Neutron and gamma-ray transport calculations in support of the design of the radiation shielding for the TOFED neutron spectrometer at LHD

• Published in: Fusion engineering and design 2021-05, Vol.166, p.112296, Article 112296
• Main Authors: Sangaroon, S., Ogawa, K., Isobe, M., Kobayashi, M.I., Conroy, S., Zhang, Y., Fan, T.S., Osakabe, M.
• Format: Article
• Language: English
• Subjects: Background radiation; Cyclotron resonance; Design criteria; Energy spectra; Fluence; Gamma rays; LHD; Neutron emission; Neutron flux; Neutron spectrometer; Neutrons; Polyethylenes; Radiation; Radiation shielding; Relocation; Thickness; Time-of-flight technique; TOFED; Toruses; Tritons
• ISSN: 0920-3796; 1873-7196; 1873-7196
• DOI: 10.1016/j.fusengdes.2021.112296

•To emphasize understanding of energetic-ions behavior in the Large Helical Device (LHD), the Time-Of-Flight-Enhanced-Diagnostics (TOFED) has been developed and newly installed at LHD.•TOFED is located at the LHD basement and shares one of the collimators with vertical neutron camera at major radius of 3.72 m at 2.5 L LHD lower port.•Neutron and gamma-ray transport calculations have been performed using the MCNP6 in order to support the design of the radiation shielding for the TOFED neutron spectrometer at LHD.•On account of design criteria and engineering constraints, 20 cm of borated polyethylene is being considered for TOFED installation as a neutron shielding and 5 cm of lead is being considered for TOFED installation as a gamma-ray shielding. The neutron emission spectrometer (NES) has been commissioned at the Large Helical Device (LHD) with a new Time-Of-Flight-Enhanced-Diagnostics (TOFED). The aim of the TOFED is to measure the neutron energy spectrum to study the fast-ions behavior such as high-energy ions created by ion cyclotron resonance heating, beam ions, and D-D fusion born 1 MeV tritons in LHD. The instrument is assembled of the first detector S1 and the second detector S2. In 2019, TOFED was installed at Z = 13.5 m below midplane without the radiation shielding and was initially commissioned. Due to neutrons streaming through a considerable number of holes in the LHD torus hall, the ambient background radiation at the TOFED was higher than expected, which causes the large number of coincidental events on the detectors. The obtained time difference between S1 and S2 provides the strong suggestion of TOFED relocation and of TOFED shielding development. In 2020, TOFED was relocated to the basement at Z = 18.5 m below midplane. In this work, a dedicated radiation shielding design is investigated using the Monte Carlo N-Particle code (MCNP6). The shielding design fulfills the design criteria on the radiation moderation capability and engineering constraints. Due to the design criteria, borated polyethylene with thickness of 20 cm is being considered as a neutron shielding and lead with thickness of 5 cm is being considered as a gamma-ray shielding. The shielding is suggested to be a rectangular polyhedron shaped roof. With the designed shielding, neutron flux is reduced by approximately two orders of magnitude at S1 and by approximately more than two orders of magnitude at S2 while prompt gamma-ray fluence is reduced by approximately two orders