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FNS Radiation Skyshine with D-T Neutron Source

 1. Name of Experiment:
    FNS/JAERI Measurement of Radiation Skyshine with D-T Neutron Source
    (March 2002 and March 2003)

 2. Purpose and Phenomena Tested:
    The D-T neutron skyshine experiments [1] have been carried out at the Fusion
    Neutronics Source (FNS) of JAERI. The radiation dose rate outside the target
    room was measured at distances up to 550 m from the D-T target point.

 3. Description of Source and Experimental Configuration:
    FNS is an accelerator based intense D-T neutron source. It has two target
    rooms: target room I has a fixed tritium target containing 3.7E+11 Bq
    tritium and the target room II has a rotating tritium target containing
    3.7E+13 Bq tritium. The  target room I was used for the shyshine
    experiments. Figure 1 shows the cross-sectional view of target room I
    along a north-south cut. The target room is 15 x 15 m wide and 9.1 m high.
    The thickness of the concrete of the roof and the vertical walls is 1.15 m
    and 2 m, respectively. The floor of the target room is an iron grating in
    order to reduce the neutron scattering from the floor. The tritium target
    is located 1.8 m above the floor, 5.5 m away from the south wall and 2.75 m
    from the west wall. The target room has a port in the roof with a size of
    0.9 x 0.9 m for the shyshine experiments.

    This port is usually closed with a concrete plug. In the skyshine experiment,
    the concrete plug was removed and only 2 mm thick stainless steel plates
    closed the port in order to keep the reduced air pressure in the target room.
    The neutron attenuation by 2 mm stainless steel plates was negligible. FNS
    was operated with an acceleration voltage of 360 kV and a target current of
    1.4 mA in this experiment. The source neutron yield was ~1.7E+11 n/s, which
    was monitored by the associated alpha particle measurement with a silicon
    surface barrier diode installed in the beam line. 

    Figure 2 shows the measurement points in the environment. FNS is located
    on a rather flat land only 150 m distance from the Pacific ocean. The north
    and east sides are surrounded with a forest of pine trees which are about
    10 m tall. The neutron dose rate was measured at the distance from 20 to
    550 m in the pine forest along the north direction, at distances from 20
    to 140 m on a road along the south direction and at 200, 230 and 300 m
    along the south west direction. 

 4. Measurement System:
    Neutron dose rates were measured with the spherical rem-counters, He-3 and BF3
    Fuji Electric NSN10002, just on the ground at all measurement points shown in
    Figure 2.

    The neutron spectra were evaluated with boner spheres with multi-moderators of
    0.5, 1, 2, 3, 5 and 9 cm thickness along the north direction, and unfolded by
    SAND-II code.

    Secondary gamma-ray spectra were measured with a high purity Ge detector and
    a NaI scintillation counter with a size of 125 cm in diameter by 125 cm long
    at distances of 20, 50 and 100 m, and with a NaI scintillation counter with
    a size of 200 cm in diameter by 200 cm long at distances of 200 and 300 m. 

 5. Description of Results and Analysis:
    The measured neutron dose in the south and south west directions are shown
    in Figure 3. The numerical data of the neutron dose are given in Table 1. 
    Neutron spectra measured at a distance of 50, 100 and 200 m and unfolded by
    SAND-II code are shown in Figure 4. They are compared with the first MCNP-4B
    calculations (forest was not included in the calculational model).

    Figure 5 shows the pulse height spectra measured with the NaI scintillation
    counter at the distances of 20, 50 and 100 m. Numerical data of Fig. 5 are
    given in the file sky-NaI.txt. The information on the detector response
    function and the detector efficiency, needed for the comparison with the
    calculation, is not yet available.
    Few characteristic gamma-ray lines can be observed in the spectra, like the
    natural gamma-ray background from K-40 and Tl-208, the energy peaks from the
    H-1(n,gamma) and Fe-56(n,gamma) reactions. A continuum spectrum in the energy
    range of 3-7 MeV was enhanced by the shyshine, which is considered to be the
    convolution of Compton spectra from some neutron capture reactions. The high
    purity Ge detector observed additional peaks of the Si-28(n,gamma) reaction
    at 3.539 and 4.934 MeV at the distance of 20 m only, which is considered to
    arise from the FNS building.

    Experiment Analysis:
    Transport calculations with the MCNP-4B code are presented in [1]. The
    calculations were performed using the JENDL-3.2 nuclear data library.
    The corresponding sample input for the MCNP-4B code is given in the file
    mcnp4b-sky.inp. The forest was not modeled in this calculation.

    More recent analysis with the MCNP-4C code and JENDL-3.2 nuclear data are
    described in [2]. The corresponding MCNP-4C input data are included in
    mcnp4c-sky.inp and the comparison of the measured and calculated neutron
    dose rate is presented in sky-mcnp4c.xls. In this model the FNS building
    and the measurement field were modelled in a simplified cylidrical geometry,
    and the pine forest was modelled by homogenised cell with a height of 10 m.

 6. Special Features:

 7. Author/Organizer:
    Experiment and analysis:
    T. Nishitani, K. Ochiai, J. Hori, S. Sato, M. Yamauchi,
    M. Nakao, M. Wada, C. Kutukake, S. Tanaka
    Japan Atomic Energy Research Institute
    Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan
    Phone: +81-29-282-6859 (for T.N)
    FAX: +81-29-282-5709 (for T.N)

    S. Yoshida, R. Tanaka, I. Murata, A. Takahashi
    Osaka University
    Yamadaoka 2-1, Suita, Osaka, 565-0871, Japan

    J. Kaneko, M. Wakisaka, T. Sawamura
    Hokkaido University
    Kita 8 Nishi 5, Kita-ku, Sapporo 060-0808, Japan
    Compiler of data for Sinbad:
    S. Kitsos
    OECD/NEA, 12 bd des Iles, 92130 Issy les Moulineaux, France

    Reviewer of compiled data:
    I. Kodeli
    OECD/NEA, 12 bd des Iles, 92130 Issy les Moulineaux, France

    The data were kindly contributed by Dr. Takeo Nishitani.

 8. Availability:

 9. References:

    [1] S. Yoshida, T. Nishitani, K. Ochiai, J. Kaneko, J. Hori, S. Sato,
        M. Yamauchi, R. Tanaka, M. Nakao, M. Wada, M. Wakisaka, I. Murata,
        C. Kutukake, S. Tanaka, T. Sawamura, and A. Takahashi:
        Measurement of Radiation Skyshine with D-T Neutron Source, 
        Fus. Eng. Des. 69 (2003) 637-641
        (22nd Symposium on Fusion Technology (SOFT-22), Helsinki, 9-13. Sept. 2002).
    [2] T. Nishitani, K. Ochiai, S. Yoshida, R. Tanaka, M. Wakisaka, M. Nakao,
        S. Sato, M. Yamauchi, J. Hori, M. Wada, A. Takahashi, J. Kaneko and
        T. Sawamura, D-T Neutron Skyshine Experiments and the MCNP Analysis,
        J. Nucl. Sci. & Technol., Sup. 4, p. 58-61 (March 2004)

10. Data and Format:

     Filename     Size[bytes]   Content
    ------------- ----------- -------------
  1 fnssky-a.htm      10,175  This information file
  2 fnssky-e.htm       3,527  Description of experiment
  3 mcnp4b-sky.inp     8,342  Input data for MCNP-4B calculations
  4 mcnp4c-sky.inp     8,565  MCNP-4C Input data (including pine forest)
  5 sky-fig1.jpg      67,850  Fig. 1: Cross-sectional view of the target room
  6 sky-fig2.jpg      73,941  Fig. 2: Measurement points in the environment
  7 sky-fig3.jpg      35,348  Fig. 3: Neutron dose distribution
  8 sky-fig4.pdf      14,901  Fig. 4: Neutron spectra at 50, 100 and 200 m
  9 sky-fig5.jpg      44,498  Fig. 5: Secondary gamma-ray spectra
 10 sky-NaI.txt      161,908  Secondary gamma-ray pulse height spectra (numerical data)
 11 sky-mcnp4c.xls    22,528  Measured and MCNP-4C calculated neutron dose rates
 12 sky-ref1.pdf     297,540  Reference 1
 13 proc_147.pdf     867,745  Reference 2

    File fnssky-e.htm contains the following table:

      Table 1: Neutron dose rate distributions.

   Figures are included in the JPG and PDF formats.

SINBAD Benchmark Generation Date: 02/2003
SINBAD Benchmark Last Update: 07/2006