The situation still remains risky at the crippled Fukushima No. 1 nuclear power plant four years after a series of meltdowns, the chairman of Japan's nuclear regulator said Wednesday, vowing utmost efforts to avoid further trouble there.

"There have been quite a few accidents and problems at the Fukushima plant in the past year, and we need to face the reality that they are causing anxiety and anger among people in Fukushima," Shunichi Tanaka told personnel at the Nuclear Regulation Authority on the fourth anniversary of the nuclear disaster.

Mishaps still occur regularly at the radiation-leaking complex in Fukushima Prefecture, where decommissioning work is continuing after it was heavily damaged in the massive earthquake and tsunami that hit the Tohoku region on March 11, 2011.

In the latest revelation of a series of contaminated water leaks from the seaside plant, late last month Tokyo Electric Power Co., the plant operator, said highly radioactive rainwater flows into the Pacific Ocean every time it rains.

The number of accidents involving plant workers has also increased as the decommissioning work has become more complex.

One worker died after falling into a 10-meter-high rainwater storage tank in January.

"There are numerous risks that could cause various accidents and problems," Tanaka said, urging Tepco to continue efforts to enhance plant safety.

The chairman also voiced concern that the lessons from the world's worst nuclear crisis since Chernobyl may fade from people's memories as time passes by.

"As regulators, we must not forget what we've learned from Fukushima," Tanaka added.

After being hit by the magnitude-9 earthquake and ensuing tsunami, the six-reactor Fukushima No. 1 plant lost nearly all its power sources and consequently its ability to cool most of the units. The No. 1 to 3 reactors suffered nuclear meltdowns, while hydrogen explosions damaged the buildings housing the Nos. 1, 3 and 4 units.