Nuclear Power Plants in Japan
Background of Fukushima Dai-ichi nuclear facility
Fukushima Dai-ichi nuclear station was commissioned in March 1971. Six boiling water reactors (BWR) were designed to produce a net energy output of 2719MW. General Electric designed Units 1 (460 MW), 2(784 MW), and 6(1100 MW). Toshiba constructed Units 3 (784 MW) and 5 (784 MW). Lastly, Hitachi devised Unit 4 (784 MW). All six reactors were managed and maintained by the Tokyo Electric Power Electric Company (TEPCO).
These six units all employed light water as a coolant and a moderator. All but unit 3 utilized low enriched uranium (LEU) for fissionable fuel. Unit 3 produced energy from mixed oxide fuel (M.O.X) which is composed of LEU oxides and fissionable Pu-239 oxides. Units 1-5 had a Mark 1 containment system while Unit 6 had upgraded to a Mark 2 containment model.
Previous observations of facility
Storage of spent fuel rods
Back up power sources
After the Tsunami
On March 11, 2011, the Fukushima Daiichi Nuclear Power Plant in Japan was badly damaged by a 9.0-magnitude earthquake and resulting tsunami. At the time, Units 4,5, and 6 were shut down for refueling and maintenance. Once the earthquake arrived at 2:46 p.m., the control rods of units 1, 2, and 4 were immediately submerged completely in their corresponding reactor vessels. Automatically, emergency generators came online to power electronics and coolant systems. However, the tsunami quickly flooded the emergency generators and cut power to the pumps that circulated coolant water through the reactors. High-temperature steam in the reactors reacted with zirconium alloy to produce hydrogen gas. The gas escaped into the containment building, and the mixture of hydrogen and air exploded. Radioactive material was released from the containment vessels as the result of deliberate venting to reduce the hydrogen pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events.
Watch the video above (13:01 minutes) and answer the questions below. This video is quite technical. Please focus on the questions and don't become overwhelmed with the comprehensive explanation of the reactor design.
1) What two natural disasters occurred prior to the Fukushima nuclear disaster? How many people lost their lives due to these natural disasters?
2) How many reactors were producing energy before the explosion? How many reactors were unloading fuel? Lastly, how many reactors were shutdown?
3) What type of reactors were used at this site (RBMK, PWR, BWR, or AP1000)? Does the United States use this type of reactor?
4) Where was the spent fuel stored in the reactor? Note: The United States does not store spent fuel in this location of the reactor.
5) When the earthquake hit, did the control rods work?
6) If a reactor site loses power, they need a backup power supply to keep the reactor core cool. Did Fukushima eventually lose back-up power? What did this do to reactors 1-3?
7) Several times, the NRC and IAEA told Fukushima that their sea walls were too low. How did this contribute to the loss of back-up power?
8) Not having enough coolant, led to the production of hydrogen gas. What is the danger of having this gas inside the reactor core?
9) To reduce the pressure of the core, workers at Fukushima vented the radioactive gas into the atmosphere (similar to Three-Mile Island situation). How else did this radiation affect the planet?
10) Workers at Fukushima used seawater to cool the core. What is the problem with using this substance?
11) Did the containment of the reactors hold?
12) What is a cold shutdown? Has this been achieved yet?
13) Do Japan force its citizens to man and then clean-up Fukushima? Do you think the people involved understood the hazards of radiation (unlike Chernobyl)?
14) At Fukushima, there were two deaths at the reactor site due to the natural disaster. There have been no reported deaths due to the meltdowns. How does this differ from Chernobyl?
Explosions and Meltdown
Immediately after the earthquake, the six external power supplies shut-down. Loss of external power could result in overheating in the reactor cores. Of the six diesel back-up generators, only one remained functional to provide cooling water. This generator cooled units 5 and 6 until it lost power. The five other diesel generators were flooded when the Tsunami hit. As a last resort, Fukushima accessed their 125 volt DC back-up batteries. One of these energy sources was able to keep unit 3 cool for 30 hours until it failed to produce any more power.
Unfortunately, hydrogen gas was produced in units 1-4. Explosions of this gas occurred in units 1,3 and 4. On March 14, hydrogen gas explosions blew to the rooftops off of Units 1 and 3. There was no damage to the 20 cm thick steel containment structure.
Evacuation of the area surrounding Fukushima Dai-chi
Around 3:40 p.m., back-up sources for the reactors lost power. Evacuation of a 2 km radial zone began at 7:03 p.m. Two hours, later the radius of evacuation was increased to 3 km. The Japanese government announced the following morning that the evacuation zone would increase to 10 km. Later that day, the final evacuation zone of 20 km was publicized. An estimated 200,000 people were evacuated from this area.
On March 16, radiation dosages inside the nuclear power station were 400 mSv per hour. Outside the facility, dosimeters measure 1.9 mSv per hour. The average TEPCO worker was exposed to 50-100 mSv per hour while on-site at the nuclear power plant.
As of 2021, there have been no reported deaths due to the partial meltdowns, explosions, or fallout from this nuclear disaster. Unlike the Chernobyl disaster, TEPCO workers (mostly composed of older males) volunteered in shifts to monitor the reactors during the accident. They were provided with protective equipment and potassium iodide pills.
Compare the radiation dosages of a TEPCO employee and outside the Fukushima Dai-ichi to that of yearly background radiation (360mrems) and Acute Radiation Syndrome (25 rem).
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