sry again for the english, in italian we call it roughtly translated "fusion of the core" in english is nuclear meltdown. Anyway there is such risk. http://en.wikipedia.org/wiki/Nuclear_meltdown#Effects
There is a risk, yes, but every plant has safety features to prevent a meltdown. Chernobyl was so bad because it was an old, badly build reactor.
"Nuclear reactors can fail in a variety of ways. Should the instability of the nuclear material generate unexpected behavior, it may result in an uncontrolled power excursion. Normally, the cooling system in a reactor is designed to be able to handle the excess heat this causes; however, should the reactor also experience a loss-of-coolant accident, then the fuel may melt or cause the vessel it is contained in to overheat and melt. This event is called a nuclear meltdown.
Because the heat generated can be tremendous, immense pressure can build up in the reactor vessel, resulting in a steam explosion, which happened at Chernobyl. However, the reactor design used at Chernobyl was unique in many ways. It utilized a positive void coefficient, meaning a cooling failure caused reactor power to rapidly escalate. All reactors built outside the former Soviet Union have had negative void coefficients, a passively safe design. More importantly though, the Chernobyl plant lacked a containment structure. Western reactors have this structure, which acts to contain radiation in the event of a failure.
Containment structures are, by design, some of the strongest structures built by mankind."
In other words, for the most part, nuclear power is perfectly safe.