>The alternatives are better. Nuclear is expensive even before you factor in the decommissioning costs.

Nuclear is very cheap for producing the base load. You also shouldn't compare the price of different energy sources in isolation. If an electricity market goes 100% solar and wind for example, it will get power outages soon. Storing the energy of varying power generators is expensive with current technology.

>Given these developments, why build new nuclear plants?

You can create cheap and clean eletricity.

>If (when?) we can design nuclear plants which are both economical, safe and don't have the externalized decommissioning costs then it would also be foolish not to include them in the mix.

Nuclear plants are economical and safe.