Vacuum vessel

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Vacuum vessel
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• Functions
• Loads
• Design criteria
• Materials
• Technological issues
• Design and manufacturing examples
• - JET vacuum vessel
• - ITER vacuum vessel

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JET Vacuum Vessel
The large vacuum vessel of the J.E.T. (Joint
European Torus) experiment has been designed as
an all metal torus of D shape cross-section. To
comply with mechanical stability and ultra high
vacuum requirements it has been designed as a
completely welded fabrication. The metal
structure consists of 32 rigid and wedge-shaped
sectors (equal in number to the B-TOR field
coils) which in turn are joined together by
parallel bellows to form a torus.
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JET Vacuum Vessel
This toroidal vacuum vessel has been designed as
a double walled structure with the bellows
linking the rigid sectors being fitted as
pairs. The rigid sectors are of box type
construction suitably ribbed to withstand the
stresses imposed upon them by the external
forces. The bellows linking these rigid sectors
determine the electrical resistance of the
metallic vacuum vessel the long way around the
torus, since currents which are induced in
parallel with the plasma current should be kept
as low as possible. Interspace used as channels
for water or inert gas to control the vessel
temperature during wall conditioning (designed up
to 500C) and experimental sessions.
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JET Vacuum Vessel
Materials Many materials in the stainless range
were critically examined in order to find one
with the required strength characteristics at
high temperatures. Of these, the high nickel
content alloys such as Inconel and Nicrofer were
found to be the most suitable, having the
required tensile properties, electrical
resistivity and workability. Inconel 625 was
finally chosen as the bellow material and Inconel
600 as the material from which to manufacture the
rigid sectors, where the requirements are not so
severe.
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JET Vacuum Vessel

• Forces
• The main forces acting on the vacuum vessel arise
  from
• the atmospheric pressure acting on the outside
  wall of the vessel and is in the order of 20
  tonnes on each of the 32 rigid sectors
• (b) the electromagnetic forces due to the eddy
  currents which are induced in the metallic
  structure when the magnetic fluxes linking the
  vacuum vessel change with time. These forces put
  an additional load of 10 tonnes per sector upon
  the vacuum vessel
• (c) force due to thermal expansion which occurs
  during the bake-out cycle, but this force is not
  present when the machine is in operation.

All forces acting on the vacuum vessel are
absorbed by the rigid sectors which also
incorporate the openings to the interior of the
machine, such as ports for pumping, diagnostics,
auxiliary plasma heating, etc...
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JET Vacuum Vessel
Support system The support system of the vacuum
vessel has been designed to minimise the built
in stresses in the torus to accommodate the
radial movement expected during the bake out
cycle, any distortion due to mechanical loading
and any movement due to electro-mechanical
stresses. This has been achieved by hanging the
torus from the mechanical structure on spring
loaded suspension units.
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JET Vacuum Vessel
Bellows manufacturing Several different methods
of forming the D shaped bellows, such as
explosive forming, hydraulic forming and rolling
were evaluated. Forming by rolling was
considered to be the best, mainly because of the
size and non circular shape. They are formed up
from a plain blank by rolling in all of the
convolutions simultaneously whilst at the same
time an axial load is applied to the edge of the
blank to assist the flow of material around the
rollers. To generate the required contour strong
D shaped flanges, of the correct contour of the
finished bellows, are welded to either end of the
blank and guided in a true D shaped path by means
of adjustable rollers.
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JET Vacuum Vessel
Rigid sectors manufacturing The fabrication is a
box type construction with the inner and outer
shells being joined by flat D shaped
diaphragms. This method of construction is
relatively easy to fabricate and provides ample
access for welding, which is by the argon arc
process.
The plates are formed to the correct contour, and
welded to form the inner shell. Stiffening
bulkheads will be welded on to this shell and the
outer shell will be then welded on to these thus
making a rigid sector, which when the horizontal
and vertical ports are welded in will be stress
relieved in an inert atmosphere. When chemical
cleaning to ultra high vacuum standards is
completed the shell interspace is pumped down and
the rigid sector will be subjected to a vacuum
leak test. Rigid sectors and bellows assemblies
are then joined together by welding to form an
octant.
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JET Vacuum Vessel
VDEs and unforeseen forces on Vacuum Vessel
upgrades of the support system To counteract
vertical forces released during vertical plasma
disruptions at JET, the so called Vertical
Displacement Events (VDEs), the following
support systems have been implemented Main
vertical port restraints Main vertical Port
restraints (locked friction brakes) Main
vertical port hydraulic damper. The above
support systems did not counteract sufficiently
the lateral forces, sometimes associated with
vertical forces during VDEs. These lateral forces
have caused damage to vacuum vessel attachments
and especially to the Rotary High Vacuum Valves.
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JET Vacuum Vessel
To counteract these lateral forces, a set of
hydraulic dampers connecting the main horizontal
ports to the mechanical structure/transformer
limbs was installed. These dampers did not have
the desired effect since the actual forces were
higher than anticipated. Further supports were
then required. The additional system comprises 32
radial dampers, 2 upper and 2 lower on each
octant. These hydraulic or viscous dampers
connect the vacuum vessel to the mechanical
structure directly.
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ITER Vacuum Vessel
The primary functions of the VV are

• to provide a high-quality vacuum for the plasma
• to be the first containment barrier for
  radioactive materials
• to be a second barrier (after the cryostat) for
  the separation of air from potential sources of
  in-vessel hydrogen generation
• radiation shielding for the magnets with in-wall
  shielding (the neutron heating is removed by the
  water circulating between the shells)
• to support blanket, divertor and other in-vessel
  components, including their loads, during normal
  and off normal operation
• a tight fitting configuration of the VV to the
  plasma aids the plasma vertical stability

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ITER Vacuum Vessel
The VV is a torus-shaped, double wall structure
with shielding and cooling water between the
shells. The VV is located inside the cryostat
and supported by the vessel gravity supports from
the toroidal field (TF) coil case. This is now
under deep revision for final design. The
blanket and divertor are mounted on the vessel
interior and all the loads are transferred to the
vessel. The blanket modules are supported
directly by the VV and the blanket cooling
channels are routed over its plasma-side
surface. The VV has upper, equatorial, and lower
port structures used for equipment installation,
utility feedthroughs, cryo-vacuum pumping, and
access inside the vessel for maintenance.
ITER radial cross section In red the vacuum vessel
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ITER Vacuum Vessel
Basic configuration of the typical VV sector and
main parameters
Size Torus outer diameter 19.4 m Torus height
11.3 m Thickness of the double wall 0.340.75
m Toroidal extent of the sector 40 Thickness of
the inner/outer shell 60 mm Rib thickness
4060 mm Surface area/volume Interior surface
area 850 m2 Interior volume (w/o ports) 1600
m3 Mass Main vessel 3755 t Port
structures 1370 t Water 235 t Total 5360 t
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ITER Vacuum Vessel

• The main components of the ITER VV are
• the main vessel
• port structures
• in-wall shielding

Re 8.9 m
Ri 3.6 m
The ITER vacuum vessel is a torus-shaped double
wall structure. The double wall structure has
stiffening ribs between the shells to give the
required mechanical strength and separate the
shells.
H 10.3 m
Vacuum vessel in green
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ITER Vacuum Vessel
The inner and outer shells are both 60 mm plates
and the stiffening ribs mainly 40 mm plate. All
are made of austenitic stainless steel. The
inner and outer shells and stiffening ribs are
joined by welding.
The shells and ribs form the flow passages for
the vessel cooling water.
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ITER Vacuum Vessel
Neutron shielding and magnetic field ripple
reduction
The space between the shells is partially filled
with in-wall shielding consisting of stainless
steel plates with 12 boron for the neutron
shielding, or ferromagnetic steel plates to
reduce the toroidal field ripple in certain
locations.
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ITER Vacuum Vessel
Blanket in-vessel supports
To support the blanket modules, housings for
blanket flexible supports are incorporated
between the shells of the main vessel. The inner
and outer shells, the housings, and the
stiffening ribs, are joined by welding.
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ITER Vacuum Vessel
Divertor support system
Welded supports

• About 70 mm for
• Tolerances
• Thermal expansions
• Deformations
• Diagnostics cables

Divertor cross section
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ITER Vacuum Vessel
The most recent updated design of vacuum vessel
sector with blanket and divertor supports,
lifting and gravity supports.
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ITER Vacuum Vessel
Cooling pipes fixing systems
Blanket modules fixing systems (keys and bolted
connections)
Blankets Vessel electrical connections
Divertor modules supports
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ITER Vacuum Vessel
Ports design
The port structures include those for the upper,
equatorial and lower ports, and the local
penetrations at the lower level. The port
components are mainly of double-wall construction
with single-wall portions included into some
ports. A typical port structure includes a port
stub, a stub extension and a port extension. The
end portion of the port extension is normally
equipped with a closure plate that provides the
primary vacuum boundary. For the upper and
regular equatorial ports, the closure plate is
integrated with the in-port componentsforming
the port plug. The port extension is connected
to the cryostat with a connecting duct.
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ITER Vacuum Vessel
Ports design
A single wall concept has been developed for the
upper and equatorial ports. For this concept, the
inner shell is kept but the outer shell is
eliminated along a certain portion of the port
length (where the neutron heating is low). To
control temperature, the double wall design
and/or active cooling is kept at both ends of the
single wall part. To preserve the shielding
performance, additional shield plates are
attached to the single wall shells.
Upper port structures
Lower port structure
Equatorial port structure
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ITER Vacuum Vessel
Ports design
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ITER Vacuum Vessel
Vacuum vessel materials
SS 316L(N)-IG Type Austenitic stainless steel
(SS) of 316 type is the main structural material
for the ITER vacuum vessel and for in-vessel
components (shielding blanket, divertor cassette
body) This steel is qualified in many national
design codes, has adequate mechanical properties,
good resistance to corrosion, weldability,
forging, and casting potential, is industrially
available in different forms and can be
manufactured by well-established
techniques. Among the 316 steel family, 316L(N)
type SS developed for the European Fast Breeder
Reactor program) has been selected for ITER
application. Borated Steel A standard borated
stainless steel is proposed for the shielding
inserts of the VV to increase the shielding
efficiency. 304B7 steel is used in the form of
plates fixed between the two VV walls.
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ITER Vacuum Vessel
Vacuum vessel materials
Precipitation-Hardened Steel High strength
precipitation-hardened stainless steel type 660
is proposed for the port stiffening extensions
and bolting the port plugs. Ferritic
Steel Inserts of ferromagnetic material are used
in the outboard area inside the vacuum vessel in
the shadow of the TF coils to reduce the toroidal
field ripple. SS 430 is recommended for the
ferromagnetic plates, because it has relatively
good corrosion resistance in spite of its
slightly lower magnetisation than the best
materials.
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ITER Vacuum Vessel
Austenitic stainless steel (SS)
Among the 316 steel family, 316L(N) type SS
(based on 316L(N)-SPH SS developed for the
European Fast Breeder Reactor program) has been
selected for ITER application. The main reasons
for the selection of this grade are the
following. The proposed grade has an optimal
combination of the main alloying elements (C, N,
Ni, Cr, Mn and Mo) with a tight specification of
their allowable composition range. The narrow
specification provides an optimal microstructure
and good control of the heat-to-heat variation of
mechanical properties. The tight control of
the carbon and nitrogen provides a satisfactory
resistance to stress corrosion cracking of base
metal and welds, and an adequate level of
material strength. The mechanical properties
of the proposed grade are better than those of
316L and 316LN steels. The higher strength is
combined with a good ductility. The design
allowable stress is higher than in the other SS
grades. The proposed grade is less prone to
delayed reheat cracking than Ti or Nb stabilised
steels. 316L(N) is less sensitive to
irradiation embrittlement than 304 steel.
There is a comprehensive database for this
grade, including heat-to-heat variations and
product size.
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ITER Vacuum Vessel
Vacuum vessel materials
Austenitic stainless steel (SS)
For ITER, only minor modifications in chemical
composition are required, in order to cope with
the radiological safety limits and with the
re-welding requirement.
Reducing the Co content from 0.25 to 0.05
decreases the total decay heat in the vacuum
vessel by 20 and helps to reduce the activation
of components. Cobalt is one of the main
components of activated corrosion products in the
water cooling system. Nb produces long-lived
radioisotopes that could become important for the
decommissioning and waste disposal of in-vessel
components. In 316 type SS, niobium is present as
a trace element picked up during the melting
process from the ferroalloy addition existing
data indicate that it is possible to reduce Nb to
lt 0.01.
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ITER Vacuum Vessel
Vacuum vessel materials
Austenitic stainless steel (SS)

• To provide a common designation and to avoid any
  confusion with similar steel grades, the
  designation 316L(N)-IGX is used, where
• 316 indicates the type of steel
• L low carbon content
• (N) controlled nitrogen content
• IG - ITER Grade
• X is a progressive number indicating the
  procurement specification, which defines the
  additional requirements in terms of product form,
  impurity content, QA procedure, and delivery
  conditions needed for the different components
• 316L(N)-IG1 for the modules of the primary wall
  should have a cobalt content
• lt0.05
• 316L(N)-IG2 for the vacuum vessel should have a
  cobalt and niobium content as low as possible
  cobalt and niobium lt 0.05 and lt 0.01,
  respectively, have been assumed as industrially
  feasible limits taking into account the
  requirement of rewelding the irradiated material,
  the boron content should be limited to 10 wppm.

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ITER Vacuum Vessel
Cooling
Heat deposited in the vacuum vessel during normal
and off-normal operations will be removed by
cooling water that is supplied by the VV primary
heat transfer system. Two independent water loops
with the same cooling capability are used in each
of the 9 sectors, feeding all sectors in
parallel. The water flow velocity and mass flow
rate for normal operation need to cope with the
nuclear heating rate in the VV in such a way that
will keep thermal stresses in the VV structure at
acceptable levels. During normal operation, the
design value of the total heat deposition in the
VV is mainly due to nuclear heating. The heat is
non-uniformly deposited in the VV. In addition,
high heat deposition is also expected in the
neutron-streaming regions, such as between
blanket modules. The non-uniformly distributed
heat in the VV is to be removed without any local
overheating of the VV. During an off-normal
event, e.g., a multiple cooling pump trip in the
blanket cooling system, the main heat load to be
considered is thermal radiation from the
blanket/divertor.

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ITER Vacuum Vessel
Two independent water loops with the same cooling
capability are used in each of the 9 sectors,
feeding all sectors in parallel. The required
water flow condition for normal and baking
operation is forced turbulent flow.
Since about 70 of the heat is deposited in the
inner shell and the first shield plate during
normal operation, the flow route is designed to
allow the water to flow mainly through the gap
between them.
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ITER Vacuum Vessel
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ITER Vacuum Vessel
ITER Vacuum Vessel fabrication Transport and on
site assembly requirements affect the main
choices for modular construction of VV at the
factory. To minimize the final assembly time on
site and to deliver a vessel structure with a
high quality, the vacuum vessel is to be
fabricated in the manufacturing factory as 9
sectors, each spanning 40. The possibility of
transporting such a large sector from the factory
to the assembly area is an important factor in
the manufacture of the vessel.
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ITER Vacuum Vessel
ITER Vacuum Vessel fabrication 3 Options are
presently considered
In the case of options 1 3, it is difficult to
correct welding deformation and to predict the
amount of welding deformation. Option 2 makes it
possible to correct and minimize the welding
deformation by machining an additional margin
before the welding of 4 poloidal segments.
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ITER Vacuum Vessel
ITER Vacuum Vessel fabrication On site assembly
sequence At the ITER site, each sector is mated
to two TF coils (and thermal shield) and
assembled. The lateral port stub halves are then
welded in place. After the sector/coils are
lowered into and positioned in the pit, 9 field
joints (located on the centre of the lateral
ports) are TIG-welded using splice plates to
compensate for the size differences of the
sectors. The final machine assembly sequence
involves a sequential attachment of adjacent
sectors until the resulting 160 and 200
segments are finally joined. Before VV
commissioning is complete, the pressure boundary
must be pressure tested as required by the code.
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ITER Vacuum Vessel
Vacuum Vessel structural analyses
Loading categories and design criteria (ASME Code)
The allowable stress factor depends on Loading
Event Category and Service Level
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ITER Vacuum Vessel
Vacuum Vessel structural analyses
Events are further classified into different
service levels A, B, C or D which relate to the
damage limits

• negligible damage, functional
• negligible damage, maintenance may be needed
• local distortion possible, repair or replacement
  may be needed
• large nonlocal distortion possible, but minimum
  safety function maintained, and replacement may
  be needed

The allowable stresses for the various types of
stress (primary, membrane, bending, etc.) as
defined by ASME VIII div.2 appendix 4 are
computed as follows

• general primary membrane lt 1.0 k Sm
• local primary membrane lt 1.5 k Sm
• primary membrane bending lt 1.5 k Sm
• primary secondary lt 3.0 k Sm (only for
  Category I and II)

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ITER Vacuum Vessel
Vacuum Vessel structural analyses
Operating states and associated load categories
Unlikely fault conditions
Extremely unlikely fault conditions
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ITER Vacuum Vessel
Vacuum Vessel structural analyses
Operating states and associated load case
combinations
Different types of VDEs are considered based on
the speed of the plasma current quench (slowS
and fastF) and on the direction of the plasma
movement (downwardD or upwardU). These VDEs are
named VDE/S-D, VDE/S-U, VDE/F-D, and VDE/F-U.
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ITER Vacuum Vessel
Vacuum Vessel structural analyses
For normal operating conditions the most severe
loads are caused by

• coolant pressure
• VV and in-vessel component weights
• seismic events
• plasma disruptions and VDEs
• TF coil fast discharge (TFCFD)

Maximum internal and external VV pressure in
off-normal events is 0.2 MPa. The loads that
mainly drive the design are due to a centred
disruption (CD), VDE, and a TFCFD.
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ITER Vacuum Vessel
Vacuum Vessel structural analyses
EM Loads
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ITER Vacuum Vessel
Vacuum Vessel structural analyses
EM Loads
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ITER Vacuum Vessel

• Structural Analyses of Main Vessel
• Primary Stresses of Vacuum Vessel
• Detailed Local Stress Analysis
• Dynamic analysis
• Seismic analysis
• Buckling Analyses
• Thermal Stress due to Nuclear Heat Load
• Structural Analyses of Port Structures
• Thermal and Hydraulic Analysis

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ITER Vacuum Vessel
Vacuum Vessel structural analyses
The global FE model static analyses of the VV
have been performed using two different models
model A and model B. Each model provides, other
than the results of the global analysis, boundary
conditions for carrying out different specific
local detailed analyses.
One of the most critical regions of the VV is the
inboard wall because of the high toroidal field,
which causes high EM forces. For this region the
most severe loading conditions are the TFCFD and
its load combination with EM loads due to a VDE,
which cause high compressive stresses in the VV
inboard wall and increase the risk of buckling.
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ITER Vacuum Vessel
Detailed Local Stress Analysis
The VV structure has several geometrical
discontinuities that cause localized
stresses. The regions that have been analyzed in
detail are - inboard wall region around the
support housings - lower and upper parts of the
inboard wall region - inboard wall triangular
support frame for the lower inboard blanket
module - outboard wall region between the
equatorial port and the lower port - VV support
region.
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ITER Vacuum Vessel
Dynamic analyses
Due to the impulsive nature of the EM loads on
the VV and in-vessel components in disruptions
and VDEs, a dynamic analysis of the structure is
required to understand the dynamic behavior of
the VV and the dynamic amplification of the
structure response for different loading
conditions. Different FE models have been
developed for dynamic analyses.

• The most important models and their main purposes
  are
• Model A a 20 sector model for the dynamic
  time-history analysis of the symmetric VDE and
  disruption cases
• Model B a 90 sector model for the modal
  analysis
• Model C a 180 sector model for the dynamic time
  history analysis of the asymmetric VDE cases.

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ITER Vacuum Vessel
Thermal Stress due to Nuclear Heat Load
The thermal stress in normal operation is
generated mainly by the nuclear heating. The
nuclear heat load on the surface of the inner
shell of the VV on the plasma side is equal to
0.1 W/cm3 in locations behind the blanket
modules. Local peaks of 0.4 W/cm3 are achieved in
correspondence of neutron streaming positions.
Assuming 500 W/m2K value for the HTC, the highest
temperature of the vessel structure is about
161C (water coolant temperature 100C nuclear
heating rate 0.4 W/cm3) and the thermal stress
is 175 MPa.
High thermal stress can be generated even in
relatively small structures attached to the VV
inner shell (i.e. shear and stub keys, divertor
support rails) if not actively cooled or properly
shielded.
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This and following slides are from ITER Vacuum
Vessel Status and Procurement - IBF, Nice 11
December 2007, K. Ioki, Tokamak Department, ITER
Organization
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