Japan loves mascots. There are enough of them out there to fill a book and many, many posts on the subject. Super-cute mascots aren't intended to be childish or condescending here; Japan's animistic, polytheistic roots seem to have given the nation the mutant power to transform nearly any object or concept, no matter how arcane, into a happy little mascot.
Japan's nuclear industry is no exception. Many of the nuclear facilities it built throughout the nation came complete with little museums -- basically, PR facilities -- to help educate and reassure locals as to the safety of the nuclear plants sitting in the middle of their neighborhoods. Looking back from the devastation of March 2011, a lot of these seem almost gross, but it's important to remember there was no special effort to brainwash anyone or specifically target children with these super-cute characters. EVERYTHING has a mascot in Japan. Even the tsunami waves themselves.
That said, I don't think anyone has actually gone to the effort of trying to collect these nuclear-related mascots in one place. Below are a sampling I found in a scan of the net. I am sure there are many more out there - post 'em if you find 'em!
First up: the notorious Pluto-kun ("Mr. Pluto"), one of the mascots used by the Japan Atomic Energy Association to calm fears about nuclear power at their Atom World PR facility. Which has been shut down to the public since March of 2011 (go figure.)
This looks like an R. Crumb character, but it's Tomarin, the mascot of the Hokkaido Electric Power Company's Nuclear PR center.
"Welcome to Onagawa Nuclear Power Plant!"
"Welcome to the Hamaoka Nuclear Power Museum!" He's still hanging out with the kids post-3/11, at least as recently as March 2012.
1W-kun ("Mr. One-Watt"), the mascot of Kinki University's teaching reactor.
Uranboya ("Uranium Boy"), another of the JAEA's helpful PR flacks.
Miss Sodium, another mascot used at Atom World to explain the workings of sodium-cooled fast reactors.
Rikki, mascot of the Shimane Nuclear Plant PR facility. (Free admission, people!)
"Super Atomic-kun," with a wave motif that looks quite unfortunate, in hindsight...