I like the Mutant Future rules on Radiation or the most part. The chart is relatively simple, Radiation causes 1d6 points of damage for each class level of radiation present and saving throws reduce damage by half. However, when it comes to mutant gaining more mutations the rules seem a bit simple and draconian. If the mutant fails five saving throws in one day (regardless of the class level of the radiation) it gains a new mutation. Also, radiation is generally given in two types, high intensity where damage occurs per round and background which occurs daily. With the rules as given in the rule book it will be impossible for a character to gain a mutation from background radiation.
In the first two editions of Gamma World, the effect of the radiation was determined by the character’s Constitution score and the Radiation levels, either causing damage, mutations, or death based on those two variables. Also in the first two editions there are two levels of radiation, high level where damage or effects occur per action turn (round) or background radiation which accumulates during the day.
In the original fourth edition of Gamma World radiation was measured in points which was designated as its intensity and the character’s body stored these points, losing one point per day. Radiation could cause damage, sickness, mutations, and death and this was determined by the intensity of the radiation versus the character’s Health (derived from Con bonus and 10) to determine the effect of the radiation on the character. Radiation was classified as high-level, low-level, and background which determine the effect of the radiation over time (once per round, once per hour, and once per day). There is actually more mechanics involved with radiation checks in fourth edition and it was possible for the effects of the radiation to be worse on consecutive days even though the overall intensity stored by the character was being reduced by one point per day.
So, Mutant Future is basically flat where characters have to fail a radiation saving throw five times within one day to gain a mutation while first and second edition Gamma World was based solely on the character’s Constitution score and the radiation level, while fourth edition was a hyrid mechanic that was probably more realistic in play be a lot more complex and trying for both the GM and the player.
I would suggest a system where the character track the number of rads his character retains from each exposure and if the exposure goes beyond his Constitution score he must make a saving throw each day versus Radiation or gain a mutation (if a mutant animal, plan, or humanoid) though a mutant may only gain one mutation per week. In addition, the character loses one rad point per day until it reaches zero. Rads can be absorbed per round, per hour, or per day based on the overall intensity (class) of the radiation. If the character suffers background-radiation through out an entire day, the character does not lose a rad point but instead gains rads based on the background-radiation.
The character would still take damage from the radiation (half for successful saving throws) but the character would gain rads based on the radiation class (regardless of saving throw). If the character gains more rads than his Constitution score he gains radiation sickness and suffers -4 for attack rolls, only heals half as many hit points as normal, and only moves at half his effective rate. If the character gains as many rads as twice his Constitution he must make a saving throw each day or he will die. If a character gains as many rads as three times his Constitution score he automatically dies (no saving throw allowed).
If tracking rads for a character seems like too much work a simpler method would require a character to simply track radiation class during the day and if the rads exceeds his Constitution score he would make a saving throw versus Radiation or gain a mutation.