when concerns were raised in Japan and by the IAEA. In accordance with GE's original specifications for the construction of the plant, each reactor's emergency diesel generators and DC batteries, crucial components in powering cooling systems after a power loss, were located in the basements of the reactor turbine buildings. Mid-level GE engineers expressed concerns, relayed to TEPCO, that this left them vulnerable to flooding. In the late 1990s, three additional backup diesel generators for Units 2 and 4 were placed in new buildings located higher on the hillside, to comply with new regulatory requirements. All six units were given access to these diesel generators, but the switching stations that sent power from these backup generators to the reactors' cooling systems for Units 1 through 5 were still located in the poorly protected turbine buildings. Meanwhile, the switching station for Unit 6 was protected inside the only GE Mark II reactor building and continued to function. All three of the generators added in the late 1990s were fully operational after the tsunami. If the switching stations had been moved to the interior of the reactor buildings or to other flood-proof locations, power would have been provided by these generators to the reactors' cooling systems and thus the catastrophe would have been averted. The nearby Fukushima Daini Nuclear Power Plant was also struck by the tsunami. However, this power plant had incorporated design changes that improved its resistance to flooding, thereby reducing flood damage. The diesel generators and related electrical distribution equipment were located in the watertight reactor building, and therefore this equipment remained functional. By midnight, power from the electricity grid was being used to power the reactor-cooling pumps. Seawater pumps for cooling were protected from flooding, and although 3 of 4 initially failed, they were restored to operation. Central fuel storage areas Used fuel assemblies taken from reactors are initially stored for at least 18 months in the pools adjacent to their reactors. They can then be transferred to the central fuel storage pond. Fukushima I's storage area contains 6375 fuel assemblies. After further cooling, fuel can be transferred to dry cask storage, which has shown no signs of abnormalities. Zircaloy Many of the internal components and fuel assembly cladding are made from zircaloy because it does not absorb neutrons. At normal operating temperatures of approximately , zircaloy is inert. However, above , zirconium metal can react exothermically with