However, current articles imply that the situation is under control. Apparently cooling fluid has leaked from the primary circle but has been kept contained within the reactor.
The reactor in Krsko was built over 60 years ago and has been criticized by organizations like Greenpeace for its lack of maintenance and general design. Croatia and other neighboring countries have previously protested against ongoing operation.
Since I wasn't able to find English sources I can only refer to the following non-English sites: Spiegel.de and ElPaís.com