This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. For more details on how Fukushima Dai-1's nuclear reactors work and what has gone wrong so far, see our explainer.
Tokyo Electric Power Co. (TEPCO) announced today its revised roadmap to bring the crippled Fukushima nuclear plant to a cold shut down. Despite changes in the roadmap’s implementation, the company still expects to achieve a cold shutdown by January.
A major change to the original roadmap issued one month ago is the way the reactors are to be cooled. TEPCO originally planned to fill the containment vessels with water before going on to install new cooling systems employing heat exchangers. But after finding that water in the No. 1 reactor has leaked out into the reactor-building basement, most likely because of damage to both the pressure vessel and containment vessel caused by the reactor’s melted fuel rods, TEPCO has abandoned this approach. The company suspects similar conditions exist for the Nos. 2 and 3 rectors. So now TEPCO says it is considering decontaminating the pooled water in the reactor buildings and using it to cool the reactors.
Following a provisional assessment based on the No. 1 reactor’s temperature and pressure data retrieved recently from the reactor’s central control room, TEPCO revealed on Monday new details of what happened immediately following the 11 March earthquake. The company now believes the fuel rods in the No. 1 reactor had melted down approximately 16 hours after the earthquake struck the plant. The company said it also suspects that an operator manually shut off the isolation condenser, which converts the steam used to drive the turbine into water for a return trip to the reactor.
The reactor automatically halted minutes after the earthquake struck at 2:46 on that Friday afternoon. Records show that that the isolation condenser system stopped working about 10 minutes later for three hours, apparently because a plant operator manually halted it when pressure and water temperature in the reactor fell dramatically. The condenser started up again after 6 p.m. and then was apparently switched off and on several times. A TEPCO official told Spectrum that the operating manual calls for the closing of the isolation condenser if the temperature drops faster than 55 degrees C in one hour.
Whatever the reason, the water level in the reactor dropped precipitously and the 4-meter-high fuel rods were completely exposed by the evening. This led to a meltdown of the rods by the following morning, as temperatures steadily climbed to an estimated 2800 degrees C. The government has ordered TEPCO to submit a report on what happened by 23 May.
In other developments the Chubu Electric Power Co. completed the cold shutdown of its Hamaoka nuclear plant on Sunday. In shutting down the plant, Chubu Electric was complying with a strong request from Prime Minister Naoto Kan to shut down the plant, given its location close to a fault line that seismologist believe will experience a major earthquake in the next 30 years. The 3.6-gigawatt plant is located in Shizuoka Prefecture almost 200 km southwest of Tokyo and provides more than 10 percent of the company’s electricity output for central Japan.
Shutdown at Hamaoka was completed after all 205 control-rods were inserted into the core of the No. 5 reactor and after bringing the temperature down below 100 degrees C. This followed the shutdown of the No. 4 reactor on Friday. The No. 3 reactor was already shut down for inspection and maintenance, while reactors Nos. 1 and 2 have been shut down since the beginning of 2009 in preparation for the decommissioning.
A company official said that Chubu hopes to restart the three operating reactors once a seawall has been built to protect the plant from a major tsunami. Construction is expected to take between two to three years. In the meantime, the company told the Yomiuri Shimbunthat it is working to restart five fossil-fuel-fired power plants now out of service. These could produce a total capacity of 1.8-million kilowatts, though not before repairs have been carried out and parts replaced. Chubu Electric is also negotiating to buy additional power from other utilities, including Kansai Electric Power Co.