of the fission reaction to produce steam, which drives turbines that generate electricity. When the reactor stops operating, the radioactive decay of unstable isotopes in the fuel continues to generate heat (decay heat) for a time, and so requires continued cooling. This decay heat amounts to approximately 6.5% of the amount produced by fission at first, then decreases over several days before reaching shutdown levels. Afterwards, spent fuel rods typically require several years in a spent fuel pool before they can be safely transferred to dry cask storage vessels. The decay heat in the Unit 4 spent fuel pool had the capacity to boil about of water per day. In the reactor core, high-pressure systems cycle water between the reactor pressure vessel and heat exchangers. These systems transfer heat to a secondary heat exchanger via the essential service water system, using water pumped out to sea or an onsite cooling tower. Units 2 and 3 had steam turbine-driven emergency core cooling systems that could be directly operated by steam produced by decay heat and that could inject water directly into the reactor. Some electrical power was needed to operate valves and monitoring systems. Unit 1 had a different, entirely passive cooling system, the Isolation Condenser (IC). It consisted of a series of pipes run from the reactor core to the inside of a large tank of water. When the valves were opened, steam flowed upward to the IC, where the cool water in the tank condenses the steam back to water that runs under gravity back to the reactor core. During a 25 March 2014 presentation to the TVA, Takeyuki Inagaki explained that unit 1's IC was operated intermittently to maintain reactor vessel level and to prevent the core from cooling too quickly, which can increase reactor power. As the tsunami engulfed the station, the IC valves were closed and could not be reopened automatically due to the loss of electrical power, but could have been opened manually. On 16 April 2011, TEPCO declared that cooling systems for Units 1–4 were beyond repair. Backup generators When a reactor is not producing electricity, its cooling pumps can be powered by other reactor units, the grid, diesel generators, or batteries. Two emergency diesel generators were available for each of Units 1–5 and three for Unit 6. The Fukushima reactors were not designed for a large tsunami, nor had the reactors been modified