One of the key words in your question is "IF".
One can postulate a scenario like you phrase above; however when you do that, you
are totally discounting all the measures taken to prevent the scenario.
As Astronuc explained, there are design features of nuclear power plants designed
to stop the scenario you envision.
One of my Professors tells the story that he gets asked that type of question all the
time; what if the core melts, and the containment fails, and...
He replies "What if two 747 airliners were to collide over Pasedena, and they crashed
into the Rose Bowl stadium, and it happened to be New Years Day while the
Rose Bowl game was being played with tens of thousands of people in attendance,
and the flaming wreckage of the 747s kill all the people in the stadium..."
At which point the person asking the question usually gets upset and says something
like "Don't be ridiculous - that scenario is totally contrived!!". The Professor would
then reply, "Not as contrived as the one YOU gave!!"
The point being, is that one can dream up strange scenarios - and that's good,
because that tells us what we have to protect against. However, you also have
to be mindful of the the probabilities of that event happening, and the measures
taken to prevent it.
If the probability of the event is so low as to be one-millionth the probability that the
Earth gets clobbered with a big asteroid that wipes out all life on the planet, then you
should spend your time worrying about the asteroid and not the power plant.
Dr. Gregory Greenman