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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