Properties of the Reactor
The Dual Fluid Reactor (DFR) differs as much from a traditional light-water reactor as a jet plane from a Zeppelin. But what exactly are these differences? Main goal during development was the construction of an energy source suited to power production in urban areas and industrial centers:
- Inherent safety: Natural laws prohibit the ultimate MCA. The DFR cannot create Chernobyl-like scenarios, much like a bucket of water will not boil in the cold of Antarctica.
- Recycling of nuclear waste: The DFR destroys its own waste as well as that of traditional nuclear plants. The only leftover are fission products, the radioactivity of which will mostly decline in the course of 300 years. Thus, the search for a geological repository becomes unnecessary.
- Very high efficiency: Any and all fission fuels can be used up completely! Nuclear waste, depleted uranium, natural uranium, plutonium, thorium may be fissioned in a DFR. This results in an energy source lasting until the sun turns into a red giant.
- Minimum costs: The DFR is the most compact possible nuclear reactor absorbing the least amount of materials, human work hours and thus — ultimately — money.
- High operating temperature: This allows for very efficient electricity generation (60% energy conversion), as well as a large number of process heat applications — e.g. sea water desalination or production of fuels without mineral oil. Thus, nuclear energy can break into the primary energy market, including cars, aircraft, heaters, etc.
Utopia or reality? Read on to find out how the reactor core is layed out and what the physical basis behind these characteristics is.
The Dual Fluid Principle
Boiled down to its most basic idea, the Dual Fluid Principle may be sketched thus:
Two liquid loops interact: Fuel and coolant loop. The first contains molten nuclear fuel, the second liquid lead. Where encompassed by the coolant, the fuel loop is widened. Size and shape of the widening are chosen such that the fuel becomes critical: A chain reaction is created and releases energy which is transmitted to the cooling loop.
While a spherical or cylindrical widening of the loop would already suffice to start the chain reaction, the generated heat could not be removed from it effectively. Instead, the conduit is split up into 10.000 lean tubes immersed in the molten lead. The following set-up results:
Each of the two liquids does what it does best — one releases energy, the other removes it: This results in a reactor of extremely high efficiency.
The DFR is a fast neutron reactor with liquid fuel and a seperate cooling loop based on molten lead: Thus the name “Dual Fluid” — two liquids.
One of its most important characteristics is its minuscule size — the accompanying image shows a 1.5 GWel-class DFR as compared to a human. Up to several gigawatts or thermal power are released in a volume of just a few cubic meters. This allows for economical construction and secure placement in an underground bunker. In order to enable the highly compact design, the concentration of fissile and fertile isotopes in the fuel must be as high as possible: Undiluted actinide chlorides oder metallic actinide melt. Thus, there are two possible configurations — the DFR/s using salt and the DFR/m with liquid metallic fuel. The DFR/m boasts a power density even higher than that of the DFR/s.
The DFR’s operating temperature clocks in at 1000 °C. This allows for very efficient electricity production, as well as using the reactor as a process heat source for industrial applications: liquid fuel synthesis, seawater desalination, concrete production, petrochemistry and some others.
Using fast neutrons, the DFR can operate alternatively as a breeder, nuclear waste eliminator or transmuter. Each and all radioactive heavy metal, which is injected into the fuel loop, will either be fissioned directly or transmuted via neutron capture into an isotope, which is fissile. If breeding is wished for, the DFR/s must be surrounded with a breeding blanket containing fertile materials (uranium 238 or thorium 232). The DFR/m is even able to breed within the fuel liquid itself.
Rising temperatures will cause the liquid fuel to expand, cooling to contract: This enables a large negative temperature coefficient, which constantly adjusts thermal power to the extracted power — mechanical control elements such as neutron-absorbing rods are unnecessary.
But if due to some reason the reactor still exceeds its nominal temperature, melt plugs at the bottom of the fuel loop liquefy, and the fuel flows into subcritical storage tanks. Fukushima-like events are prevented by the natural laws themselves!
Why two liquids?
Liquid-fueled nuclear reactors have many advantages: Reprocessing can be done on-site as well as online, i.e., next to the reactor on the plant premises while the machine is running, keeping downtimes to a minimum. Radioactive fission products are extracted continually — decay heat accidents as in Fukushima or Three Mile Island are thus ruled out. Additional safety is provided by the melt plugs.
This was realized by nuclear technologists soon after the first power reactors had been built. Therefore, liquid-fueled reactors were tested already in the 1960s by Alvin Weinberg and his team at Oak Ridge National Laboratory. A single fluid was used as fuel and coolant. Similarly, all modern liquid-fueled reactor concepts except the DFR employ one liquid for both purposes (e.g. the MSR: Molten Salt Reactor). This double duty requires disadvantageous compromises.
No substance is equally suitable as fuel and coolant. Molten salt can absorb a lot of heat, but heat transfer is too slow. To keep the reactor from overheating, the fuel has to be artificially diluted, which in turn reduces power density, resulting in MSRs being far larger and more expensive than DFRs of equal power. Durable, heat-resistant materials cannot be used due to the amount needed — thus, a temperature of 1000 °C which is optimal for process heat applications cannot be achieved with a single fluid.
Is there a solution? Yes: It is the Dual Fluid Principle. One liquid as fuel, a second one for cooling — that way, both loops can be optimized independently. E.g., is molten salt is used as fuel and lead as coolant, dilution of the salt becomes unnecessary.
Chloride salts or metal melts can be used as high-temperature capable fuel liquids (see above). Lead is the coolant of choice due to its high heat conductivity. It can be circulated by magnetohydrodynamic pumps — without moving parts and thus no material atrition. It is also an excellent neutron reflector with very low capture cross section and, thanks to high atomic weight, hardly any moderating effect (slowing of neutrons through collisions).
The circulation velocity of the fuel loop can be adjusted for different operating modi: Breeding of fissile isotopes, destruction of nuclear waste, transmutation of long-lived fission products. For the salt, ultra-pure 37Cl should be used, as the more common isotope 35Cl tends to get transmuted into the long-lived radioisotope 36Cl via neutron capture.
Nuclear waste, depleted uranium, natural uranium, thorium: The DFR is able to use every isotope with atomic weight equal to or greater than 232 (thorium) as an energy source. This is made possible by the hard neutron spectrum and the integrated reprocessing system, the Pyrochemical Processing Unit (PPU). A reactor block rated at 1 gigawatt (electric) consumes around 1.2 metric tons of these elements (actinides) per year, which would fit into a beer crate. For comparison: A coal power plant block burns 10.000 t of coal per day!
Some nuclides can be fissioned directly: e.g. uranium 235 and plutonium 239. Other, such as uranium 238 (main component of natural uranium) or thorium 232, have to be transmuted first into fissile isotopes.
Light-water reactors (LWRs) use less than 1% of the heat content of uranium, the DFR almost 100%. Even for LWRs, electricity prices are a very weak function of uranium prices, as only 160 t are needed per year; for the DFR, consuming around 1.2 t per year, the influence is completely negligible. This makes the utilization of unconventional sources with low uranium (or thorium) content economical: Even the filtration from ordinary soil, rock or seawater could be undertaken with positive energy balance. Resources are expanded to last billions of years: The DFR is the cheapest renewable energy source!
Ultimate Safety: Melt Plugs
The inherent safety properties of the DFR make it next to impossible to think up a scenario in which reactor temperature rises above the nominal value. Complete collapse of the cooling chain (including heat dissipation to the environment) during full-load operation would be a possibility or malfunction of the PPU (Pyrochemical Processing Unit) massively raising the concentration of fissile nuclei in the fuel liquid.
In such cases, the melt plugs provide ultimate safety: seals made of frozen fuel are installed at the bottom of the loop, which constantly have to be kept from melting by a cryogenic plant running at full power. If a certain temperature is exceeded, they liquify inevitably, causing the fuel fluid to flow into storage tanks, following gravity. Shape and size of the tanks make a chain reaction impossible. They are embedded in material of good heat conductivity, removing decay heat passively.
Read more about safety here.