ZA200208929B

ZA200208929B - Method and device for loading the core of a nuclear reactor with fuel assemblies.

Info

Publication number: ZA200208929B
Application number: ZA200208929A
Authority: ZA
Inventors: Philippe Joly; Patrice Jublot; Claude Hazelard
Original assignee: Framatome Anp
Priority date: 2000-05-04
Filing date: 2002-11-04
Publication date: 2003-10-07
Also published as: FR2808615B1; EP1279172B1; DE60121936D1; DE60121936T2; WO2001084557A1; FR2808615A1; CN1427995A; ATE335278T1; CN1209768C; EP1279172A1

Abstract

The invention concerns a method which consists in, prior to loading a fuel assembly (3) placing at least in a position adjacent to the position designed for the assembly (3) to be loaded, at least a means for guiding (18) the lower tip (9b) of the fuel assembly, so as to guide the lower tip of the fuel assembly on its four lateral faces, the guide means having a height, above the reactor core (8) support plate, less than 5 % of the core height. Then, it consists in moving the fuel assembly (3) downwards along a vertical axis offset relative to the vertical axis of the position of the fuel assembly in the core (6), so as to move it away from the neighbouring assemblies during the displacement; then resetting the position of the assembly (3) on the vertical axis of its loading position in the core (6) and performing the final lowering phase slowly to ensure that the lower tip (9b) is automatically guided and the assembly (3) is set down on the core (8) support plate, thereby enabling the fuel assembly to be speedily moved towards its loading position.

Description

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Method and device for loading the core of a nuclear reactor with fuel assemblies

The invention relates to a method and device for loading the core of a water-cooled nuclear reactor and in particular of a pressurised-water-cooled nuclear reactor.

Pressurised-water nuclear reactors have, inside a vessel, a core consisting of fuel assemblies of straight prismatic shape, with a square section, disposed with their vertical axes along the entire height of the core, juxtaposed along their lateral faces. Each of the fuel assemblies has a lower tip intended to come to rest on a horizontal core support plate which is disposed transversely in the vessel of the nuclear reactor and has openings passing through it for the passage of water for cooling the reactor. The core support plate has openings for the passage of water and means for positioning a fuel assembly at the level of each of the core fuel assembly positions, these fuel assembly positions forming a square-mesh lattice along the bearing surface of the core support plate. The means for positioning each of the fuel assemblies on the core support plate are generally formed by pins which project relative to the upper face of the support plate and are intended to be engaged in openings in the lower tip of the fuel assemblies in order to ensure that they are put in place in well-defined locations on the support plate.

The loading of the core of the nuclear reactor is carried out from the upper level of a containment pool of the reactor into which there opens an upper part of the nuclear reactor vessel which is now open and filled with water. The core is loaded by displacement, in the vertical direction and downwards, of each of the core fuel assemblies in succession in such a way as to set down the fuel assembly on the core support plate by means of its lower tip in a well-defined position in which two pins projecting from the core support plate engage in the lower tip of the fuel assembly. The loading position of the fuel assembly inside the core is for example delimited on two adjacent lateral sides by two boundary surfaces which are substantially continuous over the entire height of the core, are perpendicular to one another and are formed either by two faces of fuel assemblies neighbouring the assembly which is being loaded or by two perpendicular sides of a wall disposed in the nuclear reactor vessel on the periphery of the core.

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One of the problems to be solved in order to carry out the loading of the core of a nuclear reactor under satisfactory conditions relates to the guiding of the fuel assemblies in order to ensure their setting down and their positioning on the core support plate at the end of the vertical displacement using the loading machine. In fact it is necessary for the lower tip of the fuel assembly to be directed perfectly in the final lowering phase of the assembly in order to be able to engage the openings in the lower tip on the pins projecting from the core support plate, ensuring the positioning of the fuel assembly in the core.

The fuel assemblies which are reloaded into the nuclear reactor core are formed in part by used assemblies which have stayed in the nuclear reactor during operating cycles with a total duration of several months or several years. These assemblies may have been deformed under irradiation inside the core of the nuclear reactor, so that they may for example have defects of straightness in their axial direction. This results in increased difficulties in engaging the openings in the lower tip of the fuel assembly on the pins projecting from the core support plate.

It has therefore been proposed to use tools which guide the lower tip or foot of the fuel assembly while it is being set down on the lower core plate.

For example, in FR-2 588 689 a guiding shoe is proposed which is borne by the mast of the nuclear reactor loading machine which ensures guiding of the foot of the fuel assembly at the moment when it is being set down.

This document also proposes different variants of the guide means which can be put in place by the loading machine on the core support plate. These guide means extend over a substantial height above the core support plate, possible over the entire height of the core, and manipulation of these means by the loading machine is complex operation which results in periods when the loading machine is unavailable for the handling of the fuel assemblies.

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In FR-2 652 942 a tool to assist loading is proposed which comprises positioning blocks and a carrying device equipped with autonomous displacement means which enable it to move on the lower core plate. The remote guiding of the tool is not easy and the recovery of the tool can pose problems. Furthermore, the mechanism is complex and the maintenance operations have to be carried out on a contaminated device.

Furthermore, the loading of a nuclear reactor must be carried out as quickly as possible because the loading periods are situated on the critical path of the operation of shutting down the nuclear reactor for reloading, maintenance and repair. Any saving of time obtained in the core loading operation therefore reduces the immobilisation time of the central reactor and enables substantial economies to be achieved. In the case of implementation of guiding devices which necessitate the use of the loading machine, as described for example in FR- 2 588 689, the speed of use and the productivity of the loading machine are considerably reduced.

On the other hand, the use of a guiding shoe accompanying the loading machine does not provide such precise guiding as devices placed on the core support plate. The putting in place of guiding devices on the core support plate also reduces the availability and therefore the productivity of the loading machine for the loading of fuel assemblies into the core.

The object of the invention therefore is to propose a method of loading the core of a water- cooled nuclear reactor with fuel assemblies of straight prismatic shape, with a square section, which are disposed with their vertical axes along the entire height of the core, juxtaposed along their lateral faces and each having a lower tip with four lateral faces intended to come to rest on a horizontal core support plate with openings passing through it for the passage of cooling water and with means for positioning a fuel assembly, facing each one of a set of fuel assembly positions of the core, the loading being carried out from the upper level of a containment pool of the reactor into which there opens an upper part of a vessel of the nuclear reactor which contains the reactor core and is open and filled with water, by displacement in the vertical direction and downwards by means of a loading machine of each of the fuel assemblies of the core in a vertical position, in succession, in order to place each fuel ey ® assembly in an assembly position inside the core having a vertical assembly positioning axis and delimited by at least one boundary surface extending substantially over the entire height of the core, this method of loading permitting improvement of the precision and the rapidity of the loading of the fuel assemblies into the core and therefore limitation of the total time necessary for loading a nuclear reactor core.

For this purpose: - before the loading of a fuel assembly at least one means for guiding the lower tip of the fuel assembly is placed in at least one assembly position adjacent to the position of the assembly to be loaded, in such a way as to guide the lower tip along its four lateral faces in order to set it down on the core support plate, the guide means having a height less than 5% of the height of the core above the core support plate and being engaged in openings for the passage of water in order to position and immobilise it, - the fuel assembly is displaced downwards along a vertical axis offset in a direction parallel to the core support plate relative to the vertical axis of the fuel assembly position in the core, in such a way as to keep it away from at least one boundary surface during the displacement, - the vertical displacement at the first speed of the fuel assembly is stopped at the latest when the lower tip of the fuel assembly has arrived at a vertical distance from the core support plate equal to 5% of the height of the core, - the fuel assembly is displaced substantially horizontally in order to place its axis along the assembly positioning axis in the core of the reactor, and - the fuel assembly is displaced in the vertical direction and downwards at a second speed which is lower than the first, in order to set it down on the core support plate, the lower tip of the fuel assembly then being guided on its four lateral faces.

In order to make the invention readily understood, an embodiment of a device for loading the core of a nuclear reactor and its use in a method of loading according to the invention will be described with reference to the accompanying drawings.

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Figure 1 is a view in elevation and in section of the whole loading device and means for putting in place means for guiding a fuel assembly, according to a first embodiment.

Figures 2, 3 and 4 are views, respectively in elevation, in vertical section and from above, of a guide means for carrying out the method according to the invention.

Figures 5 and 6 are views in section of a device permitting the guide means to be put in place on a core support plate, in a first and a second operating position.

Figure 7 is a plan view of the lattice of assembly position of the nuclear reactor core in the course of being changed.

Figure 8 is a view in elevation and in partial section of a device for putting in place means for guiding a fuel assembly for carrying out the method according to the invention, according to a second embodiment.

Figure 9 is a view from above of the placing device shown in Figure 8 and of the machine for loading the nuclear reactor.

Figure 1 shows schematically the upper part of the vessel 1 of a nuclear reactor during an operation of loading the reactor core with fuel assemblies 3.

The vessel 1 is open on its upper part in communication with a containment pool 2 of the nuclear reactor delimited by concrete walls inside the reactor building.

During the operation of reloading the nuclear reactor core inside the vessel 1, the containment pool 2 and the vessel 1 are filled with water. Figure 1 shows the upper part of concrete walls 4 of the containment tank which support rails on which the machine 5 for loading the nuclear reactor travels.

Inside the vessel 1, the space occupied by the core 6 of the nuclear reactor is delimited by a peripheral wall 7 and, on its lower part, by the core support plate 8 on which the fuel assemblies 3 which form the core 6 are set down.

The fuel assemblies 3 are each formed by a bundle of fuel rods held in a framework of which the upper and lower end parts respectively are formed by an upper tip 9a and by a lower tip 9b. Each of the fuel assemblies of the core rests on the core support plate 8 by means of its lower tip 9b. The positioning of the fuel assembly on the core support plate 8 is ensured by pins projecting on the upper face of the support plate 8 coming into engagement in openings in the comer parts of the lower tip 9b of substantially square shape.

Furthermore, the core support plate 8 has passing through it openings for the passage of water 11 which ensure the passage of the water for cooling the reactor circulating in the vertical direction and from bottom to top following the axial direction of the fuel assemblies 3 of the core 6.

At right angles to each of the positions of fuel assemblies 3 of the core 6, the core support plate 8 has passing through it four openings 11 for the passage of water and has two fuel assembly positioning pins.

In order to carry out the loading of the core 6 of the nuclear reactor, each of the fuel assemblies 3 intended to form the core 6 is taken up by the loading machine 5 in a vertical arrangement and is lowered along a vertical displacement axis inside the vessel 1 by the loading machine until the fuel assembly is set down by means of its lower tip 9b on the core support plate 8 in a perfectly defined fuel assembly location.

The loading machine 5 has a travelling crane 12 which moves, on rails joined to the upper part of the walls 4 of the containment pool 2 of the reactor, in a first direction (perpendicular to the plane of Figure 1).

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The travelling crane 5 also has a carriage 13 which is mounted so as to be movable on the upper face of the transversely directed girders of the crane 12 in a direction perpendicular to the direction of displacement of the travelling crane 12. The carriage 13 bears a guide tube 14 with a vertical axis extending below the carriage 13 between the transverse girders of the travelling crane 12 and a tower 15 extending vertically above the carriage 13.

The fuel assembly loading mast 10 and a pole 16 of a device for placing devices 18 for guiding the lower tip 9b or foot of the fuel assemblies are mounted inside the guide tube 14 so as to be movable in the vertical direction.

The loading mast 10 has an internal movable part of which the lower end includes means for gripping the fuel assembly 3 by means of its upper tip 9a.

The loading mast 10 is suspended by means of a sheave on the lifting cable of a lifting winch 17 disposed in the tower 15 of the loading machine.

The pole 16 of the means for placing the guiding devices 18 is suspended on the cable of a lifting winch 19 passing over a suspension sheave mounted on the upper part of the tower 15.

The displacement of the travelling crane 12 and the carriage 13 of the loading machine 5 in crossed movements permits the axis of the mast 10 for loading of a fuel assembly 3 of prismatic shape with a square section to be placed vertically in line with any position of fuel assemblies in the reactor core 6 having a vertical axis for positioning of the assembly and also permits the vertical axis of the pole 16 to be placed vertically with any fuel assembly position in the core 6, this position constituting a position adjacent to the set-down position of the fuel assembly 3.

It should be noted that the vertical displacements of the charging mast 10 and of the pole 16 can be carried out independently of each other.

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In Figure 1 the loading mast 10 has been shown in a raised position in which it is entirely retracted into the interior of the guide tube 14 of the loading machine and the pole 16 is shown in an extracted position for setting down a guide means 18. The winches 17 and 19 permit displacement of the loading mast 10 and the pole 16 in the vertical axial direction between their retracted position and a low position for setting down respectively of a fuel assembly or of a guide means 18 on the core support plate 8.

In Figures 2 to 4 a guide means 18 is shown which can be placed on the core support plate 8 by the pole 16 in order to ensure the guiding of the foot 9b of a fuel assembly at the end of its vertical and downward displacement in order to set it down on the core support plate 8.

The guide means 18 has a casing 20 of pyramidal shape overall and having a square base extended by a parallelepipedal skirt for joining it to a square base plate 21. The base plate 21 has the shape and the dimensions of a square section of a fuel assembly 3.

On the base 21, projecting downwards, are fixed two, three or four pins 22 for positioning or immobilisation, the diameter of these pins being slightly less than the diameter of the water openings 11 passing through the core support plate § at the level of each of the fuel assembly positions. Furthermore, the pins 22 are placed in such a way as to be able to engage in the openings 4 in the core support plate which are present in each of the fuel assembly positions in the core 6.

The guide means 18 also has, fixed inside the casing 20 at its upper end, a gripping and lifting part 23 which has a profiled opening 24 passing through it. Finally, along two corners disposed at the ends of a diagonal of the base 21, the base plate 21 and the casing 20 have vertical openings 25 passing through them for engagement of two pins projecting from the core support plate 8 which are disposed in a fuel assembly location.

In its central part the base plate 21 has an opening 26 passing through it for the passage of guides for the glove fingers of the core instrumentation.

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The means 18 for guiding the upper tips of fuel assemblies are intended to be placed on the core support plate in a fuel assembly location. The means 18 is oriented in such a manner that the pins 22 are introduced practically without play into the openings for the passage of water of the core support plate, thus ensuring the centring and the position of the guide means 18 on the core support plate.

Moreover, two pins which project relative to the upper face of the core support plate are engaged in the openings 25 in the corners of the base plate 21 and of the skirt of the casing 20.

When the guide means 18 is placed in a fuel assembly position the four inclined faces of the pyramid-shaped casing 30 are directed towards four fuel assembly locations adjacent to the location on which the guide means 18 is disposed. The inclined outer surfaces of the faces of the pyramid-shaped casing 20 are therefore capable of ensuring the guiding of a fuel assembly in four locations neighbouring the guide means.

When a lower tip of fuel assemblies comes into contact with one of the inclined outer surfaces of the pyramidal casing of the guide means 18, this surface exerts a guiding effect in the vertical direction and in a horizontal direction on the lower tip of the fuel assembly due to the inclination of the guiding surface. These guiding effects can be optimised by acting on the angle of inclination of the surfaces of the pyramidal casing.

The opening 24 made in the gripping and lifting part 23 of the guide means 18 has a flared inlet part then a constricted neck and finally a part which is wider than the neck. Such a shape for an opening is a conventional shape in the case of parts which are to be lifted in a nuclear power station using a lifting means with manoeuvrable gripping fingers, such as clusters for control of the reactivity of the core.

The guide means 18 can be taken up and displaced by a pole such as the pole 16 having a gripping device with manoeuvrable gripping fingers.

Figures 5 and 6 show the pole 16 which has a tubular body 16a suspended on the lifting cable of the winch 19 by means of a suspension head 27 on which a compensator 26 is fixed.

The suspension head 27 is fixed on the upper part of the body 16a on which is fixed, at the level of an axially directed through opening, the body of a jack 28 which is disposed axially in the lifting head 27 and extended axially by an clongate tubular element 29 mounted coaxially inside the tubular element 16a. A tubular element 31 in the axial extension of the element 16a bears, on its lower end, a gripping assembly 30 which is manoeuvrable by means of the rod 32 of the jack 28 having a tubular support 33.

The gripping assembly 30 comprises a clamp support 34 integral with the tubular support 33 on which two gripping fingers 35, similar to gripping fingers of a control cluster of the nuclear reactor, are mounted so as to pivot about axes 35".

Inside the tubular support 33 the rod 32 of the jack 28 is movably mounted, and the rod 37 of an olive-shaped knob 37a for manoeuvring the fingers 35 is mounted on the lower part of the said rod in a coaxial arrangement and articulated about an axis 37".

The lower end of the rod 32, which has the rod 37 of the olive-shaped knob 37a mounted in an articulated manner in its axial extension, has a widened part on which the lower end part of a helical return spring 38 comes to bear, this spring being disposed in an axially directed bore of the tubular support 33 and bearing with its upper end on a shoulder of the tubular support 33.

The spring 38 is calibrated in such a way as to ensure the return of the rod 32 and of the olive- shaped knob 37a which is joined to the rod 37 to a low position shown in Figure 6 in which the olive-shaped knob 37a ensures the parting of the fingers 35.

Supplying the pneumatic jack 28 with motor fluid ensures the displacement of the manoeuvring rod 32 and of the olive-shaped knob 37a joined to the rod 37 upwards in the axial direction by compressing the spring 38, in such a way that the olive-shaped knob 37a in the high position ensures the maximum closeness of the fingers 35 as shown in Figure 5.

In this close position, the fingers 35 can be introduced into a gripping opening such as the opening 24 of a guide means 18 similar to the opening of the pommel of a nuclear reactor control cluster.

Releasing the pressure actuating the jack 28 ensures the displacement of the rod 32, the rod 37 and the olive-shaped knob 37a in the axial direction and downwards under the effect of the spring 38 which had been compressed during the upward displacement of the actuating rod 32.

The olive-shaped knob 37a then ensures the parting of the fingers as shown in Figure 6.

The ends of the fingers 35 then come to rest in the widened part of the opening 24 below the central part of small diameter and ensure the attachment of the guide means at the end of the pole 16.

The displacement of the guide means 18 suspended at the end of the gripping assembly 30 of the pole 16 can then be carried out. The attached position of the guide means 18 is maintained in a passive manner by the spring 38.

Manoeuvring of the pole 16 by the control centre of the loading machine 5 below the upper level of the containment pool permits placement of the guide means 18 on the core support plate, on a fuel assembly location, the pins 22 of the guide means being introduced into the water openings passing through the core support plate at the level of the fuel assembly location.

By actuating the jack 28 it is possible to free the fingers 35 from the opening 24 of the guide means 18 and to lift the manoeuvring pole 16 again in order to leave the guide means in place at the fuel assembly location on the core support plate of the nuclear reactor.

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In order to effect these manoeuvres, the lifting tackle 19 of the manoeuvring rod 16 has a reciprocating motor controlled by means of a speed controller which permits displacement in the vertical direction of the guide means to be put in place at a speed of approximately 15 m per minute. The lifting tackle also includes an integrated service brake, a reducer and a stainless steel take-up drum equipped with a safety brake. The manoeuvring pole 16 is suspended with the aid of stainless steel cables and the suspension compensator 26 of the pole has a dynamometric axis which permits the movement of the pole to be stopped in the event of overload or under-load and during the setting down of the guide means. Furthermore, at least one encoder is associated with the lifting tackle of the pole in order to know the exact position of the end of the pole bearing the guide means 18 in the vertical direction and in order to detect any overspeed of the tackle.

The jack has two position sensors which enable the engaged or disengaged state of the fingers of the grab to be discovered, the signals from the position sensors being transmitted to the control centre of the loading machine 5.

The upper part of the manoeuvring pole 16 is substantially the same size as a television pole customarily used on the loading machines of nuclear reactors in order to reuse the existing guide elements of a television pole in the tube 14 so that an exchange can be made between the television pole and the manoeuvring pole on the site of the nuclear reactor.

However, the lower part of the pole 16 has dimensions substantially smaller than the body of a television pole in order to permit introduction into the core with a sufficient clearance between the pole and the fuel assembly.

The means for monitoring and controlling the pole for handling the guide means 18 are integrated into the control centre of the loading machine 5.

Figure 7 shows the upper surface of the core support plate 8 of the nuclear reactor and the core 6 of the reactor in the form of a square-mesh lattice of locations of fuel assemblies 3 on the core support plate 8.

The core 6 has been shown in the conventional manner during a loading operation, the locations 40 occupied by a fuel assembly already loaded into the core being indicated by a cross and the locations 40 which are not yet occupied by a fuel assembly do not have any identification sign.

The fuel assembly locations 40 of the core 6 of the reactor are disposed in a square-mesh lattice delimited externally by a peripheral contour 39 corresponding to a transverse section of the wall 7 delimiting the core 6.

The wall 7 of the core 6 is formed by flat vertical panels having substantially the height of the core and disposed vertically along the contour 39.

The assemblies 3 situated on the periphery of the core bear on one or two lateral faces on the panels of the wall.

The drawing also shows a fuel assembly 3 in the course of being changed in a location 40 indicated by a square marked inside a location 40 with a cross in it, and squares marked in locations 40 and without a cross in them indicate guide means used during the loading of the fuel assembly 3.

As can be seen in the drawings, the compartment inside the core in which the fuel assembly 3 is placed, of which the location 40a constitutes the transverse section in the plane of the core support plate 8, is delimited on two adjacent sides by two lateral faces of fuel assemblies in positions neighbouring the compartment in which the loading of the fuel assembly is being carried out, these lateral faces of neighbouring fuel assemblies being perpendicular to one another.

The two other sides of the compartment into which the fuel assembly 3 is being loaded are not occupied lateral faces of fuel assemblies.

The loading of the core of the nuclear reactor is carried out along successive diagonals which are parallel to one another such as 41a, 41 and 41c in such a way that during the loading of any fuel assembly the compartment receiving the fuel assembly is always closed on two sides by faces extending in a substantially continuous manner over the entire height of the core and is open on its two other sides.

In the case of loading of the assemblies 3 along the diagonal 41a, each of the compartments for loading of a fuel assembly is delimited on two sides by two faces which are perpendicular to each other extending over the entire height of the core and formed by two parts of panels of the wall which are perpendicular to each other.

In the case of loading of fuel assemblies along the following diagonals 41b, 4lc, the compartments into which the fuel assemblies are introduced are delimited on two faces which are perpendicular to each other either by two wall panel parts or by a wall panel part and a face of a fuel assembly, or by two lateral faces of fuel assemblies.

The loading is carried out in a first direction for a first diagonal and in the opposite direction for the following diagonal, as shown for example by the arrows at the ends of the lines 41a, 41b and 41c.

In all cases, during the loading of the core 6 of the nuclear reactor each of the compartments into which a fuel assembly 3 is going to be placed is closed on two sides by faces of the wall or fuel assemblies containing limiting faces extending in a substantially continuous manner along the entire height of the core and open on its two other sides, each of the fuel assembly compartments having, of course, a shape analogous to the shape of a fuel assembly, that is to say a straight prismatic shape with a square section.

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A description will now be given of the operations of loading of a fuel assembly 3 into a fuel assembly compartment of the core situated vertically in line with the location 40a represented in Figure 7 within a row of locations 41h disposed along a diagonal parallel to the diagonals 41, 41b and 41c.

The diagonal row 41h has been filled with fuel assemblies 3 up to the location 40a, the corresponding compartment of the fuel assembly 3 to be loaded being delimited on two sides by neighbouring fuel assembly faces of the compartment in which the loading of the fuel assembly 3 is being carried out.

During operations prior to the reloading of the core 6 of the nuclear reactor two guide means 18, which will be used during each of the operations of loading a fuel assembly such as 3, are placed on a storage stand in the base of the containment pool 2 of the reactor.

Before the fuel assembly 3, which can be fixed on the loading mast 10 of the loading machine 5, is set down, two guide means 18 are placed in fuel assembly locations surrounding the location 40a on which a fuel assembly 3 is to be set down, in such a way as to guide the lower tip 9b of the fuel assembly on its four lateral faces during the set-down of the assembly on the core support plate. The rod 16 for handling the guide means is used for this, the operation of this rod being independent of the operation of the mast for loading fuel assemblies.

In reality it will be seen below that it is generally sufficient at each stage of loading of a fuel assembly to displace one single guide means 18 from one fuel assembly location to another in order to delimit the location for loading of a fuel assembly above the core support plate 8.

The part of the guide means 18 projecting above the core support plate 8 after introduction of the pins 22 into the interior of the holes for the passage of water in the core support plate corresponds to the height of the casing 20, this height being very small relative to the total height of the core. Thus in the case of a core of a pressurised-water-cooled nuclear reactor formed by assemblies with a height of the order of 4 to 5 metres depending upon the reactors,

® the height of the casing of the guide means 18 is less than 20 cm, that is to say less than 5% of the total height of the core.

After having placed the guiding devices 18 in the two required positions around the location 40a the loading machine is position in such a way that its vertical axis of displacement of the fuel assembly is substantially offset relative to the vertical axis of the location 40a of the fuel assembly on the upper plate of the core in a horizontal direction parallel to the plate 8, opposite the two faces of fuel assemblies delimiting the compartment of the fuel assembly 3 to be loaded.

The offset of the vertical displacement axis of the loading machine is approximately 10 cm relative to the two faces delimiting the compartment in such a way that, during the lowering of the fuel assembly into the core of the nuclear reactor, the two faces of the fuel assembly in the course of being loaded are situated at a certain distance opposite the two faces of the compartment formed by faces of neighbouring fuel assemblies, which makes it possible to avoid any shock or any friction of the fuel assembly.

It is then possible to ensure the lowering of the fuel assembly 3 at a first rapid speed inside the vessel of the nuclear reactor, for example at a speed of 15 metres per minute.

The lowering of the fuel assembly at rapid speed is stopped at the latest when the lower tip 9b has arrived at a vertical distance above the core support plate equal to 5% of the height of the core, that is to say at the level of the upper part of the guide means 18. The fuel assembly is then displaced in a substantially horizontal direction in order to dispose its vertical axis in accordance with the axis of its loading position in the core of the nuclear reactor. The vertical lowering of the fuel assembly is then resumed at a second speed lower than the first, or slow speed, which may be for example 1.2 metres per minute.

When the lower tip 9b of the fuel assembly arrives in the zone occupied by the guide means 18, slightly above the upper surface of the core support plate 8, the lower tip of the fuel assembly can come into contact with the inclined faces of the guide means 18 directed downwards the compartment of the fuel assembly in the course of being changed. These inclined faces ensure guiding of the foot of the fuel assembly not only in the vertical axial direction but also in two horizontal positions respectively perpendicular to the two faces of the assemblies neighbouring the assembly 3 which is in the course of being changed and exert forces pushing the foot of the fuel assembly towards the interior of the location 40a. The lower tip is guided during this phase not only by the guide means 18 but also by the tips of adjacent assemblies (or parts of the wall depending upon the loading position).

The lower tip of the fuel assembly 3, provided that this assembly is correctly oriented, comes to be placed on the location 40a in such a way that the openings of the tip engage on the corresponding centring and positioning pins of the core support plate 8.

Thus the loading of a fuel assembly by the method according to the invention is carried out in a rapid manner and with very great reliability.

It should be noted that the displacement of the fuel assembly in the vertical direction for it to be lowered onto the core support plate is not hindered by any adjacent element due to the fact that its faces directed towards the neighbouring fuel assemblies are spaced by a sufficient distance resulting from the offsetting of the axes (for example 10 cm) and that the other two faces of the fuel assembly are disposed on sides of the compartment which are not closed.

Such lowering of the fuel assembly with an axis substantially offset relative to the position of the axis of its compartment is called "large offset" loading.

The loading of the fuel assembly by rows disposed along diagonals of the square lattice, successively in one direction and in the other, is called "serpent" loading.

After the loading of the fuel assembly 3 into the compartment vertically in line with the location 40a, a fuel assembly is loaded into the neighbouring compartment along the diagonal 41h vertically in line with the location 40b on the upper plate of the core 8.

It should be noted that one of the sides of the location 40b is already bounded by a guiding device 18 and that it is sufficient to displace the second guiding device 18 used previously.

For this, in a first time, the mast 10 for loading fuel assemblies into the interior of the tube of the loading machine is raised again and the pole 16 for handling the guide means 18 is lowered and displaced laterally in a simultaneous manner in order to take up the guide means 18 to be displaced.

The guide means 18 which has been taken up is raised to a height permitting it to pass above the second guide means 18 left in place on the core support plate, and the guide means 18 which has been taken up is displaced as far as its new position adjacent to the location 40b.

The horizontal displacements from one location to another are carried out with the aid of the loading machine. The guide means which has been taken up is then set down on the free location neighbouring the location 40b.

After release of the guide means in its new position, the handling pole is raised again in the interior of the guide tube of the handling machine and the fuel loading machine is moved to a zone for taking up a fuel assembly which is then set down in the compartment vertically in line with the location 40b in the manner which has been described with regard to the setting down of a fuel assembly 3 in the compartment vertically in line with the location 40a.

The loading operations are continued along the line 41h as far as the end of the row, then the loading of the following row is carried out diagonally in the opposite direction.

In certain position, at the end of the rows, it may be necessary to displace the two guiding devices in order to ensure the loading of the first or the second fuel assembly of a row.

Therefore the method according to the invention makes it possible to carry out the complete loading of the core of a nuclear reactor rapidly and very reliably by manoeuvres involving minimal unavailability of the loading machine.

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Figures 8 and 9 show a second embodiment of the device permitting the method according to the invention to be carried out.

The corresponding elements in Figure 1 relating to the first embodiment and in Figures 8 and 9 relating to the second embodiment bear the same reference numerals.

In the case of the second embodiment, the pole 16 for handling of the guide means 18 which is displaced in the vertical direction by the winch 19 is borne by a carriage 43 which is displaced on a second travelling crane 42 which is itself mounted so as to be movable on the same rails as the first travelling crane 12 of the handling machine 5 which are fixed on the upper end parts of the walls 4 of the containment pool 2.

For this reason the displacements of the pole 16 for handling the guide means 18 can be carried out independently of the displacements of the loading machine 5, which enables the means 18 for guiding the fuel assemblies to be put in place on the core support plate 8 of the reactor without rendering the loading machine unavailable.

For this reason, during the operations of positioning a guide means 18 (or two guide means 18) prior to the loading of a fuel assembly 3 it is possible for the loading machine to take up the fuel assembly during dead time. The carrying out of the operations of putting the guide means 18 in place in dead time during the displacements of the loading machine, taking into account the speeds required for carrying out the different displacements of the loading machine and of the crane and of the carriage for displacement of the pole for handling the guide means 18, enables the reloading of the core to be accelerated.

Furthermore, the handling crane and the carriage for displacement of the pole for handling the guide means can be used for the displacement and the deployment of different tools and in particular of a tool for connection and disconnection of the control rods of the clusters for adjustment of the reactivity in the core of the nuclear reactor. In this case, after the reloading of the core, the guide means 18 are set down on their storage stand in the base of the containment pool and the pole 16 for handling the guide means is demounted in order to set it down on a storage stand provided in the containment pool of the reactor.

The tool for connection and disconnection of the control rods of the clusters of the nuclear reactor are mounted on the second travelling crane 42 and the carriage 43.

The storage of the second travelling crane 42 of the handling pole 16 is carried out, as shown in Figure 9, in a storage zone situated in the vicinity of the stand 44 for the storage of the upper internal equipment of the nuclear reactor.

In all the cases, the method and the device according to the invention make it possible to carry out the loading of the core of a nuclear reactor in a very rapid and very reliable manner.

The invention is not limited strictly to the embodiment which has been described.

Thus the means for guiding the fuel assembly and the pole for handling these guide means can be produced in a form different from that which has been described.

The loading of the core of the nuclear reactor can also be carried out in a different sequence from that which has been described, which has the advantage that it only necessitates a minimal number of displacements of the guide means.

In a general manner the invention applies to the loading of the core of any water-cooled nuclear reactor having fuel assemblies of prismatic shape with a square section disposed in a juxtaposed manner in the core of the nuclear reactor in a square-mesh lattice.

Claims

® Claims

1. Method of loading the core of a water-cooled nuclear reactor with fuel assemblies (3) of straight prismatic shape, with a square section, which are disposed with their vertical axes along the entire height of the core (6), juxtaposed along their lateral faces and each having a lower tip (9b) with four lateral faces intended to come to rest on a horizontal core support plate (8) with openings (11) passing through it for the passage of cooling water and with means for positioning a fuel assembly (3), facing each one of a set of fuel assembly positions of the core (6), the loading being carried out from the upper level of a containment pool (2) of the reactor into which there opens an upper part of a vessel (1) of the nuclear reactor which contains the core (6) and is open and filled with water, by displacement in the vertical direction and downwards by means of a loading machine (5) of each of the fuel assemblies (3) of the core (6) in a vertical position, in succession, in order to place each fuel assembly in an assembly position inside the core (6) having a vertical assembly positioning axis and delimited by at least one boundary surface extending substantially over the entire height of the core, characterised in that: - before the loading of a fuel assembly (3) at least one means (18) for guiding the lower tip (9b) of the fuel assembly is placed in at least one assembly position adjacent to the position of the assembly (3) to be loaded, in such a way as to guide the lower tip (9b) along its four lateral faces in order to set it down on the core support plate (8), the guide means (18) having a height less than 5% of the height of the core above the core support plate (8) and being engaged in openings (11) for the passage of water in order to position and immobilise it, - the fuel assembly is displaced in the vertical direction and downwards at a first speed along a vertical axis offset in a direction parallel to the core support plate (8) relative to the vertical axis of the fuel assembly position (40a, 40b) in the core (6), in such a way as to keep it away from at least one boundary surface during the displacement, - the vertical displacement at the first speed of the fuel assembly (3) is stopped at the latest when the lower tip (9b) of the fuel assembly (3) has arrived at a vertical distance from the core support plate (8) equal to 5% of the height of the core (6),

® ; R002’ ®t g2¢ - the fuel assembly (3) is displaced substantially horizontally in order to place its axis along the assembly positioning axis in the core (6) of the nuclear reactor, - the fuel assembly (3) is displaced in the vertical direction and downwards at a second speed which is lower than the first, in order to set it down on the core support plate (8), the lower tip (9b) of the fuel assembly (3) then being guided on its four lateral faces.

2. Method as claimed in Claim 1, in the case of loading of a core (6) of a nuclear reactor of which the fuel assemblies (3) rest on the core support plate (8) at locations (40) disposed in a square-mesh lattice, characterised in that the loading of the fuel assemblies (3) into the core of the nuclear reactor is carried out along diagonal lines (41a, 41b, 41c, 41h) of the square- mesh lattice of loading locations (40) of the core support plate (8), each of the fuel assemblies (3) being loaded in such a way that its upper tip comes to bear on a location (40a, 40b) disposed along a diagonal line following the loading location of a preceding assembly, and that the at least one guide means (18) is displaced, along a diagonal line parallel to and neighbouring the diagonal line along which loading is being carried out, between two operations of loading a fuel assembly (3).

3. Method as claimed in Claim 2, characterised in that the loading is carried out successively in one direction and then in the other along diagonal lines (41a, 41b, 41c, 41h) which are parallel to each other.

4. Method as claimed in any one of Claims 1 to 3, characterised in that the first speed of vertical displacement of the fuel assembly along the offset vertical axis is of the order of 15 m/minute.

5. Device for loading the core of a water-cooled nuclear reactor with fuel assemblies (3) of straight prismatic shape with a square section which are disposed with their vertical axes along the entire height of the core (6), juxtaposed along their lateral faces and each having a lower tip (9b) intended to come to rest on a horizontal core support plate (8) with openings (11) passing through it for the passage of cooling water and with means for positioning a fuel assembly (3), facing each one of a set of fuel assembly positions of the core (6), the loading

® being carried out from the upper level of a containment pool (2) of the reactor into which there opens an upper part of a vessel (1) of the nuclear reactor which contains the core (6) and is open and filled with water, by displacement in the vertical direction and downwards by means of a loading machine (5) of each of the fuel assemblies (3) of the core (6) in a vertical position, in succession, in order to place each fuel assembly in a loading position inside the core (6) delimited by at least one boundary surface extending substantially over the entire height of the core and having a vertical loading axis, characterised in that it comprises: - at least one means (18) for guiding the lower tip (9b) of a fuel assembly (3) having a casing (20) at least partially of pyramidal shape with a square base, a support plate on which the square casing is fixed having the dimension of the transverse section of a fuel assembly (3) on which at least two pins (22) for positioning in an opening for the passage of water passing through the core support plate (8) are fixed which project in a direction opposite the pyramid-shaped casing (20), and a gripping and lifting part (23) joined to the casing (20) in an arrangement which is opposed along the axis of the pyramid-shaped casing (20) to the base plate (21) having a gripping opening (24), along the axis of the pyramidal casing (20), - a handling pole having remotely manoeuvrable means for gripping of a guide means (18) with the aid of gripping fingers of a grab through the opening (24) of the gripping part (23) for the guide means (18), - means (19) for displacement of the handling pole (16) in the vertical direction, - means (12, 13, 42, 43) for displacement of the handling pole in a horizontal plane in two directions which are perpendicular to each other, which are disposed above the level of the containment pool (2) and means for remote control of the handling fingers (35) of the grab of the handling pole for taking up and releasing a guide means (18).

6. Device as claimed in Claim 5, characterised in that the pole (16) for handling the guide means (18) has in its upper part dimensions analogous to those of a television pole of the loading machine, in such a way that it can be exchanged for the television pole on the site of the nuclear reactor.

7. Device as claimed in any one of Claims 5 and 6, characterised in that the handling pole (16) has a gripping assembly (30) having a tubular support (33) on which gripping

® fingers (35) of a grab are pivotably mounted and in which is slidably mounted an actuating rod (32) of a jack (28) for manoeuvring the grab of the handling pole which is joined at one of its ends to an olive-shaped knob (37a) for actuating fingers (35) for their displacement between a close position and a parted position for take-up by displacement of the actuating rod (32) of the jack in the axial direction.

8. Device as claimed in any one of Claims 5 to 7, characterised in that the means for displacement of the pole (16) for handling of the guide means (18) in a horizontal plane in two directions which are perpendicular to each other are formed by an overhead crane (12) and a carriage (13) with crossed movements of a loading machine (5) having a mast (10) for loading of fuel assemblies (3), the handling rod (16) being mounted so as to be movable in the vertical direction inside a guide tube (14) for the loading mast (10) of the loading machine (5) and connected to a winch (19) for displacement in the vertical direction which is joined to the carriage (13) of the loading machine (5).

9. Device as claimed in any one of Claims 5 to 7, characterised in that the means for displacement of the handling pole (16) in a horizontal plane in two directions which are perpendicular to each other are formed by a second overhead crane (42) which is independent of a first overhead crane (12) of a nuclear reactor loading machine (5) and is movable on displacement rails situated above the upper level of the containment pool (2) of the nuclear reactor and a carriage (43) which is independent of the loading machine (5) and is movably mounted on the girder of the second overhead crane (42) in a direction perpendicular to the direction of displacement of the second overhead crane (42).