NSO Memo 8.2_020503_Rad_Env_Diag_PortJ_MS.doc 2/5/2003

# Radiation Environment at FIRE Midplane Diagnostic Port J

Mohamed Sawan

The University of Wisconsin

Neutron and gamma fluxes are expected to have impact on diagnostic
components with effects on conductivity of electrical components,
scintillation and absorption in optical components. Determination of the
radiation environment is essential for estimating shielding requirements
for diagnostic detectors and their supporting electronics. In addition
streaming through diagnostics penetrations could lead to excessive doses
outside the machine.

One of the most critical diagnostic systems with expected significant
streaming is in the midplane port plug J. A schematic of diagnostics and
penetrations in this port plug is shown in figure 1. The most serious
streaming problems occur in the clear straight tubes for the Neutral
Particle Analyzer (NPA) and Impurity Pellet Injector Guide tubes. These
tubes penetrate through the 1.1 m thick port plug and the 2.5 cm thick
port flange leading to elevated fluxes and doses behind the port flange.
The labyrinth for the Thomson scattering laser well includes several
mirrors that are sensitive to radiation level. That penetration employs
four bends that are expected to reduce the streaming problem compared to
that due to the straight clear penetrations. Two-dimensional neuronics
calculations have been performed to determine the nuclear radiation
environment at selected locations in the diagnostics penetrations and to
assess the impact of streaming on average flux outside the port flange.

The neutronics calculations have been performed using the
two-dimensional module of the DANTSYS neutral particle transport code. A
simplified geometry was modeled in r-z cylindrical geometry. In the
model both the inboard and outboard regions are modeled simultaneously
to properly account for the toroidal geometry effects. The calculations
were performed for the DT pulses with 150 MW fusion power using the
midplane radial build for the FIRE machine with 2.14 m major radius. The
front of the 110 cm thick port plug facing the plasma is at a radius of
282.2 cm. The radial distance between the front of the port plug and the
port flange is 339 cm. The port plug and flange are assumed to consist
of 80%% steel and 20% water. Three different models were used in the
calculations. The first one assumes no penetrations in the plug and
flange. The results from this calculation are used as a reference to
quantify the impact of streaming. The second case considered is for the
worst case streaming with a 10 cm straight penetration through the plug
and flange. This is representative of the NPA tube. The third case
includes the labyrinth penetration with four bends and represents the
Thomson scattering laser well. Due to the limitation of two-dimensional
modeling, all bends were modeled in the same plane as shown in figure 2.
Notice that conservative estimates are obtained using these
two-dimensional models since the pentrations are modeled as slots that
extend toroidally and attenuation by components in the penetrations is
not included. The total neutron flux (integrated over all energies), the
fast neutron flux (E \> 0.1 MeV) and the total gamma flux were
calculated at the front of the plug, along the penetrations, at the back
of the plug and at the back of the port flange. In addition, the
absorbed dose rates in silica (SiO~2~) and alumina (Al~2~O~3~), were
calculated. This includes the contributions from both neutrons and
gammas. The gamma contribution to the dose varies from \~30% at the
front of the port plug to \~80% at the back of the port flange.

Table 1 gives the neutron and gamma fluxes for the case without
penetrations at the front of the port plug, at the back of the plug and
at the back of the port flange. Table 2 gives the absorbed dose rates in
silica and alumina. The fluxes attenuate by about seven orders of
magnitudes in the shield plug. For the case with a straight clear
penetration the neutron and gamma fluxes along the penetration are given
in table 3. In addition, the average values at the three radial
locations are provided. At the back of the port plug, a flux peaking
factor of \~7 occurs due to the penetration. At the back of the flang
the peaking factor is only \~4 following neutron and gamma transport in
the large void space between the plug and flange. Notice that the
average flux values behind the plug and flange are about four orders of
magnitude higher than those in the case without the penetration. Hence,
using a straight penetration in the port plug increases the radiation
environment in the test cell area behind the flange by about four orders
of magnitude. This implies also that the biological dose rate will
increase accordingly necessitating the use of remote handling. Table 4
gives the absorbed dose rates in silica and alumina along the straight
penetration at different radial locations. The dose rates in the
penetration at the back of the port plug are a factor of \~7x10^4^
higher than in the case without penetration. In the penetration at the
port flange, the dose rates are a factor of \~4x10^4^ higher compared to
the case without penetrations.

For the labyrinth penetration with four bends, the fluxes and absorbed
dose rates were calculated at the mirrors located at each of the bends.
In addition, results are given at the front of the plug, back of the
plug, and back of the flange. Table 5 lists the neutron and gamma fluxes
along the penetration. The average values at the back of the plug and at
the back of the flange are given in table 6. At the back of the plug,
the peak values occur at the exit of the penetration with a peaking
factor of \~4. Since the penetration does not go through the port
flange, the peaking factor behind the flange is only \~2. The average
flux values behind the plug and the flange are about a factor of 200
higher than those without penetrations. Hence, the presence of the
4-bend penetration results in enhancing the radiation fluxes and dose
rates in the test cell area by about two orders of magnitude. Diagnostic
equipment in the penetration will reduce this enhancement. Table 7 gives
the absorbed dose rates in silica and alumina along the 4-bend
penetration and at the back of the port flange. The dose rates in the
4-bend penetration at the back of the port plug are about two orders of
magnitude less than in the case of a straight penetration.

Since the flux values in the test cell area behind the port flange is
increased significantly due to streaming, the biological dose rate will
be enhanced affecting the accessibility for hands-on clearing services
prior to removal/installation of port assemblies by remote handling
means. Acceptable biological dose rates behind the port flange were
obtained from activation calculations performed with a 1.1 m port plug
without any penetrations. These dose rates are \~10 mrem/hr at shutdown,
\~3 mrem/hr after 1 hr, \~0.1 mrem/hr after 1 day, and about 0.05
mrem/hr after 1 week. These acceptable dose rates can be maintained in
the cases where penetrations are employed in the shield plug by
increasing the plug thickness. Past activation calculations indicated
that the dose rate scales linearly with the total neutron flux provided
that the material used in the area where the dose is calculated is not
changed. We performed two-dimensional neutronics calculations with
different plug thicknesses for the cases with straight and labyrinth
penetrations. Table 8 gives the results for 1.1 m plug and for the case
where all the port is filled by the plug. The peak and average flux
levels are shown behind the port flange. The results for the 1.1 m plug
without penetrations are included for comparison. For the straight
penetration, the flux peaking factor increases from \~4 with 1.1 m plug
to \~15 with 3.4 m plug. On the other hand, for the 4-bend penetration,
the flux peaking factor increases from \~2 with 1.1 m plug to \~13 with
3.4 m plug. The average neutron flux behind the flange decreases by an
order of magnitude by making the shield plug with straight penetration
thicker by \~0.5 m. The same drop in flux is obtained by \~0.42 m
thicker plug for the case with 4-bend penetration. The results imply
that if a straight 10 cm penetration is employed in port plug J, the
plug thickness needs to be increased to \~3.1 m to ensure that the dose
rates after shutdown in the area behind the port flange are similar to
the acceptable levels obtained without penetrations. For the case with
the 4-bend penetration employed, the plug thickness should be increased
to \~2.1 m.

Table 1: Neutron and gamma fluxes during 150 MW DT fusion power pulses
for port plug without penetrations

+------------------------+-------------+----------------+-------------+
|                        | Total       | Fast Neutron   | Total Gamma |
|                        | Neutron     | Flux (E\>0.1   | Flux        |
|                        | Flux        | MeV)           |             |
|                        |             |                | (γ/cm^2^s)  |
|                        | (n/cm^2^s)  | (n/cm^2^s)     |             |
+------------------------+-------------+----------------+-------------+
| Front of port plug     | 8.38x10^14^ | 5.85x10^14^    | 4.94x10^14^ |
+------------------------+-------------+----------------+-------------+
| Back of port plug      | 3.51x10^7^  | 1.64x10^7^     | 3.04x10^7^  |
+------------------------+-------------+----------------+-------------+
| Back of port flange    | 1.04x10^7^  | 6.23x10^6^     | 1.04x10^7^  |
+------------------------+-------------+----------------+-------------+

Table 2: Absorbed radiation dose rate in silica and alumina for port
plug without penetrations

+----------------------------+-------------------+---------------------+
|                            | Dose Rate During  |                     |
|                            | 150 MW DT Pulses  |                     |
|                            |                   |                     |
|                            | (Rad/s)           |                     |
+----------------------------+-------------------+---------------------+
|                            | Silica            | Alumina             |
+----------------------------+-------------------+---------------------+
| Front of port plug         | 8.40x10^5^        | 9.32x10^5^          |
+----------------------------+-------------------+---------------------+
| Back of port plug          | 2.40x10^-2^       | 4.30x10^-2^         |
+----------------------------+-------------------+---------------------+
| Back of port flange        | 9.50x10^-3^       | 1.63x10^-2^         |
+----------------------------+-------------------+---------------------+

Table 3: Neutron and gamma fluxes for the case of straight 10 cm
penetration

+--------------+---------+-------+--------+-------+--------+-------+
|              | Total   |       | Fast   |       | Total  |       |
|              | Neutron |       | N      |       | Gamma  |       |
|              | Flux    |       | eutron |       | Flux   |       |
|              |         |       | Flux   |       |        |       |
|              | (n/     |       | (      |       | (γ/c   |       |
|              | cm^2^s) |       | E\>0.1 |       | m^2^s) |       |
|              |         |       | MeV)   |       |        |       |
|              |         |       |        |       |        |       |
|              |         |       | (n/c   |       |        |       |
|              |         |       | m^2^s) |       |        |       |
+--------------+---------+-------+--------+-------+--------+-------+
|              | Along   | Av    | Along  | Av    | Along  | Av    |
|              | Pene    | erage | Penet  | erage | Penet  | erage |
|              | tration |       | ration |       | ration |       |
+--------------+---------+-------+--------+-------+--------+-------+
| Front of     | 7.46    | 8     | 5.34x  | 5     | 4.14x  | 4     |
| port plug    | x10^14^ | .38x1 | 10^14^ | .85x1 | 10^14^ | .94x1 |
|              |         | 0^14^ |        | 0^14^ |        | 0^14^ |
+--------------+---------+-------+--------+-------+--------+-------+
| Back of port | 2.87    | 4     | 1.28x  | 1     | 2.08x  | 2     |
| plug         | x10^12^ | .06x1 | 10^12^ | .57x1 | 10^12^ | .79x1 |
|              |         | 0^11^ |        | 0^11^ |        | 0^11^ |
+--------------+---------+-------+--------+-------+--------+-------+
| Back of port | 5.14    | 1     | 2.75x  | 6     | 3.96x  | 1     |
| flange       | x10^11^ | .27x1 | 10^11^ | .38x1 | 10^11^ | .03x1 |
|              |         | 0^11^ |        | 0^10^ |        | 0^11^ |
+--------------+---------+-------+--------+-------+--------+-------+

Table 4: Absorbed radiation dose rate in silica and alumina along 10 cm
straight penetration

+----------------------------+-------------------+---------------------+
|                            | Dose Rate During  |                     |
|                            | 150 MW DT Pulses  |                     |
|                            |                   |                     |
|                            | (Rad/s)           |                     |
+----------------------------+-------------------+---------------------+
|                            | Silica            | Alumina             |
+----------------------------+-------------------+---------------------+
| Front of port plug         | 7.83x10^5^        | 8.35x10^5^          |
+----------------------------+-------------------+---------------------+
| Back of port plug          | 1.56x10^3^        | 2.56x10^3^          |
+----------------------------+-------------------+---------------------+
| Back of port flange        | 3.53x10^2^        | 5.53x10^2^          |
+----------------------------+-------------------+---------------------+

Table 5: Neutron and gamma fluxes during 150 MW DT fusion power pulses
for the 4-bend penetration

+------------------------+-------------+----------------+-------------+
|                        | Total       | Fast Neutron   | Total Gamma |
|                        | Neutron     | Flux (E\>0.1   | Flux        |
|                        | Flux        | MeV)           |             |
|                        |             |                | (γ/cm^2^s)  |
|                        | (n/cm^2^s)  | (n/cm^2^s)     |             |
+------------------------+-------------+----------------+-------------+
| Entrance at front of   | 7.48x10^14^ | 5.34x10^14^    | 4.15x10^14^ |
| port plug              |             |                |             |
+------------------------+-------------+----------------+-------------+
| First bend             | 1.20x10^14^ | 6.24x10^13^    | 7.68x10^13^ |
+------------------------+-------------+----------------+-------------+
| Second bend            | 2.06x10^13^ | 7.95x10^12^    | 1.37x10^13^ |
+------------------------+-------------+----------------+-------------+
| Third bend             | 2.57x10^12^ | 9.19x10^11^    | 1.79x10^12^ |
+------------------------+-------------+----------------+-------------+
| Fourth bend            | 2.87x10^11^ | 6.75x10^10^    | 2.15x10^11^ |
+------------------------+-------------+----------------+-------------+
| Exit at back of port   | 2.92x10^10^ | 6.82x10^9^     | 2.46x10^10^ |
| plug                   |             |                |             |
+------------------------+-------------+----------------+-------------+
| Peak at back of port   | 5.00x10^9^  | 1.50x10^9^     | 5.00x10^9^  |
| flange                 |             |                |             |
+------------------------+-------------+----------------+-------------+

Table 6: Average neutron and gamma fluxes at backs of plug and flange
with a penetration with 4 bends

+------------------------+-------------+----------------+-------------+
|                        | Total       | Fast Neutron   | Total Gamma |
|                        | Neutron     | Flux (E\>0.1   | Flux        |
|                        | Flux        | MeV)           |             |
|                        |             |                | (γ/cm^2^s)  |
|                        | (n/cm^2^s)  | (n/cm^2^s)     |             |
+------------------------+-------------+----------------+-------------+
| Back of port plug      | 6.98x10^9^  | 2.26x10^9^     | 5.97x10^9^  |
+------------------------+-------------+----------------+-------------+
| Back of port flange    | 1.99x10^9^  | 8.84x10^8^     | 2.10x10^9^  |
+------------------------+-------------+----------------+-------------+

Table 7: Absorbed radiation dose rate in silica and alumina along the
4-bend penetration

+----------------------------+-------------------+---------------------+
|                            | Dose Rate During  |                     |
|                            | 150 MW DT Pulses  |                     |
|                            |                   |                     |
|                            | (Rad/s)           |                     |
+----------------------------+-------------------+---------------------+
|                            | Silica            | Alumina             |
+----------------------------+-------------------+---------------------+
| Front of port plug         | 7.90x10^5^        | 8.48x10^5^          |
+----------------------------+-------------------+---------------------+
| First bend                 | 7.69x10^4^        | 9.85x10^4^          |
+----------------------------+-------------------+---------------------+
| Second bend                | 1.01x10^4^        | 1.76x10^4^          |
+----------------------------+-------------------+---------------------+
| Third bend                 | 1.24x10^3^        | 2.25x10^3^          |
+----------------------------+-------------------+---------------------+
| Fourth bend                | 1.41x10^2^        | 2.67x10^2^          |
+----------------------------+-------------------+---------------------+
| Back of port plug          | 1.65x10^1^        | 3.14x10^1^          |
+----------------------------+-------------------+---------------------+
| Back of port flange        | 3.80              | 7.07                |
+----------------------------+-------------------+---------------------+

Table 8: Neutron and gamma fluxes at the back of the port flange during
150 MW DT fusion power pulses for the 4-bend penetration

+---------------+--------+--------+--------+--------+--------+--------+
|               | Total  |        | Fast   |        | Total  |        |
|               | N      |        | N      |        | Gamma  |        |
|               | eutron |        | eutron |        | Flux   |        |
|               | Flux   |        | Flux   |        |        |        |
|               |        |        |        |        | (γ/c   |        |
|               | (n/c   |        | (      |        | m^2^s) |        |
|               | m^2^s) |        | E\>0.1 |        |        |        |
|               |        |        | MeV)   |        |        |        |
|               |        |        |        |        |        |        |
|               |        |        | (n/c   |        |        |        |
|               |        |        | m^2^s) |        |        |        |
+---------------+--------+--------+--------+--------+--------+--------+
|               | Peak   | A      | Peak   | A      | Peak   | A      |
|               |        | verage |        | verage |        | verage |
+---------------+--------+--------+--------+--------+--------+--------+
| 1.1 m plug    | 1.04   | 1.04   | 6.23   | 6.23   | 1.04   | 1.04   |
| without       | x10^7^ | x10^7^ | x10^6^ | x10^6^ | x10^7^ | x10^7^ |
| penetrations  |        |        |        |        |        |        |
+---------------+--------+--------+--------+--------+--------+--------+
| 1.1 m plug    | 5.14x  | 1.27x  | 2.75x  | 6.38x  | 3.96x  | 1.03x  |
| with 10 cm    | 10^11^ | 10^11^ | 10^11^ | 10^10^ | 10^11^ | 10^11^ |
| straight      |        |        |        |        |        |        |
| penetration   |        |        |        |        |        |        |
+---------------+--------+--------+--------+--------+--------+--------+
| 3.4 m plug    | 4.18   | 2.64   | 1.08   | 7.51   | 4.28   | 2.96   |
| with 10 cm    | x10^7^ | x10^6^ | x10^7^ | x10^5^ | x10^7^ | x10^6^ |
| straight      |        |        |        |        |        |        |
| penetration   |        |        |        |        |        |        |
+---------------+--------+--------+--------+--------+--------+--------+
| 1.1 m plug    | 5.00   | 1.99   | 1.50   | 8.84   | 5.00   | 2.10   |
| with 10 cm    | x10^9^ | x10^9^ | x10^9^ | x10^8^ | x10^9^ | x10^9^ |
| 4-bend        |        |        |        |        |        |        |
| penetration   |        |        |        |        |        |        |
+---------------+--------+--------+--------+--------+--------+--------+
| 3.4 m plug    | 9.64   | 7.04   | 1.97   | 1.63   | 1.59   | 1.33   |
| with 10 cm    | x10^4^ | x10^3^ | x10^4^ | x10^3^ | x10^5^ | x10^4^ |
| 4-bend        |        |        |        |        |        |        |
| penetration   |        |        |        |        |        |        |
+---------------+--------+--------+--------+--------+--------+--------+

![](media/image1.pct){width="8.99375in" height="5.4430555555555555in"}

Figure 1. Schematic of diagnostics and penetrations in port plug J.

Figure 2. Two-dimensional model used for neutronics calculations with
the labyrinth penetration.