Research and Development Related to the Accident at TEPCO’s Fukushima Daiichi NPS

1-9 Accurate Measurement and Evaluation of Environmental Dose Rates in Air
ᴪ Dose-Rate Evaluation in the Environment using Monte Carlo Simulation ᴪSemi-infinite

Fig.1-23 The simulation geometry of the environment
The transport of the γ-rays emitted from radionuclides
distributed in soil was simulated using the radiation-transportcalculation code PHITS. Dose rates in air were calculated withconsideration of the energies and incident directions of γ-rays
entering a detector 1 m above the ground.

AirDetector(surveymeter)

100 m

1m
SoilRadionuclides

Detector(1) Parallel beam
(2 directions)(2) ISO(3) ROT

Fig.1-24 Various irradiation conditions of γ-rays
Various irradiation conditions are shown for a CsI(Tl) scintillation detector (13 × 13 × 20 mm3), which is simulated
by calculation analysis. The irradiations of the ISO and ROT were performed under the condition where thesource was rotated around the detector.
Table 1-1 Comparison of dose rates under various γ-ray-irradiation conditions
Dose rates depend on the irradiation geometry of γ-rays. Ratios of dose rates to those in the environment(environmental dose) are also shown.
Parallel beam

Environmental
source13 × 13 mm2Dose rate (μSv/h)

5.19 ± 0.09

6.06 ± 0.20

5.12 ± 0.09

5.35 ± 0.13

5.66 ± 0.13

Ratio

1.00

1.17 ± 0.04

0.98 ± 0.02

1.03 ± 0.03

1.09 ± 0.03

Following the TEPCOʼs Fukushima Daiichi NPS (1F)
accident on March 11, 2011, radiation monitoring has been
periodically performed to obtain environmental dose rates
in air. Because scintillation-crystal-based detectors such as
survey meters are usually used for ensuring safety in nuclear
power plants, dose-rate calibration is performed for accurate
measurement under conditions where the radiation is incident
on a detector from one direction. In contrast, the radiations
that were released from radionuclides such as cesium into
the environment by the 1F accident and which were widely
deposited in soil are incident on the detector from various
directions.
Therefore, for the purpose of investigating the necessary
calibration condition of detectors for accurate measurementof the doses in the environment, the inﬂuence of the incidence
direction of radiation upon the dose-rate measurement wasanalyzed using the particle and heavy-ion transport system(PHITS). First, an environment with radionuclides existingin the soil (Ground source, Fig.1-23) was reproduced on acomputer, and the dose rates 1 m above the ground’s surface
were calculated. The CsI(Tl)-scintillation detector of crystal size
13 × 13 × 20 mm, was selected for a car-borne survey. Second,the dose rates were calculated under the following irradiation

13 × 20 mm2ROTISO

conditions: (1) a uniform parallel-beam condition against
each crystal surface (13 × 13 mm2 and 13 × 20 mm2); (2) thecondition where a detector is irradiated isotropically from alldirections (ISO); and (3) the condition where only the lateralsurface of a detector is irradiated laterally (ROT) (Fig.1-24).
Furthermore, all calculated results were normalized by thedose rates measured around 1F (Table 1-1 Dose rates for theGround source).
The results of the calculated dose rates under the irradiation
conditions are shown in Table 1-1. The dose rate for theparallel beam irradiation on the 13 × 13 mm2 crystal surface
is approximately 17% larger than that for the Ground source.
Conversely, the dose rates in the ISO and ROT geometries
agree with the dose rate for the Ground source to within 10%;this result indicates that the incident directions of gamma
rays in the ISO and ROT geometries are similar to that for theGround source. Moreover, the parallel-beam irradiation on the13 × 20 mm2 crystal surface agrees well with the dose rate for
the Ground source. It is found from these results that a simple
calibration method under unidirectional irradiation can be
sufﬁciently accurate to obtain the dose rates in the environment.
In the future, the dependence of detector configuration on
the gamma rays’ incidence direction will be analyzed.

Reference
Tsuda, S. et al., Spectrum-Dose Conversion Operator of NaI(Tl) and CsI(Tl) Scintillation Detectors for Air Dose Rate Measurement in ContaminatedEnvironments, Journal of Environmental Radioactivity, vol.166, part 3, 2017, p.419-426.
JAEA R&D Review 2016-17

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