The way to stop a nuclear fission chain is therefore to intercept and slow down the free neutrons. Nuclear reactors use control rods made of elements such as cadmium (Cd), boron (B) or hafnium (Hf) known for their ability to efficiently absorb neutrons
By John Mattson, translated from the Scientific American website www.sciam.com
How does a nuclear reactor work?
Most nuclear reactors, including the Fukushima Daiichi power plant in Japan, are actually elaborate kettles that boil water very efficiently to generate electricity. They utilize the energy released from nuclear fission: the breaking of a heavy atom into two smaller atoms. The process emits heat and free neutrons. If another atom absorbs one of these neutrons it becomes unstable and also undergoes fission, releasing more heat and more neutrons. This chain reaction is self-supporting and produces a continuous and stable supply of heat to boil the water, which drives turbines that generate electricity.
How much electricity do the nuclear power plants in Japan and other places in the world produce?
Japan operates 54 nuclear reactors that produce 280 billion kilowatt-hours per year. Japan is the third largest producer of nuclear power in the world after the United States and France, according to the International Atomic Energy Agency. Six of these 11 reactors operate under the power plant Fukushima Daiichi, which was severely damaged by the earthquake that struck northern Japan on March 2011, 54. The station was connected to the Japanese electricity grid in the 70s.
Nuclear reactors around the world produce about 15% of the electricity in the entire world. In Japan, 30% of electricity comes from its nuclear power plants. The United States produces more nuclear electricity, but its share in the total energy sources of the United States is smaller, about 20%, after coal (45%) and natural gas (23%).
What is the fuel of a nuclear power plant?
Most nuclear reactors operate on uranium that has been "enriched" with uranium 235, an isotope that undergoes fission easily. (Isotopes are atoms with a different atomic mass of a certain element, they differ from each other in the number of neutrons.) The most common isotope in nature is uranium 238 (92 protons and 146 neutrons), but this isotope does not fission, so electricity producers increase the concentration of uranium 235 ( 92 protons and only 143 neutrons) to a few percent. This concentration is enough to sustain continuous fission and regular electricity production. The enriched uranium is packed in round bars coated with a metallic protective layer, for example of zirconium alloy (Zr). In the core of the reactor, the uranium rods are immersed in water.
Reactor number 3 at Fukushima Daiichi operates on a fuel called mixed oxide (MOX) in which uranium is mixed with other fissile materials, such as plutonium produced from spent nuclear fuel or from the dismantling of nuclear bombs.
How do you shut down a nuclear reactor?
Continuous nuclear fission depends on a continuous supply of free neutrons passing from atom to atom: the neutrons released in the fission of one atom stimulate the fission of another atom. The way to stop a nuclear fission chain is therefore to intercept and slow down the free neutrons. Nuclear reactors use control rods made of elements such as cadmium (Cd), boron (B) or hafnium (Hf) known for their ability to efficiently absorb neutrons. When there is a fault in the reactor, or when the operators want to shut it down, they lower the control rods by remote control into the depth of the water in the reactor core to absorb the neutrons and stop the nuclear reaction.
Can a nuclear reactor fuse even after the nuclear reaction has stopped?
Even if the control rods faithfully fulfilled their role and stopped the fission reaction, the uranium rods treasure a lot of heat. Moreover, the fission products, the smaller atoms created from the breakdown of uranium nuclei, are radioactive and their decays release a lot of heat. Therefore, the reactor continues to produce and emit heat even when fission stops.
If the other parts of the reactor are working properly, the pumps will continue to flow coolant (usually water) to remove the heat that the shut down reactor emits. In Japan, the earthquake and tsunami that followed caused power outages that cut off the reactor's cooling systems. According to press reports, the power plants' backup generators failed immediately afterward, leaving the reactors uncooled and in great danger of overheating.
Without a constant supply of coolant, the hot reactor core could boil and vaporize the water in the core and expose the uranium rods. If the fuel rods are left exposed they may break. When this happens, hot radioactive fuel begins to accumulate at the bottom of the chamber containing the core. In the worst-case scenario of a meltdown, the pool of hot nuclear fuel could melt the steel holding the facility and penetrate through it, and through other barriers and out. In such a case, a huge amount of radioactive radiation could be released into the environment.
How can you prevent melting of the reactor?
Reactor operators in Japan have made several attempts to cool the reactors, including pumping seawater into the reactor to replace the depleted coolants. The Tokyo Electric Company also pumped boric acid, a material for absorbing neutrons, into the reactors.
How serious is the situation?
As of this writing, three reactors at Fukushima Daiichi are significantly damaged. In units 1 and 3 there were explosions that destroyed external walls. The explosions were probably caused after the ignition of hydrogen accumulated in the reactor. The hydrogen is formed in the reaction between the zirconium metal that wraps the fuel rods and the cooling water, a reaction that indicates the extremely high temperatures prevailing in the reactor. However, it appears that the inner tanks are still undamaged. A third explosion was reported on March 15 in reactor 2, and the situation there currently appears to be more serious. The water level in the storage pool surrounding the reactor tank dropped after the explosion and this indicates that the cell protecting the reactor was damaged.
In all three units, 1, 2 and 3, the water level dropped to such an extent that the groups of rods were exposed for some period of time. It is believed that these fuel rods were damaged. In addition, a fire that broke out in a storage pool for used rods in reactor 4 also endangers the few workers who remained there.
Is this event similar to previous events in Chernobyl and Three Mile Island in the USA?
Nuclear mishaps are rated on a seven-step international scale (INES scale), and Japanese officials initially rated the mishap as a level 4 mishap, meaning "accident with local impact." But physicist Frank von Hippel of Princeton University told the New York Times that the situation at Fukushima Daiichi is "much worse than it was at Three Mile Island." The accident at this reactor, which was the worst in the US, was classified as a level 5 accident, meaning "an accident with a wider impact".
The accident on Three Mile Island in Pennsylvania, which happened in 1979, was due to a combination of a fault in the cooling system and human error, and it caused a partial meltdown: about half of the reactor core was severed and created a puddle at the bottom from all the steel pressure. The steel tank was not damaged, but radiation leaked into the environment.
The accident at Chernobyl in 1986 was much more devastating and was rated a level 7, or "major accident" on the INES scale. At the Chernobyl power plant in Ukraine, which was then part of the Soviet Union, a voltage spike caused an explosion in one of the reactors, which caused a massive release of radiation into the air. Two workers died within hours, according to a US Audit Committee report. 28 workers died in the months that followed from radiation poisoning. The fallout from Chernobyl spread over a very large area, so it is very difficult to accurately determine its effects on health. A UN scientific committee report stated that 6,000 people who were under the age of 18 at the time of the explosion, and who were in Ukraine, Belarus or Russia, suffered from thyroid cancer until 2006, and that this is a small part of the people who probably contracted the disease due to exposure to radiation.
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In addition, I just saw Michael's answer and it contains more information than my first answer to you. I absolutely agree, the way and speed in which the system becomes supercritical is also very important.
Haim,
Another note, uranium 238 is not considered a fissile material in reactors, it is a sink of neutrons, so when we talk about critical mass we are talking about uranium 235, that's why we talk about uranium enrichment.
Haim:
Ehud may complete the answer and correct the necessary correction, but in the meantime:
Critical mass is not just mass.
The critical mass is also a function of the concentration of the fissile material (lower in a liquid than in a solid) and its geometric distribution (a sphere will form a critical mass much more easily than a puddle).
The rapid concentration of the mass in one area is also important because if the material is not concentrated quickly enough, a small explosion will occur in an area that has reached a critical mass and it will disperse the rest of the mass and prevent it from participating in the explosion. In the melting of the rods, the concentration (if you can call it a concentration puddle) is not fast enough. To achieve the concentration speed required in atomic bombs, very fast and synchronized mechanisms are used (such as an external explosion that sends the parts of the mass simultaneously to the center).
Haim,
A reactor also reaches critical mass in normal operation. The question is when a chain reaction gets out of control what stops it and when? When the fuel rods are separated and merged there is usually no problem of criticality since the critical mass of a system depends on the geometry. A different mass is required for a homogeneous ball of fissile material and for a surface of fissile material. In the surface (as will be created if the core breaks off) there is a very large escape of neutrons through the sides of the surface. The neutrons emitted through the surface walls do not contribute to the chain reaction. In my opinion (I'm not sure about this point) the total mass of fissile material in a reactor is not much higher than the critical mass in an optimal configuration (of course you have to "keep" some fissile material aside to create energy but I think it's not much. In summary the melting of the fuel rods is not leading to the creation of a critical mass.
Atomic explosion, meaning an explosion similar to the bomb dropped on Hiroshima and Nagasaki. The principle in nuclear bombs is to reach a critical mass of 235 or 238 pines and then uncontrolled nuclear fission occurs in the material.
This is what happens in a failed nuclear reactor. The fuel rods heat up and separate and fuse and then there is a critical mass. So why can't uncontrolled fission of the atomic nuclei occur?
Haim,
I don't know why you call it an atomic explosion. The main thing to understand is that a reactor is not a bomb (although the scientists in Nazi Germany held this idea). In a reactor, when an uncontrolled reaction occurs, it creates heat, the heat causes an explosion, usually chemical (hydrogen explosion), and then with the dispersion of the fissile material (the uranium), the root reaction stops. Reactors are built on a very delicate balance. The standard of the reactors is such that any heating will damage the nuclear reactions (the reactivity of the reactor). That is, the reactors have a negative thermal feedback (the nuclear reactivity decreases when the temperature increases). Engineering miners are safe and as they progress they become more and more safe. There was also Edward Teller's idea to build a reactor that is inherently safe, he called it Ph. D proof
So even someone with knowledge (having a PhD) cannot intentionally harm the reactor. The protection of such reactors is from the laws of nature and not from the laws of engineering. This idea gave birth to the TRIGA miners
Thank you Ehud
I understood that in a plutonium reactor a nuclear reaction could eventually occur - RUN AWAY.
Hope for the best, don't know how to prepare for the worst.
Why?
Life
In a reactor to create energy, a nuclear explosion will not occur, ever.
Why is no one answering my question?
Itzik C.
The operation of a reactor can be understood schematically and simplistically as follows:
There is the nuclear mechanism - the chain reaction that produces during the operation of the reactor the heat used to generate electricity.
There is a heat removal mechanism responsible for removing the heat from the area of the fuel rods where the nuclear reaction takes place.
Regarding the nuclear reaction, it occurs through a chain reaction. A chain reaction is described as follows: a neutron fissions a nucleus of a fissile material (uranium 235) as a result of the fission, energy (heat) is emitted, but no less important, additional neutrons are emitted which cause further fissions. It is very important that the chain reaction be stable, that is, for each neutron that causes fission and causes the emission of additional neutrons, only one of the emitted neutrons should (on average) cause additional fission. In the event and on average more than a single neutron causes additional fissions the process goes out of control and there is a nuclear malfunction. In case and on average less than a single neutron leads to fission the nuclear reaction decays.
A reactor is delicately balanced so that the neutron population remains constant in time. What affects the neutron population? There is the creation of neutrons in fission and there is the disappearance of neutrons by being absorbed by non-fissile materials (control rods for example) and the leakage of neutrons outside the reactor. All the processes have to be balanced.
To your question, you don't need to kill all the neutrons to stop the reactor, you just need to worry that more neutrons will disappear than are created and the population will die out by itself. The considerations are very similar to the considerations of population growth or decline, only that for humans there are two species and the number of pairs that can be produced must be examined. For neutrons the question is simpler.
In Japan, the chain reaction was turned off by the control rods, but even after that, heat continues to be generated in the fuel area (regardless of neutrons).
Itzik C.
What you ask for is what happens anyway.
Scientists willing to comment here voluntarily do so.
Regarding the mechanism of operation of nuclear reactors, you can get the most comprehensive information from Mahud.
He has already written several comments on your question, among them this one:
https://www.hayadan.org.il/haim-cohen-from-ariel-on-atomic-power-station-1403114/#comment-287557
And this one:
https://www.hayadan.org.il/haim-cohen-from-ariel-on-atomic-power-station-1403114/#comment-287545
Hello to Mr. Blizovsky
In many articles readers have substantive questions that would be useful to answer in order to expand understanding or clarify additional points (such as my question above)
It would be good if someone with authority would respond to them. Maybe one of your scientist acquaintances who can occasionally glance at articles on the subjects of his specialty and answer the readers.
I believe they will do it even voluntarily, if they have public opinion in their hearts.
I'm interested in knowing what needs to happen for an atomic explosion to occur?
Curriculum Vitae of Dr. Durkovich
http://www.porthopehealthconcerns.com/CV%20-%20Dr.%20Asaf%20Durakovic%20M.D,%20Ph.D,F.A.C.P.htm
Those interested in a detailed overview of what is currently happening in Japan and what to expect
Dr. Asaf Durkovic, a doctor and internationally renowned expert on the subject, is interviewed
http://www.youtube.com/watch?v=A_qtC8R20Wo
To the point, because I'm too expensive or simply because it ended 65 years ago!!
I took into account an ironic response like yours, in fact there is no nicer way to say it..they didn't start building reactors a decade ago but more than fifty years ago, so the ten years of grace are long behind us, they had to realize at some point that something was wrong with the connection between the fuel and the reactor itself and that Invites disasters like this!
Speaking of Manhattan, I tried 3 years ago (in talkbacks) to push the idea of developing a nuclear fusion mechanism in the style of the Manhattan Project...that is, to raise the urgency to such a level that it would justify international cooperation...because at the current rate, hundreds of billions have been invested for fifty years and in the meantime we only get explanations as to why not It's possible, I could have given that much cheaper - just a few million and more with a riotous party on Fridays!
point:
In Chernobyl, maybe only a few dozen died, but after those dozens, several thousand died easily.
https://docs.google.com/viewer?url=http%3A%2F%2Fwww.iaea.org%2FPublications%2FBooklets%2FChernobyl%2Fchernobyl.pdf
What matters is not whether they died with difficulty or easily but the very fact that they died.
I want to warn against overly dramatic news updates.
Behind such and other publications about energies there is a lot of money.
In short what I am saying is that there are energy companies that will make a lot of money by sowing panic and fear in the public about a nuclear reactor.
Things must be taken in proportion.
In Chernobyl, barely a few dozen were killed following the "great disaster"
Several hundred are killed in road accidents in Israel every year.
Take everything in proportion.
Everything is money.
Cyclone, I don't understand why they didn't take you to the Manhattan Project.
Clear and good article
It's an outdated reactor, it's clear that today something can be designed that will be resistant to such failures.
It's just better for all of us to get off this polluting and dangerous technology altogether.
When I need to turn off a kettle, I don't turn on rods, I just turn off the electricity!
When you want to turn off a fuel-powered power station, simply close the tap and without fuel there is no heat!
But when you want to shut down a nuclear reactor, you insert rods that absorb the free neutrons and that's how the uranium cools! To me it sounds like a serious failure in thinking!!
Someone said and did something 50 years ago and since then they haven't checked what and how it can be improved... first of all the rods have a certain heat which can be known and probably known, therefore from the moment the supply of neutrons is stopped (in any way) a known amount of liquid or other substance must be injected to absorb the heat The remaining (known in quantity)
!
Secondly, the reactor could have been designed so that the rods could be launched into a deep bottom of hundreds of meters filled with liquid that would cool the rods or just sand that would absorb the liquid uranium into it!
In general, the fact that the reactor is built as a single unit is an unwise act that does not take into account different scenarios in which there is an immediate need to disconnect the uranium from the reactor.. it's roughly like a fuel tank inside a power plant!!
There is no doubt that this is a fundamental design failure that requires a redesign of nuclear reactors based on more realistic assumptions!!
The article is good because it contains additional details - such as rods to dampen the chain response.
The article explains that without the operation of cooling pumps, the uranium rods continue to heat up despite the rods
the damping The article does not explain that when the uranium rods are separated into a kind of puddle, the temperature of the core will rise to an unbearable degree, and this is as a result of the concentration of all the uranium in one "puddle" block.
The article does not state that if this stage is reached, (stage 7) the entire 40x40 km area will be polluted for many years.
In conclusion, nuclear savings for 40 years may turn into 1600 square kilometers that cannot be lived in for 30 years.
Please take a pen and paper and do the math.
I understand that the control rods absorb the neutrons that pass through the rods and thus stop the process.
But what about the neutrons that move inside the rods and can continue the chain reaction? How do they fade?
Around 2000 – 2500 Celsius.
For those interested in the exact stages of destruction:
http://en.wikipedia.org/wiki/Nuclear_meltdown
I join Assaf to what degrees do these miners reach (from someone who really understands... I also read the article and realized that there should be degrees for steel melting, forgive Omer.)
collect:
When the cooling system is working then the reactor is probably operating at a few hundreds of degrees Celsius to create superheated steam. When the cooling system fails, then the rods are cooled by air (because that is what is in their environment) and the system reaches thousands of degrees. I believe this not because I understand reactors but only from the description in the article.
They didn't mention the temperatures... how hot is the heat?
What a beautiful article. Just a pleasure.
Written in a very simple and understandable way.
I wish all the articles were written in such a simple and understandable way.
I know it's a bit difficult because many readers are people who understand in the field of science.
But there are also quite a few people who are not knowledgeable and have not studied the field, and as a result do not know the concepts.
But still really like the scientific subjects.
I'm sure you can find some intermediate way of writing that will be a little easier to understand.
Anyway. You do a great job and I enjoy reading.
(It's just that I need maximum concentration when reading some of the articles).
Good day and good luck.
Thanks, was fascinating to read