Safety evaluation of temporary storage, etc.
1. Background and purpose
Soil and combustibles containing radioactive materials generated by decontamination(hereinafter referred to as "removed soil, etc.") should be stored in temporary or on-site storage
places (hereinafter referred to as "temporary storage, etc.") until delivered to the interim storage
facilities set up by the government. Since the amount of removed soil, etc. by decontaminationis enormous, it is difficult to secure a storage place that is large enough to store the entire
amount and isolated from the living environment. Therefore, a respective method of storing
decontaminated materials is adopted for each place or area. As these temporary storages, etc.
exist in public areas, there is concern about their impact on human health and the environment
due to the management of the removed soil, etc. in temporary storage, etc. Therefore, in the
management of temporary storage facilities, it is important to keep public exposure as low asreasonably achievable by implementing safety measures as necessary. Additionally, even after
taking safety measures for facility management, it is important to consider appropriate
responses to type-specific accidents should they occur.
On the basis of the above considerations, the purpose of this study report is to identify
important scenarios for public exposure in relation to the management of removed soil, etc., andto organize and evaluate public exposure routes, including those during accidents. Moreover, as
regards the evaluation results, it was determined that evaluation be also made of the safety of
temporary storage, etc. by comparing it with the concept of radiation protection when managing
removed soil, etc., which will be described later.
2. Method and contents of implementation
measures2.1. Evaluation target areas
Decontamination of areas contaminated withradioactive materials released in connection withthe accident of TEPCO (Tokyo Electric Power
Company)’s Fukushima Daiichi Nuclear Power
Station (hereinafter referred to as the "nuclear
accident") is to be carried out by the municipalities
and the national government. The intensive
contamination

surveyareas

designatedforFig. 1.Evaluation target areas

decontamination by municipalities are defined as shown in Fig. 1. Since the prefecture is
supposed to play a role in supporting the decontamination of municipalities, the intensive
contamination survey areas were selected as the evaluation target areas in this study.
１2.2 The current status and scope of evaluation of the management of removed soil, etc.
The removed soil, etc. containing radioactive materials generated by decontamination is to be
managed by the scheme shown in Fig. 2. The amount of removed soil, etc. shown in Table 1 hasalready been generated, and these are stored in the temporary storages and on-site storages in
the prefecture. In addition, the total amount of soil removed by decontamination including
future, follow-up decontamination is estimated by the Ministry of the Environment to be as
much as 28 million m31). In this study report, the storage process in temporary storage, etc.,
which is currently the most important management process, was evaluated.
Implementation of decontamination

Temporary storage at temporary depositories and decontamination sites

Storage in interim storage facilities (in the prefecture)

Disposal at the final disposal site (out of the prefecture)

Fig.2 The flow of removed soil, etc.
Table 1 Storage status of removed soil, etc. generated by decontamination in the prefecture intemporary storage, etc. 2),3)
Numberof places

Area

Storage status

Special decontamination areas (asof the end of March 2019)*1

Temporary storage

194

Temporary storage

616

Intensive contamination survey
areas*2 (In Fukushima prefecture
only, as of the end of March2019)

Total number ofoccurrences/stored
substance (10,000
m3)
565

529
Storage at the site

86,175

Total

1,094

*1 In addition to temporary storages, temporary depositories, temporal temporary storage, etc. are
included.
*2 Fifty-two municipalities are the target localities, except Naraha-Machi, Tomioka-Machi,
Okuma-Machi, Futaba-Machi, Namie-Machi, Katsurao-Mura, Iitate-Mura.

22.3. Structure of generally used temporary storages
Three requirements are presented for the safety measures of temporary storages, i.e.; i)shielding and separation, ii), dispersal prevention of removed soil, and iii) prevention of
rainwater intrusion, etc.4) In addition to the above facility requirements, the storage type andstructure of temporary storages are determined in consideration of workability at the time ofremoval and the site area that can be secured. Fig. 3 shows the structure of a generally usedground storage type temporary storage. This structure is designed in consideration of the factthat the target nuclide is radioactive cesium. Specific points to note include the following
items:4)･Prohibition of unnecessary entry by installing fences･Appropriate shielding by installing sandbags for shielding or separation from residential areas･Measures to prevent rainwater intrusion by using an impermeable sheet etc.
･Securing a catch basin and preventing leakage of radioactive materials with the lower
impermeable sheet･Taking measures against the generation of gas and heat storage caused by the decay of
combustibles through installation of gas removal pipes

Fig. 3 Structure of a generally used ground storage type temporary storage5)3

2.4. Concept of radiation protection in management of removed soil, etc.
Subjects affected by radiation from removed soil, etc. include workers involved in work suchas installation of decontamination and temporary storage, etc., light workers involved in work
such as regular monitoring, and the general public. In view of this, radiation protection
regulations for these subjects are summarized below.
To prevent harmful radiation exposure to workers engaged in work such as decontamination,
the Ministry of Health, Labour and Welfare has enforced the “regulations on prevention ofionizing radiation hazards related to work, etc. for decontamination of soil contaminated withradioactive materials generated by the Great East Japan Earthquake”6) (hereinafter referred to as“regulations on prevention of ionizing radiation hazards caused by decontamination”). The worktargeted for the application of the regulations on prevention of ionizing radiation hazards caused
by decontamination includes collection, transportation, or storage of removed soil, etc., which
are subject to safety evaluation, and the regulations applied to workers involved in installation
work etc. for decontamination and temporary storage, etc. On the other hand, the general public
and light workers involved in regular monitoring etc. are not subject to the regulations on
prevention of ionizing radiation hazards caused by decontamination. The Guideline for
Decontamination-related Measures stipulates that “the relevant facilities shall be designed sothat the additional public radiation exposure dose from the removed soil is less than 1 mSv per
year.” This is believed to be in line with, “it is necessary to ensure that the dose received by the
local residents does not exceed 1 mSv/year caused by treatment, etc., and (...) special
consideration is required to control the exposure of the local residents,” as shown in the“concept of ensuring safety in the current situation regarding the treatment and disposal of waste
affected by TEPCO’s Fukushima Daiichi Nuclear Power Station accident”7) by the Nuclear
Safety Commission in June 2011. Additionally, in the evaluation presented at the time of theaccident in the "simplified safety evaluation of interim storage facilities" and the "safety
evaluation method 2013 for trench disposal in shallow ground," it was estimated that excessive
radiation exposure would not be exerted unless it exceeded 5 mSv per accident. Therefore, thisstudy report summarizes the relationship between related regulations and subjects affected byradiation from removed soil, etc. in Table 2.

4

Table 2 Relationship between related regulations and subjects affected by radiation fromremoved soil, etc.

Workers involved ininstallation work etc.
for decontamination,
temporary storage, etc.
Light workers involved
in regular monitoring
etc.
General public

The regulations on prevention ofionizing radiation hazards caused
by decontamination are applied(Dose limit: 50 mSv/year, or 100
mSv in 5 years)

The regulations on prevention of ionizing
radiation hazards caused by decontamination
are not applied (Reference level: 1 mSv/year, or
5 mSv at the time of accident)

○

×

×

○

×

○

Here, workers to whom regulations on prevention of ionizing radiation hazards caused bydecontamination are applied are excluded as evaluation targets because individual doses aremanaged, and only cases to which regulations on prevention of ionizing radiation hazards
caused by decontamination are not applied are treated as evaluation targets.
2.5 Definition of a model temporary storage
There are about 800 temporary storages, etc. in the prefecture, and it is difficult to evaluate
them individually. Therefore, the safety of the entire temporary storage system is evaluated byappropriately setting an example of the typical temporary storage, etc., and evaluating it. Mainparameters and typical set values are shown in Table 3. Here, as there is a considerable
difference between storage at the site and temporary storage in respect to the scale, we defined a
representative case for each. As for the scale of temporary storages, we defined it on the basis of
the average value of the stored quantity at each temporary storage site in the prefecture. In
addition, as for the radioactivity concentration in soil, on the basis of the maximum value of air
dose rate by the 3rd aircraft monitoring outside the evacuation order zones in the prefecture, we
calculated the ground surface pollution density from the conversion factor between the groundsurface pollution density (Bq/m2) and the air dose rate (µSv/h) when the ground surface was
uniformly contaminated. Using the obtained ground surface pollution density value, we
calculated the soil concentration (Bq/kg) on the assumption that the surface layer of 5 cm wasdecontaminated, then attenuated and corrected the value to the concentration as of April 2015,
and adopted the value rounded up to the nearest multiple of 500 Bq/kg. At this time, we
assumed that the ratio of cesium-134 to cesium-137 as of March 2011 was 1: 1. For theradioactivity concentration, the value rounded out to hundreds was adopted.

5

Table 3 The model temporary storage as defined
No.
1
2
3

Parameter

Unit
m3

Scale
Radioactivity concentrationof removed soil, etc.

Bq/kg
kg/m3

Density of removed soil

Value
Temporary storage

800

Storage at the site

4

134

2500

137

8500

Cs
Cs

16007)

2.6 Identification of exposure routes
Under the premise that the target nuclides assumed from the exposure routes of removed soil,

etc. stored in the temporary storage are cesium-134 and cesium-137, we identified the exposure
routes assumed for the storage period as shown in Table 4 below.
Table 4 Scenarios assumed at the time of storage
Event
Normalconditions

At the timeof theaccident

Radiation source

Exposure type

External(Route 1)
External
Regular monitoring
In-place removed soil, etc.
(Route 2)
Inhalation(Route 3)
Dust released into the atmosphere
External(Route 4)
Release of removed soil, etc.
Removed soil, etc. deposited on External
due to fire and loss ofthe ground surface(Route 5)shielding function
Removed soil, etc. remaining on External
the premises(Route 6)
Contaminated crops (In-house
Ingestioncultivation)(Route 7)
Removed soil outflowed to the
Outflow of removed soil, etc.
External
ground surface
due to heavy rain
(Route 8)
Living in the neighborhood

In-place removed soil, etc.

Loss of leakage preventionfunction to the outside of the
Drinking water
facility through the water
systemContaminated crops (In-housecultivation)
Outflow of removed soil, etc.
Contaminated soil
due to flooding of rivers
Contaminated soil

6

Ingestion(Route 9)
Ingestion(Route 10)
External(Route 11)
Inhalation(Route 12)

Evaluation target
General public
Worker(s)General public
General public
General public
General public
General public
General public

General public

General public
General public
General public2.7 Evaluation method<Evaluation of external exposure from removed soil, etc., and route>
Dext(i) = C waste (i) ･S0･t･DFext(i)･(1-exp(-λ･t)) / (λ･t)

(1)

Here,
Dext(i): External exposure dose of radionuclide i (µSv/y)Cwaste(i): Concentration of radionuclide i in removed soil, etc. (Bq/kg)
t: Exposure time (at normal times: h/y; at the time of the accident t: h/accident)
DFext(i): Dose conversion factor for external exposure of radionuclide i (µSv/h)/(Bq/kg)
λ: Decay constant of radionuclide i (1/h)
S0: Shielding factor for external exposure (-)
The dose conversion factor DFext(i) used for evaluating the external exposure dose from volume
radiation source with an arbitrary 3D shape was obtained by MCNP code and QAD‐CGGP2R.
<Evaluation of external exposure from ground surface, and route>𝐷(𝑖) = 𝐶(𝑖) ∙ 𝑆 ∙ 𝑡 ∙ 𝐷𝐹

(2)

(𝑖)

Here,
Dext(i): External exposure dose of radionuclide i (µSv/y)Cground(i): Concentration of radionuclide i on ground surface (Bq/m2)
t: Exposure time (h)DFext(i): Dose conversion factor for external exposure of radionuclide i (µSv/h)/(Bq/m2)
λ: Decay constant of radionuclide i (1/h)
S0: Shielding factor for external exposure (-)
Meanwhile, we obtained the concentration of radionuclide on ground surface according to thefollowing method for each contamination route:･Concentration of radionuclide on ground surface through the atmospheric release route
No table of figures entries found.
As for contamination of ground surface through the atmospheric release route, the following
formula (3) was used.
𝐶(𝑖) = 𝑄 (𝑖) ∙ (𝜒⁄𝑄 ) ∙ 𝑉 ∙

𝑓
1−𝑒
𝜆

(3)

QA(i): Released radionuclide (Bq/s)
χ/Q: Relative concentration in the atmosphere (s/m3)
Vg: Emitted particle deposition rate (m/s)
f1: Percentage of remaining deposited radionuclide (-)

7

T0: Nuclide release period (h)8It needs to be noted that, while we do not describe the details of the derivation method of
relative concentration in the atmosphere (χ/Q) here, we used the plume formula based on the“Meteorological Guidelines for Safety Analysis of Power Reactor Facilities” to derive therelative concentration in the atmosphere (χ/Q). In this study report, within the area from thetemporary storage to the temporary storage site boundary, i.e. from the surface of the removed
soil, etc. to points 8 meters or more away, we selected an evaluation point with the maximum
landing concentration point due to dry deposition, and derived the relative concentration in theatmosphere of the relevant point (χ/Q).
･Concentration of radionuclide in the soil contaminated by use for irrigation in the case of
release route to water system (Outflow of removed soil into rivers)
No table of figures entries found.
The concentration of radionuclide in the soil in association with the irrigation through therelease route to water system was obtained by the formula (4). Here, instant equilibrium wasassumed for the transition of radionuclide from irrigation water to the soil.
Cground(i) =Csoil(i)･ρVCC(i) = C(i) = C(i)･(K (i) +
(i)･(
1+K

(4)
ε
)
ρ(1 − ε)
C
(i)･C

(5)

)

(6)

Css=ρ･Vwaste/(Vriver･t)(7)

Csoil(i): Concentration of radionuclide in the farm soil contaminated by irrigation (Bq/kg)
ρ: Soil density (kg/m3)
Vwaste: Volume of the removed soil, etc. outflowed into rivers (m3)
Vriver: River flow volume (m3/d)
t: The period required for all the removed soil, etc. to be washed downstream (d)Css: Suspended substances derived from removed soil, etc. in rivers (kg/m3)
Cwaste(i): Concentration of radionuclide i in removed soil, etc. (Bq/kg)
Cwater(i): Concentration of radionuclide i contained in river water (Bq/m3)
Kdss(i): Suspended substances in river water ‐Partition coefficient for radionuclide i containedin river water (m3/kg)
Kd(i): Farm soil‐Partition coefficient for radionuclide i contained in irrigation water (m3/ kg)
PV: Effective density of soil (kg/m2)

9

<Evaluation of external and inhalation exposure from dust, and route>Dsub(i) = Cair(i)･DFsub(i)･t(8)Dinh(i)=Cair(i)･Rinh･DFinh(i)･t

(9)

Here,
Dsub(i): External exposure dose of radionuclide i in the atmosphere (µSv/y)DFsub(i): Conversion factor for external exposure due to immersion from radionuclide i in theatmosphere (µSv/h)/(Bq/m3)
Dinh (i): External exposure dose of radionuclide i (µSv/y)Rinh: Absorption rate (m3/h)
DFinh(i): Dose conversion factor for inhalation exposure of radionuclide i (µSv/h)/(Bq/m3)
Cair(i): Concentration of radionuclide i in the atmosphere (Bq/m3)
t: Exposure time (h)Since it is an evaluation at the time of a short-term accident and when the half-life of theevaluation target nuclide is long, physical attenuation is not considered.
In addition, the concentration of radionuclide in the atmosphere was calculated as follows
according to the route of contamination.
･Concentration of radionuclide in the air through atmospheric release route
No table of figures entries found.
Cair(i) = QA･(χ/Q)

(10)

Here,
QA(i): Released radionuclide (Bq/s)
χ/Q: Relative concentration in the atmosphere (s/m3)･Concentration of radionuclide in the air with soaring soil contaminated by use for
irrigation in the case of release route to water system (Outflow of removed soil intorivers)
No table of figures entries found.
Cair(i) = Csoil(i)･dR

(11)

Here,
dR: Atmospheric dust concentration (kg/m3)
Here, the value of Csoil(i) was obtained through steps (5) to (7).

10

<Evaluation of ingestion exposure, and route>
Ding = Cfood(i)･Hf･DFing(i)･Gf

(12)

Here,
Ding: Ingestion exposure dose of radionuclide i (µSv/y)Cfood(i): Concentration of radionuclide i in food (Bq/kg)
Hf: Food intake volume (kg/y)
DFing: Dose conversion factor for ingestion exposure of radionuclide i (µSv/h)/(Bq/kg)
Gf: Market dilution factor of food (-)
In the exposure routes under consideration here, the foods that are assumed to becontaminated are agricultural products and drinking water. As for foods, those that are expected
to be ingested from non-distributed products in the market are targeted. For example, river fish
are excluded in view of the current situation where dietary restrictions in Fukushima prefecture
are still imposed in many areas. Livestock farming is also excluded because it is not generally
assumed that animal husbandry will be carried out for self-consumption. In addition, thespecific evaluation targets for agricultural products are roughly divided into three types: leafy
vegetables, rice, and other (fruits and non-leafy vegetables). The concentration of radionuclide
in food for each contaminated route was calculated as follows.
･Concentration of radionuclide in agricultural products due to atmospheric release route
No table of figures entries found.
𝐶𝑉 ∙ (1 − 𝑒(𝑖) = 𝑄 (𝑖) ∙ (𝜒⁄𝑄 )(
𝜆 𝜌

)

+

𝑉 ∙ TF(i)･ 1 − 𝑒
𝜆(𝑖)𝑃

)𝑓 𝑓

(13)

Crice(i) = Csoil(i)･TFsoil-rice(i)(14)

Cother(i) = Csoil(i)･TFsoil-other(i)(15)

Csoil(i) = Cground(i)/PV

(16)

Here,
C leaf(i): Concentration of radionuclide in leafy vegetables (Bq/kg)
YGround-veg: Cultivation density of leafy vegetables (kg/m2)
TFsoil-leaf(i): Transition coefficient between leafy vegetables and the soil (Bq/kg-wet / Bq/kg-dry)
Crice(i): Concentration of radionuclide in rice (Bq/kg)
TFsoil-rice(i): Transition coefficient between rice and the soil (Bq/kg-wet / Bq/kg-dry)
Cother(i): Concentration of radionuclide in other agricultural products (Bq/kg)
TFsoil-other(i): Transition coefficient between other agricultural products and the soil (Bq/kg-wet /Bq/kg-dry)
Pv: Effective surface density of soil (kg/m2)
Here, the value of Cground(i) was obtained from formula (3).

11

･Concentration of radionuclide in potable water through release route to water system(transition of radionuclide from removed soil to groundwater)
VI = RI×AP×NP×DRP(17)

RFi =1+(ρW × Kdi÷WRW)(18)

LRi = RI÷(WRW×HP×RFi)(19)

VAi=HP×AP×NP×DRP×ρW×CWi×LRi(20)

CGi = VAi÷(VI+VG)× 0.001 m3/L
Here,
VI: Outflow volume of leachate per year (Unit: m3/year)
RI: Outflow rate per unit area of leachate (m/year)
AP: Bottom area of flexible container (m2)
NP: Number of flexible containers (-)
DRP: Damage rate of flexible containers (-)
RFi: Delay factor of radionuclide i (-)ρW: Density of stored substance (kg/m3)
Kdi: Partition coefficient of radionuclide i (Bq/kg-dry per Bq/L)
WRW: Moisture content (-)
LRi: Outflow rate per year of radionuclide i (-)
HP: Height of the flexible container (m)VAi: Outflow volume of radionuclide i per year (Bq/year)
CWi: Concentration of radionuclide i in stored substance (Bq/kg)(21)

CGi: Concentration of radionuclide i in groundwater (Bq/L)･Concentration of radionuclide in agricultural products due to release route to water
system (Outflow of removed soil into rivers)
C leaf(i) = Csoil(i)･TFsoil-leaf(i)(22)

Here, the value of Csoil(i) was obtained from formulas (5) to (7). In addition, the concentrationof radionuclide in rice and other agricultural products other than leafy vegetables was obtained
from formulas (14) to (17).
2.8 Evaluation parameters
In this section, the parameters used for evaluation in each scenario are shown.
<Living in the neighborhood (Route 1)>
Evaluation of external exposure possibly occurring while living in the neighborhood of thestorage place for removed soil, etc. stored in temporary storages as specified for Route 1 iscarried out with formula (1). Table 5 shows the parameters used in formula (1). Here, the doseconversion factor for external exposure by direct radiation and skyshine radiation we used wasbased on the result derived and provided by the Japan Atomic Energy Agency using the MCNP
code.

12

Table 5 Parameters used for evaluation for Route 1
Name of parameter

Concentration ofradionuclide

134Cs
137Cs

Symbols
used in theformula

Unit

𝐶

Bq/kg

(𝑖)

Specified
value
2500
8500

Shielding factor for external
exposure

𝑆

-

0.6

Exposure time

𝑡

h/y

8760

134Cs

Dose conversion
factor for external
exposure(Temporarystorage)

2.15E-06

(𝑖)

𝐷𝐹
137Cs

µSv⁄h
per
Bq/kg

134Cs

Dose conversion
factor for external
exposure (Stored at
the site)

Decay constant ofradionuclide

5.02E-07

(𝑖)

𝐷𝐹
137Cs

134Cs
137Cs

8.17E-07

𝜆

µSv⁄h
per
Bq/kg

1.91E-07

0.336
0.023

1/y

Grounds for specifying the value

Table 3
Assuming that people will stay indoors
for 16 hours in a day, we adopted 1.0 as
the shielding factor for indoor stay and0.4 based on IAEA TECDOC-1162 as theshielding factor for outdoor stay.
A period of one year was assumed.
The analysis was performed using thefollowing system using the MCNP code.
The shielding conditions were set
according to the guideline.
Shape of the radiation source: 20 m×20
m×2 m
Density of the radiation source:1.5 g/cm3
Shielding conditions: Side shield of 30
cm
Evaluation point:8 m from the surface of
20 m×2 m
Evaluation height:1 m
The analysis was performed using thefollowing system using the MCNP code.
The shielding conditions were set
according to the guideline.
Shape of the radiation source: 2 m×2
m×1 m
Density of the radiation source:1.5 g/cm3
Shielding conditions: None
Evaluation point: General public living 4
m away from the surface of 2 m×1 m
Evaluation height:1 m
The value was set on the basis of thehalf-life defined in ICRP Publ.107<Regular monitoring (Route 2)>
The external exposure during monitoring work in the storage place for removed soil, etc.
stored in temporary storages as specified for Route 2 was evaluated by formula (1). The
parameters used in formula (1) are shown in Table 6.

13

Table 6 Parameters used for evaluation for Route 2Name of parameter

Symbols
used intheformula

Unit

𝐶 (𝑖)

Bq/kg

Shielding factor for external exposure

𝑆

-

1.0

Exposure time

𝑡

h/y

365

Concentration ofradionuclide

134Cs
137Cs

134Cs

Skyshine + Direct
radiation(temporary storage)

(𝑖)

𝐷𝐹

8500

µSv⁄h
per
Bq/kg
2.57E-06

134Cs
137Cs

2500

6.75E-06

137Cs

Decay constant ofradionuclide

Specified
value

0.336
1/y

𝜆

0.023

Grounds for specifying thevalue

Table 3
It was conservatively set to 1.
It was assumed that themonitoring in the temporarystorage would be performed for
1 hour, daily.
The analysis was performed
using the following systemusing the MCNP code.
Shielding conditions were set
according to the guideline.
Shape of the radiation source:
20 m×20 m×2 m
Density of the radiation
source:1.5 g/cm3
Shielding conditions: Side
shield of 30 cm
Evaluation point: 1 m from thesurface 20 m×2 m
Evaluation height:1 m
The value was set on the basisof the half-life defined in ICRP
Publ.107<Release of removed soil, etc. due to fire and loss of shielding function (Routes 3 to 7)>
Fig. 4 shows the assumed situation of the accident when the radionuclide was released into
the atmosphere as defined for Routes 3 to 7.

Fig. 4 An example of the situation of the accident assumed, when the removed soil, etc. wasreleased into the atmosphere14

(a) External exposure and inhalation exposure due to dust released into the atmosphere(Route 3 and Route 4)
The exposure routes shown for Route 3 and Route 4 were evaluated with formulas (8) to (10).
The parameters used in formulas (8) to (10) are shown in Table 7.
Table 7 Parameters used for evaluation for Routes 3 and 4
Name of parameter

Symbols
used in theformula

Unit

𝐶 (𝑖)

Bq/kg

Relative concentration in theatmosphere

χ⁄Q

s/m3

2.74E-02

Calculation of atmospheric diffusion was performed
based on the results of meteorological statistics inFukushima prefecture, and the maximum
concentration on the safest side was adopted.

Exposure time (Fromoutbreak to extinction of
fire)

𝑡

h/accident

12

The time required to complete the fire extinguishing
was assumed to be 12 hours.

Concentration ofradionuclide

134Cs
137Cs

Specified
value
2500
8500

Grounds for specifying the value

Table 3Weight of waste etc. burnedin a fire𝑊kg

1.28E+5

The affected ratio was conservatively set to 0.1,
which is the upper limit of the width, with reference
to Release Fractions in Table 4 of
IAEA-TECDOC-401, Trench Fire Scenario, and theweight was derived based on the parameters in Table
3.

Transition rate ofradionuclide to exhaust in a
fire

𝑅

-

1

It was assumed that the entire amount of cesium wastransferred to the exhaust.

µSv⁄h
per
Bq/m3

2.51E-04

Dose conversion
factor for external
exposure (Plume)

Dose conversion
factor for inhalation
exposure

134Cs

𝐷𝐹

(𝑖)

137Cs

134Cs

9.09E-05
6.6E-03

𝐷𝐹(𝑖)µSv⁄Bq137Cs

4.6E-03

The value of the conversion factor for plume set in
the RI clearance evaluation was used.

The exposure conversion factor for inhalation by
general public (adults) shown in ICRP Publ.72 wasused here.

Meanwhile, the relative concentration in the atmosphere was calculated by setting the main
parameters as follows based on the results of meteorological statistics in Fukushima prefecture.

Wind speed (m/s): 2 (m/s)

Atmospheric stability DRelease width0m

Release height2m

Evaluation height

1m

Here, we adopted the value of the point showing the concentration of the maximum landing
concentration in relation to the distance for the relative concentration in the atmosphere15

(b) External exposure due to removed soil, etc. deposited on the ground surface (Route 5)
The exposure route shown for Route 5 was evaluated with formulas (2) and (3). The
parameters used in formulas (2) and (3) are shown in Table 8.
Table 8 Parameters used for evaluation for Route 5Name of parameter
Concentration
radionuclide

of

134Cs
137Cs

Symbols usedin the formula

Unit

Cground(i)

Bq/kg

𝑡

h/y

Exposure time

Weight of removed soil burned in a
fire

Kg

𝑊

Transition rate of radionuclide toexhaust in a fire-Specified
value
2500
8500

8760

1

𝑆

-

0.6

Soil residue coefficient ofradionuclide

𝑓

-

1

134Cs

𝐷𝐹

(𝑖)

137Cs

Emitted particle deposition rate
(Dry deposit)

Sv⁄𝑠
per
Bq/m2

𝑉

m/s

𝜆

1/y

Nuclide release period

T0hRelative concentration in theatmosphere

χ⁄Q

s/m3

Decay constant ofradionuclide

134Cs
137Cs

16

Table 3

We assumed no decontaminationconservatively.

The affected ratio was conservatively set
to 0.1, which is the upper limit of thewidth, with reference to Release
1.28E+5 Fractions in Table 4 of
IAEA-TECDOC-401, Trench Fire
Scenario, and the weight was derived
based on the parameters in Table 3.

Shielding factor for external
Generalexposure (At the time of the
publicaccident)

Dose conversion factor for
external exposure(Nuclide-deposited soil)

Grounds for specifying the value

It was assumed that the entire amount of
cesium contained in the affected waste
was transferred to the exhaust.
It was set in consideration of theshielding effect (shielding factor: 0.4)
when staying indoors (16 hours) during
the day.
We conservatively assumed that the entire
quantity persisted.

1.6E-15 It was set based on US EPA Federal
Guidance Report No. 12.
5.7E-16
It was set based on IAEA
0.0015TECDOC-1777.
0.336
0.023

The value was set on the basis of thehalf-life defined in ICRP Publ.107

The time required to completely
extinguish the fire was assumed to be 12hours.
Calculation of atmospheric diffusion was
performed based on the results ofmeteorological statistics in Fukushima
2.74E-02
prefecture, and the maximum
concentration on the safest side wasadopted.
12

(c) Removed soil, etc. remaining on the premises (Route 6)
The exposure route shown for Route 6 was evaluated with formula (1). Here, assuming asituation where the shielding function was lost in association with the effect of the accident, weevaluated the external exposure received by neighboring residents. Parameters used in formula(1) are shown in Table 9.
Table 9 Parameters used for evaluation for Route 6
Name of parameter

Concentration ofradionuclide

134Cs
137Cs

Symbols
used intheformula

Unit

𝐶 (𝑖)

Bq/kg

Specified
value

2500
8500

Shielding factor for external exposure

𝑆

-

0.6

Exposure time

𝑡

h/y

730

134Cs

Skyshine + Direct
radiation(temporary storage)

1.78E-05

(𝑖)

𝐷𝐹

µSv⁄h
per
Bq/kg

137Cs

Decay constant ofradioactive cesium

6.79E-06

134Cs
137Cs

0.336
𝜆

1/y

0.023

Grounds for specifying the value

Table 3
Assuming that people will stayindoors for 16 hours in a day, we
adopted 1.0 as the shielding factor
for indoor stay and 0.4 based on
IAEA TECDOC-1162 as theshielding factor for outdoor stay.
We assumed it would take amonth to repair the temporarystorage.
The analysis was performed using
the following system using the
MCNP code. The shielding
conditions were set according to
the guideline.
Shape of the radiation source: 20
m×20 m×2 m
Density of the radiation
source:1.5 g/cm3
Shielding conditions: None
Evaluation point:8 m from thesurface of 20 m×2 m
Evaluation height:1 m
The value was set on the basis of
the half-life defined in ICRP
Publ.107(d) Internal exposure due to ingestion of contaminated agricultural products (Route 7)
The exposure route shown for Route 7 was evaluated with formulas (12) to (16). Here, in
association with the effect of the accident, we assumed there was internal exposure throughingestion of agricultural products contaminated through contamination of the surrounding
agricultural soil. Parameters used in formulas (12) to (16) are shown in Table 10. Meanwhile,
the value we used for Cground(i) was calculated with the parameters in Table 8.

17

Table 10 Parameters used for evaluation for Route 7
Name of parameter

Symbols
used in theformulaUnit

Specifiedvalue

Grounds for specifying the
value

𝑃v

kg/m2

280

We set the values used in U.S.
NRC Regulatory Guide 1.109.

Effective surface density of soil

Annual food intake
volumeMarket dilution
factor

Dose conversion
factorLeafy
vegetables
and the like
Rice and the
like
Other
agricultural
products

kg/y

𝐻

Leafy
vegetables
and the like

0.1
Gf

Other
agricultural
products
134Cs
137Cs

Leafy
vegetables
and the like
Other
agricultural
products

0.1

𝐷𝐹

(𝑖)

Sv / Bq

TFsoil-leas(i)TFsoil-leas(i),1.30E-8
1.90E-80.0016

Based on the safety evaluation
method 2013 for trenchdisposal in shallow ground, it
was conservatively set to 1.
The value of self-cultivation
was set based on the safety
evaluation method 2013 fortrench disposal in shallow
ground.
The value of IAEA-SS-115
was used.
We adopted the measured value(white rice) in Japan for 137Cs
of fallout shown in IAEATECDOC 1616 (P248).

(Bq/kg-wet/
Bq/kg-dry)
0.04

The value of IAEA SRS 19
was adopted.

TFsoil-others(i)

ρV

Kg/m2

𝜆

1/y

134Cs

2
0.336

137Cs

Effective attenuation 134Csconstant of
137Cs
radionuclide on
leafy vegetables
Emitted particle deposition rate
(Dry deposit)
Annual ratio of cultivation period
Residual ratio after washing before
ingestion

It was set based on the safety
evaluation method 2013 fortrench disposal in shallow
ground.

160.5

1

Cultivation density (Leafy
vegetables)
Decay constant ofradionuclide

58.5

Rice and thelikeRice and thelikeTransition
coefficient between
soil and crop

20.3

0.023
18.436

We set the value on the basis of
the dose evaluation of thegeneral public.
The value was set on the basisof the half-life defined in ICRP
Publ.107
We set the value on the basis of
the dose evaluation of thegeneral public.

𝜆

1/y

𝑉

m/s

0.0015𝑓-1

It was set based on IAEATECDOC-1777.
It was conservatively set to 1.

𝑓

-

1

It was conservatively set to 1.

18

18.123

<Outflow of removed soil, etc. due to heavy rain (Route 8)>
The exposure route shown for Route 8 was evaluated with formula (1). Here, in association
with the accident of Route 8, as shown in Fig. 5, we assumed the external exposure occurred
when people walked through gutters after the removed soil had flowed out into the road andaccumulated there. The parameters used in formula (1) are shown in Table 10.

* Scale is not accurate

Soil

Concrete

Fig. 5 Outflow accident of removed soil, etc. due to heavy rain

19

Table 11 Parameters used for evaluation for Route 8Name of parameter

Symbols
used intheformula

Unit

𝐶 (𝑖)

Bq/kg

Shielding factor for external
exposure

𝑆

-

1.0

Exposure time

𝑡

h/y

365

Concentration ofradionuclide

134Cs
137Cs

134Cs

External exposure
Dose conversion
factorDecay constant ofradioactive cesium

2500
8500

2.6E-05

(𝑖)

𝐷𝐹
137Cs

134Cs
137Cs

Specified
value

𝜆

µSv⁄h
per
Bq/kg

1/y

9.5E-06

0.336
0.023

Grounds for specifying the value

Table 3
It was conservatively set to 1.
We assumed a lifestyle of walking onroads for one hour every day.
It was assumed that heavy rain causes
10% of removed soil, etc. to flow out
and 10% of it to be deposited on in
gutters. Using the QAD-CGGP2R code,
we performed the following analysis forthe system below:
Shape of the radiation source: 0.15
m×0.3 m×180 m
Density of the radiation source:1.5g/cm3
Shielding conditions: None
Evaluation point: As shown in Fig. 5
The value was set on the basis of thehalf-life defined in ICRP Publ.107<Loss of leakage prevention function to the outside of the facility through the water system(Route 9)>
As Route 9, we set up a scenario in which the function to prevent leakage of radioactive
cesium from removed soil, etc. through water was lost, and assumed a situation where the upperand lower impermeable sheets were torn and the storage container was damaged had occurred.
As for the exposure route, we assumed that the radioactive cesium leaked to the outside of thetemporary storage would be transferred to the groundwater and then used as drinking water. Fig.
6 shows the assumed accident situation. Based on the above, the additional exposure dose that
could occur when radioactive cesium leaked via groundwater was evaluated using equations(12) and (18) to (22). The parameters used for the evaluation are shown in Table 12.

20

Radioactive cesium

Removed soil layer

Aquifer

Use of drinking water

Fig. 6 Leakage of radioactive cesium into groundwater due to rainwater intrusion
Table 12 Parameters used for evaluation for Route 9
Symbols
used intheformulaSpecified
value

Unit

Grounds for specifying the value

HP

m

1.1

Bottom area of container

AP

m2

1.1

Guideline for
Decontamination-related Measures

Damage rate of container

DRP

-

0.2

20% damage was assumed.

Width perpendicular to the flowof groundwater in temporarystorageWSm

20

The length of the long side of
temporary storage was specified.

Density of stored substance

ρW

g-dry
/cm3

1.6

Moisture content

WRW

-

0.3

Partition coefficient

Kdi

Bq/kg-dry

270

IAEA TRS-3644)

Leachate runoff rate per unit
area

RI

m/year0.4

Groundwater Handbook5)

Aquifer thickness

TG

m

5

Specified value

Effective porosity of the aquifer

EPG

-

0.3

Same as for stored substances

Groundwater flow velocity

RG

m/d

1

Same as above5)

DCFGiµSv/yearper Bq/L

Name of parameter

Height of container

Conversion factor fromconcentration of radioactive
materials related to drinking
water to exposure dose21

About the simplified safety
evaluation of interim storage
facilities
Set based on the safety evaluation
method 2013 for trench disposal inshallow ground.

134Cs:15.45

IAEA SRS-197)
137Cs:10.57

<Outflow of removed soil, etc. due to flooding of rivers (Route 10)>
As for Route 10, as a scenario for the release of removed soil, etc. due to flooding of the river,
we assumed ingestion of contaminated agricultural products, external exposure fromcontaminated soil, and insulation exposure from airborne soil through the use of river water for
irrigation, which was contaminated through accumulation of a part of removed soil, etc. on theriver bottom and the constant elution of radioactive cesium into the river water had occurred.
This exposure route was evaluated with formula (12). Furthermore, the concentration ofradionuclide obtained by formula (12) was calculated with formulas (5) to (7), (14), (15) and(23). Fig. 7 shows the accident situation we assumed. Additionally, Table 13 shows theparameters used in formulas (5) to (7). Meanwhile, the parameters used in formulas (14), (15)and (23) were the same as for Table 10.
Flow direction
Cs in dissolved state
Removed soil
Cs in suspended stateContaminated area

Non-contaminated area

Fig. 7 Sedimentation of soil removed after river flooding

22

Table 13 Parameters used to evaluate soil contamination associated with irrigation through therelease route to water system associated with river flooding
Name of parameter

Symbols used in
the formula

Unit

Specifiedvalue

Removed soil remaining time

t

d

365

Soil density

ρ

kg/m3

1600

Vwastem3

800

Cwaste(i)

Bq/kg

2500
8500

𝜆

1/y

Partition coefficient between
soil and river water

Kdss

River flow volume

Soil outflow volume
Concentration ofradionuclide
Decay constant ofradionuclide

Soil porosity

134Cs
137Cs
134Cs
137Cs

0.336

Remarks
We assumed that all removed soil
would be washed out of the sediment
in one year.
Table 3
We assumed that 100% of the totalmaterial would flow out and settle on
the riverbed.
Table 3

0.023

The value was set on the basis of thehalf-life defined in ICRP Publ.107

m3/kg

1000

It was specified from ORNL-5786.

Vriverm3/d

5.7×106

ε

-

0.38

23

The average value of the Abukuma
River flow over the past 5 years at theMotomiya Observatory was used.
It was set based on the safety
evaluation method 2013 for trenchdisposal in shallow ground.

Table 14 Parameters used for evaluation for Route 10
Name of parameter

Concentration ofradionuclide in soil

Annual food intake
volumeSymbols
used in theformulaUnit

Csoil(i)

Bq/kg

134Cs
137Cs

Leafy
vegetables
and the like
Rice and the
like
Other
agricultural
products

kg/y

𝐻

137Cs

Leafy
vegetables
and the likeTransition coefficient
between soil and crop

Decay constant ofradioactive cesium

Rice and the
like
Other
agricultural
products
134Cs
137Cs

The value was obtained byusing the parameters shownin Table 10 with formula (5).

2.3

58.5

0.1

Rice and the
like

134Cs

6.6E-1The values in the 2004
National Health and Nutrition
Survey Report were used.

160.5

Gf

-

Other
agricultural
products
Dose conversion
factorGrounds for specifying the
value

20.3

Leafy
vegetables
and the like

Market dilution factor

Specified
value

1

0.1

𝐷𝐹

(𝑖)

Sv / Bq

TFsoil-leas(i),TFsoil-leas(i)(Bq/kg-wet/
Bq/kg-dry)TFsoil-others(i)1.30E-8
1.90E-8
0.04

The value of IAEA SRS 19
was adopted.

0.0016

Measured values in Japan for
fallout radioactive cesium
shown in IAEA TECDOC
1616 (P248) (white rice) wereadopted.

0.04

The value of IAEA SRS 19
was adopted.

0.336
1/y

𝜆

24

The value of the farmer wasset based on the safety
evaluation method 2013 fortrench disposal in shallow
ground.
Based on the safety evaluation
method 2013 for trenchdisposal in shallow ground, it
was conservatively set to 1.
The value of the farmer wasset based on the safety
evaluation method 2013 fortrench disposal in shallow
ground.
The value of IAEA-SS-115
was used.

0.023

The value was set on the basisof the half-life defined in ICRP
Publ.107

<Outflow of removed soil, etc. due to flooding of rivers (Route 11)>
The exposure route shown for Route 11 was evaluated with formulas (2) and (4) to (7). Table
15 shows the parameters used in formulas (2) and (4). The parameters used in formulas (5) to(7) were the same as for Table 13.
Table 15 Parameters used for evaluation for Route 11
Name of parameter

Concentration of radionuclide on
ground surface

134Cs
137Cs

Symbols
used in theformulaUnit

Cground(i)Bq/m2Specifiedvalue
1.9E+2
6.3E+2Grounds for specifying the value

Derived from formula (4)Exposure time

𝑡

h/y

500

The value of the farmer was set
based on the safety evaluation
method 2013 for trench disposal inshallow ground.

Shielding factor for external exposure

𝑆

-

1

It was conservatively set to 1.

Dose conversion factor for
external exposure(Nuclide-deposited soil)
Decay constant of radioactive
cesiumEffective surface density of soil

134Cs

𝐷𝐹

(𝑖)

137Cs

Sv⁄𝑠
per
Bq/m2

134Cs

1.6E-15
5.7E-16
0.336

𝜆

1/y

𝑃v

kg/m2

137Cs

0.023
280

It was set based on US EPA Federal
Guidance Report No. 12.
The value was set on the basis of
the half-life defined in ICRP
Publ.107
We set the values used in U.S. NRC
Regulatory Guide 1.109.

<Outflow of removed soil, etc. due to flooding of rivers (Route 12)>The exposure route shown for Route 12 was evaluated with formulas (5) to (7), (9) and (11).
Table 16 shows the parameters used in formulas (9) and (11). The parameters used in formulas(5) to (7) were the same as for Table 13.

25

Table 16 Parameters used for evaluation for Route 12
Name of parameter

Symbols
used in theformula

Unit

dR

kg/m3

Cair(i)

Bq/m3

Atmospheric dust concentration
134Cs

Concentration of radionuclide in
the air

137Cs

Exposure time

h/y

𝑡

134Cs

Dose conversion factor for
inhalation exposure

1.1E-6500

4.6E-03

134Cs

Absorption rate

3.3E-7µSv⁄Bq

137Cs

137Cs

5.0E-7

6.6E-03(𝑖)𝐷𝐹

Decay constant of radioactive
cesiumSpecified
value

0.336
𝜆

1/y

Rinh

m3/h

0.023
1.2

Grounds for specifying the value

The value of the farmer was set
based on the safety evaluation
method 2013 for trench disposal inshallow ground.
Derived from formula (11)The value of the farmer was set
based on the safety evaluation
method 2013 for trench disposal inshallow ground.
The exposure conversion factor for
inhalation by the general public(adults) shown in ICRP Publ.72 wasused.
The value was set on the basis of
the half-life defined in ICRP
Publ.107
Calculated on the basis of ICRP
Publ.89.

3. Evaluation results
Table 17 shows the results of the dose evaluation for the scenarios shown in Table 4.
Table 17 Dose evaluation in each scenario
Exposure dose (mSv)

Target6.0×10-2

General public

1.4×10-2

General public

7.9×10-3

Monitoring worker(s)5.2×10-2

General public

At the time

1.5×10-3

General public

5of the3.0×10-1

General public

6

accident

4.6×10-2

General public

7

8.4×10-1

General public

8

4.9×10-2

General public

9

3.0×10-5

General public10

1.2×10-4

General public

11

3.2×10-3

General public

12

1.3×10-8

General public

Route
1-(a)
1-(b)
2

Situation
Normal
times

3
4

26

As a result of evaluating the assumed exposure scenarios for the model temporary storage asof April 2015, we predicted that the additional exposure dose at normal times would be less than
the reference value of 1 mSv per year. Additionally, the additional exposure dose at the time of
the accident was expected to be less than 5 mSv per reference value.
In our evaluation of radioactive nuclide at the time of the accident, the ingestion exposure
route of contaminated agricultural products at the time of the fire event was assumed to have thegreatest impact. Therefore, in the event of a large-scale fire accident in the temporary storage
that exists in the residential area, it is necessary to pay attention to this route. That is, it isparticularly necessary to measure the radioactivity of leafy vegetables, which may becontaminated by initial deposition, if air dose rates are to be measured to determine if
agricultural products are at a level of concern, or if there is concern of contamination based on
the results. By taking these measures appropriately, it is possible to prevent unnecessary
exposure even if a large-scale accident occurs.
In addition, it was shown that the external exposure route of general public, which was usedas the exposure route of normal times, can keep the additional exposure dose as low as 0.06
mSv/year by designing temporary storage according to the guideline. However, since this
scenario will inevitably occur, it is considered to be the most important scenario among thoseassumed at this time.
Meanwhile, in this study report, only the scale of the hazard in each exposure route isevaluated, and the probability of occurrence is not mentioned, but risks are evaluated based on
the hazard and the probability of occurrence, and even if the accident has a large hazard, the risk
will be limited if the probability of occurrence is small. For example, it is reported that theprobability of a fire accident, which is estimated to have the greatest impact this time, is about
0.1% in 2004-2007 in the case of the final disposal site. In addition, even in the temporarystorage of removed soil, etc. in the prefecture, only one case has been clarified so far. Therefore,
it can be determined that the risk of the outflow of removed soil, etc. into the environment, suchas through the atmosphere and water system, caused by the occurrence of fire or heavy rain in
temporary storage will be sufficiently small.
Meanwhile, since the analysis was made for the period during storage this time, we evaluateda system in which shielding and separation were properly performed. However, whentransporting removed soil, etc. in the future, the removal of shielded soil may temporarily
increase the air dose rate in the surrounding area. Therefore, it is necessary to thoroughly
consider the impact of removing the shield soil at the transportation planning stage.

27

Acknowledgments
We would like to thank Mr. Yamada of the Japan Atomic Energy Agency for his cooperationin deriving the dose conversion factor for external exposure.
References
1) "Outlook for foreseeable 5 years in relation to interim storage facility”, Ministry of theEnvironment, 2016
http://josen.env.go.jp/chukanchozou/action/acceptance_request/pdf/correspondence_160327_01.pdf (Latest browse on March 5, 2020)2) Situation of decontamination temporary storage, etc. (in Fukushima prefecture)
http://josen.env.go.jp/plaza/info/data/pdf/data_1907_02.pdf(Environmental

Regeneration

Information Plaza (Latest browse on March 5, 2020)

3) The storage status of removed soil, etc. was updated. (as of the end of March 2019)https://www.pref.fukushima.lg.jp/site/portal/hokanjyoukyou.html (Fukushima Revitalization
Station, latest browse on March 5, 2020)

4) Guideline for Decontamination-related Measures ‐ Guideline for Storage of Removed Soil,
Ministry of the Environment, 2013

5) Decontamination Information Website‐About temporary storage
http://josen.env.go.jp/soil/temporary_place.html (Latest browse onMarch 5, 2020)

6) Ordinance of the Ministry of Health, Labour and Welfare 2011, No.152, Ministry of Health,Labour and Welfare, 2011https://www.mhlw.go.jp/file/06-Seisakujouhou-11300000-Roudoukijunkyokuanzeneiseibu/000
0030858.pdf (Latest browse on March 5, 2020)

7) “Concept of ensuring safety in the current situation regarding the treatment and disposal ofwaste affected by TEPCO’s Fukushima Daiichi Nuclear Power Station accident,” Nuclear
Safety Commission, 2011 https://www.env.go.jp/jishin/rmp/conf/18/ref01_1r.pdf (Latest
browse on March 5, 2020)"Simplified safety evaluation of interim storage facilities", Ministry of the Environment, 2013
http://josen.env.go.jp/soil/pdf/safety_measure_04_02.pdf (Latest browse on March 5, 2020)28