A crack in a maintenance pit was found over the weekend, the latest confirmation that radiation continues to spill into the surrounding environment.
Fresh concrete was initially used to fill the crack, but that didn't work, so crews tried a synthetic compound earlier Sunday.
In order to get the plant's overheating reactors under control, a cooling system must be restored. Government officials conceded on Sunday that it will likely take months.
The crippled complex has caused a power crunch in Tokyo. Officials are now concerned the strain will affect air conditioning during the summer.
According to Yukiya Amano, head of the International Atomic Energy Agency, the Japanese reactor crisis poses a major challenge with enormous implications for nuclear power. Amano appeared to criticize the operator of the crippled complex.
Amano spoke at the opening session of a meeting that has drawn representatives from dozens of countries to scrutinize safety at each other's power plants.
"I know you will agree with me that the crisis at Fukushima Dai-ichi has enormous implications for nuclear power and confronts all of us with a major challenge," Amano told delegates. "We cannot take a business as usual approach."
The Associated Press contributed to this story