A few days ago there was a very strong earthquake in Japan. Amidst all the destruction one peculiar problem was caused that justifiably caught the attention of the press: a crisis in the Fukushima nuclear power plant.
As an electrical engineer I have more than some interest in how nuclear power plants work. I think it's a bit of my duty to learn about how they work and fail, and to inform others in times like this. I wanted to write a post trying to explain what is happening in Fukushima, and more importantly: why it's not and will not be a new Chernobyl, and how it may be perhaps compared to the Three Mile Island accident.
If you are really interested in the subject I recommend you read Why I am not worried about Japan's nuclear reactors . I'm doing more or less of my own version of that, with a couple of personal thoughts.
How nuclear plants work
The core of a nuclear reactor is a set of magical stones that heat up when they get close to each other. This heat is transfered to water surrounding the core. In a boiling water reactor like Fukushima and Chernobyl, this water becomes vapor, and it moves a turbine that powers an electrical generator. In pressurized water reactors this water just heats up a secondary water circuit that in its turn becomes vapor and propels the turbine.
Our magical stones are actually small cylinders made of Uranium (and whatever else), that are stacked up to form long and thin fuel rods , that are covered in a Zirconium alloy. These rods are grouped in bundles, and to control the power they generate when put together we put control rods around them. When the control rods are inserted the reaction stops, when we pull the control rods and "uncover" the fuel rods, they do their magic again.
What if there is a problem?
If there is a problem in the power plant, like an earthquake, or if the turbines get stuck or if the pumps that make the water flow and cool the core stop, the first thing we do is to scramthe reactor, insert the control rods all the way to stop the reaction. But there is a catch, the fuel rods keep producing heat for some time after that. It's unfortunately not like putting down a fire, even scrammed some radiative materials still exist inside the fuel rods that keep releasing energy for some time.
So although we "turned off" the core, it still produces power in like 7% or 3% of the original rate, and that keeps going for like 3 days... That means it's crucial that we keep cooling the rector after shutting it down. And that's why in a power plant design there are like tons of different cooling procedures, and alternative plans for cooling the core.
And what if there is a problem with that?
If we fail to cool down the core of a nuclear reactor, the water around the rods and inside the reactor will heat up more and more, getting hotter and producing more pressure. If we don't do anything the reactor will simply blow up, like a pressure pan.
If we fail to cool down the core of a nuclear reactor, the water around the rods and inside the reactor will heat up more and more, getting hotter and producing more pressure. If we don't do anything the reactor will simply blow up, like a pressure pan.
So one of the things we can do is to open up a valve to reduce the pressure, just like in the pressure pan. But there is one unpleasant aspect of that: this vapor and air that is released isradioactive !...
Wait, does that mean that whenever vapour coming from the core is released to the environment we are contaminating the surrounding environment and creating a natural catastrophe???
No!! There are different ways of being radioactive. Radioactive stuff emit their radiation at different wavelengths, power and for different times. Some stuff emit "bad" radiation at high powers and for long times. These are the dangerous stuff. But other radiative materials become inert after short periods of time. These are usually safe. This is the case with this water vapour and air that is released from the core.
So the release of "radiative materials" from a reactor does not mean that we are in immediate danger. If it's just water, we are OK.
What about the meltdown thing?
During a colling failure accident, the conditions might become such that the water level gets lower and lower, eventually uncovering the fuel rods. When that happens the temperature of the rods can get so high that they melt down. But take notice, there are two different things that melt: first the zirconium cladding, and then the actual uranium pellets.
The melting of the cladding can produce hydrogen inside the core, that may explode when expelled to the outside of the reactor. That happened in the Three Mile Island accident, and seems to be what happened in Fukushima. But I am no expert, and it seems this hydrogen gas can also be produced simply by this overheating of water... So the presence of hydrogen in the released vapour from the reactor is not alone an indication of damage to the fuel rod cladding.
The uranium pellets, after they start to be used to produce energy, end up containing lots of different elements, some of them radioactive, and some dangerous. The most famous dangerous radioactive materials that are formed are Cesium and Iodine. Their presence in the released vapor mean some fuel rods have been damaged, and we should be concerned about this Cesium and Iodine.
That is what happened in Three Mile Island, and some radioactive materials were also detected in the released vapors from Fukushima. But how much of this material was released, and where did it move to make all the difference. It could have been traces of Cesium, it could have been miligrams of Cesium... In the Goiânia accident people manipulated the radiative Cesium directly, and that is worse than if the same or larger amount of Cesium has been released into the atmosphere, moving towards the ocean. It seems what happened in Fukushima was this: there wasn't a lot of radiative material released, and it didn't even go towards the city.
A for a melting of the actual fuel, that could really mess up the reactor. The melted pellets can heat up a lot, and can even create this crazy radiative lava-like substance that can even corrode the reactor shell! It's creepy. And it happened in Chernobyl! But Fukushima, and most reactors designed and built by sensible people have a large containment structure around the reactor. This structure is designed so that even if all the core melts into this sci-fi monstrous substance it becomes safely isolated. It mixes itself with lots of graphite and concrete until it stops. Chernobyl was unfortunately not built like this, so the damage to the surrounding environment was much larger than it had to be.
But what happened in Chernobyl anyway?...
Chernobyl wasn't like TMI or Fukushima... In these two there were cooling problems, and we ended up with damaged reactors and some release of radioactive materials. The accidents happened slowly, and engineers faced problems such as equipment being found out not to be working properly, or tsunamis making secondary power sources unavailable for the cooling procedures...
In Chernobyl there was a sudden explosion. The reactor was set for full power, but not responding accordingly because of the so-called xenon-135 poisoning . When the power suddenly started to grow, operators decided to push the control rods again, but then a design flaw in the control rods resulted in a further and sudden increase of power in this very critical situation. That is what caused an explosion (if due to just vapor or hydrogen production, I don't know).
Summing up
The Fukushima accident is fundamentally different from Chernobyl, first of all because In Chernobyl there was a crazy sudden explosion of the reactor that nobody was anticipating, and in Fukushima we have a kind of "controlled crisis". We may be not sure for example of the exact temperature of each fuel rod or of the water level inside the reactor, and "plan B" failed prompting "plan C" and "plan D" etc, like in TMI. But the worst-case scenario is much better than Chernobyl.
That doesn't mean it's all OK, and all perfect. But we are dealing with it the best way possible, and there are no Chernobyl-like damages in sight.
Some people are already using the accident to preach that nuclear energy should be abandoned. What I think should be abandoned is in first place the prejudice and ignorance about the technology and its implementation in different counties and at different times. The other thing that should be abandoned are old-fashioned technologies that should be updated.The Fukushima plant is 40 years old, it was built before Chernobyl and even before TMIhappened. It may have been updated in all sorts of ways, but I would still prefer if the world only had power plants that were designed and manufactured from the ground up by people who lived through these two, now three accidents.
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