Too much fuel would estinguish the fire, similar to too much fuel in a car engine will cause it to stall. The high temperatures necessary for fusion are only a

Too much fuel would estinguish the fire, similar to too much fuel in a car engine will cause it to stall. The high temperatures necessary for fusion are only attainable at low masses, if too much fuel enters the reactor, it cools down and becomes inert. The fuel itself is not radioactive and safe for consumption in reasonable amounts (heavy water inhibits biological processes if it becomes too much in concentration, but unless you drink straight from the heavy water tanks for days, this is essentially not an issue)