The Evaluation of a Simple Measurement Method using NaI(Tl) Scintillation Survey-Meter for Radiation Safety Management of Radioactivated Armor Tiles of LHD Vacuum Vessel

Abstract

In this study, the application of a simple measurement method using NaI(Tl) scintillation survey-meter to detect the gamma-rays in the armor tiles of LHD vacuum vessel exposed to fast neutrons during deuterium plasma experiments was evaluated to control the occupational exposure for the storage of huge numbers of the armor tiles. The gamma-ray spectrometry for an activated armor tile by high-purity germanium detector (HPGe) showed the gamma-ray peaks from 58 Co, 54 Mn and 60 Co. The detection efficiencies of these nuclides for HPGe were evaluated by PHITS to estimate the concentrations of these nuclides. The radiation transport model for HPGe in PHITS were validated by the gamma-ray measurements for sealed gamma-ray sources. Then, the detection efficiencies of these nuclides for NaI(Tl) scintillation survey-meter were also evaluated by the same manner using PHITS. The predicted dose rate of the activated armor tiles in NaI(Tl) scintillation survey-meter was consistent with the actual measurement. The predicted count rate in NaI(Tl) scintillation survey-meter for 60 Co, which will be a deterministic nuclide in the armor tile due to longer half-life, in the clearance level was sufficiently higher than the minimum detectable count rate of NaI(Tl) scintillation survey-meter with a simple shielding for environmental radiation.

Evaluation of Gamma-Ray Dose Distribution in the Toroidal Direction of LHD Vacuum Vessel from Radionuclides Generated in Deuterium Plasma Experiments

“…The peak at approximately 122 keV reported in two previous papers [9,16] is not evident in Figure 3. This peak originated from the 58 Ni(n,d) 57 Co and 58 Ni(n,np) 57 Co transmutations, which emitted gamma rays of 122 keV with a half-life of 271 d. The 58 Ni(n,d) 57 Co and 58 Ni(n,np) 57 Co reactions are threshold reactions that occur with neutrons above 8 and 9.5 MeV, respectively.…”

“…At this time, the collimator (8 mm-thick tungsten polymer) was used to reduce the environmental gamma rays. Table 1 shows the measured gamma-ray energy, emission probabilities r γ , and activity concentrations of A 2 for each radioactive nuclide whose values were already reported by Kobayashi et al [9].…”

“…The radioactivity obtained by an armor tile removed from the vacuum vessel at the 1-2 U/L port was also evaluated using the same P-HPGe detector [9]. One armor tile leaned on the wall of the torus hall, approximately 30 m away from the LHD cryostat, and the P-HPGe detection port was attached to one side of the tile.…”

“…For safety management, gamma-ray spectrometric measurements and dose rate evaluations were conducted using an NaI(Tl) survey meter aimed at the activated armor tiles removed from the LHD vacuum vessel [9]. This method is highly effective for low-level activation components.…”

Gamma ray diagnostics for high time resolution measurement in large helical device

“…The model has been utilized for evaluating neutron and gamma-ray distributions in the LHD torus hall and validated with experiments using activation foil methods [52,53]. In this calculation, the plasma neutron source was assumed to be a simple torus with 99.5% deuterium-deuterium and 0.05% deuterium-tritium neutrons based on a so-called triton burnup experiment conducted in LHD [54], as performed in previous studies [55,56]. Figure 2 shows the neutron and gamma ray flux map obtained by the transport calculation.…”