosion rates in the presence of fast flowing liquid lead.

gigantic breeding ratios are possible (1.4-1.6 claimed), allowing use of neutron excess for material transmutation or further fuel production.

Key point of note is that U238 makes quite a few neutrons at high energies, and commands an excellent delayed neutron fraction up to around 2.4 MeV.

Liquid metals do have far superior Prandtl numbers relative to molten salts, but lower thermal capacities imply MUCH higher pumping rates...

Attached is a design concept document.

-------------------------------------------------

Questions:
Is this design reasonable? Materials compatibility issues seem to be #1 concern. Sacrificial lead plates are used to regulate the oxidation capacity of the coolant loop.

Is is possible to

As a general rule, the fuel should not be the coolant.... The fact that a fuel carrier must exist in signifcant volume outside of the core proper DOES imply much larger fuel inventories and reduced HX lifetimes. Do you agree this is a good design principle?

Why lead as a coolant though? Sodium has a higher thermal conductivity and is less viscous. I suppose the sodium fast reactors go that route due to the large "pool" of sodium that makes up for the relatively lower thermal capacity? I suppose the Russian Alfa submarines proved the operation of Lead cooled solid fuel reactors sufficiently...

Please any comments you have on this design!!! Thank you for your time


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DualFluidReactor.pdf [505.12 KiB]
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 Post subject: Re: Dual Fluid Reactor
PostPosted: Mar 08, 2015 10:23 pm 
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I imagine the sodium is too moderating - you have to stay away from light elements in this design.


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 Post subject: Re: Dual Fluid Reactor
PostPosted: Mar 08, 2015 10:26 pm 
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Heres the paper from behind the paywall. It actually did cover the moderating nature of the lighter metals!

Why only lead... Lead-Bismuth eutectic has also been considered for both coolant and fuel carrier in the early days of reactor designs. I know Bi loves to eat any Ni alloys for lunch, are there significant solubility differences for molybdenum in Pb vs. Bi?


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DFR_Paper-formatted.pdf [1.65 MiB]
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 Post subject: Re: Dual Fluid Reactor
PostPosted: Mar 08, 2015 11:34 pm 
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Merged with a previous topic.


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 Post subject: Re: Dual Fluid Reactor
PostPosted: Mar 09, 2015 10:50 am 
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So what is the overall opinion of this thing?

If they can make it work it is obviously a very capable reactor. Breeding ratio of ~1.6 would shorten the doubling time to a sufficient degree to render plutonium rollout issues moot.


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 Post subject: Re: Dual Fluid Reactor
PostPosted: Mar 09, 2015 1:51 pm 
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Quote:
Sodium has a higher thermal conductivity and is less viscous


They want to heat the coolant to 1000 °C , Sodium boils at 880 °C.
Sodium has also the problem of its chemical reactivity with air and water, that scares people and also requires an intermediate loop.
I also guess that with this geometry, the sodium will give a positive void coefficient due to the higher moderation.


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 Post subject: Re: Dual Fluid Reactor
PostPosted: May 25, 2015 2:53 pm 
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The DFR as shown in the documents is a parody in terms of engineering. It is on my Point of view a Star Trek design. It is fascinating that there are a couple of threads on it.

To have a 1400°C!!* hot liquid Chloride salt in small thin walled tubes (1 -3mm) in the center of the reactor, max. neutron flux, free chloride from fissions and instable fp-chlorides (depending on the pressure), is the technically worst MSR concept I have ever seen.

From the technical point of view it would be a great opportunity for mankind if there is once a magic material available that can widthstand a 1400°C chloride salt mixture with some free chlorine and on the other side a 1000°C hot lead* for >25years in Terms of corrosion resistance and mechanical properties. Usually power plants require a mechanical fatigue resistance (10000h) > 100N/mm2 and creep resistance that is at these temperatures even for molybdenum alloys challenging. It requires a great errosion resistance (thin tubes) against liquid lead. I fear it would be difficult to find such a material in our galaxy.

More realistic integral reactor concepts place the heat exchanger with its thin tubes/channels at the edges of the core or better outside the core. In this case the sensitive hx gets a lower neutron flux, there is less risk of hot spots, less free chloride.... Please be aware that the hx should have a life time of > 25years if you plan to be competitive.



*average temperature? max. temp. in the center? hot spots?


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 Post subject: Re: Dual Fluid Reactor
PostPosted: May 30, 2015 6:53 am 
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How about a DFR, the simplest possible possible, with water in tubes as moderator-coolant?
It can be highly under-moderated. There could be constant supply of neutrons by neutron initiators like Pu-Be and local moderation by water near the tubes. Boiling off of water will result in complete loss of moderation and activity. Tubes filled with liquid water will set off the fission reaction. A thermal energy multiplier. The fuel will be molten salt and never go to boiling point. Boiling of water will result in self regulation.
Reduced moderation will result in high conversion which may or may not reach breeding.
I guess it will depend on the ability of tubes to weather thermal shock like water tube boiler.


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