**5.3 Vacuum Vessel**

**5.3.1 Introduction**

The vacuum vessel, shown in Fig. 5.3.1-1, provides the vacuum
environment for the plasma as well as the first confinement barrier for
radioactive materials. The vessel also serves as the support structure
for all in-vessel components, provides the first level of nuclear
shielding, and helps provide for the passive stabilization of the
plasma. The vessel system includes the torus, the ports and port
extensions, the gravity supports, the supports for internal components,
the passive stability plates, the internal control coils, and the
integrated coolant/bake-out lines.

![](media/image1.png){width="6.933333333333334in"
height="4.6722222222222225in"}

**5.3.2 Vessel Concept**

The vessel torus is a double wall sandwich structure consisting of 15 mm
thick inner and outer face-sheets attached to poloidal ribs. The space
between the face-sheets, which varies from 20 mm on the inboard side to
540 mm on the outboard side, is filled with radiation shielding material
and coolant. Water at 20-50 C and 1 MPa is used to remove nuclear
heating during normal operation. The water temperature is raised to 150C
for heating the vessel and internals during bake-out. The shielding
material can be single sized stainless steel balls with a packing
fraction of about 60% or stacked plates with a similar packing fraction.
The vessel parameters are summarized in Table 5.3.2-1.

The primary advantages of the double wall structure include higher
bending stiffness (for a given total material thickness) and better
integration of cooling and shielding. Most vacuum vessel designs in use
(JET, JT60, DIII-D) and most designs on the drawing board (ITER, KSTAR)
use full or partial double wall vacuum vessels. Figure 5.3.2-2 shows a
cutaway of the vessel and pertinent dimensions.

There are 16 sets of access ports around the torus, which are used for
RF heating, remote maintenance, diagnostics, internal cooling, fueling,
and pumping. There are large, 1.3 x 0.7 m midplane ports, upper and
lower trapezoidal ports approximately 0.15 x 0.5 m, and upper and lower
oblong vertical ports approximately .08 x 0.15 m in size. The sets of
port openings are identical at each toroidal location to provide
structural and design symmetry, but the port extensions may be varied to
match their specific purpose. The port extensions are required to extend
the vacuum boundary past the TF coil legs and through the cryostat
region. ![](media/image2.wmf){width="5.875694444444444in"
height="4.539583333333334in"}

**5.3.3 Vessel loading and analysis**

The vessel is subjected to large gravity, seismic and electromagnetic
loads, as summarized in Table 5.3.3-1. The total vertical load is
estimated to be about 20 MN, while the net lateral load is about 7 MN.
To react these loads, the vessel is supported near the midplane on the
outboard side via vertical and lateral links to the TF coil structure.
The vertical links are attached to the radial ribs to spread the applied
loads vertically into the vessel. This minimizes the local bending
stresses in the vessel and provides a means for adjusting the vessel
location globally relative to the TF coils. Lateral supports are located
near the vertical links, and are tied to the top of the midplane ports.

The vessel must support all internal components, including the divertor
assemblies, the passive stability structure, and the first wall tiles.
The outboard divertor modules are actively cooled via pipes at each of
the upper and lower auxiliary ports. The passive stabilizing structure
is actively cooled with imbedded cooling tubes. The first wall tiles and
inboard divertor are cooled by conduction to the passive structure. All
components must have robust supports to react the electromagnetic loads
from a plasma disruption.

Preliminary structural analysis of the vessel indicates that the present
dimensions are acceptable to support the various loads. A finite element
model was developed for an earlier version of the vessel geometry, and
the stresses and deflections obtained are summarized in Table 5.3.3-2.
As seen in the table, there are some peak stresses around the divertor
supports at the top of the vessel that must be mitigated with additional
structure. Details of the stress and deflection analysis are contained
in Appendix D of this report.

**5.3.4 Passive plates and internal coils**

As indicated in Section 2, Physics, a system of highly conducting and
actively cooled passive plates and a set of internal control coils must
be incorporated into the vacuum vessel. The passive plates consist of 25
mm thick copper sheets that are bonded directly to the surface of the
vacuum vessel. The sheets are actively cooled via

internal water passages connected

through manifolds into the vessel cooling system. A bonded connection is
thought to be the most straightforward approach, since cooling can be
provided directly by the copper plates to both the

![](media/image3.wmf){width="6.501388888888889in"
height="4.706944444444445in"}

first wall tiles and the vessel, and continuous structural support can
be provided to the passive plates by the vessel. The method of bonding
has not been decided, but hot isostatic pressing (hipping) is one
possibility. The geometry of the passive plate system is shown in Figure
5.3.4-1.

In addition to the passive plates there are a pair of control coils
located between the outboard walls of the vessel above and below the
midplane ports. Multiple turns of conductors are run in permanent pairs
of conduits that are routed directly through the outboard wall. The
conductor will receive a moderately high radiation dose (\>10^9^ Rad)
and will be insulated with either MgO or a polyimide insulation system.
Redundant turns are being considered to mitigate one of the failure
modes. Each coil is designed to carry up to 75 kA.

**5.3.5 Fabrication and assembly**

The vessel is fabricated in octants, as shown in Figure 5.3.5-1. Each
octant consists of the torus, associated gravity and internal supports,
short reinforcing stubs around the major port openings and the active
and passive stabilizer systems. At assembly, each vessel octant is
rotated into the bore of a pre-assembled TF coil pair and connected via
the support links. The TF/VV subassemblies are then positioned relative
to each other with the mating joints located at radial planes between TF
coils, through the center of the ports.

When all the octants are in place and positioned, they are welded
together from the plasma side of the torus. The field joint for the
double wall structure uses splice plates on the plasma side to provide a
means for accessing the coil-side facesheet from the plasma side of the
torus. This type of joint has undergone significant, full scale testing
using remote welding equipment as part of the ITER R&D program. After
the vessel is welded, the gaps in the passive stabilizers are filled
with plasma sprayed copper to complete the upper and lower stabilizing
circuits. After the torus is welded, the port extensions are fitted and
welded to the port stubs. This completes the vessel
assembly.![](media/image4.png){width="5.990277777777778in"
height="4.586805555555555in"}