[en] Full text: According to the law for nuclear phase out in Belgium, the third unit of the Doel NPP (referred to as “Doel 3”) was permanently shut down in September 2022 after 40 years of operation. Doel 3 entered in so-called Post Operational Phase, during which the licensee prepares notably its safe dismantling. Among these preparation activities, the licensee performed a chemical Full System Decontamination (FSD) of the Doel 3 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 Bel V, notably because it was the first FSD on a PWR unit in Belgium. The current paper will introduce the FSD performed by the licensee, present the approval process applied by the regulatory body, and the associated safety assessment performed by Bel V. This assessment consisted of several pillars, i.e., the justification of the FSD from the ALARA point of view, the safe and sustainable management of the generated radioactive waste (on the short and longer terms) and the management of radiation protection and hazards during the FSD. This assessment required interrelationships between several stakeholders, for instance with the waste management organization for the aspects related to the management of the generated radioactive waste. The main lessons learned for future FSD projects will be presented.