"If there's a secondary earthquake, that could tip the whole thing over. Pipes could break, leaks could take place and even as you put sea water in, the water could bleed out, creating a full scale meltdown. That's the nightmare scenario," Kaku said.
Another nuclear complex, the Tokai Dai-Ni plant, experienced a failure after Friday's quake, the Associated Press reported. It's not clear why the incident wasn't reported by the Japan Atomic Power Co. until Sunday.
Japanese Prime Minister Naoto Kan said in a television address Sunday that the country is facing the most difficult crisis since World War II, but he is confident the nation can overcome this disaster.
"We Japanese people have overcome all kinds of hardships and were able to create a prosperous society. In the face of the earthquake and tsunami we should be able to overcome these hardships. We believe we can overcome this," Kan said.
Kan said 100,000 troops -- plus 2,500 police, 1,100 emergency service teams, and more than 200 medical teams -- have been deployed for recovery efforts.
Millions of the country's residents are grappling with food shortages, power outages and the collapse of basic services.
Ichiro Fujisaki, the nation's U.S. ambassador, said about 2.5 million households -- just over 4 percent of all households in Japan -- were without electricity Sunday, and 500,000 homes were without water.
The government is going to begin further rationing electricity by implementing rolling blackouts.
At least 49 countries along with numerous aid organizations have mobilized relief efforts.
The nuclear-powered aircraft carrier USS Ronald Reagan arrived off the coast of Honshu Saturday, one of a number of U.S. vessels diverted to offer disaster aid to the shattered country.
At least four more Navy ships will be arriving in the days to come to assist with relief efforts.
Tsunami survivors were plucked by helicopters and from rooftops, but hundreds more along the 1,300-mile stretch of coastline are waiting to be rescued.
A Navy P-3 maritime surveillance plane did a survey mission earlier Sunday and discovered a huge debris field 8 miles east of the Japanese coast line. Houses, barges, oil slicks, capsized boats, and cars fill the mile wide debris field.
The U.S. Embassy said that 100,000 Americans are known to be in Japan, and 1,300 of them live in the areas most affected by the earthquake and tsunami. There are still no known American casualties.
ABC News' Michael James, Dan Childs and Dean Schabner and the Associated Press contributed to this report.