And if in 12 years those photovoltaics somehow haven't replaced coal - maybe the efficiency improvements stop, maybe there's issues with the supply of rare earths, maybe - then we'll be in a fine fix won't we.

Nuclear isn't the only thing that tends to take longer than planned. I would confidently bet that we'll still be running coal in 12 years' time. Certainly I think it's worth hedging against when the cost is building what, a few dozen power plants that follow existing designs?