Improvements in and relating to nuclear fusion irradiation

Abstract

Apparatus for irradiating a target, and method of operating thereof comprises a nuclear fusion reactor and a target holder wherein the target holder is configured to hold a liquid target. in a preferr

Description

Field of the Invention The present invention concerns apparatus and methods for subjecting a target composition to nuclear fusion irradiation. More particularly, but not exclusively, this invention concerns apparatus and methods for exposing a target composition provided as a liquid to radiation produced from a nuclear fusion reactor. The invention also concerns apparatus and methods in which a target holder at least partially encloses a reactor vessel of a nuclear fusion reactor. The invention also concerns apparatus and methods for circulating a target composition through at least part of a target holder exposed to radiation produced from a nuclear fusion reactor. Background of the Invention Target compositions are subjected to particle irradiation for a number of applications, including the production of radionuclides and the treatment of radioactive waste. Known particle accelerators produce a focused beam to irradiate a target composition, which may be provided in a solid, liquid or gaseous form, depending on the particular requirements of the apparatus used. US2022 / 0108812A1 discloses a moderated nuclear reactor, which uses a target holder in the form of an ampoule. The irradiation method requires use of a highly complex, centralised nuclear reactor facility, limiting the availability of isotope products. Such nuclear reactors are complex, expensive to run, require fissile materials, produce nuclear waste, take a decade or more to build, and can only be constructed on very specific sites. Yet further, many of the existing reactors are scheduled for decommissioning over the next decade. WO2020 / 210147A1, WO2004079751A2 and WO9963550A1 disclose cyclotron-based processes, in which a discrete target is irradiated. A flowing liquid target for a cyclotron is discussed in Large Scale Production of 123I from a Flowing Liquid Target Using the (p, 5n) Reaction, J. G. CUNINGHAME, B. MORRIS, A. L. NICHOLS and N. K. TAYLOR AERE, International Journal of Applied Radiation and Isotopes, 1976, 27. pp. 597-603. The apparatus disclosed uses a target assembly which circulates a liquid target through a collimated proton beam of a cyclotron. A stacked-foil target also used with a cyclotron is disclosed in Excitation Functions of Deuteron Induced Reactions on 123Te: Relevance to the Production of 123I and 1241 at Low and Medium Sized Cyclotrons B. SCHOLTEN, S. TAKACS, Z. KOVACS, F. TARKANYI and S. M. QAIM, Appl. Radiat. Isot. Vol. 48, No. 2, pp. 267-271. 1997. Gas, liquid and solid targets for production of radionuclides with cyclotron targetry are discussed in Cyclotron Produced Radionuclides: Physical Characteristics and Production Methods, IAEA (International Atomic Energy Agency), Technical Reports Series No. 468. The production of high energy protons in an accelerator and their direction into an 18O enriched liquid water target is discussed in High-Powered Cyclotron Recirculating Targets for Production of the 18F Radionuclide, Jon C. Clark, A dissertation submitted to the Graduate Faculty of North Carolina State University in partial satisfaction of the requirements for the Degree of Master of Science, Department of Nuclear Engineering, Raleigh, 2004. Cyclotrons with high energy outputs can be complex and expensive to run, often requiring a centralised facility and suffering from similar drawbacks to methods relying on use of nuclear fission reactors. US2022 / 0415533A1 discloses a method of producing radioisotopes via irradiation of a stacked target assembly with charged particles or photons. Liquid targets are discussed in relation to photon beams. US2024 / 0032182A1 discloses a system that uses a D-Li-7 neutron generator to irradiate Ra-226. A Li-7 target electrode is coated with a layer of Ra-226, and will require replacement, interrupting production. There remains a need for a cost-effective, high volume apparatus for subjecting a target composition to nuclear fusion irradiation. The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved methods and apparatus for subjecting a target composition to radiation produced from a nuclear fusion reactor. Summary of the Invention According to a first aspect, the present invention provides apparatus for subjecting a target composition to irradiation. Optionally, the apparatus comprises: a nuclear fusion reactor configured and arranged to produce radiation from a radiation source in a reactor vessel; and a target holder configured to hold a liquid target composition. Preferably, the target holder is arranged to cause exposure of the liquid target composition to the radiation. It has been found that using a target holder configured to hold a liquid target composition in combination with a nuclear fusion radiation source allows for ease of extraction of the target composition. Such an apparatus may also facilitate adaptation of the configuration and / or arrangement of the target holder to provide various advantages, such as increased yield, purity, safety and / or efficiency of irradiation of a target composition. Furthermore, the apparatus may have a reduced size and / or increased mobility compared to known systems, and may allow for continuous irradiation of a target composition. Optionally, the apparatus may be configured so that the target composition may be removed and replenished without interrupting irradiation of the target composition and / or without removing the target composition in its entirety. It will be appreciated that a nuclear fusion reactor may avoid the use of fissile material (such as that present in a nuclear fission reactor), and thus provide well defined radiation and the ability to entirely shut off emission of radiation when the reactor is turned off. The radiation produced from the radiation source may be neutrons and / or protons. The target holder may be in the form of a hollow vessel suitable for holding a liquid target composition. Optionally, the apparatus may be for production of isotopes, such as medical isotopes. Optionally, the liquid target composition comprises a precursor isotope, such as a precursor isotope susceptible to radiation-induced decay to a target isotope, e.g. wherein the target isotope is a medical isotope. Optionally, the liquid target composition is present in the target holder. The nuclear fusion reactor may be configured and arranged to produce neutron and / or proton radiation (e.g. neutron radiation). Thus, the radiation source may be a neutron and / or proton source (e.g. a neutron source). It will be understood that a liquid target composition may be a liquid of any form - thus the term ‘liquid’ refers to the physical state of the target composition. For example, the liquid target composition may optionally be a mixture of a target compound or element dissolved and / or suspended in a carrier liquid, such as a solvent. Additionally or alternatively, the liquid target composition may comprise or consist of molten target material. Optionally, the target holder is configured to hold a target composition comprising a mixture of a target material and a liquid carrier (for example a solution of a target material dissolved in a solvent) and / or a molten target material, for example wherein the target holder is arranged to cause exposure of the target composition to radiation, such as neutron and / or proton, preferably neutron radiation. It will be understood that the target holder may be formed integrally with, or separate to, the nuclear fusion reactor. Optionally, the target composition is an aqueous solution of a target compound, such as an element susceptible to decay when irradiated. Optionally, the target composition comprises or consists of a carrier liquid (e.g. a solvent, such as water) and one or more salts comprising the target compound and / or element. Optionally, the target composition is a molten solution, for example wherein one or more salts are dissolved and / or dispersed in one or more molten carriers, where the carrier is in a liquid state due to high temperature. Optionally, the liquid target composition comprises an organic carrier liquid (e.g. wherein the liquid target solution is provided as an organic solution), for example wherein the carrier is an organic compound containing carbon and hydrogen atoms. Optionally, the apparatus comprises the target composition, for example wherein the target composition is disposed in the target holder, for example wherein the target holder is in the form of a hollow vessel. Optionally, the apparatus is configured so that the target composition has a thickness of 0.5 mm to 15 mm, such as 1 mm to 10 mm. Additionally or alternatively, the target holder is configured to provide and / or hold a target composition having a thickness of about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm. It will be understood that the target composition thickness does not include the target holder (for example, the thickness of the walls of any vessel holding the target composition do not form part of the thickness of the composition). Additionally or alternatively, the target holder comprises a void for holding a target composition, the void being bounded by at least a first side facing the neutron source of the nuclear fusion reactor and a second side opposing the first side, optionally wherein the first and second sides are spaced apart by about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm. Optionally, the apparatus is configured so that, during operation of the nuclear fusion reactor, a surface area of the target composition irradiated by the neutron radiation is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 cm2. Additionally or alternatively, the target holder is configured to provide and / or hold a target composition having a surface area of at least about 100 cm2, such as at least about 1000 cm2, facing the neutron source of the nuclear fusion reactor, optionally a surface area of about 100 to 4000 cm2, such as about 500 to 3000 cm2. Additionally or alternatively, the target holder is configured to present a target composition surface area of at least about 100 cm2, such as at least about 1000 cm2, for neutron irradiation, optionally a target composition surface area of about 100 to 4000 cm2, such as about 500 to 3000 cm2. Optionally, the target holder is arranged to position the target composition at least partially around a radiation source (for example a nuclear fusion reactor, more particularly around a vessel of the nuclear fusion reactor). Optionally, the target holder is arranged to extend circumferentially around the radiation source and / or to extend in an arc around the radiation source. Optionally, the target holder comprises, or is provided in the form of, a conduit that extends around at least part of a radiation source. Optionally, such a conduit is in the form of a coil wrapped around the radiation source, optionally wherein the coil has a plurality of, such as at least three, for example at least five, optionally at least ten, turns that each extend circumferentially around the radiation source (for example wrapped around a vessel of a nuclear fusion reactor, thereby at least partially surrounding the radiation source). It has been found that such an arrangement may provide a particularly convenient and effective arrangement for exposure of a large surface area of the target composition to radiation. Optionally the target holder is configured to position the target composition along at least 50%, such as at least 80%, for example at least 95% of the length of the vessel and / or electrodes of the reactor. Optionally, the reactor vessel has a length of at least about 20 cm, such as about 20 cm to about 200 cm, e.g. about 20 cm to about 80 cm, and / or a width of at least about 10 cm, such as about 10 cm to about 80 cm, e.g. about 10 cm to about 40 cm. Optionally, the reactor vessel is cylindrical for at least a portion of its length, such as for at least 15%, optionally at least 50 %, for example at least 75% (or at least 85%), of its length. Optionally, the target holder is configured to be positioned at least partially across an end of the reactor vessel. Optionally, the reactor vessel has an elongated shape with a longitudinal peripheral surface and at least two opposed end surfaces, for example wherein the end surfaces are curved or planar, and optionally the target holder is configured to position the target composition adjacent the longitudinal peripheral surface (and optionally at least one of the two end surfaces), and / or wherein such a reactor vessel is substantially cylindrical for at least a portion of its length. It will be understood that a cylindrical reactor vessel having a length of 30 cm and a diameter of 15 cm provides a surface area across which target composition may be arranged of more than 1400 cm2 excluding the ends of the cylinder, or more than 1700 cm2 including the ends of the cylinder. Optionally, the target holder extends at least partially around a portion of the reactor vessel such that the target holder at least partially surrounds at least a portion of the radiation source. For example, the target holder may at least partially wrap around at least a portion of the radiation source. It has been found that such an arrangement may provide a particularly convenient and effective arrangement for exposure of a large surface area of a target composition to radiation. Exposure of a large surface area may allow for homogenisation of the radiation fluence over the majority or entire target composition volume, which may for example avoid accumulation of unwanted activation products. Optionally, the radiation source has an elongated shape having a length along a longitudinal axis, and wherein the target holder extends along substantially the length of the radiation source. For example, the target holder may extend along the longitudinal axis of the radiation source. It has been found that such an arrangement may present a large surface area of a target composition to radiation. It has been found that a nuclear fusion reactor having an elongated radiation source may be particularly suited to use with a liquid target composition. Optionally, the radiation source is configured to emit substantially isotropic radiation about the longitudinal axis. Optionally, the apparatus comprises a target reservoir in fluid communication with the target holder and a circulator for circulating the liquid target composition between the target reservoir and the target holder, and optionally through the target holder. The target reservoir may, for example, be a vessel suitable for holding additional liquid target composition and / or extracting liquid target composition from the target holder. Optionally, the circulator may be a pump, for example a peristaltic pump. It has been found that a peristaltic pump may be advantageous for use with a liquid target composition exposed to nuclear fusion radiation. For example, a peristaltic pump may apply minimal mechanical shear to a liquid target composition, and may facilitate continuous circulation and / or easier maintenance. Optionally, the target holder extends from a first end to a second end, wherein the first and second ends of the target holder are each in fluid communication with the reservoir thereby forming a target solution circulation loop. Optionally, the circulator is configured and arranged to cause and control circulation of target solution around the circulation loop from the target solution reservoir to the first end of the target holder, through the target holder to the second end of the target holder, and from the second end of the target holder back to the reservoir. It has been found that circulation of a solution may avoid or reduce the development of ‘hotspots’ in locations with higher radiation flux, thereby helping to avoid accumulations of unwanted activation products over time. Circulation of a liquid target solution may also be particularly advantageous for continuous irradiation processes. For example, the target solution reservoir may be configured for (e.g. continuous) replenishment of target solution during irradiation. Optionally, the target solution reservoir is configured for extraction of liquid target composition and / or other constituents thereof from the target holder. Optionally, the target holder comprises a conduit, such as a conduit of any suitable shape. The conduit may comprise a tube, for example a tube having a substantially circular cross-section. Alternatively or additionally, the conduit may comprise portions with elliptical or square cross-section. Optionally, a cross-sectional orientation of a portion of the conduit may be aligned with a longitudinal axis of the target holder. For example, a portion of the conduit may comprise a first crosssectional width substantially parallel with a longitudinal axis of the target holder and a second cross-sectional width substantially normal to a longitudinal axis of the target holder, the first cross-sectional width being larger than the second cross-sectional width. In this way, a surface area of target composition exposed to radiation may be maximised, when target composition is provided within the target holder. Optionally, at least part of the conduit forms a coil having a plurality of turns extending at least partially circumferentially around the reactor vessel. The coil may have at least three, for at least five, turns that each extend circumferentially around the reactor vessel (for example at least partially surrounding the radiation source). It has been found that such an arrangement may provide a particularly convenient and effective arrangement for exposure of a large surface area of the target composition to radiation. Optionally, at least part of the conduit comprises a plurality of bends in the conduit along a length of the reactor vessel. In this way, a part of the conduit may have a serpentine configuration, with each of the plurality of bends being substantially parallel to a longitudinal axis of the reactor vessel. In the case where the conduit comprises a coil and a plurality of bends, at least a portion of the coil may double back on itself along a length of the reactor vessel. Alternatively or additionally, the coil may extend partially circumferentially around the reactor vessel and comprise a plurality of bends. In this way, the conduit may comprise a plurality of partial coiled sections, each partial coiled section extending in an arc around the radiation source. Optionally, the reactor vessel has an elongate shape having a length along a longitudinal axis, and the target holder is in the form of a shell extending along substantially the length of the reactor vessel (e.g. along at least 80%, such as at least 90% of the length of the reactor vessel) and at least partially around the longitudinal axis of the reactor vessel. The shell may have an elongate shape, for example in the form of a cylinder or an elongate prism, which may at least partially surround the reactor vessel. Optionally, the target holder is arranged to position the target composition so that it extends circumferentially around the radiation source in at least one axis and / or to extend in an arc around the radiation source wherein the arc subtends an angle of at least 90°, such as at least 180°, for example at least 270°. Optionally, at least part of the target holder is integrally formed with the nuclear fusion reactor. For example, the target holder may be integrated within a component of the nuclear fusion reactor. Optionally, at least part of the target holder is positioned within the nuclear fusion reactor. For example, the target holder may be a discrete component positioned within an outer surface of the nuclear fusion reactor. Optionally, the nuclear fusion reactor comprises a reactor shell incorporating at least part of the target holder, for example wherein at least part of the target holder is in the form of one or more channels provided in the reactor shell. Optionally, the nuclear fusion reactor comprises at least one electrode incorporating at least part of the target holder, for example wherein at least part of the target holder is in the form of one or more channels provided in the electrode. For example, an anode and / or a cathode of the nuclear fusion reactor may comprise channels therein. The one or more channels may be microfluidic channels, for example channels having an internal cross-section and / or thickness smaller than a cross-section and / or thickness of the electrode. It will be understood that a plurality of microfluidic channels may enable exposure of a large surface area of the target composition to radiation. Optionally, at least part of the target holder is formed separately from the nuclear fusion reactor. Optionally, the apparatus comprises a liquid target withdrawal valve for withdrawal of at least a portion of the liquid target. Optionally, the apparatus comprises a liquid target top-up valve for admission of additional liquid target. Optionally, the apparatus comprises a gas release valve for releasing gas produced during irradiation of the liquid target composition. Optionally, the apparatus comprises a liquid target composition held in the target holder, for example wherein the liquid target comprises an aqueous solution including a water-soluble target compound. Optionally, the nuclear fusion reactor further comprises elongate electrodes, wherein the reactor vessel and electrodes each independently have a length of at least 1.5 times their width, and wherein the target holder is configured to position the target composition along at least 50% of the length of the reactor vessel and / or the electrodes. Optionally, the apparatus comprises a coolant holder configured to hold a coolant. The coolant holder may extend at least partially around a portion of the reactor vessel such that the coolant holder at least partially surrounds at least a portion of the radiation source. Optionally, the coolant holder may be configured and arranged to have the same or similar features to the target holder. For example, the coolant holder may comprise a conduit. Optionally, the coolant holder may be positioned closer to the reactor vessel than the target holder (e.g. positioned between the reactor vessel and the target holder). For example, the coolant holder may be arranged immediately adjacent to the reactor vessel. Additionally or alternatively, the coolant holder may be arranged partially or wholly in between cooling fins of the reactor vessel, e.g. wherein such cooling fins extend outwards from the reactor shell. Optionally, the coolant holder comprises coolant. Optionally, the coolant holder and / or the coolant may be configured and arranged to reduce the energy of radiation (for example neutrons) produced from the radiation source. For example, the coolant may optimise the energy spectrum of radiation for exposure to the target composition. Alternatively or additionally, the coolant holder may be positioned further from the reactor vessel than the target holder, for example in the case where at least part of the target holder is incorporated within a reactor shell of the nuclear fusion reactor. Optionally, the target holder may comprise the coolant holder and / or act as a coolant holder. For example, the liquid target composition may be configured to provide a cooling effect to the nuclear reactor in addition to providing a target for irradiation. Additionally or alternatively, the coolant holder may be positioned at the same distance from the reactor vessel as the target holder, for example the coolant holder and the target holder may both be positioned immediately adjacent to the reactor vessel. Optionally, the coolant holder and the target holder may be positioned side by side, and optionally immediately adjacent to the reactor vessel, for example wherein the coolant holder and the target holder extend in a parallel arrangement at least partially around the reactor vessel. Examples of suitable parallel arrangements include alternating and / or intertwined positioning of the coolant holder and target holder. Optionally, the apparatus comprises a capture jacket, such as a neutron capture jacket. Optionally, the capture jacket is configured to capture radiation, such as neutron radiation, e.g. to prevent unwanted escape of radiation from the apparatus. Optionally, the capture jacket is configured and arranged to capture deuteriumdeuterium and / or deuterium-tritium fusion radiation. Optionally, the capture jacket has a thickness of up to about 1 m, such as up to about 50 cm. Optionally, the target holder is configured so that the target composition has a thickness of less than about 20 mm, for example of about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm. Optionally, the target holder is configured so that during operation a surface area of the target composition exposed to the radiation is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 cm2. Optionally, the target holder is configured so that a volume of the target composition subjected to radiation is from about 1 pl to 500 ml, for example from about 10 pl to 300 ml, such as 100 pl to 100 ml. Optionally, the target holder is a first target holder and the liquid target composition is a first liquid target composition, and the apparatus comprises a second target holder separate from the first target holder, the second target holder being configured to a second liquid target composition separate from the first liquid target composition, wherein the second target holder is arranged to cause exposure of the second liquid target composition to the radiation. It will be understood that such a second target holder may allow for multiple target compositions to be irradiated simultaneously. Optionally, the first and second target compositions comprise different isotopes, such as different precursor isotopes. Optionally, the apparatus comprises a plurality of separate target holders, such as at least three separate target holders, each independently configured to hold liquid target compositions and cause exposure of said compositions to the radiation. It will be appreciated that each target holder may independently comprise any feature described in relation to a target holder herein. For example, the apparatus may optionally comprise a corresponding plurality of circulators and / or reservoirs, and / or the target holders may optionally be configured to allow for liquid target compositions to be circulated through, added to and extracted from each target holder independently. Optionally, at least a portion of the second target holder (e.g. the portion arranged to expose the second liquid target composition to radiation) is positioned in a side by side arrangement with at least a portion of the first target holder (e.g. the portion arranged to expose the first liquid target composition to radiation). Additionally or alternatively, the first and second target holders may each comprise a portion (such as a conduit portion) extending adjacently along and / or around at least a portion of the reactor. It will be appreciated that each target holder may independently adopt any arrangement described as suitable for a target holder hereinabove. Optionally, at least a portion of the second target holder (e.g. the portion arranged to expose the second liquid target composition to radiation) is positioned between at least a portion of the first target holder (e.g. the portion arranged to expose the first liquid target composition to radiation) and the nuclear fusion reactor. Additionally or alternatively, multiple target holders may optionally be arranged in layers relative to the nuclear fusion reactor, for example wherein at least a portion of an inner target holder is positioned between the radiation source of the nuclear fusion reactor and at least a portion of an outer target holder. Optionally, the nuclear fusion reactor is configured and arranged to generate the radiation by one or more of deuterium-tritium fusion, deuterium-deuterium fusion, tritium-tritium fusion, and / or deuterium-helium fusion, preferably deuterium-tritium fusion. It will be understood that deuterium-tritium fusion produces neutrons having energies in the range of about 12 MeV to 16 MeV. It will be understood that deuterium-deuterium fusion provides neutrons and / protons having an energy of below about 4 MeV, such as in the range of about 0.5 to about 4 MeV, and tritium-tritium fusion provides neutrons having an energy in the range of about 0.5 MeV to about 10 MeV. It will be understood that deuterium-helium fusion produces protons having energies of at least 12 Mev, such as in the range of about 12 MeV to 16 MeV. It will be understood that protons and / or neutrons of other energies may optionally be produced by the nuclear fusion reactor. Optionally, the nuclear fusion reactor is configured to minimise deuterium-deuterium fusion reactions and / or tritium-tritium fusion reactions, especially deuterium-deuterium fusion reactions. Additionally or alternatively, the nuclear fusion reactor is configured to induce deuterium-tritium lattice confinement fusion to produce neutron radiation through generation of an electric field between electrodes of the nuclear fusion reactor. Optionally, at least part of a surface of one or more said electrodes of the nuclear fusion reactor is enriched with deuterium and / or tritium, for example enriched with at least 100 ppm deuterium and / or tritium by atomic percentage. Optionally, the nuclear fusion reactor is configured to produce neutron radiation comprising at least at least 50% (such as at least 60%, for example at least 80%) neutrons generated by deuterium-tritium fusion and no more than 50% (such as no more than 40%, for example no more than 20%) neutrons generated by deuterium-deuterium fusion. Optionally, the nuclear fusion reactor is configured and arranged to produce neutron radiation comprising at least 50% neutrons having an energy of at least about 12 MeV (e.g. in the range of about 12 MeV to about 16 MeV), and / or optionally no more than 40% neutrons having an energy of below about 4 MeV, such as about 0.5 to about 4 MeV. Optionally, the nuclear fusion reactor is arranged to produce neutron radiation comprising at least 75% neutrons having an energy of at least about 12 MeV and below about 4 MeV, such as in the range of about 0.5 to about 4 MeV. In other words, optionally no more than 25% of neutrons have energies outside those ranges. Additionally or alternatively, at least 90% of neutrons have an energy of no more than about 16 MeV. Optionally, the neutron radiation comprises at least 60% neutrons having an energy of at least about 12 MeV (e.g. in the range of about 12 MeV to about 16 MeV), such as at least 70% neutrons having an energy of at least about 12 MeV (e.g. in the range of about 12 MeV to about 16 MeV). Additionally or alternatively, the neutron radiation comprises at least 50%, for example at least 60%, such as at least 70% neutrons having an energy in the range of about 13 MeV to 15 MeV. Optionally, the neutron radiation comprises no more than 35% neutrons having an energy of below about 4 MeV (such as 0.5 to 4 MeV), such as no more than 18% neutrons having an energy of below 4 MeV (such as in the range of about 0.5 MeV to about 4 MeV). Optionally, the neutron radiation comprises at least 80% neutrons, for example at least 85% neutrons, having an energy in the range of about 12 MeV to about 16 Mev and below about 4 MeV (such as in the range of about 0.5 to about 4 MeV). Optionally, the nuclear fusion reactor is configured and / or arranged for multistate fusion. Optionally, the nuclear fusion reactor is a multi-state fusion reactor. It will be understood that multi-state fusion may comprise, for example, a combination of plasma fusion and lattice confinement fusion. Additionally or alternatively, the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprising a lattice substrate enriched with deuterium and / or tritium (preferably deuterium), optionally wherein the nuclear fusion reactor is configured and arranged for lattice confinement fusion in the lattice substrate. It has been found that multi-state fusion reactors provide cost, energy and space efficient systems for production of suitable neutron radiation. Optionally, the nuclear fusion reactor comprises at least one electrode having an enriched surface, the enriched surface comprises a lattice substrate enriched with deuterium and / or tritium (preferably deuterium), for example enriched with at least 100 ppm deuterium and / or tritium by atomic percentage. Optionally the nuclear fusion reactor is arranged and operated to induce lattice confinement fusion (particularly deuterium-tritium fusion) in the enriched surface. Optionally, the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. Optionally, the enriched surface is deliberately enriched. It will be understood that deliberate enrichment distinguishes from, for example, passive enrichment (through which a surface may become enriched to a minor extent during operation of a nuclear fusion reactor). It has been found that deliberate enrichment provides a significantly greater degree of enrichment as compared to passive enrichment. It will be understood that the enriched surface may be enriched by any suitable means, such as by one or more of electrolysis and high temperature, high pressure (HPHT) gas loading. Optionally, the enriched surface is enriched by electrolysis, for example electrolysis with a solution of deuterated and / or tritiated heavy water and conductive salts for electrolysis. It has been found that deliberate enrichment of an electrode surface, such as by electrolysis, provides a high level of deuterium and / or tritium enrichment, promoting effective lattice confinement fusion. It has further been found that enrichment by electrolysis results in adsorption of elements of the conductive salt into the surface, detectable for example by Energy-dispersive X-ray spectroscopy (EDX). It follows that the presence of such elements in an enriched surface max provide a characteristic fingerprint of enrichment by electrolysis. Suitable conductive salts include alkali metal-containing salts. Following enrichment by electrolysis, a sample of the enriched surface may comprise, for example, at least 0.05 %, such as at least 0.1 % of one or more elements of the conductive salt, such as one or more alkali metal elements, for example as determined by EDX. Optionally, the surface is subjected to electrolysis at a current density of least 0.1 mA / cm2 of surface for at least 10 minutes. A suitable nuclear fusion reactor having one or more enriched surface electrodes is described in WO 2022 / 263827 Al (Astral Neutronics Ltd), the contents of which are incorporated herein by reference. Optionally, the nuclear fusion reactor comprises an anode structure and a cathode structure, wherein the anode and cathode structures are substantially concentric along at least a part of their lengths and are configured such that, in operation, an electric field is provided between the anode and cathode structures, optionally wherein the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. Additionally or alternatively, at least one of the anode structure and the cathode structure comprises at least one said electrode comprising a surface enriched with deuterium and optionally tritium. Optionally, the enriched surface is deliberately enriched. Optionally, the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor. Optionally, the nuclear fusion reactor is configured to form a plasma from gas, thereby forming a plasma within the nuclear fusion reactor when in use. Optionally, the nuclear fusion reactor is operable to initiate nuclear fusion in the presence of said plasma to generate the neutron radiation. Optionally, the nuclear fusion reactor contains a gas and is operable to form a plasma from the gas. Optionally, the gas comprises at least 50 mol% tritium and no more than 50 mol% deuterium. Additionally or alternatively, the gas optionally comprises at least 80 mol% tritium (such as at least 90 mol%, for example at least 95 mol%), and optionally no more than 20 mol% deuterium (such as no more than 10 mol%, for example no more than 5 mol%). Optionally, the gas is substantially entirely tritium gas, for example about 98 mol% (such as about 99 mol%, optionally about 100 mol%) tritium gas. Optionally, the nuclear fusion reactor is configured to maintain the gas composition at a constant concentration of deuterium and / or tritium during operation of the nuclear fusion reactor. Optionally, when the gas is predominantly or entirely tritium, the enriched surface is enriched with deuterium, for example wherein the enriched surface is enriched with fusible isotope species and where the fusible isotope species enrichment is at least 95 mol% (such as at least 98 mol%, for example at least 99 mol%) deuterium, based on the fusible isotope species content of the enriched surface. Optionally, the gas is at least 95 mol% (such as at least about 99 mol%, optionally about 100 mol%) either deuterium or tritium, and the enriched surface comprises at least 95 mol% (such as at least about 98 mol%, optionally at least about 99 mol%) either deuterium and tritium, based on the fusible isotope species content of the enriched surface, wherein the gas is predominantly deuterium and the fusible isotope species of the enriched surface is predominantly tritium, or wherein the gas is predominantly tritium and the fusible isotope species of the enriched surface is predominantly deuterium. It has been found that having a gas comprising predominantly tritium and a surface enriched predominantly with deuterium, and vice versa, helps to favour deuterium-tritium fusion in preference to deuterium-deuterium and tritium-tritium fusion. Without wishing to be bound by theory, it is believed that in a multi-state fusion nuclear reactor, neutron radiation is produced predominantly by lattice confinement fusion reactions occurring between a plasma fusible isotopes species and an enriched surface fusible isotope species. Furthermore, it has been found that surface enrichment with deuterium may be more convenient than surface enrichment with tritium. According to a second aspect, the present invention provides a method for subjecting a liquid target composition to irradiation. Optionally, the method comprises operating a nuclear fusion reactor to produce radiation from a radiation source in a reactor vessel, and exposing to the radiation a liquid target composition held in a target holder. Optionally, the method comprises operating the apparatus of the first aspect of the invention. For example, the method optionally comprises operating the nuclear fusion reactor to produce radiation as described in relation to the apparatus of the first aspect of the invention. Optionally, the method is a method for production of isotopes, such as medical isotopes (i.e. isotopes suitable for medical use). Optionally, the target composition comprises one or more isotope precursors, such as a precursor isotope susceptible to radiation-induced decay to a target isotope, e.g. wherein the target isotope is a medical isotope. Optionally, the method further comprises circulating the target composition through at least part of the target holder. Optionally, the method comprises operating a pump, such as a peristaltic pump, to circulate the target composition through at least part of the target holder. Optionally, the nuclear fusion reactor is comprised in an apparatus comprising a target reservoir in fluid communication with the target holder and a circulator for circulating the liquid target composition between the target reservoir and the target holder, and optionally through the target holder. Optionally, the method comprises operating the circulator to circulate the liquid target composition between the target reservoir and the target holder, and optionally through the target holder. Optionally, the method comprises extracting at least a portion of the liquid target composition from the target reservoir, and / or adding fresh liquid target composition to the target reservoir. Optionally, the target holder extends from a first end to a second end, wherein the first and second ends of the target holder are each in fluid communication with the reservoir thereby forming a target solution circulation loop. Optionally, the method comprises operating the circulator to cause and control circulation of target solution around the circulation loop from the target solution reservoir to the first end of the target holder, through the target holder to the second end of the target holder, and from the second end of the target holder back to the reservoir. Optionally, the method is a continuous production method, for example wherein the method comprises removing a portion of the target composition, and optionally replenishing the target composition, for example without interrupting irradiation of the target composition and / or without removing the target composition in its entirety. Optionally, the method comprises operating the nuclear fusion reactor to generate the radiation by deuterium-tritium fusion and optionally by deuteriumdeuterium fusion and / or by tritium-tritium fusion. Optionally, the method comprises operating the nuclear fusion reactor as described in relation to the apparatus of the first aspect of the invention. Additionally or alternatively, the method optionally comprises operating the nuclear fusion reactor to initiate multi-state fusion. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a side cut-through schematic view of a nuclear fusion reactor in accordance with an embodiment of the invention; Figure 2 shows an end cut-through of the nuclear fusion reactor of Figure 1; Figure 3 shows a side cut-through schematic view of a nuclear fusion reactor in accordance with another embodiment of the invention; Figure 4 shows an end cut-through of the nuclear fusion reactor of Figure 3; Figure 5 shows a schematic view of an apparatus for subjecting a target composition to irradiation; Figure 6 shows an end cut-through of the apparatus of Figure 5; and Figure 7 shows a side cut through of the nuclear fusion reactor shown in Figure 5. Detailed Description Figure 1 shows a side cut-through schematic view of a nuclear fusion reactor apparatus 100 comprising a cylindrical outer vessel wall 101, high voltage stand-off component 102, cathode assembly 103, and localised fusible isotope species enriched inner anode surface 104. Specifically, Figure 1 illustrates an IELC cut-through showing the flanged cylindrical cathode 103, encompassing anode surface 104 and appended ceramic insulators 105 at either end including voltage feedthroughs 106. The anode 104 is formed on the inner surface of the reactor vessel wall 101, and is enriched with deuterium (optionally and / or tritium), thus incorporating atomic % level deuterium (optionally and / or tritium) in the anode lattice substrate. Additionally or alternatively, the surface of the cathode 103 may be enriched with deuterium and / or tritium. Cathode and anode materials may be chosen for secondary electron emission properties as well as the ability to retain high levels of fusible ion species to high temperatures whilst remaining stable. Optionally, at least one of the anode and the cathode is enriched with deuterium, and the interior 107 of the reactor vessel 101 is filled with tritium. Figure 1 represents a simplified radiation generator configuration where the deuterium or tritium gas species are released and stored in a getter material within the sealed vessel (not shown in Figure 1). The nuclear fusion reactor apparatus 100 additionally comprises a target holder 110 in the form of a conduit wrapped around the cylindrical outer vessel wall 101. The target holder conduit is configured to hold an aqueous solution of a Ra-226 salt, and has an internal diameter of about 10 mm. The conduit is arranged as a continuous spiral coil having multiple turns around the outer vessel wall 101, thus presenting a large surface area of target material to radiation emitted from the reactor when filled with the target composition. Figure 2 shows an end cut-through of the nuclear fusion reactor 100 of Figure 1, along line A-A. Features shown in Figure 2 are labelled with the same reference numerals as used in Figure 1. Figure 3 shows a side cut-through schematic view of another nuclear fusion reactor apparatus 200. Features of the nuclear fusion reactor apparatus 200 that are the same as the reactor apparatus 100 of Figure 1 are labelled with the same reference numerals. The nuclear fusion reactor apparatus 200 comprises a target holder 210 in the form of a shell around the cylindrical outer vessel wall 101. The target holder shell is configured to hold an aqueous solution of a Ra-226 salt, and has an internal diameter of about 10 mm. The shell extends around the outside of the vessel wall 101 along a length of the reactor corresponding to the length of the cathode 103, thus presenting a large surface area of target material to radiation emitted from the reactor when filled with the target composition. Figure 4 shows an end cut-through of the nuclear fusion reactor 200 of Figure 3, along line B-B. Features shown in Figure 4 are labelled with the same reference numerals as used in Figure 3. Figure 5 shows a schematic view of an apparatus for subjecting a target composition to irradiation. Features of the apparatus that are similar to the apparatus of Figures 1 and 2 are labelled with the same reference numerals. The apparatus comprises a nuclear fusion reactor 100 and a target holder 110. The target holder 110 comprises a conduit 112, which connects different components of the apparatus to enable fluid communication therebetween. The target holder conduit 112 is configured to hold a liquid target composition, for example an aqueous solution of a Ra-226 salt. The target holder conduit 112 has an internal diameter of about 10 mm. Shown at the bottom right of the drawing is a switching valve 301, which is configured to switch a flow of liquid target composition between different pathways. In a first configuration, the switching valve 301 is configured to permit additional (for example, non-irradiated) liquid target composition to enter the target holder 110 from a conduit 112 in fluid communication with a target reservoir 303. In a second configuration, the switching valve 301 is configured to permit liquid target composition (for example, depleted liquid target composition) to exit the target holder 110 via a conduit 112 in fluid communication with a vent 305. In a third configuration, the switching valve 301 is configured to permit flow of liquid target composition through the valve 301, acting as an extension of the conduit 112 of the target holder 110. In a clockwise direction from the switching valve, the conduit 112 leads to the bottom of the nuclear fusion reactor 100. The conduit 112 is wrapped around a reactor vessel of the nuclear fusion reactor 100, arranged as a continuous spiral coil having multiple turns around the reactor vessel to form a helix shape, thus presenting a large surface area of target material to radiation emitted from the reactor when filled with the target composition. The conduit 112 is shown wrapped around an exterior of the reactor vessel, more particularly around cooling fins projecting from an exterior of the reactor vessel. The conduit 112 leads from the top of the nuclear fusion reactor 100 to a peristaltic pump 307. It will be understood that the peristaltic pump 307 utilises a rotor to sequentially compress and release a flexible tube (which may be an extension of conduit 112), thereby displacing fluid (for example target composition) contained within the tube to propel it around the conduit 112 in a peristaltic motion. The conduit 112 then leads from the base of the peristaltic pump 307 back to the switching valve 301, to complete the target holder 110 circuit. Thus, the target holder 110 facilitates exposure of liquid target composition to radiation produced from the reactor 100. The peristaltic pump 307 facilitates continuous or controlled flow of liquid target composition through the conduit 112, and the switching valve 301 facilitates optional withdrawal and replenishment of liquid target composition. Embodiments therefore enable well-defined control of irradiation exposure times and / or continuous irradiation methods. Figure 6 shows an end cut-through of the apparatus of Figure 5. The cylindrical outer wall of the reactor vessel 101 is substantially circular in crosssection. The anode 104 is formed on the inner surface of the reactor vessel wall 101, while the cathode 103 is located at the radial centre of the reactor vessel 101. Cooling fins 400 are shown projecting radially outwards from the outer wall of reactor vessel 101. Each cooling fin 400 is substantially quadrilateral in cross-section, having an outer edge distal from the reactor vessel 101 that is wider than an inner edge proximal to the reactor vessel 101. Surrounding the cooling fins 400 is a coolant holder 402. The conduit 112 is wrapped around the reactor vessel 101, and the inlet and outlet portions of the conduit arranged at the top and bottom of the spiral coil are laterally displaced from one another. The conduit 112 leads to the switching valve 301 and peristaltic pump 307, connecting the loop of the target holder 110. Figure 7 shows a side cut through of the nuclear fusion reactor 100 shown in Figure 5. The multiple turns of the spiral coil of the conduit 112 are wrapped around the reactor vessel 101. Cathode assembly 103 extends along the majority of the length of the reactor vessel 101, and is aligned with a central longitudinal axis thereof. The bottom end of the cathode assembly 103 terminates within the bounds of the reactor vessel 101, while the top end of the cathode assembly 103 includes a voltage feedthrough 106 for connection to a power source. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments. References in the present disclosure to "one embodiment", "an embodiment" and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. It should be understood that, although the terms "first", "second" and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed terms. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "has", "having", "includes" and / or "including", when used herein, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof.

Claims

1. Apparatus for subjecting a target composition to irradiation, wherein the apparatus comprises:a nuclear fusion reactor configured and arranged to produce radiation from a radiation source in a reactor vessel; anda target holder configured to hold a liquid target composition, wherein the target holder is arranged to cause exposure of the liquid target composition to the radiation.

2. Apparatus according to claim 1, wherein the target holder extends at least partially around a portion of the reactor vessel such that the target holder at least partially surrounds at least a portion of the radiation source.

3. Apparatus according to claim 1 or claim 2, wherein the radiation source has an elongated shape having a length along a longitudinal axis, and wherein the target holder extends along substantially the length of the radiation source.

4. Apparatus according to any preceding claim, comprising a target reservoir in fluid communication with the target holder and a circulator for circulating the liquid target composition between the target reservoir and the target holder, and optionally through the target holder;optionally wherein the target holder extends from a first end to a second end, wherein the first and second ends of the target holder are each in fluid communication with the reservoir thereby forming a target solution circulation loop, and wherein the circulator is configured and arranged to cause and control circulation of target solution around the circulation loop from the target solution reservoir to the first end of the target holder, through the target holder to the second end of the target holder, and from the second end of the target holder back to the reservoir.

5. Apparatus according to any preceding claim, wherein the target holder comprises a conduit;wherein at least part of the conduit forms a coil having a plurality of turns extending at least partially circumferentially around the reactor vessel, and / or wherein at least part of the conduit comprises a plurality of bends in the conduit along a length of the reactor vessel.

6. Apparatus according to any preceding claim, wherein the reactor vessel has an elongate shape having a length along a longitudinal axis, and the target holder is in the form of a shell extending along substantially the length of the reactor vessel and at least partially around the longitudinal axis of the reactor vessel.

7. Apparatus according to any preceding claim, wherein the target holder is arranged to position the target composition so that it extends circumferentially around the radiation source and / or to extend in an arc around the radiation source wherein the arc subtends an angle of at least 90°, such as at least 180°, for example at least 270°.

8. Apparatus according to any preceding claim, wherein at least part of the target holder is integrally formed with the nuclear fusion reactor and / or wherein at least part of the target holder is positioned within the nuclear fusion reactor.

9. Apparatus according to claim 8, wherein the:the nuclear fusion reactor comprises a reactor shell incorporating at least part of the target holder, for example wherein at least part of the target holder is in the form of one or more channels provided in the reactor shell; and / orthe nuclear fusion reactor comprises at least one electrode incorporating at least part of the target holder, for example wherein at least part of the target holder is in the form of one or more channels provided in the electrode.

10. Apparatus according to any preceding claim, wherein at least part of the target holder is formed separately from the nuclear fusion reactor.

11. Apparatus according to any preceding claim, comprising one or more of: a liquid target withdrawal valve for withdrawal of at least a portion of the liquid target;a liquid target top-up valve for admission of additional liquid target; and / or a gas release valve for releasing gas produced during irradiation of liquid target.

12. Apparatus according to any preceding claim, comprising a liquid target held in the target holder, optionally wherein the liquid target comprises an aqueous solution including a water-soluble target compound.

13. Apparatus according to any preceding claim, wherein the nuclear fusion reactor further comprises elongate electrodes, wherein the reactor vessel and electrodes each independently have a length of at least 1.5 times their width, and wherein the target holder is configured to position the target composition along at least 50% of the length of the reactor vessel and / or the electrodes.

14. Apparatus according to any preceding claim, wherein the target holder is configured so that the target composition has a thickness of less than about 20 mm, for example about 0.5 mm to about 15 mm, such as about 1 mm to about 10 mm.

15. Apparatus according to any preceding claim, wherein the target holder is configured so that during operation a surface area of the target composition exposed to the radiation is at least about 100 cm2, such as at least about 1000 cm2, optionally wherein the surface area is about 100 to 4000 cm2, such as about 500 to 3000 cm2.

16. Apparatus according to any preceding claim, wherein the target holder is configured so that a volume of the target composition subjected to radiation is from about 1 pl to 500 ml.

17. Apparatus according to any preceding claim, wherein the nuclear fusion reactor is configured and arranged to generate the radiation by deuterium-tritium fusion, and optionally by deuterium-deuterium fusion and / or by tritium-tritium fusion.

18. Apparatus according to any preceding claim, wherein the nuclear fusion reactor is a multi-state fusion reactor, wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion.

19. Apparatus according to any preceding claim, wherein the nuclear fusion reactor comprises an anode structure and a cathode structure, wherein the anode and cathode structures are substantially concentric along at least a part of their lengths and are configured such that, in operation, an electric field is provided between the anode and cathode structures, optionally wherein the nuclear fusion reactor is an inertial electrostatic confinement nuclear fusion reactor.

20. Apparatus according to any preceding claim, wherein the target holder is a first target holder and the liquid target composition is a first liquid target composition, wherein the apparatus comprises a second target holder separate from the first target holder, the second target holder being configured to a second liquid target composition separate from the first liquid target composition, and wherein the second target holder is arranged to cause exposure of the second liquid target composition to the radiation.

21. Apparatus according to claim 21, wherein at least a portion of the second target holder is positioned in a side by side arrangement in relation to the first target holder relative to the nuclear fusion reactor, and / or wherein at least a portion of the second target holder is positioned between at least a portion of the first target holder and the nuclear fusion reactor.

22. A method for subjecting a target composition to irradiation, wherein the method comprises: operating a nuclear fusion reactor to produce radiation from a radiation source in a reactor vessel, and exposing to the radiation a liquid target composition held in a target holder.

23. A method according to claim 22, comprising operating the apparatus of any one of claims 1 to 21.

24. A method according to claim 22 or claim 23, further comprising circulating the liquid target composition through at least part of the target holder.

25. A method according to any one of claims 22 to 24, wherein the method is a continuous production method, for example wherein the method comprises removing a portion of the target composition, and optionally replenishing the target composition, for example without interrupting irradiation of the target composition and / or without removing the target composition in its entirety.

26. A method according to claim 22, comprising operating the apparatus of claim 20 or 21, wherein the method comprises: circulating the first liquid target composition through at least part of the first target holder, and circulating the second liquid target composition through at least part of the second target holder.

27. A method according to any one of claims 22 to 26, wherein the method comprises operating the nuclear fusion reactor to generate the radiation by one or more of deuterium-tritium fusion, deuterium-deuterium fusion, tritium-tritium fusion, and / or deuterium-helium fusion.

28. A method according to claim 27, wherein the method comprises operating the nuclear fusion reactor to generate the radiation by deuterium-tritium fusion and optionally by deuterium-deuterium fusion and / or by tritium-tritium fusion.

29. A method according to any one of claims 22 to 28, wherein the method comprises operating the nuclear fusion reactor to initiate multi-state fusion, wherein multi-state fusion comprises a combination of plasma fusion and lattice confinement fusion.

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