A possible breach has been detected in the reactor core of one unit of the damaged Japanese Fukushima Dai-ichi nuclear complex.
Officials fear that could mean more serious radioactive contamination.
Officials became suspicious of the breach after two workers waded into water 10,000 times more radioactive than normal and suffered skin burns when the water splashed over their protective boots
They were evacuated from the plant and rushed to the hospital.
"These workers had direct contact with highly radioactive water touching their feet, touching their skin," said Dr. Michio Kaku, leading authority on nuclear radiation. "This means they will come down with radiation burns. They know that some of them may not come out of this accident alive."
Officials said this latest threat is yet another setback in attempts to gain control of the plant two weeks after the deadly earthquake and tsunami engulfed the facility and knocked out its crucial cooling system.
Radiation from the plant has been showing up in raw milk and vegetables like broccoli and cauliflower in areas near the plant. Several countries have even halted some food imports.
Tokyo continues to wrestle with fears over tainted tap water after test results showed radioactive iodine levels had shot up above what's legally safe for infants.
Meanwhile, the death toll from the earthquake and the resulting tsunami that struck the island nation on March 11 has risen to over 10,000 with 18,000 people still listed as missing.