According to the law for nuclear phase out in Belgium, the second unit of the Tihange NPP (referred to as “Tihange 2”) was permanently shut down in January 2023 after 40 years of operation. Tihange 2 entered in so-called Post Operational Phase (POP), during which the licensee prepares notably its safe dismantling. Among these preparatory activities, the licensee performed a chemical Full System Decontamination (FSD) of the Tihange 2 primary circuit, including several auxiliary circuits (e.g. the chemical and volume control system). This activity mainly aimed at decreasing the radiation exposure of the workers during the future dismantling activities. The preparation and the realization of the FSD was a challenge for the licensee and regulatory body (RB, composed by FANC and Bel V). This was the second FSD on a PWR unit in Belgium (the first was performed in Doel 3 in 2023). 

From a regulatory perspective, as for the FSD at Doel 3, the FSD at Tihange 2 operation was considered as a temporary modification of the plant and the corresponding existing modification process, agreed by the licensee and the RB, was followed. As at Doel 3, the RB completed this process with relevant hold and witness points, allowing a stepwise implementation of the FSD operation (see Figure 2). The process allowed the development of constructive interactions between the licensee and the RB about the FSD preparation and implementation, as well as building trust between the main stakeholders in the feasibility and the safety of the FSD project. To guide the development of the safety assessment related to this modification process Bel V provided the licensee and its Health Physics Department with several expectations organised in 7 key topics. For instance: the justification of the FSD from the ALARA point of view, the safe management of the generated radioactive waste (on the short and longer terms) or the management of radiation protection and hazards during the FSD). They have helped the licensee to manage important issues for Bel V in a timely manner, compared to the planning of the project. They have facilitated Bel V's oversight of the project as well.

The Tihange 2 FSD project took into account the experience feedback from the previous FSD project at Doel 3, even though the design of Tihange 2 required some adaptations to the FSD process used at Doel 3. For instance, the material of the Steam Generators at Tihange 2 is Inconel ® 690, which is more resistant to oxidation than the material of the Steam Generators at Doel 3 (Inconel ® 800). This required FSD cycles in acidic and basic conditions at Tihange 2, instead of only cycles in acidic conditions at Doel 3.

In addition to its evaluation of the safety assessment performed by the operator, Bel V performed several specific inspections in the installations of Tihange 2 to verify that the FSD was prepared and operated as agreed in the modification process and its corresponding safety assessment. For instance, Bel V checked the preparatory works achieved by the licensee before the FSD implementation. Then, Bel V observed and checked the proper implementation of several measures foreseen in the ALARA study, and in particular the measures taken for the change of radioactive filters during the FSD. During the FSD no safety significant deviation compared to the evaluated safety assessment was noticed by Bel V. The FSD resulted in a mean Decontamination Factor (DF) of the circuits of about 16 (exceeding the initially defined DF target of 10). Globally the volume of radioactive effluents and waste estimated in the safety assessment were conservative compared to the generated volumes and no storage issues occur. The radiological characterisation by the licensee of the generated effluents and waste is ongoing.

The FSD at Tihange 2 can be considered as a success with a higher than expected DF, an acceptable volume of generated radioactive waste and an overall level of safety maintained throughout the operations. The experience feedback gained will obviously be used for the future FSD planned in the coming years in other units of Doel and Tihange NPPs.