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Old 10-21-22, 05:30 PM   #166
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DUAL FLUID - IS THIS THE SOLUTION?

published 07.08.2022 https://think-again.org/dual-fluid-ist-das-die-losung/

In the current discussions about the further use of the not yet destroyed nuclear power plants in Germany the "Dual Fluid Reactor" appears again and again. This machine seems to have many advantages over the models currently in use. But will it ever be available, and if so, when?

Not without alternatives

There are currently about 440 nuclear power plants in operation around the world, with 55 more under construction. Most of them are pressurized or boiling water reactors ("Light Water Reactor" = LWR), as already described here on the "Axis of Good". They are safe and reliable, but not without alternatives. Today, reactors of a new generation are under discussion: the SMR = "Small Modular Reactor", the MSR = "Molten Salt Reactor" and the DFR = "Dual Fluid Reactor". We want to take a closer look at the latter.

This machine is supposed to burn a large part of the fuel supplied, quite unlike the LWR, which uses only a small percentage and makes long-lived radioactive waste out of the rest. And now this miracle machine is also expected to be able to convert precisely this radioactive "waste" into energy, a nuclear waste incinerator, so to speak.

Perhaps. But the DFR is not completely free of radioactive waste either. Its operation naturally produces radioactive fission products, but with comparatively shorter half-lives, such as iodine 131 (8 days), cesium 137 (30 years) or strontium 90 (30 years).

Fission and chain reaction

Nuclear energy is based on the effect that the atomic nuclei of some heavier elements split into two lighter nuclei when bombarded with neutrons. (Neutrons, which are one type of building blocks that make up atomic nuclei. The other type is protons, which, unlike neutrons, have an electric charge). During said fission, a lot of energy is produced and a few free neutrons are also produced. They have no more place in the nuclei created during the fission, because heavy nuclei have a higher proportion of neutrons to protons compared to light ones.

The free neutrons can now be used to split more nuclei, and so we get a process in which the nuclei of the initial substance are broken into fragments in a chain reaction. In the widely used light water reactors (LWR), the initial substance is the uranium isotope 235, whose nucleus has 92 protons and 143 neutrons.

For the fission to work, however, the neutrons must not be too fast; they must first be slowed down, "moderated", otherwise they would not trigger fission. To do this, they are allowed to run through water from their point of origin in the fuel rod, where they lose their speed until they then encounter a new U235 nucleus in another fuel rod, which they fission.

Not optimal

It is a peculiarity of the U235 nucleus that it can be split only by slow, "thermal" neutrons. Many other heavy nuclei prefer fast neutrons for fission. So in such a reactor, one would not need to slow down the neutrons at all.

And another thing, the low concentration of U235 in natural uranium, which is enriched to 4% in LWR fuel, means that there is 96% of the useless heavy uranium isotope U238 in the fuel rods (which also has 92 protons in the nucleus, but 146 neutrons, hence the name "isotope"). These nuclei are also irradiated with the thermal neutrons, but instead of splitting, they capture the neutron and "transmute" into other substances, some of which are radioactive and have terribly long half-lives. They are the villains of nuclear energy, for which a final repository has been sought for years in deep salt domes so that they cannot endanger anyone with their radiation.

Today's reactors, the LWRs, are therefore anything but optimal. But why do they still dominate the scene? There are historical reasons. It could have to do with the fact that during the Cold War, people were interested in a substance that forms during the aforementioned transmutation of U238: Plutonium, the stuff bombs are made of.

The fast brother

So it happens that a lot of good experience has been gained with the LWR, knowing that it is suboptimal, but also knowing that the way to an improved reactor ready for series production is very long and very expensive.

Even earlier, reactors were built that used nuclear fuels other than U235, and that used fast neutrons. It was discovered that they were not only useful for generating energy, but that some of the abundant neutrons could also be used specifically for the transmutation of certain substances. It was thus possible to hatch a desired substance by irradiating it with fast neutrons. This type of reactor was therefore appropriately named "fast breeder reactor".

However, fast reactors do not play a role in the world's energy supply today. To change that, a group of fearless German engineers and scientists set out to do just that, among them, as an advisor, the popular author of the "Axis of Good" Manfred Haferburg.

Initially based in Berlin, now in Canada, the group is working on a concept that could one day solve all energy problems.

The Dual Fluid Reactor

So what might such a fast reactor look like? You arrange a sufficient amount of fissionable material so that a chain reaction takes place. The resulting heat is somehow transported to a boiler where steam is generated, which then drives a turbine and generator.

The higher the temperature used, say around 1000°C, the better the efficiency. Unlike in the LWR, water is no longer an option for removing the heat; it would be a nuisance anyway because it would slow down our fast neutrons. So we are looking for a liquid that does not evaporate at 1000 degrees, and that leaves our neutrons alone. Do you have a suggestion? How about liquid lead?

Now let's move on to our fissionable material. These are atomic nuclei heavier than "actinium", so-called actinides; among them also the often mentioned thorium, gladly material from spent fuel elements of LWRs. If you take the right chemical compound here, the stuff also melts at 1000 degrees. So it wouldn't be packed in solid form in fuel rods, as in the LWR, but you could put it in communicating tubes immersed in said bath of liquid lead. This would also have the advantage that new fuel could be fed into these tubes during operation.

So that's the principle of our reactor, which works with two fluids - lead and actinides - moving in separate circuits. Hence the name Dual Fluid Reactor = "DFR."

What are we waiting for?

And there's something else attractive about this design: in conventional pressurized water reactors, there is 150 atmospheres of overpressure in the reactor vessel, but in the DFR there is hardly any overpressure. So we don't need steel vessels with 20-cm walls, which greatly simplifies the construction of such a plant. In addition, the reactor vessel is much smaller because you don't need water as a moderator and because the lead transports the heat better.

So what are we waiting for?

Well, even though the physical issues with the DFR may be solved, there are still a few technical details to work out. For example, where do we get the pump that transports the many tons of 1000-degree lead between the reactor vessel and the heat exchanger at top speed? It's not available at the hardware store, and the turbine for NS1 that the chancellor visited is not available.

Or what about the material for the communicating tubes in which the fuel flows? They're suspended in hot lead and bombarded at close range with a barrage of neutrons. That must be hell. What material can withstand that for years?

Patience

And one more little mental calculation. If such a plant is to supply 300 megawatts of electricity, it will require about 1000 MW of thermal power. This is generated in a reactor vessel of - let's say - 10 cubic meters in volume. That is 100 MW per cubic meter or 100 kilowatts per liter of volume. But hello - nothing can go wrong with cooling....

Dual Fluid Energy Inc. in Vancouver, which is working on the development of the DFR, is of course aware of all these challenges and is therefore cautious in its forecasts: the reactor should be ready for operation in 2034. So Ms. Katrin Göring-Eckardt, who has demonstrated her willingness to talk about nuclear, will have to be patient.

But if it works, it will be nothing other than the beginning of a new era in energy.

(two explanations of terms:


Is energy obtained from the atomic nucleus now nuclear energy or atomic energy? More appropriate would be nuclear energy / nuclear energy. But since for many journalists atoms and nuclei and all that are somehow the same thing, the two terms are used synonymously. And even the organization that takes care of nuclear energy worldwide is called the International Atomic Energy Agency.

The abbreviation DFR for Dual Fluid Reactor could be misleading, because there has been the "Dounreay Fast Reactor" on the northeast corner of Scotland for quite some time. I was once in this plant and had a quite relaxed conversation with an engineer until I asked him where the noise in this thick pipe above my head came from. Oh he said, nothing special, that's a few hundred tons of 500 degree liquid sodium flowing through there).
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The author is Hans Hofmann-Reinecke. He studied physics in Munich and earned his doctorate in nuclear physics.
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