Recent publications discussing the role of nuclear energy in contributing to carbon emission reduction take different approaches and reach very different conclusions. For the pessimistic approach the nuclear contribution in the year 2050 would be on the unimportant level of 8 EJ/year, whilst the optimistic approach with early introduction of fast breeders sees the nuclear contribution in 2060 on the level 144 EJ/year with massive build-up of breeder reactors in the years 2030-2060, reaching the nuclear capacity of 5372 GW in the year 2060. We do not find the optimistic strategy acceptable from political, safety and technical grounds. We show instead that a technologically more conservative nuclear build-up in the years 2025-2065 with proven conventional reactors using once through fuel cycle without fuel reprocessing could reach 3300 GW on the uranium resources as known in 2008. With this concept nuclear contribution of 94 EJ/year would be reached by 2065, many times more than the pessimistic estimate, while integral CO2 emission savings would be about 500 GtCO2. This shows that essential nuclear contributions is possible without the use of plutonium and fast breeders, technology not ready for climate-critical next 50 years and not acceptable in present political environment.
