Damn interesting engineering project
But I dont understand why they would construct it on site. A small nuclear reactor small enough to be transported there in one piece could easily be built. Depends on the power they want from it though. But 10-100MWe should be plenty for a long time. They dont even need any containment structure on mars, just basic radiation shielding.
At site the main problem would be cooling it, no water and very thin atmosphere so no easy place to dump excess heat. Maby they could use nitrogen from the atmosphere as cooling gas. But there isnt very much nitrogen in the air there. They would probably need radiators.