Chernobyl was a steam explosion. When you use water - liquid water, that is - to cool a reactor you run the risk of having a steam explosion if the nuclear reaction runs away. The beauty of the pebble bed is that it uses helium as a coolant. Since helium is already a gas, it can't boil, build up pressure and explode the way Chernobyl did.
As for the graphite catching fire, you need oxygen for something to burn. Even if the graphite in the reactor did catch fire, all they'd have to do is shut all the windows, and presto! No more fire.
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Chernobyl was a steam explosion. When you use water - liquid water, that is - to cool a reactor you run the risk of having a steam explosion if the nuclear reaction runs away. The beauty of the pebble bed is that it uses helium as a coolant. Since helium is already a gas, it can't boil, build up pressure and explode the way Chernobyl did. As for the graphite catching fire, you need oxygen for something to burn. Even if the graphite in the reactor did catch fire, all they'd have to do is shut all the windows, and presto! No more fire.