page_title: Fukushima Daiichi nuclear disaster (Unit 1 Reactor)
section_title: Cooling problems and first radioactivity release
hierarchical_section_title: Fukushima Daiichi nuclear disaster (Unit 1 Reactor) / Unit 1 Reactor / Cooling problems and first radioactivity release
caption_attribution_description: English: One of the photos taken at the Fukushima I Nuclear Power Plant tour sponsored by the Tokyo Electric Power Company on June 23, 1999. 日本語: 福島第一原発　1999年6月に行われた東京電力主催の見学ツアーにて
context_page_description: The Fukushima Daiichi nuclear disaster was a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. It is the largest nuclear disaster since the Chernobyl disaster of 1986.
  The plant comprises six separate boiling water reactors originally designed by General Electric, and maintained by the Tokyo Electric Power Company. At the time of the quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. Immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to control electronics and coolant systems. However, the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through the reactor core. With the pumps stopped, the reactor cores began to overheat.
context_section_description: On 11 March at 14:46 JST, in response to the earthquake, TEPCO successfully scrammed the reactor in Unit 1, shutting down all power-producing nuclear fission chain reactions. Evacuated workers reported violent shaking and burst pipes within the reactor building. At 15:37, the quake's tsunami inundated the plant and all electrical power to the facility was lost, leaving only emergency batteries. Some of the monitoring and control systems were still operational, though Unit 1's batteries had been damaged by the flood waters. At 15:42, TEPCO declared a "Nuclear Emergency Situation" for Units 1 and 2 because "reactor water coolant injection could not be confirmed for the emergency core cooling systems." The alert was temporarily cleared when water level monitoring was restored for Unit 1 but it was reinstated at 17:07 JST. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure in the core.
  To cool the reactor, operators resorted to the plant's Emergency core-cooling systems (ECCS), including the isolation condensers, and the High Pressure Coolant Injection systems (HPCI). According to NHK, the isolation condenser system had not been activated in the previous 40 years, and no one present had ever witnessed its operation. It was also later discovered that TEPCO had made changes to the system's original design, without approval or notification of NISA. During the crisis, operators couldn't tell if one of the system's valves was open or closed.
  About 10 minutes after the earthquake, TEPCO operators removed both of Unit 1's isolation condensers from service, and instead activated the HPCI system. At 15:07, the core spray system was activated to cool the suppression pool. Both systems lost power after the tsunami struck the plant. The tsunami's arrival prevented operators from restarting the isolation condensers for more than 30 minutes. Afterwards, they were operated intermittently. Despite being designed to cool Unit 1 for at least 8 hours, the condensers' limited operation did not reduce the heat in the core and containment vessel.
  By midnight, water levels in the reactor were falling and TEPCO gave warnings of the possibility of radioactive releases. In the early hours of 12 March, TEPCO reported that radiation levels were rising in the turbine building for Unit 1. Operators were considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity. Later that morning, Chief Cabinet Secretary Yukio Edano stated that the amount of potential radiation would be small and that the prevailing winds were blowing out to sea. Pressure inside Unit 1 continued to rise. At 05:30 JST, it had reached 820 kPa, 2.1 times normal. After isolation cooling had ceased to operate, TEPCO started relieving pressure and injecting water. One employee working inside Unit 1 at this time received a radiation dose of 106 mSv and was later sent to a hospital to have his condition assessed.
  Without electricity, needed for water pumps and ventilation fans, rising heat within the containment area led to increasing pressure. In a press release on 12 March at 07:00 JST, TEPCO stated, "Measurement of radioactive material (iodine-131, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level." Dose rates recorded on the main gate rose from 69 nGy/h (for gamma radiation, equivalent to 69 nSv/h) at 04:00 JST, 12 March, to 866 nGy/h 40 minutes later, before hitting a peak of 0.3855 mSv/h at 10:30 JST. At 13:30 JST, workers detected radioactive caesium-137 and iodine-131 near Reactor 1, a sign that water levels in the coolant system had dropped so low that some of the core's fuel had melted, after being exposed to the air. Cooling water levels had fallen so much that parts of the nuclear fuel rods were exposed and partial melting might have occurred. Radiation levels at the site boundary exceeded the regulatory limits.
  On 14 March, radiation leve
caption: Unit 1 before the explosion. The joint can be seen between the lower concrete building and upper lighter cladding which was blown away in the explosion.
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