Apparatus and method for separating a contaminant from liquid metal

Abstract

Apparatus 1 comprising manifold 2, having inlet 4 for a liquid metal containing an undissolved contaminant, and outlets 6 and 8. Traps 10 and 20 are connected to the outlets of the manifold. Each trap

Description

Field of the Invention The present invention concerns an apparatus and method for separating undissolved contaminants, for example in the form of suspended or entrained particulates, from liquid metal, such as liquid sodium and liquid sodium-potassium alloy (NaK). Background of the Invention Liquid metals, such as liquid sodium and liquid sodium-potassium alloy (NaK), are used in the nuclear power industry as heat transfer fluids (HTFs), for example in fast breeder reactors. In such applications, the liquid metal circulates at temperatures of up to about 600 ’Celsius in a closed loop. Build-up of contaminants in the liquid metal can occur as a result of several processes including, for example, slow leakage of atmospheric air, progressive corrosion of pipework through which the liquid metal flows, the ingress of substances during maintenance, and by nuclear transmutation, as a result of irradiation. In order to maintain the purity of the liquid metal, one or more devices for removing such contaminants from the liquid metal are therefore usually included in a loop through which the liquid metal circulates. These types of devices include getter traps and hot and cold traps. A hot trap is a device for removing contaminants from a liquid metal without reducing the temperature of the liquid metal. A hot trap may, for example, comprise a wire mesh, through which the liquid metal flows, for trapping undissolved contaminants, such as sodium oxide and sodium hydroxide, which are held in suspension in the liquid metal. Such a device may be effective in removing insoluble solids from the liquid metal when they are initially present up to a level of contamination of no more than about 1 or 2%, above which the hot trap soon becomes at least partially blocked. This results in an unacceptable drop in fluid pressure across the trap, an effect known as "plugging". However, this problem is generally avoided in the nuclear power industry by requiring any liquid metal which is used as an HTF to always have a purity greater than about 99.95%, whereby the level of contamination never rises as high as 1 or 2%. After coarse and fine filtering of suspended or entrained solids using one or more hot traps, any contaminants which remain in the liquid metal are generally only present as dissolved substances at levels below their respective saturation solubilities. It is therefore necessary to use other techniques to remove these dissolved contaminants. Devices for removing dissolved contaminants include getter traps and cold traps. Getter traps use sorbent materials like zirconium, titanium and tantalum, which have a high affinity for various different contaminants, to remove the dissolved contaminants from the liquid metal by surface sorption. Some examples of getter traps are described in US patent nos. 3 622 303 and 3 853 700, and in GB patent no. 2 210 898. Cold traps work by cooling the liquid metal to a temperature at which a dissolved contaminant reaches and exceeds its saturation concentration in the liquid metal and therefore precipitates out from the liquid metal acting as a supersaturated solution. Some examples of cold traps are described in US patent nos. 3 831 912, 4 389 310 and 4 737 281. The capacity of a getter or cold trap to remove contaminants may be as low as 10 or 20% of their total volume before plugging occurs. In order to prevent leakage of liquid metal, all of the types of devices described above are generally sealed into the loops through which the liquid metal circulates, for example by being welded in. However, they must also be removed from time to time as part of routine maintenance to prevent plugging, as trapped contaminants accumulate within a trap, and either emptied, cleaned and replaced or substituted by a new trap. Removing, cleaning and replacing a trap or substituting it by a new one is therefore a laborious and time-consuming process. However, it only has to be performed occasionally because, as already mentioned, the level of contamination of liquid metals used as HTFs in the nuclear power industry is generally very low and therefore the rate at which contaminants accumulate in a trap is correspondingly slow. For example, the rate of precipitation of contaminants in a cold trap is typically sufficiently slow to allow contaminant crystals to grow in the cold trap. On the other hand, none of the above types of devices are suitable for handling gross quantities of undissolved contaminants when the liquid metal is already supersaturated, such that, for example, contaminants are present in the liquid metal at levels of 5% or more, because rapid accumulation of trapped contaminants would result in equally rapid plugging. This may arise when the liquid metal has been used in other applications, for example as a chemical reducing agent, resulting in large amounts of insoluble reaction products, such as particles of reduced metal and / or metal oxides, being suspended or entrained in the liquid metal as contaminants. The present invention addresses this problem. Object of the Invention It is therefore an object of the invention to provide an apparatus and method for separating undissolved contaminants from liquid metal when the level of contamination is greater than can be easily handled by the types of devices described above. Description of the Invention Accordingly, in one aspect, the present invention provides an apparatus comprising a manifold, a plurality of traps and a liquid metal flow meter associated with each trap. The manifold has an inlet for liquid metal containing an undissolved contaminant and a plurality of outlets for the same. The plurality of traps each comprises an inlet for the liquid metal containing the undissolved contaminant connected to a respective one of the outlets of the manifold, a substrate for trapping the undissolved contaminant thereon, an outlet for the liquid metal from which at least some of the undissolved contaminant has been removed, an inlet and outlet for inert gas, and a door through which the substrate can be removed from within the trap into an inert atmosphere. The liquid metal flow meter is for measuring a rate of flow of liquid metal through the respective trap. In each trap, the inlet and outlet for inert gas is located above both the inlet for the liquid metal and the outlet for the liquid metal. Furthermore, in each trap, the inlet for the liquid metal, the outlet for the liquid metal and the inlet and outlet for inert gas each comprises a respective valve which can be opened and closed. The valve of the inlet for the liquid metal is configured to close when the rate of flow of liquid metal measured by the liquid metal flow meter associated with the respective trap drops below a first predetermined threshold value when the valves of the inlet for the liquid metal and of the outlet for the liquid metal are both open. Thus, with such an apparatus, a stream of liquid metal containing an undissolved contaminant can be divided into a plurality of parallel streams by the manifold, and the undissolved contaminant in each stream can be separated from the liquid metal within a corresponding one of the plurality of traps. Although the level of contamination of the liquid metal may be high, so that each trap fills with contaminant correspondingly quickly, a trap which is approaching plugging may be quickly and easily emptied by removing the substrate with the undissolved contaminant trapped thereon from within the trap, and replacing it with a new substrate free of contaminant. The liquid metal flow meter associated with each trap indicates when the trap with which the meter is associated is approaching plugging by registering a drop in the rate of flow of liquid metal through the trap. However, by setting the first predetermined threshold value at a flow rate above that at which plugging occurs, the trap can be emptied and a new substrate can be placed within the trap before plugging occurs. The arrangement of valves of each trap allows the emptying of each one of the plurality of traps to be staggered in time, so that at least one of the traps is always operational to remove the undissolved contaminant from the liquid metal whilst another one of the traps is being emptied. Any number of traps greater than one may be employed, and a greater number of traps and / or traps of higher capacity can be used to handle higher levels of undissolved contaminant. All of the outlets for liquid metal of the plurality of traps may be connected together in parallel with each other to a common outlet, if it is desired to combine the liquid metal from which at least some of the undissolved contaminant has been removed into a single stream. Any one of the outlets for liquid metal of the plurality of traps may be connected in series with another similar trap, and / or with one or more conventional hot or cold traps or getter traps to achieve a higher level of separation of the undissolved contaminant and / or to remove dissolved contaminants from the liquid metal. Unlike prior art devices for removing contaminants from a liquid metal which are generally sealed into the loops through which the liquid metal circulates, and can therefore only be removed and emptied with difficulty, the door and the inlet and outlet for inert gas of each trap in the apparatus of the invention allow the traps in such an apparatus to be emptied with ease. In particular, since the door opens into an inert atmosphere, minor leakage of liquid metal from within the trap, which may occur at an edge of the door when the door is closed, is unproblematic, and any such minor leakage may be caught in a sump and returned to the stream of liquid metal. On the other hand, the inlet and outlet for inert gas and the valve thereof allow the trap to be drained of liquid metal before the door is opened to remove and replace a substrate, thereby preventing bulk leakage of liquid metal from within the trap when the door is opened, and for the trap to be refilled again with liquid metal after the substrate has been removed and replaced. Since in each trap, the inlet and outlet for inert gas is located above both the inlet for the liquid metal and the outlet for the liquid metal, each trap can be drained from the bottom of the trap (for example, under gravity) and refilled with liquid metal, without introducing a pocket of inert gas, or bubble, into the stream of liquid metal leaving the trap, once the trap has been emptied and the stream of liquid metal has been re-established to allow the trap to recommence removing the undissolved contaminant from the liquid metal. Instead, any inert gas remaining in the trap as a result of draining the trap, removing and replacing the substrate, and refilling the trap with liquid metal, will remain in a head space at the top of the trap. The liquid metal flow meter associated with each trap may be of a type already used to measure flow rates of liquid metal. For example, it may be a magnetic flow meter. A variety of different types of liquid metal flow meters are described in Liquid Metal Flow Measurement (Sodium) State of the Art Study by G.E. Turner, published by the Liquid Metal Engineering Centre of Atomics International as LMEC-Memo-68-9 under United States Atomic Energy Commission Contract No. AT(04-3)-700 (28 June 1968), the entire contents of which is incorporated herein by reference. The valves of the inlet for the liquid metal, of the outlet for the liquid metal and of the inlet and outlet for inert gas in each trap may all be of a type already used in liquid metal engineering. For example, they may be bellows seal valves. Preferably, the valves of the inlet for the liquid metal and of the outlet for the liquid metal are gate valves, rather than globe valves, to reduce the risk of the valves being blocked by undissolved contaminant. However, the valve of the inlet and outlet for inert gas is preferably a globe valve to allow the flow rate of the inert gas to be regulated by manipulating the valve. The inert atmosphere may consist of at least one of nitrogen and argon. Removing the substrate into an inert atmosphere has the advantage of preventing the formation of more sodium oxide from any residual liquid sodium remaining within the trap, which might otherwise cause plugging of the trap and / or cause damage to the trap as a result of the exothermic nature of the reaction between sodium and atmospheric oxygen to form sodium oxide. Moreover, if the undissolved contaminant trapped on the substrate comprises another metal which has been reduced by the liquid sodium into its elemental form (for example, into elemental iron and / or manganese), removing the substrate into an inert atmosphere has the further advantage of avoiding reoxidation of the hot metal particles. Either nitrogen or argon, or both, may be produced on site by pressure swing adsorption (PSA) of atmospheric air. An on-site PSA generator of such inert gases may be powered, for example, using waste heat derived from cooling the liquid metal from which at least some of the undissolved contaminant has been removed and / or from cooling undissolved contaminant recovered from one or more of the traps. Determining when one of the plurality of traps is drained of liquid metal may be achieved in several different ways. For example, a measured amount of inert gas may be introduced into the trap via the inlet and outlet for inert gas according to the volume of the trap and the temperature and pressure of the inert gas, to displace an equivalent volume of liquid metal. Preferably, however, when a trap is drained of liquid metal is determined using the liquid metal flow meter. This has the advantage of giving more accurate results. In some embodiments, therefore, the liquid metal flow meter associated with a trap is located at the outlet for the liquid metal. For example, this allows the volumetric flow rate of liquid metal as measured through the outlet to be integrated over time to obtain the volume of liquid metal which has left the trap. However, since the volume of liquid metal within the trap also depends on the amount of undissolved contaminant contained within the trap, preferably the liquid metal flow meter is located upstream of the valve of the outlet for the liquid metal, and the valve of the outlet for the liquid metal is configured to close when the rate of flow of liquid metal measured by the liquid metal flow meter drops below a second predetermined threshold value when the valve of the inlet for the liquid metal is closed and the valve of the inlet and outlet for inert gas is open. For example, the second predetermined threshold value can be set at a value close to zero, indicating that no more liquid metal is leaving the trap. However, since the liquid metal flow meter is located upstream of the valve of the outlet for the liquid metal, this valve can be closed whilst the valve itself still contains liquid metal, thereby avoiding the introduction of a pocket of inert gas, or bubble, into the stream of liquid metal leaving the trap. Determining when one of the plurality of traps has refilled with liquid metal after the trap has been emptied may also be achieved in several different ways. For example, a liquid metal flow meter could be located at the inlet for the liquid metal to the trap. This would allow the volumetric flow rate of liquid metal as measured through the inlet to be integrated over time to obtain the volume of liquid metal which has entered the trap when the valve of the outlet for the liquid metal is closed and the valve of the inlet and outlet for inert gas is open. However, this has several disadvantages, as follows. Firstly, the total volume of liquid metal able to enter the trap depends on the amount of undissolved contaminant contained therein, which affects the accuracy of such a method. Secondly, it is preferable for a liquid metal flow meter to be located at the outlet for the liquid metal from the trap, for the reasons described above, which would therefore necessitate a second such flow meter to be located at the inlet for the liquid metal as well. Thirdly, it risks overflowing the trap through the open valve of the inlet and outlet for inert gas. Preferably, therefore, in some embodiments, the inlet and outlet for inert gas comprises a liquid metal sensor located closer to the substrate than the valve of the inlet and outlet for inert gas, and the valve of the inlet and outlet for inert gas is configured to close when the liquid metal sensor senses liquid metal. Such an arrangement has the advantages of solving all of the above-stated problems with locating a liquid metal flow meter at the inlet for the liquid metal to the trap. In particular, since the liquid metal sensor is located closer to the substrate than the valve of the inlet and outlet for inert gas, this valve can be closed before liquid metal reaches the valve, thereby avoiding overflowing the trap. It is also cheaper and easier to implement than a second liquid metal flow meter, and can reliably indicate when the trap is full of liquid metal. In some cases, the inlet for the liquid metal of a trap may be connected to the respective outlet of the manifold via a length of pipework. This length of pipework therefore risks plugging with undissolved contaminant when the valve of the inlet for the liquid metal to the trap is closed. In some embodiments, therefore, the valve of the inlet for the liquid metal comprises an element for preventing the flow of the liquid metal containing the undissolved contaminant into the respective trap, which element, when the valve of the inlet for the liquid metal is closed, is located adjacent to the respective one of the outlets of the manifold connected to the inlet for the liquid metal. This element therefore prevents liquid metal with undissolved contaminant contained therein from entering such a length of pipework when the valve of the inlet for the liquid metal is closed, thereby avoiding plugging the length of pipework. In some embodiments, the apparatus may comprise a pump for pumping inert gas into at least one of the traps via the inlet and outlet for inert gas of the respective trap. Pumping inert gas into a trap has the advantage of making emptying a trap of liquid metal both quicker and easier, since the pressure of the inert gas can be set at a level to overcome the increased resistance presented to the liquid metal leaving the trap by the accumulation of undissolved contaminant within the trap. In some cases, the pump may be configured to heat the inert gas by compressing the gas adiabatically, or substantially adiabatically, as the pump pumps the inert gas into the trap. This has the advantage that if the inert gas is initially at a temperature below the freezing point of the liquid metal, for example if the inert gas is initially at ambient temperature, the inert gas can be heated to above the freezing point of the liquid metal before it is introduced into the trap, thereby avoiding the risk of freezing the liquid metal within the trap, and thus interrupting operation of the apparatus and / or damaging it. Heating the inert gas before it is introduced into the trap also helps to preserve the heat which is carried by the liquid metal. In some embodiments, the apparatus may comprise a heat exchanger for heating the inert gas before the inert gas enters the respective trap via the inlet and outlet for inert gas thereof, wherein the heat exchanger is configured to transfer heat from the liquid metal from which at least some of the undissolved contaminant has been removed to the inert gas. Thus if the liquid metal from which at least some of the undissolved contaminant has been removed has left the apparatus and, for example, it is desired to cool the liquid metal as well, at least some of the heat carried away by the liquid metal can be recovered and reused to heat the inert gas. Such a heat exchanger may be an alternative to heating the inert gas by adiabatic compression or additional to heating the inert gas by adiabatic compression. A combination of transferring heat from the liquid metal and heating the inert gas by adiabatic compression may be used, if desired, to heat the inert gas to a temperature similar to that of the liquid metal. However, provided that the liquid metal within the traps is kept sufficiently hot to prevent it from freezing, the traps in the apparatus of the invention can be operated as either hot or cold traps. In other words, the temperature of the liquid metal does not materially affect the operation of the traps, because the liquid metal is already supersaturated with contaminant. Therefore, changing the temperature of the liquid metal only affects the amount of contaminant able to dissolve in the liquid metal, which only changes the frequency with which the traps are emptied, rather than the way in which they operate. The apparatus of the invention may be mechanised and robotised to ensure its efficient operation. Thus, for example, in some embodiments, the apparatus may comprise one or more motors and a microprocessor. In such a case, the one or more motors are for opening and closing at least one of the respective valves of the inlet for liquid metal, of the outlet for liquid metal and of the inlet and outlet for inert gas, and the door, of at least one of the traps. If so, the microprocessor may be connected to the liquid metal flow meter associated with the respective trap, for receiving the rate of flow of liquid metal through the respective trap measured by the liquid metal flow meter. The microprocessor is then configured to compare the rate of flow with the first predetermined threshold value, and is connected to the motor or motors, for controlling their operation based on an outcome of such a comparison. If the liquid metal flow meter is located at the outlet for the liquid metal, upstream of the valve of the outlet for the liquid metal, the microprocessor may also be configured to compare the rate of flow with the second predetermined threshold value, and to control operation of the motor or motors based on an outcome of this other comparison. If at least one of the traps comprises a liquid metal sensor located as described above, the microprocessor may be connected to the liquid metal sensor of the respective trap, for receiving a signal from the liquid metal sensor indicative of the liquid metal sensor sensing liquid metal, and may be configured to control operation of the motor or motors based on receipt of such a signal. If the apparatus comprises a pump, the microprocessor may be connected to the pump and may also be configured to control operation of the pump based on an outcome of comparing the rate of flow of liquid metal through one of the traps with the first predetermined threshold value. In some embodiments, the apparatus may comprise a robotic mechanism for removing at least one of the substrates from within the respective trap into the inert atmosphere and for replacing the removed substrate by a new substrate, wherein the robotic mechanism is under control of the microprocessor. The substrate may be made of one of several different materials. For example, if the undissolved contaminant trapped on the substrate comprises sodium oxide, at least one of the substrates may be made of a ceramic material, such as one comprising zirconia and / or titania, which may, for example, be in the form of a ceramic foam. Preferably, however, if the undissolved contaminant comprises sodium oxide, at least one of the substrates is made of titanium or a titanium alloy, such as one having a composition by weight of 98.8% Ti, 0.8% Ni and 0.4% Mo. Apart from acting as a getter for sodium oxide, titanium and titanium alloys are particularly resistant to corrosion by sodium oxide and sodium hydroxide, which may result from the trapped sodium oxide subsequently being hydrated during removal of the trapped sodium oxide from the substrate. This therefore avoids subsequent contamination of the trapped sodium oxide when it is recovered from the substrate. In another example, if the undissolved contaminant trapped on the substrate comprises at least one of elemental iron and elemental manganese, then at least one of the substrates is preferably made of iron or steel. This has the advantage of allowing the substrate and the iron and / or manganese trapped thereon to be processed together since they are chemically similar, without any need to separate the iron and / or manganese from the substrate. In such a case, the substrate may also be magnetized to attract the iron to the substrate when the substrate is still inside the trap, which also has the advantage of repelling any sodium oxide, which is diamagnetic. The substrate may comprise, for example, a ceramic foam and / or a wire mesh or wool made of one or more of the materials just mentioned. A trap may contain a substrate comprising a plurality of different grades of ceramic foam and / or wire mesh or wool. If so, the different grades may be arranged from coarser to finer, with a coarser grade located nearer to the inlet for liquid metal of the trap and a finer grade nearer the outlet for liquid metal, so that larger bodies of undissolved contaminants are caught by the substrate before smaller ones. To catch the very smallest particles of undissolved contaminant, the substrate may comprise, for example, a matted material, such as sintered titanium fibre felt. If two or more traps are arranged in series, the fineness of the respective substrates in at least some of the traps in the series may increase in a downstream direction, such that a substrate in a downstream trap has a finer pore size than a substrate in another trap upstream thereof. The body of the apparatus and any associated pipework may be made of such materials as are already used in liquid metal engineering, jointed to prevent leaks. For example, appropriate grades of steel of a type already used for containing and transporting liquid sodium, such as grade 316 LN or 316 FR stainless steel, may be used. If the undissolved contaminants comprise a significant proportion of sodium oxide, the body of the apparatus and any associated pipework may be made from or lined with titanium or a titanium alloy, such as that having the composition specified above. In a second aspect, the present invention also provides a method of operating an apparatus as described herein. The method comprises the following operations. Opening the valve of the inlet for the liquid metal and the valve of the outlet for the liquid metal and closing the valve of the inlet and outlet for inert gas of a first one of the plurality of traps, and measuring the rate of flow of liquid metal through the first trap by means of the liquid metal flow meter associated with the first trap, whilst keeping the valve of the inlet for the liquid metal of a second one of the plurality of traps closed. Determining that the rate of flow of liquid metal through the first trap has dropped below the first predetermined threshold value, closing the valve of the inlet for the liquid metal and opening the valve of the inlet and outlet for inert gas of the first trap, opening the valve of the inlet for the liquid metal and the valve of the outlet for the liquid metal and closing the valve of the inlet and outlet for inert gas of a second one of the plurality of traps, and measuring the rate of flow of liquid metal through the second trap by means of the liquid metal flow meter associated with the second trap. Determining that the first trap has emptied of liquid metal, closing the valve of the outlet for the liquid metal, opening the door of the first trap, removing the substrate from the first trap into the inert atmosphere, replacing the removed substrate by a new substrate in the first trap, closing the door and opening the valve of the inlet for liquid metal of the first trap. Determining that the first trap has refilled with liquid metal, closing the valve of the inlet and outlet for inert gas and opening the valve of the outlet for liquid metal of the first trap, and returning to measuring the rate of flow of liquid metal through the first trap again. Determining that the rate of flow of liquid metal through the second trap has dropped below the first predetermined threshold value, performing the same sequence of operations thereafter for the second trap as for the first trap whilst the valve of the inlet for the liquid metal of the first trap is open, until the same sequence of operations has been performed for all of the plurality of traps, then repeating the same sequence of operations for the first trap again. This method has the advantage that emptying each one of the plurality of traps is staggered in time, so that at least one of the traps is always operational to remove the undissolved contaminant from the liquid metal whilst another one of the traps is being emptied. In some embodiments, determining that one of the plurality of traps has emptied of liquid metal may comprise determining that the rate of flow of liquid metal through the respective trap has dropped below a second predetermined threshold value, as described above. As also described above, in some embodiments, determining that one of the plurality of traps has refilled with liquid metal may comprise sensing that the inlet and outlet for inert gas of the respective trap contains liquid metal. In some embodiments, the method may comprise pumping inert gas into one of the plurality of traps via the inlet and outlet for inert gas of the respective trap when the valve of the inlet for the liquid metal of the respective trap is closed and the valve of the outlet for the liquid metal of the trap is open. The respective trap can thus be emptied more quickly and easily. If so, the inert gas may be compressed adiabatically, or at least partially adiabatically, when it is being pumped into the respective trap, in order to heat it. Alternatively or additionally, the inert gas may also be heated by transferring heat from the liquid metal from which at least some of the undissolved contaminant has been removed to the inert gas before the inert gas enters the respective trap via the inlet and outlet for inert gas of the respective trap. After a substrate has been removed from a trap, the substrate with undissolved contaminant trapped thereon may be processed in one of several different ways, depending on the nature of the undissolved contaminant and the material from which the substrate is made. For example, if the liquid metal has been used as a chemical reducing agent, the trapped contaminant may comprise particles of another metal which has been reduced by the liquid metal into its elemental form and / or of metal oxide(s). For example, if the liquid metal is liquid sodium, which has been used to reduce an oxide of another metal such as of iron and / or manganese, the undissolved contaminant trapped on a substrate may comprise the other metal in elemental form and / or sodium oxide. Accordingly, if the liquid metal comprises sodium and the undissolved contaminant comprises sodium oxide, the method may comprise spraying the removed substrate with a jet of dry air to help separate the undissolved contaminant from the removed substrate. This has the advantage that the dry air oxidizes any residual liquid sodium remaining on the substrate into sodium oxide, whilst avoiding hydrating the sodium oxide into sodium hydroxide, to leave just sodium oxide. Alternatively, if the liquid metal comprises sodium and the undissolved contaminant comprises sodium oxide, the method may comprise at least one of spraying the removed substrate with a jet of steam and washing the removed substrate in deionised water to help separate the undissolved contaminant from the removed substrate. In such a case, both any residual liquid sodium remaining on the substrate and the sodium oxide are hydrated into sodium hydroxide. If, on the other hand, the undissolved contaminant comprises iron and the removed substrate is made of a material other than iron or steel, such as from a titania- or zirconia-based ceramic, titanium or a titanium alloy, the method may comprise magnetically separating the undissolved contaminant from the removed substrate by introducing the removed substrate into a magnetic field. In such a case, since iron is ferromagnetic, whereas such materials as titanium and titania are both only weakly paramagnetic and zirconia is diamagnetic, the iron can be more easily separated from the substrate. In any event, separating trapped contaminant from a substrate may be enhanced by using a mechanical technique, for example by scrubbing, vibrating and / or centrifuging the substrate. Alternatively, if the undissolved contaminant comprises at least one of iron and manganese, and the removed substrate is made of iron or steel, the method may comprise melting the removed substrate with the undissolved contaminant still trapped thereon, without firstly separating the trapped contaminant from the substrate. For example, the removed substrate with iron and / or manganese particles trapped thereon may both be melted as part of a steelmaking process. Brief Description of the Drawings Further features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and in association with the accompanying drawings, in which: Fig. 1 is a schematic diagram of an embodiment of an apparatus for separating undissolved contaminants from liquid metal; Fig. 2A is a schematic diagram of an embodiment of a liquid metal sensor for use in the apparatus of Fig. 1, shown in longitudinal section; Fig. 2B is a cross-section through the embodiment of the liquid metal sensor of Fig. 2A, looking in the direction of the arrows labelled B-B' in Fig. 2A; Fig. 3 is a schematic diagram of an embodiment of a heat exchanger for use in the apparatus of Fig. 1; Fig. 4 is a schematic diagram of an embodiment of a control system for the embodiment of the apparatus shown in Fig. 1; Fig. 5 is a timing diagram, schematically showing an embodiment of an operating sequence for the embodiment of the apparatus shown in Fig. 1; Figs. 6A and 6B are a flow diagram of a first embodiment of a method of operating the apparatus shown in Fig. 1; Fig. 7A is a flow diagram of a second embodiment of a method of operating the apparatus shown in Fig. 1; Fig. 7B is a flow diagram of a third embodiment of a method of operating the apparatus shown in Fig. 1; and Fig. 7C is a flow diagram of a fourth embodiment of a method of operating the apparatus shown in Fig. 1. Detailed Description Fig. 1 schematically shows an embodiment of an apparatus 1 for separating undissolved contaminants from a liquid metal, which in this embodiment is liquid sodium. The apparatus 1 comprises a manifold 2, a pair of traps 10, 20, and a pair of liquid metal flow meters 31, 32, each of which is associated with a respective one of the traps 10, 20. The manifold 2 has an inlet 4 for the liquid sodium, which contains undissolved contaminants comprising sodium oxide and another metal in elemental form which is also insoluble in the liquid sodium (e.g., iron or manganese), and a pair of outlets 6, 8 for the same. The pair of traps 10, 20 each comprises a respective inlet 12, 22 for the liquid sodium containing the undissolved contaminants, which inlet is connected to a respective one of the outlets 6, 8 of the manifold 2 via a respective length of pipework 3, 5. Whereas in this embodiment, the apparatus 1 comprises only two traps 10, 20, in other possible embodiments, such an apparatus may comprise a greater number of traps connected in parallel to each other to the same manifold, in a similar fashion to that shown in Fig. 1. Each of the traps 10, 20 contains a substrate 14, 24 for trapping the undissolved contaminants thereon, and an outlet 16, 26 for the liquid sodium from which at least some of the undissolved contaminants have been removed, as well as an inlet and outlet 18, 28 for inert gas, and a door 15, 25 through which the substrate 14, 24 can be removed from within the trap into an inert atmosphere 17, 27. In Fig. 1, the door 15 of trap 10 is shown closed, and the door 25 of trap 20 is shown partially open. The substrate 14, 24 in each trap comprises a plurality of parallel layers of wire mesh, which is coarser towards the inlet 12, 22 for the liquid sodium and finer towards the outlet 16, 26 for the liquid sodium. The substrate 14, 24 rests within its respective trap under its own weight. The inert atmosphere 17, 27 in this case is a mixture of nitrogen and argon. The inlet and outlet 18, 28 for inert gas in each trap is located above both the inlet 12, 22 for the liquid sodium and the outlet 16, 26 for the liquid sodium, and the inlet 12, 22 for the liquid sodium is also located above the outlet 16, 26 for the liquid sodium, so that both traps can fill and empty under gravity. The inlet 12, 22 for the liquid sodium, the outlet 16, 26 for the liquid sodium and the inlet and outlet 18, 28 for inert gas in each trap each comprises a respective valve VI, V2, V3, V4, V5, V6, which can be opened and closed. The liquid metal flow meters 31, 32 are located at the outlet 16, 26 for the liquid sodium, upstream of the valve V3, V6 of the outlet 16, 26 for the liquid sodium in each trap. The liquid metal flow meters 31, 32 measure a rate of flow of liquid metal through the respective trap 10, 20, and the valve VI, V4 of the inlet 12, 22 for the liquid sodium is configured to close when the rate of flow of liquid metal measured by the liquid metal flow meter 31, 32 associated with the respective trap 10, 20 drops below a first predetermined threshold value when the valves VI, V3, V4, V6 of the inlet 12, 22 for the liquid metal and of the outlet 16, 26 for the liquid metal are both open in each trap. The inlet and outlet 18, 28 for inert gas of each trap 10, 20 is provided with a pump 41, 42 for pumping inert gas into the respective trap 10, 20 via the inlet and outlet 18, 28 for inert gas of the respective trap. Each pump 41,42 is also a compressor, which heats the inert gas by adiabatic compression when it pumps the inert gas into the respective trap 10, 20. Whereas in this embodiment, the inlet and outlet 18, 28 for inert gas in each trap has its own pump associated therewith, in other possible embodiments, a single such pump may be used to supply inert gas to the inlet and outlet 18, 28 for inert gas of each trap connected to an outlet of the same pump by suitable pipework and provided with suitable gas valves to direct the inert gas accordingly. To prevent plugging of the lengths of pipework 3,5 carrying liquid metal with undissolved contaminant from the manifold 2 to the traps 10, 20 when the valve VI, V4 of the inlet 12, 22 for the liquid sodium of each trap closes, these valves VI, V4 each comprise a valve element Via, V4a, which, when the valve VI, V4 is closed, is located adjacent to the respective one of the outlets 6, 8 of the manifold 2 which is connected to the inlet 12, 22. Thus not only is the flow of the liquid sodium containing the undissolved contaminant into the respective trap 10, 20 prevented when the valve VI, V4 is closed, but the flow of the liquid sodium containing the undissolved contaminant into the respective length of pipework 3, 5 is prevented as well. Whereas in this embodiment, the valve elements Via, V4a are additional to a main part of the respective inlet valves VI, V4 located adjacent to the respective one of the inlets 12, 22 for the liquid sodium of each trap, in other possible embodiments, a single valve element may move to be adjacent to the respective one of the outlets 6, 8 of the manifold 2 when the valve VI, V4 is closed. In the apparatus 1, the inlet and outlet 18, 28 for inert gas of each trap 10, 20 also comprises a respective liquid metal sensor 33, 34, which is located closer to the substrate 14, 24 than the valve V2, V5 of the inlet and outlet 18, 28 for inert gas. For improved clarity, these liquid metal sensors 33, 34 are shown in Figs. 2A and 2B in greater detail. As Fig. 2A shows, the inlet and outlet 18, 28 for inert gas of each trap is made of a material 35, which is not electrically conducting and which is represented in Figs. 2A and 2B by being shaded grey. The liquid metal sensor 33, 34 comprises a pair of electrodes 30a, 30b embedded within the insulating material 35 and which face each other diametrically across the inlet and outlet 18, 28 for inert gas on opposing sides thereof, as may be seen in Fig. 2B, which is a cross-section through the embodiment shown in Fig. 2A, looking in the direction of the arrows labelled B-B'. Thus when liquid metal rises up the respective inlet and outlet 18, 28 for inert gas, the liquid metal, which is electrically conducting, completes an electrical circuit between the opposing electrodes 30a, 30b, whereby the respective liquid metal sensor 33, 34 senses that the trap 10, 20 is full. The valve V2, V5 of the corresponding inlet and outlet 18, 28 for inert gas is configured to close when the liquid metal sensor 33, 34 senses the liquid metal, whereby the trap 10, 20 is preventing from overflowing with liquid metal, and any inert gas remaining in the trap is contained in a head space between the liquid metal sensor 33, 34 and the corresponding valve V2, V5. Fig. 3 schematically shows an embodiment of a heat exchanger 50 for heating the inert gas before it is introduced into one of the traps 10, 20. The heat exchanger 50 takes gas from the inert atmosphere 17, 27 and heats it by bringing it into thermal but not physical contact with liquid sodium from one or more of the outlets 16, 26 for the liquid sodium from each trap 10, 20, before the inert gas is introduced into the inlet and outlet 18, 28 for inert gas of a trap. Fig. 4 schematically shows an embodiment of a control system 60 for controlling the apparatus 1 of Fig. 1. The control system 60 comprises a microprocessor 61 and a plurality of motors Ml, M2, M3, M4, M5, M6, M15, M25. The motors are for opening and closing respective ones of the valves VI, V2, V3, V4, V5, V6, as well as the doors 15, 25, of the traps 10, 20. The microprocessor 61 is connected to the liquid metal flow meters 31, 32 associated with each trap 10, 20 and receives from them the rate of flow of liquid metal through the respective trap 10, 20 measured by the liquid metal flow meters 31, 32. The microprocessor 61 is configured to compare the measured rate of flow for each trap with the first predetermined threshold value, and is connected to the motors Ml, M2, M3, M4, M5, M6, M15, M25, for controlling their operation based on an outcome of this comparison. The microprocessor 61 is also connected to the pump 41, 42 associated with each trap 10, 20 and is further configured to control the pumps 41, 42 based on the outcome of the same comparison. The microprocessor 61 also compares the measured rate of flow in each case with the second predetermined threshold value, and controls the operation of the motors based on an outcome of this other comparison. The microprocessor 61 is also connected to the liquid metal sensors 33, 34 of each trap 10, 20 and receives a signal from the liquid metal sensors 33, 34 indicating when the respective liquid metal sensor 33, 34 senses liquid metal. Based on receipt of such a signal, the microprocessor 61 is further configured to control the operation of the motors. The apparatus 1 also comprises two robotic mechanisms R14, R15 for removing the substrates 14, 15 from within their respective traps 10, 20 into the inert atmosphere 17, 27 in each case and for replacing the removed substrate 14,15 by a new substrate. Since they operate in an inert atmosphere and under ambient conditions, the robotic mechanisms R14, R15 are of a conventional type. The robotic mechanisms R14, R15 are both under control of the microprocessor 61 within the control system 60. The operation of the control system 60 to control the apparatus 1 will now be described by reference to the timing diagram of Fig. 5, which shows the valve and door operating sequence for such a pair of traps 10, 20. Initially, both of the traps 10, 20 operate to trap particulates entrained in the flow of liquid sodium from the manifold 2 because both the valves VI, V4 of their respective inlets 12, 22 for the liquid sodium are open, and both the valves V3, V6 of their respective outlets 16, 26 for the liquid sodium are open as well, so that the liquid sodium is able to flow through both traps 10, 20, and undissolved contaminants are trapped on both of their respective substrates 14, 24. At time ti, for example, the microprocessor 61 determines that the flow rate through the trap 10 measured by the liquid metal flow meter 31 has dropped below the first predetermined threshold value. The microprocessor 61 accordingly instructs the motor Ml to close the valve VI and instructs the motor M2 to open the valve V2 to the inlet and outlet 18 for inert gas of trap 10. The flow of liquid sodium with entrained particles into the trap 10 is therefore cut off. Between times t2 and t3, liquid sodium remaining in trap 10 drains from the trap 10, which is speeded up and helped by the microprocessor 61 switching on pump 41 to pump inert gas into the trap 10 via the inlet and outlet 18 for inert gas. The inert gas is hot enough for the sodium to remain in its liquid state. At time tg, the microprocessor 61 determines that the flow rate through the trap 10 measured by the liquid metal flow meter 31 has now dropped below the second predetermined threshold value, instructs motor M3 to close the valve V3 of outlet 16 and switches off the pump 41, to prevent a pocket of inert gas, or bubble, from passing downstream of valve V3. Between times t4 and t5, the microprocessor 61 instructs the motor M15 to open the door 15 of the trap 10, and, under control of the microprocessor 61, the robotic mechanism R14 removes the substrate 14 bearing the trapped particles of undissolved contaminant from within the trap 10, and places an empty substrate back into the trap 10. The microprocessor 61 then instructs the motor M15 to close the door 15 again and instructs the motor Ml to reopen the valve VI. The flow of liquid sodium with entrained particles of undissolved contaminant therefore re-enters the trap 10, and the inert gas within the trap 10 is driven out via the inlet and outlet 18 for inert gas. Between times t6 and t7, the trap 10 refills with liquid sodium, until the liquid metal sensor 33 informs the microprocessor 61 that it has sensed the level of liquid sodium within trap 10 to have reached the inlet and outlet 18 for inert gas. The microprocessor 61 accordingly instructs motor M2 to close the valve V2 to prevent liquid sodium from overflowing trap 10. Finally, at time tg, the microprocessor 61 instructs motor M3 to reopen valve V3, and the flow of liquid sodium with entrained particles through trap 10 to outlet 16 is re-started. Subsequently, both of the traps 10, 20 therefore continue to trap undissolved contaminants on their respective substrates 14, 24, until the microprocessor 61 instead determines that the flow rate through the other trap 20 measured by the liquid metal flow meter 32 associated with trap 20 has dropped below the first predetermined threshold value. The same sequence of operations is then repeated for trap 20 as was already carried out for trap 10. Thus a stream of liquid sodium with a high level of undissolved contaminants entrained therein can be continuously filtered by passing the stream through a plurality of traps, such as the pair of traps 10, 20, which are connected in parallel with each other to the same manifold and emptied sequentially. In other words, whilst trap 20 continues to trap entrained particulates, the operation of trap 10 is temporarily interrupted to empty the trapped particulates from within trap 10, and vice versa. Whereas Fig. 5 shows the correct sequence of operations respectively carried out for trap 10 and trap 20, the scale of the abscissa of the timing diagram in Fig. 5 is entirely arbitrary. In other words, the time taken to carry out each of the operations described above in relation to Fig. 5 is not represented in Fig. 5. In Fig. 5, for clarity and ease of interpretation, the time taken to drain and refill each of the traps 10, 20 and the time taken to remove and replace each of the substrates 14, 15 are represented as being all the same. However, in practice, the times taken to perform any of these operations may be completely different to each other. Moreover, whereas in Fig. 5, emptying trap 10 is represented as taking approximately half the total time shown and emptying trap 20 is represented as also taking about half the total time, in practice, the amount of time taken to empty each trap 10, 20 can be much less than half the total time, so that both traps 10, 20 are operational to remove undissolved contaminants from a stream of liquid metal most of the time. The frequency with which each trap will have to be emptied will in practice depend on the level of contamination of the liquid metal and on the capacity of each trap, which can be made larger and / or greater in number to accommodate higher levels of contamination and / or to reduce the frequency with which each trap is emptied. Figs. 6A and 6B together show a first embodiment of a method 100 of operating the apparatus 1 of Fig. 1. The method 100 ensures that the traps 10, 20 do not both need to be emptied at the same time by staggering their operating sequences in time, as follows. Firstly, as box 101 shows, the valve VI of the inlet 12 for the liquid metal and the valve V3 of the outlet 16 for the liquid metal of the first trap 10 are opened and the valve V2 of the inlet and outlet 18 for inert gas of the first trap 10 is closed. At the same time, the valve V4 of the inlet 22 for the liquid metal of the second trap 20 is kept closed, 103. This ensures that whilst undissolved contaminants accumulate on the substrate 14 in the first trap 10, no undissolved contaminants are accumulating in the second trap 20. The rate of flow of liquid metal through the first trap 10 is measured 102 by means of the liquid metal flow meter 31 associated with the first trap 10 until the microprocessor 61 determines 104 that the rate of flow through the first trap 10 has dropped below the first predetermined threshold value, indicating that the first trap 10 is approaching plugging. The valve VI of the inlet 12 for the liquid metal of the first trap 10 is then closed to cut off the supply of liquid metal to the trap 10 and the valve V2 of the inlet and outlet 18 for inert gas of the first trap 10 is opened 105. Meanwhile, the valve V4 of the inlet 22 for the liquid metal and the valve V6 of the outlet 26 for the liquid metal of the second trap 20 are opened and the valve V5 of the inlet and outlet 28 for inert gas of the second trap 20 is closed, as box 106 shows. Thus whilst the flow of liquid metal to trap 10 is cut off, liquid metal continues to flow through trap 20, and the undissolved contaminants it contains can accumulate on the substrate 24 in the second trap 20. Whilst the valve VI of the inlet 12 for the liquid metal of the first trap 10 is closed and the valve V3 of the outlet 16 for the liquid metal of the first trap 10 is open, inert gas is pumped into trap 10 through the inlet and outlet 18 for inert gas. The inert gas is firstly heated 118 to above ambient temperature by transferring heat from the liquid metal leaving the second trap 20 via the outlet 26 thereof, and then further heated by adiabatic compression as the inert gas is pumped into the first trap 10. Pressurization of the inert gas as it is heated helps to force the remaining liquid metal from the first trap 10. The sequence of operations of trap 20 then proceeds as in Fig. 6B, which can be seen by following the arrow labelled C from Fig. 6A to Fig. 6B, where the rate of flow of liquid metal through the second trap 20 is measured 107 by means of the liquid metal flow meter 32 associated with the second trap 20. Meanwhile, the sequence of operations of trap 10 proceeds from box 105 as represented in Fig. 6A, wherein when the microprocessor 61 determines 108 that the first trap 10 has emptied of liquid metal, the valve V3 of the outlet 16 for the liquid metal is closed 109, the door 15 of the first trap 10 is opened, the substrate 14 is removed from the first trap 10 into the inert atmosphere 17, the removed substrate is replaced by a new substrate in the first trap 10, the door 15 is closed and the valve VI of the inlet 12 for liquid metal of the first trap 10 is reopened. The microprocessor 61 determines 108 that the first trap 10 has emptied of liquid metal by determining that the rate of flow of liquid metal through the trap 10 has dropped below a second predetermined threshold value, which is close to zero, indicating that no more than a trickle of liquid metal is leaving the trap 10 via the outlet 16. When the microprocessor 61 determines 110 that the first trap 10 has refilled with liquid metal, it closes 111 the valve V2 of the inlet and outlet 18 for inert gas and opens the valve V3 of the outlet 16 for liquid metal of the first trap 10, before returning to measuring 102 the rate of flow of liquid metal through the first trap 10 again. The microprocessor 61 determines that the first trap 10 has refilled with liquid metal by the sensor 31 sensing that the inlet and outlet 18 for inert gas of the trap 10 contains liquid metal. Then, as shown in Fig. 6B, when the microprocessor 61 determines 112 that the rate of flow of liquid metal through the second trap 20 has dropped below the first predetermined threshold value, the same sequence of operations are performed thereafter for the second trap 20 as for the first trap 10, as represented in Fig. 6B by boxes 113,114, 115, 116 and 117. These operations are all carried out whilst the valve VI of the inlet 12 for the liquid metal of the first trap 10 is open 101, as represented in Fig. 6B by the arrow labelled D from Fig. 6B to Fig. 6A. Thus the second trap 20 is only emptied when the first trap 10 operates to trap undissolved contaminant entrained in the flow of liquid metal on the substrate 14. With a continuous supply of liquid metal containing an undissolved contaminant, the method 100 can continue until the same sequence of operations has been performed for all of the plurality of traps 10, 20, then the same sequence of operations for the first trap 10 may be repeated again. If the supply of liquid metal containing an undissolved contaminant ceases at any point, the method 100 can also be stopped. Figs. 7 A, 7B and 7C respectively show further embodiments of methods 200, 201, 202 of operating the apparatus 1 of Fig. 1. Each of the methods 200, 201, 202 comprises all of the same operations as the method 100 described above in relation to Figs. 6A and 6B, but additionally comprises further operations which can be carried out after a substrate 14, 24 with undissolved contaminant trapped thereon has been removed 109,115 from within the respective one of the traps 10, 20 in the manner described above. Thus Fig. 7A shows a second embodiment of such a method 200, wherein the liquid metal comprises sodium and the undissolved contaminant comprises sodium oxide. The method 200 further comprises spraying 120 the removed substrate 14, 24 with a jet of dry air to separate at least some of the undissolved contaminant from the removed substrate 14, 24. This has the effect of oxidizing any residual liquid sodium remaining on the substrate 14, 24 to sodium oxide and separating the sodium oxide thus produced from the substrate 14, 24 along with the sodium oxide already trapped on the substrate 14, 24. Since the air is dry, no water is involved, and the production of sodium hydroxide is thereby avoided. Fig. 7B shows a third embodiment of such a method 201, wherein again, the liquid metal comprises sodium and the undissolved contaminant comprises sodium oxide. In this case however, the method 201 instead comprises at least one of spraying 121 the removed substrate 14, 24 with a jet of water vapour or steam and washing 122 the removed substrate 14, 24 in deionised water to separate at least some of the undissolved contaminant from the removed substrate 14, 24. This has the effect, in contrast, of hydrating any residual liquid sodium remaining on the substrate 14, 24 to produce at least sodium hydroxide, and of hydrating the sodium oxide trapped on the substrate 14, 24 as well. Thus one or more sodium hydrates like sodium hydroxide can be separated from the substrate 14, 24 without any admixture of sodium oxide. In both of the methods 200, 201, separating the undissolved contaminant from the removed substrate 14, 24 may be enhanced by mechanically agitating the substrate, either by direct contact with the undissolved contaminant on the substrate, for example by scraping, scrubbing and / or scouring the undissolved contaminant from the substrate, or without making contact with the undissolved contaminant on the substrate, for example by vibrating and / or centrifuging the substrate to help dislodge the undissolved contaminant therefrom, or by a combination of such techniques. Fig. 7C shows a fourth embodiment of such a method 202, wherein the undissolved contaminant comprises iron. In this case, the further operation which is carried out is determined 123 by whether the substrate 14, 24 is made of iron or steel, or not. If the substrate 14, 24 is made of a material other than iron or steel, such as from a titania- or zirconia-based ceramic, titanium or a titanium alloy, then the method 202 further comprises magnetically separating 124 the undissolved contaminant from the removed substrate 14, 24 by introducing the removed substrate 14, 24 into a magnetic field. Since iron is ferromagnetic, whereas the substrate 14, 24 is not, the iron can be separated from the substrate 14, 24 using this technique, and if, for example, the undissolved contaminant also comprises sodium oxide, which is diamagnetic, this has the effect of helping to separate the iron from the sodium oxide as well. In either case, separating the undissolved contaminant from the removed substrate 14, 24 may be enhanced by mechanically agitating the substrate as described above and / or by spraying the removed substrate 14, 24 with a jet of inert gas. Using a jet of inert gas, rather than air, has the advantage of avoiding oxidation of the iron by oxygen in the air, since the trapped iron is likely to be finely divided and therefore to have a high surface area, and may also be hot, making it susceptible to oxidation if brought into contact with atmospheric air before it has had an opportunity to cool down. If on the other hand, the substrate 14, 24 is made of iron or steel, then the method 202 instead further comprises melting 125 both the removed substrate 14, 24 and the undissolved contaminant still trapped thereon, without firstly separating them. Such an operation may be carried out, for example, as part of a steelmaking process. Moreover, the carbon content of steel from which the substrate 14, 24 is made may be selected such that after melting the substrate 14, 24 with the iron particles trapped thereon, the resulting molten mix may have a desired carbon content for steelmaking. In summary, therefore, the present invention provides an apparatus and method for separating undissolved contaminants, for example in the form of suspended or entrained particulates, from liquid metal, such as liquid sodium and liquid sodium-potassium alloy (NaK). The apparatus and method of the invention are able to handle higher levels of contamination than devices and methods known in the prior art for separating contaminants from liquid metal because a stream of liquid metal containing undissolved contaminants is divided into a plurality of streams, each of which are filtered independently of each other in a plurality of traps, and the invention provides a technique whereby the plurality of traps may be prevented from plugging, in spite of the higher levels of contamination, by being emptied of trapped contaminants at a higher frequency, but without interrupting the continuous flow of liquid metal. Whereas the present invention has been described above by reference to particular examples and embodiments, the scope of the invention should not be taken to be limited thereby and is instead defined by the appended claims.

Claims

1. An apparatus (1) comprising:a manifold (2) having an inlet (4) for liquid metal containing an undissolved contaminant and a plurality of outlets (6, 8) for the same;a plurality of traps (10, 20), each comprising an inlet (12, 22) for the liquid metal containing the undissolved contaminant connected to a respective one of the outlets (6,8) of the manifold (2), a substrate (14, 24) for trapping the undissolved contaminant thereon, an outlet (16, 26) for the liquid metal from which at least some of the undissolved contaminant has been removed, an inlet and outlet (18, 28) for inert gas, and a door (15, 25) through which the substrate (14, 24) can be removed from within the trap into an inert atmosphere (17, 27); anda liquid metal flow meter (31, 32) associated with each trap (10, 20), for measuring a rate of flow of liquid metal through the respective trap (10, 20);wherein, in each trap (10, 20):the inlet and outlet (18, 28) for inert gas is located above both the inlet (12, 22) for the liquid metal and the outlet (16, 26) for the liquid metal;the inlet (12, 22) for the liquid metal, the outlet (16, 26) for the liquid metal and the inlet and outlet (18, 28) for inert gas each comprises a respective valve (VI, V2, V3, V4, V5, V6) which can be opened and closed; andthe valve (VI, V4) of the inlet (12, 22) for the liquid metal is configured to close when the rate of flow of liquid metal measured by the liquid metal flow meter (31, 32) associated with the respective trap (10, 20) drops below a first predetermined threshold value when the valves (VI, V3, V4, V6) of the inlet (12, 22) for the liquid metal and of the outlet (16, 26) for the liquid metal are both open.

2. An apparatus according to claim 1, wherein in at least one of the traps (10, 20):the liquid metal flow meter (31, 32) associated therewith is located at the outlet (16, 26) for the liquid metal, upstream of the valve (V3, V6) of the outlet for the liquid metal; andthe valve (V3, V6) of the outlet (16, 26) for the liquid metal is configured to close when the rate of flow of liquid metal measured by the liquid metal flow meter (31, 32) associated therewith drops below a second predetermined threshold value when the valve (VI, V4) of the inlet (12, 22) for the liquid metal is closed and the valve (V2, V5) of the inlet and outlet (18, 28) for inert gas is open.

3. An apparatus according to claim 1 or claim 2, wherein in at least one of the traps (10, 20): the inlet and outlet (18, 28) for inert gas comprises a liquid metal sensor (33,34) located closer to the substrate than the valve (V2, V5) of the inlet and outlet (18, 28) for inert gas; andthe valve (V2, V5) of the inlet and outlet (18, 28) for inert gas is configured to close when the liquid metal sensor (33, 34) senses liquid metal.

4. An apparatus according to any one of the preceding claims, wherein in at least one of the traps (10, 20):the valve (VI, V4) of the inlet (12, 22) for the liquid metal comprises an element (Via, V4a) for preventing the flow of the liquid metal containing the undissolved contaminant into the respective trap (10, 20); andwhen the valve (VI, V4) of the inlet (12, 22) for the liquid metal is closed, the element (Via, V4a) thereof is located adjacent to the respective one of the outlets (6, 8) of the manifold (2) connected to the inlet (12, 22) for the liquid metal.

5. An apparatus according to any one of the preceding claims, further comprising a pump (41, 42) for pumping inert gas into at least one of the traps (10, 20) via the inlet and outlet (18, 28) for inert gas of the respective trap (10, 20).

6. An apparatus according to claim 5, wherein the pump (41, 42) is configured to heat the inert gas by adiabatic compression as it pumps the inert gas into the respective trap (10, 20).

7. An apparatus according to any one of the preceding claims, further comprising a heat exchanger (50) for heating the inert gas before the inert gas enters the respective trap (10, 20) via the inlet and outlet (18, 28) for inert gas thereof, wherein the heat exchanger (50) is configured to transfer heat from the liquid metal from which at least some of the undissolved contaminant has been removed to the inert gas.

8. An apparatus according to any one of the preceding claims, further comprising:a motor (Ml, M2, M3, M4, M5, M6, M15, M25) for opening and closing at least one of the respective valves (VI, V2, V3, V4, V5, V6) of the inlet (12, 22) for liquid metal, of the outlet (16, 26) for liquid metal and of the inlet and outlet (18, 28) for inert gas, and the door (15, 25), of at least one of the traps (10, 20); anda microprocessor (61) connected to the liquid metal flow meter (31, 32) associated with the respective trap (10, 20), for receiving the rate of flow of liquid metal through the respective trap (10, 20) measured by the liquid metal flow meter (31, 32), wherein the microprocessor (61) is configured to compare the rate of flow with the first predetermined threshold value, and is connected to the motor (Ml, M2, M3, M4, M5, M6, M15, M25), for controlling operation of the motor (Ml, M2, M3, M4, M5, M6, M15, M25) based on an outcome of such a comparison.

9. An apparatus according to claim 8 as dependent on claim 2, wherein the microprocessor (61) is further configured to compare the rate of flow with the second predetermined threshold value, and to control operation of the motor (Ml, M2, M3, M4, M5, M6, M15, M25) based on an outcome of such a comparison.

10. An apparatus according to claim 8 or claim 9 as dependent on claim 3, wherein the microprocessor (61) is connected to the liquid metal sensor (33, 34) of the respective trap (10, 20), for receiving a signal from the liquid metal sensor (33, 34) indicative of the liquid metal sensor (33, 34) sensing liquid metal, and is further configured to control operation of the motor (Ml, M2, M3, M4, M5, M6, M15, M25) based on receipt of such a signal.

11. An apparatus according to any one of claims 8 to 10 as dependent on claim 5, wherein the microprocessor (61) is connected to the pump (41, 42) and is further configured to control the pump (41, 42) based on an outcome of comparing the rate of flow with the first predetermined threshold value.

12. An apparatus according to any one claims 8 to 11, further comprising a robotic mechanism (R14, R15) for removing at least one of the substrates (14,15) from within the respective trap (10, 20) into the inert atmosphere (17, 27) and for replacing the removed substrate (14, 15) by a new substrate, wherein the robotic mechanism (R14, R15) is under control of the microprocessor (61).

13. An apparatus according to any one of the preceding claims, wherein the undissolved contaminant comprises sodium oxide and at least one of the substrates (14, 15) is made of titanium or a titanium alloy.

14. An apparatus according to any one of claims 1 to 12, wherein the undissolved contaminant comprises at least one of iron and manganese, and at least one of the substrates (14, 15) is made of iron or steel.

15. An apparatus according to claim 14, wherein at least one of the substrates (14, 15) is magnetized.

16. A method (100) of operating an apparatus according to claim 1, the method comprising: opening (101) the valve (VI) of the inlet (12) for the liquid metal and the valve (V3) of the outlet (16) for the liquid metal and closing the valve (V2) of the inlet and outlet (18) for inert gas of a first one (10) of the plurality of traps, and measuring (102) the rate of flow of liquid metal through the first trap (10) by means of the liquid metal flow meter (31) associated with the first trap (10), whilst keeping (103) the valve (V4) of the inlet (22) for the liquid metal of a second one (20) of the plurality of traps closed;determining (104) that the rate of flow of liquid metal through the first trap (10) has dropped below the first predetermined threshold value, closing (105) the valve (VI) of the inlet (12) for the liquid metal and opening the valve (V2) of the inlet and outlet (18) for inert gas of the first trap (10), opening (106) the valve (V4) of the inlet (22) for the liquid metal and the valve (V6) of the outlet (26) for the liquid metal and closing the valve (V5) of the inlet and outlet (28) for inert gas of a second one (20) of the plurality of traps, and measuring (107) the rate of flow of liquid metal through the second trap (20) by means of the liquid metal flow meter (32) associated with the second trap (20);determining (108) that the first trap (10) has emptied of liquid metal, closing (109) the valve (V3) of the outlet (16) for the liquid metal, opening the door (15) of the first trap (10), removing the substrate (14) from the first trap (10) into the inert atmosphere (17), replacing the removed substrate by a new substrate in the first trap (10), closing the door (15) and opening the valve (VI) of the inlet (12) for liquid metal of the first trap (10);determining (110) that the first trap (10) has refilled with liquid metal, closing (111) the valve (V2) of the inlet and outlet (18) for inert gas and opening the valve (V3) of the outlet (16) for liquid metal of the first trap (10), and returning to measuring (102) the rate of flow of liquid metal through the first trap (10) again; anddetermining (112) that the rate of flow of liquid metal through the second trap (20) has dropped below the first predetermined threshold value, performing the same sequence of operations (113,114, 115, 116, 117) thereafter for the second trap (20) as for the first trap (10) whilst the valve (VI) of the inlet (12) for the liquid metal of the first trap is open (101), until the same sequence ofoperations has been performed for all of the plurality of traps (10, 20), then repeating the same sequence of operations for the first trap (10) again.

17. A method (100) according to claim 16, wherein determining (108) that one of the plurality of traps (10, 20) has emptied of liquid metal comprises determining that the rate of flow of liquid metal through the respective trap has dropped below a second predetermined threshold value.

18. A method (100) according to claim 16 or claim 17, wherein determining (110) that one of the plurality of traps (10, 20) has refilled with liquid metal comprises sensing that the inlet and outlet (18, 28) for inert gas of the respective trap contains liquid metal.

19. A method (100) according to any one of claims 16 to 18, comprising pumping (105, 113) inert gas into one of the plurality of traps (10, 20) via the inlet and outlet (18, 28) for inert gas of the respective trap when the valve (VI, V4) of the inlet (12, 22) for the liquid metal of the respective trap is closed and the valve (V3, V6) of the outlet (16, 26) for the liquid metal of the respective trap is open.

20. A method (100) according to claim 19, comprising heating (105,113) the inert gas by adiabatic compression when pumping the inert gas into the respective trap.

21. A method (100) according to any one of claims 16 to 20, comprising transferring (118, 119) heat from the liquid metal from which at least some of the undissolved contaminant has been removed to the inert gas before the inert gas enters the respective trap (10, 20) via the inlet and outlet (18, 28) for inert gas of the respective trap.

22. A method (200) according to any one of claims 16 to 21, wherein the liquid metal comprises sodium, the undissolved contaminant comprises sodium oxide, and the method comprises spraying (120) the removed substrate (14, 24) with a jet of dry air to separate at least some of the undissolved contaminant from the removed substrate (14, 24).

23. A method (201) according to any one of claims 16 to 21, wherein the liquid metal comprises sodium, the undissolved contaminant comprises sodium oxide, and the method comprises at least one of spraying (121) the removed substrate (14, 24) with a jet of steam and washing (122) the removed substrate (14, 24) in deionised water to separate at least some of the undissolved contaminant from the removed substrate (14, 24).

24. A method (202) according to any one of claims 16 to 21, wherein the undissolved contaminant comprises iron, the removed substrate (14, 24) is made of a non-ferromagnetic material, and the method comprises magnetically separating (124) the undissolved contaminant from the removed 5 substrate (14, 24) by introducing the removed substrate (14, 24) into a magnetic field.

25. A method (202) according to any one of claims 16 to 21, wherein the undissolved contaminant comprises at least one of iron and manganese, the removed substrate (14, 24) is made of iron or steel, and the method comprises melting (125) the removed substrate (14, 24) with the undissolved 10 contaminant trapped thereon.s

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