The problems include a broken temperature sensor that detects heat in a reactor and leaky piping, The Mainichi Daily News reported Saturday.
Plant operator Tokyo Electric Power Co. has begun decommissioning and dismantling the plant -- a process expected to take three decades -- and in January inserted an endoscope, an instrument allowing the first direct internal observations since the earthquake and tsunami March 11, 20011, shut down the plant's cooling systems.
But after a sensor began showing temperatures at the bottom of the No. 2 reactor's pressure vessel reached alarmingly high levels, Tepco determined the sensor was faulty.
The loss of the sensor -- after it showed temperatures climbed from about 40 degrees in late January to over 80, the maximum "safe" temperature for maintaining a cold shutdown, to more than 400 degrees -- showed how little Tepco understood about conditions inside the reactors and how fragile the cold shutdown is.
The Daily News also reported that after the government said the plant had been brought to cold shutdown, water leaked from 44 locations, including seven sections of water-injection equipment -- a critical part of the water recycling and re-injection system -- and from spent fuel pools at two reactors.
Meanwhile, Japanese Prime Minister Yoshihiko Noda told CNN the country has learned much from the meltdown.
"Sharing the lessons and remorse of this nuclear accident with the international community will contribute to the overall safety of nuclear energy as well as international cooperation," he said.
In the wake of the disaster, only two of Japan's 53 nuclear reactors are operating and by the end of April, those two are to go offline, leaving the country facing the prospect of no nuclear energy into the summer.
"Our basic stance is to move away from our dependence on nuclear power. This means that we have to try and increase the use of renewable energy, in a thorough effort to save energy," Noda said.
He said the government has included $7.5 billion in its fiscal year 2012 budget for renewable energy and conservation.
Noda also said nearly a year after the earthquake and tsunami killed thousands of people and crippled the Fukushima power plant, the country is making "steady progress" in reconstruction and aid to the northeast region.