Patent Number: 044787847
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, a reactor 10 is illustrated schematically, it being housed within a larger containment building (not shown). The reactor 10 itself has an open top vessel or tank 12 which is enclosed on its sides and bottom within a guard vessel or tank 14 and shielding such as concrete walls 16. Preferably the cavity 17 located between the reactor vessel 12 and guard vessel 14 is sealed and filled with an inert gas, such as argon or helium, which could be monitored for leakage from reactor vessel 12. A deck 18 closes the vessel 12 at its open top and seals 19 maintain the vessel pressure-tight. Located within the vessel is a reactor core 20 surrounded by a radiation shield 21. The core can take any known form but generally would include a plurality of vertical passages (not shown) within which appropriate quantities of fuel and blanket materials are located. Upper internal structure 22 is located above the core and has sensors (not shown) to detect parameters of interest, such as the temperature of the primary coolant, and leads from the sensors are directed to monitor equipment (not shown) outside of the reactor vessel. The structure 22 also includes control linkages or mechanisms (not shown) for regulating the reactor power within the core. All of this internal structure 22 is suspended from the deck, and lines up vertically with the reactor core. One conventional means for supporting and aligning these components relative to the reactor core provides for three rotatable plugs of different sizes, the largest plug 24 being rotatable within the deck concentrically of the core, the intermediate plug 25 being rotatable within the largest plug 24 offset from its center, and the smallest plug 26 being rotatable in the intermediate plug 25 again offset from its center. Fuel loading and unloading mechanism (not shown) is carried by the smallest plug 26 as at circle 27 offset from its center so that rotation of the three plugs according to predetermined orientations can move it into precise vertical alignment over any of the reactor passages for loading and unloading and/or manipulation of the fuel relative to that passage. The reaction of the fuel generates heat, and the core 20 is cooled by a circulating primary coolant, typically molten sodium, which substantially fills the tank 12. Specifically, the primary coolant is circulated from a "cold pool" 28 within the vessel through pump 29 and line 30 upwardly through the core 20 to a "hot pool" 32 confined within irregularly shaped continuous wall structure 33 to inlet into one side of a primary heat exchanger 35. The primary coolant then flows through the heat exchanger back to the cold pool 28. Sliding seals 36 are located between the wall structure 33 and the heat exchanger 35 to separate the hot pool 32 from the cold pool 28 while yet allowing some thermal movement of the structural components. An intermediate coolant is circulated through the other side of the primary heat exchanger 35 (in heat conductive but fluid isolated relation relative to the primary coolant) via inlet and outlet lines 36 and 37 and a closed intermediate cooling loop (not shown) including a pump and an intermediate heat exchanger located outside the reactor vessel 12. The intermediate coolant would preferably be molten sodium also. A secondary coolant, generally water, would be circulated through the secondary heat exchanger (in heat conductive but fluid isolated relation relative to the intermediate coolant) in a closed secondary steamwater cooling loop with a power turbine (not shown) forming part of a conventional electrical power generating system. The secondary coolant is thereby essentially free of radioactive contaminants to minimize the risk of radioactive spill should any of the secondary coolant components in the steam-water loop fail and leakage occur. While reference has been made to the primary heat exchanger and other related cooling components only in the singular, most typically there would be several such primary heat exchangers and pumps, etc. located in the reactor vessel which would define parallel coolant loops to the steam utilizing turbine. Construction details need not be given since they form no part of the subject invention and are of conventional well known means. As noted above the deck 18 spans the open top of the vessel 12 and is structural in nature in that it suspends from it various reactor components including the primary coolant pumps 29 and heat exchangers 35, the rotary plugs 24, 25, 26 and the upper internal structure 22. The reactor vessel 12 can be in excess of 75 feet across its open top and the deck 18 is of corresponding size. It is yet desirable to form the deck 18 from conventional materials that are reasonably economical and easy to fabricate, while yet satisfying safety and structural requirements including thermal deflections and alignment requirements. The deck 18 (see FIGS. 2 and 3) typically has spaced upper and lower main horizontal plates 40a and 40b, flange plate 41, vertical plates 42a, 42b, etc., 43a, 43b, etc., and 45a, 45b, etc., and webs 46a, 46b, etc. In a preferred embodiment, the deck plates or webs are formed of structural plate material, such as steel, and are welded or otherwise secured relative to one another across continuous leak-proof seams so as to define a hollow but otherwise sealed unitary deck structure. With the main plates 40a and 40b and the vertical cylindrically shaped plates 45a and 45b welded together, the physical component like the heat exchangers 35 or pumps 29 can extend through and be supported by the deck. Reinforcing webs 46a and 46b can be welded in place at specific locations as needed. Thermal insulating barriers 47 of steel meshing and steel sheeting moreover are supported by walls 43a and 43b proximate the underside of the lower deck plate 40b, and radioactivity shielding 48b of ironized concrete is carried by the deck proximate the upper side of the lower deck plate. The deck 18 is exposed on its underside to the various "hot" and "cold" pools of primary coolant confined within the vessel 12. For example, the "cold" pool would typically be at temperatures in excess of 500.degree. F. and possibly up to 700.degree. F.; whereas the hot pool would typically be at temperatures in excess of 800.degree. F. and possibly up to 1000.degree. F. The upper deck plate 40a would typically be exposed to ambient air, possibly at 65.degree.-85.degree. F., in the confinement building. A preferred design for the deck structure provides that the lower deck plate 40b would be operated at temperatures less than 250.degree. F. and preferably even as low as 150.degree. F., while the upper deck plate 40a would be operated at temperatures less than 150.degree. F. and possibly even as low as 100.degree. F. This design temperature differential of 50.degree. to 150.degree. F. between the upper and lower deck plates, after once established, would thereafter have to be maintained. Otherwise, temperature differences exceeding this could cause thermal deflections which could be magnified between the components supported from the deck to create misalignment of these components, disruption of the seals between the components, or other problems. A conventional concept for cooling the deck structure has been by circulating a coolant, such as air, nitrogen, or water, within or through the deck structure. The deck would thus have many crosswise or radial passages 49 located immediately adjacent the lower deck plate and vertical passages 50 extended between and connecting these passages as coolant loops. For forced coolant circulation within the deck structure, a motor driver blower 51 is provided. The forced coolant circulation easily maintains the design temperature differential between the upper and lower deck plates. However, should the blower power source, viz., the conventional AC electrical power, standby power, or battery power be discontinued, the forced coolant circulation through the passages would cease and the design temperature differential between the upper and lower deck walls would be exceeded. Convective cooling is a possible "passive" system for use in emergency conditions, "passive" meaning that no input power is required to operate the system. The convective cooling system typically would use air as a cooling means and would have inlet and outlet passages (not shown) through the concrete barrier (with angled bends to eliminate radiation streaming from the reactor). However, convective cooling is not an attractive alternative by itself as it has low capacity and moreover requires that the deck structure be open to the atmosphere, and not sealed. This invention teaches an improved "passive" cooling means for maintaining the upper and lower deck plates within design temperature differences. The cooling means could act along or in parallel with conventional forced coolant circulating means; however, the disclosed cooling means can be designed to have adequate cooling capacity to meet most operating conditions. The subject invention thus provides thermal stability for the deck and positional stability of any reactor components carried by the deck. The invention utilizes a plurality of heat pipes 52a, 52b, 52c, etc., each of which has a vaporizing section 54 located to receive heat from the lower deck wall 40b, a primary condensing section 56 located to dissipate heat to the upper deck plate 40a, and a secondary condensing section 58 located beyond the upper deck plate and outside of the deck 18 itself to dissipate heat to the atmospheric air in the containment building. The secondary cooling section 58 could be made to be effective in only off-normal or emergency operating conditions, as will be disclosed. Each heat pipe 52 consists of a housing 60 of stainless steel, for example, having a coolant sealed therein. The coolant would be selected to vaporize at the input temperatures of the vaporizing section 54 and would condense at the output temperatures of the condensing sections 56 and 58. A coolant in the form of water or alcohol could be used for the range of operating temperature under consideration. A wick 62 would cover the inner walls of the housing 60, the wick being preferably formed of a meshed network of stainless steel having many very small pores or openings of the order of 100-150 mesh. Heat added to the heat pipe vaporizing section 54 would vaporize the liquid coolant therein which vapor would then flow axially along the center space toward the primary and secondary condensing sections 56 and 58 respectively. The primary condensing section 56, in heat dissipating relation to the upper deck wall 40, normally would condense the coolant vapors onto the wick 62. The coolant then would migrate by capillary action, and also gravity, depending on its orientation along the wick 62 from the condensing section 56 to the vaporizing section 54. The heat pipe 52 would be designed to operate within the input and output range of temperatures so that coolant condensate will always move via the wick 62 to the vaporizing section 54, and coolant vapor would move interiorally of the housing 60 to the condensing section 56 or 58; and under stabilized operating conditions, this coolant circulation would be continuous. Because vaporization and condensation are each involved in the action of the coolant in the pipe, the heat transferring capacity of the heat pipe 52 is very large, possibly 50-500 times greater than a solid copper pipe for example. However, the heat pipe 52 is yet entirely passive and requires no input electrical power. The secondary condensing section 58 is located outside of the deck 18 in the building atmosphere. However, it is housed within a small enclosure 64 having open sides, and damper doors 66 would normally close the open sides of the enclosure to isolate the condensing section from the air of the containment building. However, when the damper doors 66 are opened, air flow through the enclosure 64 is possible over the secondary condensation section 58. Fins 68 can be on the secondary condensing section 58 to provide good heat transfer with the ambient air. Each damper door 66 can be shifted between its closed and opened positions by means of a bimetal activator 70, which in a preferred embodiment would be exposed to the upper deck plate 40a to be responsive to the temperature of the deck plate. Under normal reactor operation, the temperature differential between the vaporizing section 54 and primary condensing section 56 of the heat pipe would be designed to be of the order of 50.degree.-150.degree. F. and the heat transferring capacities of the heat pipes could be sufficient to maintain the deck plates within this specified temperature differential. However, if the heat pipes are to be used only as a redundant or parallel system with the forced coolant circulation in the deck structure, the design capacity could be less. The anticipated heat withdrawn by the heat pipe system under normal reactor operation could typically only comprise 10-25% of the total deck cooling, and the forced coolant circulation would provide the balance. Upon a breakdown of the normal forced coolant circulation system, such as during a power failure, the design temperature differential between the upper and lower deck plates would be exceeded. This increase in the temperature of the deck would be sensed by the bimetal activator 70 to open the damper doors 66 to expose the secondary condensing section fins 68 to the air within the containment building. The building would be conditioned, so that the air temperature would be controlled and similar to normal atmospheric temperature of 65.degree.-85.degree. F. The secondary condensing section 58 greatly increases the cooling capacity of the heat pipe 52, although this extra capacity is used primarily for emergency only. Nonetheless, it might be possible to design the heat pipe system with sufficient overall capacity to act as the sole heat dissipating means for cooling the lower deck plate 40b and without any forced coolant circulating means in the deck. However, when acting as either the sole or as the redundant cooling means, the secondary condensing sections 58 of the heat pipes preferably would be isolated and inactive. Thus, the enclosures surround the heat pipes, and with the damper doors 66 closed during normal reactor operation, even though coolant vapor can pass into the secondary condensing section 58, little vapor condensation will take place with the doors closed as there is little or no air circulation to carry the heat away. However, the dissipating capacity of the heat pipes through the primary condensing sections and the upper deck plate 40a will be sufficient to maintain the operating temperatures balanced. It would be possible to modify each heat pipe somewhat by interposing thermally controlled valve means internally of the pipe housing at a location between the primary and secondary condensing sections. At normal operating temperatures, the valve means will be closed to isolate the secondary condensing section from the primary condensing section; whereas at elevated "emergency" temperatures, the valve means would be opened to allow coolant circulation to the secondary condensing section. Each heat pipe 52 further is designated to be removed periodically for inspection and/or maintenance, particularly as regards the integrity of the pressure confinement housing 60. Thus, separate heat conductive clamping sleeves 74 and 76 would fit over the respective vaporizing and primary condensing sections 54 and 56 of the heat pipe and would be secured also the the lower deck plate 40b and the upper deck plate 40a. This not only establishes good heat conductivity between the deck plates 40a and 40b and the heat pipe itself but also allows for the ready removal of the heat pipe. Generally, each sleeve 74, 76, preferably would extend one and possibly two feet axially along the heat pipe 52. A heat pipe 52 with a diameter of approximately an inch, for example, containing water at pressures of approximately 3 psia and working at a temperature of 140.degree. F. would have approximately one kilowatt of heat removing capacity. For a nuclear reactor having a diameter of 70 ft., for example, one hundred such heat pipes 52a, 52b, etc., distributed around the deck 18 would provide approximately 100 kilowatts of cooling capacity. The subject heat pipe cooling system can be used with minimal interferences with and/or without special designs of other components that would extend through or be part of the deck 18. This differs substantially from the typical forced coolant circulating system which requires many special interior and exterior ducts, exterior heat exchangers, as well as powered blowers. It thus might be possible to utilize the heat pipe cooling system on existing reactors as retrofit modification of the reactor cooling systems. Moreover, since each heat pipe wick 62 operates on a capillary principle, the heat pipe 52 need not be oriented vertically, but could run at an angle or even horizontally. The disclosed heat pipe cooling means can be used to cool reactor components other than the deck configuration as illustrated. For example, the heat pipe vaporizing section can be secured in heat transfer relation relative with the guard vessel 14 or to the cavity 17 between the reactor vessel 12 and the guard vessel. The condensing section can be exposed to a heat sink outside of and isolated from the vessel. A control including a temperature responsive mechanism, for example, could be used to regulate the cooling effectiveness of the heat pipe. Under normal operating reactor conditions and temperatures, little cooling would take place via the heat pipe; but above normal reactor temperatures would make the heat pipe more operative in the effort to remove such excess heat; thereby tending to avoid the adverse consequences of abnormal reactor temperatures and/or malfunctioning conditions. The heat pipe cooling means further could be designed with secondary condensing section, much like that illustrated in FIGS. 4 and 5, exposed to the same heat sink or a secondary heat sink only upon overheat situations. This would provide a redundant emergency reactor cooling system, operable passively and without the need for any secondary power. Accordingly, the invention is to be limited in scope only by the appended claims, and not by the actual specific disclosure illustrated.