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During the March 11 earthquake Units 1, 2 and 3 of the Fukushima I Nuclear Power Plant shut down automatically. Diesel generators kicked in to run the backup cooling system and extract excess heat from the core. Unfortunately, these generators were damaged by the subsequent tsunami and failed about an hour after the ‘quake struck. Moving in new generators, or giant batteries would be very difficult in the best of situations, but with the damage to infrastructure caused by the earthquake and tsunami it was impossible.
The Fukushima I Power Plant is a 4.7 gigawatt 6-unit Boiling Water Reactor (BWR) complex. Although fission reactions stopped when the reactor shut down (when it was “scrammed“) the residual heat from radioactive decay within the nuclear fuel presents a problem. Unlike other reactor designs (e.g. the Advanced Gas-cooled Reactors used in most of the UK’s nuclear power stations) the BWR design cannot cool itself passively and requires power to run the coolant systems. With no supply from the electrical grid, and no diesel generators, no power was available.
Without a functioning cooling system the temperature, and therefore the pressure, inside the reactor vessels continued to increase. The pressure inside the Unit 1 and Unit 2 containment structures increased beyond their design limits. In order to reduce this pressure some radioactive steam was released from the primary cooling circuit, though most radiation will have been removed by filters before the steam reached the atmosphere.
The steam is radioactive because neutrons from the reactor bombarding the coolant water transmute the water’s hydrogen atoms into tritium (hydrogen-3) atoms to create radioactive tritiated water. Tritium is a lightly radioactive beta emitter and beta particles cannot travel very far in air and do not usually penetrate the skin. The steam would also have contained some nitrogen-16, formed in an ‘np’ reaction when neutrons bombarded the water’s oxygen atoms, but as nitrogen-16 has a half-life of only 7.1 seconds it does not present a significant risk.
Even with the release of steam, the pressure and temperature inside the reactors continued to increase. The high temperatures inside the reactor caused the protective zirconium cladding on the uranium fuel rods to react with steam inside the reactor to form hydrogen. This hydrogen leaked into the buildings that surround the reactors and ignited, due to an aftershock in the case of Unit 1 and for unknown reasons in Unit 3. The fact that the zirconium cladding was damaged also indicates that the nuclear fuel was exposed to the air inside the reactor, not covered by coolant and this lends weight to the idea that the fuel itself has melted to some degree. Vent holes have been made in the rooves of Unit 5 and Unit 6 in case hydrogen begins to build up there.
Initially it seemed that neither the Unit 1 or Unit 2 reactor vessels had been significantly compromised. (The roof and walls of the buildings surrounding the reactors are deliberately designed to blow away, to prevent the force from being focused inwards on the reactor.) However, white smoke has now been seen coming from Unit 3 which suggests that there may be damage to the reactor or the containment structure; this would seem possible as the explosion at that site was noticeably larger than the explosion at Unit 1.
Alterations were made to the walls of the Unit 2 building to vent any hydrogen produced there, but nonetheless there was an explosion at the site. This explosion seems more serious than the first two, damaging part of the reactor’s pressure supression system. An “abnormal noise … emanating from nearby the pressure suppression chamber” caused workers to be evacuated from the area. A 20cm crack in the maintenance pit underneath the reactor is allowing radioactive contamination to seep into the sea surrounding the plant.
The problems at Unit 3 may be more dangerous than at Units 1 and 2 because Unit 3 uses mixed oxide (MOX) fuel which contains a small percentage of plutonium oxide as well as uranium oxide; this “burns” at a higher temperature and contains more radio-toxic fission fragments than pure uranium oxide fuel.
Unit 4 was shut down at the time of the quake and the reactor’s core, along with spent fuel rods, was stored in the spent fuel pond towards the top of the building. The failure of the coolant system again led to the formation of hydrogen and an explosion damaged the roof and walls of Unit 4 and caused a fire. This released radioactive material into the air, as the spent fuel ponds are not surrounded by containment vessels in the same way as the reactors are. There have been sporadic fires, and firefighting efforts are being hampered by releases of radiation from Units 1, 2 and 3, but it seems that the fires are currently extinguished.
Seawater is being pumped into the reactor core of Units 1, 2 and 3 and the containment buildings of Units 1 and 3. Workers from the fantastically named Hyper Rescue Squad and the Japanese Self Defense forces have been spraying water onto the reactors to cool them, both from the ground using water cannons and from the air using a modified lead-lined Chinook helicopter. Replacement power lines have been laid and connected to the plant but it is not clear how well the cooling systems (the ECCS and RCIC) will operate considering all the damage that has occured.
The level of radioactivity at the site is a little unclear and will vary across the site according to the location of any leaked material. The most recent reliable figure I could find reported that the border of the site is at about 265 microsieverts (µSv) per hour, significantly above the normal background but not hugely dangerous. The average exposure due to natural background in the UK is 2600 µSv/year; in Cornwall, where there is a lot of radioactive granite the background dose is 7000 µSv/year; and the residents of Ramsar in Iran receive the highest natural background radiation dose in the world at 260000 µSv/year. For a single dose received in a single event, mild radiation sickness will set in at about a million microsieverts.
There does not currently seem to be a wide dispersion of radioactive material outside the border of the plant. Workers have been moved out from, and back to, the plant according to surges in the levels of radioactivity, but current exposure levels for workers are unknown. It is known that one worker did receive a dose of 106000 µSv, enough to cause mild bone marrow supression but not enough to be fatal; and two workers received radiation burns after walking through radioactive water in Unit 3.
The roof of the Unit 1 building was blown off by the hydrogen explosion.
Fukushima will not be “another Chernobyl” or “Japan’s Chernobyl”. The Fukushima reactor does not have a combustible core made of graphite like RBMK-type (Chernobyl-type) reactors do and the Japanese Nuclear Safety Agency has said that damage to the reactor vessel is minimal. The authorities have now combined the incidents at Units 1, 2 and 3 – previously rated individually as Level 5 accidents – into one Level 7 incident.
One of the biggest concerns is that the reactor may undergo a meltdown. This is where the heat inside the reactor becomes great enough to melt the uranium oxide fuel itself. As uranium oxide melts at over 2800°C this molten fuel would causes significant damage to the reactor and make a release of radioactive material more likely. All three reactors are being successfully cooled with sea water, and boric acid is being added to absorb excess neutrons and decrease reactivity; a full meltdown at Fukushima is very unlikely.
The production of hydrogen indicates that the fuel rods were exposed (i.e. not covered by water) for at least some time and this appears to have resulted in the release of some fission fragments. Fission fragments are the “pieces” left over after splitting the big and heavy uranium atoms; they are usually either short-lived and release high-energy radioactive particles, or long-lived but lower energy. (It is the radioactive fission fragments that are largely responsible for the decay heat.)
So far interest has focused on two particular radioactive fragments: iodine-131 and caesium-137. Iodine-131 is significant because the body absorbs it just like normal iodine-127 and it accumulates in the thyroid gland where it is used to produce the thyroid hormones; this can lead to thyroid cancer. The eight-day half-life of I-131 means that it does not present a long-term risk. Caesium-137 is water soluble and has a half-life of thirty years and therefore tends to stay in the body and environment; Cs-137 is responsible for most of the remaining contamination in Chernobyl’s Zone of Alienation.
Trace amounts, billionths of a gram, of iodine-131 and caesium-137 have been detected in water around Fukushima. This level of contamination poses absolutely no danger whatsoever to human health. Were I able to get a class of Fukushima water I would gladly drink it to prove my point.
Iodine tablets are often issued to residents living around nuclear power plants. Taking the iodine tablets, which are made of potassium iodide, before exposure to radioiodine, prevents the uptake of radioiodine because the body already has all the iodine it needs. Exposure to caesium-137 is treated with Prussian blue (iron ferrocyanide) which binds to the Cs-137 and speeds its removal from the body. Both treatments are fairly successful if administered promptly and with good medical support, as is available in Japan. Nobody, in Japan or elsewhere, should take iodine tablets unless instructed to by a medical professional.
“Fallout” is a term used to refer to radioactive contamination of an area and it is fission fragments (along with the tritiated water referenced above) that would make up any fallout from Fukushima. The idea of fallout drifting across nearly 9000 km of Pacific Ocean and killing people on the West Coast of the USA is absurd. Our ability to detect radioactive contamination is very good so any contamination from Fukushima will be detected; detection does not equal danger however. Imagine dumping out a bag of flour at Fukushima in Japan – are you worried about flour contamination in Los Angeles or San Francisco?
As time goes on the situation at Fukushima improves very quickly. It is decay heat that is causing the majority of the problems and in the short term decay heat decreases very quickly. Upon shutdown decay heat is about 7% of the heat produced whilst running and this falls to about 0.3% after ten days. This fast exponential decay is related to the formation of high-energy short-lived isotopes in the fission process, as outlined above.
Whilst the situation may yet turn out to be more severe, it is worth noting that this is the first nuclear emergency ever declared in Japan, a country that produces more nuclear energy than any other country except the USA and France. The March 11 earthquake, at a moment magnitude of 9.0, was the most powerful ever to strike Japan and the fourth largest since records began. The energy released in a moment magnitude 9.0 quake is equivalent to nearly 900 000 times the energy released by the Hiroshima and Nagasaki nuclear attacks combined. It is a tribute to Japanese engineering and building codes that damage has not been more severe. The Fukushima plant has survived at least ten previous earthquakes and had the earthquake not been followed by a tsunami the shutdown would not have been nearly as problematic. The biggest concern for Japan now is not the situation at Fukushima but the damage to infrastructure, including power shortages. This video, from Russia Today, gives you some idea of the size of tsunami that the Fukushami plant had to deal with:
This whole story shows how safe nuclear energy is, and how well Japanese government is reacting to situation. I’m not very enthusiastic about relocating so many people (say 200 000), but perhaps they live so close to the reactor that such precautions are justified. In the case of Chernobyl it was clear aburd.
I really enjoyed your talk at Interesting, and have recycled some of the points you made many times. This account – from your perspective – is really interesting and educational for me, much more so than the newsy angle available elsewhere. So a long way of saying thank you twice.
Thanks for this analysis. Are you doing any media interviews? They could sure use someone like you. It seems that they got one peice of news on Friday and since then it appears to be a game of repeating terror words like “meltdown” louder and louder in the absense of any real updates or information.
Great post! Very thorough and I’m sure it’ll be very useful to generate much class discussion! Thanks for sharing it through PTNC.
Alessio
Really good info xo
We’ve used it in class discussion :)
I have to point out the distance of 150 km to the epicentre if you apply the “it was 8.9 magnitudes and survived!”-apology for nuclear reactors. The earthquake near Kobe 1995 was only of magnitude 7 and destroyed a lot more.
JNES was so far as uncooperative as any governmental agency during the last decades when it comes to releasing exact informations about the accident, so I wouldn’t give much about their statements of radiation and nuclide-levels.
You are of course right, that the world is safe from Fukushima. Unfortunately, there is always another nuclear reactor. What is even more important concering nuclear energy: as long as there are Fukushimas all over the world, there is always another processing plant dumping the waste.
What does it take? Here’s the thing, radio-active grass become radio-active milk …
Everyone is thrilled when the wind is blowing out to sea, but what about those radio-active tuna, and the Japanese rice that is all the rage now.
Who has the right to decide that they can release radio-active steam into the air … NO ONE!!!!! Morons
“Even if it seems like we have put Chernobyl “behind us”, the danger isn’t over yet. Cows on some farms in Upper Austria and in the mountains still give milk with 30 Becquerel per litre or more because it is contaminated with radioactive caesium. The highest acceptable dose of caesium in baby milk is 10 Becquerel per litre.”
There lies no danger in the contaminated “raw milk” for us because in dairies it is diluted with contamination-free milk therefore the caesium level is harmless. The Federal Ministry for Health didn’t think of one thing. Graduate engineer Antonia Wenisch of the Institute for Ecology in Vienna said: “Most of the farmers don’t know that their milk is contaminated. I would like to see the farmer who buys baby milk from the supermarket to feed his children.”
I don’t understand. How can you say the zirconium cladding react to form hydrogen which was then released AND also say the containment wasn’t breached?
The design of the Boiling Water Reactor that requires power to safely cool down is flawed. This has been known for some time, and yet its lower cost is (or was) consisdered “safe” for more than 40 years. It’s time for these types of reactors to be retired and replaced with a design that does not require emergency power to handle an emergency.
But, the almighty cost equation seems to rule. And the sad part is that there will be no push to shut these reactors down–even now.
If a substantial quantity of the fuel inside one or more damaged reactors were to reach a temperature high enough to melt, how would this result in breach of containment?
Basically, the molten core would sit at the base of the containment vessel and melt its way through. It would take a long time and Fukushima likely has a “core catcher” in place for exactly that possibility.
Containment hasn’t been significantly breached. It is a lot easier for tiny hydrogen atoms to escape from the reactor vessel than huge, heavy fission fragments.
Thank you sir.
If I may, how is it possible that a containment vessel is constructed of materials which the nuclear fuel can melt? We must be discussing a situation in which the fuel is generating heat because there is nothing to moderate the nuclear reaction after the material collects at the bottom of the vessel, so is the problem that the temperatures are practically unbounded, so that even high temperature ceramics like an induction furnace liner would eventually be consumed?
There is no material that is strong enough that will withstand the temperature of a melting fuel core. Pure tungsten or furnace liner might be able to withstand the heat, but they would not be able to withstand the forces exerted.
Again, thank you.
And, if the containment vessel were constructed with steel reinforced concrete and lined with material that can withstand the temperature of molten core material, or even jacketed with a coolant system, still this would not work, because it would have to withstand the heat without allowing it to destroy the concrete over an immense span if time?
Basically, yes. There is no material that is heat-proof in the sense that it does not transfer heat.
Japanese nuclear safety is not that good.
TEPCO operated plants have had severe security problems: systematic cover up of data showing cracks in reactors, falsifying air tightness measurements of reactor core, concealed an emergency shutdown from authorities, faking the coolant temperature data etc. This Fukushima Daiichi plant was the same plant that had falsified coolant water temperatures in the past and used them to pass inspections.
Japanese have had several nuclear accidents caused by lack of following safety standards. Most severe was Tokaimura criticality accident.
talk about being a soothsayer. really great info. you have some awesome knowledge.
My wife and I are scheduled to travel to Hawaii tomorrow for a holiday, and she’s scared. I note above that you write that the idea of fallout killing people on the West Coast of the US is absurd. Would you say the same for Hawaii? What might the affect be if there is a full meltdown of one or more of the reactors– a situation I take to be more likely after this latest (third) explosion.
Thank you for the excellent post, and for sharing your knowledge.
It looks like radioactive material is leaking out of the reactor rather than being thrown out at a great rate as it was in the explosion at Chernobyl. It’s highly unlikely that this material could drift across 6000 km in any great concentration. In any case, it looks like any material expelled from Fukushima would likely not drift over the USA.
Enjoy your holiday.
Thanks very much for your reply. I’m embarrassed to admit I forgot to mention a key detail in my earlier post: my wife is four months pregnant. I don’t suppose that changes your answer, but it did seem worth mentioning.
And if I might ask one more question: what do you mean by “any great concentrations?” Undetectable, or insignificant to human health?
Thanks again for taking the time to reply.
Radiation does pose more of a danger to pregnant women than to women who are not pregnant, so you may wish to bear that in mind.
I imagine that the US government knows far more about the situation than I do and will advise its population appropriately. They are already flying monitoring flights off the Japanese coast from the USS Ronald Reagan and I imagine that they will continue to do so; I also imagine that this monitoring programme will be expanded and will focus on Hawai’i and the West Coast in particular. I would have no qualms about heading off on holiday to Hawai’i tomorrow, but you may decide to err on the side of caution. Until more details about the amount and type of radiation released are available it is very difficult to precisely quantify the risk.
Very interesting information regarding the difference between Chernobyl’s core type and Japan’s. I feel like I am better informed. Thanks for the info!
Hello! Very nice post with clear explanation.
Just wanted to add, that it seems the INES level has been upgraded to 6 out of 7
(“The incident at Fukushima-1 nuclear power plant has been assigned the sixth level of emergency out of seven according to the INES (International Nuclear Events Scale”, http://rt.com/news/meltdown-radiation-japan/). Other reports say it is now being upgraded to 5 out of 7.
One more thing: would you please be so kind to explain the units of measure? Some use Bq/m3, some rem, some Sv… (check for instance http://inters.bayern.de/mnz/php/ifrmw.php?station=812&komp=207&tbltyp=2). I am really confused. And what appears to be the “normal” radiation level?
The move to Level 6 has been suggested by the French nuclear agency; it is not an official change.
I wrote a post about the different units used to measure radioactivity which might help you understand the different measurements. The “normal” radiation level varies depending on where you live but it’s probably about 3 millisieverts (mSv) per year or 0.3 microsieverts (µSv) per hour.
Thanks so much for all the info you have provided.
One thing i still cant understand, and havent found any info, is why its taking so long for it to cool, and how long dows it take to be cooled and completely safe?
It takes time to cool because the fission fragments produced by the fission process are decaying radioactively. As they do so, they produce heat.
Glad to see a sensible post. I would like to add a couple things also, as a nuclear engineer at an American BWR-4. Firstly, there has been confusion in the media about the design of the reactor. The “reactor” is a 6″ thick pressure vessel, with the core inside. Outside this is primary containment, a reinforced concrete structure 3 to 6 feet thick that surrounds the vessel. Outside this is the reactor building, also referred to as secondary containment. The reactor building is what exploded in units 1 and 3.
Hydrogen builds up in the reactor vessel and is vented into primary containment to reduce pressure and remove the hydrogen. The same process happens in primary containment, albeit at lower pressures. The pressures reported here are primary containment pressures, not vessel pressures. Normal reactor vessel pressures are around 1000 psi. Primary containment is normally at negative gauge pressure. An automatic shutdown occurs at 1.72 psi gauge. Primary containment is designed to handle up to 40 psi. Reported pressures were 120 psi. To reduce pressure, primary containment is vented and the hydrogen builds up in the reactor building. Also note that the vented steam is run through particulate filters which capture the vast majority of the radioactive particles.
In unit 2, there appears to have been a malfunction in the primary containment vents and the hydrogen built up and exploded inside. Due to this, Japanese officials announced there could be a leak in containment, though it is unclear whether they are referring to the reactor vessel or primary containment. They have not been able to cover the core fully since (~20 hours with about 3/4 uncovered). Prior to this the core was fully uncovered and likely dry for about 2 hours due to a pressure relief valve malfunction. I should say, that is 3/4 of the area where the core should be. There is a decent chance most of the core is being cooled because it melted down into a lower portion of the vessel.
At this point, units 2 and 3 are outputting about 3 MW, and unit 1 is about 1.5 MW. Power levels continue to decrease, and as long as they maintain current cooling in all 3 units, the situation in the reactors shouldn’t get worse. The biggest concern now is the spent fuel pools which are located at the top of the reactor building. It is unclear of the status of the pools in units 1 and 3, though radiation levels don’t seem to be nearly high enough to support their destruction. If this is so, then cooling of these pools will need to be done also, though much less significant than the reactors. Only the freshly offloaded spent fuel poses a serious concern with keeping it cool, as the older fuel can be air cooled (though at elevated radiation levels). It appears that this worry, while a huge concern, may be passing.
I would like to add that preliminary reports give the indication that this plant was not designed to handle any type of significant tsunami. The emergency electric equipment that are powered by the diesel generators in an emergency are located underground, and are still flooded. This means even if they bring more diesel generators to the site, there is no way to hook them up. This is not how these same plants were designed and built in the US, so it is unclear why they were in a tsunami-prone area.
Finally, the BWR is not a fatally flawed design, though this particular plant appears be. The newest BWR design specifically addresses the shortcomings mentioned here, specifically a core design that allows for cooling through natural convection and a gravity-fed cooling system that gives operators at least 36 hours of safe shutdown before any operator actions must be taken. Also, the fuel pool has been relocated to the bottom of the reactor building.
I will gladly answer any questions people have regarding this particular design or accident. I will do my best to respond quickly, though I may not be able to answer all questions.
P.S. I would not worry about radiation in Hawaii unless significant damage occurs to the spent fuel pools. This is the only source of radioactive material that has the potential to spread very far. As long as these don’t catch on fire for a significant amount of time, the radioactive material won’t spread to a significant degree outside of Japan.
Best post or news story I have seen or heard. As a retired Nuclear Engineer, I am glad to get this info as it generally concurs with what I have been saying to neighbors.
Question
Is it safe to travel with a 6 mnth old baby to Japan? My daughter and her husband are scheduled to leave for three weeks in Japan starting April1st. The airlines have suspended flights until the end of this week. Although their current itinerary has them changing flights in Tokyo’s Narita airport, their final destination is just outside of Hiroshima. Our son in law’s family are located there and they report that everything is quite normal right now. What questions should our son in law and daughter be asking to make an informed decision about continuing with this planned holiday? I have read all of the comments about nuclear waste floating across the Pacific to North America but what about air contamination in Japan?
Young children are at more of a risk than adults as their cells are dividing more rapidly. If they can find out, the important thing would be the radiation level at the places they intend to visit; Hiroshima is a long way from Fukushima. Unless things deteriorate drastically (if there is a large and sustained fire, for example) then I would think they are relatively safe.
I would agree, though stay tuned to the news about the situation. Radiation levels in Tokyo are elevated, though not to a significant degree. I wouldn’t even worry about a meltdown, even if the core melts through the vessel and containment it’s still only a local contamination problem. The possible widespread contamination concern is the burning fuel pools. If they can get that under control, then the effect is limited. If not, there is potentially a very serious contamination problem in a significant area of Japan. Hiroshima is very far away, so there likely won’t be seriously affected, but just keep up to date. The situation has gotten worse at every step, however, so just keep up to date.
I would also like to clarify my position about Hawaii. Even in a worst case scenario, there likely won’t be significantly elevated radiation levels on any of the islands. There will likely be detectible amounts, but nothing to worry about. That being said, see what is being said on the news, because that is due to weather patterns as much as anything else.
Well actually there is material which has sufficient heat resisting capacity to endure the pressure/temperature of a core meltdown indefinitely. It is rediculously expensive though being a product of experimental meso-atomic chemistry, specifically using carbon-12 isotope lattices with the outer shell electron orbitals compressed due to a replacement of the electrons with similarly ionic mesons; the increased relative atomic mass of which causes a partially decaying orbit made stable by the repulsive effects of the inner eletron shells. In effect this allows the creation of non-degenerate super dense materials without the need for super-conductor like temperatures. Remember I’m referring to mesons here not muons as obviously there is no lepton that can replace an electron in atomic chemistry.
Just thought I’d mention it as I am actually involved in the creation of these atomically modulated materials and the next generation of nuclear power plants intend to used this or materials like it as secondary reactor shielding. Potentially it allows for the containment of a primary fusion reaction, thus meaning that fusion power plants are a possibility in the near future.
Good article.
Questions
What are the differences between this accident and the one at Three Mile Island?
How can Fukashima be at an INES level 4 and TMI a 5?
How much total radiation was released in both the #’s are confusing?
According to Kyodo news the core has partially melted in reactors 1-3.
The containment vessel in # 2 has been compromised.
They have now found iodine and cesium in the tap water in the city of Fukushima.
Wouldn’t that mean the core had melted through to the water table?
Wouldn’t that be enough to classify this incident higher than level 4.
It seems like this is worse than Three Mile Island to me.
They have a problem with several reactors when TMI was only 1.
It does seem that matters may not have got so serious if the designed method of cooling had not failed (i.e. water pumps with diesel generator backup power). You indicated that the diesel generators failed about 1 hour after the earthquake struck. Do we know why? Were they flooded by the Tsunami? There was presumably a common problem with all the generators as you would not normally expect several generator to fail simultaneously after only an hour or so.
As I understand it the generators were located underground and were flooded out. The emergency power connections were underground with them and this made connecting alternate power sources to the station’s “emergency grid” difficult.
The accident at TMI was different because it was caused by equipment failure coupled with operator error. TMI is a pressurized water reactor, which is similar to a BWR but with key differences. Water does not boil in the core of a PWR, but gets hot (primary coolant loop) and then sends the heat to a heat exchanger. The water on the other side of the heat exchanger (secondary loop) then boils and is sent through the turbine to generate electricity. At TMI, maintenance personnel accidentally tripped the pumps that pump water in the secondary loop. The backup set of pumps automatically started, but a valve was misaligned on the lines for the backup pumps due to maintenance done weeks before, and they could not inject water into the heat exchanger. This was not realized by operators and the heat exchanger boiled dry on the secondary loop. The primary loop then started to heat up and pressure rose, causing an automatic shutdown. The primary side has a pressurizer attached to the line to maintain pressure, and when pressure rose, a valve opened that let steam in the pressurizer out to a tank. After this, operators reestablished flow in the secondary side and core cooling was restored. The valve on the pressurizer was then closed by operators, but due to a malfunction, the valve didn’t actually close. Due to a bad sensor, operators didn’t realize this. Pressure continued to lower in the primary loop until water in the core began to boil. An emergency system began pumping water into the primary loop, but because operators thought there was plenty of water in the primary loop, they manually shutdown the emergency system. This continued until the core became uncovered and fuel started melting. Eventually the malfunction was realized and the core was recovered with water. Due to the fuel melt, radioactive steam and hydrogen were vented into containment, and eventually the atmosphere after being filtered for radioactive particles. This release was very small, though radiation levels inside the plant were much higher.
This accident resulted in a complete change in US nuclear operations from a system based approach to a symptom based approach. Changes in control room layout were also done to facilitate accident prevention.
PWR plants are also different from BWRs in their containment. PWR containment is about the same size as the entire reactor building at a BWR. This is done because they surround the entire radioactive primary loop within containment instead of sending radioactive steam into the turbine building like in a BWR. This definitely has it’s advantages in certain accident scenarios, though there are more parts that can fail, and the pressures in the primary loop are about twice as high as in a BWR.
The Fukushima accident has been classified a level 6 now. This is more accurate.
The cores of units 1 and 3 are heavily damaged, though are covered with water at the moment. Unit 2 has not been covered completely in about 72 hours, though due to the longer time it was covered, less fuel damage is expected. Operators have been trying desperately to cover unit 2, and they expect a leak in the pressure vessel into primary containment. Primary containment pressure went from 120 psi to ambient after the blast in unit 2, so a leak in containment is pretty much guaranteed. A wave of radioactive steam from unit 3 forced the remaining operators to evacuate the plant for a time, though they are back now. The steam leak could indicate a crack in primary containment of unit 3, but that is unclear right now.
I would greatly appreciate people to pass on this part. There is mass confusion about what radiation is and what radioactive material is. The plant is emitting radiation, but the radioactive particles is of more concern. Radiation can be shielded from, but radioactive materials outside the plant will emit radiation after they leave the plant. This is harder to manage and is what is being talked about when they talk about contamination. The reason Chernobyl was so awful was because around 10% of the core was ejected into the atmosphere. These radioactive particles spread and emitted their radiation in the environment and continue to do so today.
As a comparison of accidents, though it is not wholly accurate in terms of accident comparison, the highest levels of radioactivity detected outside the plant at Three Mile Island was 1 milliSieverts. At Fukushima, highest levels outside the plant have been 400 milliSieverts. At Chernobyl, levels detected were 16,000 milliSieverts.
I don’t think the core melted into the water table because it likely is not molten at the moment since any melted part would have sunk to the bottom of the vessel where the water is. Also, the plant is on the ocean, so any material that leaked out should go into the ocean, not the water table. The contamination is likely from the earlier fire on the refuel floor of unit 4.
The diesel generators are located outside on the ground. The part that is underground and flooded is the electrical equipment that sends power from the generators to the plant.
In a possible sign of good news, Japanese officials say they are close to restoring offside electrical power to the plant. If they do, and the electrical equipment still functions, and the mechanical systems are not too damaged from the explosions / earthquake, they might be able to end this rather soon. After seeing the unit 3 explosion, though, I am skeptical if much of anything still works there.
Is it right that a rod is spent when 5% of the energy (if that is the right word) is used from the rod’s original energy?
If that is right and I am guessing there are at least hundreds of rods in the effected buildings there must be a massive amount of potential energy that is spending time out of any water and possibly melting. So, if there ends up being many large bodies of rods why can there not be a reaction that can not be stopped? And, if it did meltdown, could it “drill” a hole to a molten level of the planet and create a volcano?
Thanks
The idea of a molten core “drilling” its way down to the Earth’s core is absurd and cannot happen. The China Syndrome was a movie, not real life.
I worked for 40 years in the nuclear inustry, 30 years supervising on the reactor / refuelling side and also conducting ‘what if ‘experiments. All types of reactors will have serious problems of some sort if not shutdown ,contained or cooled properly in the first few days.
Japan has a mega disaster impending if cooling is not restored. I estimate days not weeks
We conducted experiments in the 80s. on loss of cooling and even if all control / safety functions are initiated, the temperatures in the reactor will continue to rise onwards and upward for a very long time. ie. meldown.
very good and helpful website used in year 10 class work on radiation and co.
I’m not sure what reactor you worked at. If it continued to heat up after being shut down, where was that energy coming from? Decay heat due to fission fragments decreases really fairly quickly.