**5.5 Thermal Shield**

## 5.5.1 Introduction

![](media/image1.png){width="2.8in" height="3.0083333333333333in"}The
main function of the thermal shield system is to provide the required
thermal environment around the coils, which operate at cryogenic
temperatures. To maintain this environment, the nitrogen gas inside the
shield must be contained (not allowed to leak out) and the air on the
outside of the shield must not be allowed to leak into the shield. The
main reason to exclude air from the inside of the enclosure is to
prevent the accumulation of ice on the cold surfaces. In addition to
maintaining the required thermal environment on the inside of the
shield, it is also important to maintain the exterior surfaces of the
shield at a high enough temperature to prevent water condensation, which
can result in corrosion and operational and safety problems.

The basic design concept for the thermal shield is to have a stainless
steel structure (channel beams) on the inside of the shield that can
support panels made of insulating materials (Figure 5.5.1-1). The SS
structure will be covered with a thin SS shell, which will form part of
the sealed boundary for the shield. Insulation will be sprayed directly
onto this shell. Penetrations will be sealed with rubber or fabric
bellows that accommodate the relative motion between the VV and thermal
shield.

## 5.5.2 Requirements

The most important requirements for the thermal shield relate to
maintaining the 80 K thermal environment inside the shield. To do this
efficiently, the maximum heat loss through the shield has been set at 15
kW. To prevent condensation on the exterior of the shield, the surface
temperature must be maintained above the dew point for air with a
relative humidity of 50%. In addition, the constraint has been imposed
to maintain the shield exterior within 10 °C of the test cell ambient
temperature.

![](media/image2.png){width="3.1847222222222222in"
height="2.936111111111111in"}

**Figure 5.5.1-1 Thermal Shield Design Concept**

Another important requirement pertains to pressure loads on the shield.
If we have an open LN~2~ system, interior pressure will be due to N~2~
gas flow through the system and out the vent. The maximum ∆p across
shield wall (maximum pressure on inside) has been set at 0.8 kPa (0.12
psi or 3 inches of H~2~O). A higher pressure on the outside of the
system could occur during cool down of the system if N~2~ flow is not
maintained. In this case, the maximum ∆p across the shield wall (maximum
pressure on outside) has been set at 0.1 kPa.

Requirements related to the flexible joints that accommodate the
relative motion between the VV and thermal shield include a maximum
integrated leak rate for all seals of 1 l/s (at 1 atm exterior pressure)
with a ∆p across the joint of 0.8 kPa. The joints must accommodate
relative motions between the components up to [+]{.underline} 25 mm.

The requirements for the thermal shield are summarized in Table 5.5.2-1.

The radiological inventories have been estimated to be modest for the
FIRE facility and lead to a requirement for the vacuum vessel to be a
highly reliable primary confinement barrier for the in-vessel
inventories, and the thermal shield to serve as a moderately reliable
secondary barrier (see Section 5.15).

In FY01, preliminary accident scenarios were defined for the FIRE
cryostat as:

-   Failure to completely purge the magnet coolant channels before a
    full power pulse

-   \- Divertor coolant leak into thermal shield leads to ice
    accumulation over an operating campaign

-   \- Nitrogen coolant leak into thermal shield accumulates nitrogen
    over an operating campaign

    Preliminary analyses have started and will continue in FY02 to
    determine the bounds on material available for activation and the
    impact of an accident of this type.

## Description

The thermal shield has three basic parts, (1) the SS structure and shell
which support the insulation and form part of the sealed boundary for
the shield, (2) the insulation itself, and (3) the flexible joints that
accommodate the relative motion between the VV and thermal shield. The
shield is semi-cylindrical in shape (formed by 16 flat facets) with a
flat top and bottom. Its OD and height are 11.2 and 11.5 m,
respectively. The total wall thickness of the structure and insulation
is 0.65 m.

## Table 5.5.2-1 Thermal Shield Design Requirements -- Summary

+-----------------------------------------------+-----------------------+
| ##### Design Parameter                        | # Value               |
+-----------------------------------------------+-----------------------+
| ###### Thermal                                |                       |
+-----------------------------------------------+-----------------------+
| 1\. Condensation on surfaces of the shield.   | None                  |
+-----------------------------------------------+-----------------------+
| 2\. Max. temp. difference between shield      | 10 °C                 |
| exterior and test cell temp.                  |                       |
+-----------------------------------------------+-----------------------+
| 3\. Max. heat flow through the shield         | 15 kW                 |
| assembly                                      |                       |
+-----------------------------------------------+-----------------------+
| 4\. Minimum gas temp. contained by the shield | 80 K                  |
+-----------------------------------------------+-----------------------+
| 5\. Max. gas temp. contained by the shield    | 150 °C                |
| (during VV baking)                            |                       |
+-----------------------------------------------+-----------------------+
| 6\. Max. temp. around feedthroughts           | 200 °C                |
+-----------------------------------------------+-----------------------+
| ###### Structural                             |                       |
+-----------------------------------------------+-----------------------+
| 1\. Max. ∆p across shield wall (max. pressure | 0.8 kPa               |
| on inside)                                    |                       |
|                                               | (0.12 psi)            |
+-----------------------------------------------+-----------------------+
| 2\. Max. ∆p across shield wall (max. pressure | 0.1 kPa               |
| on outside)                                   |                       |
+-----------------------------------------------+-----------------------+
| ###### Mechanical                             |                       |
+-----------------------------------------------+-----------------------+
| 1\. Max. integrated leak rate of all seals    | 1 l/s at 1 atm        |
| (between penetrations and thermal shield wall |                       |
| panels)                                       | (∆p- 0.8 kPa)         |
+-----------------------------------------------+-----------------------+
| 2\. Seals must accommodate relative motion    | Vertical -            |
| between penetrations and thermal shield       | [+]{.underline} 25 mm |
| panels (Initial Design Values)                |                       |
|                                               | Toroidal -            |
|                                               | [+]{.underline} 10 mm |
|                                               |                       |
|                                               | Radial -              |
|                                               | [+]{.underline} 18 mm |
+-----------------------------------------------+-----------------------+

Sixteen (16) flat panels that are centered on each row of ports form the
vertical sides of the thermal shield. Bolting the panels together forms
the complete cylinder (each panel spans 22.5 degrees). Each panel
consists of a perimeter frame, which is made with 10-inch channels. A
thin SS skin is welded to the frame to form part of the barrier for the
shield. Insulation is sprayed directly onto this skin. Holes are
provided through the skin and insulation where the vacuum vessel ports,
buswork, cooling and other services pass through. Clearance is provided
between the ports and the hole in the thermal shield to allow for the
relative motions of the VV and the shield. The maximum relative motion
([+]{.underline} 25 mm) results from the condition when the VV and
thermal shield are at normal operational temperatures and simultaneously
are under seismic conditions.

The seal around the ports is provided by a flexible joint (similar to a
bellows) that is attached to the VV port and the SS sheet part of the
structure. The joint is a single layer of silicone rubber on a fabric
base that is \~ 12 inches long (in the direction of the port axis). It
can flex and buckle to accommodate the relative motion.

The insulation is the most important part of the shield. Several types
were considered and compared before making a selection. The candidates
are shown in Table 5.5.3-1. As can be seen, the thermal conductivity and
cost of the cryo-lite and the polyurethane foam are similar. The
solimide polyimide foam has a higher thermal conductivity and cost. Due
to the ease of application of the polyurethane foam, (which is sprayed
on) this insulation has been selected for the thermal shield. This
material can be sprayed directly onto stainless steel to any desired
thickness. A rubber vapor barrier can then be sprayed onto the exposed
surface, which also enhances its abrasion resistance.

####### Table 5.5.3-1 Insulation Comparison

+-------------------------------------+-----------------+--------------+
| **Mtl Name**                        | **k**           | **Cost**     |
|                                     |                 |              |
|                                     | **(W/m-K)**     | *            |
|                                     |                 | *(\$/m^3^)** |
+-------------------------------------+-----------------+--------------+
| Polyurethane Foam                   | 0.033           | \~400        |
|                                     |                 |              |
| (Manufactured by                    |                 |              |
|                                     |                 |              |
| Foam Enterprises)                   |                 |              |
|                                     |                 |              |
| (Spray foam for tank insulation.)   |                 |              |
+-------------------------------------+-----------------+--------------+
| Cryo-Lite                           | 0.033           | \~320        |
|                                     |                 |              |
| (Manufactured by Johns Manville)    |                 |              |
|                                     |                 |              |
| (Used to insulate LN~2~             |                 |              |
| over-the-road tanks.                |                 |              |
+-------------------------------------+-----------------+--------------+
| Solimide Polyimide Foam (TA-301)    | 0.042           | \~1150       |
|                                     |                 |              |
| (Manufactured by Laporte plc)       |                 |              |
+-------------------------------------+-----------------+--------------+

The top and bottom of the TS are the same basic construction as the
vertical sides. The structural frame consists of 10 inch channels
positioned side by side and spaced \~ 0.5 m apart. The thin SS skin will
be welded to the frame and insulation will be sprayed on as described
above.

Several important thermal characteristics of the shield have been
estimated. These include the total heat loss through the shield and the
inner and outer surface temperatures. The nitrogen gas tem­perature in
the shield was assumed to be 90 K and the air temperature in the room
was assumed to be 300 K (27 °C). Given these temperatures, the natural
convection heat transfer coefficients were estimated for both the inside
and outside of the shield. The insulation thickness was adjusted so that
the out­side surface temperature would be high enough to avoid
condensation during operation. The resulting inner and outer surface
temperatures are 100 K and 290 K (17 °C) respectively, for an insulation
thickness of 0.4 m. The outer surface temperature is comfortably above
the dew point of 283 K (10 °C) (for air at 21 °C with a relative
humidity of 50 %), so condensation is not expected. The total heat flow
through the insulation is \~11 kW which is below the requirement limit
of 15 kW.

The basic parameters for the thermal shield are shown in Table 5.5.3-2.

### Table 5.5.3-2 Thermal Shield Design Parameters

+------------------------------------------------------+---------------+
| ######## Parameter                                   | **Value**     |
+------------------------------------------------------+---------------+
| ## Size                                              |               |
+------------------------------------------------------+---------------+
| #### Outside Diameter                                | 11.2 m        |
+------------------------------------------------------+---------------+
| > Outside Height                                     | 11.5 m        |
+------------------------------------------------------+---------------+
| > Total Wall Thickness                               |               |
+------------------------------------------------------+---------------+
| > Structure                                          | 0.25 m        |
+------------------------------------------------------+---------------+
| ## Insulation                                        | 0.4 m         |
+------------------------------------------------------+---------------+
| > Inside Surface Area                                | 375 m^2^      |
+------------------------------------------------------+---------------+
| ## Weight                                            |               |
+------------------------------------------------------+---------------+
| > Insulation (0.4 m thick)                           | 2400 kg       |
+------------------------------------------------------+---------------+
| > SS Structural Frame (10 inch channels)             | 24000 kg      |
+------------------------------------------------------+---------------+
| ## SS Shell Around Insulation (0.5 mm thick)         | 7100 kg       |
+------------------------------------------------------+---------------+
| ## Thermal Characteristics                           |               |
+------------------------------------------------------+---------------+
| > Nitrogen Gas Temp. Inside Thermal Shield (assumed) | 90 K          |
+------------------------------------------------------+---------------+
| > Shield Inside Surface Temp. (with natural conv.)   | \~100 K       |
+------------------------------------------------------+---------------+
| > Shield Outside Surface Temp. (with natural conv.)  | \~290 K       |
+------------------------------------------------------+---------------+
| > Air Temp. in Room Outside of Thermal Shield        | 300 K         |
| > (assumed)                                          |               |
+------------------------------------------------------+---------------+
| > Dew Point for Air at 21 C (75 F) and 50 % humidity | 283 K         |
+------------------------------------------------------+---------------+
| > Condensation Expected                              | None          |
+------------------------------------------------------+---------------+
| > Total Heat Flow Through Shield                     | \~11 kW       |
+------------------------------------------------------+---------------+