Question: Mana is the substance that makes a human capable of performing spells. In the magitocracy, mages rule the kingdom. An individual capable of magic must learn to focus his power, solidifying and controlling the Mana in order to use it. Welding Mana in this way takes years of study. Mages are of various power levels, those who master their Mana welding are among the strongest mages in society.

However, there are areas of the world in which Mana roams free and unbound by humans. This mana is unrestrained and leads to these areas being high in radiation. This could be deadly to those in the vicinity, as it can have debilitating effects, mutations, and even death.

All humans contain Mana, whether they learn to harness it or not. However, Wild Mana remains dangerous to people. How could this be the case?

Answer: 
It causes elements to transmute into other forms.

Turning lead into gold is a classic pursuit of magic, and so is turning carbon to silicon (through petrification), but both of these do not change the number of nucleons in the atom -- they change whether they are protons or neutrons.

Very little change is needed to make radically unstable atoms. And the wild magic, not being constrained by a mage, tends to be rather more random in its changes. (Iron into sodium? Why not? Which means that it's not only radioactive but before it decays, it's explosive in the presence of water.)

(Poul Anderson made use of the "silicon" idea.)
Mages are not born with magic. Mages are those born with the ability to either negate the radiation or be immune to it. Thus wield mana and no die.

Variations exist. Maybe mages are those evolved to produce a compound, or hormone or whatever, that allows them to handle the radiation.

Maybe it is radioactive because mana is a sort of sentient create or ability that only works if in a proper humans host.

Magic heals radiation, the older wizards died quickly.
Actually this ties in with the whole it takes a long time to master.
The earliest wizards died real quick. New generations were careful, they were focused on expanding their power but also protecting themselves from the radiation.
That's why it takes years. You slowly have to develop the magic to ward of the radiation.

Heck. You can have it that wizards even use pure radiation as an attack, or if they die, under certain circumstances, they have a little explosion.

Thanks god our earlier ancestors found out the Tianga plant when they did.
This little plant is almost magic. It combats radiation in all it's forms and allows people with even the most sever case of radiation poisoning to survive and heal.

Guess who has a monopoly on the thing?