Politely, this computer exists because the DoE performs stockpile stewardship.

Less politely, it exists because of The Bombs. Everything else it will do is a hobby. It is specifically designed for whatever compute needs their classified program has.

https://en.wikipedia.org/wiki/Stockpile_stewardship

https://www.energy.gov/nnsa/articles/stockpile-stewardship-a...

All of this capital is being spent because the test ban treaty prohibits the detonation of these weapons, but the US stockpile and its efficacy must be maintained and new weapons must be designed. So how do you know if an old weapon or a new design will work or not?

Some part of the answer is real data + simulations. Lots and lots of simulations. It's why Frontier exists and why the DoE has pursued exascale compute for quite some time.

(as for the real data part, it's why places like the National Ignition Factory exist, and why there's an active experimental program that exists to study dummy warheads in unexpected ways, https://www.youtube.com/watch?v=FYdAT0v4DHs )

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Addendum, re: comments arguing that the NNSA computers are separate etc., the points raised are both true in the specific sense, but not true holistically.

Quotes from the most recent report to congress, published in March 2022 about the Stockpile Stewardship and Management Plan,

Near-Term and Out-Year Mission Goals:
◼ Advance the innovative experimental platforms, diagnostic equipment, and computational capabilities necessary to ensure stockpile safety, security, reliability, and effectiveness:
– Achieve exascale computing by delivering an exascale-capable machine and modernizing the nuclear weapons code base
– Develop an operational enhanced capability (advanced radiography and reactivity measurements) for subcritical experiments
– Quantify the effects of plutonium aging on weapon performance over time
– Assure an enduring, trusted supply of strategic radiation-hardened microsystems

and,

The weapons comprising the U.S. nuclear stockpile are assessed to be safe, reliable, effective, and secure. DOE/NNSA’s scientific infrastructure is currently adequate to support stockpile actions. The DOE/NNSA plans to address near-term gaps in required scientific capabilities by deploying the DOE/NNSA’s first exascale computing platform in fiscal year (FY)2023 and by improving capabilities to conduct subcritical experiments through the Enhanced Capabilities for Subcritical Experiments project by FY 2026.

It seems they're building three of these computers, Frontier is one of them, from the budget requests and a press release,

The budget request for Advanced Simulation and Computing increased to support pursuing new validated integrated design codes and advanced high-performance computing capabilities, including the El Capitan exascale system procurement.

The press release,

Featuring advanced capabilities for modeling, simulation and artificial intelligence (AI), based on Cray’s new Shasta architecture, El Capitan is projected to run national nuclear security applications at more than 50 times the speed of LLNL’s Sequoia system. Depending on the application, El Capitan will run roughly 10 times faster on average than LLNL’s Sierra system, currently the world’s second most powerful supercomputer at 125 petaflops of peak performance. Projected to be at least four times more energy efficient than Sierra, El Capitan is expected to go into production by late 2023, servicing the needs of NNSA’s Tri-Laboratory community: Lawrence Livermore National Laboratory, Los Alamos National Laboratory and Sandia National Laboratories

El Capitan will be DOE’s third exascale-class supercomputer, following Argonne National Laboratory’s "Aurora" and Oak Ridge National Laboratory’s "Frontier" system. All three DOE exascale supercomputers will be built by Cray utilizing their Shasta architecture, Slingshot interconnect and new software platform.

Link to the congressional report, https://www.energy.gov/sites/default/files/2022-03/FY%202022...

Link to the press release, https://www.llnl.gov/news/doennsa-lab-announce-partnership-c...

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It does seem to me that it's an accurate assessment to say that this project exists because of the nuclear weapons research mandate.

They're the same computer design, replicated. Are they using these to test/mature their software architecture? Hoping that many eyes will make bugs shallow? Or, is it something else?

Perhaps it's seeing patterns where there are none, but for me the link between these projects and stockpile stewardship seems to be undeniable.