The Dual Fluid Reactor

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atw Vol. 65 (2020) | Issue 3 ı March

The Dual Fluid Reactor – An Innovative Fast Nuclear-Reactor Concept 145 with High Efficiency and Total Burnup Jan-Christian Lewitz, Armin Huke, Götz Ruprecht, Daniel Weißbach, Stephan Gottlieb, Ahmed Hussein and Konrad Czerski

1 Introduction In the early decades of nuclear fission power technology development, most of the possible implementations were at least considered in studies and many were tested in experimental facilities as have been most of the types of the Generation IV canon. Uranium enrichment and fuel reprocessing with the wet chemical PUREX process for today’s reactors originated from the Manhattan project in order to gain weapons-grade fissile material.

The use of fuel elements in light water with respect to the EROI-measure and capability and best neutronic proper- reactors originated from the propul- to passive safety standards according ties. sion systems of naval vessels like to the KISS (keep-it-simple-and-safe) Pure molten Lead has low neutron submarines and carriers. principle and with attention to capture cross-sections, a low modera- A sound measure for the overall current-state technology in mechani- tion capability, and a very suitable efficiency and economy of a power cal, plant and chemical engineering liquid phase temperature range. For plant is the EROI (Energy Return on for a speedy implementation. the fuel, it is possible to employ RESEARCH AND INNOVATION Investment). The known problem of There was a gap in the reactor undiluted fissionable material as solid fuel elements in power reactors concepts of the past with a high opposed to a MSFR that works with is fission product accumulationduring development potential for the present less than 20 % actinide fluoride, see operation requiring heavy safety and the future. A DFR power plant Sec. 4 for details. Consequently, a DFR measures to avoid a core meltdown. could exploit the potential of nuclear has increased power density, small These measures reduce the EROI for fission power with an EROI two orders core volume and very hard neutron today’s Pressurized Water Reactors of magnitude higher than fossil fired spectrum that further improves the (PWRs) to values of about 75 (Sec. 9) power plants. neutron economy. Additional benefits which is only a factor of 2 higher than of liquid metal coolant comprise the for fossil-fired power plants. This is 2 Basic principle application of magneto hydrodynamic in fact surprisingly low compared The Dual Fluid Reactor (DFR) is a techniques both for pumping and, with the possible maximum EROI for heterogeneous fast reactor with a possibly in the future, direct electricity nuclear energy of 10,000 (Sec. 9). liquid fuel and a liquid coolant generation because of the high con- Unfortunately, most Generation IV whereby both flow through the centration of charge carriers. reactor concepts except the Molten reactor core. Separation of cooling Furthermore, the reactor core and Salt Fast Reactor (MSFR, see below) and fuel supply function is achieved primary coolant loop can operate at are again based on solid fuel tech by an interconnected array of fuel normal pressure which allows for nology. For the probably most inten- conduits immersed in the coolant simple and cost regressive size-scaling. sively developed breeder technology, liquid. Both cycles can now be opti- Figure 1 explains the synergetic the Sodium-Cooled Fast Reactor SFR mized for their respective purpose. effects. The Dual Fluid Principle opens (or the Traveling-wave variant, Terra- This has many advantages to a MSFR, the possibility of a liquid fuel with power’s TP-1), sodium has been cho- where both functions must be satis- high actinide concentration in com sen as the coolant. It has aggressive fied by one liquid in a trade-off bination with a coolant with high chemical reactivity with air, water and between high-temperature fuel, low- heat transfer capability, which leads structural materials as well as a high temperature cooling, and an accept- to a high-power density. Liquid fuel neutron reaction cross section with able heat capacity. like in a MSR already reduces the the possibility of a temporary positive The coolant liquid should have the consumption of structural materials void coefficient. These properties highest possible heat transportation compared with solid fuel reactors, require a reactor pressure vessel, double-walled piping, and an intermediary cooling cycle. In effect, all this sums up to expenses which double the electricity production costs of the SFR relative to a PWR as calculated for the Superphénix class. Hence Generation III and most of Generation IV nuclear power plants are in danger of losing competition against fossil fired power plants, especially in the advent of the shale gas exploitation. | Fig. 1. The Dual Fluid Reactor (DFR) The flow chart shows the advantages of the Dual Fluid principle partially depending on each other. concept presented here is designed It is essential for the understanding of the synergetic effects.

Research and Innovation The Dual Fluid Reactor – An Innovative Fast Nuclear-Reactor Concept with High Efficiency and Total Burnup ı J.-C. Lewitz, A. Huke, G. Ruprecht, D. Weißbach, S. Gottlieb, A. Hussein and K. Czerski atw Vol. 65 (2020) | Issue 3 ı March

but the power density is limited. In the DFR, both positive properties can be combined which leads to a 146 massive reduction of structural materials. At high operating temperatures (needed when using an undiluted salt, see Sec. 7), corrosion of core structural materials limits the choices of such materials. However, corrosion resistant materials at high temperatures do exist, but they are quite expensive. Using such materials in a DFR design has little effect on its economy due to its small size, low material inventory, and the absence of | Fig. 2. any parts that need be to replaced Possible power plant based on the DFR, with the nuclear part including the core, the pyro-processing periodically. unit (PPU), disposal and decay heat dump (left hand side) and the conventional part with the heat exchanger and turbines (right hand side). The compactness allows for a subterranean installation. On the other hand, the use of such expensive corrosion resistant materials in a MSR has adverse economic effects due to its high inventory of structural material. Thus, the temperature of a RESEARCH AND INNOVATION MSR is limited and the MSR research was focused in the past on finding suitable eutectic salt mixtures, also compli- cating the production and reprocessing techniques. For the DFR, very simple state-of-art techniques can be applied, see Sec. 4.2. Another comparison can be made with the Generation IV concept of the Lead-cooled Fast Reactor, LFR. Again, due to economic reasons, the wall material of the exchangeable fuel rods must be cheap, which focused the research on finding suitable steel alloys. They yet have a higher lead corrosion susceptibility than the expensive materials intended for the DFR design, | Fig. 3. therefore also limiting the operating DFR fuel and cooling loop. The fuel circulates between the PPU (which is also connected to the short-li- temperature. Due to these material ved fission products storage) and the core whereas the coolant loop connects the fissile zone to the conventional part, also cooling the fission product storage. PPU, core and fission product storage are equip- restrictions, both, LFR and MSR, ped with a fuse plug. are not able to achieve operating temperatures suitable for economic As a result, a new concept not electric grid of industrialized coun- hydrogen production from water. fitting into one of the Generation-IV tries. But also, power-sizes even down These restrictions do not exist for reactor developments has been in to approx. 35 M GWe are possible, the DFR. vented. It foresees a compact core with depending on markets demands. Due Contrary to a MSFR, DFR’s liquid a very high power density, an operating to its compact size, the nuclear part fuel is not limited to actinide salts, temperature of about 1,000 °C, in can reside in a subterranean bunker but it is the current reference design. herits MSFR’s passive safety features, that can withstand high magnitude However, an alternative could be a and has hard neutron spectrum. The earthquakes, direct aircraft impacts solder-like melt of a metal alloy made abundant neutron excess can be used and non-concentrated conventional up of actinides and, if necessary, met- for multiple transmutation purposes, military attacks. The conventional als with low melting points in order to like nuclear waste incineration, and part can utilize supercritical water or 238 232 reduce the solidus temperature of the breeding for U and Th cycles. supercritical CO2 (see Sec. 8.1) and is alloy and gain a pumpable fluid. The All this produces a nuclear power not fortified for economic reasons, but advantage would be an even higher plant with an outstanding economic fortification to any desired degree can power density due to better heat competitiveness. easily be achieved. transportation capability, and a possible higher operating temperature due 3 System overview 3.1 Fuel and coolant loop to the lower corrosive potential of the Figure 2 shows how a DFR reference Since the cooling function is separated metal alloy. The basic design, then, al- power plant might look like. The from the liquid fuel, the circulation lows for a high degree of possibilities reference design has a power output of the fuel can be adjusted to nuclear which can be trimmed to a specific of 3 GWth and an electric output of purposes like maximum burn-up, purpose. These concepts will be dis- 1.5 GWe which is currently the typical transuranic incineration, isotope pro- cussed briefly in Sec. 4.2. nuclear power plant size for the duction, fertile material conversion

| Fig. 4. | Fig. 5. DFR core details. The cubic core (without Left: DFR core inlet region, cylindrical design. The reflector region is located directly below the lateral salt blanket here) includes a pipe system filled feed tubes, surrounded by the blanket region. with fuel salt which is connected to the fuel Right: Schematics of the inlet. In the inlet region, the salt surrounds the Lead tubes and enters the salt loop (with fuse plugs) and immersed in tubes in the core. This ensures equal pressure on all salt tubes. flowing Lead (coolant loop).

(breeding), specific deactivation of The parallel arrangement of the While passing the core region fission products, etc. Figure 3 depicts fuel tubes guarantees a quick drainage through the conduit array more and the reactor core as well as the fuel loop of the fuel liquid within minutes while more actinides are fissioned and and the primary coolant loop. The the high number of tubes provides transmuted and the fuel changes its liquid fuel enters the core vessel at the sufficient surface for the heat transfer chemical composition. The fuel RESEARCH AND INNOVATION bottom, spreads over a system of verti- to the surrounding coolant. An equal volume of the reference plant is cal tubes where it becomes critical, and flow velocity through all vertical rods only a few cubic meters, which leaves the reactor on top towards the is desirable and is achieved by a further simplifies its handling and Pyrochemical Processing Unit (PPU). horizontal-flow inlet zone with baffle processing. The Lead coolant supply pipes have plates providing equal pressure a large cross section in order to reduce differences at the vertical junctions. 3.3 Heat transfer the circulation speed and therefore An additional outer volume filled Figure 6 shows the heat transport. reducing the abrasion at the surface with Lead serves as a neutron reflector Inside the fuel tubes where the materials. It circulates with a rate of reducing the loss of neutrons and heat is generated the temperature 90 tons/s (10 m3/s). When it enters contributing to the reactivity regula- has its maximum. In a region of only the core vessel from the bottom it tion. The separation walls have small 1 mm towards the tube wall the takes the heat from the fuel duct by vents at the top and bottom in order to temperature drops by 270 °C, inside conduction and leaves the vessel on correspond with the Lead loop. A the wall by up to 85 °C, and up to top towards the heat exchanger. further fertile blanket, with simple 0.5 mm outside the wall another Depending on the power needed, structure, can increase the conversion 50 °C, so the total radial temperature part of the Lead’s heat is taken for ratio remarkably. drop is roughly 400 °C. The Lead electricity production or as process coolant moves from the bottom heat. The Lead leaves the exchanger at to the top which defines the Lead a lower temperature and is pumped temperatures at those points to back to the reactor vessel. 750 °C and 1,000 °C, respectively. This can be accomplished by an Consequently, the temperature inside impeller pump which produces a the fuel (tube center, not at the steady stream without generating walls) is 1,150 °C at the bottom sonic shock oscillations in the liquid and 1,400 °C at the top which metal. For maintenance, the Lead defines the highest absolute tem coolant can also be drained at the perature in the reactor core. Since bottom of the reactor vessel into a the bottom salt temperature at the temporary coolant storage from tube inner wall is above the melting where it can be pumped back into the temperature in all operational states, reactor vessel. the salt will not freeze out. At normal operating condition the tube inner 3.2. DFR core wall temperatures are 840 °C and The reference plant uses a mixture 1,090 °C for the bottom and top of actinide-salts as fuel. It has a region, respectively, compared to cylindrical core with diameter and the salt melting point of about height of about 3 m for the critical | Fig. 6. 800 °C. The maximum allowed varia- zone. It contains 10,000 vertical ducts Heat transfer from inside of a single fuel pipe tion of the Lead temperature is (the number is reduced in Figure 4 to the coolant. The temperature gradient is +/- 30 K, still allowing for molten calculated in three zones: The turbulence layer and Figure 5 for illustration reasons). of the fuel liquid (salt => inner pipe wall), the fuel in all cases. These tube wall Figure 4 is a simplified draft of tube wall itself and the turbulence layer of the salt temperatures are 840 °C and the core depicting the principle. An liquid Lead (outer pipe wall => Lead). Values 1,090 °C for the bottom and top actual core CAD model is depicted in are for high salt velocities and MHC pipes. region, respectively, compared to the Temperature gradient for SiC is about twice. Figure 5. salt melting point of about 800 °C.

such that the fuel inside this segment Long-lived fission products are just freezes out. sent back into the reactor core for The cooling power of the fuse is transmutation. 148 fixed, so that the plug does not yet melt at 1,000 °C. In case of an emer- 4 Liquid fuel and gency, i.e. higher core temperatures or its processing loss of power, or for an intended fuse The employment of a liquid fuel plug cooling power-off in a regular eliminates the need for the costly fuel shutdown, the fuel heat power will element infrastructure industry and melt the plug open and the fuel is replaces it with online processing of drained by gravity into the subcritical the fuel. In principal, it is possible tanks. to consider all chemical separation The subcritical tanks (see Figure methods in the reprocessing of 2) are used for fuel inventory and the nuclear fuel, since the radioactivity is concentrated highly radioactive short- a subordinated problem. This, how lived fission products from the storage ever, is not true for the presently in the main coolant loop. Each of the applied PUREX process, as shown in tanks has a capacity for a subcritical the following. mass of the liquid fuel only. They | Fig. 7. are embedded in a volume filled with 4.1 Present reprocessing Close-up of the DFR core region with part of the coolant cycle and the short- salt or metal (e.g. iron, assembled technologies lived fission products storage inside the coolant conduit ahead of the core. from ‘Lego’-like bricks, establishing Originating from the weapon pro full heat contact by temperature duction, the usual aqueous organic RESEARCH AND INNOVATION 3.4 Tank for short-lived fission expansion) which transduces the reprocessing techniques like PUREX products quickly fading heat energy passively are performed off-site. As the chemi- Highly radioactive and heat gen through the outer walls to the sur- cal processes proceed slowly at normal erating fission products with half-lives rounding. temperatures large volumes of con- of weeks to months pose the main The heat production lowers from sumed auxiliary chemicals with problem for reactors with solid fuel 200 MW (emitted from the core) medium and low radioactivity are rods and cause core meltdown unless immediately after shutdown to some required and have to be dumped. In sufficiently cooled. In the DFR like the 5 MW (from the coolant duct seg- order to limit this additional nuclear MSFR these fission products are ment) after 12 days. The salt remains waste, spent fuel elements need to be regularly separated from the fuel liquid for several days and can be stored for at least 1 year, in practice liquid so that the core contains only pumped up, entering the fuel loop rather 5–10 years, before starting the few quantities of fission products and again. After longer storage, a pre PUREX processing, otherwise the its handling in case of an emergency is heating system is to be used. expensive organic solvents are unproblematic. However, the problem destroyed by the intense radiolysis is then transferred to the storage of 3.6 Fission product treatment and therefore have to be replaced very the fission products. In the DFR, this The PPU removes the fission products often. Hence, the radioactivity of the problem is solved by storing the short- from the liquid fuel and replenishes fuel has an eminent relevance here. lived fission product salts, roughly it with fresh actinides that may come The class of aqueous organic repro- 1 m3, in the pipes of a special coolant from natural/depleted uranium, used cessing techniques is inappropriate for duct segment shown at the bottom fuel elements, and thorium at a online fuel processing. A real progress part of Figure 7, just before the Lead consumption rate of 1,200 kg/year. was made by implementation of the reaches the core, where they are Fission products are sorted by reprocessing inside the Integral Fast cooled by the liquid Lead stream chemical elements and the longer Reactor (IFR). It uses electro-refining, during normal operation of the plant. living (half-lives of years to decades) a long-known method in metallurgy, The molten salts of the short-lived are cast into small globes which are for the separation of the fission products slowly revolve through packed and hermetically sealed in products: The metallic fuel is con this tank as well as the PPU. In case of ripple tubes. The tubes are transferred verted to a salt which in turn is used an emergency or maintenance shut- to a decay storage bunker below by a for the electrolysis wherein the down, they will be drained through remote transfer system (also indicated actinides deposit at the electrode and a melting fuse plug, similar to the in Figure 2). The bunker can store all the fission products mainly remain in fuse plug used for the reactor core, see fission products, 500 kg/year,- pro the molten salt. This manageable next chapter. duced during whole life-time of the reprocessing unit was used on-site of reactor. The sorted fission products an IFR plant. After the IFR program 3.5 Melting fuse and can be removed according to their was canceled its successor, the subcritical heat storage half-life. S-PRISM reactor, inherited the pro- Melting fuse plugs, already proven 90 % of all fission products can be cess, though in a central off-site and tested in the Oak Ridge Molten removed after 100 years, providing processing facility. Salt Reactor Experiment (MSRE), are valuable and rare metals. The A possible online reprocessing used in the DFR for the short-lived medium-lived fission products decay technique was tested for the MSFR – fission products tank and for the within 300 years and may remain in a dry method with a vapor-phase reactor core (green plug below the the storage for that time. The ripple fluoride-salt distillation system as the core and the tank). It is essentially a tubes inside the storage are passively main component where the metal pipe segment which is actively cooled cooled by ambient air utilizing the salts are separated by boiling points. with a constant heat transportation stack effect. However, many fluorides have very

high boiling points so that additional Sn to a dispersion of solid actinides 239Pu fraction required for the smallest fluorination is required and yet metal and/or actinide compounds in Pb/Bi/ useful set-up can be very high and fluorides remain in a slurry needing Sn. The prospects of metallic fuels is not limited by the reactivity coeffi- further treatment steps. In a MSR, a were already investigated in the cient of the Doppler-broadening effect 149 real online fuel reprocessing conflicts 1950s. More precisely, the last option of 238U while larger cores can manage with the cooling requirements, there- would be made up of actinides which smaller fractions. The rest of the fuel fore the reactor must be shut down to are suspended in a melt of metals with is fertile material like 238U or 232Th. branch the fuel into the reprocessing low melting points with a fraction of Here, the fuel salt would consist of the facility which needs a high capacity in up to 75 atom-% which reduce the tri-chlorides of the actinides, i.e. UCl3 order to keep the outage time of the solidus temperature of the alloy below and PuCl3, which have a suitable reactor short. Nevertheless, such the operating temperature, because temperature range of the liquid state. pyrochemical processing facilities are some of the involved actinides have Purified 37Cl is to be used in order to still small in comparison to PUREX- too high melting points. Suitable met- avoid neutron losses due to their like methods. als with sufficient neutronic proper- capture by 35Cl and production of the The distillation techniques, and in ties are lead, bismuth and tin. The ac- long-lived radioactive isotope-36Cl. particular, the electro-refining tech- crued multi-component alloy does not Both previously developed and niques are subject to ongoing develop- necessarily need to be eutectic – even tested reprocessing methods of the ment activities for the Generation IV in the case the liquidus temperature is Generation IV reactors, fractional dis- reactors as well as a substitute for the above the operating temperature the tillation and electro-refining, can also complex wet chemical PUREX repro- mixture is sufficiently pumpable in be employed for the DFR. The capacity cessing plants. this pasty phase. The processing of the of the PPU can be designed even However, online does not neces metallic melt can be performed with a much smaller because of the low fuel sarily mean continuous. Batch tech- first fractionated distillation step volume. In a simple version, the niques may be used as well, provided where the metals with low boiling electro-refining method can be used RESEARCH AND INNOVATION the continuously pumped fuel fluid is points compared with actinides like in order to purify the fuel salt by intermittently stored in a small buffer Lead, Bismuth and some of the fission precipitation of a fission product while the previous batch from the products can be separated and the re- mixture. For the purpose of specific buffer is processed. maining slurry is converted to salts transmutation, a more precise parti- None of the present reactor con- and then distilled as before. Then, the tioning is required which can only be cepts of the Generation IV provides a resulting salt fractions need to be con- accomplished by fractionated distil real online fuel reprocessing. This verted to metals back again by elec- lation/rectification, which is beyond means that none of these concepts has trolysis before re-insertion into the re- the MSFR principle. all the advantages of a liquid fuel actor fuel loop. Basically, whenever liquid fuels that could be achieved with a true For the reference concept, molten are used certain preprocessing steps online fuel reprocessing like very salts are used because of their lower have to be accomplished in order to low criticality reserves which are a melting points and wider range of ex- deal with volatile and ‘noble’ fission control issue in solid-fueled reactors, perience. Unlike an MSR chlorides are products. In the case of a fuel salt and especially ADS, or MSRs with long adopted since fluoride salts have con- the fission of plutonium, significant fuel processing periods. siderable moderating quality thus sof- quantities of metals are produced tening the neutron spectrum and de- which can hardly form chloride 4.2 Fuel processing in the DFR teriorating the neutron economy. This compounds, notably Mo, Ru, and Rh. As pointed out, for online fuel pro together with the high boiling points In the frame of the Molten Salt Reactor cessing the employed technique must of many of the involved metal fluo- Experiment (MSRE) this issue was be congruously fast so only dry high rides render fluorine inapplicable. investigated in the view of the possible temperature methods can be con Higher halogens are more practical segregation problem of said fission sidered. Moreover, the fuel must be with respect to both properties. For products. It turned out that the impervious to radiolysis within the the metals in the fuel mixture chlorine segregation is not a progressive pro- process. The liquid fuel of the DFR for salts have sufficiently low boiling cess but instead an equilibrium the reference design is a molten salt, points so that a separation by boiling accrues between segregation and but could be also a metallic melt as a points in a fractionated distillation fa- solvation. This equilibrium level can future option. Therefore, the DFR con- cility alone becomes feasible. be controlled by the overall chemical cept is not an MSR variant, and the Hence, the fuel is a binary com potential of the molten salt which may reprocessing techniques are different bination of only a fertile and a fissile be adjusted by the quantity of chlorine because of the very different salts. actinide chloride which can be ions and possibly certain minor Due to the ionic nature of the bond in 238U/239Pu or 232Th/233U. It should be additives. The chemical potential also the case of the salt and the metallic clearly noticed that no carrier salt is determines the corrosive properties of bond in the case of the metallic melt, needed or desired, as opposed to cur- the salt. In preprocessing steps the the liquid is impervious to radiolysis rent MSR concepts – this is the advan- noble metals in the fuel coming from which makes it suitable for physico- tage of the Dual Fluid principle. The the reactor can be precipitated by chemical separation methods at high fraction of the initial load of reac- bubbling noble gas (He, Ar) through temperatures. These methods will be tor-grade Pu or enriched U depends the fuel salt. The metals precipitate as used in the PPU of the DFR. on the size of the reactor core because platelets at the phase boundary For the possible future concept of a of neutron losses through the surface. between the gas bubble and the salt metallic fuel melt there are several For the reference plant, it is 23 % liquid where they can be subsequently options ranging from a more hetero- (reactor-grade Pu) or 19 % (235U) retrieved by a rake. This makes it geneous system with liquid plutonium mass fraction according to first static possible to easily separate 99Mo, over a solution of actinides in Pb/Bi/ SERPENT calculations. The maximum which decays to the important

medical isotope 99mTC, see also sec. 8. and almost instantaneous because it is temperature will be reached very fast, Concurrently to the gas bubbling the determined by the speed of sound. and it cannot freeze out anymore volatile fission products Kr, Xe, Cs and Lead has a high atomic mass and 4 (melting temperature at 800 °C). 150 I2 are expelled as well and can be stable isotopes due to nuclear shell Now the reactor is regulated by removed easily. closure. Therefore, it is an excellent the described loops (see sec. 3). At Volatile iodine as well as cesium neutron reflector with low moderation the beginning the fission rate and can be removed from the fuel loop/ qualities and low isotope-weighted correspondingly the power produc- PPU and bound chemically stable. neutron capture cross section. tion is minimal. Then the coolant Since a permanent reprocessing of the These effects together with the pump starts to accelerate the circu molten salt fuel is possible, only very density change cause a strong nega- lation of the Lead. The discharge of few fission products accumulate tive temperature coefficient in the fast heat to the heat exchanger causes a so that their integration in the fuel neutron spectrum. temperature decrease in the reactor salt is unproblematic. The low fission This is in contrast to liquid Sodium (of course the heat exchanger must be product concentration in the core also as coolant which has a higher neutron able to dump the heat energy). The reduces corrosion. capture cross section, higher neutron control loops render the reactor The salt has to remain in the moderation and lower reflection supercritical until the nominal tem- liquid state during operation which is quality which means an increase of perature is regained and well- assured in the core by the criticality the neutron flux with rising tempera- balanced. This may continue until the condition and in the PPU by the ture, i.e. temporal positive tempera- nominal power output is reached. residual heat. A frozen salt would not ture coefficient in several designs. Conversely, if the Lead circulation damage the reactor but has to be Furthermore, since the most speed is decelerated (also in case of a preheated, e.g. by inductive heating. abundant Lead isotopes are each at malfunction) the temperature in the Small, possibly mobile, DFR sys- the end of a decay chain, prolonged reactor increases and it becomes RESEARCH AND INNOVATION tems could use a once through cycle, exposure to neutrons can only induce subcritical until leveled off at the i.e. they are not connected to a PPU low radioactivity. The highest stable nominal temperature but with lower and use the fuel inventory once. It can Lead isotope, 208Pb, has the lowest fission rate. In such a manner, the then be exchanged by pumping and neutron capture cross section, which fission rate in the reactor follows the processed in a PPU at a different leads back to stable Lead via 208Pb power extraction. This can be done ac- location. The fuel’s range can be (n,c) 209Pb (b) 209Bi (n,c) 210Bi (b) tively by the Lead pumping speed, or extended with a centrifuge which pre- 210Po (a) 206Pb. The stable 209Bi passively by feedback from the cipitates some of the fission product accumulates slowly, so that only 209Pb turbine’s electricity generation. There compounds by density separation. contributes remarkably to some is no need to control the fission rate activity, decaying with a half-life of directly in the reactor core (e.g. by 5 Reactor operation and only 3 h and, in contrast to Sodium, control rods). regulation free from gamma radiation. For the The equilibrium (nominal) tem- only longer living nuclide, 210Po (half- perature is determined by the fraction 5.1 Neutron absorption and life 138 days), even 50 years of reactor of the fissile material in the fuel salt. negative temperature operation and 209Bi accumulation The PPU provides the appropriate fuel feedback leads to an activity just comparable salt mixture. The PPU fabricates a fuel mixture with natural Uranium. As a result, the that is critical inside the reactor at low and gamma-free radioactivity 5.3 Shutdown procedure the desired operating temperature of makes an intermediary cooling loop For a regular shut down the coolant 1,000 °C. There are three main effects obsolete, which further reduces the circulation and the fuse cooling is which provide negative feedback to expenses, see Sec. 8.1. stopped and the fuel salt empties to the fission reaction rate by depression Due to its very strong overall the storage tanks. The same happens of the neutron flux when the tempera- negative temperature coefficient (five if the power to the entire plant fails. ture rises: times that of a TRIGA reactor) and Any other reason like malfunction and 1. Doppler broadening of the limited fuel heat capacity, the usage of sabotage increasing the fraction of the resonances in the neutron capture control rods in a DFR type reactor is fissile material raises the equilibrium cross sections increases the not necessary. temperature. For these incidents, macroscopic neutron capture cross again the melting fuse plug kicks in. section. 5.2 Startup procedure Consequently, the emergency shut 2. Density decrease of the molten salt To start up the reactor the system is down is the same as the regular shut fuel which reduces the fissilenuclei pre-heated until the coolant and the down. concentration, the far dominant fuel salt liquefy. Concurrently the effect with dk/dT >= 0.015 $/K cooling of the melting fuse plug is 6 Neutron economy assuming the density decrease of started. The fuel salt is pumped from With the U-Pu fuel cycle the fission of UCl3 for the whole salt, where the storage tanks to the reactor. At the Pu produces a high neutron yield. k is the effective neutron multi tee connector, just below the reactor Even after regeneration of the Pu fuel plication factor and T the fuel some of the fuel fluid branches to the by conversion of fertile 238U a large temperature. fuse where it freezes out and plugs it. neutron surplus remains. Neutronics 3. Density decrease of the molten As soon as the salt, preheated to simulation calculations have been Lead reduces the concentration of 900 °C, slowly moves into the reactor performed (Serpent, OpenMC); pre- the neutron reflecting lead nuclei. core it becomes critical. liminary results, though with no con- The change in reactivity due to a Thanks to the very strong negative version ratio calculations, are to be temperature induced density change reactivity coefficient, dominated published. If (besides fissile material) in the liquid fuel is by far dominant by the liquid fuel, an equilibrium only 238U is fed into the fuel this

neutron surplus will end up as doubling times. The thorium MSFR MgCl2 – KCl salts compared to additional plutonium. In this case (or (also known as liquid fluoridethorium Hastelloy-X or Iron-/Chromium-based similar for 232Th) the conversion rate reactor – LFTR or “lifter”) would have alloys. Molybdenum-based alloys is larger than one and the reactor a doubling time of about 25 years. show a high resistance against both 151 works in the breeder mode. molten fluorides and, also Niobium The neutron surplus can also be 7 Materials and alloys, against Lead. Chloride salts used for other transmutation pur fabrications are significantly less corrosive than poses, e.g. when long-lived fission As mentioned in Sec. 4.2, for a com- fluorides. products are specifically mixed in the pact nuclear core a high actinide frac- As a further option, new ceramics fuel salt by the PPU. There is still a tion is necessary to obtain sufficient may be considered, as coating and in considerable neutron surplus when fissioning and breeding capabilities. the form of new fiber backed com the reactor transmutes its own long- Thus, the fuel salt should be undiluted posite workpieces. lived fission products which can be which renders eutectic compositions Silicon carbide (SiC) is known for used to transmute fission products dispensable. This results in elevated its low neutron capture cross-section from waste fuel elements of other melting points of about 800 °C and and is therefore in the focus of today’s nuclear reactors. Only if this addi demands high operating tempera- nuclear material research. Especially tional neutron surplus is consumed tures above 1,000 °C. Therefore, the CVD-like SiC, is very resistant against otherwise, but not for breeding, the materials of the nuclear part must Lead corrosion at more than 1,000 °C, reactor works as a self-burner, i.e. withstand high-temperature corro- even when Lithium is added (Pb- conversion rate equal one. sion, a high neutron flux, and must 17Li), where pure Li would dissolve Alternatively, the PPU can mix in have a very good high-temperature SiC at 500 °C. Regarding molten salt Th or inert materials to even out the stability and creep strength. corrosion, much less data is available neutron surplus. The fission neutron These extremely resistant materi- for SiC. It was tested with NaCl which 233 RESEARCH AND INNOVATION yield of U from the Th/U fuel cycle als are known for many decades but has a similar enthalpy like UCl3 and is considerably lower than for the could not be treated in the past. This showed a good resistance up to 900 °C plutonium fission. As other fast includes in particular alloys from the even though it was a much less neutron breeders, the DFR also can be extended group of refractory metals, corrosion-resistant variant (reaction- operated in the Th/U cycle with a molybdenum- and tungsten-based bonded SiC with Si excess). Compared conversion ratio slightly larger than 1. alloys, as well as high-performance to that, CVD-SiC showed a much The transmutation of its own long- industrial ceramics. Meanwhile, how- higher corrosion resistance. Below lived fission products may be feasible. ever, fabrication methods are far 1,200 °C, this material also shows a For that, the PPU needs to separate advanced, so that such materials high irradiation resistance, whereas out and store the 233Pa until it decays find applications over a widespread SiC/SiC fibre pieces are less resistant to 233U. The PPU can handle the range in the industry, especially although the newest generation of transition from the U/Pu to the Th/U in the chemical industry, mechanical these composites showed a higher fuel cycle continuously. engineering as well as in the aviation resistance again. Micro crystalline The fissile material in the fuel (nozzles, jet vanes, balance weights). damages caused by the high neutron salt may also contain transuranium Their demand is still low but their flux as well as thermal stress will be elements from waste nuclear fuel technical feasibility has been proven automatically healed at those high elements. As in the case of fission in the past decades. For this reason, temperatures (annealing in metals) product transmutation the PPU would they are expensive, and current and ceramics are more resistant at process chlorine salts made of the fuel material research for solid-fuel based elevated temperatures. In the PPU, pellets of waste fuel elements sepa reactors (LWRs, but also most of the there are even less restrictions as rating the chemical elements by Generation IV concepts) is focused on neutron embrittlement and heat con- boiling points. Then the PPU mixes replacements like steel and Ni alloys. duction do not play a dominant role the fuel salt from the desired actinides This is in contrast to the DFR where anymore. so that the criticality condition in the higher material costs play only a Pieces from high-performance core is maintained. In this way, the minor role since the material demand alloys, even from refractory ones, can sources of fuel are natural uranium, is several times lower than for be produced by new electron welding depleted uranium, nuclear waste, and LWRs, as also pointed out in Sec. 2 processes, high-pressure sintering and thorium. The reference plant can (Figure 1) and Sec. 9. The entire laser techniques. In particular, the consume radiotoxic transuranium reactor needs only a few 100 tons of laser treatment cares for a high-purity elements from burned LWR fuel up to refractory materials, with only 20 to crystal structure (smooth melting) – a 1,200 kg per year. 50 tons for the core, while the remain- factor very important for the corro- One DFR using the U/Pu cycle can ing 80–90 percent are in a simple sion resistance. Generally, refractory provide the initial fissile charge for geometry. The durability and creep compounds are processed with the another DFR, where the doubling resistance is a central point: it requires methods of the powder metallurgy, time is comparable to the total con- but at the same time enables a core particularly because of their high struction time of a power plant and that needs not to be exchanged. melting temperatures and durability. not the limiting factor for deployment. This point is often not seen by The sintering process limits the size SFR’s (like the French Superphénix critics implicitly assuming a dispos and shape of work-parts but new laser and the Russian BN) together able material technique as equired by sintering methods might relieve many with PUREX-reprocessing plants have the solid fuel rod technology involving restrictions. Even though the fraction doubling times of 30–40 years. a very restricted view on the material of voids for today’s applications is Utilizing the Th/U cycle in water variety. still too high, sintering extruders are cooled reactors with fuel elements Tungsten and Tantalum show much capable of producing monolithic pipes would exceed even these long less corrosion in NdCl3 – NaCl-KCl or with smooth surfaces. The whole

array can be assembled with electron 8 Applications materials and are capable to get along beam and/or laser welding in vacuum. Figure 8 depicts possible application. with sulphuric acid, dust particles, For valves in molten-salt, contact- The high temperature opens the and hot steam at 1,400 °C. 152 surface seals can be used since they hydrogen-based chemistry with Another near future possibility is will only by used hourly. synthetic fuels suitable for today’s the usage of supercritical carbon The high operating temperatures vehicles. The low production costs dioxide (scCO2) turbines, leading are well above the brittle-ductile make these applications competitive to more compact machine compo- region of refractory metals hindering with fossil fuels like gasoline. Further nents with a slightly higher thermal strongly an embrittlement, best seen applications are described in the efficiency and significantly reduced on Mo-based alloys. Furthermore, following. corrosion rates and pressures com- highly-resistant coatings can be pared to scH2O turbines. Although considered. Some refractory alloys 8.1 Conventional part still in development, the experience are already ductile between 300 °C Due to the low and gamma-free radio- and outlook is promising. The corro- and 500 °C (or lower), e.g. MHC (1Hf- activity of liquid Lead (see Sec. 5.1) sion rates are monitored to be less 0.1C-Mo) or TZM (0.5Ti-0.08Zr- it is possible to extend the primary than 1 mm per year at 1,000 °C using 0.02CMo), maybe with some addi- coolant loop directly into the conven- industrial INCONEL-MA-754 nickel- tions of Rhenium in the 1 %-region. tional part of the plant. This translates base alloy, decreasing with time. The All operating temperatures (inlet into a considerable reduction of the alloys used in the DFR are significantly and outlet) are well between 850 °C reactor construction cost, as opposed more corrosion resistant so scCO2 and 1,100 °C, 1,400 °C occur only in the to Sodium cooled reactors which should be a minor problem. axial center of the fuel, not at the tube require a secondary cooling circuit walls (see Sec. 3.3 and Figure 6). The due to the high radioactive and 8.2 Process heat and electricity thermal expansion coefficients of gamma-emitting content of Sodium. If the DFR is employed for process RESEARCH AND INNOVATION refractory alloys are similar to the ones In the conventional part the heat heat generation the conventional part of ceramics not causing significant energy needs to be transduced may be modified. For process heat stress or tension, as also can be seen in from the liquid metal, a medium with generation only a heat transducer to a turbine parts or high-temperature very high heat transport capacity, secondary liquid coolant cycle or a furnaces. to a working medium with consider direct heating of a chemical reactor in The entire core (total dead weight able lower heat transport capacity close vicinity with the primary coolant is a few ten tons) can be produced in a suitable for turbines. Without further may be used. If a mixed process heat factory by the methods mentioned development, the most cost effective and electricity generation is desired, a above and deployed on site exclusively technique, nowadays, is supercritical first indirect heat exchanger which by bolting and screwing or stacking/ water (scH2O) cycle. Albeit the newest decouples heat energy at the high clamping in the case of SiC. Possibly coal fired plants work at 700 °C there operating temperature may be fol- the core must be segmented in order to is no principal problem to increase it lowed by a subsequent heat exchanger ease the exchange of possibly damaged to 1,000 °C. Generally, scH2O turbines which heats at a lower temperature parts. For the coatings, corrosion re- have more in common with gas water in a steam or supercritical water sistant materials (SiC also as structural turbines than with steam turbines cycle with a connected turbine. material, Si3N4, AlN in the core, possi- since there is no phase change bly TiB2, B4C elsewhere) exist, having throughout the whole cycle; so, oper- 8.3 Future MHD option a heat conductivity similar to Ni. For ating parameters are quite similar. A further possibility is the utilization isolation, fan and fold sheets can be The reactivity of water with respect to of an MHD generator connected to the used but because of the high neutron its ability as oxidizer increases with Lead coolant loop. Liquid metals are flux the entire core has to be sur temperature. However, modern gas particularly eligible for that because rounded by a concrete shield anyway. turbines are made of very resilient of their high concentration of free charge carriers. The efficiency of the MHD generator is chiefly limited by the nozzle which converts the internal energy of the fluid into directed stream energy which is then con verted to electricity. The still considerable residual heat after the MHD generator may be used in a subsequent heat exchanger with a water cycle as above. Such a system may be sig nificantly less costly than multiple turbines.

8.4 Radiotomic chemical production The short-lived fission products storage may be designed in an alter native way in order to enable the utilization of the intensive radiation for radiotomic induction of chemical | Fig. 8. reactions requiring high doses Possible applications for the DFR. (kGy/s). There is a constant power

level of 30 MW of the short-lived the technetium generators are feasible the denominator of the EROI). For a fission products in the reference plant which further simplifies the logistics typical 1,400 MWe PWR, a major part which may induce a γ-doserate of of the delivery to the hospitals. This of the CED is needed for the enrich- 0.1–1 MGy/s into compressed gases. could lead to a cost implosion for the ment of uranium which in the first 153 There is a small number of simple 99mTc radiotracer and therefore to an decades of nuclear power applications molecules that are the base for several inflation of applications. was dominated by the very ineffective process chains in industrial chemistry diffusion enrichment. and result from strong endothermic 9 EROI consideration This reduced the EROI to 24 which reactions which are performed with Energy Return on Investment is is comparable to fossil fired power high expenses over several steps probably the most important factor plants and is one explanation why the frequently employing costly cata to characterize the economical-expansion of nuclear power came to a lyzers. Here a γ-quantum can directly efficiency of an energy source. It is halt in the 1970s in the USA. provide the required energy by defined as the ratio of the total elec- A newly built PWR with mostly multiple excitation/ionization of the tricity output of a power plant during centrifuge enrichment has an EROI of educts resulting in a considerable its lifetime to the expended exergy for 75 to 105, with complete LASER simplification of the required equip- construction, fuel supply expense, enrichment up to 115. So the PWR ment and reduction of costs all the maintenance, and decommissioning. technology can have an advantage more the radiation source exists This should not be confused with a in the EROI factor of 4 to fossil power anyway. This possibility was already return-on-investment assessment on a but this defines also the limit of the described in the past. monetary basis. PWRs and the Generation III(++) Such basic compounds are nitrogen Unlike monetary measures, the technology in general. oxides NO2, ozone O3, hydrocyanic EROI is time invariant and inde Another costly contribution to the acid HCN, and carbon monoxide CO. pendent from the national economic low EROI are the expenses for the fuel Nitrogen oxide and ozone can be context. It requires a full life cycle element infrastructure industry which RESEARCH AND INNOVATION obtained by irradiation of compressed assessment (LCA) in order to deter- is also conceptually based on the air. Hydrocyanic acid originates mine the correct cumulative energy military logistic chain where as much from methane and nitrogen. Carbon demand CED (the energy invested, i.e. as possible is displaced from the monoxide results from radiative dissociation of carbon dioxide. The Item Units Energy Total DFR reference plant may produce (or total amount inventory inventory 1 0 4-5 tons/year tons/year of these in 1,000 kg) in TJ/(1,000 kg) in TJ chemicals. Concrete containment for reactor, fission products 21,000 0.0014 30 and turbine building 8.5 Medical Isotope High performance refractory metals and ceramics 60 0.5 30 Production (PPU and core) 99m The radiotracer Tc is a prime High temperature isolation material for PPU and 100 0.1 10 example of a medical application that core would not be possible without a Initial load, isotopically purified37 Cl + fuel 25+60 2.5/0.4 50+25 nuclear reactor. Refractory metals and ceramics for the heat 180 0.5 90 Seeking an alternative during exchanger the world-wide Molybdenum crisis 2009/2010 failed due to the high Isolation and structural materials, heat exchanger 300 0.1 30 Untreated, low-alloyed metal for fission product neutron flux required for the pro 3,000 0.033 100 duction of the 99mTc precursor 99Mo. A encapsulation cost-effective production in commer- Structural materials (steel) for non-nuclear part 1,000 0.02 20 cial reactors seems not to be possible Lead coolant 1,200 0.036 45 for several reasons, so it is mainly pro- Turbines with generators 3 40 120 duced in research reactors. An expensive separation process follows, and a Mechanical engineering parts 150 sophisticated logistic chain to finally Cooling tower (special concrete) 20,000 0.003 60 deliver the technetium generators to Refueling, 1,200 kg/a actinides over 50 years 60 0.4 25 hospitals is required due to the short 37Cl loss compensation 2 2.5 5 half-life of 99Mo of only 3 days. Maintenance, high-performance refractories + 30+50 0.5/0.1 20 The Nuclear Energy Agency (NEA) isolation for 1 new core estimates the future 99Mo world Maintenance, 50% of other reactor parts, 16 6 90+175 0.5/0.1 62.5 demand to be 4*10 6-days-Bq (10 refractories + isolation 6-days-Ci) per year, corresponding to Maintenance, 50% of mechanical engineering 135 a demand of roughly 1 kg (assuming and turbines 10 % separation efficiency) directly Maintenance electricity, 2MW over 20 days/a 182.5 from the nuclear fission in LWRs and heating, 50*0.2 TJ providing 99Mo. In contrast, one single DFR produces at least 30 kg 99Mo per Sum 1,190 Output over 50 year’s-lifetime, 1,500 MW net, year but – more important – already 2,250,000 provides it in a separated form, see 8,300 full-load hours also Sec. 4.2. This strongly reduces | Tab. 1. the handling so that a complete Input energy amounts of the DFR; bold: the sum of all inputs and the total electricity output; the ratio leads to an EROI on-site medical-clean production of of almost 2,000 for the DFR, see text.

The online separation of fission products provides presorted metals that can be used after decay as im 154 portant raw materials for the industry. Other fission products, e.g. 99Mo needed for medical diagnostics, can be quickly withdrawn in large amounts with no further processing. The liquid fuel provides the same passive safety features as already tested for the molten-salt reactor (melting fuse plug, deeply negative temperature reactivity coefficient) but the concentrated actinide fuel adds additional safety and controllability due to a higher delayed neutron fraction | Fig. 9. inside the fissile zone. The lowerfissile Energy Returned on Investment (EROI) at different electricity generating technologies. zone salt inventory means lower heat capacity leading to a faster power battlefield to factories in the back A theoretical maximum EROI reduction in the case of additional area. The utilization of fuel elements of 10,000 can be calculated as an reactivity. then again requires multiple-redun- extrapolated limit, only taking into Manufacturing the durable work- dancy elaborated active and passive account the exploitation costs at pieces for the core is feasible by RESEARCH AND INNOVATION safety systems in order to counteract 3 ppm U-content in the earth crust, state-of-the-art technical processes the risk of core meltdown, further erection of power plant, service and and well-established industrial pro reducing the EROI in effect. maintenance, dismantling and dis cedures. The complete absence of The large EROI gain of the DFR posal being neglected. Further optimi- control rods, valves or any other mainly results from two aspects: The zation of the design and extraction of mechanical parts as well as its com- loss of a costly external fuel processing fuel at basic crust concentrations pact size enables the use of expensive, infrastructure (improvement of more (10 ppm for Thorium) would lead to a corrosion-resistive materials and than a factor of 3) and the much domination of the fuel-related input modern fabrication techniques like higher compactness and simplicity and opening potential for a further laser sintering. compared to a light water reactor increase of the EROI. In essence the Dual Fluid principle (another factor of 6). Additional This all together is showing that resolves the contradiction of con minor improvements arise from lower the DFR exhausts the potential of temporary NPP concepts between a maintenance efforts and from much nuclear fission to a large extent. For high power-density which is obliga less fuel consumption as well as illustration on the relevance of the tory for the crucial economic edge to significantly lower disposal needs. EROI-definition, Figure 9 depicts the prevail in the energy market, and The higher per-mass efforts for EROIs of different currently used inherent passive safety necessary for a the refractory parts are far out- electricity producing technologies safe operation and eventually the weighed by the extreme reduction with the EROI for a DFR. public acceptance of nuclear power. of material amounts needed for construction (several 1,000 metric- 10 Final remarks tons nickel alloys and highly alloyed The Dual Fluid principle of separating Author Jan-Christian Lewitz (a,b) steels in a light water reactor the cooling and fuel function increases Armin Huke (b) compared to a few 100 metric-tons the complexity of the reactor core Götz Ruprecht (b) Daniel Weißbach (b,c) refractories in the DFR). Table 1 relative to the MSR but has large Stephan Gottlieb (b) describes the evaluation of the EROI synergetic effects in the fuel repro- Ahmed Hussein (b,d) for the DFR. cessing, the neutron economy, the Konrad Czerski (b,c) Since some materials (especially cost efficiency as well as on the (a) LTZ-Consulting GmbH, refractory metals) must be investi possible applications. This allows to Tharandter Str. 12 gated and modified for use in the combine the advantages of different 01159 Dresden, Germany DFR, their energy inventory must be Generation IV concepts (MSFR, LFR, estimated. Furthermore, the main SCWR, VHTR) in one reactor type (b) Institut für Festkörper- tenance for the nuclear part is also while considerably undercutting the Kernphysik gGmbH, Leistikowstr. 2, 14050 Berlin, Germany unknown, causing the same uncer- costs even of today’s LWRs. tainties. The good neutron economy and (c) Instytut Fizyki, Wydział The resulting EROI is therefore the hard neutron spectrum makes the Matematyczno-Fizyczny, roughly 2,000 which is 25 times DFR an effective waste incinerator Uniwersytet Szczeciński, ul. Wielkopolska 15 This document is higher than that of today’s PWR and also an excellent thorium breeder, 70-451, Szczecin, Poland based on Armin Huke technique. The very compact design outbidding even MSRs like the LFTR et al., Annals of lowers the construction energy while being more cost-effective. The (d) Department of Physics, Nuclear Energy 80 University of Northern British (2015) 225: „The Dual demand down almost to the level of high temperature combined with the Columbia, 3333 University Way, Fluid Reactor – CCGT plants on a per-watt basis, and high cost-efficiency allows the - pro Prince George, BC, Canada. A novel concept for a the fuel-related energy demands are duction of synthetic fuels in com V6P 3S6 fast nuclear reactor of tiny compared to light water reactors petition with todays refined oil and high efficiency“ due to the efficient usage. gasoline.