CA2766440C - Feeder platform for nuclear reactor - Google Patents

Abstract

A feeder platform system for installation in a reactor vault adjacent an end of a CANDU.TM.
reactor core for accessing a feeder cabinet and feeder tubes therein during a retube process. The feeder platform system includes front and rear platforms. The front platform is movable between a variety of heights in a first vertical space adjacent the reactor core. The rear platform is movable between a variety of heights in a second vertical space adjacent the first vertical space.
The front and rear platforms define respective lengths parallel to and spaced from the end of the reactor core. At least one of the lengths can be increased with a plurality of extensions releasably connectable with at least one of the front and rear platforms.
Lifting mechanisms are coupled to each of the front and rear platforms and configured to raise and lower the front and rear platforms independently within the respective first and second vertical spaces.

Description

Attorney Docket No. 027813-9029-CA
FEEDER PLATFORM FOR NUCLEAR REACTOR
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent Application No.
61/432,960, filed January 14, 2011.
BACKGROUND

[0002] The present invention relates to nuclear reactors. More particularly, the invention relates to systems used in maintenance or refurbishment (e.g., a full retube) of a nuclear reactor.
SUMMARY

[0003] In one aspect, the invention provides a feeder platform system for installation in a reactor vault adjacent an end of a CANDUTM reactor core for accessing a feeder cabinet and feeder tubes therein during a retube process. The feeder platform system includes front and rear platforms. The front platform is movable between a variety of heights in a first vertical space adjacent the reactor core. The rear platform is movable between a variety of heights in a second vertical space adjacent the first vertical space. The front and rear platforms define respective lengths parallel to and spaced from the end of the reactor core. At least one of the lengths can be increased with a plurality of extensions releasably connectable with at least one of the front and rear platforms. Lifting mechanisms are coupled to each of the front and rear platforms and configured to raise and lower the front and rear platforms independently within the respective first and second vertical spaces.

[0004] In another aspect, the invention provides a method of using a feeder platform system in a reactor vault to access and service a feeder cabinet of a CANDUTM
reactor. A reactor vault is provided adjacent a reactor core of the CANDUTM reactor, the reactor vault including a feeder cabinet, a plurality of feeder tubes positioned inside the feeder cabinet, a pair of fueling machine bridge columns, and a column support adjacent an upper end of each of the fueling machine bridge columns, the column supports supporting an overhead gantry crane having a bridge beam Attorney Docket No. 027813-9029-CA
spanning the column supports. A platform is positioned on a floor of the reactor vault adjacent the reactor core, the platform defining a width measured away from the reactor core and a length measured perpendicular to the width, the length being less than a spacing distance between the column supports. The platform is lifted between the column supports to a first height above the column supports and below a top of the gantry crane bridge beam. From the platform, with the platform at the first height, a portion of the feeder cabinet is removed to at least partially expose the plurality of feeder tubes. The length of the platform is extended with at least one releasably connectable extension to a length exceeding the spacing distance between the column supports while the platform is at the first height. The platform is lifted to a second height above the column supports and above the top of the gantry crane bridge beam. From the platform, with the platform at the second height, the feeder tubes are serviced.

[0005] In yet another aspect, the invention provides CANDUTm-type nuclear reactor site. A
reactor core includes a calandria having an end. A vault encloses the calandria end. A pair of fueling machine bridge columns are positioned in a first vertical space adjacent the reactor core, each of the fueling machine bridge columns being secured to the vault with a corresponding column support, the column supports further supporting an overhead gantry crane movable along the column supports. A front platform is movable between a variety of heights in the first vertical space. The front platform defmes a first length measured parallel to the calandria end, the first length being less than a spacing distance between the column supports. A rear platform is movable between a variety of heights in a second vertical space adjacent the first vertical space. The second platform defines a second length measured parallel to the calandria end, the second length being less than the spacing distance between the column supports. Lifting mechanisms are coupled to each of the front and rear platforms and configured to raise and lower the front and rear platforms independently within the respective first and second vertical spaces.
A plurality of extensions are releasably connectable with at least one of the front and rear platforms to increase at least one of the first and second lengths to a length greater than the spacing distance between the column supports when the at least one of the front and rear platforms is raised above the column supports by the lifting mechanisms.

Attorney Docket No. 027813-9029-CA
BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Fig. 1 is a perspective view of a reactor core.

[0007] Fig. 2 is a cut-away view of a fuel channel assembly of the reactor core of Fig. 1.

[0008] Fig. 3 is a side view of a feeder cabinet on the end of the reactor core of Fig. 1.

[0009] Fig. 4 is a perspective view of a front feeder platform section, assembled and ready for hoisting.

[0010] Fig. 5 is a perspective view of the front feeder platform section raised to a first height above a pair of fueling machine bridge column supports and having extensions attached.

[0011] Fig. 6 is a perspective view of a front wall of a lower feeder cabinet removed with the front feeder platform section at the first height.

[0012] Fig. 7 is a perspective view of the front feeder platform section raised to a second height above a vault crane with feeder cabinet side panels removed, and a rear feeder platform section, assembled and ready for hoisting.

[0013] Figs. 8-9 are a perspective view and a side view of the rear feeder platform section raised to the first height above the fueling machine bridge column supports and having extensions attached. A front portion of the feeder cabinet soffit panels are removed, and a rear portion of the feeder cabinet soffit panels are being removed.

[0014] Figs. 10-11 are perspective views of the front and rear feeder platform sections raised to the second height above the vault crane with rear wall panels of the feeder cabinet removed.

[0015] Fig. 12 is a perspective view of the front feeder platform in a first configuration.

[0016] Fig. 13 is a perspective view of the rear feeder platform in a first configuration.

[0017] Fig. 14 is a perspective view of the front feeder platform with extensions attached.

[0018] Fig. 15 is a perspective view of the rear feeder platform with extensions attached.

Attorney Docket No. 027813-9029-CA
DETAILED DESCRIPTION

[0019] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

[0020] FIG. 1 is a perspective of a reactor core of a CANDUTm-type reactor 6. The reactor core is typically contained within a vault that is sealed with an air lock for radiation control and shielding. A generally cylindrical vessel, known as a calandria 10, contains a heavy-water moderator. The calandria 10 has an annular shell 14 and a tube sheet 18 at a first end 22 and a second end 24. The tube sheets 18 include a plurality of bores that accept a fuel channel assembly 28. As shown in FIG. 1, a number of fuel channel assemblies 28 pass through the tube sheets 18 of calandria 10 from the first end 22 to the second end 24. The fuel channel assemblies 28 are generally arranged in a horizontal and vertical grid on the end faces of the reactor core. Feeder tubes 59 communicate with the fuel channel assemblies 28 to provide heavy-water to the fuel channel assemblies 28 during operation of the reactor. Each feeder tube 59 includes an upper feeder tube section or "upper feeder tube" 59A, and a lower feeder tube section or "lower feeder tube" 59B. The upper feeder tubes 59A extend from headers H
toward the reactor 6, and the lower feeder tubes 59B extend along the end of the reactor 6 to the corresponding fuel channel assembly 28. The upper and lower feeder tubes 59A, 59B are joined at a "field weld location" adjacent a bend in the upper feeder tubes 59A as shown in Fig. 9.

[0021] FIG. 2 is a cut-away view of the fuel channel assembly 28. As illustrated in FIG. 2, each fuel channel assembly 28 is surrounded by a calandria tube ("CT") 32. The CT 32 forms a first boundary between the heavy water moderator of the calandria 10 and the fuel bundles or assemblies 40. The CTs 32 are positioned in the bores on the tube sheet 18. A
CT rolled joint insert 34 within each bore is used to secure the CT 32 to the tube sheet 18.

[0022] A pressure tube ("PT") 36 forms an inner wall of the fuel channel assembly 28. The PT 36 provides a conduit for reactor coolant and the fuel bundles or assemblies 40. The PT 36, for example, generally holds two or more fuel assemblies 40 and acts as a conduit for reactor Attorney Docket No. 027813-9029-CA
coolant that passes through each fuel assembly 40. An annulus space 44 is defined by a gap between the PT 36 and the CT 32. The annulus space 44 is normally filled with a circulating gas, such as dry carbon dioxide, helium, nitrogen, air, or mixtures thereof.
The annulus space 44 and gas are part of an annulus gas system. The annulus gas system has two primary functions.
First, a gas boundary between the CT 32 and PT 36 provides thermal insulation between hot reactor coolant and fuel within the PTs 36 and the relatively cool CTs 32.
Second, the annulus gas system provides an indication of a leaking CT 32 or PT 36 via the presence of moisture, deuterium, or both in the annulus gas.

[0023] An annulus spacer or garter spring 48 is disposed between the CT 32 and PT 36. The annulus spacer 48 maintains the gap between the PT 36 and the corresponding CT
32, while allowing the passage of the annulus gas through and around the annulus spacer 48. Maintaining the gap helps ensure safe and efficient long-term operation of the reactor 6.

[0024] As also shown in FIG. 2, an end fitting 50 is attached around the fuel channel assembly 28 outside of the tube sheet 18 at each end 22, 24. At the front of each end fitting 50 is a closure plug 52. Each end fitting 50 also includes a feeder assembly 54. The feeder assemblies 54 feed reactor coolant into or remove reactor coolant from the PTs 36. In particular, for a single fuel channel assembly 28, the feeder assembly 54 on one end of the fuel channel assembly 28 acts as an inlet feeder, and the feeder assembly 54 on the opposite end of the fuel channel assembly 28 acts as an outlet feeder. As shown in FIG. 2, the feeder assemblies 54 can be attached to the end fitting 50 using a coupling assembly 56 including a number of screws, washers, seals, and/or other types of connectors.

[0025] Coolant from the inlet feeder assembly flows along a perimeter channel of the end fitting 50 until it reaches a shield plug 58. The shield plug 58 is contained inside the end fitting 50 and provides radiation shielding. The shield plug 58 also includes a number of openings that allow the coolant provided by the inlet feeder assembly to enter an end of a PT 36. A shield plug 58 located within the end fitting 50 at the other end of the fuel channel assembly 28 includes similar openings that allow coolant passing through the PT 36 to exit the PT
36 and flow to the outlet feeder assembly 54 through a perimeter channel of another end fitting 50 at the opposite Attorney Docket No. 027813-9029-CA
face of the reactor 6. As shown in FIG. 1, feeder tubes 59 are connected to the feeder assemblies 54 that carry coolant to or away from the reactor 6.

[0026] The feeder tubes 59 are substantially enclosed in a feeder cabinet 104 that extends along the end face of the reactor core, upward and away from the reactor core as shown in Fig. 3.
Access to the feeder tubes 59 is provide by a series of removable panels 106 of the feeder cabinet 104, which can include a front face panel 106A, side panels 106B adjacent the front face panel 106A, a soffit panel 106C, a rear wall panel 106D, and side panels 106E
adjacent the soffit and rear wall panels 106C, 106D. Any or all of the panels can be formed in multiple pieces. The front face panel 106A and adjacent side panels 106B generally enclose the lower feeder tubes 59B and form a "lower feeder cabinet". The upper feeder tubes 59A are generally enclosed in an "upper feeder cabinet" formed at least in part by the soffit panel 106C, the rear wall panel 106D, and the adjacent side panels 106E. The headers H are also generally enclosed within the upper feeder cabinet.

[0027] Returning to FIG. 2, a positioning hardware assembly 60 and bellows 62 are also coupled to each end fitting 50. The bellows 62 allows the fuel channel assemblies 28 to move axially. The positioning hardware assemblies 60 are used to set an end of a fuel channel assembly 28 in either a locked or unlocked position. In a locked position, the end of the fuel channel assembly 28 is held stationary. In an unlocked position, the end of the fuel channel assembly 28 is allowed to move. A tool can be used with the positioning hardware assemblies 60 to switch the position of a particular fuel channel assembly 28.

[0028] The positioning hardware assemblies 60 are also coupled to an end shield 64. The end shields 64 provide additional radiation shielding. Positioned between the tube sheet 18 and the end shield 64 is a lattice sleeve or tube 65. The lattice tube 65 encases the connection between the end fitting 50 and the PT 36 containing the fuel assemblies 40.
Shielding ball bearings 66 and cooling water surround the exterior the lattice tubes 65, which provides additional radiation shielding.

[0029] During a retube of the reactor 6, many of the major components of the reactor core are removed and replaced. Other components are inspected and/or repaired. In addition to reactor core components such as the fuel channel end fittings 50, the calandria tube inserts 34, Attorney Docket No. 027813-9029-CA
the calandria tubes 32, the pressure tubes 36, and the associated annulus spacers 48 (e.g., garter springs), the feeder tubes 59 may be inspected, and potentially removed and replaced. In order to accommodate personnel, tooling, and materials to service the feeder tubes 59, a feeder platform system 100 is provided adjacent each end of the reactor core. One exemplary feeder platform system 100 is shown in the drawings, with the understanding that a substantially similar feeder platform system can be provided in the vault on the opposite end of the reactor core.
Therefore, it will be understood that the below description relates to the illustrated feeder platform assembly 100, but multiple feeder platform assemblies 100 would typically be provided to enable servicing the feeder tubes on both ends of the reactor core simultaneously.

[0030] Feeder installation is generally comprised of four distinct mini-series: (1) header nozzle inspection and preparation, (2) feeder pipe inspection and preparation, (3) upper feeder installation, and (4) lower feeder installation. However, in at least one method of use, feeder installation is carried-out as part of a retubing process for the nuclear reactor 6, as opposed to during construction of a new nuclear reactor. During retubing, the upper feeders 59A are removed leaving a stub (several inches long) of feeder pipe attached to the corresponding header H. As described in further detail below, one or more feeder platforms 110 are positioned adjacent each end face of the nuclear reactor 6 at desired working elevations, and can be moved to various working elevations throughout different stages of feeder tube servicing during retubing of the reactor 6. The feeder platform(s) 110 are used with a lifting mechanism to provide a height-adjustable feeder platform system 100 to maximize efficiency of the feeder tube servicing.

[0031] The feeder platform(s) 110 are installed to provide a safe work surface providing access to remove the upper feeder cabinet (panels 106C, 106D, 106E), upper feeders 59A, resistance temperature detectors (RTDs, not shown) and other tubing, which may occur during the course of a retubing process of the reactor 6. The feeder platform 110 can also be used to perform inspection on any of the components in the feeder cabinet 104, including header nozzles, and can also be used for upper feeder installation and installation of the upper portion of the feeder cabinet 104. As described below, the feeder platform 110 may be provided in separate sections, such as front and rear sections, referred to herein simply as the front feeder platform 110F (Figs. 12 and 14) and the rear feeder platform 11OR (Figs. 13 and 15).
The front and rear Attorney Docket No. 027813-9029-CA
feeder platforms 110F, 11OR can be independent platforms usable separately, but may also be releasably connectable to each other as described in further detail below.

[0032] A lifting mechanism is coupled to each of the feeder platforms 110F, 110R. The lifting mechanism can be any type of device or system providing elevation control. In one construction, a plurality of wire ropes or cables 114 are coupled to each feeder platform 110F, 110R, and a corresponding plurality of strand jacks 115 (Figs. 12-15) are provided to engage the plurality of cables 114 and enable each of the feeder platforms 110F, 11OR to move up and down within the vault. Each cable 114 can be secured to a fixed structure within the vault, which can include concrete vault walls, structural (e.2., steel) attachment points on the vault walls, or the structural frame 116 of the upper feeder cabinet as shown in Fig. 9. The strand jacks 115 can be positioned at the upper ends of the cables 114 as shown, or alternately, can be positioned on each feeder platform 110F, 110R. The strand jacks 115 can be installed at pre-determined locations in the upper feeder cabinet. At these locations, the cables 114 are affixed and dropped through corresponding holes formed in the feeder cabinet. In other constructions, the feeder platform lifting mechanism can include one or more of screw drives, scissor lifts, chains, straps, pulleys, etc.

[0033] With continued reference to Figs. 12-15, each of the feeder platforms 110F, 11OR is generally constructed of a structural frame 120, a decking 122 of one or more panels forming a work surface, and handrails 124 positioned adjacent at least a portion of the perimeter. The handrails 124 can be removably coupled to the frame 120 and the decking 122, and may be rearranged in a plurality of configurations to suit various operational needs.
The front feeder platform 110F defines a length Li and a width WI, and the rear feeder platform 11OR defines a length L2 and a width W2. The lengths Ll , L2 are measured parallel to the reactor face, and the widths WI, W2 are measured away from the reactor face (i.e., perpendicular to the lengths Ll, L2). The lengths Li, L2 of the front and rear feeder platforms 110F, 11OR can be substantially equal in some constructions, each providing access to a majority of the width of the feeder cabinet 104. The lengths Ll, L2 are slightly smaller than a spacing distance D
(Fig. 4) between a pair of opposed column supports 128 coupled to the upper ends of the fueling machine bridge columns 130 to laterally brace the columns 130 against the vault walls. In the illustrated construction, the column supports 128 also serve as crane rails supportinu, the vault crane C (e.g., Attorney Docket No. 027813-9029-CA
an overhead gantry crane). The frame 120 and decking 122 of the front feeder platform 110F are configured to provide recesses 132 at the lateral sides, which are at the ends of the length direction. Each recess 132 is configured to at least partially receive one of the fueling machine bridge columns 130 as shown in Fig. 4. Thus, the front feeder platform 110F is configured to at least partially wrap around the fueling machine bridge columns 130.

[0034] As shown in Figs. 14 and 15, the front and rear feeder platforms 110F, 11OR can be reconfigured to extended lengths L3, L4 with extensions 136 at each lateral end. The extended lengths L3, L4 can be equivalent, but need not be, and can be configured to provide full access to the feeder cabinet 104 and the feeder tubes 59 therein. Each of the extended lengths L3, L4 is greater than the spacing distance D between the column supports 128. Each of the extensions 136 can be an additional platform section, constructed substantially similarly to the main portion of the platforms 110F, 11OR (i.e., of a frame 120 and decking 122), and releasably coupled to the frames 120 of the pre-assembled platforms 110F, 110R. Although not illustrated, the handrails 124 can be reconfigured and/or additional handrails 124 added to at least partially encompass the ends of the extensions 136 when attached. As shown in Fig. 14, connection of the extensions 136 on the front feeder platform 110F creates a unitary working surface with pair of apertures 140. The apertures 140 are partially formed by the recesses 132, and are enclosed by the addition of the extensions 136. Each aperture 140 is configured to receive one of the fueling machine bridge columns 130 with minimal clearance, allowing the front feeder platform 110F to travel up and down in a vertical space VS1 (Fig. 3) adjacent the reactor core that includes the fueling machine bridge columns 130, while avoiding interference. Use of the extensions 136 is discussed in further detail below with regard to the method of using the platforms 110F, 110R.
100351 Before any feeder platforms 110 are installed, the reactor face and end fitting insulation are removed, and the vault crane C is parked at its furthest location from the reactor 6 (Fig. 4). A lower majority of the front face panel 106A of the lower feeder cabinet is removed as shown in Fig. 4. This can be accomplished with one or more scissor lifts, boom lifts, or similar devices. Adjacent side panels 106B of the lower feeder cabinet may or may not be removed at this time.

Attorney Docket No. 027813-9029-CA
[0036] With the lower feeder cabinet at least partially removed as shown in Fig. 4, the front feeder platform 110F is positioned on the vault floor or on a retube platform RTP (Fig. 11) provided adjacent the calandria 10 for the retubing. The front feeder platform 110F can be assembled from a small number of pre-assembled modules to minimize assembly time. Once assembled, the front feeder platform 110F is hoisted to a first height (Fig.
5) above the column supports 128 that brace the fueling machine bridge columns 130. The first height can be just far enough above the column supports 128 to ensure clearance (e.g., 0.5 inch to 6 inches). The height of the front feeder platform 110F can be configured to substantially fill the vertical space between the column supports 128 and the upper feeder cabinet. At the first height, the front feeder platform 110F is extended to maximum length L3 by releasably coupling the extensions 136 to the assembled portion of the platform frame 120. The extensions 136 are positioned over top of the obstructing column supports 128. In this configuration, the front feeder platform 110F
can provide access to the entirety of the feeder cabinet. The remaining top portion of the front face panel 106A of the lower feeder cabinet is removed from the front feeder platform 110F (at the first height) along with the front portion of the soffit panel 106C and the side panels 106E or the upper feeder cabinet (Fig. 6). The front feeder platform 110F is then further raised to a second height (Fig. 7), which may be a maximum working elevation. At the second height, the bottom of the front feeder platform 110F is above an upper bridge beam 144 of the vault crane C, so that the vault crane C is operational and can move freely back and forth underneath the front feeder platform 110F. At the second height, the front feeder platform can be secured relative to the vault (e.g., with one or more braces 148 between the frame 120 and a fixed vault structure 150 such as a concrete vault wall, fixed structural steel beam, or other attachment point) as shown in Fig. 11, and the vault crane C is moved forward underneath the front feeder platform 110F. Securing the front feeder platform 110F to the vault provides lateral bracing that prevents any swinging of the platform during the ensuing work operations. When secured, the front feeder platform 110F is ready to provide a working surface for servicing the upper feeder tubes 59A, excluding those toward the rear portion of the upper feeder cabinet. Some or all of the handrails 124 may be removed before raising the front feeder platform 110F to the second height to avoid interference, and alternate fall prevention is provided.

Attorney Docket No. 027813-9029-CA
[0037] Two-stage assembly and lifting of the rear feeder platform 11OR is carried out substantially similar to that of the front feeder platform 110F described above. However, the rear feeder platform 11OR is positioned and made operable in a second vertical space VS2 (Fig.
3) that is adjacent to the rear or non-reactor-facing side of the front feeder platform 110F and its vertical space VS1, and is spaced away from the reactor core. The rear feeder platform 11OR is assembled on the vault floor (Fig. 7) or retube platform and is raised to the first height just above the column supports 128, and the extensions 136 are attached when the rear feeder platform 11OR is positioned at the first height (Figs. 8 and 9). At the first height, the rear feeder platform 110R is used to remove the remaining (rear) portion of the soffit panel 106C
and the rear wall panel 106D of the upper feeder cabinet.
[0038] After the extensions are added, the rear portion of the feeder platform is hoisted to the second height to match the height of the front feeder platform 110F as shown in Figs. 10 and 11.
When both platforms 110F, 11OR are at the second height, they are directly adjacent to one another and can be fastened together with one or more braces between the frames 120, such that the two sections 110F, 110R form a single rigid, conjoined feeder platform 110. Redundant and/or interfering handrails 124 may be removed to optimize the environment for removal of the upper feeder tubes 59A.
[0039] The feeder platform 110 remains in this configuration for the duration of the servicing of the upper feeder assembly (e.g., upper feeder tube removal, nozzle inspection, weld preparation and upper feeder tube installation processes). When the vault is ready for installation of the lower feeder tubes 59B, the feeder platform 110 (or only the front feeder platform 110F, detached from the rear feeder platform 110R) is lowered below the second height to a convenient elevation at or above the first height to provide a working surface adjacent the field weld location between the upper and lower feeder tubes 59A. 59B to facilitate field weld fit-up, welding and non-destructive examination (NDE). At this height, the platform 110 (or 110F) may interfere with the operation of the crane C, so movement to this height is limited to the specific operation requiring it, and proper planning can be used to minimize disturbance to any operations requiring crane operation.

Attorney Docket No. 027813-9029-CA
[0040] After servicing is completed, re-installation of the feeder cabinet 104 is performed by reversing the removal sequence described above, including disassembling portions of the front and rear feeder platforms 110F, 11OR as required.
[0041] In some constructions, the feeder platforms may offer at least one feature or advantage such as: saving time in feeder removal, maintenance and/or installation, offering a high level of safety, providing a particularly low radiation dose for work tasks on the feeders, and enabling a high quality of work to be performed on the feeders.

Claims (26)

1. A feeder platform system for installation in a reactor vault adjacent an end of a CANDU.TM. reactor core for accessing a feeder cabinet and feeder tubes therein during a retube process, the feeder platform system comprising:
a front platform movable between a variety of heights in a first vertical space adjacent the reactor core, the front platform defining a first length parallel to and spaced from the end of the reactor core;
a rear platform movable between a variety of heights in a second vertical space adjacent the first vertical space, the rear platform defining a second length parallel to and spaced from the end of the reactor core;
lifting mechanisms coupled to each of the front and rear platforms and configured to raise and lower the front and rear platforms independently within the respective first and second vertical spaces; and a plurality of extensions releasably connectable with at least one of the front and rear platforms to increase at least one of the first and second lengths.

2. The feeder platform system of claim 1, wherein the lifting mechanisms includes a plurality of strand jacks movably supporting the front and rear platforms by a plurality of cables.

3. The feeder platform system of claim 1, wherein the front platform includes a pair of opposed lateral ends at ends of the first length, and each of the lateral ends is provided with a recess configured to accommodate a fueling machine bridge column of the reactor vault located within the first vertical space.

4. The feeder platform system of claim 3, wherein the plurality of extensions are releasably connectable with the opposed lateral ends of the front platform, and wherein the plurality of extensions, when connected with the front platform, cooperate with the recesses to define a pair of apertures configured to receive the fueling machine bridge columns.

5. The feeder platform system of claim 1, wherein the plurality of extensions includes a first plurality of extensions releasably connectable with the front platform to increase the first length to a third length, and a second plurality of extensions releasably connectable with the rear platform to increase the second length to a fourth length.

6. The feeder platform system of claim 5, wherein the lifting mechanisms are configured to raise the front and rear platforms to a first height for extending to the third and fourth lengths, and to a second height above the first height for accessing the feeder tubes.

7. The feeder platform system of claim 1, wherein the front and rear platforms are releasably connectable to each other to form a rigid conjoined platform.

8. The feeder platform system of claim 7, further comprising at least one lateral brace configured to releasably secure the rigid conjoined platform relative to the reactor vault.

9. A method of using a feeder platform system in a reactor vault to access and service a feeder cabinet of a CANDU.TM. reactor, the method comprising:
providing a reactor vault adjacent a reactor core of the CANDU.TM. reactor, the reactor vault including a feeder cabinet, a plurality of feeder tubes positioned inside the feeder cabinet, a pair of fueling machine bridge columns, a column support adjacent an upper end of each of the fueling machine bridge columns, the column supports supporting an overhead gantry crane having a bridge beam spanning the column supports;
positioning a platform on a floor of the reactor vault adjacent the reactor core, the platform defining a width measured away from the reactor core and a length measured perpendicular to the width, the length being less than a spacing distance between the column supports;
lifting the platform between the column supports to a first height above the column supports and below a top of the gantry crane bridge beam;

from the platform, with the platform at the first height, removing a portion of the feeder cabinet to at least partially expose the plurality of feeder tubes;
extending the length of the platform with at least one releasably connectable extension to a length exceeding the spacing distance between the column supports while the platform is at the first height;
lifting the platform to a second height above the column supports and above the top of the gantry crane bridge beam; and from the platform, with the platform at the second height, servicing the plurality of feeder tubes.

10. The method of claim 9, wherein the platform is a front platform movable in a vertical space adjacent the reactor core, the method further comprising:
positioning a rear platform on the floor of the reactor vault in a vertical space adjacent the first vertical space and spaced away from the reactor core, the rear platform defining a rear platform width measured away from the first vertical space and a rear platform length measured perpendicular to the width, the rear platform length being less than the spacing distance between the column supports;
lifting the rear platform between the column supports to the first height;
from the rear platform, with the rear platform at the first height, removing an additional portion of the feeder cabinet to at least partially expose the plurality of feeder tubes;
extending the rear platform length with at least one releasably connectable extension to a length exceeding the spacing distance between the column supports while the rear platform is at the first height;
lifting the rear platform to the second height; and from the rear platform, with the rear platform at the second height, servicing the plurality of feeder tubes.

11. The method of claim 10, further comprising releasably coupling the front and rear platforms to each other with both platforms at the second height.

12. The method of claim 11, further comprising releasably securing at least one of the front and rear platforms relative to the reactor vault with at least one lateral brace.

13. The method of claim 10, wherein removing an additional portion of the feeder cabinet to at least partially expose the plurality of feeder tubes includes removing a soffit portion of the feeder cabinet, and the at least partially exposed plurality of feeder tubes include upper feeder tubes coupled to at least one header.

14. The method of claim 13, further comprising:
from the rear platform, with the rear platform at the second height, removing a rear wall of the feeder cabinet to further expose the upper feeder tubes.

15. The method of claim 9, further comprising detaching the extension from the platform and lowering the platform back to the floor.

16. The method of claim 9, wherein removing a portion of the feeder cabinet to at least partially expose the plurality of feeder tubes includes removing a soffit portion of the feeder cabinet, and the at least partially exposed plurality of feeder tubes include upper feeder tubes coupled to at least one header.

17. A feeder platform system for installation in a reactor vault adjacent an end of a CANDU.TM. reactor core for accessing a feeder cabinet and feeder tubes therein during a retube process, the feeder platform system comprising:
a first platform movable between a variety of heights in a first vertical space adjacent the reactor core, the first platform defining a first length parallel to and spaced from the end of the reactor core;

a lifting mechanism coupled to the first platform and configured to raise and lower the first platform within the first vertical space; and a plurality of extensions releasably connectable with the first platform to increase the first length.

18. The feeder platform system of claim 17, comprising:
a second platform movable between a variety of heights in a second vertical space adjacent the first vertical space, the second platform defining a second length parallel to and spaced from the end of the reactor core.

19. The feeder platform system of claim 17 or 18, wherein the lifting mechanism includes a plurality of strand jacks movably supporting the first platform by a plurality of cables.

20. The feeder platform system of any one of claims 17 to 19, wherein the first platform includes a pair of opposed lateral ends at the ends of the first length, and each of the lateral ends is provided with a recess configured to accommodate a fueling machine bridge column of the reactor vault located within the first vertical space.

21. The feeder platform system of claim 20, wherein the plurality of extensions are releasably connectable with the opposed lateral ends of the front platform, and wherein the plurality of extensions, when connected with the front platform, cooperate with the recesses to define a pair of apertures configured to receive the fueling machine bridge columns.

22. The feeder platform system of any one of claims 17 to 21, wherein the plurality of extensions includes a first plurality of extensions releasably connectable with the first platform to increase the first length to a third length.

23. The feeder platform system of claim 22, wherein the lifting mechanism is configured to raise the first platform to a first height for extending to the third length, and to a second height above the first height for accessing the feeder tubes.

24. The feeder platform system of claim 18, wherein the first and second platforms are releasably connectable to each other to form a rigid conjoined platform.

25. The feeder platform system of claim 24, further comprising at least one lateral brace configured to releasably secure the rigid conjoined platform relative to the reactor vault.

26. The feeder platform system of any one of claims 17 to 25 where the first platform is a front platform or a rear platform.