[gvb]

Speculations on the Nenoksa Explosion[1] is interesting. I'm not qualified to judge its accuracy.

The author's opinion is that it had to have been U-235:

"When you are concerned about weight, as a cruise missile designer must be, the places to look in these designs would be the moderator and reflector for the reactor. Highly (above 90%) enriched uranium is the only possible fuel; plutonium is too hard to handle, and lesser enrichments add too much weight. The moderators in the Tory and Rover reactors also served as structural elements."

[1] https://nucleardiner.wordpress.com/2019/08/11/speculations-o...