Patent Publication Number: US-2003227994-A1

Title: Nuclear plant

Description:
[0001] THIS INVENTION relates to a nuclear plant and to a method of operation of a nuclear plant. It also relates to a method of loading a core of a nuclear reactor.  
       [0002] In a nuclear reactor of the high temperature gas cooled type, a fuel comprising a plurality of spherical fuel elements is used. The fuel elements or spheres may comprise spheres of a fissionable material in a ceramic matrix, or encapsulated in a ceramic material. The reactor may be helium cooled. The fuel elements are known as pebbles and a reactor of this type is generally known as a pebble bed reactor (PBR). In a PBR it is known to operate a multi-pass fuelling scheme in which fuel spheres are passed through a core of the reactor more than once in order to optimise burn-up of fuel. In comparison with other fuelling schemes, a multi-pass fuelling scheme is believed to provide for a more uniform distribution of burn-up within the core and thereby flattens the axial neutron flux profile and maximises thermal power output of the reactor core. In this specification, a reactor as described above will be referred to interchangeably as a pebble bed reactor (PBR) or a nuclear reactor of the pebble bed type.  
       [0003] According to one aspect of the invention there is provided a nuclear plant which includes a nuclear reactor of the pebble bed type, the reactor including a reactor core having  
       [0004] a plurality of spherical moderator elements located in a central region of the core at least part of the central region being generally cylindrical; and  
       [0005] a plurality of spherical fuel elements located in an annular region surrounding the central region.  
       [0006] The nuclear reactor core may include a plurality of spherical absorber elements.  
       [0007] In a preferred embodiment of the invention, the moderator elements are graphite spheres.  
       [0008] According to another aspect of the invention there is provided a nuclear plant which includes a nuclear reactor which includes  
       [0009] a core containing means having at least one outlet through which moderator elements and fuel elements can be discharged from the core;  
       [0010] at least one first inlet, the or each first inlet being configured to permit moderator elements to be loaded into a first region of the core via the or each first inlet;  
       [0011] at least one second inlet, the or each second inlet being configured to permit fuel elements to be loaded into a second region of the core via the or each second inlet; and  
       [0012] a handling system intermediate the or each outlet and the or each first and second inlet for cycling the moderator elements and the fuel elements through their respective regions of the core at a predetermined rate.  
       [0013] The nuclear reactor may be a pebble bed reactor, the core containing means may be a core barrel, and the first region may be a central region with the second region being an annular region surrounding the first region.  
       [0014] The core barrel may be generally cylindrical in shape, an operatively lower end portion of the barrel tapering inwardly to provide a funnel-shaped operatively lower end, a single outlet being defined at the operatively lower end of the barrel, a single first inlet being located at an operatively upper end of the barrel, proximate the central region of the core and a plurality of second inlets being located in an angularly spaced relation about a longitudinal axis of the barrel proximate the annular region of the core and symmetrically spaced with respect to the annular region.  
       [0015] The handling system may define a flow path intermediate the outlet and each of the inlets. The flow path may include a conduit arrangement including conduit lines. Motive force for the moderator and fuel elements about the handling system may be provided, at least partly, by a gas under pressure, the moderator and fuel elements being entrained, in use, in a gas flow stream flowing through the flow path. Motive force for the moderator and fuel elements may also be provided, at least in part, by gravitational force.  
       [0016] In a preferred embodiment of the invention, the flow path of the handling system is in fluid communication with the reactor core and the gas flow stream is provided by means of reactor coolant gas. The reactor coolant gas in at least a portion of the flow stream may be at a similar pressure to the coolant gas of a reactor pressure vessel within which the core is contained.  
       [0017] The handling system may have a fuel element flow path and a moderator element flow path, the handling system further including a first sort means for separating moderator elements from fuel elements in the flow path and for entraining moderator elements in a gas flow stream of the moderator element flow path and fuel elements in a gas flow stream of the fuel element flow path.  
       [0018] The first sort means may include a first sensor means operatively coupled to a first diverter valve. The first sensor means may be a radiation sensor and be operable to detect and measure nuclear radiation emitted by moderator elements and fuel elements in the flow stream and to generate a signal containing data representative of the radiation detected and measured, the first diverter valve being operable to divert the flow stream into a first flow path, being the moderator element flow path, a second flow path, being the fuel element flow path, and a third flow path, being a discharge flow path for discharging burnt up or damaged fuel elements.  
       [0019] The moderator element flow path may include a second sort means. The second sort means may include a second sensor means operatively coupled to a second diverter valve assembly, the second sensor means being a radiation sensor which is operable to detect and measure nuclear radiation emitted by moderator elements and fuel elements in the flow stream of the moderator flow path and to generate a signal containing data representative of the radiation detected and measured, the second diverter valve assembly being selectively operable to divert moderator elements into a moderator inlet line for re-loading into the reactor core and, on detection of a fuel element in the moderator element flow path, to divert such a fuel element back into the annular region of the reactor core.  
       [0020] The moderator element flow path may further include a buffer storage means for storing elements in the moderator element flow path to provide a time delay to assist in separating misdirected fuel elements from moderator elements in the moderator element flow path.  
       [0021] The handling system may include a storage system. The storage system may include a new fuel storage system for storing new fuel elements and for feeding new fuel elements at predetermined intervals into the reactor core via the second inlets, a moderator element storage system for storing graphite moderator elements, the moderator element storage system including a moderator element storage tank having an inlet and an outlet, the inlet being operatively coupled to the second diverter valve assembly of the moderator element flow path and the outlet being coupled to the same second diverter valve assembly of the moderator element flow path. Thus, by operation of the second diverter valve assembly, graphite spheres discharged from the reactor core may be diverted to the moderator element or graphite sphere storage tank for storing, rather than being recycled back into the reactor core, thereby enabling the complete discharge of graphite spheres from the reactor core for core maintenance purposes. As required, the reactor core may be re-charged with graphite spheres from the moderator element or graphite sphere storage tank via the second diverter valve assembly and the first inlet.  
       [0022] The storage system may further include a spent fuel storage system. The spent fuel storage system may include a plurality of spent fuel storage tanks for permanent storage on site of spent and damaged fuel elements, inlets to the spent fuel storage tanks being operatively coupled to the first diverter valve of the first sort means, a third radiation sensor being located intermediate the first diverter valve and the spent fuel storage tanks to detect any misdirected moderator elements or graphite spheres.  
       [0023] The fuel storage system may further include a temporary fuel storage system. The temporary fuel storage system may include a temporary fuel storage tank for storing in-use fuel elements, the temporary fuel storage tank including an inlet operatively coupled to the first diverter valve of the first sort means and an outlet operatively coupled to the second inlets of the reactor core. Thus, as with the graphite spheres, during the maintenance of the reactor core the fuel spheres may be discharged from the reactor core and, rather than being circulated back to the core, may be temporarily stored in the temporary fuel storage tank whilst maintenance takes place. On completion of maintenance, the fuel spheres may be recharged into the reactor core via the second inlets.  
       [0024] The fuel handling and storage system may include control means operatively coupled to each of the radiation sensors and diverter valves and valve assemblies.  
       [0025] The control means may be a computer which is operable to control operation of the diverter valves to divert moderator elements and fuel elements into their respective circuits on operation of the respective radiation sensor.  
       [0026] The control means may be operable to control feeding of new fuel elements into the reactor core on discharge of spent and damaged fuel elements into the spent fuel storage system, thereby maintaining a preselected number of fuel elements in circulation, including the core and the handling system, the control means being programmed to prevent charging of a new fuel element into the reactor core where a misdirected moderator sphere is detected by the third radiation sensor of the spent fuel storage system, thereby obviating inadvertent alteration of the fuel/moderator ratio in the core.  
       [0027] According to another aspect of the invention there is provided a method of operating a nuclear plant having a nuclear reactor of the pebble bed type, the method including  
       [0028] cycling spherical moderator elements at a predetermined rate through a central generally cylindrical region defined in a core of the reactor; and  
       [0029] cycling spherical fuel elements at a predetermined rate through an annular region defined in the core surrounding the central region.  
       [0030] The method may include temporarily storing the moderator elements outside the core to facilitate maintenance of the reactor.  
       [0031] The method may further include temporarily storing the fuel elements outside the core to facilitate maintenance of the reactor.  
       [0032] According to yet another aspect of the invention there is provided a method of loading a core of a nuclear reactor of the pebble bed type which includes the steps of  
       [0033] filling the core with first moderator elements to form a bed of moderator elements; and  
       [0034] loading simultaneously, second moderator elements into a central region of the core and fuel elements into an annular region of the core at predetermined rates while removing the first moderator elements from the central and annular regions at a predetermined rate so as to form a core having a plurality of spherical moderator elements located in a central region and a plurality of spherical fuel elements located in an annular region around the central region.  
       [0035] The method may include loading the second moderator elements and fuel elements from above while removing the first moderator elements from below.  
       [0036] The invention is now described, by way of example, with reference to the accompanying diagrammatic drawings. 
     
    
    
     [0037] In the drawings,  
     [0038]FIG. 1 shows a sectional side view of a nuclear reactor pressure vessel of a nuclear reactor forming part of a nuclear plant in accordance with the invention;  
     [0039]FIG. 2 shows a process flow diagram of a handling system forming part of the nuclear plant;  
     [0040]FIG. 3 shows a schematic view of a system layout of the handling system;  
     [0041]FIG. 4 shows a schematic view of a part of the system operative in de-fuelling mode;  
     [0042]FIG. 5 shows a schematic view of a part of the system operative in re-fuelling mode;  
     [0043]FIG. 6 shows a schematic view of a part of the system operative in normal operating mode;  
     [0044]FIG. 7 shows a schematic view of fuel sphere flow during normal operating mode;  
     [0045]FIG. 8 shows a schematic view of graphite sphere flow during normal operating mode;  
     [0046]FIG. 9 shows a schematic view of spent fuel flow during normal operating model; and  
     [0047] FIGS.  10  to  12  show steps involved in the loading of a core of a nuclear reactor in accordance with the invention. 
    
    
     [0048] In the drawings, reference numeral  10  generally indicates a nuclear reactor of the pebble bed type, in accordance with the invention.  
     [0049] The reactor  10  is a high temperature gas cooled reactor, the coolant gas being helium and the reactor has a generally cylindrical pressure vessel  12 . Further, the reactor has a core barrel  14  within the pressure vessel  12  and coaxial therewith. The core barrel  14  is generally cylindrical for most of its length and has a funnel-shaped lower end portion  16  which tapers inwardly downwardly towards an operatively lower end  18 . A single outlet  20  is defined at the lower end  18  of the core barrel  14 , projecting outwardly therefrom and coaxially therewith.  
     [0050] A reactor core  22  is contained within a core region  23  defined by the core barrel  14 . The reactor core  22  comprises a plurality of spherical graphite moderator elements (not shown in detail) located in a central generally cylindrical region  26  defined in the core  22  and a plurality of spherical fuel elements (not shown in detail) located in an annular region  30  defined in the core  22  and surrounding the central region  26 .  
     [0051] The core barrel  14  has a single first inlet  32  which is configured to load spherical graphite moderator elements or graphite spheres into the central region  26  of the core  22  via the first inlet  32 . Further, the core barrel  14  has nine second inlets  34  (three of which are shown in FIG. 1, and only seven of which are indicated schematically in FIG. 3) which are configured to permit spherical fuel elements or fuel spheres to be loaded into the annular region  30  of the core  22  via the said second inlets  34 . The first and second inlets ( 32 ,  34 ) are located in an operatively upper end region  35  of the reactor pressure vessel  12 . The second inlets  34  are arranged in an angularly spaced relation about and radially spaced from a longitudinal axis of the core barrel  14  and symmetrically spaced with respect to the annular region  30 . It will be appreciated that there may be more than one graphite sphere inlet  32  and more, or fewer, than nine fuel sphere inlets  34 .  
     [0052] The nuclear reactor  10  forms part of a nuclear plant part of which is generally indicated by reference numeral  8 . The plant  8  has a handling system  40  intermediate the outlet  20  and each of the first and second inlets ( 32 ,  34 ), for cycling the graphite spheres and fuel spheres through their respective regions  26  and  30 , respectively, of the core  22  at a predetermined rate. The handling system  40  defines a flow path  42  intermediate the outlet  20  and each of the inlets ( 32 ,  34 ). The flow path  42  is defined at least in part by an arrangement of conduit lines  44 . Motive force for the moderator and fuel spheres about the handling system  40  is provided, in part, by reactor helium coolant gas from the reactor pressure vessel  12  and the moderator and fuel spheres are entrained in a gas flow stream flowing in the flow path  42 .  
     [0053] The handling system  40  has a high pressure region  45  and a low pressure region  46 , the low pressure region  46  being indicated by the dashed region labelled  46  in the drawings. The high pressure region  45  comprises those components of the handling system  40  outside the low pressure region  46 . In the high pressure region  45  of the handling system  40 , the flow path  42  of the handling system  40  is in fluid communication with the reactor core  22  and the gas flow stream is provided by means of reactor coolant gas, being helium, at the pressure of the coolant gas within the reactor pressure vessel  12 . The gas flow stream of the low pressure region  46  of the handling system  40  is provided by clean, dry air at relatively low pressure and pressure locks (not shown) are provided in the handling system conduits  44  at boundaries between the high pressure region  45  and the low pressure region  46  to bridge the said boundaries.  
     [0054] The handling system  40  has a fuel sphere flow path  50  which is operative during normal operation of the reactor  10 , illustrated schematically in FIG. 7, and a moderator sphere flow path  60  which is also operative during normal operation of the reactor  10 , indicated schematically in FIG. 8.  
     [0055] Under normal operating conditions, as shown in FIGS. 6, 7 and  8 , fuel spheres and graphite moderator spheres move continually under gravity through the core  22  of the reactor  10  from an operatively upper region  36  of the core barrel  14  to the lower end portion  16  of the core barrel  14 . At the lower end  18  of the core barrel  14  they exit the core barrel  14  and hence the reactor pressure vessel  12  via the outlet  20 .  
     [0056] A pair of sphere handling machines  48  is connected to the outlet  20  and the machines  48  are operable to feed discharged fuel and moderator spheres one at a time into a pair of discharge flow lines  52 . Each of the said sphere handling machines  48  includes a scrap separator (not shown) and scrap cask (not shown) and the machines  48  are operable to detect physically damaged spheres and to remove such spheres from the discharge flow lines  52 . On each of the flow lines  52  a first radiation and burn-up sensor  54  is arranged. The sensors  54  are operable to sense and measure nuclear radiation emitted by entrained moderator or fuel spheres in the respective flow lines  52  and to transmit a signal containing information representative of the measurements made. The sensors  54  are also operable to count entrained fuel and moderator spheres. Each of the sensors  54  is operatively coupled to a first diverter valve  56  via a computer controller (not shown). The controller is programmed to control the diverter valves  56  to divert incoming spheres to one of three ports, depending on the status and condition of each respective sphere, information representative of which is transmitted by the radiation and burn-up sensor  54  to the controller. Graphite spheres are diverted into the moderator sphere flow path  60 ; fuel spheres are diverted into the fuel sphere flow path  50 ; and spent fuel spheres are diverted into a third spent fuel storage flow line  70 , as shown in FIG. 9. Each of the diverter valves  56  also has a fourth port leading to a temporary fuel storage tank  122  via flow lines  61 .  
     [0057] Graphite spheres entering the moderator sphere flow path  60  are routed via a temporary storage and inspection region  62 . In the temporary storage and inspection region  62 , graphite spheres are delayed for a period of time, which may be of the order of five days, in order to facilitate the identification of misdirected fuel spheres which may inadvertently have entered the moderator flow path  60 . Also, in the inspection region  62 , graphite spheres are inspected for physical defects. Conduits  64  of the flow path  60  in the inspection region  62  are helical in shape (although this is not shown in the drawings) to facilitate X-ray inspection of each passing graphite sphere from all sides. From the inspection region  62 , graphite and misdirected fuel spheres are fed past third radiation sensors  66  which are operatively coupled to a third diverter valve (indexer)  68 . Both the third diverter valve  68  and the third radiation sensors  66  are connected to the controller and the diverter valve  68  is operable, under control of the controller, to forward fuel and moderator spheres to a transfer valve assembly  65 , or to divert graphite spheres to a moderator sphere storage system  90 , which will be further described below.  
     [0058] Fuel spheres exiting the outlet  20 , which are neither spent nor damaged, are diverted via the first diverter valves  56  into the fuel sphere flow path  50  and, via a pair of second inlet lines  73 , to a sphere collector  74  and sphere distributor  77  which is coupled to the controller and operable to distribute fuel spheres in a predetermined sequence to the nine second inlets  34  of the handling system  40 .  
     [0059] The transfer valve assembly  65  is operable to permit graphite spheres to travel via an inlet line  72  into the first inlet  32  of the core barrel  14 , and to divert misdirected fuel spheres into a flow line  75  leading into the sphere collector  74  and thence, via the second inlets  34 , into the annular region  30  of the core  22 .  
     [0060] The handling system  40  includes a new fuel storage system  80  for storing new (unused) fuel spheres and for selectively feeding new fuel spheres into the reactor core  22  via the second inlets  34 . New fuel spheres are introduced into the handling system  40  from a new fuel storage vessel  82  and pressure lock whence the fuel spheres are introduced to the inlets  34  via the sphere collector  74 .  
     [0061] The handling system  40  further includes a moderator sphere storage system  90  for storing graphite moderator spheres. The moderator sphere storage system  90  includes a graphite sphere storage tank  92  having inlets  93  and an outlet  94 , the inlets  93  being operatively coupled to the diverter valve  68  of the moderator sphere flow path  60  and the outlet  94  being coupled to the transfer valve assembly  65  of the moderator sphere flow path  60 . Thus, by operation of the diverter valve  68 , under control of the controller, graphite spheres discharged from the reactor core  22  may be diverted to the graphite sphere storage tank  92  for storing, rather than being recycled back into the reactor core  22 , thereby enabling the complete discharge of graphite spheres from the reactor core  22  for maintenance purposes. As required, the reactor core  22  may be recharged with graphite spheres from the graphite sphere storage tank  92  via the transfer valve assembly  65  and thence via the inlet line  72  to the first inlet  32 . The graphite sphere storage tank  92  further has a second inlet  96  coupled to a graphite and helium lock  98  via a feed line  100  through which fresh graphite spheres may be introduced into the system  40 . A fourth radiation sensor  102  is located in the feed line  100  intermediate the graphite and helium lock  98  and the graphite sphere storage tank  92  for sensing inadvertent attempted introduction of fuel spheres into the graphite sphere storage tank  92 . Graphite spheres are loaded from the graphite sphere storage tank  92  into the moderator sphere flow path  60  by means of a third sphere handling machine  104  which is connected to the transfer valve assembly  65  via line  105 . The graphite and helium lock  98  and fourth radiation sensor  102  may be portable and are shown in dotted lines in the drawings.  
     [0062] The handling system  40  further includes a spent fuel storage system  110 , which is illustrated schematically in FIG. 9. The spent fuel storage system  110  includes ten spent fuel storage tanks  112 , of which three are shown on the drawings, for permanent storage on site of spent and damaged fuel spheres. Preferably, the.capacity of the spent fuel storage tanks  112  is calculated to accommodate spent and damaged fuel spheres over the anticipated operational life of the nuclear reactor  10 . Inlets  114  to the spent fuel storage tanks  112  are operatively coupled to the first diverter valves  56  via a discharge lock  116 . Two fifth radiation sensors  118  are arranged on the spent fuel storage flow lines  70 , intermediate the first diverter valves  56  and the discharge lock  116 . The sensors  118  are operable to sense graphite spheres which may have been diverted inadvertently into the spent fuel storage system  110 . A ten port distribution controller  119  is connected to the spent fuel storage tanks  112  and is operable to divert spent fuel spheres to a predetermined storage tank  112 .  
     [0063] The handling system  40  further includes a temporary fuel storage system  120 . The temporary fuel storage system  120  has a temporary fuel storage tank  122  for storing in-use fuel spheres on a temporary basis. The temporary fuel storage tank  122  has inlets  124  operatively coupled to the first diverter valves  56  via the flow lines  61 , and an outlet  126  operatively coupled to the second inlets  34  of the reactor core barrel  14  via a re-fuelling line  128  leading to the sphere collector  74 . As with the graphite spheres, during maintenance of the reactor  10  fuel spheres may be discharged from the reactor core  22  and, rather than being circulated back to the reactor core  22 , may be temporarily stored in the temporary fuel storage tank  122  whilst maintenance takes place. On completion of maintenance, the fuel spheres are recharged into the reactor core  22  via the second inlets  34  by means of a fourth sphere handling machine  127 . Provision is made for a last core fuel cask  130  and loading station  131 , which is connected to the fourth sphere handling machine  127  and the outlet  126  of the temporary fuel storage tank  122  via a fuel line  132 . The reactor core  22  may be dumped into the last core fuel cask  130  at the end of the operating life of the reactor  10 . The loading station  131  may also be used for the dispatch of spent fuel from the spent fuel storage tanks  112 , via a series of fifth fuel handling machines  134  and a spent fuel line  136  and for the unloading of used graphite spheres via a graphite line  138  connect the third sphere handling machine  104  of the graphite sphere storage tank  92  to the loading station  131 .  
     [0064] It will be appreciated that in a plant  8  having a reactor  10  of the pebble bed type operating according to a multi-pass fuelling scheme, fuel spheres are moved through the core  22  more than once, for example up to ten times, before being exhausted (burnt-up) to the extent that they are no longer utile. The nuclear plant  8  in accordance with the invention as described herein includes a handling system  40  which is operable to keep fuel and graphite spheres separate after exiting from the reactor core  22 . The fuel and graphite spheres are fed into the reactor core  22  above the pebble bed by inlet supply tubes ( 32 ,  34 ) arranged in a specific order to ensure the two zone core loading with graphite spheres in the central region  26  and fuel spheres in the annular region  30  surrounding the graphite-filled central region  26 . The main components of the handling system  40  are preferably located in shielded, individual compartments below the reactor pressure vessel  12 . The spent fuel storage system  110 , which is designed as a lifetime spent fuel store and post operations intermediate store is located in a lower part of the reactor building. The handling system  40  provided according to the invention enables the loading of the core barrel  14  with graphite spheres and the loading of new fuel spheres into the core  22 . Further, the handling system  40  provides for the removing of misdirected fuel spheres from the moderator flow path  60  and the prevention of erroneously discharged graphite spheres initiating the loading of new fuel spheres, by means of radiation sensors  118  arranged on the spent fuel storage flow lines  70  leading to the spent fuel storage tanks  112 . Thus, while the controller is operable to trigger the loading of a new fuel sphere to replace each burnt-up fuel sphere that is diverted to the spent fuel storage tanks  112 , a graphite sphere detected by the sensors  118  will not initiate the loading of the new fuel sphere. Still further, the fuel handling and storage system  40  provides for the removal of fuel and graphite spheres from the discharge outlet  20 , the separation of damaged fuel and graphite spheres, the separation of fuel, absorber and graphite spheres, the re-circulating of graphite spheres and the recirculation of partially used fuel spheres through the core  22 . Burn-up of partially used fuel spheres is measured and spent fuel spheres are discharged into the spent fuel storage system  110 . It will be appreciated that in a PBR reactor it is anticipated that absorber spheres may be included in the core  22 . While the treatment of absorber spheres from the core  22  is not specifically described herein, it is anticipated that the handling system  40  may be readily adapted to separate, store and circulate such absorber spheres in a manner analogous to that described herein for moderator and fuel spheres.  
     [0065] Under normal operation, fuel and graphite spheres are conveyed from the core  22  through the fuel and graphite sphere discharge outlet  20  to the two sphere handling machines  48  that can deliver a continuous pebble flow from the discharge outlet  20  downstream of each machine  48 . Damaged spheres are discarded to the spent fuel storage system  110 . The graphite and fuel spheres pass through the discharge flow lines  52  and each fuel sphere or graphite sphere is released individually for radiation measurement, after which they are separated by a diverter valve  56 . The burn-up and radiation sensors  54  have the capability of measuring the burn-up of fuel spheres and of distinguishing between fuel spheres and graphite spheres. Fuel spheres are transported to the outside annular region  30  of the core  22 , while graphite spheres are transported to the graphite inspection region  62 . Further radiation sensors (not shown) are arranged in the inspection region  62 . Should a fuel sphere be detected by a buffer region radiation sensors, the normal operation of the handling system  40  is suspended. The contents of the inspection region  62  are re-circulated until the fuel spheres are removed and diverted to the sphere collector  74  by means of the transfer valve assembly  65  and flow line  75 . When a used fuel sphere is detected by the burn-up sensor  54 , the diverter valve  56  will send the spent fuel sphere to the spent fuel storage tanks  112 .  
     [0066] In the system as described, fuel and graphite spheres are conveyed in conduit lines  44 , which preferably are horizontally or vertically orientated, partly by gravity but predominantly pneumatically by using mainly the primary coolant gas at primary systems pressure. Monitoring of fuel sphere movement is performed with the aid of measurement and counting instruments ( 54 ,  66 ,  118 ), whose signals provide input to the control system which actuates the operating components in valve indexers ( 56 ,  68 ,  65 ) of the system  40 .  
     [0067] Fuel spheres are forwarded to the reactor  10  pneumatically by primary coolant. Two types of forwarding systems are used. The first forwarding system uses the extracted gas from the main gas stream. The second forwarding system is a blower system. The first forwarding system by-passes the blower (not shown) so that the blower can be maintained. In exceptional cases, such as an initial loading of the core  22  or re-filling of the core  22  with graphite spheres after emptying for inspection or repair, pneumatic forwarding is performed in air under pressure with the reactor pressure vessel  12  vented.  
     [0068] Under normal operation, the graphite and fuel spheres are separated on a continuous basis. The radiation and burn-up sensors  54  perform the functions of distinguishing fuel from graphite from absorber spheres and giving a count of such spheres passing the sensor  54  and measuring radiation and burn-up of fuel spheres. Each diverter valve  56  is operable to send a fuel or moderator sphere in one of three directions: either down the spent fuel storage flow line  70 ; or into the fuel sphere flow path line  50 ; or into the moderator sphere flow path  60 .  
     [0069] The graphite spheres are sent to a graphite inspection region  62  (buffer line) during normal operations, the buffer line  62  holding a stock of graphite spheres. The spheres in the buffer line  62  are monitored for radiation. This allows time for any misdirected fuel spheres to be detected and returned to sphere collector  74 .  
     [0070] Importantly, the handling system  40  provides for the de-fuelling and re-fuelling of the core  22  by transfer of the core inventory from the reactor  10  into separate graphite and fuel storage tanks ( 92 ,  122 ) located in an area adjacent to the reactor pressure vessel  12  during maintenance intervention requiring the venting of the main power system to atmosphere. After maintenance, the handling system  40  provides for the re-loading of the core  22  from these tanks ( 92 ,  122 ) during re-fuelling of the core  22 . The configuration of the handling system  40  during de-fuelling mode is shown schematically in FIG. 4, while the configuration of the handling system  40  during re-fuelling is shown schematically in FIG. 5. Thus, it is an important advantage of the present invention that maintenance can be carried out on the reactor core components and pressure vessel  12  during the lifetime of the nuclear reactor  10  at relatively low cost and relatively quickly.  
     [0071] The fuel handling and storage system  40  provides that the correct ratio and distribution of graphite and fuel spheres is maintained. Further, the main power system primary loop is isolated from the fuel handling and storage system  40 . The simultaneous loading of graphite and fuel spheres during re-fuelling mode, avoids horizontal movement of fuel spheres to the centre  26  of the core  22  and ensures adequate core volume is maintained.  
     [0072] De-fuelling of the core  22  will only take place if it is necessary to open the main power system to the atmosphere for maintenance. To prevent fuel corrosion, it is necessary to store fuel spheres under helium pressure in the fuel storage tank  122  adjacent to the reactor pressure vessel  12 . The reactor pressure is reduced and the low pressure region is connected to the high pressure region by the opening of pressure valves. Fuel and graphite spheres are separated by using radiation sensors  54 . The graphite spheres contained in the core  22  together with the graphite spheres which have been retrieved from the graphite storage tank  92  will be re-circulated to the core  22  and loaded into both the central region  26  and the annular region  30  thereof. The loading of the entire core region  23  with graphite spheres is to avoid horizontal movement of the fuel spheres to the central region  26  of the core  22  and to maintain adequate core volume. The fuel spheres are delivered via the inlets  124  to the water cooled and critically safe fuel storage tank  122 . During the de-fuelling mode, the spent fuel storage system  110  is out of service. Further, no new fuel loading takes place and no new graphite sphere loading or replenishment takes place.  
     [0073] After maintenance to the reactor power system, re-fuelling will commence. The required operational pressure and temperature of the helium will be maintained and the core  22  filled with graphite spheres to form a bed  200  of moderator elements or graphite spheres. The bed  200  of moderator elements is formed by loading the moderator elements or graphite spheres into the core barrel  14  from above. Once the bed  200  has been formed to the desired level, moderator elements are fed through the first inlet  32  and fuel elements are fed through the second inlets  34  into the regions  26 ,  30 . Simultaneously, the moderator elements forming the bed  200  are extracted from below through the outlet  20  at the same rate as moderator elements and fuel elements are fed into the core barrel. In this away, as illustrated in FIG. 11 of the drawings, a core can be built up having a central region  26  of moderator elements and an annular region  30  of fuel elements. This procedure is continued until all of the moderator elements of the bed  200  have been removed and the core is fully formed as indicated in FIG. 12 of the drawings. Once the two zone core  26 ,  30  is established (FIG. 12), the fuel storage tank  122  will be empty and the graphite storage tank  92  will be approximately three quarters full and a graphite buffer storage tank (not shown) will be full. At this point, start up of the reactor  10  can commence. The re-fuelling equipment is taken out of service and isolated from the high pressure components by closing the isolation valves between the low  46  and high pressure circuits  46 .  
     [0074] The process of operation of the reactor handling system  40 , including the moderator and fuel sphere storage systems  90 ,  110 ,  120  is illustrated in the process flow diagram of FIG. 2, in which legends and descriptions of major components are included for ease of use. In FIG. 2, process blocks a, b and c are together embodied in the first radiation and burn up sensors  54  of the example of the invention illustrated in FIG. 1. Further, in FIG. 2: the symbol indicated by reference numeral  140  represents a manually operated valve; the symbol indicated by reference numeral  150  represents an automatically controlled valve; and the symbol indicated by reference numeral  160  indicates a pressure relief valve.