GB2361054A

GB2361054A - Heat exchanger

Info

Publication number: GB2361054A
Application number: GB0002569A
Authority: GB
Inventors: David Victor Sherwood
Original assignee: NNC Ltd
Current assignee: NNC Ltd
Priority date: 2000-02-04
Filing date: 2000-02-04
Publication date: 2001-10-10
Anticipated expiration: 2020-02-04
Also published as: GB2361054B; GB0002569D0

Abstract

A heat exchanger and method for making such a heat exchanger suitable for use in a nuclear reactor. The heat exchanger comprises a first conduit for a first fluid and a second conduit for a second fluid. The first and second conduits are embedded in a heat conductive solid matrix formed in situ around the conduits. The matrix may be formed of copper. The use of a solid matrix between the conduits creates multiple barriers between the first and second fluids, i.e. between a liquid metal coolant used to cool a reactor core and water/steam.

Description

1 11 2361054 1 HEATEXCHANGER The present invention relates to a heat

exchanger, particularly but not exclusively for use in a nuclear reactor that is cooled by liquid metal.

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, the liquid metal and water/steam were contained within separate tubing systems in an array, heat being transferred by copper plates extending between the tubes. Failure of any one tube could be detected before a second tube failure could cause liquid metal to contact water/steam. However, this system was inefficient and very difficult and expensive to build.

In a second known heat exchanger, the consequences of tube failure were reduced by using a first heat exchanger to transfer energy from a primary coolant body of sodium into a secondary coolant body of sodium, and a second heat exchanger to transfer heat from the secondary coolant to water, to generate steam. Such an arrangement means that there are no risks of a catastrophic release of radioactive materials in the event of an explosive combination of sodium and steam as a result of tube failure in the second heat exchanger. However, the introduction of a secondary sodium circuit is very expensive, and a tube failure in the second heat exchanger could still cause serious damage.

2 It is an object of the present invention to obviate or mitigate these disadvantages with prior art devices.

According to the present invention there is provided a heat exchanger comprising a first conduit for a first fluid and a second conduit for a second fluid, wherein the first and second conduits are embedded in a heat conductive solid matrix formed in situ around the conduits.

The use of a solid matrix between the conduits creates multiple barriers between the first and second fluids, i.e. between a liquid metal coolant and water/steam. There is thus no need to use a secondary coolant circuit as failure of one of these barriers would not cause a reaction to occur provided that at least one barrier remains intact. Significant cost savings may thus be achieved over prior art systems using two heat exchangers. Furthermore, good thermal efficiency can be achieved in an economical manner.

Preferably one of the first and second conduits is bonded to the solid matrix during formation thereof, most preferably both of the first and second conduits being bonded to the solid matrix during formation thereof. This ensures excellent heat transfer and safety standards, and means that the heat exchanger is not likely to need to be repaired during its lifetime.

The solid matrix is preferably formed of copper. At least one or both of the first and second conduits are preferably formed of stainless steel.

The solid matrix and conduits may form a module of a heat exchanger, the heat exchanger being formed of a plurality of such modules. This enables easy assembly of a heat exchanger having the desired characteristics for a particular application. The conduits may be led separately from the solid matrix in segregated and protected groups to associated feed and return systems including heaters as appropriate.

A housing may be provided to enclose the solid matrix and conduits in an inert atmosphere, thus preventing fires should a leak occur.

The invention also provides a method of forming a heat exchanger comprising forming a heat conductive solid matrix around first and second conduits such that the. first and second conduits are embedded in the matrix.

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 tube arrangement for use in a heat exchanger according to the present invention; Figure 2 is a schematic illustration of the tube layout used in the heat exchanger of Figure 1; and Figures 3, 4 and 5 schematically illustrate three further tube arrangements for use in a heat exchanger according to the present invention.

Referring to Figures 1 and 2 of the drawings, there is illustrated a heat exchanger 1 comprising two separate sets of tubes 2, 3 arranged in an array. The tubes 2 carry a liquid metal coolant (Na) that has been 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 interiors of the two sets of tubes 2 and 3, the barriers being the walls of the tubes 2, 3 and the copper matrix 4.

The mechanical bonding between the tubes 2, 3 and the matrix 4 is best achieved by the use of a hot isostatic pressure (HIP) technique that may be carried out in a number of ways. In one technique, tubes 2, 3 are arranged in a mould in the desired configuration, and the space between them is filled with powdered copper. The HIP process is then used to convert the powder to metal which fuses with the outside of the tubes 2, 3.

In order to minimise the risk of loss during manufacture and to limit the size of the HIP facilities required, the copper bonded tube array may be modularised. Alternative techniques such as liquid metal casting are also feasible.

At edges of the copper matrix 4, the tubes 2, 3 are connected to plates 5 and associated supply/outlet headers by means of welds 6. The headers comprise Na supply header 7, Na outlet headers 8, water/steam supply header 9 and water/steam outlet headers 10. Although short lengths of tube will be exposed between the copper matrix 4 and the plates 5, leaks will be obvious through inspection. Fires due to leaks may be avoided by surrounding the heat exchanger 1 by an inert atmosphere such as N.,. The plates 5 and headers 7, 8, 9 and 10 are separated from one another by 4 mechanical barriers (not shown) which prevent any leaked fluid reaching the tubes and plates containing the other fluid.

Any failure of a tube wall within the matrix 4 will not result in mixing of the liquid metal and water/steam, as the fluid that was contained within the failed tube will be contained within the matrix 4. The mechanical bonding of the matrix 4 to the tube walls prevents leakage of the fluid along the tube/matrix interface. The heat exchanger may thus continue to be safely used without the need to shut the reactor down in order for repairs to the heat exchanger to be made.

Additional tubes or open passages may be moulded into the copper matrix 4 in order to provide a leakage path to indicate when one of the heat exchanger tubes 2, 3 has started to leak, thereby making it possible to take the heat exchanger out of commission before a second heat exchanger tube and the matrix could leak and produce a dangerous mixture of the two components. Alternatively, only one set of tubes 2 or 3 may be bonded to the copper matrix so that a leakage path is available to indicate failure of a tube.

The tubes 2, 3 may be arranged in a variety of ways within the copper matrix, in order to optimise the efficiency of the heat exchanger and the overall dimensions thereof. For example, the tubes may be arranged in a serpentine manner (Figure 31), or straight (Figure 4 which shows an arrangement similar to Figure 2), or coiled (Figure 5). The flow of fluid in the pipes follows the arrows shown in the Figures.

Differential expansion between the tubes 2, 3 and the copper matrix 4 may be limited by a suitable choice of material for the tubes. The maximum differential thermal expansion between the tubes and the copper matrix will occur at the hot end of the heat exchanger, where the temperature of the steam is about 500 'C. At this -6 temperature the coefficient of expansion of copper is about 19.1 x 10 / 'C and for 316 stainless steel it is about 18.2 x 10-6 / C. Youngs modulus for copper is 15.3 x 4 10. Assuming that the tubes are fully restrained by the copper matrix (which is in fact unlikely given the low strength of copper at 500 'C), then Thermal stress = (19.1 - 18.2) x 10---6x 500 x 15.3 x 104 = 68.9 MPa In order to calculate the number of steam tubes required in the heat exchanger, the heat transfer power required and the conditions must be taken into account. If the heat transfer power required in a single heat exchanger is 600 MW, with the conditions of table 1 below, Table 1

Steam generator conditions Liquid metal side Steam/water side Pressure, bar 2 185 Inlet temperature, 'C 545 240 Outlet temperature, 'C 340 490 Enthalpy change, j oule/kg 234400 2.189 x 106 Mass flow rate, kg/s 2560 274 the following calculation applies:

Treating the unit as a counter-current heat exchanger and using the expression for the liquid metal outlet temperature Tlaow -",: Tnaine-1 (1 _ M +T,.,,,,(1 _ e-k (1 - Me-) allows the evaluation of the thermal decay length, k, where 0 M- Mnacpna Milaterc peff and Mna is the liquid metal flow rate Mwater is the water flow rate cp,ff is the effective specific heat of the steam side determined from the change in enthalpy.

The quantity k is also given by 6 =_ hPL (1 - M) Mnacpna where h is the total heat transfer coefficient between the liquid metal and the steam/water P is the total heat transfer perimeter L is the length of the tubes.

The heat transfer perimeter is related to the number of steam tubes, N, by P = NirDi where Di is the internal pipe diameter, equal to 12 mm.

The total heat transfer coefficient is determined from h= h,,a hllallh,) -1 where the subscripts on the heat transfer coefficients refer to the liquid metal side, na, the metal between the tube walls and the water/steam side, s.

The heat transfer coefficient within the metal was evaluated using a finiteelement routine. A value of 4467 W/m 2 K was obtained.

The liquid metal side heat transfer coefficient was found using 0-6 Nu,, = 7.0 + 0.025Pe.

where Nu is the Nusselt number and Pc is the Peclet number. and the steam side coefficient is an average of the coefficients in the water region and the steam region, both evaluated using NUS = 0.023 Re 0.8pr 0.6 where Re is the Reynolds number and Pr is the Prandtl number.

The mean values of fluid properties used to evaluate the heat transfer coefficient are given in table 2 below, 7 Table 2 Fluid properties used to evaluate heat transfer coefficients Property Liquid metal Water Steam Density, kg/M3 839 707 102 Specific heat, 1267 6036 3174 Ag K Kinematic viscosity, 309 x 1V 1.4 x 10-1 3.25 x 10 M1/s Thermal conductivity, 68 0.57 0.13 W/m K In order to evaluate the heat transfer coefficients it was necessary to make an initial guess of the total number of tubes on each side to estimate flow velocities. It was therefore necessary to iterate until the number of tubes settled to a consistent value.

The final results for heat transfer coefficients are given in table 3 below, Table 3

Heat transfer coefficients Liquid metal Water Steam Velocity, m/s 14.84 1.886 13.07 Heat transfer coefficient, 120400 16350 8067 W/m 2 K Combining the heat transfer coefficients gave a value for the total heat transfer coefficient of 3184 W/M2 K. This gave a value of 18 18 for the number of steam tubes required. A similar number of liquid metal tubes is also required.

8

Claims

2.

A heat exchanger comprising a first conduit for a first fluid and a second conduit for a second fluid, wherein the first and second conduits are embedded in a heat conductive solid matrix formed in situ around the conduits.

A heat exchanger according to claim 1, wherein one of the first and second conduits is bonded to the solid matrix during formation thereof.

A heat exchanger according to claim 2, wherein both of the first and second conduits are bonded to the solid matrix during formation thereof.

4. A heat exchanger according to any preceding claim, wherein the solid matrix is formed of copper. A heat exchanger according to any preceding claim, wherein at least one of the first and second conduits is formed of stainless steel.

6. A heat exchanger according to any preceding claim, wherein the solid matrix and conduits forms a module of a heat exchanger, the heat exchanger being formed of a plurality of such modules. A heat exchanger according to any preceding claim, comprising a housing enclosing the solid matrix and conduits in an inert atmosphere. A heat exchanger substantially as hereinbefore described, with reference to Figs 1 and 2, Fig 3, Fig 4 or Fig 5 of the accompanying drawings A method of forming a heat exchanger comprising forming a heat conductive solid matrix around first and second conduits such that the first and second conduits are embedded in the matrix.

10. A method of forming a heat exchanger according to claim 9, wherein the solid matrix is created around the conduits by using a hot isostatic pressure technique to solidify a powdered metal placed around the conduits.

11. A method of forming a heat exchanger according to claim 9, wherein the matrix is cast around the conduits.

12. A method of forming a heat exchanger according to claim 9, 10 or 11, wherein the solid matrix is bonded to the surface of at least one conduit during formation of the matrix.

7.

8.

9.

9 13. A method of forming a heat exchanger according to claim 12, wherein the solid matrix is bonded to the surface of both of the conduits during formation of the matrix.

14. A method of forming a heat exchanger according to any one of claims 9 to 11, wherein the solid matrix is formed of copper.

15. A method of forming a heat exchanger according to any one of claims 9 to 14, wherein at least one of the first and second conduits is formed of stainless steel 16. A method of forming a heat exchanger substantially as hereinbefore described.