Fukushima Nuclear Plant in Sendai, Japan – What exactly happened?

On March 14, 2011, in Uncategorized, by jonathanfun

My heart goes out to Japan for the catastrophe that took place on March 11, 2011, when an earthquake with a magnitude of 8.9 – 9.1 on the Richter scale struck on the north-eastern coastlines, triggering a 23-foot Tsunami with an estimated power of 1.3 petawatts that battered Japan’s coast, annihilating thousands of people, sweeping away cars, houses, buildings, aircrafts, boats, and moved the Honshu island of Japan eastward by 2.5 meters on the global map.

There is another major problem looming on the horizon – A serious nuclear disaster in 5 of the nuclear power plants located in the affected areas. Three explosions already happened in the Fukushima Daiichi plant in Sendai, and more could happen soon. A nuclear expert on CNN already dubbed the event as the worst nuclear disaster after the Chernobyl and Three Mile Island nuclear disasters.

So what exactly happened? Here’s some information about Nuclear Power Plants I gathered from the internet, and I’ve summarized them in point form, Chedet’s style, for easier understanding and comprehension.

1. Many power plants in the world all operate to achieve the same objective – to boil water to generate steam, which will power a turbine which is attached to a generator. That generator generates electricity, and using transformers it supplies electricity to the power grid.

2. What differentiates a power plant from another is their methods and fuels used to generate steam to power the turbines. We all know that steam is basically generated by boiling water, and to boil water, we need heat from burning fuel. Some plants burn oil, some burn coal, but in the case of a nuclear plant, it generates, depending on the type of the plant, hot water or steam, by the fissioning of uranium.

3. There are 2 major types of nuclear power plants in the modern world. The first one is called a Pressurized Water Reactor where the water is kept at high pressure and doesn’t boil in the reactor. The heat generated in this reactor will be transferred using a heat exchanger to the steam generator, water is allowed to boil, and steam generation is achieved to power the turbine.

4. Another type of nuclear power plant is called the Boiling Water Reactor, where the water in the reactor will actually boil and generates steam directly to power the turbine.

5. So what happens in a nuclear power plant is that the fissioning of atoms (atoms splitting in half) take place (shown in Figure 1) and generates enormous amount of heat. In this process, other materials are created that continue to decay and generates more heat. Hence, if a nuclear power plant is shut down, the reactor core has to be continually cooled. Imagine generating a lot of hot water under a lot of pressure and it needs to be cooled down and because of the decay of these materials in the fuel, they will also continue to generate heat for some period of time until the decay trails off.

6. The Fukushima nuclear plant (Unit One) that we keep seeing and reading on the mass media is in the most dangerous state. That plant is a General Electric Boiling Water Reactor, which first achieved criticality (in nuclear, achieving criticality means that it has achieved a self-sustaining reaction)  in 1970, similar to many other nuclear plants located in the US.

7. When the earthquake occurred, the plant was automatically shut down. In the first hour after shut down, they were running on diesel generators to keep the pumps running so that the cooling will continue to take place. Usually when a plant shuts down, it has 2 ways to get electricity; one from the grid (provided by power companies or the government), or from emergency sources such as diesel generators or batteries.

8. In Fukushima’s case, due to the magnitude of the earthquake, the grid basically went off, and so they were operating on the diesel generators as it should be. What they didn’t anticipate, was that after the earthquake, a massive tsunami ensued, flooded the whole plant and the place where the diesel generators were, caused them to lose their diesel generated power and that ultimately reduced them to their emergency battery back up power, which could only last for hours, and that brought about the sudden difficulty to keep their plant cool.

9. In general, nuclear plants in Japan and in Western countries are built to withstand natural disasters, depending on the worst case scenario anticipated for the plant. Hydraulic dampeners, for example, are built on equipments to allow them to move back and forth without breaking in the event of an earthquake. In Japan, there’s also another requirement to build the support of these plants on bedrock.

10. There should be no doubt that what really puts them in this dire situation was the tsunami that resulted from the earthquake and not the earthquake itself. In Boiling Water Reactors, the reactor is usually full of water to a certain point, and above that point will be occupied with steam. Which means the core is always kept under water, and steam is channeled from the top section of the reactor to a turbine that is attached to an alternator to generate electricity. The exhaust steam from the turbine is then cooled, and returned back to the reactor.

12. In an emergency shut down procedure, usually only a small percentage of the water that the plant would normally pump back into the reactor for cooling will be returned to the reactor, and the water will be left to boil on its own. By allowing water to boil, heat will be taken away from the reactor, thereby cooling it.

13. But when the plant is lack of power, the plant won’t be able to use their normal and backup systems to remove the steam, cool it and return it to the reactor. To avoid overheating, what they could do is to vent this steam into the buildings at the plant, and by doing so and adding cool water at the same time, they could keep the reactor cool enough to keep it from melting down.

14. With lack of power, the plant loses a lot of instrumentation they would normally have to monitor the situation at the plant. It was reported that they had a hydrogen explosion, which means obviously hydrogen and other gases were generated and built up to an explosive level which allowed the explosion to take place.

15.  From the photos all over the internet, it looks like the entire building surrounding the reactor had collapse and the only thing left is the steel frame. That building will be usually called the auxiliary building, and according to what I’ve read, it houses a lot of emergency and safety systems for the reactor. So when the entire auxiliary building is destroyed, they are in for a very serious situation.

16. The good news is, the containment structure is still currently intact, so if there has been any release of radioactivity, it is minor, and meanwhile they have to go all out to find some way to get electricity to cool the reactor. According to reports, they’re going to use seawater to cool the reactor, which means they are going to need some pumps and mix it with boron.

17. Boron is a substance that will absorb neutrons, and that will keep the reactor from going critical again when they pump in cold seawater. Cold water is denser than hot water, and if neutrons are present, it will cause the neutrons that are still bouncing around the reactor to slow down, and they could strike the fuel rod and cause fission to take place.

18. In this case fission is not desirable at all because it generates more heat and that will cause the reactor to go critical again, which is going generate even more heat. Critical in nuclear science means self-sustaining reaction, so when a nuclear power plant operates under normal conditions, it is critical.

19. What we don’t want to hear is a term called Super Critical, and that would mean catastrophe. So in any case, boron must be added into the cold water or else it will have the potential of causing the fission level to go up in the reactor thereby causing more heat to be generated, which will put the plant into higher risks of a core melt down.

20. But that aside, it is comforting to note, that no matter what the outcome of this incident will not repeat what happened in Chernobyl, so there’s no need for a panic here.

21. The Chernobyl reactor was a completely different type of design than the one in Fukushima plant. The one in Chernobyl was a graphite  moderated reactor and the biggest difference between either a pressurized water reactor or a boiling water reactor, is that a water cooled reactor is what we call inherently stable, as opposed to the Chernobyl design which was inherently unstable.

22. However it is not good at all if the Boiling Water Reactor in Fukushima cannot be cooled, because as the water level drops, steam builds up more in the reactor. Steam is less denser than water, which means there is going to be less water molecules to slow down the neutrons and that means it will make it less likely for the reactor to be critical again. But still, the reactor needs to be cooled due to all this residual heat present.

23. Another thing to note that the Fukushima plant reactors all have a containment building. So, according to the news, although the auxiliary building had collapsed, the containment, usually a steel liner, is still intact. Hence in theory, as  long as they can maintain the pressures in that reactor, even if the core was to melt, most of the radioactivity will still be contained within the containment building. There wasn’t any containment building at Chernobyl, so when the core melted and caught on fire, all the radioactivity were spread to the atmosphere and to the countryside. In the case of Fukushima plant, that should not  occur.

So, bottom line is, we all should worry, but shouldn’t be too worried. Japan has one of the most nuclear engineers in the world, and they are working around the clock to solve this. So news reports that we see on the media can be misleading and exaggerating.

Let us just cross our fingers and pray for the best for Japan and for the rest of the world.

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