Fourteen months after the accident [at the Fukushima Daiichi nuclear plant], a pool brimming with used fuel rods and filled with vast quantities of radioactive cesium still sits on the top floor of a heavily damaged reactor building, covered only with plastic. The public's fears about the pool have grown in recent months as some scientists have warned that it has the most potential for setting off a new catastrophe ... as frequent quakes continue to rattle the region. The jury-rigged cooling system for the pool has already malfunctioned several times, including a 24-hour failure in April. Had the outages continued, they would have left the rods at risk of dangerous overheating. The No. 4 reactor is visibly damaged and in a fragile state, down to the floor that holds the spent fuel pool," said Hiroaki Koide, an assistant professor at Kyoto University's Research Reactor Institute and one of the experts raising concerns. "Any radioactive release could be huge and go directly into the environment." The worst-case situations for Reactor No. 4 would be for the pool to run dry if there is another problem with the cooling system and the rods catch fire, releasing enormous amounts of radioactive material, or for fission to restart if the metal panels that separate the rods are knocked over in a quake. That would be especially bad because the pool, unlike reactors, lacks containment vessels to hold in radioactive materials.