The life cycle GHG emissions per kWh from nuclear power plants are two orders of magnitude lower than those of fossil-fuelled electricity generation and comparable to most renewables (EC, 1995; Krewitt et al., 1999; Brännström-Norberg et al., 1996; Spadaro et al., 2000). Hence it is an effective GHG mitigation option, especially by way of investments in the lifetime extension of existing plants.
We humans can't still have our power and GHG too. Building out nuclear is not ‘effective GHG mitigation’ – unless we reduce our total energy footprint and we reduce our consumption of fossil fuels even more rapidly than that. Because using more energy people say is "better" but it isn't especially if it’s still creating GHG (and still oil-derived and oil-dependent – something that usually is conveniently forgotten). The whole life-cycle of site preparation, construction, maintenance, power production, decommission and disposal of radioactive wastes and what this ‘enables’ for the human community needs to be taken into account. Without reducing consumption and energy use in toto It’s still BAU. It’s not "mitigation" – just more emissions and creation of wastes – which we still haven't properly figured out how to get rid of or handle safely.
But if we can jump these hurdles (i.e., solve these problems and break these bad FF-dependency habits), then we should use nuclear power to replace the equivalent of fossil-fuel-derived power.