At the stricken Fukushima nuclear power plant, a robot sent into the building housing Reactor No. 1 on Saturday detected the highest levels of radiation measured since the crisis began on March 11.

According to the Japan Times, The Tokyo Electric Power Company (TEPCO) reported that radiation levels in the air around Reactor 1 were at 4000 millisieverts per hour, an exposure level equivalent to approximately 40,000 chest x-rays. TEPCO says it has no plans to send workers into the area because of its dangerously high radioactivity.

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On Friday, a spokesman for TEPCO announced that steam was rising from underneath the reactor building. That afternoon, Japanese national television carried blurry footage of smoke rising from an opening in the floor.

Underneath the reactor, an estimated 40,000 tons of “highly contaminated” radioactive water have collected in what is known as the pressure suppression containment vessel, and it’s this water that is believed to be producing the steam. TEPCO officials warn that the water will begin to overflow from the storage vessel by June 20 as it reaches its maximum capacity, sooner if there are heavy rains.

Massive tanks are being sent to contain the water from nearby Tochigi Prefecture. An estimated 370 will be needed for the job, two of which are due to arrive this weekend. The tanks each hold 100 tons of water and will continue to arrive at the plant through August.

Workers have been fighting to keep the crisis from escalating in an ongoing struggle that officials say they hope to have under control by January. Critics of TEPCO and the Japanese government say that this estimate is overly optimistic. A row has erupted in the Japanese government over Prime Minister Naoto Kan’s promise to step down when the crisis is resolved because in truth no one knows exactly when that will be.

At the plant, it’s believed that nuclear fuel rods have melted down to the bottom of three reactors’ containment vessels. Allegedly none have gone into “full meltdown” in which the fuel rods burn through the bottom of the containers.