Continuous Tritium Monitoring System for Tritium in Water Using PEM Electrolysis Cell

Jun Woo Bae, Ki Joon Kang, Hee Reyoung Kim

Department of Nuclear Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, 44919, Ulsan, Republic of Korea

INTRODUCTION

In September 2016, an earthquake measuring 5.8 on the Richter scale occurred in Gyeongju. Gyeongju is a city where Wolsong nuclear power plant is located and Wolsong Unit 1 is a pressurized heavy water reactor where frequent issues about tritium have occurred. After the earthquake, the concentration of tritium in the surrounding air of Wolsong Unit 1 temporarily increased, and tritium was detected in the urine samples of nearby residents [1]. Through these events, interest in tritium has increased in Korea and technology for monitoring tritium has been demanded. Korea currently has a tritium concentration control criterial (40,000 Bq/L) in the effluent, but there is no tritium criterion in drinking water and it is recommended by the World Health Organization (WHO) recommendation (10,000 Bq/L) [2].

Tritium in water is generally analyzed using a liquid scintillation counter (LSC) [3]. LSCs have the advantage of low minimum detectable activity (MDA) and high tritium detection efficiency, but they require work such as bringing samples to the laboratory and mixing them with cocktails in vials. In addition, it is not suitable for continuous analysis of large amounts of water samples because it continuously generates organic wastes. Some commercial products continuously detect tritium in the water, but these products have a disadvantage that the detection time is too long to satisfy the MDA that meets the tritium concentration criteria because of low detection efficiency.

The energy generated by tritium is extremely low (maximum energy: 18.6 keV), thus, the detection efficiency is very low due to the self-absorption effect when measured in the liquid state. In order to overcome this problem, this study used electrolysis to extract gaseous hydrogen from water samples. There was a case in which the tritium gas using electrolysis was measured using a gas-flow proportional counter [4]. However, in this study, the sample gas was diluted by flowing the gas due to the characteristic of the proportional controller, and the gas had to be continuously consumed for the continuous measurement. To overcome this problem, a scintillation-based detection system was designed for continuous tritium monitoring, the gas generated by electrolysis of the tritiated water sample was calibrated, and the tritiated water sample was used to verify the developed system.

RESULTS

The developed system was designed to take in water and electrolyze it and inject the electrolyzed hydrogen gas into a detector optimized for tritium detection and detect it. This chapter consists of the developed system configuration, design of detector, and the electrolysis characteristics of tritiated water.

System Configuration

Figure 1 shows a conceptual system configuration for continuous water sampling and tritium detection and figure 2 shows the picture of the detection system. The system consisted of deionization (D.I.) filter, water sample tank, pump, DC supply, proton-exchange membrane (PEM) electrolysis cell, water trap and tritium detector. The water sample should be deionized to physically and chemically protect and ensure the lifetime of the PEM electrolysis cell using the D.I. Filter. A pump was used to cool the PEM cell and prevent degradation of electrolysis efficiency due to bubbles generated in the anode. Flow rate of the pump was 1.8 L/min. A 10-stack of PEM assembly for water electrolysis was used. The geometry of the membrane was 60 mm × 80 mm and anodic and cathodic catalyst of the membrane were iridium and platinum, respectively. 7 A of current was applied, and the hydrogen production rate was 0.53 L/min. The water trap was equipped because small amount of water was overflowed through the PEM cell to the cathode.

Fig. 1. Conceptual system configuration for continuous water sampling and tritium detection.

Transactions of the American Nuclear Society, Vol. 119, Orlando, Florida, November 11–15, 2018

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Detector Design for Measuring Tritium Gaseous tritium is detected by an ionization chamber in

general. However, background fluctuation of the ionization chamber due to external comic ray is high. Therefore, it is not suitable for detecting the tritium gas with environmental levels of concentration. A tritium measurement using a proportional counter would be effective. If a proportional counter is used, the gaseous sample is necessarily mixed with the detecting gas. In this process, dilution is required, and the detecting gas should be charged periodically. In this respect, scintillation-based detector was adopted in this study.

The detection system was based on the plastic scintillator. Plastic scintillator was useful to detect the beta-ray because its effective atomic number and density were low compare to other material. Plastic scintillator shows low external cosmic ray background counting rate, and high efficiency for beta-ray. Plastic scintillator is also advantageous in that they are easy to process, inexpensive, and have good chemical resistance.

The detector assembly consisted of plastic scintillators, detecting chamber, photomultiplier tubes and sockets, and nuclear instrument modules(NIMs). Two plastic scintillators with diameter of 50 mm faced each other and were attached to the detecting chamber using resin cement. The detecting chamber was made of acrylic with connection port to be connected to the gas flow path, and two holes which the two plastic scintillators were inserted. The plastic scintillator and photomultiplier tubes were optically coupled. Figure 3 shows the NIM configuration for coincidence signal processing. Coincidence circuit was highly recommended for measurement of low-energy ionizing radiation because the pulse height of the signal too low to discriminate with the thermal noise signal. Timing characteristic of the coincidence circuit was carefully calibrated.

Fig. 2. Picture of the developed detection system.

Fig. 3. NIM configuration for coincidence signal processing Characteristics of Electrolysis of Tritiated Water

An electrolysis has been used to enrich the tritium in a

water sample before counting using LSC. Enrichment means that the tritium is less likely to be converted to gas than hydrogen atoms during electrolysis. However, there is a special characteristic in PEM cells that this enrichment factor is relative low rather than the conventional electrolysis cell [5]. When calculating the liquid tritium concentration from the concentration of gaseous tritium produced by electrolysis, it is necessary to characterize this enrichment factor as well as the volume ratio from liquid to gas. Figure 4 shows the measured concentration of hydrogen gas produced by electrolysis of tritiated water. The measurement was carried out using calibrated tritium monitor based on ionization chamber (Portable tritium monitor β-ionix, PREMIUM Analyse). The gaseous tritium concentration was proportional to the liquid tritium concentration. The errors for each measurement were not proportional to the concentration of liquid or gaseous tritium concentration because the major contributor for the errors was a fluctuation of the measured concentration due to electrical noise, not the statistical error.

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Fig. 4. Relationship between concentration of liquid tritiated water and of hydrogen gas produced by electrolysis

Discussion and Future Works

An electrolysis based continuous monitoring system for tritium-in-water was developed. The electrolysis characteristics of tritiated water were analyzed using a commercial detector. This will be applied to the developed system and the quantitative analysis of relationship between gaseous tritium counting rate in the developed detector assembly and tritiated water sample will be carried out in the future.

REFERENCES

1. S, YOON, W.H. HA, and S.S. Lee. "Tritium analysis ofurine samples from the general Korean public." AppliedRadiation and Isotopes, 81, 276-278 (2013).2. C. ABBOT et al., “Guidelines for Drinking-WaterQuality, Fourth Edition” World Health Organization (2011).3. J. Y. YOON et al., “The Annual Report on theEnvironmental Radiological Surveillance and Assessmentaround the Nuclear Facilities” Korea Institute of NuclearSafety (2015).4. A.M. SOREEFAN and T. A. DEVOL, “ProportionalCounting of Tritium Gas Generated by Polymer ElectrolyteMembrane (PEM) Electrolysis” Journal of RadioanalyticalNuclear Chemistry, 282, 517 (2009).5. A.M. SOREEFAN and T. A. DEVOL, "Determination oftritium enrichment parameters of a commercially available PEM electrolyzer: a comparison with conventional enrichment electrolysis." Journal of radioanalytical and nuclear chemistry, 282, 511 (2009).

Transactions of the American Nuclear Society, Vol. 119, Orlando, Florida, November 11–15, 2018

1016 Radiation Protection and Shielding: General—I