２ Major Research Results
Priority Subjects ― Establishment of Optimal Risk Management

Quantitative Evaluation of Low-Dose Radiation Risk and its
Reflection on Radiation Protection

Background andObjective

Main results

The environmental contamination by radioactive
materials which has arisen from the accident
at the Fukushima Daiichi nuclear plant hasgiven rise to public concern regarding radiation
exposure, which showed that radiation protection
for the general public in such situations have notbeen adequately developed. An epidemiologicalresearch study of residents in high naturalradiation areas suggested that there is no
increase in radiation risk at a prolonged lowdose-rate, in other words, the existence of a dose

1P r op o s a l of a r a d i a t i o n p r o t e c t i o n f r a m e w o r k w i t h a g r a d e d a p p roa c h i ne x is t in g e x p o s u re s i t u a t i o n s r e s u l t i n g f r o m n u c l e a r a c c i d e n t s

The International Commission on RadiologicalProtection (ICRP) recommends that radiation
protection be optimized by the applicationof reference levels selected according to thesituation by taking into account economic
and social factors in emergency exposure
situations* 1 and existing exposure situations* 2 ,whereas dose limits apply to normal planned
exposure situations. In this study, a concept ofreference level application for optimization ofprotection in the management of radioactive
waste and food safety regulation is proposedin light of the fact that several issues in these
sectors were revealed by the accident at the2rate effect. Properly understanding this effectcould contribute to the establishment of rational
protection criteria and alleviate public concern
regarding radiation exposure.
This project is aiming to construct rationalradiation protection systems which achieve
accountability to society; specifically to propose
for the improvement of protective measures under
the situation after accidents, and to elucidate
biological mechanisms of dose-rate effect byexperimental studies.

Fukushima Daiichi nuclear power plant.
A radiation protection framework for themanagement of radioactive waste generated
in decontamination is proposed, in whichintermediate reference levels for such waste
management are adopted gradually according
to environmental remediation progress andthe reduction in the existing ambient dose, inorder to make the waste management activities
reasonably practical (Fig. 1). Also gradedreference levels are specifically proposed to
improve food safety regulations, according to
the timeframe and exposure situations after anuclear accident (Fig. 2).

Q u a n t it a t i v e a n a l y s i s o f t i s s u e s t e m c e l l t u r n o v e r t o e l u c i d a t e m ec ha ni s ms
f o r d os e r a t e e f f e c t s

Cancer is considered to be initiated by anaccumulation of lesions in tissue stem cells(TSC* 3 ). We are exerting efforts to clarify
mechanisms for dose-rate effects, under theworking hypothesis that the effects would
attribute to the mechanisms for eliminating
damaged TSCs from a living body by cell death
and/or turnover.
We established an experimental system toquantitatively analyze TSC turnover afterirradiation, focusing on intestinal stem cells, ofwhich the behavior has been well studied (Fig. 3).

Using this system, we clarified that the turnoverof stem cells in small intestines was stimulated byhigh dose-rate exposure (Fig. 4). This behavior
suggested that cancer risk would increase after
high dose-rate irradiation which can make lesions
in all cells simultaneous, resulting in a reductionof TSCs and reproliferation of surviving damaged
TSCs to maintain tissue function. We will extend
the application of this method to clarify thedifference in the case of low dose rate exposure,which can induce a small number of damaged
cells among healthy TSCs.

*1 Exposure situations that require urgent action to avoid or reduce risks.
*2 Exposure situations that already exist when a decision on control has to be taken. Also include long-term exposure situations following a nuclear
or radiological emergency.
*3 Cells in which tissue-forming cells originate. They are also considered as origins of cancer because of their proliferative characters.

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Fig. 2：Comparison of graded reference levels versus index
values for food safety regulations
The period of emergency and existing exposure
situations are divided into early, intermediate andlate phase for which the time scale is in weeks,months and years, respectively, and gradedreference level values are assigned for each phase.
This approach allows realistic implementation of
restrictions on distribution and consumption while
the balance between various risks is kept, enabling
stepwise reduction of the dose to members of thepublic. In contrast to index values for which thesame values were assigned independent of phases
after the accident, implementation of the graded
triphasic reference level system permits optimizationof the food safety regulation depending on situationsof the accident and exposure.

Priority Subjects - Establishment of Optimal Risk Management

Under an existing exposure situation with a certain
ambient dose due to contamination resulting fromthe emission of radionuclides by a nuclear accident,
the first reference level for radioactive waste
management as a source-related restriction is selected
below the reference level selected for the existing
annual ambient dose in the environment, taking
into account the practicability of the environmental
remediation including the waste management. Whenthe existing annual ambient dose is reduced to below
the first reference level, the second reference levels
for the existing annual ambient dose and for waste
management are selected to be lower than the firstvalues. This procedure is repeated on the basis of
the principle of optimization until the existing annual
ambient dose in the environment is reduced to thenormal dose level.

2 Major Research Results

Fig.1: Conceptual diagram of application ofintermediate reference levels for waste
management in existing exposure situations

Fig. 4: TSC turnover after high-dose-rate irradiation

Fig. 3: Experimental system to analyze TSC turnoverafter irradiation
TSCs and their progenies (functional cells) werevisualized as labeled crypts (shown in blue in thefigure) from the bottom (crypt) to top (villi) of theintestines of transgenic mice. We found that thepercentage of labeled crypts in cross sections could
be a useful indicator to understand the frequency ofTSC turnover.

The percentage of labeled (blue) crypts in thecolon was evaluated after high-dose rate (1.5 Gy/min) X-irradiation (1 Gy). X-irradiation stimulated
TSC turnover (loss of TSCs). Since the loss of TSCs
has to be reproliferated by surviving (damaged)
TSCs, radiation damage induced by high-dose-rateirradiation may accumulate in the tissue cells. On
the other hand, we hypothesize that normal TSCs
may compensate a loss of TSCs after low dose rateirradiation to prevent the accumulation of radiation
damage.

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