Not sure why this is being downvoted jettison the core would likely be the go-to solution you can put into a stable orbit or even set on course with the sun and

Not sure why this is being downvoted jettison the core would likely be the go-to solution you can put into a stable orbit or even set on course with the sun and let it burn off... There is no much need for a complex control mechanism in fact keeping those core as orbital modules on mars would likely allow you to skip many of the complications of having a complex control system to prevent a run away effect and modulate power output no one really cares if the core would go critical even at low orbit on Mars as there is so much radiation there from natural sources anyhow. If the core goes critical mid way then either have a fallback core or just jettison it and wait for a rescue mission. The vessels would likely have enough supplies for such occasions and as far as risk to the crew goes nuclear propulsion in space is much less dangerous than chemical propellants that can actually explode. With a nuclear core you can safely vent hydrogen and xenon to space not worrying that much about radiation or long term contamination the cores can be more or less exposed with large fin stacks for radiative cooling.