Nuclear technology |
proportion of the mined uranium (around 1%) into useful energy. The rest of it, mostly mixed with fission products, must be disposed of as nuclear waste.
Power generation technology employing today’s light water reactors is, therefore, not a high-yield system. High investment costs and regulatory requirements completely offset the energy advantage over fossil-fired power plants. On the whole, the potential of nuclear fission remains mostly unused, Dual Fluid argues.
A new generation of reactors (“Generation IV”) may achieve evolutionary but not fundamental increases in efficiency. This is because either the concept of fuel rods is maintained, or the concepts build on older liquid-salt reactor designs. In the latter, the same liquid both carries the fuel and provides heat removal, leading to suboptimal results for both functions. (Moltex Energy’s design is an exception to this. It opts for liquid fuel contained in solid fuel rods.)
The Dual Fluid reactor design — which its developers describe as “Generation V” – with concentrated liquid fuel and lead cooling, combined with the recycling of the fuel, reduces the energy expenditures related to the fuel to a fraction (blue areas, Figure 3). Further efficiency gains result, as already noted, from the relative compactness of the system, due to the high power density (green areas, Figure 3).
Overall, the energy expenditure for a DF300 power plant drops to a tenth of that for a typical LWR, and this inevitably lowers costs.
The energy return increases, depending on the size of the reactor, to 800-1000 for the DF300 and around 2000 for the DF1500. Figure 4 compares the energy return on investment for Dual Fluid with that for other power sources.
Material issues
The material separating the two fluids of the Dual Fluid reactor must have sufficient thermal conductivity and corrosion resistance, both for lead and for the fuel, which is a molten liquid metal. Compared to the conditions in thermal reactors, there is a wide choice of materials for the structural wall mainly because of the low neutron capture cross sections for fast neutrons. Materials that are suitable in principle have in fact existed for decades, but they contain rarer and more expensive chemical elements. This may be a problem for conventional reactor technology and for modern molten salt concepts since they require large quantities of structural materials due to relatively low power density.
Right: Figure 6. Possible timeline to serial production. *Technology readiness level **Forecast
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22 | April 2022|
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This does not apply to Dual Fluid and the entire spectrum of modern industrial materials can be used, the company says, noting that even noble metals can be used as components of the alloys with little impact on the overall cost of the system.
Examples of such materials are the alloys of refractory metals or highly corrosion resistant ceramics such as silicon, titanium or zirconium carbide, which have been increasingly used in industry in recent decades for applications under extreme conditions.
In addition, coatings with substances such as yttrium oxide, which is resistant to pure uranium up to 1500°C, are also possible. Since the temperatures in the reactor core are significantly lower than this, and moreover the fuel does not consist of pure uranium but of a uranium-chromium mixture, the identification of the most suitable material represents a “solvable development task”, says Dual Fluid.
Safety features
The most important safety feature of Dual Fluid is the reactor´s instantaneous self-regulation, made possible by the very negative temperature coefficient. If the temperature increases, the nuclear fuel expands. As a result, the reactivity immediately subsides and the temperature drops.
The reactor is thus completely self- regulating; a power excursion such as that which occurred at Chernobyl is ruled out. If the system nevertheless heats up beyond the normal operating temperature – “conceivable only due to incorrect fuel composition” – melting fuse plugs provide additional protection.
The fuse plug (Figure 5) is an actively cooled section of the fuel line near the lowest point. The fuel is actively cooled there from the outside, so that it freezes locally and closes the downstream outlet. If the fuel overheats, the frozen fuel plug melts and the liquid drains downward (due to gravity) into subcritical tanks. The chain reaction stops immediately and the decay heat is removed
Above: Figure 5. Sketch of the fuse plug
purely passively (ruling out accidents where decay heat is not removed (eg, Fukushima)). This simple control system concept cannot be compromised and has already proven its worth in the US molten salt reactor experiment of the sixties, Dual Fluid observes. For effective protection against violent impact and earthquakes, the nuclear part of the plant would be located underground in a thick-walled bunker. Even in the worst possible accident scenario – a leak associated with the fuel cycle – no radioactive material would escape to the outside, since there is no significant pressure and nothing could explode.
Route to serial production Following around ten years of groundwork, done principally at the Institute for Solid- State Nuclear Physics, Berlin, the Dual Fluid technology is estimated to be at TRL3. The next step is component testing. See Figure 6. Academic partners are pursuing analyses of the stability of the system as a basis for licensing.
The seed funding round was successfully completed in June 2021.
If everything goes to plan, the hope is to produce a prototype within a decade and start series production soon after.
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