Principal Research Results

Development of a Clearance Measurement System for Concrete Waste−Development of Correction Method of BG Count Rate Due to Natural
Radionuclides in Concrete−
Background
Regarding the wastes generated from decommissioning of a nuclear power plant, they can be reused or recycled as resources
when their activity levels are low enough; i.e. their activity levels are below the clearance levels ＊ 1. We therefore have developed aclearance level inspection system, named CLALIS (Clearance Automatic Laser Inspection System), which can measure very low-level
activity using gamma-ray detector, laser shape measurement and Monte-Carlo calculation techniques. The activity detection ability of
CLALIS for metal waste has been experimentally clarified using mock-metal waste and standard radioactive sources so far. To apply
CLALIS into the clearance measurement of concrete waste, it is required to correct the BG (background) count rate ＊ 2 during gammaray measurement since it may be increased by natural radionuclides in concrete.

Objectives
To develop a method which can correct the influence of BG count rate due to the natural radionuclides in concrete.
To evaluate the accuracy of the correction method using mock-concrete waste samples.

Principal Results
1. Development of correction method of the effect due to natural radionuclides
A new correction method of BG count rate during gamma-ray measurement was developed using gamma-ray emission rateof natural radionuclides in concrete and Monte-Carlo calculation. The gamma-ray emission rate of natural radionuclide is pre-analyzed
by representative sample and HPGe semiconductor detector (Fig.1).
2. Correction accuracy of BG count rate for concrete samples of different activity concentration andratio of radionuclides
The natural activity concentration and ratio of natural radionuclides of concrete components, such as cement and aggregate
vary according to the production site. Using six types of concrete samples from different production site, the accuracy of correction of
BG count rate was evaluated. As a result, it was clarified that this method can correct the BG count rate during measurement within agood accuracy of +/- 1% (Fig.2).
3. Correction accuracy of BG count rate for concrete samples of various shapes and amountsConsidering the actual waste, such as segments, generated from nuclear power plant, the correction accuracy of BG countrate was evaluated using various shapes and amounts of mock-concrete wastes. Consequently, it was revealed that this method could
estimate the BG count rate during gamma-ray measurement within a good accuracy of +/- 4% (Fig.3).
Since the detection limit ＊ 3 of CLALIS for concrete waste is evaluated as 110 Bq, it is indicated that CLALIS could beapplied for clearance measurement of concrete waste above 1.1 kg when the key radionuclide is Co-60.

Future Developments
We will study the application of CLALIS for concrete waste generated from nuclear power plant not only in decommissioning, but also in operation.
Main Researchers: Michiya Sasaki, Ph. D., Research Scientist; Takatoshi Hattori, Ph. D., Senior Research Scientist;
Radiation Safety Sector, Nuclear Technology Research Laboratory
Reference
M. Sasaki and T. Hattori, 2006, “Application of Clearance Automatic Laser Inspection System -Development of correction method ofbackground count rate due to natural radionuclides ”, CRIEPI Report L05006 (in Japanese)＊1：Activity concentration level of material to judge whether it should be treated as radioactive or nonradioactive material.
＊2：Count rate due to ambient radiation (radiation from radionuclides in atmosphere, or cosmic radioation etc.).
＊3：The lowest limit of amount that a system can determine.

885. Nuclear - Nuclear fuel cycle

Ambient
BG count
rate (A)

BG count rateduring measurement (B)Count rate of detector

Count rate
due to contaminationor activation
Naturalradionuclides

Ambient
BG

Ambient(A)BG

Monte-Carlo calc. using
shape and gamma-rayemission data

Shape measurement

Efficiency(C)BG countrate during
measurement (B)Gamma-rayemission rate
Pre-analysis ofrepresentative sample
using HPGe detector

To estimate the BG count rate during measurement (B),
it is required to estimate the increase of count rate (C)due to the natural radionuclides in concrete.

Estimation of (C)Newly developed correction method

BG count rate [cps]800780760
740
720

Ambient BG count rate(A)AB count rate during measurement(B)
BGCCorrected
BG count rate((A)+(C))

700
680660640620600Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6Fig.2 Correction results of BG count rate for mockconcrete samples of different activity concentration(A) Ambient BG count rate without mock-concrete sample.
(B) BG count rate with mock-concrete sample.
((A)+(C)) BG count rate corrected by the newly developed
method. Since BG count rate during measurement (B) can beappropriately estimated, count rate due to contamination oractivation can be estimated within a good accuracy.

89

Ratio of BG count rate (Estimation/Actual)Fig.1 Increase of BG count rate due to concrete waste measurement and newly developed
correction method

Correction accuracy{(A)+(C)}/(B)

1.2
1.1
1.0
0.9
0.8
1.2

Sample 2± 4%

1.1
1.00.9Sample 30.805101520Run No.

2530

Fig.3 Correction accuracy of BG count rate
for various shapes and amounts