Are they trying to model every single atom?
Is this a case where the physicists in charge get away with programming the most inefficient models possible and then the administration simply replies "oh I guess we'll need a bigger supercomputer"
Given all nuclear physics happens inside atoms, I'd hope they're being more precise.
Note that a frontier of fusion physics is characterising plasma flows. So even at the atom-by-atom level, we're nowhere close to a solved problem.
What are you basing this on? Plasmas don't flow like gases even absent a magnetic field. They're self interacting, even in supersonic modes. This is like saying you can just model gases like liquids when trying to describe a plane--they're different states of matter.