Capacity isn't the problem, usage is. If Europe reduced its usage to about 10% of current usage, full tanks would last a full year.

Nuclear is unsuited for this problem. Europe needs something that will supply approximately all of its power needs for only a few days a year. Europe already has enough natural gas storage to do this. If you built enough nuclear to cover all its power needs for a few days a year you'd have enough nuclear to supply all its power needs 365 days a year. That would work great, but it would cost tens of trillions.