The Klystron is not a secret. It’s design is available everywhere. The neutron source for the grid is hard for non-nuclear countries to maintain in large quantities. I’ve always thought it pretty cool that we still (maybe) use vacuum tubes to ignite the low explosives in thermonuclear warheads. Kind of like how tube amps are preferred for good amplification and effects.
Used to be a secret until B Clinton gave it to the Chinese as a good will gesture in the 90s for more ‘campaign cash’. The point is without one small thermonukes are not possible - as far as I know.