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In reactors 1, 2 and 3, complete core meltdowns have occurred. Japanese authorities have admitted the possibility that the fuel may have melted through the bottom of the reactor core vessels. It is speculated that this might lead to unintended criticality (resumption of the chain reaction) or a powerful steam explosion – either event could lead to major new releases of radioactivity into the environment. Reactors 1 and 3 are sites of particularly intense penetrating radiation, making those areas unapproachable. As a result, reinforcement repairs have not yet been done since the Fukushima accident. The ability of these structures to withstand a strong aftershock earthquake is uncertain. The temporary cooling pipes installed in each of the crippled reactors pass through rubble and debris. They are unprotected and highly vulnerable to damage. This could lead to a failure of some cooling systems, causing overheating of the fuel, further fuel damage with radioactive releases, additional hydrogen gas explosions, possibly even a zirconium fire and fuel melting within the spent fuel pools. Reactor No. 4 building and its frame are serious damaged. The spent fuel pool in Unit 4, with a total weight of 1,670 tons, is suspended 100 feet (30 meters) above ground, beside a wall which is bulging outward. If this pool collapses or drains, the resulting blast of penetrating radiation will shut down the entire area. At the Fukushima Dai-ichi nuclear power station, the spent fuel pools alone contain an amount of cesium-137 that is 85 times greater than at Chernobyl.