Radiation is natural and found everywhere – it comes from outer space, it's in the air, and on the earth. It's even in the human body; naturally occurring radioactive elements in the bones irradiate on average 50000 times per second in this world. In fact, most radiation is natural and life on this planet wouldn't be possible without it.
Perhaps, the drastic increase in nuclear energy within this city has to do with the death of the millions of people that previously resided in the city and that of the few hundred titans that had died during the defense of the city.
The corpses of several Titans littered the ground external to the city walls gasp from the students as the train shot explosively past the massive lifeless bodies. The train suddenly connected to a railroad, moving along it which then lead to an underground train station.