"There are, or were, three chief sources of fissionable ores," Harkaman said.
The fissionable material was too valuable for other purposes.
Do you know why that planet was so extensively colonized by the Federation, when there don't seem to be any fissionable ores?
Both uranium 235 and plutonium are fissionable and can be used to produce an atomic explosion.
What makes the whole thing terrible is that the freighter is loaded with fissionable material exported from Ganymede.
The neutron source, the neutron reflector, and one piece of fissionable material were in place.