Student or Learner
For years, he had been handling six crises of international meltdown range at once.
Does this "meltdown" have got any special meaning here?
To me it just means that for years he had to cope with 6 crises of international range at once, that's all.
It's about a special CIA agent.
Some more context, though I don't know whether it will help:
Now his official home was a specialized piece of that agency that John Q. Public had never heard of and never would. He had two sets of creds: one for the public that showed him to be with Homeland Security and was suitably intimidating; and another set that he showed only to certain fellow federal agents. That evidenced his association with OSM, the Office of Special Matters. It was made up of personnel from five of the major intelligence agencies, though it was controlled by a handful of folks at Langley. "Office of Special Matters" sounded a little hokey, Knox thought, but what they did was far from it. Knox had been up to his ass in "special matters" for years, often handling six crises of international meltdown range at once.
I would hazard a guess that the author of your book was not Jane Austen.
It appears to mean something like: six crises that were so serious that they might have brought about a new world war.