Nuclei

The Fukushima Incident

	A powerful earthquake at 9.0 Richter scale hit Northern Japan (130 km East of Sendai) on March 11, 2011 at 2:46 pm (JST). The quake and subsequent tsunami severed cooling water to 3 of the 6 nuclear reactors (Unit 1, 2, and 3, see Figure 14-16e) in the Fukushima nuclear plant causing explosions that blown off the
Figure 14-16e Fukushima Incident [view large image]	concrete shielding (in unit 1 and 3). The other 3 units were shutdown before the quake for regular inspection. Following is a summary of the sequence of events leading up to the crisis (as of March 15, 2011):

1. Immediately after the earth quake, the Fukushima reactors (BWR type) went into an automatic shutdown mode.



2. Although the neutron induced fission has been stopped by inserting the control rods automatically, elements transmuted into radioactive isotopes in the nuclear fuel continue to generate a great deal of heat by spontaneous decay (about 1.5% of the full production power), and there is no way to turn them off.



3. The water pumps used to carry the excessive heat away stopped working with the failure of the backup generators as its fuel tanks and power lines were washed away by tsunami. The core began to heat up.



4. The zirconium alloy holding the fuel pellets is inactive at temperature up to hundreds of degree Celsius. However, when temperature reaches to two thousand degrees, it reacts with the steam and created hydrogen gas according to the formula : Zr + 2H₂O ZrO₂ + 2H₂. The hydrogen molecules combine with oxygen in the air to explode violently - obviously a fault in design that has reportedly been fixed in the latest generation called ESBWR (Economic Simplified Boiling Water Reactor).



5. This kind of explosion blew the concrete shielding (outer casing) of the reactors apart. Unit 1 went off on March 12, followed by unit 3 on March 14. At 6:14 a.m. on March 15, the pressure-suppression pool beneath the containment vessel (the donut-shaped structure at the bottom of the core used to vent overpressure, see Figure 14-16e) in unit 2 was rocked by a blast. Unlike previous explosions, this one was much closer to the reactor core, releasing more radioactive substances and raising fears of damage to the core and the surrounding containment vessel. At around the same time, unit 4 unexpectedly caught fire when old fuel rods, supposedly stored in deep water pools, became exposed and overheated, releasing explosive hydrogen.



6. Seawater mixed with boronic acid (boron is excellent neutron absorber to slowdown any possible neutron induced fission) are pumped into the emergency cooling system by fire trucks to prevent a complete meltdown. This action effectively ruins the reactors forever, but prevented a much worse catastrophe.



7. From the release of hydrogen and the fission products, it is fairly clear that all of these reactors have probably had fuel rods exposed for significant periods of time over a portion of their length and may have suffered partial meltdown. The real danger is the melted fuel to gather at the base of the reactor. It could burn through the concrete containment vessel. In a worse case scenario, the fuel could form a critical mass outside the core and restart the power-generating process, but in a completely uncontrolled way leading to a full-scale nuclear meltdown. However, the danger will subside with each passing day, as radioactive elements in the fuel decay, and cool down gradually.

Table 14-04 Update on the Fukushima Reactors

Figure 14-16f1 Fukushima Clean-up [view large image]	Figure 14-16f2 Clean-up Flowchart [view large image]	Figure 14-16f3 Robot at Work [view large image]	Figure 14-16f4 Robot at Work, 2015 [view large image]

As the immediate threat of further damage has receded in mid April, the Fukushima power plant faces a massive clean-up task that some said will take many decades and even up to a century to complete. Meanwhile in the short term there are four areas that needs to be considered as shown in Figure 14-16f1. A flowchart for the tasks has been published by the Tokyo Electric Power Company (TEPCO) (Figure 14-16f2). Following is a brief description for each of the clean-up task (in work flow sequence) :


1. Water - As of April 30, cooling water has to be injected through the Emergency Core Cooling system producing contaminated water in the process. The task is to restore the "heat exchange function" (meaning normal cooling water flow). But in the meantime maintains the flooding up to top of active fuel (to prevent overheat). The contaminated water meanwhile will be de-contaminated by running it through zeolite filter which removes the Cs-137 isotope, and will be stored in tanks or barges.



2. Reactors - While continues to cool the active fuel, robots are sent into the containment area to examine the radioactive level etc. for drawing up an action plan (see Figure 14-16f3, it is not clear why the reflection of 1 or 2 workers are shown in the photo as well). Eventually, new buildings are required to anchor new cranes for the final removal of damaged fuel rods, and to cover up the damaged structures.



3. Spent fuel - The goal is to stabilize the cooling and then have them removed from the site.



4. Debris - Remote-controlled excavators and transporters are being used to clear debris around the site, but some people will most likely be needed to work inside the reactor buildings. New cover-up buildings will help to prevent further release of radioactive materials to the environment. There is also plan to solidify the contaminated soil.

The Tokyo Electric Power Company (TEPCO) deployed a remote-controlled robot on April 10 2015 inside one of the damaged reactors to collect data on radiation levels and investigate the spread of debris (see Figure 14-16f4, and the April 15 2015 update from CNN News). The robot has stalled after traveling about 10 meter inside. Nevertheless, it shows that the radiation levels inside the three damaged reactors are still extremely high and remain unsafe for people to enter. Decommissioning work is estimated to cost $50 billion and will take years to complete.

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