GB2176924A - Decommissioning nuclear reactor - Google Patents

Abstract

A machine for decommissioning nuclear reactors comprises a shield structure (12) to be mounted rotatably at the open top of the reactor pressure vessel, a mast (23) mounted on the shield (12) for extension through an opening (17) in the shield and a tool-carrying manipulator (42) mounted on the mast (23). A slew beam (25) provided with a hoist (36) is provided beneath the shield (12) for handling dismantled material created by operation of the manipulator (42). The shield opening (17) can be closed by a number of slidable shield sections (18, 19, 20) and the opening (17) can be increased in cross-section to allow the manipulator (42), when retracted, to be drawn upwardly through the shield into a decommissioning module (50, 51) in which the manipulator can be extended to permit tool changes to be made. <IMAGE>

Description

SPECIFICATION Machines for dismantling decommissioned nuclear reactors This invention relates to the decommissioning of nuclear reactors which are considered to have completed their operational life, and in particular, to machines for effective dismantling of the reactor structure in a controlled and safe manner.

When a nuclear reactor has reached the end of its planned operational life, the alternatives, after removal of the nuclear fuel and reactor coolant, are to cover the whole structure with adequate shielding, such as a mound of soil, and take precautions for the covered reactor to be preserved so shielded for all time as a permanent "memorial", or to dismantle the structure of the reactor and dispose of the dismantled material in small amounts in conventional safe storage facilities, and as a result, restore the reactor site to its pre-building state. The latter course is to be preferred, especially in view of doubts concerning the practicability of preserving the shielding integrity of the whole reactor structure over the period of thousands of years necessary before biologically unsafe amounts of radioactivity will have decayed away.

A conventional gas-cooled, graphite moderated, nuclear reactor for which, within a pressure vessel, a core including the moderator and the fuel elements, the latter being capable of being charged and discharged by a refuelling machine, and surrounding the core, and heat exchangers which remove heat from coolant heated by circulation through the core and employ the removed heat to generate electricity for example employing steam and turbine. When such a reactor has completed its operational lifetime, it may be decided that the reactor should be dismantled and the site restored.

After normal discharge of the fuel elements and blow-down of the coolant gas it is considered acceptable from a safety point of view to employ conventional dismantling techniques for removal of the turbines, heat exchangers, refuelling machine and, with adequate temporary shielding, the top dome of the pressure vessel so as to expose the core. In order to dismantle and remove the pressure vessel and to unload and transfer to a disposal facility the graphic moderator (which is in discrete block form) and other vessel internals, a machine is required which will perform these oper ations safely and expeditiously, and it is an object of the present invention to provide such a ma chine.

According to the invention there is provided a machine for dismantling, unloading and transfer to a disposal facility nuclear reactor structure and/or components which have been irradiated during op eration of the reactor, said machine comprising a rotary radiation shield installed, above the pressure vessel, in or on a rigid part of the reactor which has not yet been dismantled, said shield having an offset opening therein and at least one movable section for varying the size of the opening, drive means for effecting rotation of the shield, structure mounted on the rotary shield for supporting a generally vertical mast which can pass through the shield and extend into the pressure vessel, said mast being retractable to such an extent that it can be brought wholly above the shield, and a tool-carrying manipulator mounted on the mast for extension and contraction laterally thereof, the lateral extent of the manipulator, when contracted, being such that it can be withdrawn upwardly through said opening in the shield when said opening size is maximised by suitable positioning of themovable shield section(s).

The machine preferably further includes a generally horizontal beam mounted below said shield for angular movement about a generally vertical axis (which may coincide with the rotary axis of the shield) independently of the shield, the beam serving as a track to mount hoist means for use in lifting dismantled reactor structure and/or components, the beam being movable into at least one angular position in which it registers with a fixed beam or beams forming continuation(s) of said track whereby the hoist can be transferred to a disposal station or stations to discharge its load.

Means may be provided for controlling the drive means associated with the shield and the movable beam in such a way that neither can foul the other at any time.

A feature of the invention is that the mast can be fixed in offset relation to the shield and access to all points within the pressure vessel can be achieved by means of the extendible manipulator simply by rotating the shield to bring the mast into a position in which the manipulator can be extended to reach any desired point. This avoids the need for the complication of a cross-travel mounting for the mast. In addition because the shield supports the mast and is itself mounted on the reactor structure, the mast and its supporting and guiding structure may be readily demounted from the shield and the latter can be left in place as a permanent shield if desired.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which; Figure 1 is a vertical section through the rotary axis of a shield forming part of a decommissioning machine in accordance with the invention, the drive motors for the shield and hoist beam being shown displaced 90 from their true position in order to facilitate understanding of their operation; Figure 2 is a sectional view in the direction A-A in Figure 1; Figure 3 is a plan view; Figure 4 is a view similar to Figure 1 but showing additional details; Figure 5 is an end view on arrow 5 in Figure 4; Figure 6 is a plan view on arrow 6 in Figure 5; Figure 7 is a schematic side view in section of a gas cooled nuclear reactor intended to be dismantled by the machine of Figures 1 to 3; and Figure 8 is a plan view of the reactor.

Referring firstly to Figures 7 and 8, these Figures show in diagrammatic form the gas-cooled, graphite moderated, nuclear reactor known as the Windscale Advanced Gas-Cooled Reactor (WAGR), which has now completed its useful life and is to be dismantled. The reactor core (not shown), which includes the graphite moderator and fuel element channels, and other internals such as core support (not shown), hot box 1 and refuelling standpipes 2, are contained in a pressure vessel 3, the upper dome and the upper parts of the standpipes 2 of which are shown removed. The volume which the upper dome occupied is intended to be employed for a dismantling, unloading and transferring machine, such as one of those subsequently described herein.The reactor also has four heat exchanger housings 4, from which the heat exchangers have been dismantled and removed by conventional means with shielding precautions, since the radioactivity level of these structures is such as to permit such operations. One of the buildings 4 (when emptied) is employed as a temporary repository 5 for dismantled material, in which sorting and designation for packaging in a communicating building 6 is carried out, small size material which can be disposed of in bulk being dropped into a waste container 7 beneath the temporary repository 5. The outer containment 8 of the reactor is retained and helps to retain integrity during dismantling and related operations. The packaging building 6 is outside the containment 8 and communicates via an air lock (not shown).The refuelling floor level is designated 9 in Figure 1, and is provided with rails 10 for the reactor refuelling machine (already removed) to run on.

Figures 1-3 show the reactor with a decommissioning machine 11 in position. The machine 11 comprises a radiation shield 12 which is rotatably mounted in the floor 9 through the agency of a fixed annular shield structure 13 (which may be segmented for convenience of handling) and annu lar bearing arrangement 14. The rotary shield 12 is arranged so that its axis of rotation is generally vertical and substantially coaxial with the centre line of the pressure vessel 3. A drive motor 15 serves to effect rotation of the shield 12 via a rack and pinion drive 16, the rack being arcuate and extending around the entire perimeter of the shield 12. The shield 12 has an eccentrically disposed opening 17 and a number of sliding shield sections 18, 19, 20 mounted by wheels 21 and rails 22 for movement between the solid outline positions and phantom line positions shown in Figure 3.In the latter position, the shield sections are clear of the opening 17 whereas in the former position they re strict the opening to a cross-sectional area corre sponding to that of a vertical mast 23, with gaps 24 also through which a series of cables can pass.

A slew beam 25 is mounted below the shield 12 and is supported for rotation about a vertical axis (which may be coincident with the rotary axis of the shield 12) via a framework comprising ring gir der 26 and uprights 27 in turn mounted, via annu lar bearing arrangement 28, on an annular support 29 instailed in the biological shield 30. Rotation of the slew beam 25 is effected by means of drive motor 31 via vertical shafting 32 extending through fixed shield 13 and a rack and pinion gearing 33, the rack extending around the full circumference of the ring girder 26. As shown in Figure 1, the slew beam can be rotated into a position in which it registers with one or more fixed beams 34, 35 which may extend to respective heat exchanger housings 4 acting as repositories for dismantled material.A motor driven hoist 36 for raising dismantled material and components is suspended from the lower flanges of the slew beam 25 for movement lengthwise of the beam 25 and can transfer from beam 25 to a selected beam 34, 35 (when aligned) for transporting the dismantled material to the desired repository. Service cables 37 for the hoist (eg power supply and control signal cabling) is routed from a cable feed reel 38 on the rotary shield 12 via a sleeved throughbore 39, centrally located pully arrangement 40 and guide 41.

The mast 23 is built up from sections bolted or otherwise secured together end-to-end and provides a guide for the plafform 43 of a manipulator 42 which can be traversed along the mast from a lower position (see Figure 1) and an upper position (see Figures 1 and 4) by hoists 44 mounted on a platform 45 which is mounted on pillars 46 above the opening 17. The platform also mounts a number of drum reels 47 for paying out and taking up various service cables 48 for the manipulator 42 and tools carried thereby. Guides, such as sets of rollers, 49 serve to locate and guide the mast 23 as it is lowered section-by-section by means of an overhead crane (not shown).

The manipulator comprises an arm having multiple degrees of freedom and as shown in Figure 4 it can be folded into a compact structure to allow it to be withdrawn through the opening 17 when the shield sections 18, 19 and 20 are drawn clear.

When not is use, the manipulator is raised to the position shown in Figures 1-4 to minimise contamination. It is also raised into this position, and then fully extended as shown in phantom outlined in Figure 1, when tool changes are necessary. The area beneath the platfrom 45 and the area into which the manipulator projects for tool change purposes are enclosed within demountable enclosures 50, 51 with access doors 52, 53. A glove box enclosure 54 with access part 55 is provided above the platform 45.

As previously mentioned, the mast 23 comprises a number of sections (see 56, 57 in Figure 4). The mast is built up one section at a time by lowering each section, using the overhead crane, until an aperture therein registers with a locking pin unit 58 whereupon the pin 59 is inserted to retain the section while the next section is assembled above it; the pin 59 is removed, the assembly is lowered, the next section is pinned and so on. In practice, the mast 23 will be built up in stages over a period of time so that, as the mast descends, the reactor structure and internals can be dismantled using the manipulator 42 and hoist 36. The mast will then be raised, involving removing the sections one by one. After the mast 23 has been dismantled, the opening 17 may be completely plugged and the shield 12 may be left in position.

Claims (4)

1. A machine for dismantling, unloading and transfer to a disposal facility nuclear reactor structure and/or components which have been irradiated during operation of the reactor, characterised in that said machine comprises a rotary radiation shield installed, above the pressure vessel, in or on a rigid part of the reactor which has not yet been dismantled, said shield having an offset opening therein and at least one movable section for varying the size of the opening, drive means for effecting rotation of the shield, structure mounted on the rotary shield for supporting a generally vertical mast which can pass through the shield and extend into the pressure vessel, said mast being retractable to such an extent that it can be brought wholly above the shield, and a tool-carrying manipulator mounted on the mast for extension and contraction laterally thereof, the lateral extent of the manipulator, when contracted, being such that it can be withdrawn upwardly through said opening in the shield when said opening size is maximised by suitable positioning of the movable shield section(s).

2. A machine as claimed in claim 1 further including a generally horizontal beam mounted below said shield for angular movement about a generally vertical axis independently of the shield, the beam serving as a track to mount hoist means for use in lifting dismantled reactor structure and/ or components, the beam being movable into at least one angular position in which it registers with a fixed beam or beams forming continuation(s) of said track whereby the hoist can be transferred to a disposal station or stations to discharge its load.

3. A machine as claimed in claim 1 or 2 including means for controlling the drive means associated with the shield and the movable beam in such a way that neither can foul the other at any time.

4. A machine as claimed in claim 1 substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.