Development of on-Line Monitoring Device for Marine Radioactive

Pollution

Xinsheng Lv, Guoxiu Qin

, Xiaoli Lin, Keyu Meng and Jinming Ma

Shenyang Institute of Engineering, 18 Puchang Dr, 110136 Shenyang, Liaoning, China

Keywords: Nuclear Accident, Seawater Radioactivity, on-Line Monitoring, Plastic Scintillator

Abstract: On-site measurements are indispensable to swiftly and precisely evaluate the magnitude of radioactive

contamination triggered by a nuclear accident or terrorist attack in the ocean. Consequently, an on-line

monitoring device for marine radioactive pollution has been developed. A plastic scintillator was used as the

detector of the device, and a pump was used to extract the seawater around the measuring region, ultimately

enabling continuous on-line measurement. The device measures sea water for 10 minutes, and the minimum

detectable activities of

137

Cs and

Co are 1.7 Bq/L and 2.1 Bq/L, respectively. It can promptly and efficiently

the detect events of excessive radioactivity in wastewater and give an alarm signal. The device will be critical

for monitoring radioactive pollution in the maritime region around nuclear power plants, and it has the benefits

of a quick measuring time, a high monitoring frequency, and a high degree of real-time monitoring, allowing

for the early identification of accidents and the implementation of emergency strategies.

1 INTRODUCTION

Developing nuclear power is an excellent technique

to alleviate environmental problems and optimize the

energy structure. In the process of nuclear power

development, the safety of nuclear reactors is very

important. Once a nuclear leakage accident occurs, it

will have a great impact on the environment (Arvela

et al. 1990; Lee et al. 2017; Fereshteh et al. 2021). In

2011, a serious nuclear accident occurred at the

Fukushima nuclear power plant in Japan, resulting in

the release of huge quantities of radionuclide into the

environment (Kinohita et al. 2011; Nakano and

Povinec, 2012). As a nearby territory, China

immediately carried out a vast number of radioactive

monitoring projects. Traditional marine radioactivity

monitoring consists of collecting samples of seawater,

biological organisms, and sediments from critical

sites 1 to 4 times per year, which are then transported

back to the appropriate laboratory for processing and

analysis (Vlastou et al. 2006; Tsabaris 2008). Using

γ-ray spectrometer to measure samples in laboratory

is a time consuming method, which requires chemical

pretreatment of the measured samples. This non real-

time and discontinuous method is difficult to

effectively monitor the marine radioactive pollution,

Corresponding author

and it is even more difficult to realize the early

warning of marine radioactive pollution.

On April 13

, 2021, the Japanese government

officially agreed to discharge sewage from the

Fukushima Daiichi Nuclear Power Plant into the

ocean at a cabinet meeting that day. Since then, Japan

intends to discharge 1.25 million tons of nuclear

sewage into the sea, with the goal of starting to

discharge in two years. While TEPCO has claimed

that after treatment, most of the radionuclides in the

nuclear sewage except

H can be removed. However,

monitoring data reveal that there are radioactive

material residues such as

Co and

Sr that are

difficult to completely remove in the nuclear sewage

after "filtration". β radionuclides, such as

C and

Sr, cannot be monitored online by γ spectrometry,

hence the question of how to monitor β radionuclides

in the water near nuclear power plants in real-time

and precisely has emerged as an essential one.

Furthermore, with the development of nuclear power,

the construction of real-time monitoring network for

radioactivity in sea areas around nuclear power plant

has also been put on the agenda. It will be a trend to

develop a radiation monitoring network along the

whole coast with online monitoring, real-time early

Lv, X., Qin, G., Lin, X., Meng, K. and Ma, J.

Development of on-Line Monitoring Device for Marine Radioactive Pollution.

DOI: 10.5220/0011889200003536

In Proceedings of the 3rd International Symposium on Water, Ecology and Environment (ISWEE 2022), pages 35-39

ISBN: 978-989-758-639-2; ISSN: 2975-9439

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

warning and other functions (Wedekind et al. 1999;

Tsabaris 2005; Katsumi and Pavel, 2020).

2 INSTRUMENT DESIGN

The designed online monitoring device for marine

radioactive pollution is mainly composed of a

detector, a sampling chamber, a lead chamber and

signal processing unit (see Figure 1). To complete the

continuous measurement and indication of seawater,

the device is required to have the following

characteristics: 1) high sensitivity is required of the

detector; 2) a superior shield layer and fewer

background count; 3) cleanliness and anti-fouling

should be priorities while designing the sampling

chamber; 4) easy to manufacture and reasonable cost;

5) stable, dependable performance over an extended

period of time, as well as simple maintenance.

Figure 1: Structure diagram of the online monitoring device

for marine radioactive pollution.

2.1 Detector Selection

Plastic scintillator is a kind of organic scintillator that

may be used to detect α particles, β particles, γ rays,

neutrons and fission fragments (Suffian et al. 2020;

Kagami et al. 2020). It is widely used in the

measurement of some low-level radioactive or liquid

samples. Plastic scintillator has several benefits,

including its ease of production, low cost, great

mechanical strength, ability to be molded into a

variety of forms, stable performance, strong radiation

resistance, and high corrosion resistance. Plastic

Scintillators are thus perfect for β radionuclide

monitoring in the marine environment. To detect

radioactivity in seawater in real time, the plastic

scintillator is built as a nested mode of circular rings,

with the outermost detector separating the sampling

chamber into an inner and an outer portion (see Figure

2). Seawater enters from the bottom of the detector,

and there are some overflow holes with a diameter of

5 mm on the top of each circle ring detector. When the

pumped seawater exceeds the overflow hole, it will

gradually fill the whole sampling chamber through the

overflow hole, and finally flow out of the sampling

chamber through the water outlet.

Figure 2: Structure of the plastic scintillator detector.

2.2 Design of Lead Chamber

The concentration of radionuclides in the measured

seawater is low most of the time. To reduce the

influence of background, the typical low-background

lead chamber is used as shield. The designed lead

chamber is a cylinder with a size of Φ 40×50 cm, the

thickness of lead is 5 cm. Since natural lead materials

contain

210

Pb and other radionuclides, a 2-mm copper

layer was added to the inside of the lead chamber to

shield the radiation of radioisotopes of lead.

2.3 Design of Sampling Chamber

The sampling chamber is positioned within the lead

chamber, and its size is Φ 25×30 cm. Its main

function is to continuously collect seawater into the

sampling chamber. The sampling vessel is composed

of ABS plastic that is resistant to impact, heat, and

low temperatures. The bottom is equipped with an

input and an outlet for water, enabling the continuous

collection and discharge of seawater. The inner wall

of the sampling chamber (light-transmitting at the

top) is covered with aluminum foil to isolate the

interference of the outside world to the detector and

the photomultiplier tube, and reflect the fluorescence

produced by the interaction of radiation with the

detector back to the photomultiplier tubes.

2.4 Data Processing Unit

The data processing unit consists of signal

discrimination, filtering shaping, and counting

display, which is mainly used to process the nuclear

signal output from the detector and display the results.

Pump

Seawater

Data

processing

Discharge

Lead chamber

Photomultiplier

tube

Light guide

Detector

Sampling

chamber

Pump

Seawater

Data

processing

Pump

Seawater

Data

processing

Discharge

Lead chamber

Photomultiplier

tube

Light guide

Detector

Sampling

chamber

ISWEE 2022 - International Symposium on Water, Ecology and Environment

Firstly, the nuclear signals are filtered by signal

discrimination, then it is turned into a Gaussian

waveform and noise interference is minimized. In the

counting display module, the nuclear signals are

classified and counted after filtering, and the result

are displayed.

3 TECHNICAL INDICATORS OF

THE INSTRUMENT

3.1 Minimum Detectable Activity

One of the key performance indicators of the

measuring equipment is the minimum detectable

activity (MDA). It refers to the smallest amount of

radioactivity in a sample or medium that may be

detected with a particular degree of confidence (Choi

et al. 2019). In general, the smaller the MDA in the

same time, the better the detection performance of the

instrument. However, MDA characterizes the

qualitative analysis ability of the system for the

existence of nuclides, which cannot characterize the

quantitative measurement accuracy of the system for

nuclide activity. At the 95% confidence level, MDA

can be expressed as:

2.71 4.65 B

MDA

⋅

(1)

where B is the background count; t is the live time of

counting; ε is the detection efficiency. The change

trend of MDA of

137

Cs and

Co in seawater with

measurement time by this device is shown in Figure

Figure 3: The change trend of MDA of

137

Cs and

Co in

seawater with measurement time by the designed device.

The seawater containing

137

Cs and

Co with an

activity concentration of 5 Bq/L were measured

respectively in the laboratory, and the technical

indicators when the device was used as the "activity

detection" function were obtained: the MDA of

137

and

Co were 1.7 Bq/L and 2.1 Bq/L respectively

after 10 minutes of measurement. For the seawater

containing

137

Cs and

Co with an activity

concentration of 5 Bq/L, the statistical uncertainty of

measurement for 10 minutes is less than 10%.

3.2 Monitoring Performance of the

Instrument

The MDA test results indicate that the device is

capable of detecting β radionuclide contamination

events in a short time frame. As a real-time

monitoring device, the measurement results must be

provided continuously in a short time. The shorter the

measurement interval, the more precisely the data

may be represented throughout time. In the case of

real-time monitoring, after deducting the background,

the count of the device is less and obeys the Poisson

distribution. The false alarm rate and the missing

alarm rate of the monitoring equipment are the two

most crucial and contradictory technical indicators.

According to the calibration results of the detection

efficiency of the device, based on the Poisson

distribution law, we have formulated the exceeding

threshold and compiled the real-time monitoring

program.

The measured results show that when the activity

concentration of

137

Cs is 5 Bq/L, the total count rate

of the monitoring device is 2.7 cps, or an average of

162 counts per minute. This means that the

mathematical expectation for the device to count in

two minutes is 324. According to Poisson

distribution, the corresponding probability density

was calculated with 324 as the expected value. For

setting different exceeding threshold y (the alarm will

be given if the counts detected in any 2 minutes is ≥

y), "probability of reaching the threshold" is equal to

the curve area on the right side of the medium

threshold.

Set the monitoring cycle of the designed device to

2 minutes. When the count of two adjacent cycles is

greater than or equal to the exceeding threshold, the

device will automatically give an alarm signal. By

setting the threshold value in this way, the number of

false alarms per year can be controlled below 0.1. For

radionuclides with activity concentration greater than

5 Bq/L in seawater, the missing alarm rate is less than

1% after 20 minutes of measurement.

To test the performance of the device, seawater

containing

137

Cs with an activity concentration of 5

Bq/L was measured and monitored continuously for

100 cycles. The monitoring results were analyzed,

and it was found that 60 cycles sent out alarm signals,

Development of on-Line Monitoring Device for Marine Radioactive Pollution

with an average delay of 4.52 minutes and a

maximum delay of 7 minutes. Count 100 cycles, and

the results are shown in Figure 4. It can be seen from

Figure 4 that the count obtained by the device is in

good agreement with the Poisson distribution, and it

is effective to use the Poisson distribution law to

design the alarm program.

Figure 4: Comparison of counting statistics of 100 cycles

with the theoretical Value of Poisson distribution.

4 CONCLUSIONS

The designed online monitoring device for

radioactive contamination in seawater has a high

degree of sensitivity, can provide information on

radioactive contamination in seawater in a short

period of time, and has the function of rapid

measurement of radioactive activity concentration in

seawater. In the "detection" aspect, the plastic

scintillators with a low price, stable performance,

good radiation resistance and corrosion resistance.

Combined with the 14.7 L sampling chamber in the

lead chamber, excellent MDA was obtained by

reducing the background. Taking

137

Cs as an

example, MDA measured for 10 minutes can reach

1.7 Bq/L.

As for the "Alarm" feature, we have adopted the

optimized design of inlet and outlet waterways, so

that the seawater in the sampling chamber can be

updated quickly. We have also developed a set of

"short time and less counting" design method based

on Poisson distribution to determine the alarm

threshold and minimize the false alarm rate and

missing alarm rate within a reasonable and acceptable

detection time. For seawater containing

137

Cs with an

activity concentration of 5 Bq/L, it takes only 4.52

minutes on average to give an alarm, and the

probability of missing an alarm after 20 minutes of

monitoring is less than 1%.

ACKNOWLEDGEMENTS

This work was supported by the Shenyang Science

and Technology Bureau (No. 20-206-4-03). The

authors would like to express thanks to the China

Institute of Atomic Energy for its support of this

work.

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Development of on-Line Monitoring Device for Marine Radioactive Pollution