２ Principal Research Results
Project Research ― Establishment of Optimal Risk Management

Rational Radiation Safety Technology

Background andObjective

Main results

Methodology on how to ensure radiation safety (i.e.,
no health effects due to radiation) for the public has
been pointed out as a significant issue of interest
since the Fukushima Daiichi nuclear accident
occurred. It is required now more than ever before
to explain the validity of criterion regarding
radiation safety, maintaining transparency forsociety.

1V e r i f i c a t i on o f t h e Sc r e e n i n g L e v e l f o r t h e D e c o n t a m i n a t i o n I m p l e m e n t e dafter the Fukushima Daiichi Nuclear Accident

The Fukushima Daiichi nuclear accident caused
the release into the environment of a substantial
amount of radioactive materials, and a screening
level for decontamination was implemented forthe surface of the human body and for handling
contaminated objects. In order to verify thescreening levels for decontamination, the dosesthat arise from external irradiation, ingestion,
inhalation, and skin contamination were assessed(Table 1).
The results show that the annual effective dosesthat arise from handling objects contaminatedwith the screening level for decontamination(i. e., 100,000 cou nt s per mi nut e ) ar e lowe r
than 1 mSv y -1, which can be considered as theintervention exemption level in accordance with2

In this project, a rational concept and assessment
method for ensuring radiation safety are proposedto the relevant experts including newly developed
criteria after the Fukushima Daiichi nuclear
accident for the purpose of the improvement ofaccountability to society regarding radiation safety.

the International Commission on RadiologicalProtection (ICRP) recommendations. Furthermore,
the screening level for decontamination is also
found to protect the skin from the incidence ofa deterministic effect because the absorbed doseof the skin that arises from direct deposition on
the surface of the human body is calculated to belower than the threshold of the deterministic effectassuming practical exposure duration. Therefore,
it can be considered that the emergency response
regarding the screening level for decontamination
implemented after the Fukushima Daiichi nuclear
accident was the appropriate countermeasurefrom the viewpoint of risk management under such
emergency conditions.

Clarification of the Low Diffusivity in the Radioactive Waste Disposal System

As for the radiation safety of a radioactive
waste disposal system, it is necessary to clarify
the accuracy of parameters for the performanceassessment of the repository in the new guidelinesof the licensing safety review. In a sub-surface
disposal system, the closely packed concrete
layer is expected to retard the migration ofradionuclides from the repository to the biosphere.
Therefore, the diffusion of radionuclides in FAC*1,
which is a candidate material for the constructionof a sub-surface repository, is a very importantparameter. In this study, focusing on the pore
structure of FAC, the diffusion of trace ions (whichare hardly sorbed on cementitious materials) in
hardened cement pastes was examined throughdiffusion experiments. As a result, the effective
diffusion coefficients, De, of the trace ions for FAC
were 1–3 orders of magnitude smaller than thosefor OPC* 2 (Fig. 1) because of the continuouschange in the pore structure (Fig. 2) (L11008).

*1 Low-heat Portland cement containing fly ash*2 Ordinary Portland cement

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Other reports: H. Ogino et al., Radiat. Prot. Dosim., Vol. 151, No. 1, pp. 36-42, 2012

Ratio of radioisotopes（I-131：Cs-134：Cs-137）

Manually handling
objects [0.1 m 2]
Contaminated Closely handling
objects
objects [1 m 2 ]Remotely handling
objects [10 m 2 ]Inhalation [mSv y -1]
Skin [mSv h-1]

Case 3

Case 4

1:1:1
150
150
150
0.24
56
0.35
0.00190.17
0.00028
1.1

I-131 poor ratio ‑ >0.1:1:1
0.01:1:1
23
2.4
230
240
230
240
0.11
0.085
83
87
0.51
0.53
0.00240.00250.24
0.25
0.000360.00038
1.1
1.1

Case 5

Table 1: Calculation of doses that arise from contaminated objects and direct deposition to skin regarding
the screening level for decontamination (i.e., 100,000 counts per minute)Fig. 1: Diffusion of Cl, Br, and I throughthe hardened cement pastes

2.0
1.0745

305

140

74

43

24

148

The pore size
became smaller<Porosity>
Before: 28%
After: 18%745

Pore size/nm

380

0.02058000

Before experiment
After experiment(b) FAC

120

4000
6000
Time/hour

4.03.074

2000

Pore size/nm

4800.0310.0000

1.0210.0005<Porosity>
Before: 16%
After: 14%

No difference14

Cl: 10-13m2 s-1
Br: 10-14m2 s-1
I: 10-15m2 s-1

2.09

De in FAC(a) OPC

40.00103.0

610-12m2 s-1 order

Before experiment
After experiment

4.0

4Tracer conc. / mol dm-3

De in OPC

FAC-ClFAC-Br
FAC-I
OPC-ClOPC-Br
OPC-I

Pore volume (%)

0.0015

Pore volume (%)

On 19 th March 2011, the Nuclear Safety Commission of Japan technically suggested the screening level fordecontamination to be 100,000 cpm by assuming the use of a typical GM survey meter with a 5-cm bore. In
this study, the surface contamination densities that correspond to the screening level for decontamination werecalculated by assuming five cases for the ratio of radioisotopes, and the doses that arise from external exposure,internal exposure (inhalation and ingestion), and skin contamination were calculated. Even assuming that asimilar deterministic effect can be observed in the case where the time integration of the absorbed dose rate of
the skin from chronic irradiation (e.g., 1.0 mSv h -1) reaches the lowest threshold of acute irradiation (>3 Sv), theimplemented screening level for decontamination can be considered to prevent the skin from the incidence of adeterministic effect such as temporal epilation (3 Sv), erythema (5 Sv), and permanent epilation (7 Sv).

Project Research - Establishment of Optimal Risk Management

Direct deposition to skinCase 2<‑ I-131 rich ratio
100:1:1
10:1:1
410
360
4.1
36
4.1
36
0.50
0.441.7130.0410.11
0.000760.00099
0.028
0.058
0.000110.000151.0
1.0

2 Principal Research Results

Surface contamination density[Bq cm-2]

I-131
Cs-134
Cs-137
Ingestion [mSv y -1]
Skin [mSv y-1]
External [mSv y -1]
Inhalation [mSv y -1]
External [mSv y -1]

Case 1

Fig. 2: Pore size distribution

The effective diffusion coefficient (D e ) of anion (chloride, bromide, and iodide) depends on the pore
structure of the hardened cement pastes. D e in FAC is much smaller than that in OPC (Fig. 1). As shown in
Fig. 2, the pore size distribution and porosity of FAC changes to become a more closely packed structure
because of the pozzolanic reaction in which pozzolanic materials react with portlandite to form insoluble
silicate compounds. Such changes of the micro structure of FAC lead to the low diffusivity of FAC.

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