Institute for Solid-State Nuclear Physics
Institut für Festkörper-Kernphysik
Simply spoken if only enough fissile material is stockpiled the pile becomes an operating nuclear reactor. This happens in nature, too (Oklo natural reactor in Gabon, Africa). One can imagine the DFR as a liquid metal cooled fast reactor with just one fixed fuel element. In such a manner the DFR ist very similar to the world wide operated sodium cooled fast reactors. In the SFR an assembly of fuel elements filled with uranium plutonium dioxide pellets is the active core which is cooled with streaming liquid sodium. If the fuel rods are interconnected and filled with uranium plutonium halide instead of the oxide pellets one arrives almost at the DFR. Liquid lead cooled reactors are constructed similar, only exising reactors are much smaller than the sodium cooled types.
Yes. Not even this, the neutron spectrum is very hard, thanks to the low moderation properties of lead and the absense of dilution of the fuel. The very hard neutron spectrum is also the reason for the high neutron excess which can be used for breeding, transmutation or isotope production.
Fluorine salts have still have considerable moderating quality thus softening the neutron spectrum and deteriorating the neutron economy. Furthermore, many of the involved metal fluorides have a boiling point too high for an effective online reprocessing in the PPU.
Higher halogens are more practical with respect to both properties. For the metals in the used fuel mixture chlorine salts have sufficiently low boiling points yet still higher than 1000 °C as required in the DFR core.
No. Most material problems exist for thermal reactors, but since the DFR is a fast reactor, the choice of materials opens widely. In principal the material problems were already solved by the MSRE development. In the past more durable and resistant materials were applied in industry. Those materials are rarer and rather expensive. Indeed they are affordable for the DFR because of the high powered small core and the abandonment of fuel elements the required amounts are low.
Thanks to the separation of fuel and coolant loop the fuel can be reprocessed online in the PPU. Dry high temperature processing can be used in combination with the fuel cycle. Due to the ionic nature of the bond, the used fuel salt is impervious to radiolysis and as such is directly suitable for physicochemical separation methods at high temperatures.
Two such methods have been proven in the past: the molten salt electrorefining method of the IFR and the high temperature distillation of the MSR. Both can be utilized in the DFR. The capacity of the pyrochemical facility can be designed even much smaller because the processing is performed online continuously. In a simple version, the electrorefining method can be used to purify the fuel salt by precipitation of a fission product mixture. For the purpose of specific transmutation a more precise separation is required which can be accomplished by fractionated distillation / rectification which is beyond the MSR principle.
Thanks to the low boiling points (but still higher than 1000 °C as required in the DFR core) a separation in a fractionated distillation facility alone becomes feasible.
The shape of the reactor is a cube because the durable metal alloys for the fuel ducts are harder to process. So unbowed parts are easier to manufacture into the fuel duct assembly. The size of the reactor vessel is approximately 2 m edge length for a power output of 1 GWth.
Nominally, The fuel consists of undiluted actinide salt. However, the composition of the fuel is very flexible and depends on the particular application. At least enough fissile material (i.e. 233U, 235U, 239Pu, 241Pu) needs to be contained in order to keep the reactor critical. Minor Actinides (which are fissionable) may contribute, too. The other fractions are fertile material (i.e. 238U, 232Th) and possibly a small fraction of to be transmuted material like long-lived fission products. A small system with 1 GWth working with U-Pu cycle has a concentration of 35% plutonium and 65% (depleted) uranium. With increasing reactor size the concentration of the fissile material is declines.
Fission products produce decay heat, the fresher they are, the more. Therefore, they must be stored thin in space to ensure sufficient passive cooling.
In solid-fuel reactors as they are in use today, there is no way to avoid the accumulation of fission products in the fuel rods while the reactor is running. For this reason, active cooling is required, even when the reactor is shut down, depending on the time the reactor was running with those fuel rods. The requirement of active cooling is the biggest safety challenge für solid-fuel driven reactors and lead to the problems known from Harrisburg and Fukushima.
This is totally different for the DFR. Since the liquid fuel circulates through the PPU outside the reactor core, fission products can be continuously separated from the fuel and therefore can not accumulate in the reactor core. Outside the reactor core they can easily be stored according to the usual safety standards for radioactive waste handling. When the DFR is shut down, no active cooling is required.
Furthermore, since the DFR is able to transmute fission products in a very effective manner, medium-lived fission products like 90Sr could also be de-activated in the system which would further reduce the waste storage size. Assessments for these possibilities are in progress.
Breeding additional pure 239Pu (DFR running in breeder mode) for nuclear weapons is not possible, because there is no seperated breeding zone containing pure 238U. Using a fractional-distillation PPU with the U/Pu fuel cycle, you would have to remove 239Np (half-life about 2 days) very fast. This is hardly possible. Since the DFR's nuclear part is fully enclosed and watched telemetrically by anti-proliferation authorities, there is no way to extract weapons-grade material. Using an electro-refining PPU, makes things even harder for bomb makers, because it can only distinguish between actinide salts and fission product salts. The highly radioactive 239Np must be chemically purified in a separate facility within hours to one day, which is nearly impossible, and because such facilities are not necessary for the civil use of the fuel cycle.
When using the Th/U cycle, the 232U isotope, synthesized via (n,2n) reactions, is generating intense hard gamma radiation, which can be detected easily and causes serious damage to weapon electronics. The fractional-distillation PPU is secured in the same way as it is in the U/Pu cycle, preventing the 233Pa being captured by bomb makers.
Compared to the well-known standard method for bomb making, the enrichment of weapons-grade uranium from natural uranium, both possibilities mentioned above are far more difficult to realize.
Cheaper than from the DFR? No way!
Long answer: Gas power plants have the lowest construction effort of all power plant technologies, since it is not much more than a turbine with a generator a turbocompressor and a comparatively tiny combustion chamber directly in front of the turbine. Devices which require most other power plants, too. However the "combustion chambers" of coal plants are much larger and more complex. Similar for a nuclear plant with additional abundant safety equipment. Therefore the investment costs for gas plants are low but the effort to provide the gas is much higher than for coal plants let alone nuclear plants. This is reflected in the ERoEI's which are similar for gas and coal plants and almost 3 times lower than for nuclear power plants.
However ERoEI's are higher than market prize relations. The prizes for natural gas are artificially superelevated by the cartel of the few countries which possess abundant easy extractable gas deposits. Thus gas power plants have the highest production costs of the conventional power plants. (unconventional) shale gas deposits are more even distributed on earth and despite their considerable higher extraction costs they reduce the market price of gas because they break the ascendancy of the cartel. Despite of the even lower ERoEI of a gas plant fueled with shale gas it can be attractive to investors because the investment costs are low and the electricity production costs competitive though higher in comparison to coal plants. This means a shorter financial amortization time than for other power plants. The head start in the ERoEI of PWR plants is diminished by political costs like exuberant licensing procedures.
It is therefore necessary to exploit the vast potential of nuclear fission to a larger extend than existing nuclear technologies. That can be achieved by a high powered reactor core with a overall simplified design and inherently safe and simple operation. Also the costly fuel cycle industry needs to be abandoned. Precisely that is achieved by the DFR with an ERoEI of 1000.