FtA: Leveling mechanics

I've been wracking my brain trying to figure out a way of tying experience to the leveling system in such a way that would provide a logical explanation for the growing levels of radiation in the PCs body, leading up to Level 10, where they expire.  I think I may have found a system that works.

The mutations that exist are as a result of an unstable recessive genetic trait, and each time a power is used, it further destabilizes the genetic makeup of the mutant.  As the mutants' DNA deteriorates, more mutations manifest, until they hit critical mass at Level 10.  My thought is that every time a power is used, a box is checked, and at the end of each session, a roll is made, and the number of checks is subtracted from the roll.  If the roll comes in above a target number, the character advances a level and rolls again on the mutation chart.  This penalty stacks, and gradually the PC becomes weaker and weaker, until they hit Level 10 and expire.

I'm also contemplating figuring a way of making skill usage and high stress situations (like combat) factor in as well.  The goal here is to really drive home the idea that actions matter.  I want the PCs to be thinking about what their characters do, putting those actions in the context of their goals, and making cost/benefit analyses.  How important is this task?  Further, by making combat detrimental to their health even if they win, it should force the PCs to consider all options, viewing violence as a last resort.

I think this should add up to a very different gaming experience, one that keeps the PCs thinking about their actions, rather than looking for things to hit.