Principal Research Results

New Surface Contamination Measurement Method at Nuclear Power Plants−Demonstration of Applicability of Clearance Automatic Laser
Inspection System for Surface Contamination Measurement−BackgroundIn the clearance level inspection ＊ 1 of solid materials such as metal and concrete scrap generated from nuclear power plants,
the activity level of waste must be compared with not only the clearance level (0.1 Bq/g for Co-60) but also the surface contamination
density standard (4 Bq/cm2 for beta or gamma emitter). Usually, a manual GM survey meter based on beta-ray detection is used forsurface contamination density inspection at present.
Recently, the clearance automatic laser inspection system (CLALIS) has been developed for low-level radioactive wastemeasurement. The radiation detection ability for scrap metals and concrete segments has been clarified in a large number of experiments using mock metal and concrete waste samples and standard radioactive sources. It has been experimentally shown that thedetection limit of CLALIS is approximately 100 Bq for Co-60, which is sufficiently low not only for clearance measurement, but alsofor surface contamination density measurement.
If clearance level and surface contamination density standards can be confirmed in a single radiation measurement withCLALIS, rational clearance-level inspection can be achieved (Fig. 1). For this, it is necessary to demonstrate the applicability of
CLALIS for surface contamination measurement using actual waste generated from a nuclear power plant.

ObjectiveThis study aims to demonstrate that CLALIS can be used for the measurement of surface contamination density by comparing the measurement results of CLALIS with those of conventional GM survey meter using various shapes and radiation levels ofmetal scrap generated from a nuclear power plant.

Principal Results
1. Experiment at nuclear power plant.
The 97 measurement samples of various shapes and different radiation levels (less than detection limit, less than 4 Bq/cm2
and above 4 Bq/cm2 by GM survey meter measurement) were chosen from scrap metal waste dump at Kashiwazaki-Kariwa Nuclear
Power Station of Tokyo Electric Power Company. By comparing the measurement results of CLALIS with those of the GM survey
meter, it is clarified that the CLALIS has equivalent surface contamination density classification ability of GM survey meter for all
measurement samples (Fig. 2, Table 1).
2. Verification of applicability
To discuss the applicability of CLALIS as surface contamination monitor in practical use, detection limit and measurement
time were estimated using the average BG count rate during the experiments at the nuclear power station. As a result, it was found that
detection limit of CLALIS would be approximately 0.8 Bq/cm2 in the case where the measurement time is approximately 1 minute,which is equivalent ability of present surface contamination monitors.

Future Developments
We aim to achieve practical application of CLALIS to a nuclear power plant in operation, and standardization of the surface
contamination measurement method based on gamma-ray detection.
Main Researchers:Michiya Sasaki , Ph. D., Research Scientist, Radiation Safety Research Center, Nuclear Technology Research Laboratory
Takeshi Ichiji, Research Scientist, Radiation Safety Research Center, Nuclear Technology Research Laboratory
Haruyuki Ogino, Research Associate, Radiation Safety Research Center, Nuclear Technology Research Laboratory
Takatoshi, Hattori, Ph. D., Senior Research Scientist, Radiation Safety Research Center, Nuclear Technology Research Laboratory
Reference
M. Sasaki, et al., 2009, “Verification of surface contamination density standard using Clearance Automatic Laser Inspection Systemfor objects from a nuclear power plant ”, CRIEPI Report L08007 (in Japanese)＊1：Nuclear power plant operators implement the measurement to confirm that the measurement target is to be “an object which is

non-radioactive material”, and the regulatory authority is involved in the judgment appropriately (Clearance Inspection Report,
Nuclear Safety Comission of Japan, 2001). The target of clearance inspection is limited to solid materials.

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Fig.1 Rational clearance inspection using CLALIS

5Fig.2 Verification test at Kashiwazaki-Kariwa Nuclear Power Station of Tokyo Electric Power Company
Table 1 Comparison between numbers of samples with different surface contamination density
classifications for CLALIS and GM survey meter

CLALISLTD

Less than 400 Bq
by total gamma
measurement※1

Above 400 Bq by
total gamma
measurement※1

Less than detection
limit (LTD)518 ※23 ※2Less than 4Bq/cm20312

Above 4Bq/cm20020

GM
survey meter※1

For the 35 samples
with surface
contamination higher
than the detection
limit of the GMsurvey meter,
CLALIS could also
detect the activity.

Co-60 equivalent activity. Surface contamination density, in unit of Bq/cm2, can be estimated divided by the constant surface area of 100 cm2,
which is the averaging unit used in the Japanese Industrial Standards.
※2
CLALIS gives more conservative values than the GM survey meter because it uses the total count rate and the conservative source position
assumed in the Monte Carlo calculation for the calibration.

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