GB2180091A - Apparatus for use in refuelling nuclear reactors - Google Patents

Abstract

A fuel element assembly has a tie rod (4) which is arranged to pass vertically downwards through the fuel element assembly and engage an assembly bottom support (8), the tie rod and support having cooperating engagement surfaces which can be automatically released and re-engaged, thereby enabling the tie rod to be removed from the fuel element assembly when the latter is placed within the reactor core, and subsequently reintroduced when it is desired to remove the assembly from the core. <IMAGE>

Description

SPECIFICATION Apparatus for use in refuelling nuclear reactors The invention relatesto apparatusforuse in refuelling nuclear reactors, being especially, but not exclusively, concerned with refuelling nuclear reactors of the Advanced Gas Cooled type, and providing a method of loading new fuel into, and removing irradiated fuel out of, the nuclear reactor core.

In Advanced Gas Cooled Reactors the nuclearfuel is handled in the form of a fuel assembly,consisting of a plug unit and a fuel stringer,thefuel stringerconsisting of a top reflector element, a vertically aligned stack offuel elements, a bottom reflector element and a fuel stack bottom support, the fuel stringer3 being supported from the plug unit 2 by means of a tie rod 4which passes through the assembly and engages the bottom support 8.

Apparatus of this form may be used also with other types of nuclear reactors, and such apparatus will hereinafter be called apparatus ofthe kind referred to.

An object ofthe invention isto provide a refuelling system ofthis kind in which the tie rod is withdrawn from the fuel assembly during its residence in the reactor core and is reinserted prior to fuel assembly discharge, thus avoiding the need to rely on the in tegrityofan irradiated tie rod during fuel handling operations and providing an opportunity to proof load the tie rod subsequentto its insertion into the fuel assembly.

According to the invention, in apparatus of the kind referred to for use in refuelling nuclear reactors the bottom support and co-operating tie rod incorporate co-operating engagement surfaces which are urged into gripping engagement to causethetie rod to supportthefuel assembly on introduction into a recess in the bottom support, a relative movement between the tie rod and bottom support resulting in disengagement of the tie rod from the bottom support, to permit its withdrawal from the assembly, and the tie rod and bottom support being arranged to reengage two permitthe tie rod to supportthefuel assembly on re-introduction into said recess.

Preferably the engagement surfaces ofthe tie rod and the bottom support are urged into frictional engagement sufficient to enable the tie rod to support the fuel assembly on introduction into said recess.

Preferably also one ofthe engagement surfaces is provided by a cam which is urged into gripping engagementwith the other engagement surface on introduction of the tie rod into said recess, the relative movement ofthe tie rod and bottom support causing the shearing of a pin which effects the release of the tie rod from the recess and the cam being arranged to engage a second engagement surface ofthe tie rod orthe bottom support on reintroduction ofthetie rod into the recess.

Thus the bottom support may incorporate a cam arrangement initially restrained by the shear pin and having a surface urged into a gripping engagement with a part ofthe tie rod of a relatively small cross section, the application of an increased axial load on the tie rod relative to the assembly producing an increased pressure on the shear pin, sufficientto cause the pin to shearand thecam arrangementto release the tie rod, the tie rod having a region of relatively largerdiameterwhich, on the re-introduction ofthe rod into the recess, is itself frictionally engaged by the cam to an extent which permits the tie rod to raise the fuel assembly.

Afeature ofthe invention is the provision of a route through the plug unit such that, with the plug unit engaged with the top of the fuel stringer, a tie rod guide tube carried by the plug unit is axiallyaligned with the guide tubes carried by the fuel elements and reflector elements. The fuel stack bottom support also includes a guidance feature, axially aligned with the tie rod guide tubes, into which the lower end of the tie rod is arranged to be inserted and locked to the bottom support.

Preferably the plug unit is provided with a tie rod support into which a co-operating feature at an appropriate position on the tie rod engages, the purpose of the tie rod support being to transferthe load of the fuel stringer, when suspended on the tie rod, through to the plug unit body, the tie rod support conveni- ently being mounted on springs which maintain the tie rod in tension whilst its lower end is locked within the fuel stack bottom support.

One form ofapparatus in accordance with the invention for use in refuelling an advanced gas cooled nuclear reactor will now be described by way of example with reference to Figures 1 to 4 of the accompanying schematic drawings, in which Figure 1 illustrates a sectional view india- grammatic form, of a fuel element assembly embodying the invention, and Figures2to4illustrate in an enlarged form, part of apparatus in accordance with the invention illustrating its manner of operation in handling the fuel element assembly illustrated in Figure 1.

Referringfirstto Figure 1 the fuel assembly 1 con sistsofa plug unit 2 arranged to provideasupportfor a fuel stringer 3 as will subsequently be described, the fuel stringer comprising a top reflector element5, a vertically arranged stack of fuel elements 6, a bottom reflector7 and afuel stack bottom support 8.

The plug unit2 incorporates a guide tube 9carrying at its lower end a tie rod support 12 in theform of an annulardisc,the central opening of which is in registerwith the bore ofthe guidetube 9. Thetop reflector element also carries a guide tube 11 disposed coaxiallywith the plug unitguidetube 9, and each of the fuel elements 6, as well as the bottom reflector7, also have guide tubes 10 which align with one an other and the guide tubes 9,11.

The bottom support8 also has a cylindrical recess which aligns with the guide tubes. In ordertosupport the assembly, a tie rod 4 is arranged to be lowered through the respective guide tube 9,10,11 until its lower end is inserted into the recess 18 in the bottom support 8. The top ofthe plug unit guide tube 9 is provided with an access plug 13 to maintain the reactor pressure boundary or containment.

The bottom support 8 incorporates within it a locking unitcomprising a double cam 15,17,initially restrained by a shear pin 14 and located in a cam housing 16 at the side of the recess 18 of the fuel stack bottom support 8. The diameter of a section 24 at the bottom end ofthe tie rod 4 is reduced to less than that ofthe section immediately above it, and initiallythe reduced diameter section of the tie rod 4 is inserted into the fuel stack bottom support 8, through the fuel assemblytie rod guide route, by an apparatus located in an associated tie rod insertion machine (not shown); this engages the inner cam 15 and deflects it against a spring load (also not shown), the inner cam 15 being restrained by the cam housing 16 and the outer cam 17 by the shear pin 14, such thatthe larger section ofthe tie rod 4 may 4may not be inserted beyond the cam arrangement, as shown in Figure 2. The tie rod 4 is retained within the cam arrangement by the resultantfrictional reaction to the tie rod tension, the latter being maintained initially by the spring loaded tie rod support 12 and subsequently, during fuel assembly handling operations, by the fuel stringer 3 weight itself.

To release the tie rod 4, the tension is increased by an apparatus which applies an axial load tothetie rod 4, sufficientto cause the shear pin 1 4to fail, the apparatus being located in the tie rod insertion machine, the cams being constrained bythe cam housing 16 in the configuration shown in Figure 3, such that the tie rod4 may be withdrawn.

To re-engagethe locking mechanism, the tie rod 4 is inserted to engage its larger diameter section in the cam system, as shown in Figure 4.

The tie rod insertion machine seals to the top ofthe plug unit 2, extending the reactor pressure boundary, when the tie rod access plug 13 is removed by a mechanism loacted in the tie rod insertion machine; the plug unit 2 being located in the reactor pressure vessel.

A mechanism,located within the tie rod insertion machine, positions a tie rod 4 above, and axially aligned with,the plug unit tie rod guide tube 9, and lowers it through the fuel assembly 1 until a feature provided on the upper end ofthe tie rod 4 engages in an appropriate feature in the tie rod support 12, the lower end ofthe tie rod 4 being provided with a tapered end section to ensure that it is adequately guided between the various guide tubes constituting the fuel assembly tie rod insertion route.

The mechanism continues to lowerthetie rod 4 against the spring load provided by the tie rod support 12, until such time asthetie rod4engages in the fuel stack bottom support 8.

The mechanism is also capable of applying a proof load to the tie rod 4 prior to fuel handling operations, the lower end of which is locked within the fuel stack bottom support 8; the proof load being in excess of the normal and maximum anticipatedfauitloads.

The system described, therefore, facilitates insertion and withdrawal ofthe tie rod 4 priorto, and immediately following fuel assembly 1 handling oper ations, ensuring thatthetie rod 4 itself is in an essentially unirradiated condition when supporting the fuel stringer 3 load. In addition, the tie rod 4 may be proof loaded, priortofuel assembly 1 handling operations, the proof load being in excess of both normal and maximum anticipated fault loads; the system as whole providing a high degree of structural integrityforfuel assembly 1 handling operations.

However the tie rod engagement in the fuel stack bottom support 8 may be effected in any convenient manner which recog nises the basic requirements of the system. Firstly, the tie rod 4 must be capable of locating in, and engaging with the fuel stack bottom support8 with a high degree of reliability, the integrity offuel handling operations depending largely on thisfeature. Secondly, the tie rod 4 must be capable of disengaging from the fuel stackbottomsup- port 8, prior to withdrawal of the tie rod 4from the reactor core. Finally, the tie rod 4 must beableto reengage in the fuel stack bottom supportS, within the reactor core, as a precursorto irradiated fuel discharge.

Although the locking mechanism provided by the double cam arrangement is shown located within the fuel stack bottom support it may in some cases be located within the tie rod itself; or alternatively, both the fuel stack bottom support 8 and the tie rod may incorporate mechanisms which co-operate to achieve the required locking and unlocking functions.

Claims (9)

1. Apparatus ofthe kind referred to for use in refuelling nuclear reactors, wherein the bottom support and co-operating tie rod incorporate cooperating engagement surfaces which are urged into gripping engagement to cause the tie rod to support the fuel assembly on introduction into a recess in the bottom support, a relative movement betweenthetie rod and bottom support resulting in disengagement ofthetie rod from the bottom support,to permit its withdrawal from the assembly, and the tie rod and bottom support being arranged to re-engage to permit the tie rod to support the fuel assembly on re-introduction into said recess.

2. Apparatus according to Claim 1 wherein the engagement surfaces ofthetie rod and the bottom support are urged into frictional engagement suf- ficientto enablethetie rod to supportthefuel assembly on introduction into said recess.

3. Apparatus according to Claim 2 wherein one of the engagement surfaces is provided by a cam which is urged into gripping engagement with the other en- gagementsurface on introduction ofthetie rod into said recess, the relative movement ofthe tie rod and bottom support causing the shearing of a pin which effects the release of the tie rod from the recess and the cam being arranged to engage a second engage ment surface of the tie rod or the bottom support on reintroduction ofthetie rod into the recess.

4. Apparatus according to Claim 3 wherein the bottom support incorporates a cam arrangement initially restrained bythe shear pin and having a surface urged into a gripping engagement with a part of the tie rod of a relatively small cross section, such that the application of an increased axial load on the tie rod relative to the assembly produces an increased pressure on the shear pin, sufficient to cause the pin to shear and the cam arrangement to release the tie rod, the tie rod having a region of relatively larger diameterwhich, on the re-introduction of the rod into the recess, is itself frictionally engaged by the cam to an extent which permits the tie rod to raise the fuel assembly.

5. Apparatus according to any preceding claim wherein a route through a plug unit is provided such that,with the plug unit engaged with the top of the fuel stringer, a tie rod guide tube carried by the plug unit is axially aligned with the guide tubes carried by the fuel elements and reflector elements.

6. Apparatus according to Claim 5 wherein the fuel stack bottom support includes a guidance feature, axially aligned with the tie rod guide tubes, into which the lower end ofthe tie rod is arranged to be inserted and locked to the bottom support.

7. Apparatus according to Claim 5 or 6 wherein the plug unit is provided with a tie rod support into which a co-operating feature at an appropriate position on the tie rod engages, the support being arranged to transfer the load of the fuel stringer, when suspended on the tie rod, through to the plug unit body.

8. Apparatus according to Claim 7 wherein the tie rod support is mounted on springs which maintain the tie rod in tension whilst its lower end is locked within the fuel stack bottom support.

9. Apparatus ofthe kind referred to substantially as herein described with reference to Figures 1 to 4 of the accompanying drawing.