Patent Number: 043022960
Section: claims

1. In a pool-type nuclear reactor which uses liquid sodium as a coolant, comprising a vertically extending main reactor vessel; a generally horizontally extending structural load bearing arrangement located within and extending across said vessel; a reactor core located directly on top of and being supported by said arrangement; a first plenum located within and generally horizontally across said vessel above said load bearing arrangement and core for containing a relatively turbulent supply of liquid sodium; and a second plenum located within said vessel above and directly adjacent to said load bearing arrangement around said core such that the latter and said second plenum together separate said first plenum and load bearing arrangement from one another, said second plenum containing a stagnant quantity of sodium serving as a thermally insulating fluid barrier between the first plenum and said load bearing arrangement. 2. A reactor as in claim 1 wherein said second plenum has substantially no net flow of sodium therethrough and achieves temperature stratification of the sodium therein as a result of variations in density of the sodium due to a temperature gradient. 3. A reactor as in claim 1 wherein said second plenum includes a horizontal baffle for reducing thermal currents within the second plenum due to irregular temperature gradients between the sodium in the first plenum and the structural arrangement. 4. A reactor as in claim 1 wherein said second plenum includes a horizontal baffle forming a top wall of the second plenum, said first plenum being located generally above said baffle, said sodium in the second plenum being in fluid and temperature communication across the horizontal baffle with the sodium in said first plenum. 5. A reactor as in claim 7 wherein said first plenum receives hot sodium discharging upward from the reactor core during operation. 6. A reactor as in claim 1 wherein the sodium in said first plenum has a temperature greater than about 800.degree. F. 7. A reactor as in claim 1 wherein said second plenum includes means separating the latter into a first sub-plenum adjacent said first plenum and a second sub-plenum adjacent said load bearing arrangement, said sub-plenums being in fluid communication with one another sufficient to eliminate any pressure drop therebetween. 8. A reactor as in claim 1 wherein said first and second plenums are in sufficient fluid communication with one another to eliminate any pressure drop therebetween while maintaining the stagnant nature of the sodium within the second plenum. 9. A reactor as in claim 1 including a third plenum containing a relatively tubulent supply of liquid sodium lower in temperature than the sodium in said first plenum, said third plenum being located within said vessel adjacent said load bearing arrangement opposite said second plenum whereby said stagnant quantity of sodium in said second plenum also serves as a thermally insulating fluid barrier between the first plenum and third plenum. 10. In a pool-type nuclear reactor which uses liquid sodium as a coolant, said reactor comprising: a vertically extending main reactor vessel; a reactor core located within said vessel; a generally horizontally extending structural load bearing arrangement located within and extending across the vessel below said core for supporting the latter; an upper generally horizontally extending plenum located with said vessel above said load bearing arrangement and around a top section of said core for containing a supply of hot liquid sodium; a generally horizontally extending lower plenum located within said vessel below said structural arrangement for containing a supply of cooler liquid sodium; means including at least one heat exchanger and one circulation pump for providing a continuous stream of liquid sodium from said lower plenum through said core and thereafter to and through said heat exchanger and into said upper plenum whereby the sodium in each of the upper and lower plenums is in a state of turbulence; and an intermediate plenum located within said vessel between said upper plenum and said load bearing arrangement and around said core for containing a stagnant quantity of sodium serving as a thermally insulating fluid barrier between the upper plenum on one side thereof and the load bearing arrangement and lower plenum on the other side thereof, said upper plenum and intermediate plenum being in sufficient fluid communication with one another to eliminate any pressure drop therebetween while maintaining the stagnant nature of the sodium within the intermediate plenum, regardless of the turbulence in the upper plenum.