It doesn't matter, as the only infrastructure available for refining fissile isotopes is on Earth. Even if you found appropriate ores directly adjacent to the reactor site, they would still have to be shipped back to Earth to turn them into reactor fuel.
You need fully mature steel, aluminum, and electronics industries on Mars before you can think about UF6 centrifuges. The rocket equation just murders any idea anyone might have about shipping any kind of factory-in-a-box to any other rock in the Solar system--except one that takes in the local regolith and produces a copy of itself, or a similar factory-in-a-box with a different combination of inputs and outputs.
You're trying to jump right from putting the ore in at one end, and getting reactor fuel out at the other, but you apparently need at least a North Korea-sized economy inside the black box to do that, and Mars currently has an economy smaller than Sealand. There are no humans on Mars, and the robots currently there don't mine or farm anything in excess of their own immediate needs.
NASA's Kilopower uses sodium coolant, so it's a fast reactor and gets much more energy out of its fuel. It needs enough U235 to kick off the reaction but after that it will fission the U238 as well.
Burnup could still be limited due to fission products poisoning the reaction.
I'm pretty sure that conventional reactor split around as much fuel as the U-235 content.
For conventional reactors it is around 3% and you are left with around 3% fission products.
Natural uranium has around 0.7% U-235. But I0m pretty sure that you would enrich the uranium befor you send it to mars. The 16TJ/kg is for enriched uranium with 20% U-235 content.
Anyway, energy density of nuclear fuel is extremly high. Even if you pay 10000 USD/kg for the transport of fuel to mars, you'd pay around 1 cent/kwh for the transport.
