GB2218255A - High power fast reactor - Google Patents

Abstract

The core (30) of a gas cooled nuclear reactor (7) comprises two or more mutually parallel coolant flow regions (56, 66) which are divided from each other within the core and are preferably concentrically arranged, coolant flows in adjacent regions (56, 65) running counter to each other. The arrangement enables the materials and dimensions of each core region to be optimised for their duty and facilitates the design of a reactor having compact size. The reactor uses a supply of cryogenically stored gas coolant, such as hydrogen, and is particularly for use in a space-based power plant of high power output (fig 1, not shown), the cold gas being used to cool a secondary coolant circuit of the spacecraft prior to entering the reactor and being discarded after driving one or more turbines. Each fuel element comprises a casing with metal reflector blocks 44-47 at its ends, the blocks having coolant flow control holes. The casings are welded to each other and to a shroud 48 dividing the core regions, the core being suspended from ring 60 via the shroud. Control is by means of pivoted reflector flaps 34 and hexagonal shut-down moderator rods 26. Seal ring 90 has apertures to allow cooling of the pressure vessel wall,and the top dome is provided with ceramic foam insulation. Exit duct 68 has an inner ceramic insulation layer. in a second embodiment (fig 6, not shown) there are three concentric core regions. <IMAGE>

Description

HIGH POWER FAST REACTOR The present invention relates to fast nuclear reactors capable of sustaining high power outputs for brief periods at short notice, and in particular it relates to such reactors which are gas-cooled and of relatively small size.

Currently, a requirement can be envisaged for space -based electrical power systems which consume large quantities of electrical power for brief but repeated periods, the power being available at short notice. In such a situation,the power plant should of course be as compact and lightweight as possible.

One possible type of power plant capable of meeting such requirements uses a fast nuclear reactor with a gas coolant, the gas being a cryogenic fluid before entry to the reactor and being gaseous at a high temperature on exit from the reactor so that large volumes of hot, high velocity gas are available when required to drive turbines for electrical power generation.

The most suitable gases to use as coolant are those which have good heat transfer characteristics, but availability is also an important consideration.

Consequently, hydrogen is thought to be the most likely candidate for use, due to its satisfactory heat transfer characteristics and also due to its likely use for other cooling and propulsion purposes in space-based systems. In any case, use of a cryogenic fluid as coolant, combined with the high power demand, dictates that the reactor must be started (and may operate throughout) with very low inlet temperatures of a few degrees Relvin and with very high outlet temperatures of 0 well over 1000 K.

Two of the problems we have identified in the design of such a reactor are that, firstly, the extreme temperature range from inlet to outlet of the reactor requires the use of at least two materials with contrasting properties at the inlet and outlet, with consequent difficulties in joining and thermal compatibility of the materials, and that secondly, the large change in the specific volume of the coolant between inlet and outlet may require the coolant flow area at outlet to substantially exceed the flow area at inlet in order to ensure acceptable gas velocities at outlet, with consequent complication in the internal design of the reactor core due to the need for diffusing coolant passages.

It is an object of the present invention to contribute to the solution of the above two problems.

Accordingly, the present invention provides a gas cooled nuclear reactor comprising a pressure vessel a fuel core contained in the pressure vessel, and duct means for supplying and removing a gas coolant to and from the core and pressure vessel; the core comprising a plurality of mutually parallel coolant flow regions which are divided from each other within the core and connected to the duct means such that coolant flows in adjacent regions run counter to each other.

This cooling flow configuration allows the different regions of the core to be constructed from different materials according to the temperatures experienced in those regions. Furthermore, since the regions, which are preferably concentric, are in a side-by-side parallel relationship to each other, the constructional difficulties due to differing thermal characteristics are eased and moreover, the designer is at liberty to choose different flow areas for different regions so as to provide suitable step increases in flow area between regions in flow series with each other.

Other aspects of the invention and the advantages arising from them will be apparent from a perusal of the accompanying description and claims.

A further problem in the design of such a reactor is the way in which the core responds to the large amount of heat generated when the power plant is called upon to supply power; the resulting large radial temperature gradients can cause fuel modules in cores which are constructed, supported and restrained in the usual ways to bow towards each other due to thermal growth effects.

This causes a positive feedback to core reactivity and can make the reactor unstable.

It is a further object of the present invention to contribute to the solution of the above problem.

Accordingly, the present invention further provides a gas cooled nuclear reactor comprising a pressure vessel and a fuel core within the pressure vessel, the fuel core having a plurality of mutually parallel elongate fuel modules each module comprising a plurality of fuel elements and a fuel wrapper containing the fuel elements, said fuel modules being secured to each other at one end of the core and being otherwise free to thermally expand relative to each other and the pressure vessel, the core being supported and restrained within the pressure vessel by suspension means mounted from the pressure vessel and attached to the core at the same end thereof as that at which the fuel modules are secured to each other.

The above construction permits the radial temperature gradients to cause the fuel modules to bow away from each other by allowing expansion of the core at its "free" end, thereby providing negative feedback to core reactivity and enabling the reactor to remain in a stable power generating condition.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig 1 is a block diagram showing part of a power plant, of which a fast reactor in accordance with the present invention is a part; Fig 2 is a schematic sectional side elevation showing details of the construction of the fast reactor, the section being that defined by section line II-II in Fig 3; Fig 3 is a plan view of the reactor as seen on section line III-III in Fig 2; Fig 4 is an enlarged view of the lower end of the reactor as seen in Fig 2; Fig 5 is an enlarged view of the right hand upper end of the reactor as seen in Fig 2; and Fig 6 is a schematic sectional side elevation of an alternative cooling configuration for a reactor in accordance with the present invention.

Referring first to Fig 1, a power plant capable of a high electrical power output comprises, in terms of its major hardware items, cryogenic hydrogen storage tanks 1, a cryogenic hydrogen pump 3, a heat exchanger 5, a fast reactor 7, a pressure reducer 9, a turbine set 11, a number of electrical power generators 13, of which only two are shown, and power conditioning apparatus 15. Also included, but not shown, are automated command, control and safety systems for these items, which are beyond the proper scope of this description.

Items 1 to 11 are in flow series with each other with respect to the flow of hydrogen 17 through the system, with the hydrogen pump 3 delivering supercritical hydrogen to the inlet of the fast reactor 7 via the heat exchanger 5, the latter's purpose being to utilise the supercritical hydrogen as a heat sink for a further coolant in a separate coolant circuit 19 for circulation to other apparatus (not shown).

Depending upon the heat input from the heat exchanger 5, and upon the pressures in the system, the hydrogen coolant 17 may still be close to its critical point temperature when it enters the fast reactor 7, but it is required that the hydrogen be heated up to well over 10000K during its passage through the reactor in order that adequate power be obtained from the turbine/generator combination 11, 13. Upon exit from the reactor 7, the hot hydrogen gas passes through the pressure reducer 9 immediately before entry to the turbine set 11, the pressure reducer comprising a set of nozzles designed to convert some of the pressure of the hot gas into kinetic energy and thereby increase the efficiency of the turbine set, whose blading is conveniently designed to have a low degree of reaction.

Spent hydrogen gas 17 is exhausted from the turbine 11 into the environment, or used for other purposes.

The turbine set 11 drives the generators 13. These feed their output into power conditioning equipment 15 to produce power having the right current and voltage characteristics for the use envisaged.

It may be noted that the turbine set 11 preferably comprises several turbines each linked to a generator 13, the hot gas outflow from the reactor 7 being divided into a corresponding number of separate portions by a flow divider and each portion being passed through its own pressure reducer before being passed to a corresponding turbine. One of the turbine/generator combinations may then be sized smaller than the others, the smaller one being utilised on its own when the system is in an idling mode between peak power requirements.

Having briefly described the context in which the fast reactor 7 is situated, the reactor itself will now be described with reference first of all to Figs 2 and 3.

In broad terms, the reactor 7 comprises a cylindrical pressure vessel 20 with domed ends 22, 24, the upper domed end 22 being modified to house seven hexagonal shut-down moderator rods 26, of which only two are shown in Fig 2 due to the route taken by the section line II - II in Fig 3. The lower domed end 24 is modified to blend its contour with a large diameter coaxial duct 28 through which the hydrogen coolant is supplied to, and taken from, the core 30 of the reactor.

The fuel zone of the core 30 is shown by thick hatching lines.

Surrounding the cylindrical side wall of the pressure vessel 20 is an external neutron absorber and reflector 32. This reflector 32 is constructed from berylium and comprises a fixed cylindrical reflector 33 and six pairs of moveable reflector elements 34 which lengthwise are coextensive with the interior longitudinal dimension of the core 30. Hinged at top and bottom pivot points 36 and 38 respectively to the fixed reflector 33, the pairs of moveable reflectors 34 are arranged circumferentially of the fixed reflector in "double door" configurations as shown in Fig 3. In the closed position the moveable reflectors 34 occupy an annular recess in the side of the fixed reflector which is of the same length and thickness as the moveable reflectors, but in the open position they swing outwards on pivots 36, 38 so that most of their bulk stands clear of the fixed reflector.Their purpose is to vary the reflectivity and absorption of reflector 32 so as to control the neutron flux within core 30 to allow for the gross power variations required, and to this end they are powered via drive shafts 40 from a control system (not shown, but already known in principle) which measures the neutron flux in the core 30 and opens or closes the moveable reflectors 32 to acheive the desired reactor criticality.

It will be seen from Fig 3 that the reactor core 30 comprises a honeycomb-like array of hexagonal fuel modules 42, each comprising a hexagonal sheet metal fuel wrapper containing an assembly of fuel pins. The wrappers are attached to each other as described later.

For purposes of illustration in Fig 3, one fuel module 42 is shown in section to reveal its complement of fuel pins. The number of fuel pins within each wrapper is a matter of design choice, but 127 has been chosen for the present design. Seven of the wrappers, shown densely shaded in Fig 3, are empty for receiving the shut-down rods into the core 30.

An important design consideration for the fuel modules 42 is that the spacing between adjacent fuel pins should be sufficient to allow adequate flow of the hydrogen coolant through the fuel modules 42, the pins of course extending longitudinally through the modules and being held apart from each other and the wrapper by spacers or other knawn suitable means in such a way that they are securely held against vibration.

At the top and bottom of each fuel module 42 and held within the fuel wrappers by virtue of being attached thereto, e.g. by welding or by mechanical means, are respective pairs of inner and outer hexagonal berylium reflector plates 44, 45 and 46, 47. The members of each pair of reflector plates are vertically spaced apart by a small gap as shown. The innermost ones 44, 46 of each pair of reflector plates positively retain the fuel pins within their fuel modules 42. In order to allow for ingress and egress of the hydrogen coolant to the core's fuel zone, the reflector plates 44 to 47 are perforated, each being provided with a grid of inlet and outlet passageways (not shown) through their thickness, the passageways in reflector plates 44 and 46 being of smaller diameter than the fuel pins to ensure that the latter cannot pass through them.The grids of passageways in the outer reflector plates 45, 47 are staggered with respect to those of the inner reflector plates 44, 46 in order to prevent unwanted "shine-through" of neutrons from the core 30.

It should be noted here that, if desired, the passageways in outer reflector plates 45,47 could be bigger than those in inner reflector plates 44,46. In any case, the proportion of the area of the reflector plates occupied by the passageways should vary over the diametral extent of the core in order to provide the proper flow distribution between fuel modules 42 and the proper flow resistance for each region of the core. In this connection it will be seen that a shroud 48 divides the core 30 into two regions, these being an outer hexagonal ring 56 of twenty four fuel modules with their reflector plates and an inne-r hexagonal group 66 of thirty fuel modules with their reflector plates.

Returning now to a consideration of the special cooling flow configuration adopted by the present invention, it should be observed in connection with Figure 2 that hydrogen 17 enters the reactor assembly through the feed duct 50 which opens into a hemi-toroidal feed-ring 52 in the wall of the outer annular duct 54 of the coaxial duct 28, the hydrogen then passing upwards as shown by the arrows through the outer ring 56 of fuel modules 42 with their associated reflector plates, this path comprising a "first pass" through the reactor core 30. At the top of the core 30 the flow out of the outer ring 56 of fuel modules exits from the immediate vicinity of the core 30 through a circular array of flow holes 58 provided in an annular plate 60 which supports the shroud 48 previously mentioned.

Hydrogen coolant exiting from the holes 58 is turned in reactor head space 62 by a flow turning shroud 64, as shown by the arrows, the hydrogen then passing downwards through the inner group 66 of fuel modules, this path comprising a "second pass" through the reactor core 30.

It should be noted that the shroud 48 (which as can be seen from Figure 3 is interposed between, and conforms to, the matching contours of the outer ring 56 and inner group 66 of fuel modules) divides the two core regions from each other within the core itself. In order to cope with the temperature gradient across the shroud 48 caused by the temperature difference between the relatively cooler gas in its first pass through the core 30 and the hot gas in its second pass, the shroud should be made of a nickel-base superalloy. At the end of its second pass through the core 30, the hydrogen coolant exits the core through the grids of outlet passageways in the reflector plates 46,47 and passes as hot gas at temperatures above 1000 0K into the central hot duct 68 of the coaxial duct 28, from whence it passes on to the rest of the power plant as described in relation to Figure 1.

The hot duct 68, but not the shroud 48, should be provided on its inside with a ceramic coating for insulation purposes.

Refering now to Figure 4, additional details of the lower end of the reactor, comprising both the inlet and outlet for the coolant, will be described.

It should first be noted that after it has passed through the reflector plates 46,47, the shroud 48 is extended by being joined to a transition fabrication 70 which is then joined at 72 to a circular bell-mouth 74 leading into the hot outlet duct 68, the constructional materials being the same for items 70,74 and 68 as for the shroud 48. The inlet duct 54 is structurally to be considered as an extension or continuation of the domed end 24 of the pressure vessel 20 and is sealed to the hot outlet duct 68 by means of a pressure-tight seal 76 comprising an annular thickened land 78 on the periphery of the outlet duct 68, a mating thickened internal flange 80 which terminates the lower end of the pressure vessel/duct 54, and a metallic pressurised toroidal seal ring 82 between land 78 and flange 80, retained in a groove in the flange.A further resilient clamping ring assembly 84, comprising a rigid ring 86 and a circumferential array of resilient fingers 88 embracing a heel on flange 80, may be necessary in order to maintain seal 76 suitably pressure tight under all conditions of temperature expansion and contraction of the ducts 54,68.

Seal 76 can be kept cool if necessary by a controlled leakage of cold hydrogen through it.

A further feature of interest is the seal support plate fabrication 90 which is generally 'U'-shaped in section and bridges the variable gap (seen best in Figure 3) between the periphery of the outer ring 56 of fuel modules and the inner surface of the pressure vessel 20, one leg of the U-shape being welded to the sheet metal wrappers of the fuel modules and the other bearing resiliently against the pressure vessel. That part of the U-shape which links the legs has holes therethrough to provide a controlled leakage flow of cold gas up the outside of the outer ring 56 of fuel modules to keep the pressure vessel wall cool. This leakage flow of cold gas rejoins the main flow at the top of the outer ring 56 of fuel modules.

Looking now at Figure 5, the parts of the reactor at the top of the core 30 near the periphery of the upper dome 22 will be described in greater detail.

Firstly, the mode of assembly of the core 30 should be understood. It involves the individual hexagonal wrappers of the "second pass" central group 66 of fuel modules being welded together by means of line welds positioned a little way down from their upper ends to form a thirty-seven cell honeycomb structure (seven of the cells are not fuel modules because they are available for insertion of the shut-down rods). The wrappers of the outermost ones of this central group 66 of fuel modules are welded to a flat inward corrugation 92 of the shroud 48, which is shaped to match the external perimeter of the central group 66. Likewise, the fuel module wrappers of the outer ring 56 of fuel modules are welded to each other and to a flat outward corrugation 94 of the shroud 48.

Although welding is mentioned in the above paragraph, other forms of attachment may be preferred.

Above the corrugations 92,94, an extension of the shroud 48 is joined by welding to a core suspension member in the form of the thick annular plate 60, the shroud and plate together supporting and restraining the whole of the core 30 by virtue of the plate 60 being encastred at its perimeter in an annular recess 96 in the side of the pressure vessel. The top side of the recess 96 is defined by the underside of a thickened rim portion 98 of pressure vessel end dome 22, which is bolted to an upper thickened flange portion 100 of the cylindrical wall of the pressure vessel 20, thereby also clamping the plate 60 in the recess, the other sides of which are defined by an increased diameter stepped portion of the internal bore of flange 100.The joint between dome rim 98 and flange 100 of the cylindrical wall is sealed by means of a gas-filled toroidal metal seal ring 102, which is held in a compressed condition in a rebate 104 formed in the joint surface of flange 100.

Joining and mounting of the fuel modules at one end only of the core 30pin the above way permits the core to expand at its "free" end due to the fuel elements bowing away from each other as power and therefore radial temperature gradients increase. This causes negative feedback to core reactivity rather then positive feedback and hence makes the reactor stable when delivering high powers and during power transients.

Whereas the incorporation of shroud 48 in the core 30 and its attachment to the support plate 60 and to the fuel modules in the way described above is particularly advantageous in connection with supporting and restraining the core within the pressure vessel, the shroud 48 is not essential to implement the division of the core into separate counterflow regions, because the fuel wrappers themselves are impermeable and may well provide an adequate division between regions, subject to any consequent modification in the materials of which the first pass fuel modules are composed; such modification may be necessitated by the increased heat flow from the second pass region to the first pass region due to the absence of the shroud 48.

Regarding the upper end of the fixed external reflectbr 33, it can be seen that this tapers to a relatively thin-walled portion 106 surrounding the thickened rim and flange portions 98 and 100 of the pressure vessel wall, and is retained axially by means of a thick annular retainer plate 108 which is bolted to the rim 98 and the reflector 33. The drive shafts 40 for the movable reflector elements 34 pass through the retainer plate 108.

Returning to the interior of the reactor, the upper boundary of the head space 62, which the hydrogen coolant traverses between its first and second passes through the core 30, is defined by the flow turning shroud 64, which as previously mentioned turns the flow of gaseous hydrogen towards the central part of the head space 62 after it exits the flow holes 58 in support plate 60. At exit from its first pass through the core 30, the hydrogen gas may be at temperatures of up to 8000K, and the flow turning shroud 64 prevents this hot gas from coming into contact with the pressure vessel's domed end wall 22, with a view to preventing distortion and low cycle fatigue due to repeated operation of the reactor.

Isolation of the dome 22 from the effects of the hot gas in head space 62 is increased by the fact that the shroud 64 is in fact the facing of an expanded ceramic foam cellular insulation material 110 which fills the space between the shroud 64 and the dome 22. However, provided that the temperature of the hydrogen gas at exit from the flow holes 58 is substantially lower than 8000R, it may be feasible to delete the shroud 64 and the insulation 110 and allow the inner surface of the dome 22 to perform the flow turning function.

Having looked at the detailed construction of the reactor, certain observations can be made. Because the reactor's coolant flow configuration is such that (a) the cold inlet duct 54 surrounds the hot outlet duct 68 and (b) the cooler first-pass ring 56 surrounds the hot second-pass inner group 66 of fuel modules, the reactor's pressure vessel wall is therefore effectively isolated from the high-temperature gas exiting from the reactor.

Consequently, the temperatures of the pressure vessel wall can be maintained at a level sufficiently low to allow its construction from a stainless steel or nickel-based alloy, rather than a superalloy.

Furthermore, the fact that the temperature of the hydrogen is raised in two stages, during two concentric passes through the core of the reactor, means not only that the design can be made compact but also that the ring 56 of fuel modules comprising the first pass can be constructed with stainless steel (type 316L) fuel pin cladding and wrappers, since these can operate satisfactorily at temperatures up to : 000K, while the inner group 66 of fuel modules comprising the second pass can be constructed with molybdenum or molybdenum alloy pin cladding and wrappers, which can operate at temperatures well over 1000 0K. Hence, different materials are used in each pass, matched to the temperature range which will be experienced, yet the parallel concentric counter-flow configuration of the core avoids in large measure the compatibility problems which would otherwise be experienced in joining these different materials together.

It would of course be possible to devise a concentric counter-flow configuration for the core of the reactor which reverses the arrangement in the above preferred embodiment, in that cool gas could be passed up a central coaxial inlet duct and a central region of the core and hot gas could be passed down the outer region of the core and an outer coaxial exit duct. While this would preserve the compact nature of the reactor, it would lose the advantage of enhanced isolation of the pressure vessel wall from the hottest part of the core and the hot outlet gas: provision would have to be made for a considerable cold gas cooling flow along the cylindrical sides of the pressure vessel in order to keep the pressure vessel wall cool.

In order to emphasise that the invention embraces not only a reactor having a double pass core coolant flow configuration, but also cores having several passes, Fig 6 shows in diagramtic form a flow configuration for a reactor 600 having coaxial inlet and outlet ducts 602, 604 respectively. These have the same general advantageous arrangement as previously described in relation to Figs 2 and 4. Cold gas passes up the outer inlet duct 602, the outer wall 603 of which joins the domed lower end 606 of the pressure vessel 608. However, unlike the embodiment described in relation to Figs 2 to 5, the cold inlet gas does not immediately flow through the first pass region of the core, but instead bypasses the core and flows through an annular passage 610 between the outer periphery of the core and the cylindrical wall of the pressure vessel 608.In this way, effective isolation of the pressure vessel from the hottest parts of the core is obtained, together with excellent cooling of the wall without the need for a separate flow of coolant to achieve it.

After passing up the annular passage 610, the cool gas emerges into the head space 612 of the reactor and is turned as shown by the arrows to pass downward through a central "first pass" region 614 of the core. At the bottom of the first pass 614 the by now partially heated gas emerges into an inverted head space 616 bounded by a part-spheroidal shroud 618 which turns the gas to pass upwards through an annular "second pass" core region 620 surrounding and coaxial with the first pass region 614.

Again, the further heated gas emerges from the second pass region 620 into a small head space 622 having the form of a hemi-toroid separated from the large head space 612 by a hemi-toroidal shroud 624 which turns the gas to pass downwards through a final annular "third pass" region 626 of the core. After emerging from the bottom of the third pass region 626, the downward annular flow of hot gas converges into the circular outlet duct 604 and passes on to other components of the power plant in which the reactor 600 is installed. Apart from differences imposed by the different coolant flow configuration, the structure and principles of construction of the reactor 600 are similar to those described in relation to the embodiment of Figs 2 to 5, and will consequently not be further described.

Whereas the above description has focussed upon reactors having cores in which counter flow regions are arranged concentrically, it is also possible to conceive of such regions which are not arranged concentrically but which, e.g., are arranged as adjacent larger and smaller sectors or segments of the core. However, the concentric inlet and outlet duct arrangements would preferably be retained.

A suitable sequence of operation for starting up the reactor as part of the power plant of Fig 1 is as follows: (i) Remove six of the shut-down rods 26 completely from the core's fuel zone to the position shown in Fig 2, but leave the seventh, central rod partly within the fuel zone.

(ii) Shut the moveable reflector elements 34 to acheive criticality, and increase the hydrogen coolant flow rate in step with reactor power as indicated by the sensed neutron flux to obtain the required electrical power level from the generators 13.

(iii)Obtain coarse control of power level by controlling the positions of the moveable reflector elements 34.

(iv) Obtain fine trimming of power level by small movements of the central shut-down rod up and down within the core's fuel zone.

The reactors described above may be modified in many particulars without departing from the scope of the present invention. For example, other gases having suitable heat transfer and cryogenic characteristics could be used as a coolant instead of hydrogen. Again, the fuel modules 42 described in relation to Figs 2 to 5 could be square in section instead of hexagonal, and although it is thought that the coaxial inlet and outlet duct arrangement is preferable for reasons of design convenience, structural integrity and compactness, it would nevertheless be possible to separate the inlet and outlet ducts, e.g. by situating them at different ends of the pressure vessel, whilst still maintaining the invention's advantages in terms of optimisation of core material properties for first and subsequent passes of coolant and ease of joining the different core materials together.

Claims (14)

Claims:

1. A gas cooled nuclear reactor comprising a pressure vessel a fuel core contained in the pressure vessel, and duct means for supplying and removing a gas coolant to and from the core and pressure vessel; the core comprising a plurality of mutually parallel coolant flow regions which are divided from each other within the core and connected to the duct means such that coolant flows in adjacent regions run counter to each other.

2. A gas cooled nuclear reactor according to claim 1 in which the duct means comprises coolant inlet and outlet ducts in flow series with each other via the coolant flow regions in the core.

3. A gas cooled nuclear reactor according to claim 1 in which the duct means comprises two ducts, one surrounding the other, the outer duct wall being joined to the pressure vessel wall as a continuation thereof and the inner duct being connected to one of the regions in the core.

4. A gas cooled nuclear reactor according to any one of claims 1 to 3 in which the regions in the core are arranged concentrically.

5. A gas cooled reactor according to claim 3 in which the regions in the core are arranged concentrically, theouter duct conveying cold gas to an outer one of the regions in the core and the inner duct conveying heated gas from an inner one of the regions.

6. A gas cooled nuclear reactor according to claim 3 in which the regions in the core are arranged concentrically, the outer duct conveying cold gas to an inner one of the regions in the core and the inner duct conveying heated gas from an outer one of the regions.

7. A gas cooled nuclear reactor according to claim 5 or claim 6 arranged such that at least a portion of the cold gas from the outer duct flows past an internal surface of the pressure vessel to cool same.

8. A gas cooled nuclear reactor according to any one of claims 1 to 7 in which different regions of the core comprise different materials whose properties are optimum for the temperature conditions therein.

9. A gas cooled nuclear reactor according to any one of claims 1 to 8 in which the members of each adjacent pair of counter-flowing regions in the core are linked by a corresponding head space within the pressure vessel.

10. A gas cooled nuclear reactor according to claim 9 in which the head space is arranged to assist in turning the direction of coolant flow between one region and the other.

11. A gas cooled nuclear reactor according to claim 10 in which the head space is bounded by barrier means interposed between the core and the pressure vessel to assist in turning the direction of coolant flow and to protect the pressure vessel from contact with the coolant.

12. A gas cooled nuclear reactor according to any one of claims 1 to 11, the gas being hydrogen supplied to the reactor in a cryogenic state.

13. A gas cooled nuclear reactor substantially as herein described with reference to and as illustrated by Figs 2 to 6 the accompanying drawings.

14. A power plant incorporating a gas cooled nuclear reactor according to any one of claims 1 to 13.