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Abstract The Leibstadt nuclear power plant (KKL) in Switzerland was originally commissioned in 1984 with a nominal power output of 1000 MWel. Since then, there have been several uprates of the boiling water reactor (BWR) which have increased the power to 1275 MWel. After the nearly four decades of operation, the original condenser was impacted by advanced erosion corrosion and by progressive performance deterioration. In addition, the thermal power uprates required load reduction during hot summer periods to avoid high condenser back pressure. Consequently, KKL asked GE to carry out a comprehensive retrofit study to identify potential improvements on both lifetime and performance. These included: robust material selection considering cycle chemistry and cobalt limitations; optimized extraction of non-condensable gases by means of a bespoke air cooler section of the condenser; and applying GE Steam Power’s latest innovative tube pattern design which is aligned to BWR requirements and simultaneously optimizes the condensation area together with the steam flow paths. However, site constraints prevented the use of a typical modular condenser retrofit as both the local access in the machine house was very restricted and modifications of the concrete walls of the radiation shielding had to be avoided. Hence, working in a close cooperation between KKL and GE Steam Power, a customized retrofit concept was developed which used semi-prefabricated skeletal modules and an on-site rebuild of the condenser. This paper provides details of the solution, particularly of the improvements which are specific to condensers operating in a BWR plant. Also described is the preparatory and complementary refurbishment work which was executed at the system interfaces. This includes replacement of strongly eroded condenser neck parts, an external reinforcement of the condenser walls, and modification of the continuous tube cleaning system. The construction site work itself is also described. This includes the required safety measures, the complex work logistics, the development of specific installation tools for handling and turning the skeletal modules, and the compliance with treatment rules for contaminated material. In total, approximately 475 tons of old condenser material were dismantled, 24 new semi-prefabricated skeletal modules were installed, a condenser with 51’600 tubes was rebuilt on-site and 8 waterboxes were replaced with an improved design made of stainless steel. More than 350 professionals contributed to the successful retrofit with an overall site effort of 270’000 working hours — all without any major accident. Key success factors for this retrofit have been the close and efficient cooperation between an experienced and knowledgeable customer with an OEM supplier bringing their expertise in engineering and construction during all project phases, and the very detailed planning and preparation phase.