Dual fluid reactor

Abstract

The present invention describes a nuclear reactor with a loop for liquid nuclear fuel, which, contrary to similar systems like the Molten-Salt Reactor of the Generation-IV canon, does not use the fuel loop for the heat transport at the same time. Instead, cooling is provided by an additional coolant loop, which is intensively coupled to the nuclear fuel duct for heat transport. That way, the advantages of liquid fuel can be utilized while optimizing the coolant loop performance, so the complexity of safety systems can be reduced significantly. This reactor design further includes an optimized neutron economy and is able to deactivate long-lived fission products generated by its own, so only short-lived radiotoxic waste has to be stored. With the neutron surplus it is also possible to deactivate long-lived radiotoxic waste from used fuel of today's light water reactors or to produce medical radioisotopes.

Claims

1. A nuclear reactor including a primary fuel duct continuous insertion and discharge of liquid fuel into and out of a core vessel, wherein the primary fuel duct is lead through the core vessel, and a secondary coolant duct for a liquid coolant, wherein the coolant enters the core vessel via an inlet, wherein the coolant passes and bathes the primary fuel and leaves the core vessel via an outlet.

2. The nuclear reactor according to claim 1, wherein the primary fuel duct for liquid fuel contains at least one pump for liquid fuel circulation, at least one pyrochemical processing unit, at least one buffer volume, one fuse plug, one storage volume for holding and providing of the liquid fuel, and at least one valve for fuel flux control; and wherein the secondary coolant duct for a liquid coolant contains at least one pump to circulate the liquid coolant, at least one valve for liquid coolant flux control and at least one volume unit for storing, providing and volume-buffering of the liquid coolant, and wherein the nuclear reactor can be optionally operated as a sub-critical system, and wherein the design of the nuclear reactor enables MHD generator operation.

3. The nuclear reactor according to, claim 1 wherein molten-salt is used as liquid nuclear fuel.

4. The nuclear reactor according to claim 1, wherein halides/halogenides are used for liquid fuel.

5. The nuclear reactor according to claim 1, wherein chlorides are used for liquid fuel.

6. The nuclear reactor according to claim 1, wherein a molten metal containing actinides are used for liquid fuel.

7. The nuclear reactor according to claim 6, wherein metal elements are added to the molten metal to decrease the solidus temperature of the molten metal below the operating temperature to ensure that the melt is pumpable.

8. The nuclear reactor according to claim 7, wherein the metal elements are selected from an ensemble consisting of Lead, Bismuth and Tin.

9. The nuclear reactor according to claim 1, wherein the liquid coolant is a liquid metal.

10. The nuclear reactor according to claim 9, wherein Lead is the liquid coolant.

Description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(1) FIG. 1 schematically shows a preferred embodiment of the reactor core. FIG. 2 shows different shapes of the fuel duct, and FIG. 3 shows the total system with the cooling and fuel loops.

(2) Reactor Core

(3) The reactor core shown in FIG. 1 comprises a reactor core vessel filled with the coolant (1) and the fuel pipe (7) where the liquid fuel is pumped through. The fuel pipe (7) is formed in a manner to fill the reactor core vessel (4) as compactly as possible while the coolant must still be capable of removing the heat sufficiently and uniformly.

(4) The top view of one level of the fuel pipe (7) in different possible embodiments is shown in FIG. 2. The pipe shape shown in FIG. 7a) is most simple to manufacture. For an ADS system, the central level has a different shape (7b) in order to be able to direct a particle beam (10) from an external accelerator into the center of the reactor core onto a neutron producing target (12). Instead of the target (12), also neutron sources working without an accelerator can be mounted. The fuel pipe can also be spiral-like as indicated in (7c), which allows for a cylindrical vessel shape.

(5) The coolant in this preferred embodiment is liquid Lead, circulating at a temperature of around 1000 C. and at atmospheric pressure. Lead incoming from the heat exchanger (22) has a lower temperature. For this reason, the core volume (4) is surrounded by a good heat conducting separation wall (3a). Between this wall (3a) and the outer wall of the reactor core vessel (1) an additional volume, the reflector volume (3), is formed. The cold Lead incoming at (2) first moves down through the reflector volume (3) where it heats up by conduction from the inner vessel. There, it also serves as a neutron reflector reducing the loss of neutrons. At the bottom it now moves preheated into the core volume (4). When it moves up it takes the heat from the walls of the fuel duct (7) and leaves the inner vessel on top (5) at a higher temperature level.

(6) For this favored design a liquid salt fuel is used which circulates at a temperature of 1000 C. at normal pressure. The liquid fuel enters the core region (4) at the bottom inlet (8). Inside the core volume, the high neutron flux will cause an appropriate amount of nuclear fission events in the fuel's actinides. The released fission energy heats the fuel, which deposits its thermal power to the coolant via the fuel tube walls. The fission events will generate fast neutrons at the necessary rate to maintain the nuclear chain reaction inside the core volume. While moving slowly through the long tubes more and more actinides will be fissioned causing a chemical composition change of the liquid fuel at the outlet (9) where it is further directed into the pyrochemical processing unit (PPU) (28).

(7) Cooling Cycle and Fuel Cycle

(8) FIG. 3 shows the outer assembly, the fuel cycle and the coolant cycle.

(9) In the reactor core one primary pipe duct, also referred to as the fuel duct, includes two pumps (30, 34), a pyroprocessing unit (28), a pre-buffer volume (27), a post-buffer volume (29), an actively-cooled fuse plug (32), three sub-critical storage tanks (33) and two multi-way valves (31, 35) wherein the said primary duct enters the core via the inlet (8), is lead through the core volume (4) and leaves the core via the outlet (9). A secondary pipe duct, also referred to as the cooling duct or coolant loop, contains a pump (24), a multi-way valve (23), a coolant storage for temporary disposal (26) and a heat exchanger (22) wherein said secondary duct enters the core vessel (1) via at least one inlet (2), is lead through a reflector volume (3) between a heat-conducting barrier (3a) and the outer wall of the core vessel (1) and additionally through the core volume (4) bathing the fuel duct (7) and leaves the core vessel (1) via the outlet (5).

(10) The heated Lead enters the heat exchanger (22) after leaving the reactor. Depending on the power needs a part of the Lead's heat is taken for electricity production or as process heat. The Lead leaves the exchanger at a lower temperature and, after passing a multi-way valve (23), is being pumped back (24) to the reactor vessel. For maintenance the Lead coolant can also be drained through a valve (6) at the bottom of the reactor vessel into a temporary coolant storage (26) from where it can be pumped back into the reactor vessel passing the multi-way valve (23) from the bottom.

(11) A direct-contact heat exchanger is used as a heat exchanger (22) in a preferred variant of this invention. Here, the direct-contact heat exchanger replaces the combustion chamber of a gas turbine wherein the heat transfer is done by spraying the liquid coolant, favorably a liquid metal, especially Lead, in the gas jet from the turbo compressor in a drift dynamic optimized chamber. The coolant droplets are then removed by a directly connected centrifugal separator before the heated gas enters the turbine.

(12) In a preferred design, the coolant coming from the reactor is slowed down in a special volume to adapt the flow speed to the throughput of the injection pumps for a steady deposition into the intermix chamber. This chamber consists of an array nozzles which spray the high-pressured coolant from the injection pumps as sufficiently small droplets into the high-pressure gas jet. In the case of a power turndown, periodic nozzle sub-arrangements can be deactivated and the droplet size can be adjusted by variable cone drift pins inside the nozzles. So it is possible to maintain the separation efficiency of the centrifugal separator at a lower gas flux, because the droplet size is enlarged, too. The intermix chamber's cross section is congruent along its axial direction and is placed nearby the following separator. The liquid metal, especially Lead, may serve as a lubricant for the injection pump. Lead is preferred as a coolant in this prevention.

(13) The intermix chamber is followed by a separator where the metal droplets are removed from the heated gas. Because the gas moves with a high velocity at high pressures, separators based on the centrifugal principle like cyclone separators or vortex tubes are favorable. These can be arranged as cascades or multiple parallel units to achieve a high separation efficiency. The cleaned gas enters the turbine via nozzles, which provide the thermo-mechanical conversion.

(14) The separator cascade is arranged in a way that a separator with a large diameter is followed by several ones with a smaller diameter, thus having a higher centrifugal force. Therefore, the gas flow is spread to a suitable number of smaller cyclones to remove droplets of decreasing size.

(15) The separated liquid metal from the cyclones is removed by a gearwheel lock, which also transforms the turbine's high pressure to the liquid coolant's ambient pressure, and is deposited in a intermediate storage. From there, the liquid metal is pumped back into the reactor core. Because of the compact arrangement, this storage can be placed directly next to the storage volume for the injection pumps or one bigger storage with a removable barrier to create the two sub-storages can be used. In the case of emergency the barrier can be opened (even manually if the electricity supply is offline) which allows natural convection cooling as a pure liquid metal coolant loop.

(16) The aforementioned heat exchanging processes with the droplets, the removal and deposition of the metal into the storage will even work when the metal droplets are cooled below the melting point, solidifying them. For that, the metal inside the storage must be reheated, e.g. heating by indirect bypassing the hot liquid metal, to liquify the metal particles.

(17) The gas has to be sufficient chemically inert against the liquid metal to prevent chemically stable compounds between them. Nitrogen gas would fulfill this for many metals. However, gas turbines use the Joule-Brayton thermodynamic cycle where single-atomic gases provide efficiency advantages due to the isentropic exponent because of missing molecule's degrees of freedom, which then would absorb energy. For this reason Helium is used in high-temperature gas-cooled reactors wherein Helium also has favorable neutronic properties. The Helium's heat transfer capability is twice as high that of Argon which reduces the needed heat exchanging area. In an indirect-contact heat exchanger which has the appropriate large size and weight the material cost are so high that the usage of Helium would be worthwhile compared to Argon. This point is irrelevant for the said direct-contact heat exchanger proposed in this invention. Furthermore, Argon is significantly cheaper than Helium. Both, the reduced material costs of the heat exchanger and working gas costs of the cheaper Argon reduces the overall costs considerably. In the proposed DFR design using Lead coolant the usage of Lead combined with Argon is preferable.

(18) Usually a separation before the turbine is not fully possible. Modern gas turbines already consist of very resistant materials and are able to handle sulfuric acids and dust particles. The latest developments for increasing the efficiency aim for an operation with direct coal dust firing where the turbines have to handle large amounts of ash. Compared with this, Lead droplets are less problematic, in particular as the gas temperature even behind the turbine is still above the melting point of Lead. Lead adhering on the blades of the rotor and stator would certainly produce an unbalance leading to vibrations of the rotor blades w.r.t. the gas flow which shakes off the droplets. This is particularly true for the side of the turbo compressor. The Lead droplets remaining in the gas flow would freeze out latest in the residual heat exchanger where the working gas dumps its residual heat. Since the Lead has a high heat conductivity, the function of the residual heat exchanger would not be influenced by this process, except for a slow fill-in of Lead which requires regular maintenance. In order to stretch those intervals, it is advantageous to install separators effectuating the already strongly delayed gas flow short before, or in constructional combination with, the residual heat exchanger, like lamella separators, impact separators, receiver separators, and demisters.

(19) The liquid fuel leaving (9) the reactor core is first collected in a pre-buffer (27). From there an amount that can be reprocessed is branched into the PPU (28). The amounts reprocessed there are collected in the post-buffer (29) from where they are pumped (30) through a multi-way valve (31) and through the inlet at the bottom of the reactor core (8) back into the core volume (4). The purpose of the buffers (27) (29) is to temporarily compensate for different throughputs in the reactor and the PPU (28); for the some purpose the sub-critical storage tanks (33) can also be included in the cycle. This is especially necessary if a batch processing technique like electrorefining is employed. The pre-buffer (27) may also be used to purge from the noble gases.

(20) In ADS mode the fuel mixture is retained just below criticality by the PPU (28) so that merely a few per mille of the total neutron flux must be provided by the accelerator neutron source in order to lift the reactor into criticality. In such a way a small accelerator is sufficient instead of a high-energy accelerator with a spallation source.

(21) For maintenance or in case of an emergency a sub-critical fuel storage (33) is provided. It comprises several tanks each of which has a capacity of only a deep sub-critical mass of the molten-salt. The tanks can be filled either through the melting fuse plug at the bottom of the reactor vessel (32) or through the multi-way valve (31) from the post-buffer (29) and the pump.

(22) The actively-cooled melting fuse plug (32) can also be used for a regular shutdown of the system, as it was used in the MSRE at the Oak-Ridge laboratory. It is essentially a pipe segment, which is cooled with a constant heat transportation. Because of the non-negligible heat conduction capability of the molten-salt fuel the heat produced in the core volume (4) is also conducted into the melting fuse plug (32). The constant heat deposit is balanced in a manner that the salt is yet not melting if it has a temperature of 1000 C. in the core. For higher core temperatures or if powered off the heat conducted through the salt will melt the plug, which opens, and drains the fuel to the sub-critical tanks (33). From there it can be pumped up (34), entering the fuel loop again through the multi-way valve (35), either to the pre-buffer (27) or to the post-buffer (29).

(23) The fuel is a combination of a fertile and a fissile actinide salt which can be uranium-238/plutonium-239 or thorium-232/uranium-233. When the uranium-plutonium fuel cycle is utilized the reactor requires an initial load of plutonium (alternatively highly enriched U-235 may be utilized if no Pu is available). The fraction of plutonium depends on the size of the reactor core because of neutron losses through the surface. The maximum is a Pu-239 fraction of 35% required for the smallest useful set-up while larger cores can manage smaller fractions. The other fraction is U-238 as fertile material. The fuel salt would here consist of the trichlorides of the actinides, i.e. UC13 and PuC13, which have a suitable temperature range of the liquid state. Purified C1-37 should be used in order to avoid neutron losses by capture at C1-35 and production of the long-lived radioactive isotope C1-36.

(24) Negative Temperature Coefficient

(25) The PPU (28) fabricates a fuel mixture that is critical inside the reactor at the desired operating temperature of 1000 C. There are three main effects that provide negative feedback to the fission reaction rate that reduces the neutron flux when the temperature rises: Doppler broadening of the resonances in the neutron capture cross sections increases the macroscopic neutron capture cross section. Density decrease of the molten-salt fuel reduces the fissile nuclei concentration. Density decrease of the liquid Lead reduces the concentration of the neutron reflecting Lead nuclei.

(26) Because of its high atomic mass and its many stable isotopes due to nuclear shell closure, Lead is an excellent neutron reflector with low moderation qualities and low neutron capture cross sections. These effects together cause a strong negative temperature coefficient in the fast neutron spectrum. This is in contrast to liquid sodium as coolant which has a much higher neutron capture cross section, higher neutron moderation and lower reflection quality which means an increase of the neutron flux with raising temperature, i.e. positive temperature coefficient. A further consequence is that the low radioactivity of Lead renders an intermediary cooling loop superfluous different to sodium.

(27) Startup Procedure

(28) To start up the reactor, the system is pre-heated until the coolant and fuel salt becomes liquid. Concurrently, the cooling of the melting fuse plug (32) is started. The fuel salt is pumped from the storage tanks (33) to the reactor core volume (4). At the tee connector just below the reactor some of the fuel fluid branches to the melting fuse (32) where it freezes out and plugs it. Inside the reactor core volume (4) the fuel becomes critical.

(29) Now, the reactor is regulated by the earlier described physical control loops. At the beginning the fission rate, and correspondingly the power production, is minimal. Then, the coolant pump (24) starts to accelerate the circulation of the Lead. The discharge of heat to the heat exchanger (22) causes a temperature decrease in the reactor (of course the heat exchanger must be able to dump the heat energy). The control loops render the reactor supercritical until the nominal temperature is regained and well-balanced. This may continue until the nominal power output is reached. Conversely, if the Lead circulation speed is decelerated (also in case of a malfunction) the temperature in the reactor increases and it becomes sub-critical until leveled off at the nominal temperature. In this way, the fission rate in the reactor always follows the power extraction.

(30) This equilibrium temperature (operating temperature) will be set and controlled by regulating the fissile inventory fraction (Pu fraction) of the fuel salt. The PPU (28) cares for an appropriate mixture of the fuel.

(31) Shutdown of the Reactor

(32) For a regular shutdown, the coolant circulation and the fuse (32) cooling is stopped, so that the liquid fuel drains onto the sub-critical tanks (33). The same happens if the power to the plant's aggregates fails. If the PPU, for any reason like malfunction and/or sabotage, should mix in too high fractions of fissile material, the equilibrium temperature raises, too, so that again the melting fuse plug kicks in.

(33) Consequently, the emergency shut down is the same as the regular shut down.

(34) Possible Accidents

(35) The PPU (28) continuously removes the fission products from the fuel salt and replaces them with fertile material, e.g. U-238. The residual decay heat of the few fission products in a core load can easily passively be dissipated from the storage tanks (33). In summary, for all known typical dangerous reactor accidents like loss of power accident, loss of coolant accident, criticality accident, or decay heat the DFR behaves well mannered as for a regular shutdown.

(36) Application of the Neutron Surplus

(37) With the uranium-plutonium cycle, the fission of plutonium produces a high neutron yield. Even after regeneration of the Pu-239 by conversion of U-238 a large neutron surplus remains. If only U-238 is fed into the fuel, this neutron surplus will end up as additional plutonium. The conversion rate is now greater than 1 and the reactor works in breeder mode.

(38) The neutron surplus can also be used for other transmutation purposes, such as when long-lived fission products are specifically mixed in the fuel salt by the PPU (28). There is still a considerable neutron surplus when the reactor transmutes its own long-lived fission products, which can be used to transmute fission products from used fuel elements of other nuclear reactors. Only if the neutron surplus is consumed ulterior the reactor works as a self-burner, i.e. conversion rate equal to 1.

(39) Alternatively the PPU (28) can mix in thorium or inert materials to even out the neutron surplus.

(40) The fission neutron yield of U-233 from the thorium-uranium fuel cycle is considerably lower than for the Pu-239 fission. It is possible to operate the DFR as a fast neutron Th-U breeder with a conversion rate slightly larger than 1. The transmutation of the own long-lived fission products may be feasible. For that, the PPU (28) needs to separate out and store the Pa-233 until it has decayed to U-233. The PPU can frame the transition from the U-Pu to the Th-U fuel cycle continuously.

(41) The fissile material in the fuel salt may also contain transuranium elements from used nuclear fuel elements. As in the case of fission product transmutation, the PPU (28) would process chlorine salts made of the fuel pellets of used fuel elements separating the chemical elements by boiling points. Then the PPU (28) mixes the fuel salt from the desired actinides so that the criticality condition in the core is maintained. In this way the sources of fuel are natural uranium, depleted uranium, nuclear waste, and thorium.

(42) Further DFR Design Variants

(43) The main reasons for Lead as coolant selection are low moderation as well as low neutron capture cross sections, high neutron reflection capability and good thermal conductivity. Other materials like tin or complex alloys may reduce structural material corrosion but may have worse thermal or neutronic properties, requiring to find an optimum here.

(44) Using a coolant with lower nucleus mass numbers, such as Lithium, and a moderating reflector in the DFR results in a softer, thermal or epithermal, neutron spectrum. This makes a smaller variant of the DFR with less power for mobility applications possible, causing a worse neutron economy and reducing the conversion ratio less than 1 so this variant looses its transmutation capability.

(45) The notation fuel cycle also means open fuel cycle where the fuel is stored in sub-critical storage tanks (33) after once-through usage in the core volume (4). The fuel then can be processed offline or offsite. This variant offers advantages for mobility applications, too, since it is not so sensitive to vibrancy. Inside the reflector volume (3), where the neutron spectrum is softer, additional ducts can be inserted in which transmutable but non-fissionable materials can be transported. These can be separated products from the PPU (28) or inserted independently from elsewhere. In the reflector zone the transmutation rate for some materials could be significantly higher than inside the core volume (4) due to resonant neutron capture.

(46) The liquid fuel can also be a liquid metal alloy whose melting point is below the operation temperature. Due to the higher heat transfer capability and lower corrosion capability compared to a liquid salt the power density as well as the operating temperature can be further increased thus obtaining the maximum economic efficiency of the DFR concept. Because many actinides have a too high melting point, additional metals with sufficient neutronic properties and low melting points must be added to lower the solidus temperature. This composition does not necessarily need to be eutectic. Even if the liquidus temperature is higher than the operating temperature the fuel remains sufficiently pumpable. Suitable metal additives are Lead, Bismuth, and, if necessary, Tin, up to a fraction of 75 mol-%. Higher fuel processing efforts are the main drawback. Two additional processing steps are needed: the conversion of metals into molten metal chlorides and reconversion of the separated chlorides into metals, e.g. via electrolysis. It is also possible to apply pyrochemical separation techniques on the metal alloy fuel and one just has to work only the fuel components further, which cannot be processed that way. Lead, Bismuth and other materials with low boiling points can thus be separated from the fuel via distillation and the remaining slurry must be processed as a metallic salt.

FIGURE CAPTIONS

(47) FIG. 1: reactor core of the DFR FIG. 2: Possible shapes of the fuel duct FIG. 3: Overall schematics of the DFR

LEGEND

(48) (1) Reactor vessel (2) Lead inlet (3) Reflector volume (3a) Lead separation barrier (4) Core volume (5) Lead outlet (6) Lead drain valve (7) Fuel duct (7a) Normal view of the fuel duct (7b) Beam view of the fuel duct (7c) Spiral variant of the fuel duct (8) Fuel inlet (9) Fuel outlet (10) Particle beam (11) Particle beam direction (12) Neutron-generating target or source (22) Heat exchanger (23) Lead valve (24) Lead pump (26) Lead storage tank (27) Fuel pre-buffer volume (28) Pyrochemical processing unit (PPU) (29) Fuel post-buffer volume (30) Fuel pump (31) Fuel inlet valve (32) Fuse plug (33) Sub-critical storage tanks (34) Fuel redirection pump (35) Valve for fuel redirection