Project Subjects
Rational Radiation Safety Technology
Background and Objective
Radiation safety in this country has been ensured through a deterministic approach to assessing individual
dose with conservative parameters. On the other hand, a new methodology for dose assessment is being developed adopting probabilistic and risk-informed approaches toward optimization of resources (human, facility andperformance of equipment) required for compliance with international safety standards.
In this project, we propose a methodology to reasonably ensure radiation safety standards such as exemption and clearance1 of extremely low-level radioactive waste through enhancement of measurement and safetyassessment techniques.

Main results
1. Evaluation of measurement performance for clearance of concrete materials
The methodology of metal waste measurement for compliance with clearance level has been alreadyestablished in this country. On the other hand, in the case of concrete waste for clearance, there is a need todevelop measurement techniques to appropriately subtract counts of radiation due to natural radioactive nuclides
such as potassium-40 (K-40) from the total counts. From the above reason, detection limit was estimated as afunction of scattering (relative standard deviation) of probability distribution of natural radioactivity, which wasobtained from measurement results of representative samples from a nuclear power plant (Fig. 1). As a result,it was made clear that detection limit satisﬁes a target value (equivalent to one tenth of the clearance level for
Co-60) when the measuring unit is beyond approximately 20 kg, even though there is large scattering (e.g. therelative standard deviation is 0.3). This shows that a reasonable measurement technique for clearance of concretematerial was established and completed [L09003].
2. Proposal of rational exemption level for surface contamination
The current radiation safety standard for surface contamination was established through conservative
parameters using two categories for alpha emitters and the others, based on past knowledge on radiation protection. In this study, the rational nuclide-speciﬁc exemption level for surface contamination was derived considering treatment types of contaminated objects (manually, closely and remotely handled), calculating the individual
dose due to contamination and normalizing the individual dose to 0.01 mSv/y. As a result, it was clariﬁed that thecurrent standard has a remarkably large safety margin and therefore could be eased, although the current standard
for Co-60 (most severe case) is almost the same as the derived exemption level1) (Fig. 2).
Other reports

1) Appl. Radiat. Isot, 67, 1283-1286 (2009)

1 Exemption is not to be considered under any radiological regulation since the radiation risks to individuals caused by theexempted practice or source are sufﬁciently low as to be of no regulatory concern. Clearance is to remove radiologicalregulation in the same way.

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Nuclear Technology

Detection limit activity for Co-60 [Bq]

Required reference level for detection limit activity

200

Averaging unit = 20kg

σ= 0.1σ= 0.2σ= 0.3180160200Bq140120
100
80
0.5

1.0

1.5
4

Natural activity
x10 of K-40[Bq]

2.0[×104]

Fig. 1 Estimated results of the detection limit activity for Co-60 at a normal natural activity
range of K-40
This indicates that the small amount of Co-60 is detectable even though natural radioactivity concentration is slightly high.

Fig. 2 Result of calculation of exemption levels for surface contamination
Exemption levels for surface contamination, which was derived based on the latest scientiﬁc knowledge, were found to behigher than current criterion, except for Co-60. The results suggest the possibility that current radiation protection standards
for surface contamination could be eased.

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