El Capitan: New supercomputer is the fastest

I fail to understand how these nuclear bomb simulations require so much compute power.

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"

Well there's a fair bit of chemistry related to the explosions to bring the sub-critical bits together. Time scales are in the nanosecond range. Then as the subcritical bits get closer obviously the nuclear effects start to dominate. Things like berrylium are used to reflect and intensive the chain reaction. All of that is basically just a starter for the fusion reaction. That often involved uranium, lithium deturide, and more plutonium.

So it involves very small time scales, chemistry, fission, fusion, creating and channeling plasmas, high neutron fluxes, extremely high pressures, and of course the exponential release of amazing amounts of energy as matter is literally converted to energy and temperatures exceeding those in the sun.

Then add to all of that is the reality of aging. Explosives can degrade, the structure can weaken (age and radiation), radioactive materials have half lives, etc. What should the replacement rate be? What kind of maintenance would lengthen the useful lives of the weapons? What fraction of the arsenal should work at any given time? How will vibration during delivery impact the above?

Seems like plenty to keep a supercomputer busy.