GB2468846A - A magneto-plasma-dynamic nuclear fusion reactor - Google Patents

Abstract

A magneto plasma-dynamic (MPD) generator having: a supplier operable to supply a gaseous nuclear fusion fuel 104, such as a deuterium-tritium (D-T) mixture, at velocity; a duct 100 arranged to convey this operating gas; a magnetic field generator arranged to generate a magnetic field 102 across the duct 100 and the direction of travel of the operating gas 104; and electrodes 108 & 110 arranged in the duct 100 to conduct electrical current 106 induced in the gaseous nuclear fusion fuel as it is conveyed by the duct 100 through the magnetic field 102. By short circuiting the electrodes 108 & 110, an electrical current is generated through the fuel in the duct 100 sufficient enough to bring about heating and compression of the fuel (by means of a magnetic pinch) to the point of nuclear fusion of the fuel.

Description

NUCLEAR FUSION

The present invention relates to methods of producing nuclear fusion and to nuclear fusion reactors.

In the prior art, extensive efforts have been made to produce nuclear fusion in Tokamak (torus) fusion reactors.

In a Tokamak reactor a gaseous mixture of fusion fuel is contained in a toridal vacuum vessel. A transformer iron core passes through the hole in the torus. An AC winding on the transformer core induces an electric current through the gaseous mixture. This electric current heats the mixture and the magnetic "pinch" effect compresses the mixture, increasing its density. The increased density and raised temperature produces fusion.

A Tokamak does not generate electrical power itself.

Rather, it needs some form of heat exchanger in the walls of vacuum vessel to extract heat to raise steam to generate electric power. Such a heat exchanger needs careful design to avoid eddy current losses.

Tokamak fusion reactors require pulses of AC power of several hundreds of megawatts to produce electric currents large enough to heat the gaseous fuel and produce magnetic forces that compress the gaseous fuel to produce fusion.

The pulsed power required presents great engineering challenges and connection to a high voltage transmission system of at least 400 000 V is required. To date, no reactor has produced more power than it consumes.

The present invention provides a departure from the prior art Tokamak approach to the production of nuclear fusion.

MPD (magneto-plasma-dynamic) generators are known in

the prior art. As background information, Figure 1

schematically illustrates the principle of a Faraday NPD (magneto-plasma-dynamic) generator. An ionised operating gas 20 is passed at velocity through a magnetic field 22 perpendicular to the velocity. A voltage (MPD voltage 24) is developed perpendicular to the magnetic field and velocity according to Fleming's right hand rule. Current (MPD current 26) is collected by positive and negative MPD electrodes 28, 30, termed anode and cathode and labelled + and -respectively.

As well as the Faraday MPD generator illustrated in Figure 1, other designs are possible such as the Hall effect MPD generator and the disc MPD generator. These other designs work on similar principles to the illustrated Faraday MPD generator.

After generation of MPD power (for example MPD voltage 24, MPD current 26 passing through an electrical load 32 in Fig. 1), the exhaust gas is usually still hot enough to be used in other processes, typically to raise steam for conventional turbo-generators. MPD has therefore been regarded as a "topping cycle" which can increase the thermal efficiency of electric power generation by using the temperature difference between the temperature of combustion and the temperature that most engineering materials can withstand. MPD is sometimes termed "magneto-hydro-dynamics (MHD) but strictly speaking MHD refers to the interaction of liquid, rather than gas or plasma, with magnetic fields. References which deal with MPD/MHD power generation include: [1] . The future for MHD power generation L.H.T.

Rietjens 1979 Physics in Technology 10 216-21.

[2] . Stanqeby (1974) : A review of the status of MHD power generation technology including suggestions for a Canadian MHD research programme, Institute for Aerospace Studies, University of Toronto, UTIAS Review No.39.

The present invention now provides a new departure in MPD technology.

The present invention provides a method of operating a magneto-plasma-dynamic generator having a supplier operable to supply operating gas at velocity, a duct operable to convey the operating gas,

a magnetic field generator arranged to generate a

magnetic field across the duct, across the direction of travel of the operating gas, such that the operating gas passes through the magnetic field when conveyed by the duct, and electrodes arranged in the duct to be operable to conduct electrical current induced in the operating gas as it is conveyed by the duct through the magnetic field, the method comprising supplying to the magneto-plasma-dynamic generator as operating gas a gaseous nuclear fusion fuel and in operation generating electrical current through the fuel in the duct to bring about heating and compression of the fuel to the point of nuclear fusion of the fuel. The present invention also provides a corresponding nuclear reactor for generating nuclear fusion in a magneto-plasma-dynamic generator.

Thus, the present invention uses an MPD generator to produce large currents in a gaseous fusion fuel. These currents raise the temperature and pressure of the gaseous fusion fuel to produce nuclear fusion. The large currents not only heat the fuel but also compress it due to the magnetic pinch effect. The MPD duct becomes a fusion generator.

The gaseous fusion fuel may firstly be compressed, for instance in a conventional gas compressor of a design known in aircraft engines and the like. The compressed gaseous fusion fuel is then passed through the MPD duct at high pressure and velocity to generate voltage between the electrodes.

In embodiments of the invention electrical current through the fuel, to bring about heating and compression of the fuel to the point of nuclear fusion of the fuel, may be generated by short-circuiting the electrodes.

The electrodes may be of a material which, when exposed to nuclear fusion of the fuel, breeds a component of the fuel.

The electrodes may comprise lithium and the fuel may be a deuterium-tritium mixture, the lithium breeding tritium when exposed to nuclear fusion of the deuterium-tritium mixture.

The electrodes may be of a material which contains a component of the fuel.

The electrodes may comprise lithium deuteride, the fuel being a deuterium-tritium mixture.

The electrodes may be of liquid metal or extruded solidified liquid metal, such as lithium, or may be of liquid or molten metal salt, such as lithium hydride or lithium deuteride.

Gas exhausted from the duct may be admitted to a turbine, to expand through the turbine which is arranged to drive an electrical generator.

The turbine may be arranged to drive a compressor operable to re-compress exhausted gas expanded through the turbine, for re-supply to the duct.

Electrical current electrical current induced in the operating gas in the duct may be supplied via the electrodes to an external circuit.

Reference is made to the accompanying drawings which show:-Figure 1: a schematic diagram of a Faraday MPD generator, Figure 2: a schematic illustration of an MPD duct in accordance with an embodiment of the present invention, using liquid metal electrodes, in a view into the MPD duct, Figure 3: a schematic plan view of the lower surface of the MPD duct of Figure 2, Figure 4: a schematic side view of the MPD duct of Figure 2, Figure 5: a schematic illustration of the use of a connection pipe structure with the MPD duct of Figure 2, and Figure 6: a schematic illustration of a self-breeding MPD fusion reactor in accordance with an embodiment of the present invention.

Figure 2 schematically illustrates an MPD fusion generator in accordance with an embodiment of the present invention. The generator comprises a duct -MPD duct 100 -through which, in operation, a vertical (in the drawing) magnetic field 102 passes and a fast-flowing stream of operating gas 104 flows horizontally (into the plane of the drawing) . An MPD voltage is developed perpendicular to both the gas flow and the magnetic field, that is, horizontally also. This voltage passes an MPD current 106 between electrodes 108 and 110, known as anode and cathode according to their polarity.

Recognising the need for good electrical insulation at high temperatures, the MPD duct 100 is made of refractory electrically insulating material, for example fused silica, fused alumina, fireclay or concrete, in order to prevent a short-circuit of the MPD voltage.

The operating gas 104 is a gaseous fusion fuel such as a deuterium-tritium (DT) mixture. As mentioned above, the MPD current 106 is a large current which raises the temperature and pressure of the gaseous fusion fuel to produce nuclear fusion.

In the illustrated embodiment the electrodes 108, 110 are liquid metal electrodes set in wells at the bottom of a channel 112 with a sloping base. Electrical connections are made to each electrode.

In general, MPD generators suffer from erosion of the electrodes due to the fast flowing stream of gas used, which may contain abrasive contaminants, and high temperatures. Erosion can be managed by replenishing solid electrodes in a similar way to carbon arc lamps which had carbon rod electrodes that were driven by electric motors feeding in fresh carbon electrodes. However, this is mechanically complex. Liquid electrodes, for example liquid metal electrodes, can offer a simplification of the mechanical complexity and supply of the electrode material.

Thus, preferred embodiments of the invention use liquid metal electrodes 108, 110 to make contact with the gaseous fusion fuel 104 in the MPD duct 100. The main advantage of liquid metal electrodes is that the electrode material can easily be replaced if the electrodes are consumed by erosion, since liquids can be pumped. Thus, in operation, the liquid metal electrodes 108, 110 are supplied with liquid metal to make good loss caused by evaporation and erosion.

Another advantage is that liquid metal electrodes offer a convenient way of "seeding" the gaseous fusion fuel in the duct with metal ions with suitable electrical properties to promote ionization and current flow.

The use of gaseous or vapour electrodes may also be contemplated in embodiments of the present invention.

Figure 3 shows a schematic view looking down on the bottom of the MPD channel 112. Fig. 4 shows a schematic side view of the MPD duct 100.

As mentioned above, the electrodes 108, 110 may be short-circuited to produce a large current through the gaseous fusion fuel 104, to bring about fusion in the fusion fuel. The electrodes may be short-circuited either by a suitable switch or circuit breaker connected to the terminals of the electrodes. Alternatively, a connection pipe 200 containing liquid metal could be used to connect the metal electrodes 108, 110 as shown in Figure 5. The connection pipe 200 has a valve 202 made from electrically insulating material so that closing the valve breaks the electrical connection between the electrodes and opening the valve makes the electrical connection.

The use of a connection pipe means that both electrodes may be supplied with liquid metal from a common source.

The fusion reactor of the embodiment illustrated in Fig. 2 is based upon an MPD duct 100 with liquid metal electrodes 108, 110 located at the base of the duct.

However, alternative embodiments could use liquid metal electrodes located differently, for example in vertical walls of an MPD duct. The use of liquid metal electrodes in the vertical walls of the MPD duct can allow orientations of the MPD duct that have a flow of gas in a direction different to the horizontal, for example a vertical flow of gas (with suitable direction of the magnetic field) . The metal electrodes in the vertical walls of the MPD duct could also be connected together by a liquid metal connection, as described in Figure 5.

The use of liquid metal electrodes may limit the location and deployment of the electrodes, in consideration for example of avoiding leakage of the liquid metal. These restrictions may be overcome in alternative embodiments which use, in place of liquid electrodes, solid extruded electrodes. For example, to provide solid extruded electrodes, liquid electrode metal is cooled and solidified before it enters the MPD duct. The metal can be extruded as a solid into the MPD duct using known extrusion techniques similar to those used to extrude sectored solid aluminium conductor for electric cables. This may add to mechanical complexity but may also offer design freedom in other respects. In fact, the extrusion of a solid alkali metal such as lithium is easier than aluminium since the alkali metal is more ductile than aluminium.

Extruded metal electrodes could also be connected together by a liquid metal connection, as described in Figure 5.

In a preferred embodiment of the invention the metal electrodes are lithium electrodes, for example liquid lithium electrodes. As will be understood from the description below, using lithium offers the opportunity to breed tritium from reactions between the lithium and neutrons produced by the nuclear fusion.

Figure 6 illustrates a self-breeding MPD reactor using an MPD duct 100 in accordance with an embodiment of the present invention.

The operating gas is a fusion fuel such as suitable isotopes of hydrogen or helium, for example a DT mixture as mentioned above. The preferred liquid/extruded electrode metal is lithium, which allows the breeding of nuclear fuel. For reasons of clarity and simplicity, means of electrically breaking down the gas in the MPD duct and establishing a short circuit between the anode and cathode are not shown in Figure 6.

A turbine 300 and a compressor 302 are mounted on the same shaft 304 and are coupled to an electric generator 306 which is designed so that it may also operate as a motor.

To start the MPD fusion reactor, electric power is supplied to the generator 306 so that it acts as a motor, driving the compressor 302 and compressing the fusion fuel gas. The gaseous fusion fuel enters a nozzle 308 where it is expanded to form a fast flowing stream of gas 104 at low pressure that passes through the MPD duct 100. A magnetic field 102 is applied across the MPD duct 100 and an MPD current is established between the liquid/extruded lithium electrodes 108, 110. The fast flowing gas leaving the MPD duct 100 is slowed down in another nozzle 318 before being admitted to the turbine 300 to generate mechanical power which offsets some of the power required to operate the compressor 302.

At this stage, no net electric power is generated, in fact, energy has to be supplied to the system to establish the MPD current. After expansion through the turbine 300, the gas is cooled in a condenser heat-exchanger 320 and returned to the compressor 302. The condenser heat-exchanger 320 may provide cooling by means of a cold water coil (CW in, CW out) and allow gas to be added or removed as needed, and also allow make-up of lithium if needed.

Lithium in the gas passed to the condenser is condensed from the gas and returned to the liquid/extruded metal electrodes 108, 110 via a lithium storage tank 322 (possibly using a pump 324) . This tank 322 supplies liquid lithium for the MPD duct electrodes 108, 110 by gravity.

Gravity flow may be used to avoid pumping the liquid lithium. This simplifies the flow management by avoiding earthing and short circuits. Gravity flow electrical isolators may be used to prevent the liquid lithium storage tank from electrically short circuiting the electrodes in the MPD duct 100. A gravity flow electrical isolator may, for example, use any known technique that breaks up a liquid flow into drops that are surrounded by insulating fluid that does not react with the liquid metal, for example an inert gas such as argon or an electrically insulating liquid such as mineral oil used in electrical transformers or electrical switchgear.

Once an MPD current has been established, the pressure of gaseous fusion fuel is raised by admitting more fuel into the cycle through a suitable valve or valves and increasing the power delivered by the compressor 302. The objective of this procedure is to increase the MPD current and the gas density through which the current flows in order to achieve thermonuclear fusion. The energy generated by the fusion imparts more energy into the fast flowing stream of gas and this extra energy is extracted by the turbine 300 to produce an overall net output of electric power.

One example fusion reaction which can be put to use in an embodiment of the invention is that between deuterium and tritium. This has one of the lower activation energies of fusion reactions. Deuterium, 2H, and tritium, 3H, are isotopes of hydrogen and the reaction yields 17.59 MeV per atom of deuterium and tritium: + 2H 4He + n. 17.59 MeV.

Deuterium may be obtained from sea-water. Tritium is not naturally occurring as it is radioactive with a half-life of about 12 years: short in geological terms. However, tritium may be bred from lithium using neutrons as follows: Lithium 6 absorbs slow neutrons: 6Li + n -4He + 3H. 4.8 MeV.

Lithium 7 absorbs fast neutrons: 7Li + n (fast) 4He + 3H + n (slow) . Endothermic.

Lithium 6 comprises up to 7.4% of natural lithium.

The self-breeding NPD fusion reactor uses lithium electrodes so that tritium fuel can be bred from neutrons produced by the nuclear fusion reaction and the lithium vapour that will be released from the lithium electrodes.

The self-breeding MPD fusion reactor may be operated in either if two ways.

The MPD duct may be used as a fusion reactor and an electrical generator. In this case the MPD duct produces heat from the fusion reactions taking place in the MPD current, and also supplies the MPD current via the electrodes to an external circuit, generating electric power. In this case, fusion reactions may be initiated by short-circuiting the electrodes to generate a large MPD current whereafter short-circuiting is discontinued to allow current supply from the electrodes to the external circuit, whilst maintaining fusion reactions in the MPD duct.

The MPD duct may be used as a fusion reactor alone. In this case, once an MPD current is established, the electrodes are short-circuited to generate a large MPD current suitable for achieving nuclear fusion. No attempt is made to supply an MPD current to an external electrical system.

A pulsed mode of operation may be adopted, in which the electrodes are periodically short-circuited to generate large MPD currents suitable for achieving nuclear fusion.

The turbine-compressor operates with the fusion reaction providing heat for its driving. For example the turbine-compressor may operate on the Brayton cycle.

In the embodiments described above, use is made of liquid metal electrodes or electrodes extruded from cooled liquid metal. However, in other embodiments of the invention, molten salts may be used instead, since they also conduct electricity. For example, lithium hydride (LiH) may be used, or better still, lithium deuteride which is similar to lithium hydride but contains deuterium and so offers a way of fuelling the reactor. Fluxes could be used to lower the melting point of the lithium deuteride.

The present invention preferably uses a short-circuited MPD duct as a fusion reactor. In embodiments of the invention, an MPD generator may be employed, in addition to the MPD duct used as a fusion reactor. The MPD generator may be placed downstream of the MPD duct to generate electric power from the hot fast flowing gas before that gas enters the turbine. The MPD generator would be used as a topping cycle to improve the overall efficiency of generation of electric power.

The present invention can offer advantages, for example over prior Tokamak (torus) fusion reactor configurations: - 1. Less electrical pulsed power requirement. In fact the power supplied to initiate the fusion reaction in an embodiment of the present invention having a compressor/turbine arrangement as illustrated in Figure 6 could be almost entirely mechanical, for example using a starter motor to turn over the turbine compressor. Some electrical power would be needed to provide the magnetic field, and also for pumps and sundry electrical loads.

2. Easier method of generating electric power. As mentioned above, a Tokamak would not generate electrical power itself. Instead a Tokamak would need some form of heat exchanger in the walls of vacuum vessel to extract heat to raise steam to generate electric power. Such a heat exchanger would need careful design to avoid eddy current losses. By contrast, the heat produced by the fusion reaction in the MPD duct of an embodiment of the present invention is coupled directly into the working fluid of the turbine.

3. In an embodiment of the present invention using lithium electrodes and, for example, a deuterium-tritium fusion fuel, intimate mixing of the lithium with the fusion reaction offers good opportunity for neutron capture by the lithium and consequent breeding of tritium.

Claims (15)

1. Claims 1. A method of operating a magneto-plasma-dynamic generator having a supplier operable to supply operating gas at velocity, a duct (100) operable to convey the operating gas,a magnetic field generator arranged to generate amagnetic field (102) across the duct (100), across the direction of travel of the operating gas, such that the operating gas passes through the magnetic field (102) when conveyed through the duct (100), and electrodes (108, 110) arranged in the duct (100) to be operable to conduct electrical current (106) induced in the operating gas as it is conveyed through the duct (100)through the magnetic field (102),the method comprising supplying to the magneto-plasma-dynamic generator as operating gas a gaseous nuclear fusion fuel and in operation generating electrical current through the fuel in the duct (100) to bring about heating and compression of the fuel to the point of nuclear fusion of the fuel.
2. 2. A method as claimed in claim 1, wherein electrical current through the fuel in the duct (100), to bring about heating and compression of the fuel to the point of nuclear fusion of the fuel, is generated by short-circuiting the electrodes (108, 110)
3. 3. A method as claimed in claim 1 or 2, wherein gas exhausted from the duct (100) is admitted to a turbine (300), to expand through the turbine which is arranged to drive an electrical generator (306)
4. 4. A method as claimed in claim 3, wherein the turbine (300) is arranged to drive a compressor (302) operable to re-compress gas exhausted from the duct and expanded through the turbine, for re-supply to the duct (100)
5. 5. A method as claimed in any preceding claim, wherein electrical current (106) induced in the operating gas in the duct (100) is supplied via the electrodes (108, 110) to an external circuit.
6. 6. A nuclear reactor comprising a magneto-plasma-dynamic generator having a supplier operable to supply operating gas at velocity, a duct (100) operable to convey the operating gas,a magnetic field generator arranged to generate amagnetic field (102) across the duct (100), across the direction of travel of the operating gas, such that the operating gas passes through the magnetic field (102) when conveyed by the duct (100), and electrodes (108, 110) arranged in the duct (100) to be operable to conduct electrical current (106) induced in the operating gas as it is conveyed by the duct (100) throughthe magnetic field (102),wherein the supplier is operable to supply to the magneto-plasma-dynamic generator as operating gas a gaseous nuclear fusion fuel, the magneto-plasma-dynamic generator generating in operation electrical current through the fuel to bring about heating and compression of the fuel to the point of nuclear fusion of the fuel.
7. 7. A nuclear reactor as claimed in claim 6, comprising a short-circuiting arrangement operable to short-circuit the electrodes (108, 110) to generate electrical current through the fuel, to bring about heating and compression of the fuel to the point of nuclear fusion of the fuel.
8. 8. A nuclear reactor as claimed in claims 6 or 7, further comprising a turbine (300) arranged for admitting gas exhausted from the duct (100), to expand through the turbine, and further comprising electrical generator (306) arranged for driving by the turbine.
9. 9. A nuclear reactor as claimed in claim 8, further comprising a compressor (302) arranged for driving by the turbine (300) and operable to re-compress exhausted gas expanded through the turbine, for re-supply to the duct (100).
10. 10. A nuclear reactor as claimed in any of claims 6 to 9, further comprising an electrical circuit to which electrical current electrical current (106) induced in the operating gas is supplied via the electrodes (108, 110)
11. 11. A method or a nuclear reactor, as the case may be, as claimed in any preceding claim, wherein the electrodes (108, 110) are of a material which, when exposed to nuclear fusion of the fuel, breeds a component of the fuel.
12. 12. A method or a nuclear reactor, as the case may be, as claimed in claim 11, wherein the electrodes (108, 110) comprise lithium and the fuel is a deuterium-tritium mixture, the lithium breeding tritium when exposed to nuclear fusion of the deuterium-tritium mixture.
13. 13. A method or a nuclear reactor, as the case may be, as claimed in any preceding claim, wherein the electrodes (108, 110) are of a material which contains a component of the fuel.
14. 14. A method or a nuclear reactor, as the case may be, as claimed in claim 12, wherein the electrodes comprise lithium deuteride and the fuel is a deuterium-tritium mixture.
15. 15. A method or a nuclear reactor, as the case may be, as claimed in any preceding claim, wherein the electrodes (108, 110) are of liquid metal or extruded solidified liquid metal, such as lithium, or are of liquid or molten metal salt, such as lithium hydride or lithium deuteride.