Last updated on 28-Aug-2001 2:11 PM

Cryostat Disassembly

1   Before Starting

2   Removing the Warm Electronics and Calibration Unit

3   Opening the Vacuum Vessel

4   Removing the Front (Imager) Cover and Radiation Shield

5   Removing the Array

6   Removing the 4K Arm
7   Removing the Joule-Thomson Cooler
8   Removing the Front Optical Bench
9   Removing the Slit and Field Rotator Mechanisms
10 Removing the Closed Cycle Cooler
11  Removing the rear vacuum cover and the Grating Drum
12  Removing the Main Optical Bench

1   Before Starting

It takes two to three people a full working day to complete the disassembly procedure to the end of the section 'Front Optical Bench'. Approximate timescales for each major step are shown below for planning purposes.
The procedure assumes that the instrument has been warmed up to ambient temperature and backfilled with clean, dry nitrogen gas to atmospheric pressure. (see Warm Up).  It also assumes it has been removed from the Telescope (see UKIRT Telescope installation/removal) and transferred to the Handling Manipulator (see ../cryohandling/cryohandling.htm).  The procedure is also based on the cryostat in UKIRT model.

 

The Berger-Lahr motor drive racks must be depowered before the motor cables are unplugged, otherwise they will suffer damage.

 

As the cryogenic stepper motors are revealed they should be protected by placing some protective plastic film over the yellow label to prevent dirt getting into the bearing.

 

Many of these procedures require use of cranes and some manual lifting.   It is recommended that at least one person trained in slinging techniques is present throughout.

 

2   Removing the Calibration Unit (20 min.)

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2.1   Unbolt and remove the section of the calibration unit which covers the cryostat window. This section contains the virtually irreplaceable polarimetry optics and so it should be treated extremely carefully. They should be removed from the unit and stored safely as soon as possible.

 

 

2.2   Protect the cryostat window from water vapour damage by mounting the window cover plate with its six mounting screws.

 

2.3   Remove the flux collector assembly followed by the optics housing and store in a suitable safe, dry place.

 

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3   Opening the Vacuum Vessel

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3.1   Mechanism Check

Using CCS dm screens do a force datum on the mechanisms in turn.   One person should observe the mechanism itself for motion, noise etc., whilst the other should observe the dm screen for datum switch operations etc.

Before driving the motors remove all perspex films used to cover the motor shafts.

 

Open the (nitrogen gas filled) cryostat to atmosphere, preferably using a controllable rotary valve rather than the gate valve on the turbo pump.   

  Be sure to check that all sensors have been stable at or above ambient temperature for several hours, to ensure that condensation will not form in the opened cryostat.

Mechanism

Test 1

Test 2

Test 3

Test 4

Slit Wheel
Field Rotator
Filter Wheel A
Filter Wheel B
Imager Inject
Imager Extract
Focus Drive
Translation Drive

 

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Temperature Sensor Check

Indicate temperature reading either directly from scanners or from CCS

Temperature Monitor A

Temp (K)

Temperature Monitor B

Temp (K)

1 Rad Shield Nitrogen Can 1 JT Stage 1
2 GM Stage 1 2 JT Stage 2
3 Rad Shield Grating Cover 3 JT Stage 3
4 GM Stage 2 Unused
5 Rad Shield Front Unused
6 Grating Assembly   6 Filter Wheel Motor
7 Slit/Field Rotator housing Unused
8 Imager Stop Unused

 

 

 

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3.2   Remove the window assembly, inspect for water damage and store in a dry atmosphere.

 

 

4   Removing the Front (Imager) Cover (1 hour)

 

4.1   Rotate the cryostat until the interface between the centre section and the front section is horizontal.   Pull out the two alignment dowels at the bottom corners which align the front cover and hence calibration unit to the rest of the cryostat.

 

4.2   Because access to bolts along the side nearest the manipulator is restricted, these should be removed first by rotating the cryostat such that the front face is vertical.

 

4.3   Now rotate the cryostat until the front face is horizontal.   And attach the four corner eye bolts and using two slings, one along each side, attach to the overhead crane.   Remove the remaining bolts retaining the front cover.

 

4.4  Lift off the front cover (mass of front cover = 220kg) and lower it onto its flat storage plate for later evacuation and storage. Note that the unused set of fore-optics could be stored on the flat cover so you may wish to leave the front section suspended until you have removed them.

 

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4.5   Check the front section MLI blanket for damage, undo the tape and velcro pads which secure it to the MLI on the centre section of the cold optics, and remove for storage , ideally under vacuum.

 

Figure 4.4 The front section multi-layer insulation.

 

 

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Radiation shield cover

4.6   Remove the sensor plate hiding the Temperature sensor on the Front Radiation shield and disconnect. The loose connector and lead can be taped to the focus translation stage for safety.   Replace the plate on the front for safe keeping.   With the front face vertical, undo the sixteen screws on periphery of shield, the three shorter screws at the copper thermal wick and lastly the eight quick release catches to remove the front section.

Store the screws and associated belleville washers in the box provided, keeping the shorter wick screws separate.

Place the perspex cover over the UKIRT fore-optics by reusing some of the screws holding on the front shiny plate.

 

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   At this point in the process it is recommended that all personnel wear gloves to minimise dirt and contamination to the cryostat innards.

 

5 Removing the Array Housing (30 minutes)

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This procedure should only be undertaken by competent personnel who have had previous instruction from a member of the Michelle electronics team.

 

It is not necessary to remove the whole of the 4K arm in order to remove the array housing.

 

First check that the instrument is earthed through a low impedance safety earth connection (see installing the detector box and array)

 

 

Connect an ESD lead from a conducting mat to the copper block on the corner of the detector Box, ensure that the detector box assembly is properly grounded through the cryostat and that the person who will be removing the array assembly is wearing a connected wrist strap, gloves and will be standing on the mat

 

 

Remove the section of radiation shield in front of the 4K arm.

 

Undo the screws for the four clock and bias connectors, there are two screws per connector, remember to slacken a screw on either side of each connector alternately.

 

Remove the earth tags connected to the circular copper cold block, disconnect the temperature sensor connector and remove the right angle copper block which is attached to the centre of the circular copper block at the end of a wick from the 4K arm, replace the single M6 screw and invar bush in the circular copper block for safe keeping.

 

 

 

 

Carefully move the cables, wires and wicks now hanging from the end of the 4K arm to the side, remove the baffle plate (three M3 socket screws) from along the gap between the detector box radiation shield and the front optical bench before undoing the eight M3 button head screws to remove the main detector box radiation shield.  
Do not undo the four M3 socket cap screws.

 

Refit an ESD lead to the circular copper cold block.

 

To remove the detector box cover loosen all fourteen M3 slotted screws, Then remove the eight connector screws first, followed by the six screws holding the cover to the detector box, It will be necessary to temporarily disconnect the ESD lead in order to remove this cover.

 

Fit shorting plugs to the four connector positions before finally removing the ESD lead connection to the circular cold block.

 

 

 

Undo the four M3 socket cap screws at the corners of the inner gold array housing and carefully remove it from the detector housing. Note the positions and arrangement of the copper washers, G10 spacers and mylar insulators these should be replaced in the detector housing along with the screws for safe keeping.

 

 

Cover over the square hole into the array housing and transfer it to an ESD dissipative bag then store it away safely, preferably in a clean room environment or if one is not available then in a protected dry atmosphere.

 

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5.3  Disconnect the array cables at the four connectors (2 jackscrews each) at the back of the detector box, the connector to the array temperature sensor and heater and the small black connector to the DT470 temperature sensor. Immediately attach four 1 million Ohm grounding paths, running from the detector focus and translation mechanism to locations as listed below, and as shown in Figure 5.3.

  1. To the top of the Joule-Thomson cooler, which should in turn be connected to the telescope ground.

  2. To a conducting mat placed on which the person installing the detector box should stand.

  3. To a wrist strap on the installer.

  4. To the grounding stud on the gold detector box or (as shown in the Figure) to the base of the detector cold finger.

    Figure 5.3 Grounding the detector box when the detector cables are not connected. (Cooling arm has been removed).

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5.4 Unscrew the single bolt with its Invar washer which holds the copper block to the back surface of the array, and remove the 25K shield by undoing its 3 screws.

 

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5.5 Undo the screws which hold the array connectors to the inside of the array housing cover, and then remove the array housing cover by undoing its 6 screws. (see Figure 5.3).

 

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5.6 Remove the gold detector box by undoing its four screws and their insulating washers, and transfer it to a grounded antistatic tray or bag for immediate transfer to a clean and electrically safe area. The aperture in the gold box over the array should be covered.

Figure 5.6 The gold detector box ready for removal and storage (should be grounded!).

 

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6   Removing the 4K arm (30 minutes)

   This Procedure should only be undertaken by competent personnel who have had previous instruction from a member of the Michelle electronics team.

 

   First check that the instrument is earthed through a low impedance safety earth connection (See installing the detector box and array)

 

 

Connect an ESD lead from a conducting mat to the copper block on the corner of the detector box. Ensure that the detector box assembly is properly grounded through the cryostat and that the person who will be removing the 4K arm is wearing a connected wrist strap, gloves and will be standing on the mat.

 

 

Undo the button head screws from end section of radiation shield and remove both components, replace the screws in copper arm and secure the two pieces of radiation shield together for safe keeping.

 

 

Remove the front and side sections of the box surrounding the gold 4K plate at bottom of JT radiation shield by undoing the socket head screws from along the bottom and back edges.

 

 

Remove last three pieces of the radiation shield around the copper arm and the curved pieces from around the gold 4K plate.      Note that one of these pieces is fixed by acorn nuts to studs on the plate and the other is only guided by screws at the rear left between the JT radiation shield and the main optical bench.

 

 

All the pieces of the 4K arm radiation shield should now have been removed, the curved shaped bracket attached to the bottom of the JT radiation shield need only be removed if you intend removing the JT system from the cryostat.

 

                        

 

Undo the screws and disconnect the four clock and bias connectors, there are two screws per connector, remember to slacken a screw on either side of each connector alternately.

 

Remove the earth tags connected to the circular copper cold block, disconnect the temperature sensor connector and remove the right angle copper block which is attached to the centre of the circular copper block at the end of the wick from the 4K arm, replace the single M6 screw and the Invar bush in the circular copper block for safe keeping.
Carefully remove the cable, wired and wick now hanging from the end of the 4K arm to one side in order to fit shorting plugs to the four connector positions.
Attach another ESD lead to the circular copper cold block, so that the ESD lead to the copper block on the corner of the detector box can safely be removed, This then allows you to undo the two M5 socket head screws with Invar spacers and remove the copper block which joins the 4K arm to the detector box by a wick. 
 Remember to replace the screws and spacers in the detector box for safe keeping.
Slacken the three jam nuts then carefully disconnect the right-angled connectors from the bottom of the JT system wiring tubes. Also disconnect the 15 way D type connector which is located on the gold 4K plate.
Support the weight of the arm and undo the seven M6 screws with Invar spacers which are securing it to the angled face on the bottom of the 4K plate.  Then carefully remove the arm.   REMEMBER to replace the screws and spacers.
Clean off any cryogenic conducting grease from the arm and angled face of the 4K plate, before storing the arm somewhere safe.
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6.   Remove the front and right hand side covers (Al sheet) from the J-T cooler. (Note that one of the covers is only guided by the screws at the rear left - IRB). Then remove all shields from the copper arm.

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6. Remove the 7 fixing screws holding the arm to the base of the J-T cooler. They all have Invar washers and there is an Indium sheet sandwiched between the arm and the cooler.
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7   Removing the Joule-Thomson Cooler

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   First Ensure that the 4K arm and its radiation shield have been removed.

 

 

Remove the curved shape bracket from the bottom of the JT radiation shield.

 

 

Undo 8 M4 screws from slit ring inside the cryostat at the cold head end of JT system.  Access for the rear two screws can be obtained by retracting the top connector panel at the services are adjacent to the grating drum cover.

 

 

Remove the two halves of split ring and the 0-ring through front of cryostat.

 

 

Ensure that any pressure is released from the JT system then remove the Helium pressure relief bends and fit protection caps.

 

 

Rotate cryostat until JT system is horizontal.

 

 

Position the JT handling frame on the lifting trolley then line up the frame with the six hexagonal stand offs on the JT plate then secure in place using nuts.

 

 

Undo the nine M5 screws with Invar spacers, G10 washers and G10 bushes from cryostat plate fitting the alignment pins provided sequentially as the screws are removed.

 

 

Slowly withdraw the JT system from the cryostat using both the frame and trolley, slightly raise or lower the trolley if required and guide from beneath.

 

 

Once JT system has been retracted, slide the frame back on the trolley and secure with clamps.

 

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8   Removing the Front Optical Bench

This procedure assumes that the array and the 4K arm have been removed.

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If desired, confirm the operation of the Front End mechanism before disassembly.

Mechanism Check

 

8.1 Remove all six stepper motor cables (check first that the control racks are depowered) and the one status cable at the the interface to the radiation shield.  Also remove the long panel covering the JT side of the joint between the FOB and the MOB
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8.2 Disconnect the twisted copper strands at the filter wheel motors.
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8.3 Rotate the instrument until the interface between the front optical bench and the main optical bench is 5 degrees past vertical with Array end sticking out most.
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8.4 Attach an eyebolt and shackle to the upper-centre of the front optical bench and support the bench on a single hook using the overhead crane (~50kg).
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8.5 Undo the six M8 bolts, with the invar washers, three down each side of the FOB that attach it to the MOB and swing it clear using the two handles, and the trolley facility on the crane.  Replace the bolts and washers in the MOB for safe keeping.  Lower the FOB gently to lie horizontally on a storage table by lifting the array end whilst the rear end is lowered. This needs one person per handle.   The FOB has four feet to stand on.

Figure 4 (201954 bytes)

Figure 8. Removing the Front Optical Bench.

 

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8.6 Cover the exposed field rotator optics and the opening to the spectrometer camera with the perspex covers provided.  (Spectrometer cover needs to be taped in place.)

 

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Service operations and removing modules from the front optical bench are described in Front Optical Bench.

 

9   Removing the Slit and Field Rotator Mechanisms

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9.1   Following removal of the Front Optical Bench a perspex cover should have been fitted to the front of the slit and field rotator. If not find and fit.   Disconnect and remove the two motor cables.   Disconnect the two status cables at the Radiation Shield end only and tape to the base of the assembly.   Disconnect the thermal wicks at the motor end, replacing the screws in the motor housing for storage.   Screws for the wicks should be replaced in the motor housings for storage.

 

 

 

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9.2   Remove the tape at the rear of assembly used for light sealing.  Rotate the instrument until the Slit/Field Rotator assembly is well supported, as shown in Figure 9.2. Undo the three bolts which secure the assembly to the MOB and remove it, taking care not to damage the cabling on the Radiation Shield, in particular where the field rotator motor passes the bracket on the radiation shield.

Figure 9.2 The orientation of the field rotator and slit wheel assembly for removal.

 

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9.3 Cover the newly exposed hole in the spectrometer collimator primary mirror with the appropriate perspex cover using the slit FROT screws to retain.   Store the slit/FROT assembly in an appropriate place.
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10   Removing the Closed Cycle Cooler (20 minutes)

 

10.1 Undo the 12 screws holding the two copper coloured flat plates which connect the copper wicking between the adapter plate at the end of the second stage of the cooler and the main optical bench, (as shown in Figure 10.1). Remove the wick.

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Figure 10.1 The closed cycle cooler second stage flexible wicking. The position of the temperature sensor on the adapter plate(now copper) is shown inset.

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10.2 Unscrew and remove the 2nd stage temperature sensor (seen between the two wick bundles in Figure 10.1).
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10.3 Undo the six 6-32UNC screws holding the 2nd stage adapter plate onto the end of the 2nd stage pillar and remove the adapter plate. This is an awkward operation requiring a short Allen key.
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10.4  Remove the eight screws holding the copper sleeve from the coldhead 1st stage to the radiation shield.   The cooler should now be free to be removed from the outside of the cryostat.

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Figure 10.4 Unbolting the CTI1020 cold head from the vacuum vessel.

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10.5 Undo the six bolts which mount the cooler onto the outside of the vacuum vessel (Figure 10.5) and lift the cooler clear (Figure 10.6).

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Figure 10.5 Removing the CTI1020 cold head from the vacuum vessel. Note that the copper wicking has been replaced by a solid copper sleeve.

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11   Removing the Rear Vacuum Cover and the Grating Drum

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11.1 Position the cryostat with the grating drum at the top and the interface between the grating and the centre section horizontal. Remove the bolts connecting the grating drum vacuum cover to the centre section.

   Note that as with the front cover, the screws nearest the manipulator may need to be removed first with the interface surface vertical to ease access.   

Install eyebolts in the grating drum cover, lift it off (mass = 100kg), and lower it onto a table. (see Figure 11.1).

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Figure 11.1 Removing the grating drum vacuum cover

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11.2 Inspect grating multi-layer insulation (MLI) blanket for damage and remove for possible storage under vacuum.  (Retained by velcro pads.)
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11.3 Undo bolts connecting grating radiation shield to centre section radiation shield and remove radiation shield.   Store the screws and belleville washer pairs in the box provided.
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11.4 Unplug the two motor cables, one status cable and two thermal wicks (shown in Figure x) from the motor side of the grating drum. All cables are green shrouded.
  • Ensure all motor drive cards have been depowered before disconnecting motor cables.
  • Leave the wavelength drive motor wick attached at the motor end to help in drum installation.
  • The wick attached to the motor which rotates the drum is tricky to access
               (see Figure 11.4).

 

Fig 2a (224364 bytes)     Fig 2b (84209 bytes)

Figure 11.4 The thermal wicks on the grating drive motors. The hand is pointing to the wavelength drive wick at the left (note the grey snubber bar on the radiation shield). The right hand picture shows the difficult to access grating exchange motor wick.

 

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11.5 Fit five grey coloured bar shaped snubbers to the centre section radiation shield using the radiation shield screws to finger tightness. (they act as guides when withdrawing the drum). Each snubber is numbered as is its location on the radiation shield.

 

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11.6   Fit two lifting shackles (the third is not required) to the drum on the side nearest to the motors, undo the four captive bolts holding it to the centre section using the special tools provided (two Allen key ended bars 57cm and 67cm long) and lift it out of the centre section as shown in Figure 10.6 (drum mass 135 kg).

 

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Figure 11.6 Removing the grating drum.

 

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11.7   As soon as the drum is raised far enough mount the protective perspex cover over the open face of the centre section to protect the spectrometer collimator and camera optics.   Four short screws are needed.

 

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11.8   Unplug and remove the three grating exchange motor drive and datum sensor cables from the centre section.

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12   Removing the Main Optical Bench

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12.1 Remove the vacuum fittings and the supporting collars from the MOB nitrogen fill tubes where they enter the vacuum vessel. (Figure 11.1).

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Figure 12.1. Removing the support collars on the nitrogen fill tubes.

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12.2 Rotate the instrument to access the pipes from the inside of the instrument and undo the 6 bolts which hold each onto the radiation shield. Remove the collets by pushing the pipes from the outside of the instrument (Figure 3.11.2).

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Figure 12.2 Removing the collet clamps from the MOB nitrogen fill tubes (Joule-Thomson cooler removed).

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12.3 Rotate the instrument until the truss feet (and the face of the vacuum vessel) are horizontal. Screw three lifting shackles into the MOB and then undo the eight M6 x 70mm screws which hold it down onto the truss feet (Figure 3.11.3).

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Figure 12.3 Undoing the bolts holding the MOB onto the truss.

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12.4 Lift the MOB clear, (as shown in Figure 3.11.4), with someone guiding the fill tubes through the radiation shield.

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Figure 12.4 Lifting the MOB from the vacuum vessel, with the fill tubes being guided through the radiation shield.

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