Monitoring system of the fuel-Casette-free STATE of the control rod sleeves on the PAKS NPP.

The article focuses on the recalibration and operation of the monitoring system used to detect fuel cassette adhesion to control rod sleeves (CRS) during refueling of the VVER-440-213 type reactors at the Paks Nuclear Power Plant in Hungary. The system, originally calibrated for a 12-month fuel cycle in 1999, was recalibrated in 2018 to accommodate a new 15-month fuel cycle and comprises dose-rate meters and high-purity germanium (HPGe) gamma spectrometers that measure radiation near the reactor vessel. The monitoring system can reliably detect cassette adhesion events by identifying characteristic increases in radiation, particularly gamma peaks from the isotope ^140La, and can localize the adhesion position within the reactor core with an uncertainty comparable to the size of a single cassette. The combined coverage of the dose-rate meters and gamma spectrometers ensures full monitoring of the active zone, enhancing worker safety by preventing unplanned radiation exposure during CRS lifting.