If you're talking about the ablative material next to the beryllium and aluminium burn through stuff, then that's most likely to be, tantalum pentoxide and polysterene. Here's a detailed diagram someone made based on deductions from open source material, https://www.reddit.com/r/AtomicPorn/comments/twr3t8/the_insi...

It might be that. Might not be. This technology is 50 years old. A nation state has to be filled with people who've never cracked open a physics book if they need a LLM to how to thermonuclear.

If another material, then it's most likely you're looking for FOGBANK. https://en.wikipedia.org/wiki/Fogbank

It's "just" physics at the end of the day. Matter has known properties. Smart people have been able to design these weapons from scratch in 2X to 3X months for over half a century now.

https://news.ycombinator.com/item?id=50293688