Liquid metal primary coolants are used in nuclear reactor cores, and there is a need to remove heat from such a primary coolant, either by direct heat exchange with water to generate steam, or by means of heat exchange with a secondary liquid metal coolant, which may then be used in a second heat exchanger to generate steam. The heat exchangers used in nuclear reactors must be extremely robust in order to prevent contact between the liquid metal and water/steam which would cause a violent reaction, potentially allowing radioactivity to escape to the atmosphere. Such a violent reaction between the liquid metal and water/steam would also create a pressure pulse in the heat exchanger, with the products formed from such a reaction causing considerable damage to the remainder of the coolant circuit, making repair of the heat exchanger very difficult and requiring a long shut down period to enable repairs to take place.

In one known heat exchanger for transferring heat directly between a liquid metal primary coolant (sodium) and water/steam, tubes of circular cross section arranged in an array are embedded in a solid copper matrix. In use, some of the tubes carry sodium, some water/steam. This known heat exchanger has the disadvantage that because tubes of circular cross section are used, the number of possible tube size and distribution patterns which give a good packing fraction is limited. Additionally, using tubes of circular cross section requires large volumes of copper to fill spaces between the tubes. Tubes of circular cross section can not be allowed to touch as narrow voids formed at the contact points are difficult to fill with copper. Thus the spacing between tubes cannot be reduced beyond a minimum limit.

Although the known embedded heat exchanger is highly reliable as a continuous body of copper is formed between any two adjacent tubes, the safety critical issue is the provision of a body of copper between tubes carrying different fluids, that is sodium and water/steam. Accordingly some of the characteristics of the known embedded tube heat exchanger are not necessary to meet safety critical requirements.

It is an object of the present invention to obviate or mitigate the problems outlined above.

According to a first aspect of the present invention, there is provided a heat exchanger comprising a plurality of first conduits for a first fluid and a plurality of second conduits for a second fluid, wherein the first conduits and the second conduits are embedded in a heat conductive solid matrix formed insitu such that the first and second conduits are separated by the solid matrix, and the spacing between adjacent first and second conduits is substantially greater that the spacing between adjacent first conduits and the spacing between adjacent second conduits.

Adjacent first conduits may abut one another and/or adjacent second conduits may abut one another or alternatively a foil may be placed between adjacent first conduits and/or adjacent second conduits.

Each of the first conduits and/or each of the second conduits is preferably of rectangular cross-section. The solid matrix may define a plate between the first and second conduits, each conduit being in contact with the plate. The solid matrix may be formed of copper. Each of the first conduits and/or each of the second conduits may be formed of stainless steel.

At least one conduit may be bonded to the solid matrix during formation thereof.

Preferably, each first conduit and each second conduit is bonded to the solid matrix during formation thereof.

The first conduits may be formed to have a larger flow area than the second conduits.

The first and second conduits may have a substantially equal surface area adjacent the plate.

According to a second aspect of the present invention there is provided, a method of forming a heat exchanger as described above, wherein a solid matrix is formed between the first conduits and the second conduits using a hot isostatic pressure technique to bond metal placed between the first conduits and the second conduits to the first and second conduits.

The solid matrix may be bonded to the surface of at least one conduit during formation of the solid matrix. Preferably, the solid matrix is bonded to the surface of each of the first and second conduits during formation of the solid matrix.

A known heat exchanger, and embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a known heat exchanger tube arrangement ; Figures 2 and 3 are cross sectional views of alternative tube arrangements in a heat exchanger according to the present invention; Figure 4 is an illustration showing an overview of a system in which a heat exchanger is accordance with the invention is used; Figure 5 is a side view on line 5-5 of figure 4; Figure 6 is a cross-sectional view on line 6-6 of figure 5; and Figure 7 is a cross sectional view of a heat exchanger tube configuration used in the system of figures 4 to 6.

Referring to figure 1, there is illustrated a known heat exchanger 1 comprising two separate sets of tubes 2,3 arranged in an array. The tubes 2 carry a liquid metal coolant (sodium) that is used to cool a nuclear reactor core, whilst the tubes 3 carry water. The tubes 2,3 are embedded in a solid copper matrix 4 that is mechanically bonded to both sets of tubes. Three barriers are thus created between the interior of the two sets of tubes 2,3, the barriers being the walls of the tubes 2,3 and the copper matrix 4.

It can be seen from figure 1 that this arrangement of tubes 2,3 results in large gaps between tubes which must be filled with copper. The pipes can not be allowed to touch one another, so as to reduce the gaps, as the resultant small voids between pipes would be difficult to fill. This makes this heat exchanger configuration expensive.

Additionally, as three barriers are required only between a tube carrying sodium 2 and a tube carrying water/steam 3 but not between two pipes carrying the same material, the arrangement has considerable redundancy.

Referring now to figure 2, there is illustrated an improved heat exchanger according to the present invention. Three tubes 5 of rectangular cross section carrying a metal coolant (sodium) are provided adjacent one another. Similarly, four tubes 6 of rectangular cross section carrying water/steam are provided adjacent one another.

Between the two sets of tubes 5,6 there is provided a copper plate 7, which is mechanically bonded to the two sets of pipes 5,6. Three barriers are thus provided between the interiors of tubes 5 carrying sodium and tubes 6 carrying water/steam, the barriers being the walls of the tubes 5,6 and the copper plate 7. Only two barriers are provided between the interior of any two tubes 5 carrying sodium or any two tubes 6 carrying water, these barriers being provided by the walls of the tubes 5,6. Thus the additional barrier is provided where it is necessary to ensure safety, but not between two tubes carrying the same material. In a preferred embodiment of the present invention, tubes 5,6 are formed of stainless steel, although other suitable materials may be used.

The creation of the copper plate 7 and the mechanical bonding between the copper plate 7 and the tubes 5,6 is best achieved by use of a hot isostatic pressure (HIP) technique that may be carried out in a number of ways. In one technique, tubes 5 are arranged such that they abut one another. A solid copper plate is then placed on top of the tubes 5, and water carrying tubes 6, again in abutment, are placed on top of the solid copper plate. The resultant assembly is then arranged in a mould and a HIP process is used to bond the copper plate to the two sets of tubes 5,6 thereby forming the plate 7 bonded to the tubes 5,6.

In an alternative technique a space for the formation of the plate 7 is defined between the two sets of tubes 5,6. The defined space is then filled with powdered copper, and the HIP process is then used to convert the powder to metal which fuses with the outside of the tubes 5,6.

The thickness of the copper plate 7, acting as a third barrier between sodium and water/steam carrying tubes, needs only be sufficient to arrest any crack propagation that may emanate from one of the tubes 5,6. The present invention therefore greatly reduces the amount of copper necessary to form a heat exchanger as compared with the arrangement of figure 1. Preliminary calculations indicate that whilst steel content would be similar in the known heat exchanger using pipes of circular cross section of Figure 1 and a heat exchanger according to the present invention, the copper content may be reduced by as much as 57%. This leads to an overall cost saving of about 30%, relative to the known arrangement.

In the known arrangement of figure 1, highest thermal stresses in the copper matrix 4 caused by differential expansion of the different materials of the tubes 3 and the matrix 4 occur between two adjacent steam tubes 3 or two adjacent sodium tubes 2.

Therefore, adopting a rectangular configuration as illustrated in figure 2, in which there is no copper between adjacent sodium tubes 5 or adjacent water/steam tubes 6, will reduce peak stress levels in the copper.

Although the illustrated embodiment of figure 2 shows tubes 5 carrying sodium in abutment and tubes 6 carrying water/steam in abutment, it may in practice be convenient to provide a thin copper foil between adjacent tubes 5 carrying sodium and adjacent tubes 6 carrying water/steam.

Referring now to figure 3, an alternative configuration for a heat exchanger according to the present invention is shown. Here tubes 6 carrying steam are formed such that they have a relatively large surface area adjacent the copper plate 7, and a relatively small cross sectional area, thereby increasing their heat transfer potential. Although the tubes 6 carrying steam are formed with a relatively small flow area, a higher pressure drop may be used to maintain good flow stability in the case of once-through steam generators. In contrast, the tubes 5 carrying sodium have a relatively large flow area, and operate with a lower pressure drop. Thus heat transfer area is independent of flow area, allowing pressure drop and surface area to be selected as appropriate to the application.

An installation using a heat exchanger according to the present invention will now be described with reference to figures 4 to 7.

Referring to figure 4, an overview of a cooling system incorporating the heat exchanger of the present invention is illustrated. Tubes carrying sodium are arranged vertically and communicate with headers 8. A series of tubes 9 carrying water enter the installation towards the bottom of the diagram. The tubes 9 are connected to tubes that run vertically parallel to the sodium tubes within the installation and which in turn are connected to tubes 10 that emerge from the installation, forming a number of U-shaped loops. This allows a lower module 11 and an upper module 12, each carrying an array of sodium tubes and water tubes, to be conveniently joined e. g. by welding at a joint 13. Tubes 14 emerge from the module 12 below the upper header 8.

Each tube 14 is connected to a respective tube 10 and a respective tube 9 through the modules 11 and 12. h use, water enters the installation through the series of pipes 9 and flows in a vertically upwards direction. The water is heated by the hot sodium as it passes through the installation generating steam which passes out of the lower module 11 through the U-shaped tubes 10, and continues into the upper module 12 where the steam is heated further by hot sodium. The steam leaves the installation through the series of tubes 14.

Referring now to figure 5, it can be seen that six parallel linear arrays of steam tubes 15 are provided, each array in the lower module. 11 comprising eight tubes. Between each array of tubes 15, an array of tubes 16 carrying sodium (only one tube in each array being illustrated in the view of figure 5) is provided. The interface of each array of tubes 15 carrying water/steam and each array of tubes 16 carrying sodium is provided by a copper plate 17.

Referring now to figure 6, a section on line 6-6 of figure 5 is shown. It can be seen that each pipe 15 of square cross section carrying water/steam and travelling in the vertically upwards direction through the lower module 11 is connected to a pipe 10 of round cross section. The pipes 10 do not pass through the welded joint 13, thereby allowing access about the full diameter of each pipe 16 to form a weld.

Referring finally to figure 7, the tube layout in the installation of figures 4 to 6 is shown. The tubes are arranged such that two relatively large cross section sodium tubes 16 are placed alongside four relatively small cross section water/steam tubes 15.

A plate 17 is provided between the two rows of tubes. It can therefore be seen that the tubes illustrated travelling through the lower module 11 of figure 6 will in fact be made up of twelve modules as illustrated in figure 7.

The plates 17 are mechanically bonded to the tubes 15,16. This is achieved by arranging the tubes 15,16 in a mould such that all tubes 15 are in abutment and all tubes 16 are in abutment. A space defined between an array of tubes 15 and an array of tubes 16 is filled with a copper plate or powdered copper. A HIP process is then be used to bond the tubes to the matrix as described above.