From what I’ve read about pebble bed reactors, it’s almost impossible for them to go critical.
Since "go critical" means "start the chain reaction", I certainly hope that they can do so. What you are probably thinking of is "melt down", as the fuel elements can take higher temperatures without melting.
Unfortunately, pebble bed reactors have other problems.....it has recently found that the fuel elements are not nearly as good at containing the radioactive fission products as was postulated, so are MORE DANGEROUS than current uranium-fueled reactors.