Radiation is capped at 5.0 at the moment and it has a direct affect on a nations environment. Your environment rating affects both population size and happiness.
The environment rating is pretty complex and is affected by the following:
Imported coal, oil, and uranium resources
Radiation cleanup bonus resource
Infrastructure to land ratio
Military to population ratio
Access to water
Government type (+ for Capitalist, Democracy, and Republic. - for Anarchy, Communist, Dictatorship, and Transitional)
Responses to the drug and immigration government policy questions
Owning nuclear weapons
Population is also very complex as it's affected by environment, resources, govt. position, improvements, wonders, etc.
Vince specific advice is hard to give because I can't run the numbers for every alternative but changing from Monarchy to Democracy could boost environment at the cost of the 5% discount in infra purchase, or to Republic for the cost of 1 point in happiness.
A useful tip for everyone, increase land to greater that 50% of your infrastructure for +1 to environment.
Hope that helps.