United Nations Scientific Committee on the Effects of Atomic Radiation

ATTACHMENTCOMPARISON OF MEASUREMENTS OF IODINE-131
CONCENTRATIONS IN AIR WITH ESTIMATES USING
ATDM
Developments since the 2013 UNSCEAR report on the levels and effects ofradiation exposure due to the nuclear accident following the great east-Japan
earthquake and tsunami
A 2016 white paper to guide the Scientific Committee’s future programme of work

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©

United Nations, October 2016. All rights reserved, worldwide.

This publication has not been formally edited.

ATTACHMENT: COMPARISON OF MEASUREMENTS OF IODINE-131 CONCENTRATIONS IN AIR WITH ESTIMATES USING ATDM

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Contents
I. AIM ........................................................................................................................................ 3
II. BACKGROUND .................................................................................................................... 3
III. APPROACH........................................................................................................................... 4
IV. RESULTS............................................................................................................................... 6
REFERENCES ............................................................................................................................. 9

I.

AIM

1.
The aim of this paper is to compare measurements of 131I concentrations in air asreported by Hirayama et al. [H1] with 131I concentrations in air as estimated using atmospherictransport, dispersion and deposition models (ATDM) alone (and in combination withmeasurements of iodine-131 deposited on the ground) for the 2013 UNSCEAR report on thelevels and effects of radiation exposure due to the nuclear accident following the great eastJapan earthquake and tsunami [U1].

II.

BACKGROUND

2.
Hirayama et al. [H1] assessed the distribution of 131I concentration in air with time frommeasurements made in March 2011 at several monitoring posts in Fukushima Prefecture using
peak count rates and the calculated response of NaI(Tl) detectors. The locations of themonitoring posts are indicated by red squares in figure I. Hirayama et al. [H1] estimated 131I
concentrations in air from the measurements as hourly values. The data provide a new andvaluable source of information on 131I concentrations in air during the first days of the accident
for which very few direct measurements have previously been published.
3.
For the 2013 report two different approaches had been used by the Committee toestimate 131I concentrations in air (and resulting inhalation doses):− For settlements within the evacuated areas, 131I concentrations in air had been evaluatedusing an estimate of the source term (including the temporal and spatial patterns of
release) [T1] together with appropriate ATDM;− For all other locations, the Committee had relied on measurements of the deposition
densities of radionuclides as the basis for its estimates of 131I concentrations in air.
ATDM results were used to evaluate the concentrations of radionuclides in air frommeasured deposition densities on the ground by scaling the measured deposition
densities using the ratio of the concentration in air and deposition density estimated by
ATDM.
The ATDM calculations had been carried out by a task team of the World MeteorologicalOrganization (WMO). The ATDM calculations had used a regular grid with a grid spacing of4

ATTACHMENT: COMPARISON OF MEASUREMENTS OF IODINE-131 CONCENTRATIONS IN AIR WITH ESTIMATES USING ATDM

0.05° and the results were in the form of time-integrated concentrations in air over three-hourintervals. The grid cells located close to the monitoring posts considered by Hirayama et al.
[H1] are shown in figure I as green circles.

III. APPROACH
4.
For all monitoring posts, a comparison has been made of measurements and estimatesof 131I concentrations in air using the following approach:− For each monitoring post (considered by Hirayama et al. [H1]), the closest ATDM gridcell or cells was/were identified, see red lines in figure I;− For each monitoring post, the hourly estimates made by Hirayama et al. [H1] of 131I
concentration in air were integrated over the same three-hour intervals used in the
ATDM calculations and also over the whole monitoring period;− For each ATDM grid cell, the ATDM results were also integrated over the whole time
period covered by the ATDM calculations (i.e. from 00:00 on 12 March 2011 until
03:00 on 4 April 2011);− The time integrals (over relevant periods) of 131I concentrations in air were thencompared, see figures II–IV and table 1.
5.
For monitoring posts located beyond the evacuation area, an additional comparison has
been made between the time-integrated concentrations derived from hourly estimates made by
Hirayama et al. [H1] and those estimated in the 2013 report using ATDM in combination withmeasured levels of 131I deposition densities.
6.
The former comparisons are, in general, relevant to the estimation of doses to thoseevacuated, which were based on ATDM calculations alone. The latter are relevant to theestimation of doses in non-evacuated areas that were based on ATDM combined withmeasured deposition densities.

ATTACHMENT: COMPARISON OF MEASUREMENTS OF IODINE-131 CONCENTRATIONS IN AIR WITH ESTIMATES USING ATDM

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Figure I. Location of monitoring posts considered in Hirayama et al. [H1] (red squares) andclosest UNSCEAR ATDM grid cells (green circles). The red lines indicate the closest ATDM gridcell for each monitoring post

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ATTACHMENT: COMPARISON OF MEASUREMENTS OF IODINE-131 CONCENTRATIONS IN AIR WITH ESTIMATES USING ATDM

IV. RESULTS
131

Figure IIa/b. Comparison of I concentrations in air integrated over three-hour intervals for
Okuma (green points are ATDM results)

131

Figure IIIa/b. Comparison of I concentrations in air integrated over three-hour intervals for
Futaba and Naraha (green points are ATDM results)

ATTACHMENT: COMPARISON OF MEASUREMENTS OF IODINE-131 CONCENTRATIONS IN AIR WITH ESTIMATES USING ATDM

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131

Figure IVa/b. Comparison of I concentrations in air integrated over three-hour intervals for
Hirono and Fukushima City (green points are ATDM results)

7.
Figures II to IV show a comparison of 131I concentrations in air integrated over threehour intervals for Okuma, Futaba, Naraha, Hirono and Fukushima City. From the figures, it isapparent that, in some of the comparisons, there are significant differences. Specifically, themeasurements indicate an earlier arrival time of a smaller radioactive plume for three of themonitoring posts in Okuma and the two in Futaba (see figures IIa and IIIa), and a lack ofmeasurements for the monitoring posts in Okuma and Futaba for the period when the ATDM
calculations predict the passage of a larger radioactive plume (see figures IIa, IIb, IIIa). In thesecases, Hirayama et al. [H1] have stated, in an additional publication [H2], that data for thesemonitoring posts after 20:00 on 14 March “could not be analysed because of a drastic increaseor a lack of data”. Given the very large increases in airborne radionuclide concentrations
predicted by ATDM after the 14 March, a reasonable inference would be that the absence ofmonitoring data after this time was due to saturation or other common-mode failure of thesemonitors. Meaningful comparisons with ATDM predictions cannot, therefore, be made forthese monitoring posts after 14 March. The comparison of 131I concentrations in air for Okuma
and Futaba is also limited by the grid resolution of the ATDM calculations, as well as that of
the underlying meteorological data, being of the same order of magnitude as the distance
between the closest monitoring points and the release point.
8.
The remaining comparisons (see figure IIIb for Naraha/Shoukan, figure IVa for
Hirono/Futatunuma, and figure IVb for Fukushima City/Momijiyama) show better agreement,
consistent with (a) the uncertainties inherent in ATDM predictions, (b) the differences between
the locations of the monitoring posts and those for which ATDM predictions were made, and(c) the uncertainties associated with the method used (and acknowledged) by Hirayama et al.
[H1] to estimate radionuclide concentrations in air from measured count rates.
9.
Table 1 presents a comparison of the 131I concentration in air integrated over the wholetime period covered by the ATDM calculations (i.e. from 00:00 on 12 March 2011 until 03:00
on 4 April 2011) with time-integrated concentrations measured at these three monitoring posts.
This comparison indicates agreement within a factor of about two for the monitoring posts at
Shoukan (Naraha) and Futatunuma (Hirono) and within a factor of about eight for themonitoring post at Momijiyama (Fukushima City). This level of agreement is again in line withthe uncertainties underlying the comparison.

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ATTACHMENT: COMPARISON OF MEASUREMENTS OF IODINE-131 CONCENTRATIONS IN AIR WITH ESTIMATES USING ATDM
131

Table 1. Comparison of time-integrated I concentrations in air from Hirayama et al. [H1] withaestimates based on ATDM carried out for the 2013 report
Data from [H1]2013 report ATDM predictions [U1]131

Monitoring post

Time-integrated I
concentration in air(Bq d/m3)

Town/city

ATDM grid cellTime-integrated 131I
concentration in air(Bq d/m3)

6 000

Naraha37.3N 140.95E

4 100

6 000

Naraha37.3N 141.0E

15 000Futatunuma

6 000

Hirono37.25N 141.0E

12 000Momijiyama

2 700

Fukushima City

37.75N 140.45E

340

Shoukana

All values quoted to two significant figures.

10.
For Hirono and Fukushima City, 131I concentrations in air (and resulting inhalation
doses) were estimated in the Committee’s 2013 report, not from ATDM estimates alone, but incombination with measured deposition densities (i.e. using ATDM to scale measurements ofdeposition density). Table 2 shows a comparison of the results from Hirayama et al. [H1] forthe monitoring posts closest to these two locations with the time-integrated 131I concentrationin air estimated in this way. For both locations, the agreement is good given the inherent
uncertainties in the measured and estimated values: the estimated concentrations in air are a
few tens of per cent lower than those measured. The agreement is better than that between
measured levels and estimates based on ATDM alone (see table 1). This supports the approach
adopted in the 2013 report to assess radionuclide concentrations in air (and subsequently
inhalation doses) primarily based on the measured deposition densities, with ATDM (together
with an assumed source term) used to derive scaling ratios.
131

Table 2. Comparison of time-integrated I concentrations in air from Hirayama et al. [H1] withaestimates carried out for the 2013 report based on ATDM scaling of deposition measurements
Estimates based on method used in 2013 report [U1](using ATDM to scale measured deposition densities)

Data from [H1]

Monitoring postaTime-integrated 131I
concentration in air(Bq d/m3)

Town/city

ATDM grid cellTime-integrated 131I
concentration in air(Bq d/m3)

Futatunuma

6 000

Hirono37.25N 141.0E

4 100

Momijiyama

2 700

Fukushima City

37.75N 140.45E

2 200

All values quoted to two significant figures.

11.
The measurements also provide support for the estimates made in the 2013 report ofradionuclide concentrations in air (and subsequently doses) to which evacuees were exposed
prior to their evacuation from Futaba and Okuma. These evacuations were completed before 12
March, and the ATDM estimates indicated that there were no exposures prior to this time. The
measurements at Futaba and Okuma show the arrival of air masses with elevatedconcentrations of 131I at varying times during the day of 12 March (i.e. after the evacuation had
been completed) and, as such, confirm the estimates in the 2013 report.

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9References
H1

H2

T1

U1

Hirayama, H., H. Matsumura, Y. Namito et al. Estimation of time history of I-131 concentration in air
using NaI(Tl) detector pulse height distribution at monitoring posts in Fukushima prefecture. Trans At
Energy Soc Japan 14(1): 1-11 (2015). (Japanese).
Hirayama, H., H. Matsumura, Y. Namito et al. Method to separate plume- and accumurated surroundingcontributions to peak count rate using pulse height distribution time histories at monitoring post. KEK
Internal 2014-7. High Energy Accelerator Research Organization (KEK), Japan, 2015.
Terada, H., G. Katata, M. Chino et al. Atmospheric discharge and dispersion of radionuclides during the
Fukushima Dai-ichi Nuclear Power Plant accident. Part II: verification of the source term and analysis ofregional-scale atmospheric dispersion. J Environ Radioact 112: 141-154 (2012).
UNSCEAR. Sources, Effects and Risks of Ionizing Radiation. Volume I: Report to the Genera Assemblyand Scientific Annex A. UNSCEAR 2013 Report. United Nations Scientific Committee on the Effects ofAtomic Radiation. United Nations sales publication E.14.IX.1. United Nations, New York, 2014.