[Elv13]

I am pretty sure material science went a long way. No need to put a foot of lead behind the reactor. The space race provided quite a bit of funding toward lightweight protection. It was too late for those plane project as they were already ramping down in the mid 60's.

Neither that or the plane powerplant are unsolavle problem, it's more of a "why would we invest this much to solve something this silly". Apparently the russians are trying to resurrect their SLAM clone and tested one. How much of this is a realistic military project versus propaganda isn't super clear to me. It's up there along with the manned military space stations in the scale of pointless deterrent PR.