GB2234849A

GB2234849A - Nuclear reactor-based power source

Info

Publication number: GB2234849A
Application number: GB8717137A
Authority: GB
Inventors: William Pek Seng Tan
Original assignee: National Nuclear Corp Ltd
Current assignee: National Nuclear Corp Ltd
Priority date: 1987-07-28
Filing date: 1987-07-28
Publication date: 1991-02-13
Anticipated expiration: 2007-07-28
Also published as: GB2234849B; GB8717137D0

Abstract

A nuclear reactor-based power source operates on a closed Brayton Cycle using helium or helium/xenon coolant gas with a particle fuel reactor 18. The reactor 18 is selectively operable to provide different output modes, one of which involves the production of relatively short duration, high energy output bursts. The power source is provided with thermal storage materials having a range of melting points and high latent heats of fusion so that large amounts of waste heat can be absorbed during the burst mode of operation. The reactor 18 has a selectively operable radiator 47 associated with its coolant circuit so that the extent of waste heat dissipation can be varied according to the mode of operation of the reactor 18. Heat pipes 44 couple waste heat to the radiator 47. The core comprises circular fuel rods each containing enriched fuel particles dispersed between concentric porous tubes of a frit material (Fig. 5A, not shown). Tube core is surrounded by an annular reflector in which rotatable drums of reflector material are arranged, each drum including a peripheral sliver of neutron absorber for control purposes (Fig. 5, not shown). Additional control is provided by a central control rod comprising a neutron multiplier section and a neutron absorber section. <IMAGE>

Description

Nuclear Reactor-Based Power Source This invention relates to nuclear reactor-based power sources especially, but not necessarily exclusively, for extra-terrestial applications and in situations where large power outputs are required over relatively short time spans, eg a hundred seconds or more, and where there may also be a requirement for a lower power output to be maintained either for much longer time spans or continuously.

According to the present invention there is provided a nuclear reactor-based power-generating system comprising a nuclear reactor core, means for selectively varying the reactivity of the core, means for circulating a fluid coolant around a closed loop including the reactor core, means downstream of the reactor core for converting the heat content of the fluid coolant into electrical power, means downstream of the converting means for absorbing and storing waste heat from the fluid coolant when the reactor is caused to operate in a high reactivity mode, and radiator means thermally coupled with said absorbing means for dissipating heat by radiation.

The converting means may comprise a turbine which may in addition to driving a generator for producing electrical power may also drive a compressor located upstream of the reactor core. The system is preferably based on a Closed Brayton Cycle.

The coolant fluid may comprise helium or a mixture of helium and xeron.

The absorbing means is preferably in the form of a material which remains in the solid phase when the reactor core is operating in a lower output mode but melts and hence absorbs heat energy when the reactivity of the reactor core is increased to provide a higher output. Operation of the reactor core may be controlled in such a way that the higher output phase is maintained for relatively short intervals, eg one hundred seconds or more, to provide bursts of energy (for instance of the order of one hundred megawatts or more) and during the lower output phases the electrical output may be maintained continuously at levels of 10 MWe or lower.

The absorbing means preferably comprises a range of materials having differing melting points, the arrangement preferably being such that the higher melting point materials are thermally coupled with the exhaust fluid coolant at locations upstream of the locations at which the lower melting point materials are thermally coupled with the fluid coolant. The absorbing materials may comprise mixtures of high melting point compounds, the mixtures being varied in order to provide different melting points. Preferred materials are those having high latent heats of fusion.

The thermal coupling between the exhaust coolant and the absorbing means may be provided by a multiplicity of heat pipes. The heat pipes may be arranged with their evaporator sections in direct heat exchange relation with the exhaust coolant and with their condenser sections associated with the abosrbing means. The condenser sections of the heat pipes may in addition be thermally coupled with the radiator means. The absorbing materials may be incorporated in the heat pipes and the arrangement may be such that the heat pipes are each thermally coupled with the exhaust coolant at successive locations downstream of the converting means, those heat pipes which are further upstream being associated with the higher melting point absorbing materials.

The fluid coolant circuit may include a recuperator section upstream of the reactor core and the recuperator section may be thermally coupled with a section of the coolant circuit downstream of the converting means. Such coupling may be affected by a multiplicity of heat pipes.

The radiator means may be selectively operable to vary the area exposed for the emission of radiation. In one embodiment, there is at least one radiator assembly comprising panels or panel sections (which may be hingedly or otherwise connected together) so arranged that the panels or sections can be moved between a retracted position in which the panels or sections overlie one another in face-to-face relation and an extended position in which the panels or sections are substantially co-planar. The panels or sections may incorporate part of heat pipe units connecting the radiator means to the coolant circuit.

The heat pipes are conveniently finned so as to facilitate heat dissipation.

The invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic view in longitudinal section of a nuclear reactor-based power generating system in accordance of the invention, only one half of the system being shown; Figures 2, 3 and 4 are respectively sections in the directions A-A, B-B and C-C in Figure 1; Figure 5 is an enlarged view corresponding to Figure 2 and Figure 5A is a further enlarged view of one of the fuel elements; Figure 6 is a view similar to that of Figure 1 showing a modified form of the system; and Figure 7 is an end view of a duplex form of the system with the radiators shown retracted.

Referring to Figure 1, the system comprises a closed recirculating fluid flow circuit which in cross-section is of annular configuration. The fluid flow circuit comprises a zone 10 accommodating a compressor unit 12, a zone 14 constituting a recuperator, a zone 16 accommodating a nuclear reactor core 18, a zone 20 accommodating a turbine 21 and a zone 22 extending between the turbine and compressor sets 12, 22 for recirculating the coolant exhausted from the turbine set 22 back to the compressor set 12 via a return path in which waste heat can be extracted from the fluid. The fluid which serves to extract heat from, and cool the reactor, may be helium or a mixture of helium and xenon.

The curved sections 24 contain guide vanes 26 for minimising turbulence in the fluid around these sections.

The turbine and compressor sets each comprise two stages 28, 30 and 32, 34 coupled by shafts 35, 36. The shaft 36 also serves to drive an electrical generator 39.

The shafts 35, 36 may, by suitably arranging the turbine blades, contrarotate with respect to each other so that their torque reactions counteract each other. The system operates on a Closed Brayton Cycle in which: the working fluid undergoes compression at the compressor stages 32, 34: the fluid passes through the recuperator zone 14 where, as explained hereinafter, heat exchange occurs between the fluid in the recuperator zone 14 and the adjacent part of outer annular zone 22; the fluid traverses the reactor core 18 and, in doing so, acts as a coolant and is heated to a high temperature, eg of the order of 210000K; the hot fluid drives the turbine stages 28,30 and then returns to the compressor zone via the outer annular zone 22.

As the fluid travels along the annular zone 22, it passes over the evaporator sections 40 of a first array 38 of heat pipes (depicted diagrammatically in Figure 3) arranged in succession lengthwise of the zone 22 over the region which is substantially co-extensive with the recuperator zone 14. The heat pipes 38, which may be finned, and contain an alkali metal, extend inwardly and have their condenser sections 42 exposed to the fluid flow in the recuperator zone 14 so that the fluid in the zone 14 can be heated by heat transfer from the zone 22.

If desired, the recuperator zone 14 may be omitted as shown in Figure 6 where it will be seen that the compressed fluid passes directly from the compressor set 12, 22 to the reactor core 18.

After flowing past the heat pipe array 38, the coolant then passes over the evaporator sections 46 of a second array of heat pipes 44 (depicted diagrammatically in Figure 4) which serve to dissipate heat by radiation and may be finned for this purpose. The heat pipes also serve to transfer heat from the fluid circuit to radiation means. The radiator means is operable selectively to vary the area thereof available for heat dissipation by radiation to the surroundings. The radiator means may, as can be seen more clearly in Figure 7, comprise a number of sections 47 hingedly or otherwise interconnected so as to be foldable in concertina fashion in the manner shown in Figure 7 and extendable into substantially co-planar relation shown in Figures 1 and 6. Movement of the radiator assemblies is effected by suitable means such as multiple piston/cylinder units 48.

The panels or sections 47 forming the radiator assemblies have high thermal emissivities (eg 0.85 or greater) and may be fabricated from beryllium or titanium alloys or carbon-carbon materials (depending on the temperatures involved) coated, if necessary, with materials, such as metal oxides, enhancing emissivity.

As shown in Figure 6, the condenser sections 50 of the heat pipes 44 as illustrated extend over only part of the extended area of the radiator assemblies. However, in practice, the condenser sections 50 will be distributed at intervals across the entire area of each radiator assembly. Typically the outward extent of each radiator assembly when fully extended will be of the order of 15 metres.

Referring to Figures 5 and 5A, the reactor core 18 may comprise a plurality of axially extending fuel elements 52 based on particle fuel, the fuel being enclosed within the space between concentric porous tubes 54 composed of a frit material and enclosing particles of for example enriched uranium carbide or uranium carbide/zirconium carbide. The coolant gas (ie helium or helium/xenon) after compression and pre-heating in the recuperator zone 14 (if present) enters the outer tube of each fuel element 52 and permeates through the mass of particles and the porous inner tube 54 into the central passageway with consequent heating to temperatures of the order of 21000K before entering the turbine stage 20.

The frit material may be a carbon-carbon material so as to withstand temperatures of the order of 21000K and for compatability with the fuel. The fuel may comprise particles, about 500 microns in diameter, each consisting of a kernel of enriched UC2 (or UC.ZrC), buffer pyrocarbon layers (PyC), a silicon carbide diffusion barrier and an outer layer of isotropic PyC to provide erosion resistance. A particle fuel of this type is considered advantageous because of its ability to provide a large thermal output in a short time without fracturing.

As shown in Figure 5, the array of fuel elements 52 encircling the shaft assembly 35, 36 is encircled by an annular zone 56 forming a neutron reflector fabricated from beryllium oxide and enclosing a series of control drums 58 each of which contains beryllium oxide and around part of its periphery is provided with a sliver 60 of a neutron absorbing material such as boron carbide (10B4C). The drums 58 are each selectively rotatable about their axes by motors 59 (see Figures 1 and 6) to vary the position of the sliver 60 relative to the core thereby allowing core reactivity to be controlled.Further control of core reactivity is available by means of a cylindrical plug 62 comprising axially successive sections respectively composed of a neutron multiplier (eg beryllium oxide) and a neutron absorber (eg boron carbide), the plug 62 being accommodated within the turbine shaft assembly 35, 36 at the centre of the reactor core and being selectively displaceable to bring the neutron absorber section or the neutron multiplier section into operative relation with the core, according to whether the reactivity is to be reduced or increased.

Core reactivity may be controlled by means of the control drums 58 and the plug 62 to provide different modes of operation, for example: 1. a first "pulse"-producing mode in which a burst of energy of the order of 100 MWe or more is produced for about 100 seconds or more.

2. a second mode giving a continuous output of the order of 2 MWe or more for tens of hours for instance; and 3. a third mode in which the output is reduced to 200 KWe or more and maintained continuously for a number of years.

When operating in the third mode, the radiator assemblies may be fully retracted or only partially extended and, in this mode, it will be seen that they may act as protective shields for the power source. When in the first and second modes, the radiator assemblies may be fully extended to maximise dissipation of waste heat.

When the core reactivity is increased to produce the higher outputs required by the first mode, and possibly also the second mode, the heat generated is substantial.

To cope with such situations, the system incorporates thermal storage means for absorbing waste heat generated during high output operation. The thermal storage means can be implemented by the provision of a range of thermal storage materials having differing latent heats of fusion. The materials may be accommodated either in annular zones around the outer surfaces of the heat pipes of the array 44 or in rod-like elements extending axially along (and, if desired, within) the heat pipes. The materials are organised in such a way that those having the higher melting points are upstream in the coolant circuit of those having lower melting points, ie the melting points may decrease in a progressive manner from the heat pipe 44 which is furthest upstream to the heat pipe 44 which is furthest downstream.The thermal storage materials may comprise lithium hydride/lithium hydroxide compositions in which the melting points can be varied according to the proportions of the hydride and hydroxide employed. For example, 60% LiH/40% tiOH provides a melting point of just below 8000K whilst 90% LiH/10% LiOH provides a melting point of the order of 9500K. The arrangement may be such that in the first mode, the thermal storage materials absorb approximately 908 of the waste heat generated during the energy burst whilst the radiator dissipates approximately 10% of the waste heat. After the burst output has ceased, the thermal storage materials cool down by giving up the stored heat to the heat pipes and hence to the radiator assemblies.The melting points of the thermal storage materials may be such that the temperatures prevailing in the zone 22 during the second and third modes of operation are insufficient to cause melting.

As shown in Figure 7, the reactor/turbine/ compressor/coolant circuit arrangements may be duplicated and arranged to minimise torque disturbance.

A feature of the invention is that the outlet annular return path of the coolant circuit forms an annular space 64 so that if any gas leaks develop over the compressor - recuperator - reactor- turbine zones, the leaking gas enters the space 64 and develops a counter-pressure which will automatically counteract leak. If desired, at the time of fabrication, the space 64 may be charged with a gas which will exert a counterpressure to counteract any leaks.

The described power source is intended for extra-terrestial applications, eg on spacecraft or as a moon or planet-based power source. However, terrestial applications are also within the scope of the invention.

In one terrestial application, the power source may be carried by an aircraft and, in this event, the source may be modified by the use of ram air-cooling either in addition to, or in place of, cooling by radiators and with or without high latent heat thermal storage materials.

Claims

1. A nuclear reactor-based power-generating system comprising a nuclear reactor core, means for selectively varying the reactivity of the core, means for circulating a fluid coolant around a closed loop including the reactor core, means downstream of the reactor core for converting the heat content of the fluid coolant into electrical power, means downstream of the converting means for absorbing and storing waste heat from the fluid coolant when the reactor is caused to operate in a high reactivity mode, and radiator means thermally coupled with said absorbing means for dissipating heat by radiation.

2. A system as claimed in Claim 1 in which the converting means comprises a turbine which, in addition to driving a generator for producing electrical power, also drives a compressor located in the fluid flow path upstream of the reactor core.

3. A system as claimed in Claim 1 or Claim 2, the system being based on a Closed Brayton Cycle.

4. A system as claimed in any one of the preceding Claims in which the coolant fluid comprises helium.

5. A system as claimed in any one of Claims 1 to 3 in which the coolant fluid comprises helium and xenon.

6. A system as claimed in any one of the preceding Claims, in which the absorbing means is in the form of a material which remains in the solid phase when the reactor core is operating in a lower output mode but melts and hence absorbs heat energy when the reactivity of the reactor core is increased to provide a higher output.

7. A system as claimed in Claim 6 in which the absorbing means comprises a range of materials having differing melting points,

8. A system as claimed in Claim 7 in which the arrangement is such that the higher melting point materials are thermally coupled with the fluid coolant at locations upstream of the locations at which the lower melting point materials are thermally coupled with the fluid coolant.

9. A system as claimed in Claim 7 in which the absorbing materials comprise mixtures of high melting point compounds, the mixtures being varied in order to provide different melting points.

10. A system as claimed in Claim 8 in which the thermal coupling between the coolant and the absorbing means is provided by a multiplicity of heat pipes.

11. A system as claimed in Claim 10 in which the heat pipes are arranged with evaporator sections thereof in direct heat exchange relation with the coolant and with condenser sections thereof associated with the absorbing means.

12. A system as claimed in Claim 11 in which the condenser sections of the heat pipes are in addition thermally coupled with the radiator means.

13. A system as claimed in Claim 11 in which the abosrbing materials are incorporated in the heat pipes.

14. A system as claimed in Claim 13 in which the absorbing materials comprise mixtures of high melting point compounds, the mixtures being varied in order to provide different melting points, the thermal coupling between the coolant and the absorbing means is provided by a multiplicity of heat pipes, the arrangement being such that the heat pipes are each thermally coupled with the coolant at successive locations downstream of the converting means, those heat pipes which are more upstream being associated with the higher melting point absorbing materials.

15. A system as claimed in any one of the preceding Claims in which the fluid coolant circuit includes a recuperator section upstream of the reactor core and the recuperator section is thermally coupled with a section of the coolant circuit downstream of the converting means.

16. A system as claimed in Claim 15 in which the thermal coupling between said recuperator section and said downstream section of the cooling circuit is effected by a multiplicity of heat pipes.

17. A system as claimed in any one of the preceding Claims in which the radiator means is selectively operable to vary the area exposed for the emission of radiation.

18. A system as claimed in Claim 17 in which there is at least one radiator assembly comprising panels or panel sections so arranged that the panels or sections can be moved between a retracted position in which the panels or sections overlie one another in face-to-face relation and an extended position in which the panels or sections are substantially co-planar.

19. A system as claimed in Claim 18 in which said panels or panel sections are hingedly connected together.

20. A system as claimed in Claim 18 in which the panels or sections incorporate parts of heat pipe units connecting the radiator means to the coolant circuit.

21. A nuclear reactor-based power-generating system substantially as hereinbefore described with reference to Figures 1 to 5A.

22. A system as claimed in Claim 20 or Claim 21, and substantially as hereinbefore described with reference to Figure 7.

22. A modification of the system of Claim 21 substantially as hereinbefore described with reference to Figure 6.

23. A system as claimed in Claim 21 or Claim 22, and substantially as hereinbefore described with reference to Figure 7.

Amendments to the claims have been filed as follows 1. A nuclear reactor-based power-generating system comprising a nuclear reactor core, means for selectively varying the reactivity of the core, means for circulating a fluid coolant around a closed loop including the reactor core, means downstream of the reactor core for converting the heat content of the fluid coolant into electrical power, means downstream of the converting means for absorbing and storing waste heat from the fluid coolant when the reactor is caused to operate in a high reactivity mode, said absorbing means being in the form of a material which remains in the solid phase when the reactor core is operating in a lower output mode but melts and hence absorbs heat energy when the reactivity of the reactor core is increased to provide a higher output, and radiator means thermally coupled with said absorbing means for dissipating heat by radiation.

4. A system as claimed in any one of the preceding Claims in which the fluid coolant comprises helium.

5. A system as claimed in any one of Claims 1 to 3 in which the fluid coolant comprises helium and xenon.

6. A system as claimed in any one of the preceding Claims, in which the absorbing means comprises a range of materials having differing melting points, 7. A system as claimed in Claim 6, in which the arrangement is such that the higher melting point materials are thermally coupled with the fluid coolant at locations upstream of the locations at which the lower melting point materials are thermally coupled with the fluid coolant.

8. A system as claimed in Claim 6, in which the absorbing materials comprise mixtures of high melting point compounds, the mixtures being varied in order to provide different melting points.

9. A system as claimed in Claim 7, in which the thermal coupling between the coolant and the absorbing means is provided by a multiplicity of heat pipes.

10. A system as claimed in Claim 9, in which the heat pipes are arranged with evaporator sections thereof in direct heat exchange relation with the coolant and with condenser sections thereof associated with the absorbing means.

11. A system as claimed in Claim 10, in which the condenser sections of the heat pipes are in addition thermally coupled with the radiator means.

12. A system as claimed in Claim 10, in which the abosrbing materials are incorporated in the heat pipes.

13. A system as claimed in Claim 12, in which the absorbing materials comprise mixtures of high melting point compounds, the mixtures being varied in order to provide different melting points, the thermal coupling between the coolant and the absorbing means is provided by a multiplicity of heat pipes, the arrangement being such that the heat pipes are each thermally coupled with the coolant at successive locations downstream of the converting means, those heat pipes which are more upstream being associated with the higher melting point absorbing materials.

14. A system as claimed in any one of the preceding Claims, in which the fluid coolant circuit includes a recuperator section upstream of the reactor core and the recuperator section is thermally coupled with a section of the coolant circuit downstream of the converting means.

15. A system as claimed in Claim 15 in which the thermal coupling between said recuperator section and said downstream section of the cooling circuit is effected by a multiplicity of heat pipes.

16. A system as claimed in any one of the preceding Claims, in which the radiator means is selectively operable to vary the area exposed for the emission of radiation.

17. A system as claimed in Claim 16, in which there is at least one radiator assembly comprising panels or panel sections so arranged that the panels or sections can be moved between a retracted position in which the panels or sections overlie one another in face-to-face relation and an extended position in which the panels or sections are substantially co-planar.

18. A system as claimed in Claim 17, in which said panels or panel sections are hingedly connected together.

19. A system as claimed in Claim 17, in which the panels or sections incorporate parts of heat pipe units connecting the radiator means to the coolant circuit.

20. A nuclear reactor-based power-generating system substantially as hereinbefore described with reference to Figures 1 to 5A.

21. A modification of the system of Claim 20 substantially as hereinbefore described with reference to Figure 6.