Proceedings of ITC/ISHW2007

Activation Analysis for LHD Experiments with Deuterium Gases
Kiyohiko Nishimura, Hirokuni Yamanishi and Akio Komori
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
Identification of radionuclides and evaluation of dose rate level have been carried out on the structural materialsof the Large Helical Device and the Experimental Hall. The neutron fluence was calculated using
two-dimensional transport code DOT-3.5. Energies of neutron sources are 2.45 MeV (D-D reaction) and 14 MeV(D-T reaction). Generations of radionuclides were calculated using CINAC code. Radionuclides of 93mNb, 63Ni
and 60Co for helical coils, 55Fe and 60Co for stainless steel, 55Fe, 60Co and 93mNb for poloidal coils, 40K and 55Fe forfloor concrete were dominant after a series of experiments with deuterium gases. Evaluation of dose rate level forthe structural materials and air were calculated taking account a present experimental schedule.
Keywords: Large Helical Device, deuterium operation, Superconducting coils, radioactivity, dose rates,
DOT-3.5 code, CINAC code, cobalt 60

1. Introduction

2. Large Helical Device

In fusion devices such as JT-60U, TFTR and JET
neutron production generated by deuterium-deuterium
reactions is increased accompany with the increase infusion power and the devices are subsequently activated.
As a result, maintenance and repair works during
off-operational periods become difficult because an accessaround and/or into a vacuum vessel is so limited from ahealth physics point of view. A safety analysis is also amajor concern of experiments with deuterium gases in theLarge Helical Device (LHD) [1-4] and a design of DEMO
reactors with high neutron yield operations.
Several calculations concerning induced activities forthe tokamak devices have been reported. In thesetokamaks, varied structural materials are used so thatcharacteristics of radioactivity differ with each device.
For this reason, it is clear that information of inherent
property of activation on LHD is essential to anunderstanding of the dose rate around the device inmaintenance works.
The radiation protection concepts of the LHD wascarried out at the building design phase [5] and revised at
the building construction phase [6]. The neutron fluence
was calculated using two-dimensional transport code
DOT-3.5 [7]. Generations of radionuclides caused by
2.45 MeV (D-D reaction) and 14 MeV (D-T reaction)
were calculated using CINAC code [8].
In this paper, identification of radionuclides andevaluation of the dose rate on the LHD are described.

LHD is the largest superconducting heliotron typedevice with l=2/m=10 continuous helical coils and threepairs of poloidal coils. The major and minor radii of theplasma are 3.5-3.9 m and 0.6-0.65 m, respectively.
Schematic figure of LHD are shown in Fig. 1. The
maximum magnetic field strength is about 3 T at themagnetic axis.
The LHD project is aimed at exploring thefeasibility of helical plasmas for fusion applications. In
particular, to demonstrate the steady-state currentlessplasmas confined in the helical fields generated bysuper-conducting coils.

author’s e-mail: nisimura@LHD.nifs.ac.jp

Fig. 1. Schematic figure of LHD.

Proceedings of ITC/ISHW2007

3. Calculation
Standard numbers and energies of neutron sources
using calculations are shown in Table 1. One shot is 10
seconds pulse and 1000 shots are operated in a year.
Table 1. Parameters of neutron
Energy (MeV)
Number (1/shot)
2.45
2.4x1017
14.0
4.3x1015
The most of 2.45MeV neutrons (D-D neutron) aregenerated by D plasma and D0 beam reaction, which iscalled TCT effect.
The14.0 MeV neutrons (D-Tneutron) are generated by the reaction of deuterium withtritium (1.01 MeV) through D-D reaction. The numberof D-D thermonuclear reaction is 1 – 3 % of that of TCT
effect.
The neutron fluence data and the distribution ofγ-ray were calculated using two-dimensional transport

code DOT-3.5 by means of the FUSION-40 nuclear data
set [9]. Generations of radionuclides were calculatedusing CINAC code. Effects of chain disintegration weretaking account in this code. Since the LHD has
three-dimensional structure, it is difficult to model thisgeometry on a two-dimensional configuration. Twomodels (a horizontal port model and a vertical portmodel) with R-Z geometry were used in thesecalculations. Figure 2 shows cross sections of these twomodels near the LHD. Calculated area was divided intotwelve regions; plasma, helical coil, helical coil case,
supporting shell, IV poloidal coil, IS poloidal coil, OV
poloidal coil, cryostat, vacuum vessel, wall and ceilingconcrete, floor concrete and air. LHD experimental hall
has 45m x 75m of floor area and 40m of height.
Thickness of walls and floor concrete is 2m and thicknessof ceiling concrete is 1.3m. In R-Z model calculation, Rof 2.5m and Z of 40m were assumed.

Fig. 2. Cross sections of models.
a) vertical port model, b) horizontal port model.

4. Results
Detail analysis was carried out to identify theradionuclides, which give the contribution to the γ-ray doserate, and to calculate time evolution of the dose rate after astop of D-D experiments.
Figures 3 and 4 show results of specific radionuclide
activities as a function of time after 1 shot pulse. Two
figures show averaged activities of the cryostat (SS-316)
and the floor concrete. In the cryostat, 56Mn (half-life of2.6h) generated by a reaction of 55Mn(n,γ) is dominant by
the time of one day. After disappearance of 56Mn, 51Cr(27.7d) becomes dominant. After that, 60Co (5.27y)generated by a reaction of 59Co(n,γ) becomes dominant. Infloor concrete, many radionuclides with short half-life aredominant by the time of one day. After disappearance of
these shot life time nuclides, 24Na (15.0h) generated by areaction of 23Na(n,γ) becomes dominant for a week. After
that, 40K (1.28x109y) becomes dominant. Most of the 40K
are nuclides naturally existing in a concrete.

Proceedings of ITC/ISHW2007
In this calculation, actual experimental plan were used; 3seconds pulse in each 15 minutes, 30 shots in a day, 4 daysin a week, 3 weeks experiment and 3 weeks intermission.
Using this curve, time evolution of γ-ray dose rate duringexperimental series was calculated by summing up
contributions of each shot. Figure 6 shows a result of one
experimental series. During three days in weekend, theγ-ray dose rate decreases one order. This level of 10μSv/h
is lower than the limitation for worker (20mSv/y) whoworks for 2000 hours per year.

Fig. 3. Specific activity in the Cryostat (SS316) after 1 shot.

Fig. 6. Time evolution of γ-ray dose rate duringexperimental series.

Table 2-1. Activities of typical radionuclides on stainlesssteel structures 1.

Fig. 4. Specific activity in the floor concrete after 1 shot.

Fig. 5. Time evolution of total γ-ray dose rate after 3
seconds pulse.
To estimate the γ-ray dose rate around the vacuum
vessel, total dose by all structural materials was calculated
at the front of L-port of LHD. Figure 5 shows this result.

Proceedings of ITC/ISHW2007
Table 2-2. Activities of typical radionuclides on stainlesssteel structures 2.

Since neutron flux through the big vertical port gives strong
contribution to the floor concrete, result of the vertical port
model is shown for floor concrete. Others are results of
the horizontal port model.
From one to ten years after 1000 shots operation, mainradionuclides contributed to the γ-ray dose are 55Fe (half-lifeof 2.73 y) and 60Co (5.27 y) for structures of stainless steel,and 40K (1.28x109 y) and 55Fe for concretes. However,almost of 40K are natural nuclides existing in concrete.

5. Conclusions
In this paper, the 2-D neutron transport code DOT-3.5
and the induced radioactivity calculation code CINAC wereemployed to identify the radionuclides and to evaluate thedose rate during and after experimental series in LHD.
Detailed analysis of activation properties and the dose rate
for individual structural materials of LHD were carried out.
Major results are as follows:(1) In the SS-316 materials, dominant radionuclides justafter shots are 56Mn. Finally 60Co becomes dominant.
(2) In concrete, dominant radionuclides just after shots are24
Na. Although 40K becomes dominant finally, it isalmost natural nuclides.
(3) γ-ray dose rate decreases one order during weekend
interval. This level of 10μSv/h is lower than thelimitation for worker (20mSv/y) who works for 2000
hours per year.
Table 3. Activities of specific radionuclides on concrete
structures.

References

Tables 2, 3 show results of the typical radionuclide densities
on the stainless steel (SS-316) structures and concretes.
Two kinds of data, one-year interval and ten years
interval after 1000 shots operation, were shown. In thesetables, data less than 1.0x10-14 (Bq/cc) were omitted.

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