FR2618014A1 - Process and plant for reconstitution of nuclear fuel assembly - Google Patents

Abstract

Process for reconstitution of nuclear fuel assembly and plant for making use of this process, according to which: an assembly 21 from which the fuel rods 35 are to be extracted is placed in a first vertical cell 20, a new skeleton assembly 26 with its nozzles removed is placed in a second cell 25, the new skeleton 26 is loaded with inert false rods 52 the upper ends of which have a cavity 53 of shape complementing that of the lower end 11 of the rods of the assembly 21 of the first cell 20 and whose length is more than twice the distance between two grids 4, each of the rods 35 is extracted from the assembly in the first cell and is lowered into the new skeleton while vertically pushing back the false rod 52 until the said rod to be transferred 35 adopts its definitive position while ejecting the false rod.

Description

Method and installation for nuclear fuel assembly reconstruction
The invention relates to the reconstitution of a nuclear fuel assembly of the type comprising a skeleton formed by two end pieces connected by tie rods fixed to the two end pieces and grids distributed between the end pieces along the tie rods and a bundle of fuel rods held by the skeleton between the end caps. Such assemblies are conventionally used in cooled and moderate reactors with natural water, in particular in so-called "PWR" pressurized water reactors.

 It appeared necessary in certain cases, to dismantle an irradiated assembly after its stay in a reactor and to remove all or part of the rods which it contains. This appears in particular when a part of the skeleton of an assembly is damaged to such an extent that it is necessary to reconstitute an assembly by removing the pencils to transfer them to a new skeleton. This is also the case when some of the rods in the assembly are themselves damaged and must be eliminated to avoid contamination of the water in the primary circuit or the storage pools.

Another situation is that where it is desired to transfer assembly rods into a new assembly according to a different arrangement or by combining them with new or other rods (for example containing a neutron absorbent or an inert material).

 Methods and installations are already known for reconstituting nuclear fuel assemblies making it possible to dismantle an assembly and to reconstitute one with all or part of the rods contained in the assembly which has been dismantled. In patent application FR-A-2 553 226 there is described a method of reconstitution of an assembly in which there is an assembly from which fuel rods have to be extracted in a first vertical cell, the upper end piece is removed, the above the first cell and in alignment with it a second cell containing and retaining a new assembly skeleton in the same orientation as the assembly of the first cell and the pencils to be extracted from the assembly are drawn into the new skeleton. The skeleton is then completed by fitting end pieces. This process and the installation which implements it have numerous advantages; however, it is only applicable to an assembly of which the pencils are provided in the upper part with a groove making it possible to ensure good grip of the pencil by the clamp having to ensure its extraction.

 All the assemblies of the prior art are not provided with such pencils, these having not been intended initially to be handled under such conditions.

 The invention aims in particular to remedy this shortcoming of the prior art, and provides a method and an installation making it possible to reconstitute a nuclear fuel assembly by transfer, to a skeleton, of fuel rods not initially intended to be handled and not not having, in particular, a gripping groove in their upper part. The process for transferring these pencils, generally highly irradiated and often damaged, uses only simple means and presents only a very low risk of errors or breakdowns.

To this end, the invention proposes in particular a method for reconstituting a nuclear fuel assembly of the type defined above, method according to which
an assembly is put in place having a removable end-piece and from which the fuel rods must be extracted in a first vertical cell, the removable end-piece of said assembly being directed upwards,
said removable end piece is removed,
a new skeleton of assembly stripped of its end pieces is put in place in a second cell retaining said skeleton and in the same vertical orientation as the assembly of the first cell,
the fuel rods of the assembly placed in the first cell are transferred into the new skeleton and, when the new skeleton is completely refilled with fuel rods,
we put in place the tips of said skeleton, characterized in that
before placing the pencils in the new skeleton, said new skeleton is loaded with inert false pencils whose upper ends have a recess of a shape complementary to those of the lower ends of the pencils of the assembly of the first cell and whose lengths are greater than twice the distance between two grids, each pencil to be transferred is removed from the assembly of the first cell and it is lowered into the new skeleton by pushing the false pencil vertically with said combustible pencil, the lower end of said pencil transfer coming into abutment on the head of said false pencil which provides centering during the positioning, and this until the pencil to be transferred assumes its final position by ejecting said false pencil.

Advantageously, and to complete the new skeleton containing the transferred pencils, the following operations are also carried out
the first end piece is fixed on the upper part of the tie rods of the reconstituted assembly, then a plug is fixed on the upper opening of the cell containing said reconstituted assembly, the said cell is turned over and the second end piece of said reconstituted assembly is fixed.

 Advantageously, there is also a mask for guiding the false crayons to be ejected on the lower part of the second cell before inserting the crayons to be transferred into the new assembly skeleton.

 The invention also proposes an installation making it possible to implement the method defined above; this installation is advantageously supplemented by means also making it possible to repair an assembly of which certain pencils are to be replaced.

 The invention will be better understood on reading the following description of an installation which constitutes a particular embodiment of the invention, and of the process used in this installation, given by way of non-limiting example .

The description refers to the accompanying drawings in which
- Figure 1 is a simplified view of a fuel assembly for a light water reactor capable of being reconstituted according to the method of the invention.

FIG. 2 is a partial exploded perspective view of a pencil belonging to the assembly of FIG. 1,
FIG. 3 is a simplified view partially in section of an installation according to the invention,
FIGS. 4 and 5 show in section the gripping pliers used to extract the pencils from the assembly contained in the first cell according to the invention, respectively with and without pencil,
FIG. 6 is a partial view of a false pencil according to the invention,
FIG. 7 is a partial section view of the mask for guiding the false pencils and of the receptacle of said false pencils placed under the cell containing the new assembly skeleton to be filled according to the invention,
- Figure 8 is a block diagram showing the different steps followed for the implementation of the method according to the invention.

 Figure 1 shows a partial view of a fuel assembly 1 for reactor & light water. This assembly 1 comprises an upper end piece 2 and a lower end piece 3 connected by a structural skeleton constituted by grids 4 and tubes or tie rods 5 fixed to said end pieces 2 and 3 in a removable manner, for example by screws 6.

 The grids 4 moreover hold the rods 7 in place, an exploded perspective view is given in FIG. 2.

 Each pencil consists of a sheath 8 inside which are stacked pellets 9 of uranium oxide enriched in uranium 235 and is terminated on each side by an upper plug 10 and a lower plug 11. A expansion 12 containing a spring 13 is provided at the head of the rod in order to allow differential expansion between the interior of the sheath and the sheath itself during the stay in the reactor.

 The plugs 10 and 17 of the pencils shown in FIG. 2 do not have a groove suitable for allowing easy gripping thereof and above all allowing, when they are gripped by pliers, to exert on the entire plug and the pencil a significant tensile force (such as that which would be necessary to pull the pencil through a new skeleton).

 FIG. 3 shows an installation according to the invention for reconstituting a nuclear fuel assembly of the type described above, and this schematically.

 This installation comprises a first cell 20 for receiving the fuel assembly 21, the upper end of which has been removed. FIG. 3 shows an assembly comprising seven spacing grids, but this figure is in no way limiting. The assembly can in particular comprise only six grids.

The cell 20 comprises means 22 for immobilizing the skeleton of the assembly 21. It has in the lower part a shim 23. A piece 24 for centering and guiding the head of the assembly is removably placed on the upper part of the cell 20.

The installation further comprises a second cell 25 for receiving a new skeleton 26 of fuel assembly, devoid of nozzle, placed near the first cell and provided with means 27 for immobilizing the new skeleton 26 of assembly in said cell 25.

 The two cells 20 and 25 are themselves supported by a structure 28 which keeps them in a manner known per se in a vertical position at the bottom of the pool 29 in which the reconstitution of the nuclear fuel assembly takes place.

 The transfer means 30 constituted. by a gripper 31, vertical transfer means 32 controlled by an automaton 33 actuable by a control console 34 operated by an operator located for example at the level of the rolling beam of the means 30 are provided.

 These transfer means 30 are suitable for extracting one by one the fuel rods 35 from the assembly 21 arranged in the first cell 20 and for introducing them into the new skeleton 26 of the fuel assembly placed in the second cell 25.

 The gripper 31 of the fuel rods which, as we have seen, are not provided with a gripping groove but on the contrary have a smooth plug whose upper part is frustoconical, is for example constituted as appearing in FIGS. 4 and 5 .

 FIG. 4 shows the gripper 31 for gripping the pencils to carry out a transfer according to the invention.

 It comprises a sleeve 40 in which a coaxial piston 41, which ends in the lower part by a deformable cap 42, can move. The cap 42 is provided with a bulged edge 43 capable of coming into abutment with a lower part 44 of the sleeve 40. When a vertical upward pulling force is exerted on the piston 41, the protrusion 43, coming s 'pressing against the lower part 44, tightens the cap on the pencil, thereby pinching the cap and therefore gripping the pencil with the clamp 31. By pushing the piston 41 downwards, inside the sleeve 40 , the pencil is however released, the clamp coming into the open position as indicated in FIG. 5. It is well understood that with such a clamp, the tensile forces which can be exerted on the pencil which has been grasped cannot not be very large. The risk of jamming a pencil during its extraction without being able to take it back, risk that would be encountered with a device such as that of the aforementioned prior art, would therefore be too great.

 Guide means 50 for the rods 35 extracted from the first assembly and introduced into the new skeleton 26 of the second assembly are provided. They include a removable centering piece 51 and inert dummies 52. The false pencils (FIG. 6) each have an upper end having a recess 53, constituted by a chamfered cavity in the upper part of a shape complementary to that of the end of the plug II of the pencils 35 of the assembly of the first cell, so to ensure mutual centering of one by the other when one comes into abutment with the other. The false pencils 52 are at least greater than twice the length existing between two grids 54 of the new skeleton 26.They are terminated at the bottom by a nose 55 suitable for guiding and centering the false pencil 52 when it is introduced in the openings or cells for receiving and holding pencils practiced in a known manner in the successive grids of the assembly which is reconstituted.

 Means 56 for recovering the false pencils when they have been ejected after placing the pencils in the new skeleton are provided under the cell 25. These means 56 comprise a recovery tank 57 provided with housings 58 into which the ejected pencils. These housings are for example constituted by holes of diameter slightly greater than that of the rods and drilled in spacer plates 59 spaced longitudinally at a distance identical to that separating two grids of the reconstituted assembly. The holes 58 are in identical number to the number of rods of the reconstituted assembly, for example 179 in the case of a distribution of the rods according to a 14 x 14 network with a square pitch, and distributed according to the same arrangement plumb pencils of said assembly. The receptacle 57 is provided with a bottom 60 on which the false pencils abut.

The receptacle 57 is also provided with an upper part 61 for centering with respect to the cell 25 and the assembly structure 26 which it contains.

The means 56 can be movable, for example by means of a carriage 62 on which they are fixed and which can be actuated via the area of hydraulic cylinders 63 or any other means known per se
A guide mask 64 makes it possible to guide the false rods when they descend into the receptacle 57.

In a preferred solution of the invention, as shown in FIG. 7, the guide mask 64 is fixed to the lower part 65 of the cell 25 by means of screws 66 put in place when the cell 25 is returned 180 *. The mask has passages 67 for guiding the false rods corresponding to the location of these in the assembly and arranged opposite the housings 58 of the receptacle 57.

 In FIG. 7, the mask is integral with the cell 25 in which a new assembly structure 26 appears (mixed line). A pencil 21 and a false pencil 52 have also been shown in dotted lines. The lower grid 70 is arranged opposite the passages 67 provided for guiding the pencils by the mask 64. Legs 71, integral with the mask 64, enclose the grid 70. The upper guide part 61 of the receptacle is centered on the mask 64 and the socket 25 by means of guide pins 72.

 The operation of the installation as described and the method that it implements is set out below. For clarity, reference will also be made to FIG. 8 giving schematically from a to h the steps followed.

 With a conventional clamp 80 for gripping the assembly mounted on a bridge, the assembly 21 is put into place in the first cell 20 (step g).

 The upper end piece 71 is then removed (step b) by means known per se (milling of the welding of the guide tubes after centering of the skeleton for example).

This operation can be carried out at a lower pool depth than the following operations of the assembly reconstitution process provided that a sufficient level of biological protection water is maintained between the operators and the fuel.

 After various additional preparation operations (for example setting up a pre-piece 24 on the top of the cell 20 so as to facilitate guiding during extraction), the assembly is then ready to be disassembled.

 Step ç consists in preparing the new assembly structure 26 by introducing the dummies 52 therein manually or automatically. If necessary, new identical or other pencils are also placed there (for example pencils from other assemblies, new pencils comprising absorbent material or fissile material enriched differently, etc.). If there is no risk of contamination and irradiation, this operation can advantageously be carried out dry.

 The new assembly structure 26 is then introduced into the second cell 25 (step d) after placement under said cell of the receptacle 57 for recovering false rods and fixing of the guide piece 64 on the cell 25.

 Step e is the actual reconstitution step. Successively, one by one, each pencil 35 is extracted from the assembly 21, it is transferred by means of the clamp 31 to above the new assembly structure 26 placed in the second cell 25 and we introduce the pencil 35 by pushing the corresponding false pencil 52 to the final position of said pencil 35 in the new assembly structure. A map of the position of the pencils in the assembly, memorized by the automaton 33, allows the positioning of each pencil to be carried out without error. Lighting and display means (cameras) allow control of the x, y, z handling of the clamp by the operators during operation in manual, semi-automatic or automatic mode chosen for the reconstitution of the assembly. false pencil 52 falls into the recovery tank 57 located below the cell 25 while being guided by the part 64.

 The operation is repeated until the new assembly structure has been completely filled.

 Steps f, q, k give the end of the process of reconstituting a fuel assembly according to the method described in the present invention. After repeating step e until complete reconstitution of a new assembly, an upper end piece 82 is placed on the reconstituted assembly (step f), then a plug 83 on the cell 25 (step q). The cell 180 is then turned over so as to have access to the lower part of the reconstituted assembly, the part 64 is removed, and the second assembly end piece 84 is fixed (step h). The assembly is then completed.

 The inversion of the cell is done in a manner and by known means such as those which have been described in the document FR-A-2 553 226.

 As is obvious and as it follows from the above, the present invention is in no way limited to the description more particularly described in the text above. On the contrary, it embraces all variants.

Claims (6)

 1. A method of reconstituting a nuclear fuel assembly of the type comprising a skeleton formed by two end pieces (23) connected by tie rods (5) fixed to the two end pieces and grids (4) distributed between the end pieces along the tie rods, and a bundle of fuel rods (7, 35) held by the skeleton between the ends, method according to which  an assembly (21) is put in place, the fuel rods (35) of which must be extracted in a first vertical cell (20), the removable end piece (81) of said assembly being directed upwards,  said removable end piece (81) is removed,  a new skeleton (26) of assembly stripped of its end pieces is placed in a second cell (25) retaining said skeleton and in the same vertical orientation as the assembly (21) of the first cell (20),  the fuel rods (35) are transferred from the assembly (21) placed in the first cell in the new skeleton (26) and, when the new skeleton (26) is fully supplied in fuel rods (35),  the end pieces (82, 84) of said skeleton are put in place, characterized in that  before placing the pencils (35) in the new skeleton (26), said new skeleton (26) is loaded with false inert pencils (52) whose upper ends have a recess (53) of shape complementary to that of the lower end (11) of the rods of the assembly (21) of the first cell (20) and which are of length greater than twice the distance between two grids (4), each of the rods (35) to be transferred is extracted of the assembly of the first cell and it is lowered into the new skeleton by vertically pushing the corresponding false pencil (52) with said pencil (35) to be transferred, the lower end (11) of said pencil (35) to be transferred coming abutment on the head of said false pencil (52) which ensures centering during the positioning, until said transfer pencil (35) assumes its final position by ejecting said false pencil.  2. Method according to claim 1, characterized in that, to complete the skeleton (26) containing the transferred pencils (35), the first end piece (82) is fixed on the upper part of the tie rods of the reconstituted assembly, it is fixed then a plug (83) on the upper opening of the cell (25) containing said reconstituted assembly, said cell (25) is turned over and the second end piece (84) of said reconstituted assembly is fixed.  3. Method according to claim 1 or 2, characterized in that there is a guide mask (Ç4) of the false pencils (52) to be ejected on the lower part of the second cell (25) before inserting the pencils (35 ) to be transferred to the new assembly skeleton (26).  4. Installation for reconstituting a nuclear fuel assembly of the type comprising a skeleton formed of two ends (2,3) connected by tie rods (5) fixed to the ends and grids (4) distributed between the ends along the tie rods and intended to hold a bundle of fuel rods (7, 35) trapped between the ends, installation comprising a first cell (20) for receiving the fuel assembly (21) provided with means for immobilizing (22) the skeleton of assembly in said cell1  a second cell (25) for receiving a new fuel assembly skeleton without a nozzle, placed near the first cell and provided with means (27) for immobilizing the skeleton (26),  transfer means (30) suitable for extracting one by one the fuel rods (35) from the assembly (21) arranged in said first cell (20) and introducing them into the new skeleton (26) of fuel assembly placed in the second cell (25), characterized in that the installation also comprises,  guide means (50) of said rods during their introduction into said skeleton comprising inert false rods (52) whose upper ends have a recess (53) of a shape complementary to that of the lower end (11) of the rods the assembly of the first cell so as to ensure mutual centering and of length at least greater than twice the distance between two grids of said skeleton, and  recovery means (56) of these so-called false pencils.  5. Installation according to claim 4, characterized in that it further comprises a guide mask (64) of the false pencils suitable for being fixed on the lower part of the second cell (25) to guide the false pencils (52) during of their ejection.  6. Installation according to claim 4 or 5, characterized in that each false pencil (52) has an upper cavity (53) chamfered and a lower nose (55) of substantially conical shape suitable for guiding and centering said false pencil when is introduced into an assembly structure grid cell.