Dismantling of the BR3 Reactor Pressure Vessel

Abstract

The BR3 PWR is a small nuclear power plant (thermal power 40.9 MWth, net electrical power output 10.5 MWe), designed in the late fifties and started in 1962. It was definitely shut down in 1987. In 1989 the BR3 was selected by the European Union as pilot decommissioning project in the framework of its RTD programme on the decommissioning of nuclear installations. A pre-dismantling decontamination of the reactor primary loop was carried out and allowed to save doses to the operators. The savings are estimated to be up to about 4 to 7 man-Sv. The decommissioning project concerns mainly: • The dismantling of the highly radioactive reactor internals. Different techniques were used and compared on a first actual piece called the thermal shield: from plasma arc torch cutting to mechanical sawing, including also electric discharge machining. Based on the experience gained during this part of the project, the mechanical cutting techniques were promoted for the segmentation of both sets of internals, the desolidarisation and the segmentation of the RPV. • For the dismantling of the reactor pressure vessel, wet and dry dismantling were studied and compared. For economical and feasibility reasons, the wet dismantling was selected. Afterwards, two underwater segmentations were also studied: in-situ segmentation and a segmentation after having removed the RPV out of its cavity. • Mainly for technical reasons, the reactor pressure vessel was removed in one piece out of its cavity in order to be cut in the former refuelling pool. The disconnection of the RPV from the other parts of the plant was followed by the reinstallation of the watertightness of the pool in order to allow remote underwater segmentation. The disconnection, the watertightness reinstallation and the segmentation represented important challenges. The subtasks will be extensively described in the paper: disconnection from the pools floor, removal of the thermal insulation from the legs, decoupling from the primary loop at two levels, from its supporting structure, the reinstallation of the watertightness of the pool and testing, the removal of the RPV out of its cavity, the remote dismantling of its surrounding thermal insulation (which led to an annoying pool water turbidity) and, finally the effective RPV dismantling. • For the segmentation, two main cutting equipments were used: the milling cutter for cutting the RPV into rings and the bandsaw machine for cutting each ring into segments. The bandsaw machine was also used in order to cut the RPV upper flange into pieces vertically as well as horizontally. • The last generated pieces, the highest radioactive ones, were evacuated at the end of 2000. • Waste characterisation, minimization and management is an important part of the task in order to reduce evacuation and storage costs. • ALARA approach was applied from the early beginning of the project. • For each “key operation” cold tests were organized in order to optimize the work and to take benefit of the learning effect of such operation. Results of the operations will be presented, the lessons drawn for the technical choices, dose uptake minimization, waste reduction and the technical problems met will be highlighted. As a pioneering project, the dismantling of the BR3 Reactor Pressure Vessel has shown the technical feasibility of such an operation in a safe and economical way as well.