Then the reactor would shut down automatically - without a grid load there is nowhere for the power to go and the reactor has to shutdown or the lines would melt.
This is what happened in Florida a few weeks ago, the power grid failed and 3 reactors shutdown safely- although the TV news told it as 'nuclear plant failure puts state into darkness'.
A fire is going to put a lot of particles of radioactive heavy metals over a large area but this isn't going to devastate it - even a huge explosion like chernobyl didn't damage very much forest. Most of the animals didn't die, main victims were those very close to the explosion and embryos of pregnant animals, within a generation they are pretty much back to normal.
Uranium and even plutonium aren't too much of a danger in the enviroment, they form mostly insoluble compounds it's the highly active short lived compounds that would cause the fire.