The operators have been paying into a fund since the 80s which is meant for decommissioning and spent fuel storage.

Basically it is a legal requirement (some small fraction of a cent for every kWh produiced)

This is really the only sane way to do it.

Also we got lucky with the ground under Olkiluoto being good spot for nuclear waste storage so not much NIMBY stuff for that as it is already the site for the biggest nuclear power plant in the country. It is also small town so a huge % of the population there work at the plant or its sub contractors.