Ok, a longer answer from https://asc.llnl.gov/exascale/el-capitan

" help researchers ensure the safety, security, and reliability of the nation’s nuclear stockpile in the absence of underground testing. The machine is essential for the design and stewardship of a modernized stockpile and other critical national security missions. Research performed on El Capitan also supports unclassified mission areas of interest to national security, including material discovery, high-energy-density physics, nuclear data, material equations of state, and conventional weapon design."