Patent Number: 048790893
Section: claims

1. A liquid metal cooled nuclear reactor, of the kind in which the liquid metal coolant is circulated from a relatively cool region (herein called the cold pool) of the reactor primary vessel through the reactor core where the coolant is in heat exchange relationship with nuclear fuel and emerges at a higher temperature in a higher region (herein called the hot pool) of the reactor vessel, the coolant then being returned from the hot pool to the cold pool via heat exchangers which serve to extract heat from the hot coolant for conversion to other forms of energy, the reactor having an intermediate plenum encircling the reactor core to provide a thermal barrier between the hot and cold pools of the reactor, characterised by means intercommunicating an upper part of the intermediate plenum and the hot pool superimposed in relation thereto to establish therebetween a permanently open, convectively acting communication capable of allowing rapid interchange of coolant convectively and unforced between the intermediate plenum and the hot pool in response to rapid temperature reduction in the hot pool and in a sufficient amount so as to avoid substantial temperature differentials which would otherwise occur between the upper part of the intermediate plenum and the hot pool but incapable of allowing, when the reactor is in a steady state condition, any more than a substantially smaller interchange to occur such as will not materially affect the thermal barrier function of said intermediate plenum. 2. A reactor as claimed in claim 1 in which the means comprises coolant interchange routes between the intermediate plenum and the hot pool, which routes are provided with deflector means for deflecting inward coolant flow to the intermediate plenum in a predominantly horizontal direction. 3. A reactor as claimed in claim 1 in which said means comprises thermosiphon means to effect transfer of coolant from the intermediate plenum to the hot pool. 4. A reactor as claimed in claim 3 in which said thermosiphon means comprises a thermosiphon leg which opens at its opposite ends into the intermediate plenum and the hot pool respectively and is in thermal contact with the hot pool so that, when a fall in temperature occurs within the hot pool, a pressure head is developed to create coolant flow along the thermosiphon leg from the intermediate plenum into the hot pool. 5. A reactor as claimed in claim 4 in which, to compensate for the outward coolant flow along the thermosiphon leg, the intermediate plenum includes one or more openings in communication with the hot pool to admit coolant from the hot pool into the intermediate plenum. 6. A reactor as claimed in claim 4 in which the thermosiphon leg is constituted by one or more standpipes which communicate at the lower end with the interior of the intermediate plenum and open at the upper end adjacent the normal operating level of the hot pool. 7. A reactor as claimed in claim 4 in which the thermosiphon leg comprises an annular passage which encircles the hot pool and is defined between an inner tank, which is located within the reactor primary vessel and encloses the upper regions of the coolant pool, and a radially inwardly spaced, generally vertical annular wall which terminates below the normal operating level of the hot pool. 8. A reactor as claimed in claim 7 in which the intermediate plenum is bounded along its generally vertical outer side by the inner tank and the generally vertical annular wall penetrates below the top of the intermediate plenum and is formed with a series of circumferentially spaced port holes. 9. A reactor as claimed in claim 1 in which openings for said interchange of coolant are provided in the intermediate plenum and are in the form of clearances between penetration holes in the top of the intermediate plenum and structures passing through the intermediate plenum. 10. A reactor as claimed in claim 1 in which the top of the intermediate plenum is of permeable construction. 11. A reactor as claimed in claim 10 in which the intermediate plenum top is perforated with a large number of relatively small holes or comprises a structure of generally vertical honeycomb channels. 12. A reactor as claimed in claim 10 in which the intermediate plenum top comprises an array of generally horizontally disposed plates. 13. A reactor as claimed in claim 12 in which the plates are supported in generally co-planar relationship and in spaced relation to one another to form an openwork structure permitting coolant interchange between the hot pool and the intermediate plenum. 14. A reactor as claimed in claim 13 in which the array of plates is superimposed above an array of generally horizontal deflector plates which overlap with the clearances between the plates above so as to deflect inward coolant flow to the intermediate plenum in a predominantly horizontal direction. 15. A reactor as claimed in 13 in which the plates forming the openwork structure are in relatively closely spaced relation and unsupported at their edges such that, in steady state conditions, flow of coolant into the intermediate plenum is severely restricted and when transient conditions develop and tend to create a temperature differential through the thickness of the plates, the plates deform to enlarge the gaps between adjacent plates and permit increased flow of coolant into the intermediate plenum. 16. A reactor as claimed in claim 1 which the intermediate plenum is divided into at least two vertically superimposed sections, the upper of which is in communication with the hot pool and the lower of which is isolated from any convection flow of coolant within the upper section.