Satsuki Katayama is an LDP member of the Upper House of the Japanese Diet and a former career bureaucrat in the Ministry of Finance.
In her official blog on January 13, 2012, Katayama relates the following rumor:
Just then, there was a communication from the ex-council member of Minami Soma City to the mayor [of Soma City] that there was some kind of explosion at Fukushima I Nuclear Power Plant on January 9 (probably at Reactor 4 building) and the government is hiding it. I don't know whether it's true or not, but if it is, it is probably a hydrogen explosion, not a nuclear explosion.
Katayama says she was speaking with Hidekiyo Tachiya, mayor of Soma City (north of Minami Soma City) when the mayor related the "rumor" to her. I have no idea why she's allowed to talk loose like this in her official blog.
So far, it doesn't seem to have gathered much attention in Japan, and many on Twitter ridicule her. But some people are connecting this "rumor" to the massive deployment of the Self Defense Force helicopters on January 9.