 R1, R3, and particularly the never finished R4 project at Marviken were heavy water reactors, motivated by the option to use Swedish uranium without isotope enrichment and by the possibility to use the reactors to produce weapons grade plutonium for Swedish nuclear warheads.The Swedish nuclear weapons program was eventually shut down, however, and Sweden signed the nuclear non-proliferation treaty in 1968.Six nuclear reactors began commercial service in the 1970s, another six through 1985.