Earlier quoted context omitted.
What is the specific power of a nuclear reactor + heat engine + crew shielding + radiators? Naval reactors are on the order of a few watts to tens of watts per kg and they have the benefit of unlimited cooling. Seems an order of magnitude short of PV.
Possibly north of 200 W(elec.)/kg [0]. (I can't assess this stuff personally, but it's some MIT person's PhD thesis, so I assume some baseline credibility). Key distinction: you're looking at a conservative kilowatt-scale proposal, one which isn't intended for propulsion. But propulsion-optimized designs are on the megawatt scale: and the mass/power scaling is sub-linear. [0] https://dspace.mit.edu/handle/1721.1/6755…
A mylar mirror is extremely cheap and light (kW or tens of kW of heat can be trivially concentrated per kg), so the fact that there are no even semi-serious proposals for space based CSP should be fairly telling as to the viabity.
At 200W/kg, a hypothetical fission generator is strictly worse than a current generation solar array in the inner planets for space based acceleration.
Seems useful outside mars though.