Patent Number: 043572979
Section: summary

This invention generally relates to nuclear reactors and, more particularly, to cooling systems for primary vessels. In general, the temperature of the walls of a primary vessel or tank is controlled by the thermal resistance of the insulation in combination with the heat removal systems which cool the supporting structure of the reactor and its surrounding cavity. The thermal insulation protects the walls of the primary tank against axial temperature gradients, transient temperatures, and high temperatures per se. Such insulation is necessary in order to protect the tank and its internal structures from thermal stresses which could endanger the integrity of the primary coolant boundary. The present invention has application in any nuclear reactor including but not limited to pressurized water, boiling water, gas cooled and liquid metal cooled reactors. The preferred embodiment is disclosed in connection with a liquid metal cooled nuclear reactor because such reactors operate at higher temperatures. Typically, these reactors operate within a temperature range of between 650.degree. F. and 950.degree. F. In addition, sodium has a very high coefficient of heat transfer so that during the operation of these reactors thermal transients rapidly propagate through the system and can cause severe thermal stresses. The general problem of insulating a primary tank in order to limit temperature level, gradients, and transients, occurs in both loop and pool type reactor designs. The temperature gradient problem, however, increases with the size of the reactor vessel. The axial gradient stress increases as the square root of the product of the wall radius and the thickness. Also, the transient radial gradient stress increases with the wall thickness. FIG. 1 illustrates a typical sodium cooled nuclear reactor. This reactor is a pool type reactor and includes a reactor core 4 which is supported within a primary tank 6. The primary tank is suspended from a steel support 7 which is mounted on top of the side wall of the concrete reactor cavity. The reactor further includes a plurality of pumps 8 which circulate the sodium coolant through the reactor during operation. One such pump is illustrated in FIG. 1. The pumps draw relatively cool sodium from a cold pool 9 located in the bottom of the primary tank and discharge it into the bottom of the reactor core 4. The sodium then flows upward through the core while being heated therein and is discharged into the hot pool 10. The sodium thereafter flows downward through a plurality of intermediate heat exchangers 11 and is discharged back into the cold pool. One such heat exchanger is illustrated in FIG. 1. Within the intermediate heat exchanger 11, FIG. 1 the heat from the core is transferred to a secondary sodium coolant which is circulated between the intermediate heat exchanger 11 and a plurality of steam generators (not shown). The hot and cold pools are separated by a horizontal structure 12 that forms the boundary between the two pools and includes a plurality of vertical wells 16 that thermally separate each pump 8 from the hot pool. Only one of the pump wells is shown in FIG. 1. The reactor operates at nominally atmospheric pressure and has a level of sodium 13 in the hot pool which is substantially below the reactor cover 12. The space above the hot pool is filled with an inert gas such as helium. In the past the principal technique for cooling the walls of primary tanks or vessels has been bypass cooling. In bypass cooling a portion of the flow of the relatively cold coolant is directed against the side walls of the primary tank. In the reactor of FIG. 1, the bypass coolant is obtained from the cold pool 9 and is directed against the inner side wall of the primary tank. In effect, a moving barrier of relatively cold sodium is interposed between the hot pool 10 and the primary tank wall 6. In one bypass flow design a portion of the discharge from the coolant circulating pump is first directed against the inside of the primary tank in an upward direction. The coolant is thereafter redirected downward by a baffle and flows back into the cold pool. In this design there is an upward flow of sodium from the cold pool, thermal contact between the cold sodium and the primary tank wall and a downward flow of sodium in the space between the hot pool and the upward flowing cold sodium. An alternative approach has been to direct the cold sodium from the discharge of the pump upward along the sidewall of the hot pool and then downward along the inner side wall of the primary tank. In this alternative design there is a wide, annular space between the side wall of the hot pool and the primary tank. Although bypass cooling is an accepted technique, there are several problem areas which cause concern. The principal problem with bypass cooling is that a large temperature difference is developed across the structural boundaries between the hot pool and the bypass cooling flow. This temperature difference is typically about 300.degree. F. during normal operation. This difference can cause severe thermal stresses to be developed during accident conditions. Another problem with bypass cooling is that the system cannot be designed to optionally function at constant temperature at every power level. Most reactors are operated with a fairly constant cooling outlet temperature so that the cooling requirement for the vessel wall is preferably constant and independent of power level. The rate of bypass coolant flow, however, is a function of the output of the circulating pump. The result is that the flow of bypass coolant varies with power level and the amount of heat transfer and resultant vessel wall temperatures likewise vary. A further problem with bypass cooling is that the bypass coolant itself is subject to temperature transients. The bypass flow is typically taken from the cold pool which is subject to temperature transients such as the stoppage of the flow of secondary sodium through the intermediate heat exchanger. This stoppage causes a sharp transient in the temperature of the cold pool because the primary sodium flowing through the intermediate heat exchanger is no longer cooled and hot sodium is dumped into the cold pool. Still another problem with bypass cooling is that it decreases the overall output of the reactor. Bypass cooling requires the diversion of a portion of the flow of coolant away from the reactor core. This diversion represents an efficiency loss because the net electrical output of the reactor is decreased. An additional technique for cooling the walls of nuclear reactors is disclosed in U.S. Pat. No. 4,055,465 issued on Oct. 25, 1977 to La Mercier. A principal object of the present invention is to reduce the amount of axial temperature stresses in the primary tank of a nuclear reactor. In FIG. 1, there is a vertical temperature gradient between the point of support 7 and the horizontal structure 12. This temperature gradient, which is in the axial direction, and the rate of change of this temperature gradient causes meridian bending stresses. Reduction of axial temperature stresses is a principal object of this invention because it has been observed in large breeder reactors that this is the controlling stress consideration. If adequate provisions are made to control axial temperature stresses, then both steady state and transient radial temperature gradients usually provide no difficulties. A further object of the present invention is to reduce the temperature of the primary tank at the point of support to approximately 100.degree. F. In the embodiment of FIG. 1, the point of support is the ledge 7. Such a low temperature is necessary at the point of support in order to reduce differential thermal expansions which occur at that point. Typically, in the design of a top supported primary tank the change of the temperature gradient near the point of support produces the most intense stresses in the primary tank. A further object of the present invention is to design a passive temperature insulating apparatus suitable for all power levels and for constant temperature operation. In addition, the apparatus must limit the stresses caused by thermal gradients to acceptable values for approximately 40 years. The apparatus must also resist chemical attack from sodium and must avoid the formation of particulate materials that could circulate through the reactor. An additional object of the present invention is to insure that the integrity of the primary tank of the reactor is maintained during emergency conditions and that the reactor reliably functions without requiring in-vessel maintenance for a period of approximately 40 years. The objects and advantages of the present invention are achieved by an apparatus for thermally insulating a primary tank in a nuclear reactor. The apparatus includes a plurality of vertically oriented, reflective metal plates located within the primary tank and around the outside of the reactor core for thermally insulating the side walls of the tank from the temperatures generated by the reactor. The reflective metal plates are radially spaced apart and each has an arcuate cross section.