Am I wrong here? I thought 238 was weapons-grade uranium? I thought reactors used 235, I'm not suggesting, I'm literally asking because I don't know.
Not hardly.
Weapons-grade nuclear material - Wikipedia
"Standard weapon grade plutonium requires a
Pu-240 content of no more than 6.5%." which is 65,000 PPM, and the analogous
Pu-238 was produced in levels of 0.5% (5000 PPM) or less).
Weapons grade plutonium is made in research reactors by exposing U238, removing it, waiting for the U239 to turn to Pu239, separating it, rinse-lather-repeat, since it isn't practical to separate Pu238, Pu239, and Pu240.
Pu 239 has a half-life of 24,110 years.
You want a long half-life so the pit doesn't spontaneously detonate during the compression phase, but only when triggered at peak compression by what is effectively a tiny atom smasher (a neutron source). Also the short half-life elements would make the warhead too radioactive to maintain and unreliable.
A primary only detonates for about 60 ns. The energy released starts driving it apart at that point and you want maximum efficiency.
The parts of the pit are accelerated to 200 km/s (almost 1/1000 of the speed of light) in about 2 meters.
The secondary is a short distance from the primary so the prompt and xray radiation compress it and the neutron radiation fuses it before it gets hit by pieces of the primary.
As a side note - the pressure (mostly radiation pressure) compresses the secondary 1:10 in 3 dimensions (a secondary the size of a soccer ball gets compressed to the size of a golf ball).
50,000 times the pressure of the RDX/HMX used for the primary.