Student seminars

TRITIUM: Design, construction and commissioning of automatic station for real time monitoring of low radiactivity levels of tritium in water

by Marcos Martinez (Contrato temporal)


In this talk, the design, construction and set-up of a near real-time monitor intended to measure low levels of tritium in water is presented. This monitor will be installed in the Arrocampo dam in which Almaraz nuclear power plant (Cáceres, Spain) releases cooling water, which flows afterwards into the Tagus river, which pro- vides drinkable water to a wide area of Spain and Portugal. The Council Directive 2013/51/Euratom requires that the maximum level of tritium in water for human consumption be lower than 100 Bq/L. This level is much higher than the level due to the natural or cosmogenic component of tritium in the environment. However, it can be easily exceeded in the cooling water of nuclear power plants in normal operation. This tritium monitor should be able to measure every 10 minutes the tritium level of water with the sensitivity required by the Council Directive (100 Bq/L). Although there is a number of techniques, like liquid scintillator systems, capa- ble of measuring low levels of tritium in water, sample collection and measurement time are not suitable for providing in-situ and near real-time alert signals for water quality monitoring in rivers near nuclear power plants. Furthermore, this technique generates chemical residues (as toluene) that should not be released in the environment. Plastic scintillators read out by photomultiplier tubes are robust and clean, and have been used in the past for this task, but their detection limit, around 10 kBq/L in a 10 minutes measurement, is quite higher than the required detection limit for drinkable water. Our goal is to improve the current limit of scintil- lating plastic technique by two orders of magnitude. To reach this goal, a monitor consisting in scintillating fibers read out by silicon photomultipliers has been designed. Silicon photomultipliers have better photon de- tection efficiency than photomultiplier tubes and do not need high voltage, which is important from the point of view of autonomy of the monitor. A data acquisition system, originally intended for positron-emission to- mography imaging, allows the measurement of both energy and time of the signals in both ends of the fibers, and to implement triggers to reduce electronic and environmental backgrounds. Background due to both cosmic rays and environmental radioactivity is suppressed with the help of a lead shielding and an active cosmic veto, made of plastic scintillator. To avoid frequent maintenance of the monitor, a water purification system has been designed and implemented which purifies water to the hyperpure grade of purity before providing it to the tritium detector. Detailed Geant4 simulations of the prototype have been carried out. Be- sides the employment of this monitor in nuclear power plants, it will be useful when commercial fusion nu- clear reactors are available, for which tritium is the main radioactive product released. Our tritium monitor is planned to be installed in Almaraz by spring 2019. This project is funded by the INTERREG SUDOE program of the European Economic Community with reference SOE1/P4/E0214, in which a consortium of six institu- tions from three southwestern Europe countries are involved: Spain, Portugal and France.

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