A METHOD FOR PROTECTING THE GUIDE TUBE MEMBERS OF A NUCLEAR FUEL ASSEMBLY AND GUIDING PIN FOR CARRYING OUT THE METHOD

Abstract: A method for protecting the guide tube members (2) of a nuclear fuel assembly during removing and reinstalling of the top tie plate of the fuel assembly and during intermediate stages, said guide tube members (2) having a vertical centre axis (1) and a circular cross-section, and being each inserted, with a slitted upper end portion (2′), in a withdrawable manner into a respective, substantially circular-cylindrical through-hole (4) in said top tie plate (3), said slitted end portion having an annular protrusion (5) adapted to snap into an annular slot (6) provided in the wall of said through-hole (4). For removing the top tie plate from the guide tube members, according to the in­ vention a guiding pin (8) is inserted from above into each said through-holes (4), said guiding pin being formed with a lower portion (8′), an upper portion (8″), and an intermediate, rela­ tively thin portion (8‴), said upper portion (8″) and said lower portion (8′) being dimensioned to be inserted with slip fit into said through-hole (4) and into said guide tube member (2), re­ spectively, said intermediate portion (8‴) having a diameter that is by so much smaller than the inner guide tube diameter (d1) as to allow the adjacent upper portion (8″) to be formed with an annular, downwardly facing surface (18) with an annu­ lar, downwardly facing slot (9). The insertion of the guiding pin (8) from above into said through-hole (4) is blocked by a con­ tact made between said downwardly facing surface (18) of the guiding pin (8) and the upwardly facing surfaces of a plurality of axially directed fingers (10) formed by said slitted end portion of the guide tube member (2) located in the through-hole. When thereafter said top plate (3) is lifted, said fingers (10) - because said annular protrusion (5) leaves said annular slot (6) - are bent radially inwardly and slip into said annular, down­ wardly facing slot (9). They are held here in position until the top tie plate is re-applied.

Description: A method for protecting the guide tube members of a nuclear fuel assembly The invention relates to a method for protecting the guide tube members of a nuclear fuel assembly according to the precharacterising part of claim 1. A nuclear fuel assembly for which this method can be used is known, for example, from FR-B-8 211 575 (publication No. 2 529 704). In the fuel assembly disclosed in the afore-mentioned publication the slitted end portions of the guide members are very vulnerable. By the slitting of said end portion a plurality of axially directed flaps are formed, which can easily be broken when the slitted portion is not in its place in the top tie plate, that is, during removal and reinstallation of the top tie plate, and at intermediate stages. The invention aims at developing a method of the above-mentioned kind which provides good protection for the axially directed flaps or fingers of the guide tube member during removal and reinstallation of the top tie plate and during intermediate storage of a guide tube member without demanding time consuming special measures having to be taken. In order to achieve this aim the invention suggests a method according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. A guiding pin for carrying out the method according to the invention is characterized by the features of claim 2. The invention will now be described in greater detail with reference to the accompanying drawings showing in Figures 1-5 a guide tube member in partial vertical section through the centre axis of a substantially cylindri cal through-hole in a top tie plate included in a nuclear fuel assembly designed for a pressurized wa ter reactor, whereby the different figures show in numerical order for one and the same guide tube mem ber the consecutive stages during the removal and reinstallation of the top tie plate; Figure 6 in vertical section along line VI-VI in Figure 7 an upper end portion of the same guide tube member prior to removal or after reinstallation of the top tie plate; Figure 7 a partial horizontal section along line Vil-Vil in Figure 6. In the drawings, 3 designates a top tie plate in a fuel assembly for a pressurized water reactor. The top tie plate has a plurality of substantially cylindrical through-holes 4 with a vertical centre axis 1. A guide tube member,2 for a control rod pin is inserted with an upper end portion into each through-hole 4. The guide tube member 2 has a vertical centre axis 1' and contains a guide tube 11 with an inner diameter dl, and a top sleeve 12. The lower top sleeve portion surrounds the upper end portion of the guide tube 11 and is fixed thereto. In normal condition the top sleeve 12 is inserted into a through-hole 4 provided in the top tie plate 3. The substantially circular-cylindrical wall 4' of this through-hole is provided which a horizontal, annular slot 6. The upper end portion of the top sleeve 12 is provided with six vertical slits 13 being substantially evenly distributed along the circumference of the sleeve. The sleeve portions which are located between the slits have a substantially constant wall thickness and form six resilient fingers 10. Each finger 10 has a circle arc-shaped, radial projection 14 with a radially outwardly facing surface of a convex, double-curved shape and a radially inwardly facing surface of a concave, double-curved shape. Together, these projections 14 form a substantially annular sleeve portion 5, divided into annular segments having a double-curved, convex, radially outward facing surfaced and a doublecurved, concave radially inward facing surface. When inserting a guide tube member 2 into a corresponding through-hole 4, the fingers 10 are first bent resiliently inwardly and then - when their circle arc-shaped projections arrive at the level of the annular slot 6 - move back radially outwardly thus causing the above-mentioned, substantially annular portion 5 of the top sleeve 12 - which portion is divided into annular segments and has a relatively large outer diameter - to snap into the slot 6. The sleeve portion 5, preferably the entire top sleeve 12, has a substantially constant wall thickness. The top sleeve 12 is made from a thin-walled, metallic tube material, for example a tube produced by rolling or drawing. For the purpose of retaining the annular sleeve portion 5 of the top sleeve 12 in the slot 6, a metallic, substantially hollow-cylindrical locking sleeve 7 with a substantially constant wall thickness is inserted into the top sleeve 12 without any significant play. The locking sleeve 7 is fixed in the axial direction by means of at least one bulging or projecting part which is arranged in engagement with the concave side of the annular sleeve portion 5 of the top sleeve 12. The projecting part of the locking sleeve 7 may be annular or may consist of at least one wart embossed in the wall of the locking sleeve 7. In the embodiment shown in Figures 6 and 7, three radially outwardly directed warts 15 are embossed in the wall of the locking sleeve 7 by means of an expansion tool which is inserted into the locking sleeve 7 when the latter has been inserted into the top sleeve 12. The locking sleeve 7 may be made from a material the ductility of which is either considerably higher than or equal with the ductility of the material from which the top sleeve 12 is made. In the latter case the locking sleeve may consist of the same material as the top sleeve 12, with the average wall thickness of the locking sleeve being chosen preferably smaller than 80 % of the average wall thickness of the top sleeve 12. When the top tie plate 3 is to be removed - for example, for the purpose of inserting new fuel rods - the locking sleeve 7 must first be withdrawn. This involves a destructive deformation of the locking sleeve 7, whereas the top sleeve 12 remains intact. After the top tie plate 3 has been re-applied, a new locking sleeve 7 is inserted and expanded. The withdrawal of the locking sleeve is performed by means of a tool having a plurality of withdrawal hooks and being inserted into the locking sleeve 7 in such a way that the withdrawal hooks engage with the lower end surface of the locking sleeve 7. In a method according to the invention, the locking sleeve 7 in each one of the guide tube members 2 is first removed. Thereafter, a corresponding guiding pin 8 is inserted into each of the holes 4, said guiding pin being oriented in the direction of the guide tube 11. The guiding pin 8 has a lower circular-cylindrical portion 8', an upper circularcylindrical portion 8", and a relatively thin, intermediate circular-cylindrical portion 8"', which is positioned immediately adjacent to the upper portion 8". The portion 8"'has an axial length which is preferably longer than the thickness H of the top tie plate 3 and shorter than 3H. Further, the guiding pin 8 is formed with a circular-cylindri cal flange 16, located immediately below the relatively thin portion 8"'. The portions 8", 8' and the flange 16 are dimensioned with such maximum diameters as to be able to be inserted with slip fit into the hole 4, into the guide tube 11, and into the top sleeve 12, respectively. A circularcylindrical part 17 of the guiding pin 8, located between the flange 16 and the lower portion 8', has a diameter which is somewhat smaller than the maximum diameter of the lower portion 8' but larger than the diameter of the relatively thin portion 8"'. The upper portion 8" has a horizontal annular end surface 18, in which an annular, downwardly-facing slot 9 is provided. The radially inner wall of this slot 9 consists of an extension of the circular-cylindrical surface of the relatively thin portion 8"'. During the above-mentioned insertion of a guiding pin 8 into each one of the through-holes 4 (see Figures 1-5), the downwardly-facing annular surface 18 of each guiding pin is first brought into contact with the upper end surfaces of the fingers 10. Thereafter, the top tie plate 3 is lifted. This movement causes the fingers 10 of each guide tube member 2 to be bent resiliently radially inwards by the fact that the annular protrusion 5 is forced out of the slot 6. When the contact between the surface 18 and the end surfaces of the fingers 10 is broken, the guiding pin 8 falls down, and the end portions of the fingers 10 slip into the annular slot 9 (Figure 2). The fingers 10 maintain this position in relation to the slot 9 during the continued lifting of the top tie plate 3 to a level above the upper ends of the guiding pins 8, and also after the top tie plate 3 has been removed as well as during subsequent corrective maintenance for example, exchange of fuel rods - and as well during the reinstallation of the top tie plate 3. Thus the delicate slitted part of each of the top sleeves 12 is well protected against contact with the lifting devices used during the repair work and the components handled by said lifting devices. During reinstallation, the top tie plate 3 is lowered down towards all the guide tube members 2 while at the same time these members are to be guided into a respective through-hole 4. This is done without difficulty since each guiding pin 8 - above the slot 9 - tapers in the direction towards its upper end. When the top tie plate during its reinstallation has been brought approximately into its normal position in relation to the guide tube members 2 - as shown in Figure 4 - all the guiding pins 8 are removed, whereafter the radially outwardly protruding sleeve portion 5 of each guide tube member 2 snaps into a corresponding slot 6. Finally, a new locking sleeve 7 is inserted into each one of the guide tube members 2 and is fixed in the same way as the original one.

Claims: CLAIMS 1. A method for protecting the guide tube members (2) of a nuclear fuel assembly during removing and/or reinstalling of the top tie plate of the fuel assembly and/or during intermediate stages, said guide tube members (2) having a vertical centre axis (1) and a circular cross-section, and being each inserted, with a slitted upper end portion (2'), in a withdrawable manner into a respective, substantially circular-cylindrical through-hole (4) in said top tie plate (3), said slitted end portion having an annular protrusion (5) adapted to snap into an annular slot (6) provided in the wall of said through-hole (4), said end portion being locked in said snapped position by means of a locking sleeve (7) inserted in the end portion, c h a r a c t e r i z e d in that for removing the top tie plate from the guide tube members the locking sleeves (7)are first removed, that thereafter a guiding pin (8) oriented in the direction of the guide tube (11) is inserted from above into each one of said through-holes (4), said guiding pin being formed with a lower portion (8'), an upper portion (8"), and an intermediate, relatively thin portion (8"') located immediately adjacent to said upper portion, said upper portion (8") and said lower portion (8') being dimensioned to be inserted with slip fit into said through-hole (4) and into said guide tube member (2), respectively, said intermediate portion (8"'), at least in the vicinity of said upper portion, having a diameter that is by so much smaller than the inner guide tube diameter (d1) as to allow the adjacent upper portion (8") to be formed with an annular, downwardly facing surface (18) in which an annular, downwardly facing slot (9) is provided large enough to allow the insertion of said slitted upper end portion 2', that the insertion of the guiding pin (8) from above into said through-hole (4) is blocked by contact made between said downwardly facing surface (18) of the guiding pin (8) and the upwardly facing surfaces of a plurality of axially directed fingers (10) formed by said slitted end portion of the guide tube member (2) located in the through-hole, that thereafter said top tie plate (3) is lifted and said fingers (10) - because said annular protrusion (5) leaves said annular slot (6) - are bent radially inwardly and are inserted with their ends into said annular, downwardly facing slot (9) and are held in position in said slot during the continued removal of the top tie plate (3) in relation to said guide tube member (2). 2. A method according to claim 1, c h a r a c t e r i z e d in that, after the top tie plate has been removed, the guiding pins are further held in their position relative to their corresponding guide tube members (2) until the reinstallation of the top tie plate and up to the point where a normal relation between the top tie plate (3) and the guide tube members (2) is restored, that the guiding pins (8) are then removed, and that each one of said slitted guide tube portions (2') is again provided with a corresponding locking sleeve (7). 3. A guiding pin for carrying out the method according to claim 1 or 2, c h a r a c t e r i z e d in being formed with a lower portion (8'), an upper portion (8"), and an intermediate, relatively thin portion (8"') located immediately adjacent to said upper portion, said upper portion (8") and said lower portion (8') being dimensioned to be inserted with slip fit into said through-hole (4) of the top tie plate and into said guide tube member (2), respectively, said intermediate portion (8"'), at least in the vicinity of said upper portion, having a diameter that is by so much smaller than the inner guide tube diameter (d1) as to allow the adjacent upper portion (8") to be formed with an annular, downwardly facing surface (18) in which an annular, downwardly facing slot (9) is provided large enough to allow the insertion the slitted upper end portion 2' of the guide tube member.

Inventors: ALMEN, OLLE, COLLIN, PER, Almén, Olle
Applicants: AB ASEA-ATOM
Application year: 1985
Main classification: G21C3
IPC codes: G21C3, B25B27