Reactor grade graphite isn't flammable. The Chernobyl rescue workers reported seeing a red glow when they flew overhead (an activity undertaken for some reason known only to the KGB) and assumed it was the graphite, hence how the myth got started. In fact, the fire at Chernobyl wasn't a graphite fire, it was the fuel that was burning.
Chernobyl was graphite moderated, but it is nothing like the PBMR. The configuration of the reactor is totally different, the coolant is different, the fuel is totally different.