FIELD OF THE INVENTION

The invention provides a method for producing a tile-substrate assembly. Also provided is a tile-substrate assembly obtainable by the method described herein and a nuclear fusion reactor comprising a tile-substrate assembly as described herein. Also provided is a tile-substrate assembly precursor and a tile-substrate-spacer assembly.

BACKGROUND

Spacers are needed to hold tiles in precise positions during the manufacture of tile- substrate assemblies. Tiles may be bonded to the substrate by methods including brazing, diffusion bonding, hot isostatic pressing and welding or through the use of an adhesive. During diffusion bonding with hot isostatic pressure (HIP), there are requirements to ensure high geometric tolerance of assembled components is maintained prior to, and during, the bonding process, as well as in the final product. Solid spacers may be used, but tight requirements on chemistry and preventing bonding of the spacer to the tiles/substrate mean that there is often a coefficient of thermal expansion mismatch that results in difficulty in removing spacers after bonding, for instance spacers j amming between tiles.

In one example, tile-substrate assemblies may be used in a nuclear fusion reactor. The reactor lining of a nuclear fusion reactors is known as the “first wall”. The first wall is a plasma facing component, so must be resistant to the conditions used inside the reactor to create the plasma, whilst also not contaminating the plasma. In one form, plasma facing components consist of arrays of precisely positioned tiles attached to the front face of a substrate. Such tile-substrate assemblies may be referred to as armour tiles. These assemblies are manufactured by either brazing, diffusion bonding or hot-isostatic pressing, all of which require the tiles to be held in position with spacers during the process. First wall tile placement for HIP-DB results in a large number of spacers being placed between the tiles that then require complex, time consuming and high-risk removal strategies to ensure the tile-substrate assembly is free of contaminants for application. Current processes use graphite spacers which are hard to extract and can fracture leaving residual material which contaminates the tile-substrate assembly.

Hot pressing tools for powder (metallic and ceramic) often use lubricants, low adhesive materials and draft angles to reduce parts sticking following treatments at high pressures and temperatures. However, draft angles can only be effectively utilised in larger components. For a part of 1 mm thickness the maximum length of a 5° taper is 10 mm.

This is not expected to be stable enough to ensure that a spacer of this small size remains rigid during use. Further, draft angles are not permitted in certain applications, for instance in the manufacture of plasma facing components. Release agents may be used to lubricate and reduce adhesion of solid spacers in a wide range of applications. Through the use of a release agent, solid spacers could be mechanically removed following FHP-DB. However, spacers require additional height or other features in order to provide a suitable grip for removal. Further, such a release agent would be difficult to remove from a narrow gap after HIP, and may leave some residue if not bonded to the spacer directly, meaning that a strict purging process would be required to remove all trace of the release agent.

There therefore exists a need to develop a spacer that remains stable and thereby ensures a high geometric tolerance of assembled components is maintained during the bonding process, but which can be easily removed from the tile-substrate assembly afterwards, without leaving behind contaminants.

SUMMARY OF THE INVENTION

The invention relates to a method for producing a tile-substrate assembly which comprises an array of tiles bonded to a substrate, said method comprising: (a) arranging the tiles in an array on the substrate;

(b) bonding the tiles to the substrate to form a tile-substrate assembly, wherein the step of bonding is performed while adjacent tiles in the array are separated by spacers comprising a spacer material which remains solid during bonding of the tiles to the substrate, and (c) removing the spacers from the tile-substrate assembly by a removal process which comprises physically or chemically altering the bulk structure of the spacer material.

The invention relates to a method for producing a tile-substrate assembly using a spacer comprising a spacer material that is compatible with the bonding process, i.e. which remains solid and intact and for example does not melt, disintegrate, weaken or otherwise lose structural integrity under the conditions employed to bond the tiles to the substrate to any extent that might affect the precision of the tile positions in the tile-substrate assembly, but which may easily be removed from the tile-substrate assembly after bonding, using a removal process that physically or chemically alters the bulk structure of the spacer material. To achieve good tolerances in the tile assembly, the spacers may be selected to withstand the physical conditions (e.g. temperatures, pressures and the chemical environment) of the bonding process. Unlike the bonding process, the removal process does physically or chemically alter the bulk structure of the spacer material, thereby disrupting the physical integrity of the spacer material and facilitating the removal of the spacer from the gaps between tiles. For instance, the spacer material may be melted, burnt away, decomposed, sublimated, dissolved or reacted with a chemical during the removal process, such that physical integrity of the spacer material is lost and the spacer can easily and cleanly be removed from between the tiles, without leaving any residue behind on the tile-substrate assembly. Thus, the spacer material is selected such as to be inert with respect the conditions used in the bonding process but to be physically or chemically reactive in response to the conditions used in the removal process.

The invention also provides a tile-substrate assembly obtainable by the methods described herein.

The invention also provides a nuclear fusion reactor comprising a tile-substrate assembly as described herein.

The invention also provides a tile-substrate assembly precursor comprising tiles as described herein, a substrate as described herein, and spacers as described herein, wherein the tiles are arranged in an array on the substrate, and are separated by the spacers.

The invention also provides a tile-substrate-spacer assembly comprising tiles as described herein, a substrate as described herein, and spacers as described herein, wherein the tiles are arranged in an array on the substrate, and are separated by the spacers, and wherein the tiles are bonded to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS Figs la- Id shows a schematic of the method of the invention, where the spacer material is dissolved to facilitate removal of the spacer. Fig. la shows the inserting of a spacer between tiles in an array of tiles on a substrate. Fig. lb shows the step of bonding the tiles to the substrate to form a tile-substrate assembly. Fig. lc shows the spacer removal process. Fig. Id shows the completed tile-substrate assembly. Fig. 2 shows the tiles, substrate and spacers inside a HIP canister base.

Fig. 3 shows a cross section of the tiles, substrate and spacers inside a sealed HIP canister.

Fig. 4 shows the substrate comprising a base and an interlayer. DETAILED DESCRIPTION

The invention relates to a method for producing tile-substrate assembly using a removable spacer, the spacer comprising a spacer material which remains solid during bonding of the tiles to the substrate, but which is sensitive to a removal process which comprises chemically or physically altering the bulk structure of the spacer material.

Figs la- Id illustrate a method for producing a tile-substrate assembly 1 comprising tiles 3 bonded to a substrate 2.

In a first step shown in Fig. la, the tiles 3 are arranged in an array on the substrate 2. Spacers 4 are used to separate adjacent tiles 3 in the array.

In a second step shown in Fig. lb, the tiles 3 are bonded to the substrate 2 to form a tile-substrate assembly 1. The step of bonding is performed while adjacent tiles 3 in the array are separated by spacers 4. The spacers 4 comprise a spacer material which remains solid during bonding of the tiles 3 to the substrate 2.

Bonding process

The step of bonding the tiles 3 to the substrate 2 may be performed by a bonding process which comprises applying heat and/or pressure and/or contact and/or hold duration to the tiles 3 arranged in an array on the substrate 2, typically, the step of bonding the tiles 3 to the substrate 2 may be performed by a bonding process which comprises applying heat and/or pressure to the tiles 3 arranged in an array on the substrate. The step of bonding the tiles 3 to the substrate 2 may be performed by a bonding process which comprises applying heat only to the tiles 3 arranged in an array on the substrate 2. The step of bonding the tiles 3 to the substrate 2 may be performed by a bonding process which comprises applying pressure only to the tiles 3 arranged in an array on the substrate 2. Typically, the step of bonding the tiles 3 to the substrate 2 may be performed by a bonding process which comprises applying heat and pressure to the tiles 3 arranged in an array on the substrate 2.

The step of bonding the tiles 3 to the substrate 2 may be performed by a bonding process selected from the group consisting of hot-isostatic pressing, brazing, welding, sintering, casting, adhering by an adhesive or moulding to bond the tiles 3 to the substrate 2

For instance, when the bonding process comprises welding, the welding may be selected from the group consisting of diffusion welding, solvent welding, tungsten inert gas (TIG) welding, laser welding, electron beam welding, friction welding, electro-resistive welding and ultrasonic welding. When the bonding process comprises brazing, the brazing may be selected from the group consisting of vacuum brazing, torch brazing, furnace brazing, laser brazing and electron beam brazing.

Preferably, the bonding process comprises hot isostatic pressing to bond the two or more tiles 3 to the substrate 2. Thus, the step of bonding the tiles 3 to the substrate 2 may be performed by hot-isostatic pressing. Typically, hot-isostatic pressing is performed at a temperature of at least 200 °C, for instance at least 400 °C, at least 500 °C, at least 600 °C preferably at least 700 °C. Hot isostatic pressing may be performed at a pressure of at least 50 MPa, preferably at least 100 MPa. For instance, hot-isostatic pressing may be performed at a temperature of at least 700 °C, preferably from 700 °C to 800 °C and at a pressure of at least 100 MPa, preferably of between 125 MPa and 150 MPa.

Hot isostatic pressing may be performed at a temperature of at least 500 °C, for instance at a temperature from 500 °C to 700 °C, preferably from 550 °C to 650 °C. Hot isostatic pressing may be performed at a pressure of at least 100 MPa, for instance at a pressure of from 100 to 150 MPa, typically at a pressure of from 100 to 125 MPa. Hot isostatic pressing may be performed at a temperature of between 500 °C and 700 °C and a pressure of from 100 to 150 MPa, preferably at a temperature of from 550 °C to 650 °C and a pressure of from 100 to 125 MPa. For instance, hot isostatic pressing may be performed at a temperature of about 580 °C and a pressure of about 105 MPa. Fig. 2 shows the tiles 3, substrate 2 and spacers 4 inside the canister base 5 for hot- isostatic pressing. Fig. 3 shows a cross section of the tiles 3, substrate 2 inside the sealed canister 6 for hot-isostatic pressing. A graphite foil cover 7 may be placed over the tiles 3 and substrate 2 and the canister lid 8 is welded on.

The bonding process may comprise adhering the tiles 3 to the substrate 2 using an adhesive. In this instance, the bonding process may not comprise applying heat and/or pressure to the tiles 3 arranged in an array on the substrate 2. The skilled person would be well aware of suitable adhesives for bonding tiles 3 to substrates 2, and would be able to select a suitable adhesive based on the combination of tile and substrate materials.

In a third step shown in Fig. lc, the spacer 4 is removed from the tile-substrate assembly 1 by a removal process which comprises physically or chemically altering the bulk structure of the spacer material. In the example shown in Fig. lc, the removal process comprises dissolving the spacer material in a solvent 9. Fig. Id shows the final tile- substrate assemblyl following removal of the spacer 4. More generally, the removal process could be any process which physically or chemically alters the bulk structure of the spacer material.

Removal process

Typically, physically or chemically altering the bulk structure of the spacer material disrupts the physical integrity of the spacer, thereby facilitating removal of the spacer from between the tiles 3. The term “bulk structure” distinguishes from processes which superficially cause minor alterations at the surface of the spacer material, such as chipping away the spacer to remove it from between the tiles 3, but which do not alter the bulk of the spacer material in any way. The removal process therefore does not encompass methods which consist of mechanically prising, scraping or chipping the spacer to remove it from between the tiles 3.

The removal process comprises physically or chemically altering the bulk structure of the spacer material. A physical alteration to the bulk structure of the spacer material is a change to the physical form of the spacer material, but not the chemical composition of the spacer material. By way of example, a physical alteration to the spacer material could be melting the spacer material, vaporising, decomposing or subliming the spacer material or dissolving the spacer material. On the other hand, a chemical alteration to the bulk structure of the spacer material involves changing the chemical composition of the spacer material, for instance changing or destroying the molecular or crystal structure, and/or changing the oxidation state of the atoms and/or ions present in the spacer material. By way of example, a chemical alteration to the spacer material could comprise oxidising or reducing the spacer material, or reacting the spacer material with an acid or alkali.

The removal process typically comprises physically or chemically altering the bulk structure of the spacer material such that at least part of the spacer material is no longer solid. For example, the removal process may comprise melting the spacer material, dissolving the spacer material or reacting the spacer material with a chemical so as to disrupt or weaken the solid structure of the spacer material.

Preferably, the removal process is compatible with the tile-substrate assembly 1. Thus, the removal process usually does not cause chemical or physical change to the tile- substrate assembly 1. Typically, the tile-substrate assembly 1 is inert with respect to the removal process. For instance, the removal process should not cause any chemical reaction or physical change to the tiles 3 or substrate 2, or effect the bond formed between the tiles 3 and the substrate 2 in the bonding step. Typically, the geometric tolerances in the positions of the tiles on the substrate are not affected by the removal process.

Preferably, the removal process completely removes all traces of the spacer from the tile-substrate assembly 1. Typically, the removal process comprises heating the spacer 4, dissolving the spacer material in a solvent or reacting the spacer material with a chemical.

Preferably, as shown in Fig. lc, the removal process comprises dissolving the spacer material in a solvent 9. The solvent 9 may be selected from the group consisting of a polar solvent, an nonpolar solvent, a protic solvent, an aprotic solvent, a liquid metal, an ionic liquid, an organic solvent or an inorganic solvent. Examples of such solvents are known to the skilled person. The skilled person would also be aware of possible spacer materials and their solubility in various solvents and would readily be able to determine combinations of spacer material and solvent which would be compatible in the removal process. The solvent 9 may be a polar solvent, for instance a polar protic solvent such as an alcohol or water. The solvent 9 may comprise isopropanol (2-propanol), methanol, or ethanol. Typically, the solvent 9 comprises water i.e. is an aqueous solvent. Preferably the solvent 9 is water.

The removal process may comprise heating the spacer 4 to chemically alter the bulk structure of the spacer material. For instance, the removal process may comprise heating the spacer 4 to chemically alter the bulk structure of the spacer material by burning the spacer material, decomposing the spacer material or pyrolysing the spacer material. Typically, burning and pyrolysis are performed in the presence of oxygen. Thus, the removal process may comprise heating and oxidizing the spacer material. The removal process may comprise heating the spacer 4 to physically alter the bulk structure of the spacer material. For instance the removal process may comprise heating the spacer 4 to physically alter the bulk structure by melting, vaporising or subliming the spacer material. Typically the removal process comprises heating the spacer material to physically alter the bulk structure by melting the spacer material or laser ablating the spacer material.

The removal process may comprise reacting the spacer material with a chemical selected from the group consisting of an acid, an alkali, an oxidising agent or a reducing agent. The removal process may comprise reacting the spacer material with a chemical during electrolysis on the spacer material. The chemical is typically selected from chemicals that are unreactive with respect to the tile-substrate assembly 1, but which react with the spacer material.

Reacting the spacer material with a chemical typically causes a chemical change in the bulk structure of the spacer material which facilitates removal of the spacer from the tile-substrate assembly 1. Typically, reacting the spacer material with a chemical causes a chemical change in the bulk structure of the spacer material which causes the spacer material to weaken and disintegrate. For instance, the removal process may comprise reacting the spacer material with an acid, wherein the acid corrodes the spacer material. Spacer Material

Typically, the spacer material is selected from materials that are compatible with the bonding step, but which are chemically or physically altered during the removal process.

The spacer material remains solid during the bonding step, shown in Fig. lb. Typically, the spacer material is inert to the conditions used in the bonding step, and remains unchanged during the bonding step. Thus, typically the spacer preserves geometric tolerances in the tile-substrate assembly during its manufacture.

The spacer material may be soluble in a solvent 9. For instance, the spacer material may be soluble in a solvent 9 selected from the group consisting of a polar solvent, an nonpolar solvent, a protic solvent, an aprotic solvent, a liquid metal, an ionic liquid, an organic solvent or an inorganic solvent. Typically, the spacer material may be soluble in a polar protic solvent. The polar protic solvent may comprise an alcohol and/or water. Typically, the spacer material may be soluble in an aqueous solvent. Preferably the spacer material is soluble in water. The spacer material may be selected from materials that remain unchanged when subjected to heat and or pressure. Thus, when the step of bonding the tiles 3 to the substrate 2 comprises applying heat and/or pressure to the tiles 3 arranged in an array on the substrate 2, the spacer material remains unchanged during the bonding step.

The spacer material may be selected from materials that remain solid when subjected to heat and or pressure. Thus, when the step of bonding the tiles 3 to the substrate 2 comprises applying heat and/or pressure to the tiles 3 arranged in an array on the substrate 2, the spacer material remains solid during the bonding step.

The spacer material may be selected from materials which remain unchanged when subjected to a pressure of at least 10 MPa, at least 50 MPa, preferably at least 100 MPa. The spacer material may be selected from materials which have a melting point of at least 100 °C, or from materials which have a melting point of at least 250 °C, or from materials which have a melting point of at least 500 °C. In some instances, the spacer material may be selected from materials which have a melting point of at least 700 °C or at least 800 °C.

For instance, the spacer material may be selected from materials which have a melting point of at least 100 °C and which remain unchanged when subjected to a pressure of at least 10 MPa. The spacer material may be selected from materials which have a melting point of at least 250 °C and which remain unchanged when subjected to a pressure of at least 50 MPa. Preferably, the spacer material is selected from materials which have a melting point of at least 500 °C and which remain unchanged when subjected to a pressure of at least 100 MPa. For instance, the spacer material is selected from materials which have a melting point of at least 700 °C, preferably at least 800 °C and which remain unchanged when subjected to a pressure of at least 100 MPa.

For instance, the spacer material may be selected from materials which have a melting point of at least 100 °C and which remain solid when subjected to a pressure of at least 10 MPa. The spacer material may be selected from materials which have a melting point of at least 250 °C and which remain solid when subjected to a pressure of at least 50 MPa. Preferably, the spacer material is selected from materials which have a melting point of at least 500 °C and which remain solid when subjected to a pressure of at least 100 MPa. For instance, the spacer material is selected from materials which have a melting point of at least 700 °C, preferably at least 800 °C and which remain solid when subjected to a pressure of at least 100 MPa.

The skilled person would be aware of various possible spacer materials and their fundamental properties, for instance melting point, resistance to pressure and solubility in various solvents, and would be able to select an appropriate material based on the removal process. Typically, the spacer material is selected from the group consisting of an ionic solid, a polymer, a solid organic material, or a metal. In the context of the present application, the term “metal” refers to both an elemental metal and an alloy comprising two or more metals.

Typically, the spacer material is an ionic solid. The spacer material may be selected from a halide, an oxide, a hydride, a nitrate, a carbonate, a sulfate or a hydroxide, typically, from a halide salt, an oxide, a hydride or a hydroxide. Typically, the spacer material is a halide salt comprising one or more anions selected from fluoride, chloride, bromide and iodide. Typically, the ionic solid comprises a metal cation. Thus, the spacer material may be selected from a metal halide, a metal oxide, a metal hydride, a metal nitrate, a metal sulfate or a metal hydroxide, typically, from a metal halide, a metal oxide, a metal hydride or a metal hydroxide. Usually, the spacer material comprises a group I (alkali) or group II (alkaline earth) metal cation. For instance, the spacer material may comprise a metal cation selected from the group consisting of lithium, sodium, potassium, rubidium, caesium, beryllium, magnesium, calcium, strontium or barium cations.

Typically, the spacer material is a metal halide. Preferably the spacer material is an alkali metal halide. Thus, the spacer material may comprise a lithium, sodium, potassium, rubidium or caesium cation and a halide anion. For instance, the spacer material may be an alkali metal fluoride, chloride, bromide or iodide. Typically, the spacer material is selected from the group consisting of lithium fluoride, chloride, bromide or iodide; sodium fluoride, chloride, bromide or iodide; or potassium fluoride, chloride, bromide or iodide. The spacer material may be selected from the group consisting of lithium fluoride, sodium fluoride, sodium chloride, potassium fluoride or potassium chloride. Preferably the spacer material is sodium chloride.

When the spacer material is an ionic solid, typically an alkali metal halide salt, preferably lithium fluoride, sodium fluoride, sodium chloride, potassium fluoride or potassium chloride, the removal process typically comprises dissolving the spacer material in a solvent 9, as shown in Fig. lc. Preferably the solvent 9 is a polar protic solvent such as an alcohol or water. The solvent 9 may comprise isopropanol (2 -propanol), methanol, or ethanol. Typically, the solvent 9 is comprises water i.e. is an aqueous solvent.

Preferably the solvent 9 is water. For example, the removal process may comprise dissolving the spacer material by submerging the spacer material in water, preferably deionised water, and applying ultrasound.

For instance, the spacer material may be selected from the group consisting of lithium fluoride, sodium fluoride, sodium chloride, potassium fluoride or potassium chloride and the removal process may comprise dissolving the spacer material in an aqueous solvent, preferably the removal process comprises dissolving the spacer material in water. For instance, the spacer material may be selected from the group consisting of lithium fluoride, sodium fluoride, sodium chloride, potassium fluoride or potassium chloride and the removal process may comprise dissolving the spacer material by submerging the spacer material in water, preferably deionised water, and applying ultrasound.

The spacer material may be solid organic material selected from a flammable organic material, a sugar, a wax or a polymer. Typically, when the spacer material is solid organic material, the removal process may comprise heating the spacer 4 to physically alter the bulk structure by melting, vaporising or subliming the spacer material. Typically, when the spacer material is solid organic material, the removal process comprises heating the spacer 4 to physically alter the bulk structure by melting the spacer material.

The spacer material may be a flammable organic material selected from the group consisting of wood, cardboard and flammable foams. Typically, when the spacer material is a flammable organic material, the removal process comprises burning or pyrolysing the spacer material.

The spacer material may be a wax. Typically, when the spacer material is a wax, the removal process comprises melting, burning or pyrolysing the spacer material, preferably melting the spacer material.

The spacer material may be a polymer. Typically, when the spacer material is a polymer, the removal process comprises melting, burning, decomposing or pyrolysing the spacer material. The method may typically comprise:

(a) arranging the tiles 3 in an array on the substrate 2;

(b) bonding the tiles 3 to the substrate 2 by applying heat and/or pressure to the tiles 3 arranged in an array on the substrate 2 to form a tile-substrate assembly 1, wherein the step of bonding is performed while adjacent tiles 3 in the array are separated by spacers 4 comprising a soluble spacer material which remains solid during bonding of the tiles 3 to the substrate 2, and

(c) removing the spacer from the tile-substrate assembly 1, wherein removing the spacer 4 comprises dissolving the spacer material in a solvent.

For instance, the method may comprise: (a) arranging the tiles 3 in an array on the substrate 2;

(b) bonding the tiles 3 to the substrate 2 by hot-isostatic pressing to form a tile- substrate 2 assembly, wherein the step of bonding is performed while adjacent tiles 3 in the array are separated by spacers comprising a soluble spacer material which remains solid during bonding of the tiles 3 to the substrate 2, and (c) removing the spacer 4 from the tile-substrate assembly 1, wherein removing the spacer 4 comprises contacting the spacer material with an polar protic solvent, preferably an aqueous solvent, for instance water, to dissolve the spacer material.

The method may comprise: (a) arranging the tiles 3 in an array on the substrate 2;

(b) bonding the tiles 3 to the substrate 2 by hot-isostatic pressing to form a tile- substrate assembly 1, wherein the step of bonding is performed while adjacent tiles 3 in the array are separated by spacers 4 comprising a spacer material which is a soluble ionic solid which remains solid during bonding of the tiles 3 to the substrate 2, and

(c) removing the spacer from the tile-substrate assembly 1, wherein removing the spacer 4 comprises contacting the spacer material with an polar protic solvent, preferably an aqueous solvent, for instance water, to dissolve the spacer material.

Typically, the method comprises (a) arranging the tiles 3 in an array on the substrate 2;

(b) bonding the tiles 3 to the substrate 2 by hot-isostatic pressing to form a tile- substrate assembly 1, wherein the step of bonding is performed while adjacent tiles 3 in the array are separated by spacers 4 comprising an spacer material that is a metal halide salt, preferably an alkali-metal halide salt, more preferably where the spacer material is selected from the group consisting of lithium fluoride, sodium fluoride, sodium chloride, potassium fluoride and potassium chloride, and

(c) removing the spacer 4 from the tile-substrate assembly 1, wherein removing the spacer comprises contacting the spacer material with an aqueous solvent, preferably water, to dissolve the spacer material.

Nature of Assembly

Typically, the tile-substrate assembly 1 is an armour component for a nuclear fusion reactor. Thus, the tile-substrate assembly 1 may be an armour component for facing a plasma. In this situation the tiles 3 and substrate 2 comprise materials that are resistant to the conditions inside a nuclear fusion reactor.

The tiles 3 typically comprise, consist essentially of or consist of a metal, an alloy or carbon. The tiles 3 typically comprise, consist essentially of or consist of a metal or carbon. The tiles 3 typically comprise, consist essentially of or consist of a metal. The metal may be selected from the group consisting of beryllium, tungsten and molybdenum. The metal alloy may be a copper-chromium-zirconium alloy. Preferably, the tiles 3 comprise, consist essentially of or consist of beryllium, tungsten or molybdenum, or a copper-chromium-zirconium alloy. For instance, the tiles 3 may consist of beryllium, tungsten or molybdenum, or a copper-chromium-zirconium alloy. The substrate 2 typically comprises, consists essentially of or consists of a metal.

Preferably the substrate 2 comprises, consists essentially of or consists a copper alloy, for instance a copper-chromium-zirconium alloy, or steel.

The substrate 2 may comprise at least one layer, for instance two layers. For instance, the substrate may comprise a base 21 and an interlayer 22, wherein the tiles 3 bond to the interlayer 22 such that in the tile-substrate assembly 1 the interlayer is disposed between the tiles 3 and the base 21. A substrate 2 comprising a base 21 and an interlayer 22 is shown in Fig. 4. Typically, the base 21 comprises, consists essentially of or consists a copper alloy, for instance a copper-chromium-zirconium alloy, or steel. For instance, the base 21 may comprise, consist essentially of or consist a copper-chromium-zirconium alloy. Typically, the interlayer 22 comprises, consists essentially of or consists of a metal. Preferably the interlayer 22 comprises, consists essentially of or consists of copper. For instance the interlayer 22 may consist of copper metal.

The invention also provides a tile-substrate assembly 1 obtainable by the method as described herein. The invention also provides a nuclear fusion reactor comprising a tile-substrate assembly 1 as described herein. Typically, the nuclear fusion reactor is a tokamak.

The invention also provides a tile-substrate assembly precursor comprising tiles 3 as described herein, a substrate 2 as described herein, and spacers 4 as described herein, wherein the tiles 3 are arranged in an array on the substrate 2, and are separated by the spacers 4.

The invention also provides a tile-substrate-spacer assembly comprising tiles 3 as described herein, a substrate 2 as described herein, and spacers 4 as described herein, wherein the tiles 3 are arranged in an array on the substrate 2, and are separated by the spacers 4, and wherein the tiles 3 are bonded to the substrate 2. Typically, in the tile-substrate assembly precursor and the tile-substrate-spacer assembly, the tiles 3 comprise a metal or carbon, the substrate 2 comprises a metal and the spacers 4 comprise a spacer material which is a metal halide, preferably an alkali metal halide. For instance, in the tile-substrate assembly precursor and the tile-substrate-spacer assembly, the tiles 3 comprise beryllium, tungsten, molybdenum, or carbon, the substrate 2 comprises a copper alloy or steel and the spacers 4 comprise a spacer material selected from the group consisting of lithium fluoride, sodium fluoride, sodium chloride, potassium fluoride or potassium chloride, preferably sodium chloride.

Example 1

The following example sets out how various spacer materials may be tested to assess their suitability for the method of the present invention.

Spacer manufacture

Spacers 4 will be produced from several materials to approx. 36 ^c 10 ^c 1 mm sheets. These materials will be produced in the following ways:

• Mica - Procured as solid sheet. This will be scored and broken to size and layered if required to reach approximate thickness;

• Lithium Fluoride - Procured as solid sheet. This will be scored and broken to size and layered if required to reach approximate thickness;

• Sodium Chloride - Procured as solid sheet and powder. Sheets will be scored and broken to size and layered if required to reach approximate thickness. Powder will be pressed into circular sheets of approximate thickness. At least one pressed and one sheet spacer will be encapsulated in foil to limit contamination;

• Copper-Tin alloy - Procured as Cu-20Sn powder and pressed into circular sheets of approximate thickness. Spacers will be encapsulated in stainless steel foil to limit bonding and contamination;

• Partially sintered powder - Procured as a commercially available bronze (CulOSn) filter and cut to thickness using a metallurgy saw. Spacers will be encapsulated in foil to limit bonding and contamination;

• Graphite - Spacers used for the current design will be used;

• Solid - Solid spacers will be made of 1 mm 316 stainless steel sheet, cut to size and coated in a release agent;

• Release agents - Boron nitride and or yttrium oxide will be procured as paintable release agents;

• Foil - Foil used to reduce contamination will be AISI 310 or 316 steel foil 0.025 mm thickness. Uniaxial compaction die - Sodium chloride will be pressed into 1 mm sheets using a uniaxial press. The tool design uses a hard stop to ensure thickness cannot be lower than 1 mm. The base is removable in order to aid removal of the pressed spacer. The tool will be made of tool steel to reduce contamination through tool wear. Calculations based on desired green density will be used to set filling mass. The powder will be vibrated until a packing density of at least 60% has been achieved (measured using the press height). Spacer thickness and mass will be measured to calculate green density.

Manufacture of substrate, tiles and HIP canister

The tiles 3 will be machined 36 x 36 x 12 mm copper cuboids, with triangular half- tiles. This matches the smallest length of beryllium tiles use in the real first wall. The smallest length was chosen to provide a representative size tile whilst optimising use in the HIP, and ensuring that all spacers would be subjected to the same conditions whether placed in the x or y direction. Half tiles were used to further optimise available space.

The substrate 2 will be made as an octagonal Copper Chrome Zirconium plate. The plate will be 20 mm thick and hold 5 tiles and 4 half tiles with 1 mm spaces on either side. This will provide up to 14 places for spacers 4 in each assembly 1.

The canister 6 will be a grade 316L stainless steel sheet fabrication conforming to the shape of the substrate 2. The wall will be attached directly to a base 5 and a lid 8 will seal the top. Two canisters will be produced with evacuation tubes extending from the top and side in order to allow stacking within the HIP.

Method of assembly

Two canisters will be HIPed. This will allow spacer methods which are likely to interact in a detrimental way to be separated. A third assembly will also be produced as mitigation for the unlikely event of a failure during HIP. The wall will first be fitted around the substrate 2 and attached to the base, the fill-tube will be attached to the bottom canister. Tiles 3 and spacers 4 will then be laid out as shown in Fig. 2, to limit interaction between differing spacer methods, whilst optimising space and cost. 3 x 3 spacers will be used for each concept and 3 graphite spacers will be used to baseline the process. There is also space for an additional 2 spacers in the middle sections, for concepts with additional variants. The precise layout of the spacers is shown in Table 1 Table 1

During assembly, the thickness of each spacer 4 will be given a unique number and measured using a digital micrometre or calliper. The test number and material will be scribed onto the corresponding tile. Due to the varying thickness and tolerance of spacers, two perpendicular ends will be packed with stainless steel shim in order to reduce movement.

Once all spacers 4 have been added the top of the assembly will be scanned for geometric information using structured light scanning. As shown in Fig. 3, a graphite foil cover 7 will be placed over the tiles 3 and substrate 2 and the canister lid 8 will be welded on. The top canister will have the fill tube welded to the lid previous to this step.

The canister can then be checked for leaks using helium, evacuated and then crimped. The canister will be transported for HIPing on a flat trolley and slowly loaded into the HIP to reduce any risk of spacers moving.

Removal

Once HIPed, the canister lid 8 and base 5 will be removed. The assemblies will then be scanned using structured light once more to compare pre and post HIP location of the tiles 3.

The materials will then be removed in a specific order to reduce the risk of different removal methods interacting. Once a spacer has been removed the spacer and its HIPed location will be photographed. The order of removal is as follows:

• Attempt to manually remove loose sintered spacers with thin tweezers: Measure the thickness of any spacers that are easily removed.

• Attempt to physically remove remaining sinter-based spacers with additional force: Measure any solid pieces.

• Attempt to remove the solid release agent-coated spacers including two graphite spacers;

• Use liquid nitrogen to remove CTE based spacers and any remaining solid spacers (including the remaining graphite spacers); • Wash soluble spacers thoroughly with deionised water, until all soluble spacers are removed.

Analysis

Analysis of test pieces will be as follows:

• All spacers will be measured before and after HIP if possible;

• All spacers will be photographed before and after HIP;

• All assemblies will be inspected using structured light before and after HIP to analyse location and size of gaps;

• All spacer-gaps will be photographed before and after HIP to analyse any damage or contamination;

• All spacers will be graded on pre-HIP integrity;

• All spacers will be graded on difficulty to remove post HIP;

• A segment of each gap will be sectioned for analysis using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis to measure diffusion or contamination.

Example 2

The use of NaCl spacers 4 in a HIP bonded tile assembly was demonstrated. A small-scale HIP bonded assembly with NaCl spacers 4 was produced, and the spacers were removed post HIPing via dissolution in deionised (DI) water. The substrate consisted of a 20mm thick octagonal CuCrZr and a 2mm Cu interlayer. An octagonal array of nine CuCrZr tiles 3 - five cuboid tiles with four triangular half tiles - were positioned on the Cu interlayer. The tiles 3 were degreased and the bond surfaces ground prior to assembly.

The tiles 3 were separated by bespoke NaCl lens blanks as spacers 4. These spacers 4 were 36x12 mm with a thickness of 1.3mm (+0.00mm, -0.08mm). In a first instance, the

NaCl spacers 4 were in direct contact with the tiles 3. In a second instance, the NaCl spacers 4 were separated from the tiles 3 with a 316 stainless steel foil barrier.

The tiles 3 were packed tightly in the cannister 6 using 316 stainless steel shims between the cannister wall and tile assembly, which applied a small compressive load. The locational stability was tested prior to welding the cannister lid 8. A graphite layer was positioned on top of the tile assembly, and the canister lid 8 with the central evacuation tube was welded shut.

The canister 6 was HIPed at 580°C and 105 MPa for 125 minutes. The tiles 1 successfully bonded to the substrate, and the NaCl spacers 4 maintained a gap between tiles 3. Prior to HIPing, a gap between spacers 4 was present at the intersection of tiles. During HIPing the spacer material was extruded into this gap. Additionally, in two locations the spacer material extruded out the side of the assembly. The soluble spacers 4 were then removed by submerging the diffusion bonded assembly in an ultrasonic bath filled with deionised (DI) water. This water was regularly replaced every 20 minutes.. After 40 minutes all of the spacers 4 were visibly removed.

The assembly 1 was finally rinsed with deionised water and immediately dried with hot air. No visible contamination was noted. The use of 316 stainless steel foil barriers did not appear to be required.

The gap width was measured at three locations along the tiles 3using a feeler gauge or metal shims in conjunction with the feeler.

The NaCl spacers 4 deformed during HIPing causing the gaps between the tiles 3 to shrink. The smallest shrinkage occurred in the spaces around the central tile (3.35 percent change), while the outermost gaps displayed the greatest shrinkage (17.11 percent change).

The angular misalignment of the tiles 3 was also calculated from the variation in gap size along the width of the tile. Similar to gap shrinkage, the tiles surrounding the centre tile had the smallest misalignment (0 degrees), while the outermost had the largest (0.398 degrees). As noted, the inner spacers were found to have been extruded into the intersection gaps. This resulted in gap shrinkage along the innermost spaces. The outer spacers extruded into the gap between the outer tiles and the canister 6, an area much larger than the intersection gaps. This accounts for the higher shrinkage and misalignment observed in the outer tiles. These issues could be addressed by filling the intersectional open gaps and reducing the ability for NaCl spacers to extrude out of the assembly, thereby improving the gap and alignment tolerances in all locations.