Patent Number: 048511838
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, the reactor core is disposed within a comparatively low-cost pressure vessel (10) at the bottom of a vertical shaft (20). The shaft is of sufficient diameter to receive the horizontal cross section of the reactor pressure vessel and may possibly but not exclusively have a depth of 600-1500 feet. That depth which constitutes a safe distance for isolating nuclear contaminants from the atmosphere depends on details of the local lithology and stratigraphy. A possible depth cited in the technical literature for underground siting of a conventional nuclear power plant is 340 feet (Bowman, Watling, and McCauley, op. cit.). To be safe against the worst conceivable reactor accident, namely the nearby explosion of a nuclear weapon, the reactor must be situated at a depth of at least 800 feet. The shaft (20) is lined with a borehole liner (30) of impermeable material which in a preferred embodiment of the invention comprises a concrete-encased, thermally insulated steel pipe. Heat transfer means (40) remove heat from the reactor core and transport it to heat utilization means (50) located at or near the surface of the earth. The heat utilization means (50) may typically comprise a boiler, turbine generator, and cooling tower. In one possible embodiment of the invention, as shown in FIG. 1, the means (50) for exchanging and utilizing heat from the reactor occupy a central generating facility on or near the surface of the earth, where they can readily be serviced like those of any conventional coal- or gas-fired power plant, and where they do not require any extra and costly pressure containment vessels and safety cooling system. The heat exchange and utilization means (50) at the surface of the earth are surrounded by a plurality of shafts (20), (60), (70). At any one time during the operative lifetime of the invention, one or more operative reactors at the bottoms of their respective shafts are thermally connected to the centrally located heat exchange and utilization means (50) by underground, vertical heat pipes which are thermally connected to the central heat exchange means (50). As an individual reactor completes its operative lifetime, it is cut off from the heat exchange means and sealed in situ within the lower portion of the casing by activating valves and underground mechanical closures (80) and possibly explosive closures (90). Thus the old reactor is abandoned in place in a deactivated hole (60). New reactors can be installed in previously unused shafts (70). Prior to becoming operative, new reactors are thermally connected to the central heat exchange means and electrically heated to liquefy the solidified working fluid and the liquefiable neutron-reflecting material. Referring now to FIG. 2, the reactor is of the type known in the art as a "self-regulating, heat-pipe controlled, reflector-critical, compact, fast reactor." The reactor comprises a core (100) which in a preferred embodiment is conical, and nested, inert-gas buffered heat pipes (110). The heat pipes (110) are arranged preferably in primary, secondary, and higher-order arrays. The primary heat pipes, which extend into the core of the reactor and remove heat directly therefrom, are conical in the preferred embodiment of the invention. The evaporator sections (120) of the primary heat pipes are received within the reactor core (100). The condensor sections (130) of the primary heat pipes are received within the evaporator sections (140) of the secondary heat pipes. Similarly, the condensor sections of the secondary heatpipes may be received coaxially within the evaporator sections of the tertiary heat pipes, and so forth. The heat pipes emerging from the reactor core pass through a reflector region and extend vertically in the space above the reactor. The reflector region comprises a liquid-reflector reservoir (145) and a neutron-reflecting mantle (150). A liquefied neutron-reflecting material is transferred to the liquid-reflector reservoir from a storage reservoir (155). The design and use of a reactor of this kind is described in V. Hampel, U.S. Defensive Publication No. T101,204, "Compact Fast Nuclear Reactor Using Heat Pipes." The pressure vessel (10) enclosing the core is composed of heat-resistant material chosen to maintain its structural strength at the operating temperature of the reactor core, which possibly but not exclusively may lie in the range between 1400 and 2500 Kelvin degrees. The operating pressure lies in the range of vapor pressures of suitable working fluids employed in heat pipes in the range of operating temperatures. A partial list of possible working fluids includes lithium fluoride, lithium, and beryllium difluoride. Over the temperature range from 1400 to 2500 Kelvin degrees, the vapor pressure of lithium fluoride ranges from 0.01 to 15 bars. Over the same temperature range, the vapor pressure of lithium ranges from 0.1 to 50 bars. Over the same temperature range, the vapor pressure of beryllium difluoride ranges from 1 to 10,000 bars. The pressure vessel (10) is continuous with a casing (160) which extends vertically upward within the borehole liner between the reactor core and the surface of the earth. The pressure vessel (10) is enclosed within and thermally insulated from the borehole liner (30). A thermally insulating layer (170) is disposed within the annular space (180) defined between the outer surface of the pressure vessel and the inner surface of the borehole liner. In a preferred embodiment of the invention, the insulating layer comprises multiple layers of reflective foil in an evacuated space. In an alternative embodiment, the annular space (180) may additionally contain a liquid coolant for circulation as a carrier of low-grade heat in a cogeneration loop. The annular space (180) additionally contains temperature sensors and chemical sensors to assure that the contents of the pressure vessel and casing are thermally insulated from the ground and that there is no exchange of material between the pressure vessel or casing and the ground. A series of at least two thermally connected heat pipe arrays (190) (i.e., the primary and secondary arrays) extends within the casing (160) from the reactor core to heat utilization means (50) situated at or near the surface of the earth. Additional stages of heat-pipe arrays may be interposed within the casing between the secondary heat-pipe array and the heat-exchange means. Each higher stage is added by coaxially receiving the condenser section of a lower-stage heat pipe within the evaporator section of the corresponding next-higher-stage heat pipe. Referring now to FIG. 3, heat may additionally be transferred between vertical heat-pipe stages by the use of a thermal coupling manifold (200). The manifold (200) is a thermally insulated enclosure (210) filled with a heat-conductive fluid (220). The condensor ends (230) of the lower-stage heat pipes enter from the bottom of the manifold and terminate within the heat-conductive fluid. The evaporator ends (240) of the higher-stage heat pipes terminate within the heat-conductive fluid and exit through the top of the manifold. This arrangement decouples every individual heat pipe of a higher-stage array from any specific heat pipe in the lower-stage array. This offers distinct advantages in the case of failure of a heat pipe. In that case, the remaining heat pipes share the load previously carried by the failed heat pipe. In a preferred embodiment of the invention, semi-helical baffles (250) within the heat pipes deflect the evaporating gas as it condenses and drive the condensate towards the evaporator end of the heat pipes. This pumping action enhances the passive gravitational return of the heat-pipe fluid, reducing the likelihood of burnout over a range of power levels, and thus providing substantially fail-safe operation. The condensor section of the highest-stage heat pipe array is thermally connected to the heat-utilization means. Referring now to FIG. 4, the heat pipes in one or more arrays may contain, in addition to the working fluid, quantities of inert buffer gas to enhance the operation of the passive self-controlling mechanism and to provide remote, fast-acting, active reactor power control. The action of the buffer gas which may enhance passive self-control is described by Hampel (U.S. Defensive Publication No. T101,204, "Compact Fast Nuclear Reactor Using Heat Pipes"), and has been used in radioisotope space-power heat sources to regulate the temperature of the thermionic diodes over time. A preferred embodiment of the invention additionally incorporates active means, essentially as described by Hampel (U.S. Defensive Publication No. T101,204, "Compact Fast Nuclear Reactor Using Heat Pipes"), to control the reactivity of the reactor core by adjusting the pressure of buffer gas in one or more heat pipe arrays. Accordingly, a preferred embodiment of the invention additionally comprises high- and low-pressure gas lines (260) extending vertically within the casing (160) between the vertical heat pipes to means located at suitable depths below the surface of the earth for supplying, storing, and controlling the inert buffer gas. Each pair of high-pressure and low-pressure gas lines terminates at its lower end in a three-way valve (270) communicating with a gas-flow inlet (280) through the wall of a secondary or higher-stage heat pipe (290) near the condensor section of said heat pipe. The setting of the valve can be electro-pneumatically adjusted to either open the inlet from the high-pressure gas line to the heat pipe, effectively raising the operating temperature of the heat pipe, or to open the low-pressure inlet, permitting the venting of buffer gas and effectively lowering the operating temperature, or to close off both gas lines from the heat pipe. Sensors within the pressure vessel, casing, annular space, heat pipes, and manifold transmit information about temperature, pressure, chemical composition, and other operating parameters to a control center at or near the surface of the earth. Referring now to FIG. 5, at various depths along the shaft there are disposed closure means (90) and (80) to sealingly close off the casing and all heat pipes and electro-pneumatic control lines and sensors contained therein. The positions of the closure means are chosen to be optimally effective in isolating from the atmosphere such gaseous and particulate contaminants as might issue from the reactor system in the case of malfunction, and as might be expected to issue during the cooling-off period of a reactor that has been permanently shut down. In a preferred embodiment of the invention, the closure means include first-acting high-explosive-actuated pipe closures (90), and later-acting mechanically or pneumatically driven butterfly valves, sliding gates, and/or miter valves (80). The mechanically or pneumatically driven closure means (80) are disposed between the explosive closures (90) and the surface of the earth. The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, the reactor may employ a cylindrical core and cylindrical heat pipes in place of the conical elements described herein. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.