Patent Number: 048790893
Section: summary

This invention relates to liquid metal cooled nuclear reactors 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, e.g. by way of steam generating plant. Typically, in normal operating circumstances, the mean operating temperatures of the hot and cold pools are of the order of 540.degree. C. and 370.degree. C. respectively. In this kind of reactor, it has been previously proposed to introduce a thermal barrier between the hot and cold pools by way of an intermediate plenum encircling the reactor core and enclosing a substantially stagnant volume of coolant which undergoes thermal stratification, thereby providing thermal resistance between the hot and cold pools and protecting the pool separator structure which provides the pressure boundary between the hot and cold pools. The intermediate plenum also serves to establish an extended thermal gradient for reactor internal structures, such as the shells that contain the heat exchangers and coolant pumps which pass through the plenum. One proposal for an intermediate plenum is disclosed by G. Kenworthy et al in a paper "Study of natural convection phenomena in LMFBR intermediate plenum"--Liquid Metal Engineering and Technology (April, 1984), Vol 1, Page 179, British Nuclear Engineering Society. A potential drawback with the design disclosed in the above Kenworthy reference is the low speed of response to thermal transients within the hot pool. For instance, in the event of a reactor trip, the temperature of the coolant entering the hot pool from the reactor core is reduced considerably and, as a result, the temperature within the hot pool may fall within a very short space of time to 400.degree. C. or below. With the previously proposed intermediate plenum design, this could lead to a considerable temperature differential across the upper plate of the intermediate plenum with the risk of unacceptable thermal stressing of the structure. The object of the present invention is to provide a nuclear reactor of the kind specified in which the intermediate plenum is constructed so as to allow rapid response to temperature changes within the hot pool. According to the present invention there is provided 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. Thus, in contrast to the previous proposal in which the intermediate plenum is isolated from the strong forced convective flow of coolant within the hot pool so as to ensure a totally stagnant volume of coolant within the intermediate plenum, interchange of coolant between the intermediate plenum and the hot pool is provided for so that, in the event of a rapid thermal transient within the hot pool, interchange of lower temperature coolant from the hot pool and higher temperature coolant from the intermediate plenum can occur thereby enabling the coolant enclosed within the intermediate plenum to respond promptly to temperature changes within the hot pool. The arrangement is such that the rate of flow of coolant that may occur from the hot pool into the intermediate plenum when the reactor is in a steady state condition is substantially smaller than that which may occur during thermal transient conditions. Preferably, the arrangement is such that the rate of flow of coolant that may occur from the hot pool into the intermediate plenum when the reactor is in a steady state condition is substantially smaller than that which may occur during thermal transient conditions. Preferably the coolant interchange routes between the intermediate plenum and the hot pool are provided with deflector means for deflecting inward coolant flow to the intermediate plenum in a predominantly horizontal direction so as to minimise disruption of the thermal stratification conditions within the intermediate plenum during steady state operation of the reactor. In one embodiment of the invention, the interchange of coolant is effected by thermosiphon means. Thus, the thermosiphon leg may open at its opposite ends into the intermediate plenum and the hot pool respectively and may be 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. To compensate for the outward coolant flow along the thermosiphon leg, the intermediate plenum may include one or more openings in communication with the hot pool to admit coolant from the hot pool into the intermediate plenum. The thermosiphon leg may be 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. Preferably however the thermosiphon leg comprises an annular passage which encircles the hot pool and may be 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. The intermediate plenum may be bounded along its generally vertical outer side by the inner tank and the generally vertical annular wall may penetrate below the top of the intermediate plenum and be formed with a series of circumferentially spaced port holes. The previously-mentioned openings in the intermediate plenum are conveniently in the form of clearances between the penetration holes in the top of the intermediate plenum and structures, such as the containment shells for the primary pumps and/or heat exchangers, passing through the intermediate plenum. The deflector means in this instance may comprise annular plates encircling such structures and disposed immediately beneath the clearances. In a second embodiment of the invention the top of the intermediate plenum is of permeable construction and may be realised in a number of ways. For example, the top may be perforated with a large number of relatively small holes or it may comprise a structure of generally vertical honeycomb channels. Alternatively, the top may comprise an array of generally horizontally disposed plates. In one such arrangement, the plates may be 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. In this arrangement, the array of plates may be 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. The plates forming the openwork structure may be 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. This effect may be accentuated by arranging the plates so that juxtaposed plates differ substantially in size. In a further development of the invention, applicable to each of the embodiments described above, the intermediate plenum may be 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. Thus, the intermediate plenum may be divided into two sections by a generally horizontal plate located beneath the top of roof of the intermediate plenum, which plate may be impermeable to the coolant except for clearances for components such as heat exchanger shells.