matching the assessment given to the 1986 Chernobyl nuclear disaster. Since that time the Fukushima reactors have continued to emit radiation; including atmospheric, water, and gamma ray releases. At Chernobyl, approximately 10 times the amount of radiation was released into the atmosphere as was released from Fukushima I through 12 April 2011. The total amount of radioactive material still stored at Fukushima is about 8 times that stored at Chernobyl, and leakage at Fukushima continues. After cooling efforts at spent-fuel pool 4 were halted, due to an erroneous warning about the pool filling up, the temperature of the pool rises to 90 °C and the dose rate 6 meters above the pool spikes at 84 mSv/h. Friday, 15 April Nuclear fuel is reported to have melted and fallen to the lower containment sections of reactors 1, 2 and 3. The melted material is not expected to breach a container (which might cause a massive radiation release). Instead, the melted fuel is thought to have dispersed fairly uniformly across the lower portions of the containers of the three reactors, which would make the resumption of the fission process, to the extent of a recriticality accident, 'most unlikely'; however, it is only during future dismantling of the three damaged reactors that it would be possible to verify this hypothesis and to know what really occurred inside the reactor cores. Saturday, 16 April Plans are announced for a large-scale study on the environmental and health effects of radioactive contamination from the nuclear plant. Academics and researchers from across Japan will work with the Fukushima Prefectural Government starting in May. Monday, 18 April The Associated Press reports that two PackBot ground robots from iRobot have entered unit 1 and unit 3 of the crippled Fukushima nuclear power plant and performed temperature, pressure, and radioactivity measurements. The remote-controlled robots entered the two reactors over the preceding weekend. The devices opened closed doors and explored the insides of the reactor buildings, coming back with radioactivity readings of up to 49 mSv/h inside unit 1 and up to 57 mSv/h inside unit 3. TEPCO officials say that the radiation data from the robots do not change their plans for shutting down the plant by the end of this year. Though more robots will be used, a TEPCO official, Takeshi Makigami, says that robots are limited in what they can do, and eventually "people must enter the buildings".