Perhaps the best time to build one is 20 years from now with the hope that by then someone else has sorted out how to do it without exceeding an already enormous budget by a factor of four as with Olkiluoto (so far, it's still in trials that it failed on the first attempt). Plus, to get out of gas you'd need tons of storage anyway as current nuclear reactors aren't very reactive, that's what gas plants do. But if you do invest in storage and make that work well, might just overbuild renewables and limit nuclear reactors to specific niches.