[rcxdude]

>Are they trying to model every single atom?

Modelling a single nucleus, even one much lighter weight than uranium, is a captital-H Hard Problem involving many subject matter experts and a lot of optimisation work far beyond 'just throw it on a GPU'. Quantum systems get non-tractable without very clever approximations and a lot of compute very quickly, and quantum chromodynamics is by far the worst at this. Look up lattice QCD for a relevant keyword.