Patent Number: 048790893
Section: description

Referring firstly to FIG. 1, the reactor primary vessel 10, which is shown in part only, encloses an inner tank 12 which encloses the hot pool 13 of the reactor, a cylinder 14 supported from the base of the primary vessel by a strongback structure (not shown) and a reactor core 16 mounted on a diagrid 18 also supported by the strongback structure. The cold pool 19 of the reactor occupies the volume of the primary vessel outboard of and beneath the cylinder 14. The inner tank 12 also encloses the above-core structure 20 which incorporates control rods for use in reducing the reactivity of the core when called for and various other instrumentation which need not be described here. In conventional pool-type fast reactor designs, the hot and cold pools are separated by a pool separator deckplate 22 which encircles the core 16 and various reactor internals, such as the primary pumps and intermediate heat exchangers, extend downwardly from the reactor roof and pass through the separator deckplate 22. The primary pumps serve to pump coolant from the cold pool 19 through the reactor core 16 (via the sub-assemblies of which the core is composed) and the hot coolant discharges into the hot pool 13. The intermediate heat exchangers (of which one is shown in outline and is depicted by reference numeral 23 in FIG. 1) draw hot coolant from the hot pool to heat a secondary flow of sodium which is passed to steam generating plant and the cooled primary coolant is discharged back into the cold pool 19. In the embodiment of FIG. 1, the deck separator plate 22 forms the bottom of an annular intermediate plenum 24 having sides bounded by the cylinder 14 and the inner tank 12 and a roof formed by an upper deckplate or baffle plate 25. The intermediate plenum forms an annular chamber encircling the reactor core and encloses a volume of sodium which, ideally, will be totally stagnant and will stratify to form a thermal barrier between the hot and cold pools. The innermost vertical wall of the intermediate plenum is separated from the core itself by a neutron shield assembly which to some extent acts as a thermal barrier to the heat generated in the core and may even act as a heat sink with respect to the intermediate plenum. The outer vertical wall of the intermediate plenum is thermally insulated by the gas space between the inner tank and the primary vessel, the latter space also accommodating a layer 27 of thermal insulation. Even where the sodium within the intermediate plenum is totally enclosed, some convective flow (which is undesirable where stratification is required) will tend to take place as a result of heat flux transmission through its inner vertical wall depending on the extent to which the neutron shield region acts as a heat source of sink. Nevertheless, substantial stratification can be achieved even in the presence of convective flow within the intermediate plenum and the thermal gradient between the hot and cold pools can, in this way, be extended across a substantial vertical distance corresponding to the height of the intermediate plenum. Where the sodium is totally enclosed however, the response of the intermediate plenum to temperature changes in the hot pool will be poor and could, for example in the case of a reactor trip, lead to a very steep temperature gradient through the thickness of the upper deckplate 25 with a risk of unacceptable levels of thermal stress. The severity of the temperature differential between the hot pool and the sodium at the top of the intermediate plenum could be reduced by employing a laminated deckplate 25 arrangement but this will tend to significantly increase the complexity of the structure leading to greater costs and fabrication difficulties and still leaves the difficulty of a large vertical temperature gradient along the vertical components and structures. To improve the response of the intermediate plenum to thermal transients, in the embodiment of FIG. 1 the plenum 24 is provided with thermosiphon means for creating a flow of coolant from the plenum 24 into the hot pool when, for example, the hot pool undergoes rapid cooling as a result of a reactor trip. The thermosiphon means in this case comprises a generally vertical siphon leg 30 of annular form defined between the inner tank 12 and a radially inwardly spaced cylindrical wall 31 which extends below the upper deckplate 25 and is closed at its bottom end 32 but is formed with a series of circumferentially spaced inlet ports 33 immediately below the outer circumference of the deckplate 25 to enable coolant at the highest levels in the plenum to flow into and out of the thermosiphon leg. The wall 31 extends for a substantial distance above the deckplate 25 and terminates a short distance below the normal operating level 34 of the hot pool to provide an annular opening 35. It will be seen that, when the hot pool cools as a result of a thermal transient, a thermosiphon action results which leads to an upwardly directed coolant flow along the thermosiphon leg 30 so that hotter coolant from the upper region of the intermediate plenum is discharged into the upper region of the hot pool. At the same time, flow of coolant from the hot pool into the intermediate plenum is induced via openings provided for this purpose. In the present case, such openings are provided around the standpipes for the various reactor internals penetrating through the intermediate plenum. For example, as shown in FIG. 1 the stand pipes 36 for the intermediate heat exchangers 23 and associated gas sealing cylinders 37, 38 and surrounded by clearances 39 and upstanding collars 40 through which coolant can enter the intermediate plenum. In this way, the coolant interchange induced by the thermosiphon action serves to cool the upper regions within the intermediate plenum so that rapid temperature changes in the hot pool are accompanied by rapid temperature changes within the intermediate plenum. In the steady state condition of the hot pool, coolant flow may take place into the intermediate plenum via the clearances 39 and also via the thermosiphon leg 30. To prevent undue disruption of the thermal stratification within the intermediate plenum, such inward flows are deflected generally along the undersurface of the upper deckplate by annular deflectors 42 located beneath the clearances 39 and ports 33. The upper and lower deckplates 25 and 22 extend generally horizontally but advantageously they include inclined sections as shown which slope upwardly towards the penetration holes in the deckplates so that any gas accumulating at the undersurfaces of the deckplates tends to migrate towards the penetration holes thereby preventing the formation of gas layers at these undersurfaces which would otherwise adversely affect heat transmission. Referring now to FIGS. 2-4, in this embodiment the roof of the intermediate plenum is of openwork structure to provide for interchange of coolant with the hot pool during transient conditions. The roof comprises a grid structure supported within the primary vessel 10 between the inner tank 12 and the inner cylinder 14. The grid is made up of a large number of generally radial bars, e.g. 54, intersected by a large number of intersecting generally circumferential bars, e.g. 56. A generally rectangular roof plate 58 is mounted at alternate grid intersections (both in the radial and circumferential directions) by a depending stud 60 so as to form an array of generally co-planar roof plates with gaps 62 between them, the array being interrupted by various openings 64 for the standpipes of reactor internals such as the heat exchangers, coolant pumps and instrumentation bundles. An array of generally co-planar deflector plates 66 is also supported from the grid structure in superimposed relation with the roof plate array so that the deflector plates 66 overlap horizontally with the gaps 62 and serve to deflect any inward flow of coolant through the gaps 62 in a predominantly horizontal direction (see arrow C in FIG. 4) thereby minimising disruption of the thermal stratification of the coolant within the intermediate plenum under steady state conditions. Each deflector plate 66 is of generally cruciform shape and is suspended from the grid by studs or pins 68 located at alternate grid intersections (both radially and circumferentially). FIG. 5 illustrates a modification in which the roof plates 58 are retained and the separate deflector plates are replaced by deflector extensions 70 along two of the four sides of each roof plate, the arrangement being such that the gaps 62 are in superimposed relation with the extensions 70 to input a predominantly horizontal flow component to coolant entering the intermediate plenum during steady state conditions. FIGS. 6A and 6B illustrate an embodiment similar to those of FIGS. 2-4 in which an array of grid mounted roof plates 72 are used. In this case they are mounted, via studs of pins 73, above the grid. In the steady state condition when the temperatures prevailing immediately above and below the intermediate plenum roof are substantially the same, as shown in FIG. 6A the roof plates 72 are all substantially co-planar and present restricted gaps 74 for ingress of coolant from convention currents (see arrow C in FIG. 6A) circulating in the hot pool. When, however, a thermal transient occurs in the hot pool the temperature differential developed through the thickness of the roof plates 72 causes them to deform in the somewhat exaggerated manner shown in FIG. 6B since they are unsupported along their edges thus allowing increased coolant flow from the hot pool into the intermediate plenum. This effect can be accentuated if, as shown in FIGS. 6A and 6B, the juxtaposed plates differ substantially in size. In FIG. 7, the intermediate plenum is shown divided into two zones 80, 82 by a middle deckplate 84 which separates the lower zone 82 from the upper zone 80 so that heat is transmitted between the two by conduction rather than convection. The middle deckplate 84 may be impermeable apart from clearances for components such as heat exchanger shells. The upper zone is bounded by a roof 86 which is coolant permeable or incorporates a thermal siphon arrangement as shown in FIG. 1, to allow coolant interchange between the hot pool and the upper zone 80 of the intermediate plenum, particularly when thermal transient conditions prevail in the hot pool.