http://www3.nhk.or.jp/nhkworld/english/news/20130501_36.html
There are 3 major points that the Authority wants to investigate about the cause of the crisis at the Fukushima nuclear power plant.
The first question is whether the earthquake impacted the plant, and if so, how.
TEPCO and the government-appointed panel of experts maintain that the plant's radiation containment system withstood the quake and remained intact until the tsunami reached the plant.
But the Diet panel says there still is no evidence available to prove that the quake didn't cause any damage to major equipment. No detailed inspections have been conducted inside the contaminated reactor buildings.
The second question is how the fuel rods melted down and damaged the reactors, and where and how the melted fuel is now in the reactors.
The third question is from where radioactive substances leaked into the atmosphere.
It is estimated that they leaked through sections connecting the containment vessels and the pipe networks that had been damaged by high temperatures and pressure. But it remains to be proved exactly which sections were damaged to allow the leak.
All experts agree that they need to identify possible weaknesses and shortcomings in the reactors and containment vessels that helped cause the crisis.
With those vital questions still unanswered for the past 2 years, it remains difficult to be certain of the safety of nuclear power generation.