Colin is writing a story in which a spy damages a fusion nuclear reactor, the reaction runs out of control, and the reactor explodes like a fusion bomb. why is his idea for the story unlikely to be accurate?

Because even though fission and fusion reactions both produce humongous amounts of energy, they're completely opposite in an important way.

A fission reaction, you might say, is teetering "on the edge".  Once you get it going, then as long as there's some fuel lying around, the reaction keeps going on its own, and can run out of control.  This is the reason why fission reactions are so desirable for commercial power generation.  The reaction keeps itself going, and all WE have to do is keep it supplied with fuel, and keep it SLOWED DOWN so it doesn't run out of control.

A fusion reaction is exactly opposite.  It needs extreme temperature and pressure to keep going.  If either the temperature or the pressure falls off, the fusion reaction dies out.  This is a big part of the reason why we're not at the point yet of being able to use fusion reactions for commercial power generation.  We don't yet have dependable, reliable ways to CONTAIN a fusion reaction, maintaining the temperature and pressure it needs to keep going.

There are NO fusion reactors that run continuously.  If there WAS one, and a spy damaged it, the worst thing he could do might be, let's say, to drill a hole in it or break a piece off of it.  Either way, as soon as the temperature or the pressure in the vessel dropped off, the "fire would go out", and the "fusion reactor" would turn into a big tank of cold hydrogen and helium.  If the spy stuck around and set fire to it, then Big Deal !  The hydrogen would burn in air and produce ... water !  The helium would do exactly nothing, because helium is totally non-reactive.  It would just float out of the tank, through the hole, up and away into the sky.  The reaction would just totally stop, and the whole event would be a big bunch of nothing.