“Any nuclear reactions would immediately cease.”
That’s the beauty of fusion versus fission.
But a superheated plasma of millions of degrees Farenheit is still a concern. It will light the surrounding atmosphere, the air will catch fire.
I think that’s still an acceptable risk but there is no indication of how expansive such a ball of fire would be. If it is several miles wide, it is not gonna work.
I'm not sure what you mean by "catch fire." Do you mean burn (oxidize), in the chemical sense?