Spain's seven operating nuclear plants currently provide more than 20% of its electricity. Each of these began operation in the 1980s and is approaching the end of its 40-year design life. Extending their lives will require additional investments. Should Spain make the investment and extend their lives, or should they be retired at the end of their design life? We show that investing in nuclear plant life extensions is the least-cost alternative for further reducing GHG emissions. We also show that in assessing the cost of renewable alternatives it is critical to take into account the time profile of the available renewable resource. Solar PV and especially wind capacity were expanded significantly since 2000, and significantly greater penetration, especially of solar PV, is promised out to 2030 in order to reduce GHG emissions still further. We show that at these expanded penetration levels, curtailment becomes a significant determinant of system cost. This significantly improves the relative value of nuclear life extensions as a contributor to reducing GHG emissions.