ITER VV supports

Cadarache 6 September 2007

A. Capriccioli

Outline

1. Pot bearings actual design: possible solution with

a) new 40 MN downward force

b) new 10 MN upward force

c) toroidal restraint system

2. Flexible Plates, alternative solution

3. Conclusions

With reference to the new 40 MN downward force (total value: dead weight plus vertical and horizontal electromagnetic forces) and

with reference to the actual design (net rubber diameter equal to 800 mm),

the average pressure on the rubber (Neoprene) component results equal to 79.6 MPa.

If the reference max value of 80 MPa, during downward transient VDE, can be assumed for the rubber component in the pot bearing pads, we obtain a safety margin SM = 1.

Several solutions can be adopted to increase the SM and the easier, cheaper and feasible seems to be the upward translation of the pot bearing itself:

1.a) New electromagnetic downward vertical force:

Port

Bearing Pad

Pedestal Ring

Ref. 960 mm

1800 mm

1300

mm

1158 mm

max 1400 mm

1200 mm

While in the toroidal direction

seems feasible to increase the

bearing pad size from 960 up to

1700 or more (see previous

figure), in the radial direction the

maximum neoprene diameter

should be around 1200 mm.

In every case, changing the

neoprene diameter from 800 to

1200 mm induces an area

increment of about 2.3 times and

an average pressure less then 35

MPa (against the previous 80

MPa, with an increment of the SM

from 1 to about 2.3).

1330 mm

1.b) New electromagnetic upward vertical force:

The max value of 10 MN upward per support was

estimated

The figure on the right side shows the long rods

and ropes groups foreseen to prevent vertical

detachment between VV and bearing pads.

With reference to the previous meeting (28 June

2007), the use of not preloaded tie-rods is to

avoid.

The use of very stiff rods only can replace the previous

one.

Another solution is the use of vertical dampers (the Fig.1

next page shows an example of shock absorbers).

In this case, two dampers 5000 kN each are necessary

and diameters around 550 mm with minimum 1.7 m length

are the standard dimensions.

Fig.1

The only way to reduce the dampers dimensions is the reduction of the

axial force, through the amplification of the displacement PortDamper.

A proposal of alternative

solution is shown in the

scheme of Fig.2a.

Only one damper is necessary

(the horizontal device); the

other elements are

connections between Port and

Ring. These last connections

form an articulated structure

attached to the Port/Ring side

through Cardan or spherical

joints (see Figures 2a and 2b).

When the port tries to move

vertically upward an axial force

acts on the damper and its

value is related to the slope of

the stiff connections.

Fig. 2b

With INCONEL rods diameter of 200 mm and a vertical force of 5 MN, the

max vertical displacement results about 0.35 mm per meter rod length.

~1400 mm L ____ mm

Tvf

/2

Tvf/2*0.4

20°

2*(cos - cos )yb

xa

(sin - sin )

b

y

xaTvf/2 = Total electromagneticvert. upward force /2

PORT

Fig. 2a

1.c) Toroidal restraint system:

The actual Toroidal restraint system seems to show

potential seizing risks and when the vertical supports allow

toroidal displacements, as in actual pot bearing (or

spherical bearing) pads, the “pendulum” restraint system

seems to be a really good choice.

M

The figures on the left side show the W7-X Auto Centering

System (see A.Cardella “ITER Vacuum Vessel Support System”,

Working Group Meeting, Cadarache 28 June 2007).

In the W7-X reactor no electromagnetic forces act on the

Vacuum Vessel and the “pendulums” geometry is not a

critical point.

In the figure it is possible to note the relative high L/D ratio

The system allows Vacuum Vessel vertical displacements

and radial thermal expansion with small toroidal rotations of

the whole VV itself.

The “pendulum” solution for ITER reactor has to take into account several basic points:

• the presence and the entity of the net horizontal force;

• the presence of the radial restraint system;

• during the disruption event, the opportunity to spread the reaction forces with different weights

between radial and tangential ports (to minimize the stress level in the Ports-VV connection).

At the moment no possibilities there are to

perform a detailed design and analysis of a

proper toroidal restraint style W7-X and only

a scheme of “pendulum” with variable axial

stiffness is shown in figures:

It will be possible to change with continuity the axial stiffness at constant pendulum length, if the

screws pitches are identical.

This could be fixed on one end to the lower port and on the other end to the pedestal ring.

2. Flexible plates, alternative solution

Tab.1: an example with the Actual Allowable Space (Width =960 mm and Length = 1200 mm)

40 MN Vertical Force + Radial Displacement:   Allowable

20 mm   

Thickness = 16 mm Membrane stress, σm 43.4 MPa < 141 MPa

n° of plates = 60 Membrane + Bending, σm + σb 149 MPa < 212 MPa

Width= 960 mm Buckling margin , mcr 2.5 < 3

Length = 1200 mm SM against collapse (Elas-Plas) 3.3 > 3

Radial space, ΔR 1430 mm    

If width = 1100 mm and n° of plates = 70 then Buckling margin, mcr = 3.3 >3 and ΔR = 1810 mm

Tab.2: the same example with 2 plate thicknesses

40 MN Vertical Force + Radial Displacement:   Allowable

20 mm    

Thicknesses = 14 / 20 mm Membrane stress, σm 29 / 39.2 MPa < 141 MPa

n° of plates = 70 Membrane + Bending, σm + σb 138.8 MPa < 212 MPa

Width= 1000 mm Buckling margin, mcr 3.3 > 3

Length = 1200 mm SM against collapse (Elas-Plas) 3.5* > 3

Radial space, ΔR 2090 mm    

A point to point out is the toroidal stiffness of the flexible plates system: in this case its value is

basically high and cannot be easily changed (for example it is possible to divide each plate

vertically in two or more parts).

The values of the toroidal restraint and radial restraint stiffness are two basic characteristics to

evaluate together with the tangential and radial stiffness of the Port / VV shell connection.

Tab.3: Flexible plates 2.4 m height

25 MN Vertical Force + Radial Displacements:   Allowable

20 mm 30 mm 40 mm    

Thickness = 43 mm Membrane stress, σm 16.2 MPa 16.2 MPa 16.2 MPa < 141 MPa

n° of plates = 30 Membrane + Bending, σm + σb 88.5 MPa 131 MPa 162 MPa < 212 MPa

Width= 1200 mm Buckling margin, mcr (*) 12 (µ=0.5) > 3

Length = 2400 mm SM against collapse (Elas-Plas) 10.8 > 3

Radial space, ΔR 1580 mm

(*) see formula (3) from X.Wang, K.Ioki - ITER, August 7, 2007 "Preliminary Assessment of Multi Flexible Plates VV Support"

Tab.2: Flexible plates 2.4 m height with 40 MN

40 MN Vertical Force +Radial Displacement:   Allowable

20 mm 30 mm    

Thickness = 43 mm Membrane stress, σm 26 MPa 26 MPa < 141 MPa

n° of plates = 30 Membrane + Bending, σm + σb 97.3 MPa 135 MPa < 212 MPa

Width= 1200 mm Buckling margin, mcr 7.4 (µ=0.5) > 3

Length = 2400 mm SM against collapse (Elas-Plas) 6.8* > 3

Radial space, ΔR 1580 mm

3. Conclusions

The two analyzed VV Support systems are:

(1) Pot bearing + vertical upward restrain + toroidal system → Vertical up/down + toroidal

(2) Flexible plates → Vertical up/down + toroidal

- Both seem feasible (and for both other analyses are necessary).

- The system (1) foresees common industrial use devices (pot bearings, shock absorbers) while the

“pendulum“ (W7-X type) toroidal restraint system has to be analyzed.

- The system (2) results easier, because in a single block are present all the restraints but it is not a

commercial device (R&D), has a fixed toroidal stiffness and more vertical space would be

necessary (see the buckling margin mcr).

- Common to both the systems (1, 2) is the radial restraint.

- With the new radial forces during the downward vertical plasma disruption (73 MN against the

previous 25 MN), the radial restraint system must be reviewed.

- The Vacuum Vessel and Ports global model is essential to the evaluation of all the restraint

systems.