But wouldn’t that irradiate certain objects in the area, any ferrous metal for example? And there are always certain amounts of the U235 or Pu that do not fission, yes?
Also, such a device is pretty far down the development chain.
Tested in 1973-1975 by US govt. Probably used in Kosovo-Serbia by US jets to knock out tank crews but leave the tank intact by bombarding background secondary radiation with soft neutrons. Even the Ruskies have it. Guess where they are testing it?