Patent Number: 043426212
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention is susceptible of various modifications and alternative constructions, there is shown in the drawings and there will hereinafter be described in detail a description of the preferred embodiment of the invention. It is to be understood, however, that the specific description and drawings are not intended to limit the invention to the specific form disclosed. On the contrary, it is intended that the scope of this patent include all modifications and alternative constructions thereof falling within the spirit and scope of the invention as expressed in the appended claims. In a preferred embodiment of the present invention, as depicted in FIG. 1, a nuclear reactor 10 having a vessel 14 and an internally mounted reactor core 12 is suspended within a reactor chamber 24 formed within the base 21 of the containment building 20. The reactor vessel 14 is supported by a cantilevered support member 22 upon which rests the reactor coolant piping 18 as it leaves the reactor vessel 14. Reactor coolant piping 18, in the case of a pressurized water reactor, fluidically connects the interior of the reactor vessel 14 with a steam generator 16 which generates steam for its ultimate delivery to a steam turbine. The general function of the present invention is to protect the concrete walls of chamber 24 from the damaging effects of both the extreme heat and radiation generated by a nuclear core 12 which has melted and dropped through the bottom of the reactor vessel 14 as shown generally at 38. Of primary concern are both the cavity floor 28 and the vertical walls 26. Damage to these walls is to be avoided if at all possible on the occurance of such an accident in order that the radioactive materials associated with a molten core 38 are prevented from escaping either to the exterior of chamber 24 or the exterior of the containment building 20. Therefore, the present invention provides a system of heat pipes 40 and 50 which respectively protect the chamber floor 28 and the chamber wall 26 by collecting a portion of the heat generated by the core in chamber 24 and transporting the heat to a location exterior to the containment building 20. It has been found that heat pipes are well suited for this function in that they can be designed to transfer large quantities of heat with very little thermal resistance, can remain passive for large periods of time without maintainance and can automatically begin their heat transfering function without need of human or mechanical intervention. Furthermore, a heat pipe is ideal for accomplishing these functions inasmuch as each heat pipe constitutes a hermetically sealed unit which is independent from all the other heat pipes of the system. The hermetically sealed feature of the heat pipe permits the heat pipe to penetrate from the interior of chamber 24 to the exterior of the containment building 20 without running the risk of pumping radioactive material to the exterior of the containment envelope in the event that one end of the heat pipes is breached. Heat pipes 40 and 50 are arranged with their evaporator sections 42 and 52 adjacent to the reactor chamber's floor 28 and wall 26 respectively so as to shield the concrete base 21 from the damaging heat and radiation of the molten core 38 and so as to contain the core. Each of these heat pipes has its condenser section 46 and 56 respectively located in a water reservoir 60 external to the reactor containment building 20. The condenser sections and the evaporator sections of the heat pipes are fluidically connected by adiabatic sections 44 and 54 respectively which penetrate through the reactor containment building 20 through base 21. Each of the adiabatic sections 44 and 54 are surrounded by thermal insulation 64 in order that the concrete of the base 21 and the containment building 20 not be exposed to excessive heat which could possibly cause concrete dehydration and subsequent failure. As may be seen in FIG. 1, water reservoir 60 is vented by a vent pipe 62 directly to the atmosphere exterior to the containment building 20. As may also be seen from FIG. 1, condenser sections 46 and 56 of heat pipes 40 and 50 respectively are positioned at elevations higher than the elevations of their respective evaporator sections so that the working fluid does not have to work against a gravitational head in its return to the evaporator section. An angle of slant no less than 30.degree. is preferred in order that the maximum capability of the heat pipe be achieved. As can be seen in FIG. 1, evaporator ends 42 of heat pipes 40 are disposed vertically below reactor vessel 14. A slightly conical upwardly facing shallow basin 30 is also disposed below evaporator sections 42. Basin 30 is provided to prevent direct contact between molten core 38 and base 21. Basin 30 desirably consists of a refractory metal having a high melting point such as tungsten, tantalum carbide, zirconium carbide, niobium carbide, hafnium carbide or graphite. However, it is predicted that the temperature of molten core 38 would exceed the melting point of refractory basin 30 so that refractory basin 30 must be either directly cooled by a cooling system or shielded from the elevated temperatures of the core. The present invention chooses the latter arrangement. Accordingly, heat pipes 40 are arranged in a star-like pattern radiating outwardly from a position under the core 12 in a manner which best shields refractory basin 30 from the temperature of the core. The preferred arrangement of the invention is illustrated in FIG. 2 in which heat pipes 40 are arranged in an outwardly radiating star-like array. Each heat pipe 40 includes thermal conducting fins 48 attached to its evaporator end 42. As can be seen, fins 48 are shaped to butt one against another to almost completely cover the upwardly facing surface of basin 30. Adjacent fins 48 however, are separated by a slight gap in order to accomodate the thermal expansion expected when heated by a molten core 38. Evaporator sections 42 of heat pipes 40 are vertically supported by but not anchored to underlying basin 30. In this manner, heat pipes 40 are permitted to operate at a temperature in excess of the temperature of basin 30 without incurring the significant problems of differential thermal expansion. Thermal expansion of the heat pipes 40 are further accommodated by the provision of anchoring heat pipes 40 to base 21 of the containment building 20 at only one point: the point at which the heat pipes enter the concrete foundation 21. At this position, the heat pipe is hermetically sealed to the base 21 by seal 34 so that the passage through which the adiabatic section 44 of the heat pipe passes is hermetically isolated from the interior of chamber 24. Seals 34 must be of such a nature as to be able to withstand radial expansion of heat pipe 40. As can be seen, with this arrangement, heat pipes 40 are permitted axial growth in both the inward and outward direction from the attachment point at seal 34 and the hermetic containment envelope is maintained. While the above described arrangement effectively shields basin 30 from excessive temperatures, it is still expected that the basin 30 will be exposed to an extreme elevated temperature. Accordingly, basin 30 will also experience thermal growth. In anticipation of the thermal growth expected in basin 30, the basin is attached to base 21 only in one centrally located point 32. This arrangement permits basin 30 to undergo unrestricted radial expansion so that warping effects are minimized. As can be seen in FIG. 1, a layer of thermal insulation 36 such as a layer of alumina bricks may be placed under basin 30 in order to protect the underlying concrete of base 21 from thermally caused dehydration. The above described arrangement exposes the evaporator sections 42 of heat pipes 40 directly to the molten core 38. It should be recognized, however, that basaltic blocks as taught in U.S. Pat. No. 3,702,802 may also be placed above the heat pipes in order to reduce the thermal shock placed on the heat pipes 40 as well as to reduce the heat release per unit volume of core material by diluting the molten core material with material from the basalt blocks. An additional measure which may be taken to avoid the excessive concentration of heat of the molten core is the formation of basin 30 and evaporator sections 42 of heat pipes 40 in a nearly horizontal manner. The molten core material would then be expected to spread out in a relatively thin layer. Other measures may be taken such as those taught in U.S. Pat. No. 4,036,688 in order to prevent the molten core from forming a critical geometry. In a manner similar to that described above for heat pipes 40, heat pipes 50 are provided with thermally conducting fins 58 which spread out and shield the inner surface 26 of chamber 24 at portions which are not directly shielded by the heat pipe evaporator section 52 itself. While not shown in the drawing of FIG. 1, it may also be desirable to line the inner surface of chamber 24 with a thermal insulator such as refractory bricks in order to prevent the dehydration of the concrete base 21. Heat pipes 50 are preferably arranged in a star-like pattern which radiates outwardly from the reactor. As can be seen from FIG. 1, an inwardly projecting "knee" of heat pipe 50 is provided in order to intercept and absorb the upwardly directed radiations emminating from molten core 38. In this manner, the upper portions of the cavity 24 are protected against the damaging heat and radiations emmited by the core. Heat pipes 50 are also anchored at that point at which each heat pipe enters the concrete wall of the chamber at seal 34 so that heat pipes 50 may undergo unrestricted thermal expansion in both the inward and outward directions. Turning now to an examination of FIG. 3, a typical cross-section of the evaporator sections of heat pipes 40 is shown. Also shown are a portion of the base 21, the thermal insulation 36, the refractory metal basin 30 and the fins 48. Wicking material 66 resides on the interior of the heat pipes 40. As is well understood, the materials from which heat pipes 40 are constructed depend upon a number of factors including the amount of heat which must be transported, the maximum temperatures expected, and the compatability of the materials used in the pipe which included the pipe itself, the working fluid, and the wicking material. It has been calculated that for a 3,800 megawatt thermal core, 136 six inch sodium filled heat pipes with evaporator lengths of eight feet and condenser lengths of twelve feet would be adequate to effectively remove the heat generated by molten core 38 so that basin 30 is protected from melting and core 38 is prevented from boiling. Other possible candidates for the working fluid of the heat pipes 40 and 50 includes potassium, cesium, mercury, and one of the eutectic alloys such as NaK. If the selected working fluid were to be liquid sodium, suitable heat pipe materials might include one of the alloys having trade names Nickel 200, Monel 400, Inconel 600, or Inconel 800. In addition, long heat pipes containing a plurality of working fluids are possible. Such heat pipes, when called upon to operate, would automatically separate themselves into zones determined by the latent heat of evaporation of the various working fluids as well as the temperature of the evaporation of the various working fluids.