(a) shows the measurement flow with the liquid scintillation counter (LSC) used for conventional measurements. (b) shows the measurement flow with the tritium monitor developed in this work. The developed monitor employs plastic scintillation (PS) pellets as the detector, eliminating the need for a measurement pretreatment process.

Fig. 2 shows the analysis results of the measurement time and detection limits when measured by the tritium monitor. It has been found that the tritium monitor has the ability to achieve the target values in a shorter period of time as compared to the conventional methods.

As a part of the works for TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS) decommissioning, the release of water treated by the Advanced Liquid Processing System (ALPS) to the sea began in 2023. Continuous monitoring of the water is necessary to ensure that the tritium concentration in the ALPS-treated water does not exceed the standard value of 1,500 Bq L⁻¹. The conventional measurement method uses a liquid scintillation counter (LSC) as a monitor. This process encounters some problems such as the need for a time-consuming pretreatment process and generation of waste liquid due to the use of organic solvents in the scintillator. We developed a practical tritium monitor to continuously supervise the tritiated water concentration in ALPS-treated water. The monitor comprises a flow cell detector consisting of plastic scintillation pellets, a coincidence circuit composed of three photomultiplier tubes (PMT), lead shielding designed to mitigate the impact of environmental gamma radiation, and a VETO detector. By employing a plastic solid scintillator, we developed a detector that enables measurements without requiring a pretreatment process and without generating organic waste liquid. In addition, a low background dose rate (0.082 cps) was achieved by adding a shielding structure and background subtraction function for conducting measurements in environments, such as the FDNPS, with relatively high ambient dose rates.

This system is designed to allow continuous measurement by pumping water through it. In this test, the lower detection limit of 593 Bq L⁻¹ was achieved within a short measurement time of one hour, without the need for a complicated pretreatment process. This value is well below the release standard of 1,500 Bq L⁻¹ for ALPS treated water.

Sanada, Y., Abe, T. et al., Development of a Practical Tritiated Water Monitor to Supervise the Discharge of Treated Water from Fukushima Daiichi Nuclear Power Plant, Nuclear Instruments and Methods in Physics Research Section A, vol.1062, 2024, 169208, 7p.

Paper URL: https://doi.org/10.1016/j.nima.2024.169208

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