R4 nuclear reactor was a nuclear reactor built at Marviken and the fourth nuclear reactor built in Sweden. It was heavy water moderated and intended for the dual role of 130MWe of power generation and also plutonium production. It had a central role in the Swedish nuclear weapon programme. During the mid 1960s the social democratic government officially abandoned the project of designing Swedish nuclear weapons and the Marviken-plant became derelict. It was never loaded with fuel, and the project was cancelled in 1970.