Level measurement of borated water in a nuclear spent fuel pool

The EDF Group is the world's leading nuclear operator. In France, EDF operates a network of nuclear power plants across the nation, which are essential for maintaining energy security and advancing the company’s goal of achieving a net-zero energy future. Among these facilities is the Tricastin plant, located in the Auvergne-Rhône-Alpes region.

The handling of spent fuel rods from pressurised water reactors (PWR) requires precise and reliable monitoring and control. Once depleted, the fuel rods are removed from the reactor and stored in a spent fuel pool (SFP). This pool, located in the fuel building (BK), is filled with borated water. This ensures that the heat-emitting rods are cooled and adequately shielded from the environment, protecting both personnel and nearby equipment from radiation. The spent fuel can only be transported to reprocessing plants once its decay heat has been sufficiently dissipated. Boron is added to the water due to its ability to reduce the reactivity of nuclear fuel, thereby enhancing the overall safety of the plant.

For safety reasons, the water level in the spent fuel pool must be continuously monitored. Following the Fukushima Daiichi accident, additional measures were implemented across the industry to enhance SFP safety. The primary objective of this level measurement is to maintain the water inventory in the SFP and improve the response to loss-of-coolant accidents (LOCA). The instrumentation must comply with stringent nuclear regulations regarding radiation resistance and hold certifications for seismic and accident qualifications. During recent maintenance procedures, the existing level probes were identified as candidates for replacement with state-of-the-art level transmitters.

The nuclear operator selected the POWERFLEX 2200 level transmitter because of KROHNE's extensive experience in developing, supplying, and servicing process instrumentation for nuclear environments. The TDR guided radar has a proven track record in spent fuel pool applications at nuclear power plants.

For this application, the guided radar was provided with an extension between the electronics and the probe (S). It features a single cable probe that is immersed in the borated pool water for level measurement. The probe is installed at the edge of the pool using a DN80 flange connection. Both the probe and flange are located in the nuclear zone, while a coaxial cable connects them to the electronics unit placed in the non-irradiated zone. From there, the readings are transmitted to the control room.

The use of specific materials and the flexible design of the POWERFLEX 2200 allowed the operator to install the converter of the guided radar remotely, up to 100 m (328 ft) away from the probe. This configuration ensures the converter is protected from radiation exposure.

By reliably and accurately measuring the level of borated water in the SFP, the POWERFLEX 2200 ensures that the spent fuel remains fully immersed. This enhances both process safety and occupational safety for the plant operator. The KROHNE device is designed to continue operating and transmitting measurement data even in the event of a loss-of-coolant accident (LOCA). Its design allows for remote operation of the main electronics unit via the coaxial cable, eliminating the need for manual intervention or verification in the nuclear zone. Consequently, the installation fully complies with the safety enhancement and requirements introduced after the Fukushima accident.

Thanks to its compliance with international nuclear standards, such as ASME Section III and RCC-M, and its adherence to seismic and accident qualifications, including IEEE Std 323, IEEE Std 344, and RCC-E, the KROHNE guided radar has established itself as a benchmark device in the nuclear industry. The operator now plans to deploy it across all nuclear power plants.