Principal Research Results

Development of Inspection System for Clearance Level (CLALIS) andProposal of Reasonable Methods to Judge Satisfaction of Clearance
BackgroundIn recent years, basic concepts for compliance with clearance level, that is distinction level to separate the solid material to
be treated as radioactive, have been examined in Japanese regulatory authorities. The detail of methods for the verification of clearance level was described in a standard entitled “Monitoring for Compliance with Clearance level: 2005” and released in August 2005,
by the Standard Committee (SC) in the Atomic Energy Society in Japan (AESJ). In drafting this standard, reasonable methods wererequested, which can measure quite a low level radioactivity with high reliability and judge the satisfaction of the clearance level
without setting an excess safety margin in the judgment.

Objectives
To develop an automatic clearance level monitor which can measure radioactivity in metal wastes accurately without humanfactor. To propose a reasonable method to judge satisfaction of the clearance level appropriately considering uncertainties of measurement data for the objective nuclides such as Co-60 and Cs-137 and scattering of relation between the objective and the other nuclides.

Principal Results
1. Development of clearance level automatic laser inspection system (CLALIS)A new clearance monitor, CLALIS has been developed (Fig.1). CLALIS enables low level measurement with high accuracy. The full automatic measurement that could not be achieved by the former style, realizes the removal of human factor from themeasurement results.
2. Verification of performance of CLALISAs a result of the performance tests for approximately 260 patters of mock metal wastes, it was made clear that the accuracyof the CLALIS is within +/- 20% (Fig.2). Using the result of the accuracy, detection limits for Co-60 and Cs-137 have been estimated100 Bq and 240 Bq, respectively, which enables simultaneous judgment of Japanese legal criteria for surface density level in additionof the satisfaction of the clearance level.
3. Proposal of reasonable method to judge satisfaction of clearance level
Some nuclides, which are impossible to measure easily, are to be estimated using the previously prepared nuclide ratio to a
key nuclide, for example Co-60. A new method has been proposed and adopted to the SC in the AESJ as follows; If the uncertainties
of nuclide ratios are within the upper limits of the scattered nuclide ratios determined using a probabilistic approach, it would not benecessary to treat the clearance level as lower level considering some margin for uncertainties of measurement and nuclide ratio (Fig.
3). In addition, a probability distribution calculation system (PDCS), which is required for the application this method, has been developed and opened from our CRIEPI web site for free use.

Future Developments
In the future, extension of the measurement target of CLALIS to concrete wastes in addition to metal wastes will be carriedout after developing a method to remove the effects of natural radioactive nuclides.
Main Researcher:Takatoshi Hattori, Ph. D., Senior Research Scientist and Michiya Sasaki, Ph. D., Research Scientist,
Radiation Safety Sector, Nuclear Technology Research Laboratory
Reference
M. Sasaki and T. Hattori, 2005, “Development of clearance level automatic inspection system, Nuclear Technology Research Rep.
L01 (in Japanese)
T. Hattori, 2004, “An Approach to Safety Margin for Measurement Error and Scattered Nuclide Spectrum on the Monitoring for
Compliance with Clearance Level”, Transactions of the Atomic Energy Society of Japan, Vol.3, No.4, pp363-368 (in Japanese).

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Laser scanners

Gamma ray measurement
Mass and shape measurement
Fig.1 Clearance level automatic laser inspection system (CLALIS)

Estimated/Actual Radioactivity

2.0Co-60
Cs-137

1.5
+20 %
1.0
-20%
0.5

0.0
50

100

150Run No.

200

250

Fig.2 Accuracy of CLALIS Possible to measure within +/- 20%

Large scattering
Geometric standard deviation

Small scattering

100
1 nuclide2 nuclides
3 nuclides
4 nuclides

1010.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Importance of Co-60

Fig.3 Concepts for upper limit for scattering of nuclide spectrum (an example of simple
method)Large scattering in nuclide spectrum is permissible in the case that Co-60 is dominant nuclide.

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