I have been poking at the idea of an eventual re-emergence of the PACER concept, where energy is obtained by detonating nuclear explosions... but not exactly in the same form. After humans become space-faring, the main technical and economic limitations of PACER (related to managing a large containment chamber, or massive underground cavities) are lifted, because there exists a large industrial base in microgravity. In such a world, building sufficient containment and radiation management becomes essentially trivial with scaling up the yield of the explosions. Nuclear explosions, for the record, have tremendous scaling ability. Twice the yield does not require twice the cost. Because of this, PACER eventually (I can not say exactly when) becomes outrageously cheap energy compared to all alternatives.
While this establishes the technical and economic argument, I am completely empty-handed regarding what social, political, and historical (accidental) factors could enable an organization to re-embark on program for nuclear explosives. Considering the shifts that humanity will undergo as we become interplanetary, what changes would be important for the re-enablement of PACER-type projects?
Making vague references to "technical" elements, I left out an important detail of how I picture these nuclear explosions being used. In order to extract the heat to run engines, it must still occur in a closed space. This could be satisfied by a pressure vessel in microgravity. This pressure vessel, if built in microgravity, could be miles across, instead of a few 100 of meters, as nuclear containment structures are necessarily limited to on Earth. That does incur an additional construction cost. Chambers could also be built below the surface of a planet, moon, or asteroid. On the moon, with 1/6th the gravity, the chamber could be built approximately 6 times as large as on Earth, with possibly 6^3=216 scale up in yield. These are small potatoes. Ceres and Vesta are large, but sufficiently small such that robotic equipment can have pressure tolerance to operate in its very center. Sealing is needed in these cases, but no rigid vessel. The upper limit on yield you could detonate (saving more and more money) would be mind-numbingly large. A Ceres society could extract energy efficiently from the largest, continent-destroying, bomb that have very rarely even been discussed in human history.