Patent Number: 048658022
Section: summary

BACKGROUND OF THE INVENTION The present invention broadly relates to an accumulator which allows for the thermal expansion of a fluid. It particularly relates to such an accumulator which allows for the thermal expansion of a liquid metal in a zero gravity environment. There is currently a major combined effort being made by certain government agencies to develop a high-power, light-weight nuclear reactor power source (SP-100 reactor) for use on future spacecraft. The program will require the resolution of numerous technological challenges. One of those challenges will be to provide a means for allowing for thermal expansion of the liquid metal reactor coolant. The SP-100 reactor utilizes liquid lithium as a reactor coolant and heat transport medium. The temperature of the reactor coolant may range from as low as ambient prior to initiation of reactor power to as high as 1000.degree. Kelvin at maximum power. In addition the means providing for thermal expansion must be reliable since it must last for many years. To accommodate the thermal expansion of a liquid in a terrestrial environment is a relatively simple and straightforward matter. Specifically, one need only provide an enclosed vessel having an opening in a lower portion thereof. Gravity will result in the more dense liquid staying in a bottom portion of the vessel and air or gas trapped therein will be an upper portion of the vessel by virtue of its lesser density. As the liquid expands the gas will be compressed. Conversely, as the liquid cools, the gas will expand forcing the liquid back out into the system from which it was drawn. Providing a liquid accumulator for use in a zero gravity environment, such as would be encountered in space, is not so easy a task. In the absence of a gravity gradient, the gas and liquid would intermingle in a device such as just described with the result that the combination of gas and liquid could be anywhere in the system. If this were allowed to occur, the gas could accumulate as a large bubble anywhere in the system for example in a liquid-cooled, nuclear reactor core and interfere with the heat transport of the liquid metal coolant. Specifically, it could result in overheating and failure of individual fuel pins within the reactor core. Obviously, therefore, some means must be provided to accommodate the increase in volume of liquid metal coolant which results from thermal expansion and it must be accomplished in a manner to insure that there is not entrainment of gas in the liquid metal coolant. It has been proposed to use a bellows to provide for such thermal expansion. The liquid would be on one side of the bellows and the gas on the other such that the flexible movement of the bellows would accommodate expansion and contraction of the liquid while still maintaining the two physically separated. A disadvantage of this approach is that the flexing of the bellows could result in a structural failure or crack the bellows which could in turn result in mixing of the gas and liquid. In addition, the temperatures which would be encountered by the bellows from the coolant in a space reactor environment are such that it is doubtful if a suitable bellows material would be available. It also has been proposed to use a piston with seals to maintain a gas and liquid separate, while movement of the piston would accommodate expansion or contraction of the liquid. An obvious disadvantage of this approach is that any failure of the seal would again result in a mixture of the gas and liquid with the attendant risk of overheating and failure of fuel pins within the reactor core. Thus, there clearly is need for a reliable accumulator to allow for thermal expanson of a liquid metal coolant which would be suitable for use in a substantially zero gravity environment. OBJECTS OF THE INVENTION It is an object of the invention to provide an apparatus which will accommodate thermal expansion of a liquid and which utilizes the surface tension of the liquid to maintain a gas liquid interface. It is another object of the invention to provide an apparatus which will accommodate thermal expansion of a liquid and provide a storage device which utilizes the surface tension of a liquid coolant from a nuclear reactor. It is still another object of the invention to provide an apparatus which will accommodate thermal expansion of a nuclear reactor coolant which requires no moving parts. It is still another object of the invention to provide an apparatus which will accommodate thermal expansion of a liquid and maintain a gas-liquid interface in a substantially zero gravity environment. Another object of the invention is to provide an apparatus for accommodating thermal expansion of a reactor coolant in a space environment which will reliably function for the life of the reactor. These and other objects of the invention will be more apparent from the drawing and following description thereof. SUMMARY OF THE INVENTION The present invention provides an apparatus for accommodating thermal expansion of a liquid utilizing the compression of a gas and further provides for maintaining the gas and liquid separate in a substantially zero gravity environment. The present invention is particularly applicable to allowing for the thermal expansion of a liquid metal coolant used in a nuclear reactor which is required to operate in space. It is another advantage of the present invention that it utilizes the surface tension of the liquid and requires no moving parts. Broadly, the essence of the present invention resides in the use of the surface tension of a liquid to form a meniscus which acts as a gas-liquid interface, the meniscus providing a barrier to the passage or entrainment of gas into the liquid. Thus, in accordance with a particularly preferred embodiment, the apparatus comprises a plurality of elongated tubes which are supported at one end in a grid member. The grid member is in sealing engagement with the outer periphery of the tubes such that an interior passgeway through each of the tubes provides the sole fluid communication through the grid member. The apparatus further includes a vessel having outer walls which surround and are in sealing engagement with an outer periphery of the grid member. The vessel further includes a bottom member which is spaced apart from a face of the grid member opposite the tubes such that the wall member, bottom member and grid member form a liquid zone. A conduit member is provided for the introduction and withdrawal of liquid from said liquid zone. Located within the liquid zone, and extending at least part way into each of the tubes is a body of liquid, the surface tension of which forms a meniscus in each of the tubes. Each of the tubes also contains a body of a gas. The body of gas may be contained within each individual tube by sealing an end of the tube opposite the grid. Alternatively, the grid and tubes may be totally confined within the vessel.