# **5. 10 Decay Heat, and Radiation Exposure**

### 5.10.1 Introduction

Activation analysis was performed using toroidal cylindrical geometry
with the inboard and outboard sides modeled simultaneously. Calculations
assumed peak neutron wall loadings during D-T shots of 1.8 and 3.6
MW/m^2^ for the divertor and outboard first wall, respectively. The
machine is assumed to have an operation schedule of four pulses per day
for one full power year (FPY). The D-T pulse burn is 20 seconds with 3
hours between pulses. Calculations were performed for D-T and D-D pulses
with 200 MW and 1 MW of fusion power, respectively. The first wall is 5
cm thick and consists of 0.5 cm layer of Be coating as a plasma facing
component (PFC), 1.8 cm of a CuCrZr tiles and 0.2 cm Cu gasket.

The vacuum vessel structure is made of 316 SS with inner shield made of
a mixture of 304 SS and water as a vacuum vessel shield. The vacuum
vessel thickness varies poloidally from 5 cm in the inboard region to 54
cm in the outboard region at the midplane. A 2.5 cm water cooled Cu
cladding is attached to the plasma side of the vacuum vessel. The magnet
winding pack is modeled using BeCu and OFHC in the inboard and outboard
sides, respectively. The magnet uses a 316 SS coil case with 4 cm front
thickness and 6 cm back thickness. The divertor consists of three
layers. The front layer consists of 0.5 cm thick tungsten rods followed
by 2 cm of a CuCrZr/water mixture as a heat sink, and a 3 cm thick layer
of a mechanical attachment made of CuCrZr/316 SS mixture. Finally, the
mechanical attachment connects the heat sink to a 7 cm thick layer of
316 SS/water mixture which is used as a backing plate. Using of a plug
(80% steel and 20% water) to stop neutrons streaming through
penetrations at the midplane and using an additional shield at the top
of the machine were assessed to allow for hands-on maintenance.

### 5.10.2 Activity and Decay Heat

The neutron flux used for the activation calculations was generated by
the one-dimensional discrete ordinates neutron transport code DANTSYS
3.0 \[1\]. The activation analysis was performed using the activation
code DKR-PULSAR2.0 \[2\]. The code combined the neutron flux with the
FENDL/A-2.0 \[3\] cross section library to calculate the activity and
decay heat as a function of time following shutdown. Figures 5.10-1 and
5.10-2 show the specific activity values for the inboard and outboard
regions as a function of time following shutdown, respectively. Figures
5.10-3 and 5.10-4 show the specific decay heat values for the inboard
and outboard regions as a function of time following shutdown,
respectively. Finally, figures 5.10-5 and 5.10-6 show the specific
activity and decay heat generated in the divertor.

As shown in figures 5.10-1 to 5.10-6, the plasma facing components,
first wall on the inboard and outboard sides as well as the divertor,
produce the highest levels of specific activity and decay heat. However,
the favorable operational schedule allows for the decay of short-lived
radionuclides between pulses resulting in low levels of activity and
decay heat at shutown.

![](media/image1.pct){width="5.430555555555555in"
height="3.9444444444444446in"}

**Fig. 5.10-1**. Activity induced in the inboard side.

![](media/image2.pct){width="5.430555555555555in"
height="3.9444444444444446in"}

**Fig. 5.10-2**. Activity induced in the outboard side.

![](media/image3.pct){width="5.430555555555555in"
height="3.9444444444444446in"}

**Fig. 5.10-3**. Decay heat induced in the inboard side.

![](media/image4.pct){width="5.430555555555555in"
height="3.9444444444444446in"}

**Fig. 5.10-4.** Decay heat induced in the outboard side.

![](media/image5.pct){width="5.4847222222222225in"
height="3.9770833333333333in"}

**Fig. 5.10-5**. Activity induced in the divertor.

![](media/image6.pct){width="5.430555555555555in"
height="3.9444444444444446in"}

**Fig. 5.10-6**. Decay heat induced in the divertor.

At shutdown, the decay heat induced in the first wall following D-T
shots is less than 0.25% of the nuclear heating generated in the first
wall during operation. In the mean time, the ratio between the shutdown
decay heat and nuclear heating generated in the vacuum vessel during
operation is on the order of 0.1%. The activity and decay heat generated
following D-D shots are more than three orders of magnitude lower than
their values following D-T shots due to the lower D-D fusion power and
the significant reduction in the number of high energy neutrons.

**Table 5.10-I**. List of Dominant Nuclides.

+-----------+----------------------------+-----------------------------+
| ##        |                            |                             |
| ## ***Sho |                            |                             |
| rt-term\< |                            |                             |
|  1 day*** |                            |                             |
+-----------+----------------------------+-----------------------------+
| ###       | ### Activity               | ### Decay Heat              |
+-----------+----------------------------+-----------------------------+
| FW        | ^62^Cu, ^64^Cu,^66^Cu      | ^62^Cu, ^64^Cu, ^66^Cu      |
+-----------+----------------------------+-----------------------------+
| VV        | ^56^Mn, ^58^Co,^51^Cr      | ^56^Mn, ^58m^Co             |
+-----------+----------------------------+-----------------------------+
| Mag.      | ^62^Cu, ^64^Cu,^66^Cu      | ^62^Cu, ^64^Cu, ^66^Cu      |
+-----------+----------------------------+-----------------------------+
| Div.      | ^187^W,^185^W,^181^W       | ^187^W, ^185^W              |
+-----------+----------------------------+-----------------------------+
| #         |                            |                             |
| ##### *In |                            |                             |
| termediat |                            |                             |
| e-term \< |                            |                             |
|  1 month* |                            |                             |
+-----------+----------------------------+-----------------------------+
| ###       | ### Activity               | ### Decay Heat              |
+-----------+----------------------------+-----------------------------+
| FW        | ^60^Co, ^63^Ni             | ^64^Cu, ^60^Co              |
+-----------+----------------------------+-----------------------------+
| VV        | ^55^Fe, ^51^Cr, ^57^Co     | ^58^Co,^54^Mn,^58m^Co       |
+-----------+----------------------------+-----------------------------+
| Mag.      | ^60^Co, ^63^Ni             | ^64^Cu, ^60^Co              |
+-----------+----------------------------+-----------------------------+
| Div.      | ^185^W, ^181^W             | ^185^W, ^181^W              |
+-----------+----------------------------+-----------------------------+
| #         |                            |                             |
| ##### *Lo |                            |                             |
| ng-term \ |                            |                             |
| > 1 year* |                            |                             |
+-----------+----------------------------+-----------------------------+
| ###       | ### Activity               | ### Decay Heat              |
+-----------+----------------------------+-----------------------------+
| FW        | ^63^Ni                     | ^63^Ni                      |
+-----------+----------------------------+-----------------------------+
| VV        | ^63^Ni                     | ^60^Co, ^63^Ni              |
+-----------+----------------------------+-----------------------------+
| Mag.      | ^63^Ni                     | ^63^Ni                      |
+-----------+----------------------------+-----------------------------+
| Div.      | ^91^Nb, ^63^Ni             | ^94^Nb, ^39^Ar              |
+-----------+----------------------------+-----------------------------+

The decay heat induced in the first wall at shutdown is dominated by the
two copper isotopes ^62^Cu(T~1/2~ = 9.74 min) and ^66^Cu(T~1/2~ = 5.1
min). The low decay heat induced in the first wall at shutdown is due to
the fact that the short lifetimes of the ^62^Cu and ^66^Cu isotopes
allow them to decay during the three hours between pulses. The decay
heat induced in the vacuum vessel at shutdown is dominated by
^52^V(T~1/2~ = 3.76 min) and ^56^Mn(T~1/2~ = 2.578 hr) isotopes. Due to
the short lifetime of ^52^V, its entire radioactivity also decays
between shots, resulting in a low overall radioactivity generated in the
vacuum vessel at shutdown. In general, the short-term activity and decay
heat values at shutdown are almost fully dominated by activation during
the last pulse. Table 5.10-I shows a list of nuclides that dominate the
induced radioactivity in the different machine components.

### 5.10.3 Biological Dose Rates

In order to assess the feasibility of hands-on maintenance, biological
dose rates were calculated at different locations following shutdown.
The gamma source from radioactive decay was determined at all mesh
points and transported, using the DANTSYS 3.0 code, to calculate the
dose rate at different locations following shutdown. The dose rates were
calculated at the following locations:

-   Behind the outboard vacuum vessel and magnet at the midplane.

-   Behind the magnet at the machine top.

-   Behind the additional shield at the machine top.

![](media/image7.pct){width="5.875in" height="4.222222222222222in"}

**Fig. 5.10-7**. Biological dose rates at the midplane following D-T
shots.

![](media/image8.pct){width="5.875in" height="4.222222222222222in"}

**Fig. 5.10-8**. Biological dose rates at the midplane following D-D
shots.

![](media/image9.pct){width="5.875in" height="4.120833333333334in"}

**Fig. 5.10-9.** Biological dose rates at the machine top following D-T
shots.

![](media/image10.pct){width="5.875in" height="4.222222222222222in"}

**Fig. 5.10-10.** Biological dose rates at the machine top following D-D
shots.

Figures 5.10-7 and 5.10-8 show the biological dose rates at the midplane
as a function of time following shutdown for D-T and D-D shots,
respectively. As shown in figure 5.10-7, the biological dose rates
behind the vacuum vessel remain high for several years following
shutdown for D-T shots. On the other hand as shown in figure 5.10-8, in
the case of D-D shots, the dose rates behind the vacuum vessel are five
order of magnitudes lower than after D-T shots. The significant drop in
the dose rate allows for hands on maintenance behind the vacuum vessel
following D-D shots. The dose rates behind the magnet and at the
midplane are acceptable for both D-D and D-T shots. Dose rates behind
the magnet are caused by the ^62m^Co(T~1/2~ = 13.9 min) isotope and are
independent on the number of pulses due to the fact that ^62m^Co decays
between pulses. One week following shutdown, the dose rates are
dominated by the ^60^Co(T~1/2~ = 5.27 yr) isotope. The dose rates caused
by the ^60^Co isotope almost increase linearly with the increase in
number of pulses. The calculations showed that using a 110 cm long steel
plug in the midplane port will stop neutron streaming through
penetration and provide adequate shielding that allows for hands-on
maintenance.

The dose rates at the top of the machine (at the maximum divertor
thickness) following D-T shots (figure 5.10-9), drops to an acceptable
level within a day following shutdown. Adding a 20 cm thick POLY/CAST
shield at the top of the magnet drops the dose rates on the top of the
shield to acceptable levels only few hours earlier. However, since the
divertor thickness is much smaller at other locations, it is essential
to maintain the same shield thickness to guarantee hands-on maintenance
at all locations at the top of the machine. The shield is composed of a
POLY/CAST mix placed inside a steel tank (the tank wall is 1 cm thick).
The activation of the outer wall of the steel tank results in the
generation of ^56^Mn. As shown in the figure, the ^56^Mn(T~1/2~ = 2.578
hr) isotope results in a slightly higher dose (in comparison to the no
shield case) outside the 20 cm thick shield during the first couple of
hours following shutdown. As shown in figure 5.10-10, the dose rates at
the top of the machine following D-D shots are very low, allowing for
immediate access to that space any time following shots. Both the
midplane port plug and the top shield were included in the FIRE basline
design.

**5.10.4 Routine Release of ^13^N to the Environment**

Normal conducting copper magnets that are inertially cooled with LN2 are
being considered in the near term ignition machines such as FIRE. In
this case, liquid nitrogen is used to cool the magnets between shots.
Nitrogen gas will exist inside the cryostat during the shots and will
get irradiated. One radiological concern is the generation of
radioactive ^13^N. The ^13^N is a major source of radioactive hazard.
Activation calculations were performed for nitrogen gas at room
temperature and at different locations inside the machine. The
activation of nitrogen gas by D-T shots will produce ^13^N as well as a
small amount of ^14^C.

Table 5.10-II gives the amount of ^13^N and ^14^C generated at different
locations inside the cryostat following each D-T shot. The results
indicate that the largest amount of ^13^N and ^14^C is generated in the
space between the IB magnet and the IB VV. This is due to the fact that
the shield thickness on the IB side is much smaller than that on the OB
side. The thinner shield results in exposing the nitrogen gas in the IB
region to a larger and harder neutron flux resulting in higher
probabilities for the (n,2n) reaction that produces the radioisotope
^13^N from the natural ^14^N isotope and the (n,p) reaction that
generates ^14^C. As a result, the specific activities in the IB side are
much larger than those in the OB side. Although the gas volume in the IB
side is smaller than in the OB side, the total activity is still about
two orders of magnitude larger. Since ^13^N has a short half-life of
9.97 minutes, the option of designing a nitrogen-holding system that
allows for a significant decay of ^13^N before releasing it to the
environment is adopted by the FIRE team.

**Table 5.10-II**. Amount of Activity (Ci) Generated in the Nitrogen per
D-T Shot.

+--------------------------+---------------------+---------------------+
| Location of              | Activity (Ci)       |                     |
|                          |                     |                     |
| Nitrogen Gas             |                     |                     |
+--------------------------+---------------------+---------------------+
|                          | ^13^N               | ^14^C               |
+--------------------------+---------------------+---------------------+
| Between IB               | 1.2                 | 1.7x10^-6^          |
|                          |                     |                     |
| Magnet and               |                     |                     |
|                          |                     |                     |
| IB VV                    |                     |                     |
+--------------------------+---------------------+---------------------+
| Between OB Magnet and    | 1.4x10^-2^          | 2.4x10^-8^          |
|                          |                     |                     |
| OB VV                    |                     |                     |
+--------------------------+---------------------+---------------------+
| Between OB Magnet and    | 8x10^-9^            | 7x10^-11^           |
| Cryostat                 |                     |                     |
+--------------------------+---------------------+---------------------+

Another source of radioactive ^13^N is the activated air inside the
building (outside the cryostat). Activation analysis of the large amount
of air present inside the building showed that the amount of ^13^N
generated per D-T shot is only 2.5x10^-7^ Ci. In addition, air
activation results in the generation of 8x10^-11^ and 2.8x10^-6^ Ci of
^14^C and ^41^Ar per D-T shot, respectively. These are extremely low
levels of activity and should not cause any radiological concern.

**5.10.5 Waste Disposal Ratings (WDR)**

The radwaste of the different components of the machine were evaluated
according to both the NRC 10CFR61 \[4\] and Fetter \[5\] waste disposal
concentration limits. The 10CFR61 regulations assume that the waste
disposal site will be under administrative control for 100 years. The
dose at the site to an inadvertent intruder after the 100 years is
limited to less than 500 mrem/year. The waste disposal rating (WDR) is
defined as the sum of the ratio of the concentration of a particular
isotope to the maximum allowed concentration of that isotope taken over
all isotopes and for a particular class. If the calculated WDR ≤ 1 when
Class A limits are used, the radwaste should qualify for Class A
segregated waste. The major hazard of this class of waste is to
individuals who are responsible for handling it. Such waste is not
considered to be a hazard following the loss of institutional control of
the disposal site. If the WDR is \> 1 when Class A WDL are used but ≤ 1
when Class C limits are used, the waste is termed Class C intruder
waste. It must be packaged and buried such that it will not pose a
hazard to an inadvertent intruder after the 100 years institutional
period is over. Class C waste is assumed to be stable for 500 years.
Using Class C limits, a WDR \> 1 implies that the radwaste does not
qualify for shallow land burial. Fetter developed a modified version of
the NRC\'s intruder model to calculate waste disposal limits for a wider
range of long-lived radionuclides which are of interest for fusion
researchers than the few that currently exist in the current 10CFR61
regulations. Fetter\'s model included more accurate transfer
coefficients and dose conversion factors.

The waste disposal ratings for the 10CFR61 and Fetter limits are shown
in Table 5.10-III. These values are at the FIRE end-of-life with total
of 5TJ D-T and 0.5 TJ D-D fusion energy. Results in the table are given
for compacted wastes. Compacted waste corresponds to crushing the solid
waste before disposal and thus disallowing artificial dilution of
activity. The dominant nuclides are given between brackets. At the end
of the machine life, all components would qualify for disposal as Class
C low level waste according to the two waste disposal concentration
limits used in the analysis. As shown in the table, according to Fetter
limits, the WDR are dominated by the silver impurities in the CuCrZr
alloy and the niobium impurities in the 316 SS and 304 SS alloys. The
10CFR61 limits indicate that the WDR of components made of the CuCrZr
alloy are dominated by ^63^Ni which is produced from copper by the
^63^Cu(n,p) reaction. On the other hand, the WDR of components made of
the steel alloys are dominated by their niobium impurities. Due to the
reduced neutron environment following D-D shots, all components will
easily qualify for disposal as Class C LLW.

**Table 5.10-III**. Class C WDR.

+------------------+------------------------+--------------------------+
| ### Zone         | ### Fetter             | ### 10CFR61              |
+------------------+------------------------+--------------------------+
| IB FW            | 0.2 (^108m^Ag)         | 2.16e-2 (^63^Ni)         |
+------------------+------------------------+--------------------------+
| IB VV            | 9.21e-2 (^108m^Ag,     | 3.46-2 (^94^Nb,^63^Ni)   |
|                  | ^94^Nb)                |                          |
+------------------+------------------------+--------------------------+
| IB Mag.          | 1.96e-4 (^108m^Ag)     | 1.1e-3 (^63^Ni)          |
+------------------+------------------------+--------------------------+
| OB FW            | 0.21 (^108m^Ag)        | 2.36-2 (^63^Ni)          |
+------------------+------------------------+--------------------------+
| OB VV            | 1.06e-2 (^108m^Ag,     | 3.23e-3 (^94^Nb,^63^Ni)  |
|                  | ^94^Nb)                |                          |
+------------------+------------------------+--------------------------+
| OB Mag.          | 2.26e-6 (^94^Nb)       | 2.56e-6 (^94^Nb,^63^Ni)  |
+------------------+------------------------+--------------------------+
| Divertor         | 3.4e-2 (^108m^Ag)      | 1.33e-2 (^94^Nb)         |
+------------------+------------------------+--------------------------+

***[References for Section 5.10:]{.underline}***

\[1\] R.E. Alcouffe et al., "DANTSYS 3.0, One-, Two-, and
Three-Dimensional Multigroup Discrete Ordinates Transport Code System,"
RSICC Computer Code Collection CCC-547, Contributed by Los Alamos
National Lab, August 1995.

\[2\] M. J. Sisolak, Q. Wang, H. Khater and D. Henderson,
\"DKR-PULSAR2.0: A Radioactivity Calculation Code that Includes
Pulsed/Intermittent Operation,\" to be published.

\[3\] A. Pashchenko et al., \"FENDL/A-2.0: Neutron Activation
Cross-Section Data Library for Fusion Applications,\" Report
INDC(NDS)-173, IAEA Nuclear Data Section, March 1997.

\[4\] Nuclear Regulatory Commission, 10CFR part 61, \"Licensing
Requirements for Land Disposal of Radioactive Waste,\" Federal Register,
FR 47, 57446 (1982).

\[5\] S. Fetter, E. Cheng and F. Mann, \"Long Term Radioactive Waste
from Fusion Reactors,\" Fusion Engineering and Design, 13, 239-246
(1990).