Probably an app running wild. Radiation...no, I don't think so. Perhaps a factory reset without google back up selected is in order.
Once you get the notification that the update is available or has already downloaded, BUT has yet to install:
1. Stay plugged into charger for the entire process
2. Charge to 100% and back up music, photos, files etc...contacts should be saved to Google Account and use Google Drive or Google Photos to back up with as well and apps are saved in a list in your My Apps screen in the Play Store app and online using the desktop page.
3. DISABLE Google backup and restore of apps and app data in your Backup and Reset tab in your phone's main settings screen.
4. Factory Reset the phone
5. Download and/or Install update from notification bar (may have to manually check for update in About Phone/System Update to kick start it) at next boot/power on after skipping set up if possible
6. Phone will reboot after installation. Factory reset one more time as soon as possible before setting up the phone (you may have to press the back button a few times to escape the set up screen, or you may be able to skip it. Just go back to the Backup and Reset tab and FDR again.
7. Unplug from charger at 100% recharged
8. Set up your phone however you like to include apps from your My Apps screen in either the Play Store App or from the PC webpage version
9. Leave phone on until it powers off
10. Leave off and recharge to 100%
This is my usual process and I have had no issues.
The reasons for disabling app data is due to the fact that DAVLIK is now ART and the Android Optimization process might not go as planned if you just slap the new OS over the old OS. Also, when you go back to the Play Store to download apps, they will be the correct version this time for this OS.