Didn't the plants all get safely shut down before the Russians got there? If so, they can't cause a meltdown unless they actually try to start the reactors.
Not to say they can't cause something bad to happen, especially since they are storing munitions in there but it wouldn't be a meltdown. I would assume fuel material wouldn't get thrown as far/the core wouldn't be compromised but that would take someone more knowledgable than me on this.