FALSE

The claim inaccurately describes the mechanics of a thermonuclear weapon. In the standard Teller-Ulam design, the fission reaction occurs in the primary (first) stage, while the second stage (secondary) is where nuclear fusion takes place. Although the energy from the primary stage is released as X-rays that travel at the speed of light, these rays are directed by a radiation case, or hohlraum, made of high-atomic-number (high-Z) materials such as uranium, lead, or tungsten. These dense materials are necessary to reflect and contain the X-ray flux; beryllium is a low-atomic-number (low-Z) metal that is relatively transparent to X-rays and would fail to reflect them. Beryllium is instead used as a neutron reflector surrounding the plutonium pit within the primary stage. This specific phrasing appears to be a verbatim quote from a technically flawed popular science video titled "The Imitator of God," which conflates the role of neutron reflectors with the X-ray-reflecting radiation case.