 The groundwater must be periodically monitored before and after decontamination and decommissioning (D&D) of nuclear facilities. Long-term groundwater radiation monitoring is required in the case of restricted release after restoration of D&D sites. Beta radionuclides generated in nuclear facilities need to be managed from the point of view of health physics and require technology to quickly and accurately detect the degree of contamination. However, because of the short range of low-energy beta radionuclides, measurement equipment such as liquid scintillation counter (LSC) must be used during the pretreatment after sampling. LSC that require long sampling and analysis times, including pretreatment, cannot be used in the field.

 Therefore, a plastic scintillator-based detection system has been developed that increases the detection efficiency of gross-beta by directly contacting the sample and the detector. In addition, since tritium has very low detection efficiency in the liquid state, a system for gasification trough a PEM (Polymer Electrolyte Membrane) electrolysis method has been developed. In this study, the characteristic experiment is conducted by integrating the individually developed gross-beta and tritium detection systems.

 The detection system consists of four steps. First, for more accurate measurement, the groundwater goes through the membrane to extract pure water. The groundwater around nuclear facilities contains various components such as debris, suspended matter, organic matter, heavy metals, microorganisms and ionic substances including radioactive materials. To remove these substances, the pretreatment system uses an A/C (Activated Carbon) filter and an ion exchange resin, including a membrane filter, to treat the water samples on site with pure water. The groundwater, which has passed through the pretreatment system, then enters the first detector to measure the gross-beta. The detector was constructed with a lead shielding box that blocks the gamma background and light in the field, and a background reduction measurement system through a coincidence circuit. After gross-beta measurement, the tritium electrolysis collection pretreatment system is then used for tritium measurements. The electrolysis method used can be applied to pure water by the PEM electrolysis method. Finally, the vaporized pure water is finished passing through a second detector that measures the same tritium as the gross-beta detector.

 The MDA (Minimum Detectable Activity) target of 10,000 Bq/L which is WHO (World Health Organization) drinking water standard for tritium and 1,000 Bq/L which is conservative value of ^90^Sr clearance level for gross-beta. Through this detection system, the detection time to reach the target MDA will be matched with real-time monitoring within about 5 minutes. The background measurement time required to satisfy the MDA using the current system was 10 minutes and 1 minute for tritium and gross-beta, respectively. In the case of gross-beta, real-time monitoring is expected to be possible enough but in case of tritium, it is necessary to derive a method that can sufficiently reduce the time below 5 minutes for real-time monitoring by reducing the background and increasing the detection efficiency through such as light guides.


Development of Gross-Beta and Tritium Monitoring System in Decommissioning Site Groundwater

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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