Pat Flannery wrote:

You heard it here first- a gun assembly nuclear device employing
plutonium, not uranium, as its fissile material.
That was supposed to be impossible due to a unstable isotope of
plutonium (Pu-240) that would cause it to pre-react if gun assembly was
used, and which was supposed to not be separable from the Pu-239.
Well, either the Orion team made a major slip in their report, or by
1964 we knew how to separate Pu-239 and Pu-240.

.... or maybe that pre-reacting is perfectly acceptable
in the low yield situation here?

Paul