27aC09

ＬＨＤにおける総発生中性子計測と計測器校正
Neutron Flux Monitoring and Calibration of the Neutron Detectors in LHD
瓜谷章1，仲野裕次1，磯部光孝2,3，小川国大2,3，西谷建夫2，渡辺賢一1，山﨑淳1
A. Uritani1, Y. Nakano1, M. Isobe2, K. Ogawa2, T. Nishitani2, K. Watanabe1 and A. Yamazaki1
名古屋大学1、核融合科学研究所2、総合研究大学院大学3
Nagoya Univ.1, NIFS2, SOKENDAI3
Introduction
Deuterium
plasmaexperiments(DDexperiments) are in plan at the Large Helical Device(LHD) in NIFS. Deuterium thermonuclear fusion
reactions generate neutrons with specific energy of2.45 MeV. Neutron monitoring is quite important
not only for evaluation of the plasma properties but
also for the radiation safety managements. The
neutrons will be measured by three fission
chambers and a neutron activation method. It isnecessary to calibrate the sensitivities of theseneutron monitors in prior to the DD experiments.
The in situ calibration will be done using a 252Cf
neutron source placed on a model train that will run
on a circular railroad truck set in the LHD. There
are some differences of conditions between the DD
experiments and the calibration ones, as shown inTable 1. The influences of these differences on thesensitivities of the neutron monitors must beevaluated. In this work, the influences wereevaluated using the Monte Carlo neutron transport
code MCNP5.

degrees rotation in the toroidal direction, the coils
rotate 90 degrees in poloidal direction. Figure 1
shows a schematic view of the LHD geometry usedin the MCNP simulations. In this work, we adopted
the simulation models with 360 divisions from aviewpoint of the simulation precision and therequired calculation time.
Results of simulation
Figure 2 shows the neutron energy spectra at
500 cm from the equator and on the center axis ofthe LHD where the fission chamber will be placed.
These two spectra are almost the same but there areslight differences. These slight differences cause thedifference in the sensitivity of the fission chamber
for DD and 252Cf neutrons. The ratio of thesensitivities of the fission chamber at that position
for DD and 252Cf neutrons was 0.92±0.06.
(a)(b)
O-port18°
0°

Table 1 Differences of conditions between the DD
experiments and the calibration ones.
13.5m

2.45 MeV
mono-energetic

252

Cf fission
spectrum~3.9 m3.74 m~60 cm1 cm

exist
Not exist

Not exist
exist

Monte Carlo Simulation
In MCNP, a geometry is defined with cells
districted with many surfaces such as flat planes.
Because no surface defining helical shape isprepared in MCNP, we have constructed an
approximation model by dividing parts with a small
angle in toroidal direction. We made the helical coilstructure by rotating the poloidal position of coils.
The LHD has a periodic structure with ten cycles(every 36 degrees). In other words, for every 18Fig. 1. Schematic view of the LHD geometry used in
MCNP code. (a) Horizontal section on the equatorial
plane of the LHD. (b) Vertical sections at different
toroidal angle positions. Left upper figure; 0 degrees,
left lower figure; 18 degrees. The helical coil structures
gradually rotate.
2

Neutron fluence

Calibration[10-8 /cm2 /incident/lethargy]

Neutronenergy
Major
RadiusMinorradiusLiquid He
Railroad

DD Experiments

DD experiment
Calibration

1010 -9 10 -8 10 -7 10 -6 10 -510 -4 10 -3 10 -210 -1 10 0 10 1
Energy [MeV]Fig. 2. Neutron energy spectra at 500 cm from theequator and on the center axis of the LHD.