Assuming it hasn't changed added radioactivity is something like this. This is from my tweaks mod.
--Makes Plutonium and Breeder cells plus stacked versions radioactive for K2
--script.on_init(function()
-- if game.active_mods["Nuclear Fuel"] then
-- remote.call("kr-radioactivity", "add_item", "plutonium")
-- remote.call("kr-radioactivity", "add_item", "breeder-fuel-cell")
-- remote.call("kr-radioactivity", "add_item", "used-up-breeder-fuel-cell")
--
-- end
--end)
--script.on_configuration_changed(function()
-- if game.active_mods["Nuclear Fuel"] then
-- remote.call("kr-radioactivity", "add_item", "plutonium")
-- remote.call("kr-radioactivity", "add_item", "breeder-fuel-cell")
-- remote.call("kr-radioactivity", "add_item", "used-up-breeder-fuel-cell")
--
-- end
--end)