The steam lines at Chernobyl were operated beyond their design limits and ruptured. The force of the steam eruption battered the reactor and damaged it, and spent fuel was pulverized, carried up with the steam. It was the coolant, which was graphite, that caught fire and sent up smoke.
Chernobyl was a Light Water Reactor...it used water as coolant and moderator. The fire was the result of the lack of coolant...the fuel got so hot because there was no water to remove the heat...it burned.