[en] Spent fuel reprocessing operations at UP1 ceased at the end of 1997, and the clean-up and dismantling programme began soon thereafter. In this paper, the criticality safety cases which have been implemented and the lessons learnt during the first years of the clean-up and dismantling operations are presented, including: -) An overview of the UP1 reprocessing plant: UP1 operation and history, and criticality safety principles during reprocessing; -) An overview of the clean-up and dismantling operations which have been carried out: rinsing the chemical process sectors, dismantling operations; -) An overview of the main criticality safety cases and rules implemented during decommissioning: general principles, criticality control modes, reference fissile media, safety limits. Some examples of lessons learnt and practical considerations regarding the implementation of criticality control will also be presented. The methods used to measure or estimate the remaining masses of fissile material in some vessels: Some of the methods which have been used are presented, such as sampling the deposit in order to estimate its Plutonium content, measuring the volume of deposit, or using in-situ gamma spectrometry. Concerning continuous fuel dissolution facility (MAR 200): in some of this facility's vessels, the total Plutonium masses remaining in deposits with a low percentage of Plutonium content required the use of neutron poisoning. Concerning plutonium processing facilities: in these facilities, some lessons learned from the rinsing and dismantling operations are presented. (authors)

[en] The PHENIX Life Extension Project groups together all the actions required to pursue operation of the reactor, particularly with a view to performing irradiation experiments in the framework of the back-end of the fuel cycle programs. As such, it comprises a series of investigations whose objective is to assess the state of the reactor after about one hundred thousand hours of operation. The following points have been particularly investigated: the materials behavior (austenitic, austeno-ferritic, and ferritic steels base metal, welds, heat affected zones) in terms of thermal aging and its effect on mechanical properties, embrittlement, sensitiveness to corrosion (in normal and incidental environment), and radiation effect on the potentially exposed structures. Furthermore, specific programs have been devoted to the assessment of thermo-mechanical response of some particular components. This concerns some types of welds with regard to fatigue or creep fatigue, some parts of large shells with regard to ratchetting and buckling, and main secondary piping. An extensive program was dedicated to the recovery of the thermo-mechanical damage undergone by the structures and its extrapolation to the future. This has led to consider in details thermo-hydraulic effects such as fluctuations in streams and bedding zones. Some intergranular cracking of welded joints had to be closely examined; this was achieved by a research work that has produced important advancements in that field. With the aim of evaluating potential defects, real progresses have been made in the knowledge of large defect's behavior in thin shells. The feedback of the examination and studies was also derived in terms of relevance of manufacturing, exploitation and monitoring conditions. It is believed that this experience will be useful for future design rules. (author)