The visit was part of the ambassador's visit to Fukushima Prefecture on January 16. It was his first visit to Fukushima Prefecture ever.
The visit to the nuclear power plant was not open to the press, and details are unknown.
From Kyodo News (1/16/2012):
Us Ambassador to Japan Roos visited Fukushima for the first time, says "Danger to the residents are not over"
US Ambassador to Japan John Roos visited Fukushima Prefecture for the first time on January 16 and visited Fukushima I Nuclear Power Plant (located in Futaba-machi and Okuma-machi) ten months after the start of the accident. At temporary housing in Iwaki City, he spent more than one hour talking to 6 residents from Okuma-machi. He later told the press, "The danger to the people who live here is not over. The US government, in collaboration with the Japanese government, will do all it can to support them."
The ambassador's visit to Fukushima I Nuclear Power Plant was closed to the public. The ambassador is said to have surveyed the progress for restoration work with experts from the US Department of Energy and the Nuclear Regulatory Commission.
After the visit to the plant, he visited Hisanohama District of Iwaki City where 409 homes were flooded by the tsunami. He dedicated flowers and stood in silence for a few moments on the coastal embankment.
Well, spending more than 1 hours with the evacuees is much more than what the Japanese politicians have done.
The news of Fukushima I Nuclear Power Plant has almost totally dried up ever since PM Noda's silly declaration of "cold shutdown state". What I suspect though is that the silly declaration was a signal to the Japanese media to stop reporting on the plant regularly (which they did) and the real work at the plant could start, away from the media's eyes. (Not that they were seeing much to begin with.)
In that regard, the fact that the experts from the US DOE and the NRC accompanying the ambassador is very interesting. After all, they may know more about the plant than the Japanese, having received SPEEDI information since March 14, 2011 (Japanese people were told at that time SPEEDI was not working) and other technical information from the Japanese government and TEPCO which haven't been disclosed to the Japanese public.
TEPCO is supposed to start the effort to probe inside the Containment Vessel of Reactor 2 with an endoscope on January 17, first by drilling into the CV. They will insert the endoscope on January 19.