Technical Field

The present invention relates to a system and a method of separating hydrogen isotopes including two or more of protium (H), deuterium (D) and tritium (T) from a gaseous mixture comprising diatomic molecules thereof. Such a gaseous mixture may be obtained from the exhaust of a fusion reactor, for example.

The challenge of producing fusion power is hugely complex. One route to fusion involves confining a deuterium-tritium (D-T) or deuterium-deuterium (D-D) plasma at temperatures that are so high that the nuclei fuse together, releasing highly energetic neutrons. The deuterium-tritium reaction is summarised in the following equation:

The helium “ash” produced by the reaction, as well as any other impurities that have entered the plasma, generally needs to be removed so that the “unburnt” deuterium and tritium fuel can continue to react. This typically requires removing the ash and unburnt fuel from the plasma, separating the unburnt fuel from the ash and then returning the unburnt fuel to the plasma.

Some fusion reactors, such as tokamaks and stellarators, use magnetic fields to confine the plasma within a plasma chamber at the high temperatures needed for fusion to occur. The deuterium and tritium fuel is typically injected into the plasma at high speeds in a frozen pellet or as a pulsed molecular beam. However, a significant fraction of the injected fuel may never even reach the plasma core where fusion occurs. It is therefore desirable for this fraction, which may be referred to as the fuelling inefficiency, to be recovered from the exhaust gases of the reactor in order to be reinjected into the plasma core. In addition, deuterium and tritium ions that are confined initially in the core of the plasma diffuse to the edges (extremal surfaces) of the trapping potential and collide with a device called a divertor, where they are neutralised and subsequently pumped away from the plasma chamber. Exhaust gases from fusion reactors such as tokamaks and stellarators, therefore typically comprise a mixture of hydrogen isotopologues comprising deuterium and tritium atoms (i.e. D2, T2 and DT, and also HD, HT and H2 as a result of residual protium), as well as other gases, such as helium or plasma enhancement gases, e.g. Ar, Ne, N, etc.

Efficient recovery of unburnt tritium is crucially important for fusion reactors because: (i) tritium is difficult to obtain in the quantities needed for fusion as it is radioactive (with a half-life of 12.3 years); and (ii) no readily extractable sources of tritium exist naturally on Earth. Efficient tritium recovery is also important in order to meet strict limits for the amount of radioactive material that can be discharged into the environment. A variety of isotope separation systems have therefore been proposed that allow tritium to be separated from protium or deuterium. In particular, cryogenic distillation using a series of cryogenic distillation columns has been proposed to separate hydrogen isotopologues based on the difference in boiling points of H2 and T2 (20 K and 25 K, respectively). While this approach may allow large quantities of gas to be processed and separate the isotopes efficiently (i.e. achieve high separation factors), the cooling, liquefaction and distillation of gases is a slow process, typically requiring timescales of hours. Existing isotope separation systems often therefore require large quantities of tritium to be stored for long times. In addition, a reactor using such isotope separation systems may require large quantities of tritium to be obtained before the reactor can be operated i.e. a large “startup inventory” of tritium is needed.

The conventional view of the tritium fuel cycle in most reactor designs is that the isotope separation system should be provided relatively “late” in the return leg of the cycle, e.g. after the exhaust gases from the reactor have been processed to remove gases other than the hydrogen isotopologues. Many designs also propose storing the tritium in uranium getters for fuel processing after the separation process has been completed.

US4976938 describes absorbing deuterium and tritium isotopes using a getter operating at relatively low temperatures to absorb substantially all the deuterium and tritium from a gas and then heating the getter to selectively release gas comprising tritium (i.e. DT and T2), whilst retaining a proportion of deuterium atoms in the getter. Whilst this approach allows, at least in theory, the gas to be enriched in a controlled manner, the time and energy required to repeatedly cycle the temperature of the getter may be a significant disadvantage in many applications. For example, when the gas containing the isotopes is initially produced at high temperatures, significant cooling of the gas may be required before it is introduced into the getter.

Summary

The present application seeks to address, or at least alleviate, at least some of the problems described above.

According to a first aspect of the present invention there is provided a method of separating hydrogen isotopes including two or more of protium (H), deuterium (D) and tritium (T) from one or more of (i) a gaseous mixture comprising diatomic molecules thereof, and (ii) a lithium-containing material comprising the hydrogen isotopes. The method comprises passing the gaseous mixture or lithium-containing material over or through one or more getters, each getter comprising getter material to preferentially absorb, from the gaseous mixture or lithium-containing material, a first of the hydrogen isotopes relative to a second of the hydrogen isotopes.

The gaseous mixture may, for example, comprise diatomic molecules of H2, D2, T2, HD, HT, and DT. The relative amounts of each of the diatomic molecules in the gaseous mixture may be (essentially) statistical based on the proportions of each hydrogen isotopes. For example, in a gaseous mixture in which hydrogen (H) predominates, with only relatively small amounts of deuterium (D) or tritium (T) present, then under equilibrium conditions most of diatomic molecules will be H2, HD, and HT, with only small amounts of D2, T2 and DT. In other cases, the gaseous mixture may contain non- statistical amounts of the isotopomers, i.e. the partial pressures of H2, D2, T2, HD, HT, and DT may differ from their equilibrium values.

The first of the isotopes is protium or deuterium and the second of the hydrogen isotopes is tritium.

The lithium-containing material may comprise lithium tritride and one or more of lithium hydride and lithium deuteride, i.e. LiH, LiD and/or LiT. The lithium-containing material may be liquid and/or comprise liquid lithium metal. In some implementations, the liquid lithium-containing material may be in contact with solid getter material, such that H (or D) atoms and T atoms are exchanged across the solid-liquid interface between the getter and lithium-containing materials. The temperature of the getter material and/or the liquid lithium-containing material may be controlled to control (i) the rate of exchange, e.g. the rate of diffusion of H (or D) and/or T within and between the materials; and/or (ii) the equilibrium concentrations of H (or D) and T within the two materials. In some cases, a pressure applied to the materials may also (or alternatively) be varied to control these properties. In some implementations, a gaseous mixture may be present in addition to the liquid lithium-containing material, in which cases there may be an exchange of H (or D) atoms across the solid-gas interface as well.

The getter material may be maintained at temperatures above a desorption temperature, defined at one reference pressure in a range from 0.01 bar to 2.0 bar, of the getter material for the second of the hydrogen isotopes, the pressure of the gaseous mixture in the one or more getters being less than or equal to the reference pressure.

In some implementations, the reference pressure may be chosen according to operational requirements. For example, the reference pressure may be 1 bar, or it may be less than 1 bar, e.g. from 0.01 bar to 1 bar, or from 0.1 bar to 1 bar, or from 0.01 bar to 0.1 bar, or from 0.01 bar to 0.5 bar, or from 0.1 bar to 0.5 bar, and so on. In some implementations, the reference pressure may correspond to a plateau pressure of the getter material for the first or the second of the hydrogen isotopes. The plateau pressure may correspond to a pressure at which the concentration of hydrogen in the getter material can be increased without a rise in the equilibrium pressure of hydrogen. The plateau pressure may correspond to the co-existence of two hydride phases in the getter material. In some alternative methods, the pressure of the gaseous mixture in the one or more getters may be greater than the reference pressure, e.g. 10%-50% greater than the reference pressure in some examples.

The preferential absorption of the first of the hydrogen isotopes (e.g. deuterium) relative to the second of the hydrogen isotopes (e.g. tritium) may occur through a thermodynamic isotope effect, which in this context refers to an increased thermodynamic favourability for the getter material to absorb the first of the hydrogen isotopes compared to the second of the hydrogen isotopes. This preference is reflected by the getter material having a different desorption temperature (which is a measure of the equilibrium constant for the absorption reaction) for the different hydrogen isotopes, with the desorption temperature of the first of the hydrogen isotopes being greater than the desorption temperature of the second of the hydrogen isotopes.

The desorption temperature is defined as the temperature at which reaction of 1 mole of hydrogen gas with a stoichiometric amount of the getter material yields, at equilibrium, a pressure of hydrogen gas equal to the reference pressure. Desorption temperatures may be measured using conventional methods based on temperature programmed desorption, such as temperature programmed desorption mass spectrometry (TPD-MS), or calorimetry. The reference pressure may be at the standard pressure of 10 ⁵ Pa (1 bar), in which case, the desorption temperature is referred to as a standard desorption temperature.

The desorption temperature does not, in general, correspond to a rapid change in the amount of gas absorbed by the getter material. Accordingly, the temperature of the getter material may be greater or less than the desorption temperature defined at the reference pressure, of the getter material for the first of hydrogen isotopes.

The term getter is used to refer to a device into which a gas can be introduced and which comprises getter material, i.e. material that is able to absorb a gas (in the present case, hydrogen isotopologues), and thereby lower the pressure of the gas in a volume adjacent in the getter material. The getter may comprise an inlet for receiving the gas and an outlet through which the gas is expelled, with the getter material being provided between the inlet and the outlet such that the gas flowing through the getter flows over or through the getter material. Absorption of a hydrogen isotope by the getter material may be referred to as “hydriding” (irrespective of which of the hydrogen isotopes is being absorbed). Conversely, desorption of any of the hydrogen isotopes by the getter material may be referred to as “dehydriding”. Preferably, the getter material absorbs the hydrogen isotopes by chemisorption (i.e. through a chemical reaction) as chemisorption typically leads to larger thermodynamic isotope effects, as compared to physisorption, for example.

By maintaining the getter material at temperatures above the desorption temperature of the getter material for the second of the hydrogen isotopes, absorption of the second of the hydrogen isotopes by the getter material is suppressed. The gaseous mixture at the outlet of each the one or more getters is therefore enriched in the second of the hydrogen isotopes relative to the gaseous mixture at the inlet of the getter as a result of the preferential absorption of the first of the hydrogen isotopes.

Depending on the getter material, the desorption temperature of the getter material for the second of the hydrogen isotopes may be greater than 400°C, greater than 500°C or even greater than 600°C. In applications where the gaseous mixture to be isotopically purified is obtained at high temperatures (e.g. greater than 400°C), it may therefore be possible to avoid or minimise the amount of cooling of the gaseous mixture before it is introduced into the one or more getters, thereby reducing the processing time needed by the isotope separation system. Smaller processing times are particularly beneficial when the isotope separation system is used to separate tritium from other hydrogen isotopes in systems where the purified tritium is re-used (such as in the inner fuel loop of a fusion reactor) as the overall amount of tritium that the system requires to operate continuously may be reduced.

In some implementations, the desorption temperature of the getter material for the second of the hydrogen isotopes may be greater than 400°C, greater than 500°C, greater than 600°C, greater than 700°C, or greater than 800°C, e.g. from 400°C to 800°C, from 500°C to 700°C, or from 600°C to 700°C, or from 600°C to 800°C.

The method may further comprise obtaining measurements of respective amounts of one or more of the hydrogen isotopes in the gaseous mixture or lithium-containing material, within or after the getter and adjusting, according to the measurements, the temperature of the getter material of the of the one or more getters to control (e.g. via a feedback loop) the respective amounts of the one or more hydrogen isotopes in the gaseous mixture. For example, the temperature of the getter may be adjusted to keep the ratio of the second of the hydrogen isotopes to the first of the hydrogen isotopes at a predetermined value (i.e. within a predetermined range about the predetermined value).

The method may further comprise adjusting, according to the measurements, the pressure of the gaseous mixture in the one or more getters to control the respective amounts of the one or more hydrogen isotopes in the gaseous mixture or lithium- containing material. Preferably the pressure in the one or more getters is maintained within 5% or 10% of a target pressure The gaseous mixture or lithium-containing material may be provided to the one or more getters from a source, e.g. a plasma chamber or breeder blanket (e.g. a breeder blanket comprising liquid lithium). The method may further comprise returning the gaseous mixture or lithium-containing material to the source after the gaseous mixture has passed through the one or more getters. The gaseous mixture or lithium-containing material may flow continuously from the source, through the one or more getters, and back to the source, i.e. the gaseous mixture or lithium-containing material from the source is provided to the one or more getters at the same time as the gaseous mixture or lithium- containing material that has already passed through the one or more getters is returned to the source.

The method may further comprise maintaining the getter material at temperatures within 100°C (or within 50°C) of the desorption temperature of the getter material for the second of the hydrogen isotopes.

The method may further comprise passing the gaseous mixture or lithium-containing material through one or more secondary getters. Each secondary getter comprises secondary getter material maintained at temperatures below a desorption temperature, defined at the reference pressure, of the secondary getter material for the second of the hydrogen isotopes. The secondary getter material may be different from or the same as the getter material of the one or more getters. The method may then comprise heating the one or more secondary getters to desorb gas comprising the second of the hydrogen isotopes from the secondary getter material. Use of one or more secondary getters in this way allows the second of the hydrogen isotopes to be stored or processed separately from other components of the gaseous mixture or lithium-containing material.

The getter material and/or the secondary getter material may be a metal or an alloy. For example, the getter material and/or the secondary getter material is or comprises one or more of zirconium, yttrium, and gadolinium. In another example, the getter material and/or the secondary getter material is an alloy comprising zirconium and cobalt, and optionally nickel, the alloy having a chemical formula ZrCoi. _x Ni _x , where x is from 0 to 0.3.

In some implementations, the getter material and/or the secondary getter material is a high entropy alloy. For example, the high entropy alloy may comprise 5 to 11 metallic elements, each having a mole fraction in the alloy from 5% to 30%. The metallic elements may have a single-phase microstructure (e.g. a solid solution) and/or be present in equiatomic or near-equiatomic amounts. The high-entropy alloy may additionally comprise other elements having smaller mole fractions than the 5 to 11 metallic elements (e.g. mole fractions less than 5%). In some cases, the metallic elements may be selected from: beryllium, magnesium, aluminum, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, platinum, gold, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium and terbium. In some preferred examples, the high entropy alloys (HEAs) may be or comprise Mo-Nb-Ta-V-W or Ti-Zr-Nb-Hf-Ta

A high entropy alloy may be a preferred choice of getter material because hydrogen isotopes (H, D or T atoms) may be absorbed and accommodated by the material without significant or severe deformation of the material, which may e.g. avoid the material being pulverised during absorption and/or desorption of hydrogen isotopes. Thus, H or D atoms may be preferentially absorbed when the gaseous mixture or lithium-containing material flows over the surface of the getter material without the getter material forming a powder (i.e. the structural integrity of the material may be retained or deteriorate less than for other types of alloy).

Getter material in the form of a (e.g. refractory) high entropy alloy may be resistant to lithium corrosion or degradation. It may therefore be used to extract tritium from a liquid lithium breeder blanket, such that, in use, the high entropy alloy absorbs tritium from the tritium-containing material present in the breeder blanket. In particular, the liquid lithium- containing material may comprise lithium hydride and lithium tritide, which contacts an exterior surface of the high entropy alloy, such that hydrogen isotopes are exchanged across the interface between the liquid lithium-containing material and the getter material. Alternatively or additionally, the liquid lithium may comprise entrained or dissolved diatomic molecules of hydrogen isotopes (e.g. H2, HT, T2) which are absorbed by the getter material.

The gaseous mixture may be or may comprise exhaust gas from a plasma chamber. The temperature of the getter material and the temperature of the exhaust gas may differ by less than 300 K, preferably less than 200 K, more preferably less than 100 K. Accordingly, the amount of cooling power needed to match the temperature of the gaseous mixture to the temperature of the getter material may be relatively low. In some implementations, this may mean that the one or more getters may be provided close to the exhaust of the plasma chamber and/or coupled to the exhaust of the plasma chamber such that the exhaust gas is not actively cooled before it reaches the one or more getters.

The plasma chamber may be operated as a fusion reactor. The method may then comprise receiving the gaseous mixture from the plasma chamber whilst the plasma chamber is operating as a fusion reactor and re-introducing a proportion of the gaseous mixture into the plasma chamber after it has passed through the one or more getters. Operating the plasma chamber as a fusion reactor in this context means operating the plasma chamber under conditions that cause deuterium and tritium nuclei (ions) confined in the plasma chamber to fuse.

The method may further comprise breeding tritium using neutrons from the plasma chamber operating as a fusion reactor, entraining the tritium in a carrier gas and mixing the carrier gas and tritium with the exhaust gas to form the gaseous mixture.

The method may further comprise subsequently heating the getter material to desorb the first of the hydrogen isotopes (e.g. deuterium) therefrom.

According to a second aspect of the present invention there is provided an isotope separation system for separating hydrogen isotopes including two or more of protium (H), deuterium (D) and tritium (T) from one or more of (i) a gaseous mixture comprising diatomic molecules thereof, and (ii) a lithium-containing material comprising the hydrogen isotopes. The system comprises one or more getters, each getter comprising getter material for enriching the gaseous mixture or lithium-containing material by preferentially absorbing, from the gaseous mixture or lithium-containing material, a first of the hydrogen isotopes relative to a second of the hydrogen isotopes. The first of the isotopes is protium or deuterium and the second of the hydrogen isotopes is tritium.

The system may further comprise a control system configured to maintain the getter material at temperatures above a desorption temperature, defined at one reference pressure in a range from 0.01 bar to 2 bar, of the getter material for the second of the hydrogen isotopes, and to maintain the pressure of the gaseous mixture in the one or more getters at less than or equal to the reference pressure.

The system may further comprise an analyser to measure respective amounts of one or more of the hydrogen isotopes in the gaseous mixture or lithium-containing material before, within or after the getter. The control system may be configured to receive measurements of the amount(s) from the analyser and adjust, according to the measurements, the temperature of the getter material of the one or more getters to control the respective amounts of the one or more hydrogen isotopes in the gaseous mixture.

Alternatively or additionally, the control system the control system may be configured to receive measurements of the amount(s) from the analyser and adjust, according to the measurements, the pressure of the gaseous mixture or lithium-containing material in the one or more getters.

According to a third aspect of the present invention there is provided a system comprising a plasma chamber and the isotope separation system of the second aspect of the invention. The isotope separation system is configured to receive the gaseous mixture from the plasma chamber. The plasma chamber may be a tokamak or stellarator, for example.

The isotope separation system may be configured to return some of the gaseous mixture to the plasma chamber after the gaseous mixture has passed through the one or more getters.

The plasma chamber may comprise one or more divertors for extracting and neutralising ions from a plasma confined with the plasma chamber. The isotope separation system is configured to receive a gaseous mixture comprising the neutralised ions from the one or more divertors.

The isotope separation system may be configured to return at least some of the gaseous mixture to the plasma chamber after the gaseous mixture has passed through the one or more getters. According to a fourth aspect of the present invention there is provided a fusion reactor comprising the system of the third aspect of the invention.

The fusion reactor may comprise a tritium breeding module configured to generate tritium from neutrons produced by fusion reactions occurring in a plasma chamber. The tritium breeding module is configured to introduce the generate tritium into the gaseous mixture before the gaseous mixture enters the one or more getters

Brief Description of the Drawings

Figure 1 is a graph showing hydrogen and deuterium desorption temperatures for alloys of composition ZrCoi. _x Ni _x ; and

Figure 2 is a diagram of the inner fuel cycle of a fusion reactor according to an embodiment of the present invention.

Detailed Description

The absorption of gaseous (diatomic) hydrogen by a metal or alloy (M) to form a metal hydride (MH _y ) may typically be described by a chemical equilibrium of the following form:

M + y/2 H ₂ MH _y .

Corresponding equilibria exist for diatomic deuterium (D2) or tritium (T2) reacting with the metal. Thus, for a closed, equilibrated system at constant temperature (T), the metal hydride will co-exist with an equilibrium pressure of hydrogen (p _Hz ), which can be expressed in terms of the standard enthalpy and entropy changes for the absorption of 1 mole of hydrogen gas as follows (in which p° is the standard pressure and R is the molar gas constant):

The temperature at which the hydrogen pressure (p _Hz ) equals the standard pressure (p°), i.e. a pressure of 10 ⁵ Pa (1 bar), is referred to as the standard desorption temperature. The standard desorption temperature can be calculated from the ratio of the standard enthalpy and entropy changes (as the logarithm in the above equation vanishes), or measured experimentally e.g. by temperature programmed desorption mass spectrometry (TPD-MS). Desorption temperatures can also be defined for pressures different from the standard pressure. For example, a reference pressure falling with a pressure range determined by operational requirements may be used.

As absorption generally occurs with dissociation of the hydrogen molecule, the enthalpy change for the absorption reaction depends on the bond strength of the diatomic molecule being absorbed, which in turn depends on the masses of the hydrogen isotopes in the molecule. For example, the bond strength of D2 is greater than that of H2 predominantly because the lower mass of hydrogen (H) compared to deuterium (D) means that the vibrational zero-point energy in H2 is greater than in D2. The same effect means that the bond strength of T2 is stronger than that of D2. The equilibrium constant for absorption of H2 by the metal is therefore typically greater than for D2 and T2. Thus, the enthalpy change for absorption is less exothermic for molecules of the heavier hydrogen isotopes than for molecules of the lighter hydrogen isotopes. This ordering is referred to as a “normal” isotope effect. The entropy change for the absorption reaction is generally dominated by the loss of entropy of the gas phase hydrogen molecules upon absorption (which depends inversely on the mass of the molecules) and therefore leads to the same (i.e. “normal”) isotopologue ordering as the enthalpy change. Therefore, overall, absorption of hydrogen by a metal is normally more thermodynamically favourable for the lighter hydrogen isotopes than the heavier hydrogen isotopes.

The enthalpy and entropy changes for formation of the solid metal hydride may also affect the size of the isotope effect, or even in some cases, give rise to an “inverse” isotope effect in which the relative stabilities of the metal hydrides follows a different ordering, e.g. with absorption of tritium being energetically preferred compared to deuterium or protium. More complex behaviour may also be found when the metal hydride can adopt more than one phase.

Similar equilibria exist when there is exchange of H atoms (or D or T atoms) between two solid phases, a solid and a liquid phase, or two liquid phases. Equilibrium constants can be determined (e.g. based on the free energies associated with the equilibria) from which the equilibrium ratio of the concentrations of H (or D) and T in the different phases can be determined. For example, one of the phases may have a higher affinity for H (or D) relative to T compared to the other phase, such that when equilibrium is reached following exchange of hydrogen atoms across an interface between the two phases, the concentration of H (or D) in the phase with the relatively higher affinity for H (or D) is increased, while the other phase becomes enriched with T. In general, both entropy and enthalpy may contribute to each of the phases having different affinities for H (or D) relative to T.

Figure 1 is a graph showing the different desorption temperatures for hydrogen and deuterium exhibited by various alloys of zirconium, cobalt and nickel having a composition ZrCoi. _x Ni _x . The measured decomposition temperature is shown on the Y axis, whilst the X axis shows the nickel content (x, where x is from 0.0 to 0.3) of the alloys. The desorption temperatures in this case are for a reference pressure of 1 bar (100 kPa) of hydrogen/deuterium. The solid line 101 of the graph shows the standard desorption temperatures for the deuterides of the various alloys, whilst the broken line 103 shows the standard desorption temperatures for the corresponding hydrides. The standard desorption temperatures of the hydrides are approximately 5 K higher than those for the deuterides for all of the ZrCoi. _x Ni _x alloys. For these particular alloys, the standard desorption temperatures for both hydrides and the deuterides decrease monotonically with increasing nickel content (x), with the maximum desorption temperature (for ZrCo) being about 683 K for the hydride and 678 K for the deuteride. Although not shown on the graph, the corresponding desorption temperature for the tritide may be 1-3 K lower that the desorption temperature, i.e. about 676 K.

Figure 2 shows schematically an inner fuel loop 200 of a fusion reactor 201 comprising a tokamak plasma chamber 203 in which a plasma 205 comprising deuterium and tritium can be confined. Alternatively, another type of plasma chamber can used, such as a stellerator. Exhaust gases leave the plasma chamber 203 through one or more outlets and then flow through an isotope separation system 207 comprising a primary getter 209, in which getter material for absorbing hydrogen from the exhaust gases is provided. The getter material is disposed between an inlet and an outlet of the primary getter 209 so that the exhaust gases flow over or through the getter material. The getter material is provided in a form that has a large surface area to maximise the rate of absorption of the hydrogen from the exhaust gases, e.g. on or more of a mesh, wire, sheet, powders, pebbles, foams and sponges. In this particular embodiment, the getter material is a zirconium cobalt alloy (ZrCo). The temperature and pressure of the primary getter is controlled by a temperature and pressure control system (not shown) which maintains the temperature of the getter material at temperatures above about 676 K (which as noted above is the estimated standard desorption temperature of the alloy for tritium), which allows the getter material to absorb more deuterium than tritium. The pressure of the exhaust gases is maintained at or below 1 bar in this example. An exhaust gas pressure below 1 bar is generally preferred to reduce the risk that tritium is expelled into the environment in the event of a leak. Although Figure 2 shows the isotope separation system as having a single primary getter 209, in practice more than one primary getter 209 may be used (e.g. 2 to 5 getters, more than 10 getters, or more than 100 getters).

The gas leaving the getter 209 is enriched with tritium relative to the exhaust gas entering the getter 209 from the plasma chamber 203. In the present example, the isotope separation system 207 comprises a secondary getter 211 downstream of the getter 209 to absorb tritium from the tritium-enriched gas, although it will be appreciated that the output of the getter 209 may in some applications be fed back into the plasma chamber 203 without passing through any further getters 211. The secondary getter 211 in the present example is the same as the upstream primary getter 209, except that it is operated at a temperature that is less than the standard desorption temperature of the alloy for tritium (i.e. less than 676 K), preferably significantly lower than this temperature (e.g. 500 K) order to maximise the amount of tritium that can be adsorbed by the secondary getter. The tritium adsorbed by the secondary getter 211 is released by heating the getter material. The released gas may be re-introduced into the plasma chamber 203 or else provided to a tritium storage module 213 for storage (e.g. using uranium getters) and/or further processing.

The inner fuel loop 200 of the fusion reactor 201 may be operated whilst the reactor is online (i.e. operating as a fusion reactor). For example, the exhaust gas from the plasma chamber 203 may flow continuously through the primary getter 209, with the enriched gas then being re-introduced into the plasma chamber 203. In this mode of operation, the secondary getter 211 may be operated in the same way as the primary getter 209 (i.e. so as to preferentially absorb deuterium as compared to tritium) or otherwise be absent from the inner fuel loop 200.

Other getter materials may be used in addition or as an alternative to ZrCo, with the temperature at which the getter material is maintained being adjusted to be less than the standard desorption temperature of the getter material for tritium. For example, the getter material may further comprise nickel, i.e. the getter material may be ZrCoi. _x Ni _x , with x > 0, as described above. Other examples of getter material include: zirconium; yttrium; alloys of zirconium and aluminium (e.g. ZrAI), optionally including vanadium (e.g. ZrAIV); ZrCoo.gFeo.i; and Pdo.77Ago.1oCuo.13. High entropy alloys (HEAs) such as Mo-Nb- Ta-V-W or Ti-Zr-Nb-Hf-Ta high entropy alloys may also be used as getter materials.

The isotope separation system 207 may comprise more than one primary getter 209, arranged in series, i.e. with the exception of the last getter, the outlet of the each getter is connected to the inlet of the next getter, with the inlet of the first getter being connected to the exhaust of the plasma chamber 203 and the outlet of the last getter being connected to a part of the inner fuel loop 201 downstream of the getters. Alternatively, the primary getters 209 may be connected in parallel, such that the inlet of each getter 209 is connected to the exhaust of the plasma chamber 203 and the outlet of each getter 209 is connected to the same part of the inner fuel loop 201 downstream of the getters. Similarly, the isotope separation system 207 may comprise more than one secondary getters 209 arranged in series or parallel with respect to one another.

A gas purity measurement system (not shown) may be used to monitor the ratio of tritium to deuterium at various locations of the inner fuel loop 207 and adjust the temperature of the getter material in the primary getter 209 and/or the temperature of the getter material in the secondary getter 211 accordingly, e.g. to maintain the ratio of tritium to deuterium in the gas leaving the primary and/or secondary getter with a certain range of a desired value. The gas purity measurement system comprises an analyser for measuring the concentrations of deuterium-containing and tritium-containing molecules in the gaseous mixture. The analyser may, for example, make the measurements via mass spectrometry (e.g. using a quadrupole mass spectrometer to rare gas analyser), electrochemical methods, and/or Raman spectroscopy.

The isotope separation system 207 may also be used extract tritium produced in a breeder module through the action of fusion neutrons on a lithium containing material. The produced tritium is released from the breeder into a helium carrier gas, which carries the tritium to the isotope separation system 207. The gas from the breeder module may be combined with the exhaust from the plasma chamber 203. Alternatively, separate isotope separation systems 207 may be provided for the exhaust of the plasma chamber 203 and the breeder module, with the tritium recovered from each being injected into the plasma chamber 203 through separate inlets or the through the same inlets. In some implementations, the breeder module may comprise liquid lithium, which contains hydrogen and tritium atoms or molecules, e.g. in the form of dissolved or entrained gas, or as lithium hydride and lithium tritide. The liquid lithium may be passed over the getter material (e.g. a high entropy alloy) so that the tritium can be preferentially absorbed, e.g. the liquid lithium may flow over the surface of the getter material.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It would be apparent to one skilled in the relevant art(s) that various changes in form and detail could be made therein without departing from the spirit and scope of the invention.