THERMAL BRIDGE

Abstract

A thermal bridge for improving thermal transfer between a fuel element to a fuel block wherein there is provided a high temperature gas cooled nuclear reactor fuel block comprising a fuel channel and a coolant channel wherein the fuel channel comprises a fuel element, the fuel channel further comprising a thermal bridge thermally linking the fuel element and the fuel channel, wherein the thermal bridge comprises a melting point greater than the working temperature of the fuel block, thereby improving thermal transfer from the fuel element to the fuel block, thereby improving thermal transfer to the coolant channel.

Claims

1. A high temperature gas cooled nuclear reactor fuel block comprising: a fuel channel; and, a coolant channel, wherein the fuel channel comprises a fuel element, the fuel channel further comprising a thermal bridge thermally linking the fuel element and the fuel channel, wherein the thermal bridge comprises a melting point greater than the working temperature of the reactor fuel block, thereby improving thermal transfer from the fuel element to the fuel block, thereby improving thermal transfer to the coolant channel.

2. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge is resiliently compressible.

3. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge is a powdered material.

4. The high temperature gas cooled nuclear reactor fuel block according to claim 3 wherein the powdered material are particles, with a particle size less than 100 m.

5. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge is selected from a group consisting of metals and their alloys, metalloids, carbon, and thermally conductive ceramics.

6. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge is graphite.

7. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge further comprises a burnable poison.

8. The high temperature gas cooled nuclear reactor fuel block according to claim 7 wherein the burnable poison is boron carbide.

9. A high temperature gas cooled nuclear reactor system comprising the fuel block according to claim 1.

10. The high temperature gas cooled nuclear reactor system according to claim 9 wherein the coolant channel comprises a gas having a thermal conductivity lower than 0.1 W/mK at 25 C.

11. The high temperature gas cooled nuclear reactor system according to claim 10 wherein the gas is nitrogen.

12. The high temperature gas cooled reactor system according to claim 9 wherein the reactor system is a direct cycle system.

13. A method of improving cooling in a high temperature gas cooled reactor system that includes a coolant channel and a fuel channel, the method comprising: providing a thermal bridge between a fuel element within the fuel channel and the fuel channel; and, causing a low conductivity gas to flow in the coolant channel.

14. A fuel block of direct cycle high temperature nitrogen cooled reactor system, the fuel block comprising: a fuel channel; and, a coolant channel, wherein the fuel channel comprises a fissile material, the fuel channel further comprising a thermal bridge in the form of graphite powder thermally linking the fissile material and the fuel channel, wherein the thermal bridge comprises a melting point greater than the working temperature of the reactor fuel block, the thermal bridge further comprising a burnable poison in the form of boron carbide, thereby improving thermal transfer from the fuel element to the fuel block, thereby improving thermal transfer to the coolant channel.

15. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge comprises metals and their alloys, metalloids, carbon, or thermally conductive ceramics.

16. The high temperature gas cooled nuclear reactor fuel block according to claim 1 wherein the thermal bridge includes a resiliently compressible powdered material.

17. The high temperature gas cooled nuclear reactor fuel block according to claim 16 wherein the resiliently compressible powdered material includes particles, with a particle size less than 100 m.

18. The high temperature gas cooled nuclear reactor fuel block according to claim 16 wherein the resiliently compressible powdered material includes a graphitic powder and a compound of boron or gadolinium.

19. The high temperature gas cooled nuclear reactor fuel block according to claim 16 wherein the resiliently compressible powdered material is a homogenous blend of graphitic powder and boron carbide.

20. The high temperature gas cooled nuclear reactor fuel block according to claim 16 comprising a compound of boron or gadolinium, adjacent to the thermal bridge.

Description

FIGURES

[0037] Several arrangements of the invention will now be described by way of example and with reference to the accompanying drawings of which;

[0038] FIG. 1 shows an arrangement of a high temperature gas cooled reactor fuel block.

[0039] FIG. 2 shows a plan view of a fuel channel comprising a fuel element packed with a thermal bridge according to the invention.

[0040] FIG. 3 shows a thermal conductivity diagram of a fuel channel to a coolant channel.

[0041] FIG. 4 shows a graph comparing nitrogen coolant with and without a thermal bridge versus helium gas coolant without a thermal bridge

[0042] FIG. 5 shows a high temperature gas cooled reactor system.

[0043] Turning to FIG. 1, there is provided an example of a fuel block arrangement 100 in the prior art. In this particular example, the fuel block 102 is a General Atomics GT-MHR high temperature gas cooled nuclear reactor fuel block comprising a plurality of fuel channels 106 and a coolant channels 104 wherein the fuel channels 106 comprise a plurality of fuel elements (not shown). The fuel block 102 comprises a hexagonal cross section which allows a plurality of adjacent fuel blocks (not shown) to tessellate in a reactor core. In use, a coolant gas flows in the longitudinal axis Z defined by the fuel channels 106 and coolant channels 104 extending through the fuel block 102. The coolant gas also flows in the gaps between the fuel channel 106 and the fuel elements as there is a fuel channel gap provided to allow thermal expansion and neutron irradiation expansion of the fuel element and fuel channel 106 in addition to accommodation manufacturing tolerance errors of the fuel element.

[0044] Turning to FIG. 2, there is provided a plan view arrangement 200 of a fuel channel 206 comprising a fuel element 208, the fuel element 208 surrounded by a thermal bridge 210. In the present arrangement, the fuel channel is a TRISO uranium oxide based fissile material compacted into a cylindrical compact and the fuel channel 206 is made of graphite. In the present arrangement, the thermal bridge is a solid graphite powder with a spherical particle size of 50 m packed around the fuel element. In the present arrangement, the thermal bridge 210 is resiliently compressible owing to gaps between powder particles thereby allowing thermal expansion and contraction of the fuel block, fuel channel and fuel element and volumetric changes of the fuel channel due to neutron irradiation. The thermal bridge 210 further comprises a burnable poison (not shown) in the form of boron carbide which helps offset additional reactivity of the fuel element 208 at the beginning of its life, the poison gradually burning over its lifetime as neutrons are absorbed.

[0045] Turning to FIG. 3, there is provided a thermal conductivity diagram 300 between a fuel channel 306 and a coolant channel 304. In use, the fuel element 308 produces thermal energy due to fission upon splitting of the fissile product, for example uranium 235. This thermal energy must be transferred from the centre of the fuel element 308 to the edge of the fuel element 308 denoted by resistance R1. Without a thermal bridge, the heat must then traverse a fuel channel gas gap denoted by resistance R2. This gas gap is filled with the coolant gas which also flows through the coolant channel 304. Such a gap significantly reduces the thermal transfer between the fuel element 308 and the fuel channel wall 306 due to the low thermal conductivity of the cooling gas relative to the fuel block 302 and fuel compact 308. The transferred heat then traverses the fuel block 302 having a resistance of R3 to the coolant channel 304 wherein coolant gas is heated by convection. In the present invention, resistance R2 is significantly reduced due to the provision of a thermal bridge 310 which is in effect an extension of the fuel block to abut the fuel element 308. In this example, the resistance of R3 is greater than R2 due to the provision of voids between graphite particles in the thermal bridge. In practice, the resistance R2 may be slightly higher than R3 due to very small gaps between powder particles in the thermal bridge 310. Where R2 is a thermal bridge 310, the resistance is always lower than if R2 were merely a coolant gas therefore it can be seen that heat transfer is vastly improved between the fuel element 308 to the coolant channel 304 by provision of a thermal bridge 310.

[0046] Turning to FIG. 4, there is provided a graph 400 comparing nitrogen coolant with and without a thermal bridge versus helium gas coolant without a thermal bridge. The Y axis shows a difference in kelvin between the fuel element and the fuel channel wall plotted against an X axis of fuel block power in kilowatts. Line 420 denotes a fuel block without any thermal bridge utilising nitrogen as a coolant gas. Line 430 denotes a fuel block without any thermal bridge utilising helium as a coolant gas. Line 440 denotes a fuel block with a thermal bridge of the present invention utilising nitrogen as a coolant gas. As can be seen from the graph 400, the utilisation of a nitrogen coolant without a thermal bridge 420 is significantly less efficient than a helium coolant without a thermal bridge 430 owing to the nitrogen possessing a thermal conductivity of around .sup.th that of helium. As such, there is shown a significant temperature difference across the fuel channel gap as a function of fuel block power. Where a simulation is run using a thermal bridge with a nitrogen coolant 440, it can be seen that the temperature difference is less than that of the high conductivity helium coolant therefore the use of a thermal bridge can yield higher thermal efficiencies than use of a helium coolant alone and much more efficient than a nitrogen coolant without a thermal bridge 420.

[0047] Turning to FIG. 5, there is provided a high temperature gas cooled reactor system 500 comprising a reactor 516, the reactor comprising at least one graphite fuel block 502. In the present invention, the graphite fuel block 502 is a General Atomics GT-MHR fuel block comprising a fuel channel 506 and a coolant channel 504 wherein the fuel channel comprises a plurality of TRISO fuel elements 508, the fuel elements 508 formed into cylindrical capsules mounted in series within the fuel channel 506. The fuel channel 306 further comprises a thermal bridge 510 which surrounds each fuel element 508 wherein the thermal bridge is a graphite powder. In the present arrangement, the high temperature gas cooled reactor system 500 is a direct cycle system wherein there is provided a primary circuit loop 512 which directly drives turbomachinery 514 without the need for a secondary coolant loop or associated heat exchangers. In this example, the coolant gas is nitrogen.

[0048] Although a few preferred arrangements have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[0049] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0050] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[0051] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0052] The invention is not restricted to the details of the foregoing arrangement(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.