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Fukushima Nuclear Accident – a simple and accurate explanation

Twitter updates: @BraveNewClimate

New 15 MarchFukushima Nuclear Accident – 15 March summary of situation

New 14 MarchUpdates and additional Q&A information here and Technical details here

福島原発事故-簡潔で正確な解説 (version 3):(東京大学エンジニアリング在学生の翻訳) (thanks to Shota Yamanaka for translation)

Other translations: Italian, Spanish, German, 普通话


Along with reliable sources such as the IAEA and WNN updates, there is an incredible amount of misinformation and hyperbole flying around the internet and media right now about the Fukushima nuclear reactor situation. In the BNC post Discussion Thread – Japanese nuclear reactors and the 11 March 2011 earthquake (and in the many comments that attend the top post), a lot of technical detail  is provided, as well as regular updates. But what about a layman’s summary? How do most people get a grasp on what is happening, why, and what the consequences will be?

Below I reproduce a summary on the situation prepared by Dr Josef Oehmen, a research scientist at MIT, in Boston. He is a PhD Scientist, whose father has extensive experience in Germany’s nuclear industry. This was first posted by Jason Morgan earlier this evening, and he has kindly allowed me to reproduce it here. I think it is very important that this information be widely understood.

Please also take the time to read this: An informed public is key to acceptance of nuclear energy — it was never more relevant than now.


NOTE: Content Updated 15 March, see:

We will have to cover some fundamentals, before we get into what is going on.

Construction of the Fukushima nuclear power plants

The plants at Fukushima are Boiling Water Reactors (BWR for short). A BWR produces electricity by boiling water, and spinning a a turbine with that steam. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water returns to be heated by the nuclear fuel. The reactor operates at about 285 °C.

The nuclear fuel is uranium oxide. Uranium oxide is a ceramic with a very high melting point of about 2800 °C. The fuel is manufactured in pellets (cylinders that are about 1 cm tall and 1 com in diameter). These pellets are then put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1200 °C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form assemblies, of which several hundred make up the reactor core.

The solid fuel pellet (a ceramic oxide matrix) is the first barrier that retains many of the radioactive fission products produced by the fission process.  The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.

The core is then placed in the pressure vessel. The pressure vessel is a thick steel vessel that operates at a pressure of about 7 MPa (~1000 psi), and is designed to withstand the high pressures that may occur during an accident. The pressure vessel is the third barrier to radioactive material release.

The entire primary loop of the nuclear reactor – the pressure vessel, pipes, and pumps that contain the coolant (water) – are housed in the containment structure.  This structure is the fourth barrier to radioactive material release. The containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. To aid in this purpose, a large, thick concrete structure is poured around the containment structure and is referred to as the secondary containment.

Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in. (this is the part that was damaged in the explosions, but more to that later).

Fundamentals of nuclear reactions

The uranium fuel generates heat by neutron-induced nuclear fission. Uranium atoms are split into lighter atoms (aka fission products). This process generates heat and more neutrons (one of the particles that forms an atom). When one of these neutrons hits another uranium atom, that atom can split, generating more neutrons and so on. That is called the nuclear chain reaction. During normal, full-power operation, the neutron population in a core is stable (remains the same) and the reactor is in a critical state.

It is worth mentioning at this point that the nuclear fuel in a reactor can never cause a nuclear explosion like a nuclear bomb. At Chernobyl, the explosion was caused by excessive pressure buildup, hydrogen explosion and rupture of all structures, propelling molten core material into the environment.  Note that Chernobyl did not have a containment structure as a barrier to the environment. Why that did not and will not happen in Japan, is discussed further below.

In order to control the nuclear chain reaction, the reactor operators use control rods. The control rods are made of boron which absorbs neutrons.  During normal operation in a BWR, the control rods are used to maintain the chain reaction at a critical state. The control rods are also used to shut the reactor down from 100% power to about 7% power (residual or decay heat).

The residual heat is caused from the radioactive decay of fission products.  Radioactive decay is the process by which the fission products  stabilize themselves by emitting energy in the form of small particles (alpha, beta, gamma, neutron, etc.).  There is a multitude of fission products that are produced in a reactor, including cesium and iodine.  This residual heat decreases over time after the reactor is shutdown, and must be removed by cooling systems to prevent the fuel rod from overheating and failing as a barrier to radioactive release. Maintaining enough cooling to remove the decay heat in the reactor is the main challenge in the affected reactors in Japan right now.

It is important to note that many of these fission products decay (produce heat) extremely quickly, and become harmless by the time you spell “R-A-D-I-O-N-U-C-L-I-D-E.”  Others decay more slowly, like some cesium, iodine, strontium, and argon.

What happened at Fukushima (as of March 12, 2011)

The following is a summary of the main facts. The earthquake that hit Japan was several times more powerful than the worst earthquake the nuclear power plant was built for (the Richter scale works logarithmically; for example the difference between an 8.2 and the 8.9 that happened is 5 times, not 0.7).

When the earthquake hit, the nuclear reactors all automatically shutdown. Within seconds after the earthquake started, the control rods had been inserted into the core and the nuclear chain reaction stopped. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions.

The earthquake destroyed the external power supply of the nuclear reactor. This is a challenging accident for a nuclear power plant, and is referred to as a “loss of offsite power.” The reactor and its backup systems are designed to handle this type of accident by including backup power systems to keep the coolant pumps working. Furthermore, since the power plant had been shut down, it cannot produce any electricity by itself.

For the first hour, the first set of multiple emergency diesel power generators started and provided the electricity that was needed. However, when the tsunami arrived (a very rare and larger than anticipated tsunami) it flooded the diesel generators, causing them to fail.

One of the fundamental tenets of nuclear power plant design is “Defense in Depth.” This approach leads engineers to design a plant that can withstand severe catastrophes, even when several systems fail. A large tsunami that disables all the diesel generators at once is such a scenario, but the tsunami of March 11th was beyond all expectations. To mitigate such an event, engineers designed an extra line of defense by putting everything into the containment structure (see above), that is designed to contain everything inside the structure.

When the diesel generators failed after the tsunami, the reactor operators switched to emergency battery power. The batteries were designed as one of the backup systems to provide power for cooling the core for 8 hours. And they did.

After 8 hours, the batteries ran out, and the residual heat could not be carried away any more.  At this point the plant operators begin to follow emergency procedures that are in place for a “loss of cooling event.” These are procedural steps following the “Depth in Defense” approach. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator.

At this time people started talking about the possibility of core meltdown, because if cooling cannot be restored, the core will eventually melt (after several days), and will likely be contained in the containment. Note that the term “meltdown” has a vague definition. “Fuel failure” is a better term to describe the failure of the fuel rod barrier (Zircaloy).  This will occur before the fuel melts, and results from mechanical, chemical, or thermal failures (too much pressure, too much oxidation, or too hot).

However, melting was a long ways from happening and at this time, the primary goal was to manage the core while it was heating up, while ensuring that the fuel cladding remain intact and operational for as long as possible.

Because cooling the core is a priority, the reactor has a number of independent and diverse cooling systems (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and others that make up the emergency core cooling system). Which one(s) failed when or did not fail is not clear at this point in time.

Since the operators lost most of their cooling capabilities due to the loss of power, they had to use whatever cooling system capacity they had to get rid of as much heat as possible. But as long as the heat production exceeds the heat removal capacity, the pressure starts increasing as more water boils into steam. The priority now is to maintain the integrity of the fuel rods by keeping the temperature below 1200°C, as well as keeping the pressure at a manageable level. In order to maintain the pressure of the system at a manageable level, steam (and other gases present in the reactor) have to be released from time to time. This process is important during an accident so the pressure does not exceed what the components can handle, so the reactor pressure vessel and the containment structure are designed with several pressure relief valves. So to protect the integrity of the vessel and containment, the operators started venting steam from time to time to control the pressure.

As mentioned previously, steam and other gases are vented.  Some of these gases are radioactive fission products, but they exist in small quantities. Therefore, when the operators started venting the system, some radioactive gases were released to the environment in a controlled manner (ie in small quantities through filters and scrubbers). While some of these gases are radioactive, they did not pose a significant risk to public safety to even the workers on site. This procedure is justified as its consequences are very low, especially when compared to the potential consequences of not venting and risking the containment structures’ integrity.

During this time, mobile generators were transported to the site and some power was restored.  However, more water was boiling off and being vented than was being added to the reactor, thus decreasing the cooling ability of the remaining cooling systems. At some stage during this venting process, the water level may have dropped below the top of the fuel rods.  Regardless, the temperature of some of the fuel rod cladding exceeded 1200 °C, initiating a reaction between the Zircaloy and water. This oxidizing reaction produces hydrogen gas, which mixes with the gas-steam mixture being vented.  This is a known and anticipated process, but the amount of hydrogen gas produced was unknown because the operators didn’t know the exact temperature of the fuel rods or the water level. Since hydrogen gas is extremely combustible, when enough hydrogen gas is mixed with air, it reacts with oxygen. If there is enough hydrogen gas, it will react rapidly, producing an explosion. At some point during the venting process enough hydrogen gas built up inside the containment (there is no air in the containment), so when it was vented to the air an explosion occurred. The explosion took place outside of the containment, but inside and around the reactor building (which has no safety function).  Note that a subsequent and similar explosion occurred at the Unit 3 reactor. This explosion destroyed the top and some of the sides of the reactor building, but did not damage the containment structure or the pressure vessel. While this was not an anticipated event, it happened outside the containment and did not pose a risk to the plant’s safety structures.

Since some of the fuel rod cladding exceeded 1200 °C, some fuel damage occurred. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started failing. At this time, some of the radioactive fission products (cesium, iodine, etc.) started to mix with the water and steam. It was reported that a small amount of cesium and iodine was measured in the steam that was released into the atmosphere.

Since the reactor’s cooling capability was limited, and the water inventory in the reactor was decreasing, engineers decided to inject sea water (mixed with boric acid – a neutron absorber) to ensure the rods remain covered with water.  Although the reactor had been shut down, boric acid is added as a conservative measure to ensure the reactor stays shut down.  Boric acid is also capable of trapping some of the remaining iodine in the water so that it cannot escape, however this trapping is not the primary function of the boric acid.

The water used in the cooling system is purified, demineralized water. The reason to use pure water is to limit the corrosion potential of the coolant water during normal operation. Injecting seawater will require more cleanup after the event, but provided cooling at the time.

This process decreased the temperature of the fuel rods to a non-damaging level. Because the reactor had been shut down a long time ago, the decay heat had decreased to a significantly lower level, so the pressure in the plant stabilized, and venting was no longer required.

***UPDATE – 3/14 8:15 pm EST***

Units 1 and 3 are currently in a stable condition according to TEPCO press releases, but the extent of the fuel damage is unknown.  That said, radiation levels at the Fukushima plant have fallen to 231 micro sieverts (23.1 millirem) as of 2:30 pm March 14th (local time).

***UPDATE – 3/14 10:55 pm EST***

The details about what happened at the Unit 2 reactor are still being determined.  The post on what is happening at the Unit 2 reactor contains more up-to-date information.  Radiation levels have increased, but to what level remains unknown.

By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

874 replies on “Fukushima Nuclear Accident – a simple and accurate explanation”

Great c/m in place until this type of catastrophic event ( Tsunami) happened. The question is how come we over look the simple aspect that a country that gets hundreds of earthquakes with intensity range between from 1-9 would still consider nuke plant/energy? Japan should get nuke energy but not necessarilry produce it on its soil. IAE should review pre requisites for a nuke plant. Developing nations face the heat when coming to asking for uranium, what is happening here?


IT would be good if you stuck to the technical portion
of the conversation instead of chastising my grammar!


yes, you seem to take the technical portion pretty serious. i am not chastising your grammar. i just told you that the punctuation distracts from what you write. as you seem to comment on the technical portion how you write becomes a part of that, because people have to read it if they want to follow.


[…] The Japanese reactors are BWR or Boiling Water Reactors, very different from the Russian Chernobyl reactor design which did NOT conform to international safety standards or design. Despite the intensity of the earthquake and the power of the follow-on tsunami, everything in the Japanese nuclear facilities functioned correctly. And while the tsunami knocked-out diesel generator backup replacements, this turn of events triggered other backups and “depth in defense” measures and these are still operating. (See a detailed explanation here) […]


Ok I’ve said my peace and It’s logged!
To late now any way!
Sopia I hope you go on to be a great Grammar teacher!
This site is yours now take care of it and make sure everyone adheres to your rules of engagement touche’


[…] Fukushima Nuclear Accident – a simple and accurate explanation BraveNewClimate Fukushima Nuclear Accident – 15 March summary of situation BraveNewClimate Poster of the Great Tohoku Earthquake (northeast Honshu, Japan) of March 11, 2011 – Magnitude 9.0 As I used to work at the DOE, thought I'd pass on a few things from today's mailbox: Quote: […]


i find it hilarious how much controversy, and then controversy on top of controversy, and then skepticism on top of controversy this single event is creating. Everyone’s right, yet everyone’s wrong. Controversy cleared


Thank you for the article. I can’t help but notice that its present tone is a lot more humble than the March 13 version dismissing the concerns of uninformed laypeople.

That overconfidence is something experts always have to watch out for.


William Fairholm, on 16 March 2011 at 7:19 AM said:
“I find it somewhat astounding that there was a fire in the spent fuel rod pond. ”

It seems the spent fuel rods sit in a deep pit full of water, covered by many metres of water. If the rods are recently removed, they will still be very hot for a long while, and it would take days for the water to boil away to the point that the rods were uncovered. Nevertheless this seems to be what has happened in the #4 pit, releasing Hydrogen, which caused the fire.

The JAIF report for 19:00 15 March:

Click to access ENGNEWS01_1300189582P.pdf

shows they are filling the #4 pit with water (presumably seawater). It also reports the temperature in the #5 and #6 pits is increasing, which implies the water level is going down.

These reports now seem to have been stopped:
which only compounds the uncertainty and confusion. If anyone has an alternative quotable source, do post the link here.


Hello everybody.

I stumbled across this site whilst doing some internet research (i.e. trying to educate myself!) on the unfolding events at the stricken nuclear plant in Japan. The explanation in the original post and the following replies have been a fascinating read and it’s obvious that there are some very knowledgeable posters, some of whom have had first hand experience in the the nuclear industry.
With that in mind I would be extremely grateful if some of you could provide me with answers to some of the technicalities of the subject that are puzzling me. Please feel free to provide links etc ….

1. What sort of size is the primary (steel?) containment vessel? Height, diameter etc. What is the composition of the alloy it’s made of and how resistant is it to direct contact with hot nuclear fuel?

2. How do the control rods interact with the fuel rods? I understand what both the fuel and control rods do, but how do they interact? What is there length, diameter etc? How many are there? Does a fissile reaction take place between separate fuel rods or inside them? If it’s inside them does that imply that the control rods are inserted inside the fuel rods? I’m struggling to get my head round this!

3. What is the typical runtime of a nuclear reactor (commercial power generation) on a single fuel load? How long is the shutdown whilst a refueling takes place?

4. What volume of water is passed through the reactor primary containment vessel during normal power generation? What’s the volume of water that’s drawn from the sea {or in some instances a river) for the cooling/condensing of the steam after it exits the turbines? Also, I understand the basic difference between a BWR and a PWR.

5. On reactor start-up, how is the initial fissile reaction started? I understand the basics of nuclear fission chain reaction but how does it work in practice? What do the reactor operators do to the fuel and control rods to begin the initial fissile process?

6. Finally …… what does it look like inside a reactor? I’ve seen lots of graphics and schematics on the web and in the news media but they are just that, drawings and pictures. Are there any actual pictures that show, in detail, the size and scale of the primary containment vessal and the fuel and control rods?

Sorry for all the questions but I’m just trying to understand what’s going on. It’s a fascinating subject that will dominate the future debate on energy provision for years to come ( not that it hasn’t already! ) The comment and debate on this site is in stark contrast to the mainstream media which is obsessed with soundbites and sensationalism. To all of you, whether you are pro nuclear or against, I say this ….. keep talking ….. keep arguing ….. keep debating!

Thankyou to all of you who have been willing to share your opinion.

Kind regards.




i know that and it’s just funny that you feel for commenting on that, when there are a lot more other things to discuss.

if you would have followed the comments you probably could have realised that this wasn’t the only thing i contributed here and as your remark is the only thing you wrote here.

commenting on something that it is not important seems pretty important to you. i don’t get the logic here, but anyway.

just check some posts i made before and maybe you can answer some of my questions. thanks.


Tepco just said that in Reactor One 70% of the rods are damaged; in reactor Two it is one third. They don’t know in which though. Could mean they have holes (?) or the cores are about to melt.

What does it mean if a rod got a hole?


sophia, on 16 March 2011 at 8:32 AM said:
yes, you seem to take the technical portion pretty serious. i am not chastising your grammar. i just told you that the punctuation distracts from what you write. as you seem to comment on the technical portion how you write becomes a part of that, because people have to read it if they want to follow

To criticise someone’s grammar in a response written quickly and then not use proper capitalization and punctuation yourself is the height of hypocrisy. I’m sure it was only because you were rushed.


@William Fairholm:

i didn’t criticise his grammar. that was his personal interpretation. i just was making a (probably not so) funny remark about his ‘power’ usage of exclamation marks (which you would have recognised if you would not just pick up a random post from somewhere and comment on it).

it is amazing how this gets this part of the discussion all of a sudden. i have to ask you, Mr Fairholm, is this really all you have to contribute?

(and as a english is not my mothertongue i wouldn’t dare to criticise someone’s grammar in my dreams…)


I heard an expert on TVO Agenda program saying that the fresh water they are flying in by helicoptor is for the spent fuel pond. They do not want to pump seawater in there, as that would corrode the fuel bundles. Very limited containment for the spent fuel pond.


im right in the middle of tokyo. ive done so many research among this topic and heard so many different informations…i truly dont know which information i should rely on, and i am very confused at this point. would anybody tell me if it is really true to stay in tokyo and be safe? we have detected 20 times as much radioactive materials in couple of places among tokyo. although the amount of radioactive materials at this point is said to be harmless to human, is it possible to say it will stay harmless tomorrow?


Well, they have upgraded to INES 6 which puts Fukushima between Three Mile Island(5) and Chernobyl (7) on a scale of 1-7. And I read an lovely bit of news that explains that Oehmer is a business scientists, NOT a Nuclear one.

You have to scroll down to find it. Title: Debunking a viral blog post on the nuke threat But this article he wrote that we have been discussing was posted before the meltdown happened, Oehmer is also not taking interviews.


RE: “French Greens have anti-Nuclear field day « French News Online Newsroom”

It’s really amazing how these spin masters just can’t let go of an idea. They keep repeating that the situation in Fukushima is not like Chernobyl. No joke!. As if the technical aspects of the event that causes the reactors operators to lose control of the reactor is somehow significant.

This is basically a rehash of what GE has been saying about their reactors for year: “what happened at Chernobyl could not happen to GE reactor.” No joke! It was a different design. Something might and has happened.

Indeed this piece also implies a piece of fiction about the Chernobyl accident. It implies that there was some specific flaw in the reactor that caused the reactor to go out of control. This is completely wrong. In fact the safety and control mechanisms of the plant operated perfectly well. What happened is that through a series of errors the plant operators deliberately created the accident themselves by making the reactor do things that caused the accident.

They did not know they were doing these things, and the problems would likely not have occurred had the reactor staff had better instruction on the plants design, and if the regulations had been clearer. There was no malfunction at the plant caused by the design.

The only real design flaw that might have helped in the Chernobyl situation was the use of an outer containment. An outer containment of the kind that has been blown off by hydrogen explosions at the GE plants at Fukushima. There is no reason to suspect that had the Chernobyl system had this outer containment, that the they too might have had a similar explosion inside it, after the initial accident.

The people who built these Fukushima plant are no less guilty of making fundamental design errors than those who built the RMBK 1000 at Chernobyl.

In fact, they apparently failed to take into account local environmental factors that were fundamental to this accident.

1) The reactors were not designed to withstand the earthquake that occurred.

2) The back up generators were not housed in an environment that protected them from massive amounts of water that might come into the plant as a result of a Tsunami caused by an earthquake.

3) Their final line of defense, the battery power was not sufficient to run the plant through the entire cool down phase after the reactor was shut down, leaving the plant with what amounted to one and a half redundant power supplies for the reactors.

There are other problems here that we could talk about, but the bottom line is that the possibility of a large earthquake creating a large tsunami was entirely predictable, and this indicates inadequate safety features at the plant, bad government supervision, and at the end of the day massive dereliction of duty by the companies that contracted the construction, and the contractors themselves, since these environmental factors were easily foreseeable.

It seems they chose to ignore these factors, in more or less the same way that those who designed and built the RMBK at Chernobyl, decided to ignore the added safety features provided by an outer containment building.

Saying that this event is “not like Chernobyl” on purely technical grounds amounts as an attempt to distract from the nature of the major nuclear accident that is unfolding.

Ok, this is not like Chernobyl. So what?


Workers have now left the plant. Kyodo News reports that the very unspent and fully functional fuel rods that were being stored in the number 4 spent fuel pond have been exposed. They also note a reaction is not possible because these rods are contained in boron. What happens if the boron fails for some reason? Reports say this pond is burning, and possibly the source of the fire. Would this fire be spewing radioactive material into the air?


Ok ,fine a 10″ pipe line can still be laid to a fresh water source.
I’m pretty sure a seasoned crew can lay a mile or more in 24 hrs.
We have done 3000 ft in 7 hrs.
So with a crew working 24 hr shifts that’s 9000′
But we had to have a crane to lift the pipe in place.
Temporary is light weight no crane involved, so I say that production can be tripled easily.
There no exclamation points!!!!!!
They are also used when I am angry!
And this situation could have been avoided if someone just used common sense.


This is not like Chernobyl.
It is a new Nuclear crisis, and with every crises comes a new set of problems.
The case in Russia the core did not have a containment cell it should have had.
Also you cannot knock the Reactor itself it has done everything it was supposed to do!
The back up emergency system is the cause of this effect.
That is all over.
There is no need to try to save it anymore . Now is the time to contain it!


Have a look at which I have added an extra label to, indicating what I think is the Spent Fuel Pit. If they lose water to that and it boils dry, you get a Hydrogen fire that blows the roof off, and you should be able to see the exposed rods from a helicopter. Yes, the fire/explosion would be liberating all kinds of toxic/radioactive particles.

And the water in the pits in #5 and #6 is heating up too. If water isn’t added, it might take hours or days or weeks to boil off. It probably wasn’t their first priority.


If they are unable to cool the reactor cores what time-frame are we looking at for them to reach total melt-down? Also, should this happen and the fuel rods melt to the bottom of the containment vessel is there a time limit on how long the containment vessel will remain intact?


If the fuel melts and forms a glob, will there be fission heating within the glob or will the heating just be the result of radioactive decay?


What part of Destroy Nuclear waste is not comprehended!
You cannot destroy it you can only contain it!
This is not a Nuclear explosion,
But devices that will be radioactive for all time!


3 glaring mistakes by the engineers/scientists that designed this plant. whoever sanctioned the placement of all structures is also guilty.
1. they placed a limit on what size earthquake nature could deliver.
2. they anticipated small tsunamis only.
3. they built the backup generators where they could be destroyed by a tsunami and failed to enclose them within an effective safe barrier.
if this is the best that clever people can do then god help us all. obviously the politicians who approved it all are dumber than rocks.


The exact condition of the core in reactors is not known as long as it can not be confirmed that all decay heat is removed safely from the core to the coolant in the containment and further on from the containment to the surroundings.

Currently nobody knowns if this is the case, as all that is known is that fuel rods have been damaged, but it is not known if the core has remained in coolable geometry, or if it has melted partially forming a lumped mass, which is hard to cool completely (as in Three Mile Island). If the latter option has taken place, it will finally depend on durability of pressure vessel whether the accident will progress even further. In the Three Mile Island the pressure vessel remained intact even after the melt poured into the vessel bottom, although analyses carried out after the accident can not explain why.

Should core damages in Fukushima progress to pressure vessel breach, it is very difficult to progress what will happen. In particular if the pressure vessel is ruptured in high system pressure, also the containment structure could be violently ruptured. The contaiment structure in Fukushima plant units is not designed for such loads.

However, if the containement containing pressure vessel is flooded so that also pressure vessel is wetted from outside, it may remain intact even if the core has melted and the melt flows (or has already poured) into pressure vessel bottom. Chances for this increase as the time goes by, as the decay heat level decreases. However, there are control rod mechanisms that penatrate the pressure vessel bottom, and it is it very difficult to estimate how they would survive if molted core flooded the primary pressure vessel bottom. In this respect the design is very different from Three Mile Island, where there was no such penetrations in the primary pressure vessel bottom.



Sorry but I think this is very wrong indeed:

“For those who are reading this in Japan, the event is not over yet and the findings of airborne radioactivity plumes 100 mi. from shore and 30 km down from Fukushima are alarming – however they are not an emergency, especially if they are only the short-lived I-129 and 131”

Iodine-129 is not short lived. It’s actually the opposite, all though not primodial it actually has a half life of 15.7 million years.



haven’t read all the comments but i think nuclear has a future in Thorium reactors, hopefully MoltenSalt will be back as the new black


Great summary of the events. But this only reinforces the fact that we cannot precisely predict everything in the world. Japan is the World’s most advanced nuclear nation, and regardless of how this all ends it is still clear that what was highly unlikely did in fact happen. Because highly unlikely things do.


It doesn’t seem that the situation is improving, and despite adding sea water, it seems the temperatures are rising inside. The core, may or may not melt into a blob, and penetrate below the containment. Some kind of nuclear excursion, may or may not happen, but if it does it may spew whatever amount of radioactive material into the area, and into the atmosphere.

One way or the other, they are just going to have to figure out how to bury the whole damn thing. Better sooner than later.


Just seen this on BBC –

“Following reports that several governments plan to reconsider their nuclear strategy after the events in Japan, science journalist Angela Saini tells the BBC World Service that this makes sense in seismically active zones. “But it would would just be reckless to throw energy policy up in the air because of an incident that affected one country, [it’s] almost freakishly rare for there to be an earthquake and a tsunami and for emergency services to be overwhelmed like that,” she says.”

Now I sort of agree and think we need time to look at this, however events like this are not that rare even taken in context of 1000 years. Lisbon comes to mind:

How about the possibility of a Tsunami generated from Iceland and effecting European sites. Are we able to cope with that at our installations?

What is up with these people!!!!!!!


It is amazing to see all the media prognosticators trying to “analyze” the severity of the situation going on at Fukushima. It would be one thing if CNN and FOX News would bring in nuclear physicists who have experience in plant design and operation, however most of these “analysts” have no qualifications whatsoever to be commenting on nuclear issues. None. Worse yet, the clueless, feckless media has been giving plenty of airtime to talking heads from political agenda groups who are masquerading as impartial “scientists.”


What I just began wondering about is
if a reactor has holes in it, how much
effect has watering in it then? Couldn’t
the watering make holes even bigger
by a kind of erosion?


Eric, there are plenty of retired physicists who would have excellent credentials to talk about the situation, and who have no obligations or present-day financial ties to the nuclear power industry, or conversely, for an anti-nuclear group with a special agenda. As a “normal person” myself, I have no problem with people taking an interest in the situation, such as yourself, and discussing the situation on this forum. The proper place for laypersons’ speculation is online forums, not national television by un- or under-qualified individuals, that’s all.


even China does not want to build new nuclear power plants right now.

and the helicopter plan failed. they now want the police to shoot with water guns on it (no joke!).


The police have water cannon – it can accurately shoot a stream of water hundreds of feet – Gamma radiation is attenuated by water and the water’s depth was apparently too shallow to permit the helicopter to safely fly over and drop water. The water also would have been sea water and the water cannon may be able to use fresh since it has a self-contained tank of water on board. The sides of the pool also would be a good shield against the gamma radiation, whereas the lowered water level was not.


Chris is correct, the media are parading a motley array of prognosticators and prophets before our eyes and, naturally enough, they are all disagreeing with each other. But, worse, even the people with apparently good credentials can’t seem to agree. The spectrum of opinions and after-the-fact ‘advice’ given right here also reflects this. Reactor physics is pretty complex and I begin to suspect that a lot who work in the field don’t fully understand their profession. So how can the public have confidence in the nuclear industry?



“Some kind of nuclear excursion, may or may not happen”

Are you saying that fission may be the predominant source of heat in the case of a molten blob – and then, again, it may not?

This is such a fundamental question, with such huge implications, that I would be horrified if it is truly an unknown.


There Headline says USAF dispatching US Navy
Pumping equipment now!!!!!!!
Finally after how many days ?
So sad!


Folks: some of you may have seen Robert Alvarez very scary spent fuel pool scenario. He makes it sound like Chernobyl will be nothing compared to this.

On alvarez and chernobyl, alvarez claims that the reactor core released 6 million curies. Not quite right. It released 300 million curies, half of which were xenon 133, inert gas, much of the rest the more dangerous iodine 131 and Cesium 137. Info from Garwin and Charpak, Megawatts and Megatons.

How wrong can you be in the service of creating fear?

Or am i wrong about something?


The radiation monitoring system around the plant is down apparently due to power outage. Could be I suppose. Maybe battery backup is a consideration in these circumstances. I would have thought radiation monitoring would be one of the most important requirements for the authorities.


Chris. Scientists also have opinions. Indeed, some of them have opinions and then do things like join the anti-nuclear lobby. Its a kind of weird inversion to think that once they join the anti-nuclear lobby that they then represent an “agenda” that doesn’t have anything to do with their scientific appraisal of the value of nuclear power. The same is true of those scientist who support nuclear power — they think it is safe, or at least safe enough.

Some scientists are in-between, they think nuclear power would be safe, if implemented differently. You just aren’t going to find a lot of informed people who don’t have an opinion on the subject. I don’t see where this idea that informed people should not have opinions comes from.

It is their study and research that informs their opinion. Dollars to donuts almost any working scientist who supports nuclear power has a job in the nuclear industry, somewhere, and any working nuclear physicist who doesn’t have a job in the nuclear industry, doesn’t have that job because they don’t support nuclear power — likely they support groups, or work with groups that are against it.

You have it backwards, they are not fronting for an agenda, the agenda is formed from their opinions, based on their research and analysis.

Other than that, if you don’t like CNN, stop watching it. There are plenty of news sources out there. My first exposure to this event was through Russia Today, and they featured a rather pale faced, nuclear scientist working in the Tokyo area, who said nothing reassuring, or definitive — that is part of the problem with scientists, they just don’t fit the American media mold. He spoke in detail, in a very abstract way about this event, from a technical point of view, and laid out numerous possibilities, from the worst to the best, but, (and this is an important point) any scientist worth his salt, who is not directly in the loop, is not likely to give a firm opinion about outcomes in this case, because it is simply too difficult to come to a firm conclusion based on the very scant information that is available publicly.

It is possible to make conjecture of course, but too much conjecture from a position of authority can be irresponsible from a professional point of view.


Gregory. I don’t get your point. You are saying Alvarez was attempting to foment fear by understating the severity of the release of radiation from the plant?

Events are moving quickly, at the time that Alvarez originally wrote his article, bringing attention to the spent fuel pool issue long before it became current in the media, the release from the reactor may have only been 6 million curries.

It may now be a total much higher than that as described by your source.

As for the threat of the spent fuel rods, numerous scientists confirm the problem here. It is quite simple to understand if you think about it this way: A lot of people have been talking about the importance of keeping the fuel rods inside the containment from being exposed in an uncontrolled environment. In fact, GE went through considerable effort to ensure that they were sealed within three different containments.

The spent fuel rods in the spent-fuel pool are not in a containment at all, the containment having been blown off by a hydrogen explosion.

Hence, neither TEPCO nor the Japanese government seems as sanguine as you are about the issue of the spent-fuel pools.



The spent fuel cells in the pools were never in any containment. The containment system only exists around the reactor. I believe that the hydrogen explosions have destroyed some of the buildings that were outside the containment vessels.


Stupid question on the original article: If there is no hydrogen in the plant it won’t explode and therefore not spread around radioactive material? Is that correct? If so, why not let (from a certain point) let the reactor fall dry and have all that radioactive stuff nicely accumulated lying in the containment – or down in the earth? Wouldn’t do big damage, I guess.


The Chernobyl reactor was not built like these and did not have a primary containment cell!
Thus the radioactive release!


Can someone help me with the math? The latest report from JAIF says that the radiation level is 1937 micro sieverts.

The chart a commenter elsewhere linked to:

is in milli sieverts.

As a resident of the Denver area, it says I get 1.8 milli sieverts a year from background radiation.

According to Wikipedia, the conversion is 1000 mSv = 1,000,000 milli sievert.

So the workers are exposed to about a year’s worth, right?


Whatever the final outcome at Fukushima Daiichi, and let’s remember it’s the only plant that got swamped by the tsunami, and all of the other 49 reactors in Japan have not been catastrophically damaged by the earthquake, this event is not the ‘death of the nuclear industry’.

If Chernobyl did not stop the development of better and safer nuclear reactors, then I seriously doubt this tragedy will either. What one could safely assume is that reactors will no longer be sited anywhere that’s just some metres above sea level if they are in a region like Japan.

As horrible as this event is, we still need to look at the real causes and make better judgments about managing risk, but blaming the reactor design seems a bit odd when it was never designed to withstand being flooded by the ocean.


Typical MSM and PPS for political gain!
Worst case scenario and jam nuclear power.
once again i will say the reactor did it;s job and was safe until the backup coolant sys failed.
It’s like paying your baby sitter to watch your kids and the house burns down!


@Common Sense, on 17 March 2011 at 7:46 AM

1937 microsieverts (μSv) is equal to 1.937 millisieverts (mSv)

Technically this a dose, not a level. The report will almost certainly have been 1937 microsieverts per hour.

Residents of Denver get 3.2 mSv natural exposure per year, compared to national average of 2.4 mSv

So the worker’s dose depend on how long they stay in the area that has high levels, what form of radiation it is and whether their protective gear is suitable for that exposure. You can be pretty sure they’re all breathing filtered air with full body cover. So their actual dose will be much lower than an open-air environmental reading would suggest.



the 6 million curies figure was alvarez describing the chernobyl release.

the chernobyl release was much higher according to charpak.

I thought the effect of underplaying the amount was then to compare the current situation to his description of the chernobyl situation.

but you know, the shit is so confusing and crazy. I saw the alvarez reference this morning on the keith yost site.


I lived in the Fukushima area for several years and have had close work-related contact with nuclear power plants for over 10 years, so I am not completely ignorant about what is happening by any means, but admittedly am not a trained engineer or technician. I am following the news as best I can but can easily see that there is a lot of mistaken terminology being bandied about and unsubstantiated assertions / uninformed “expert” opinions made in the international media, which is frustrating, when one only wants to get an accurate picture of the situation. Of course, I understand that the people on the ground themselves are probably short of a lot reliable data needed to work with…they have an incredibly difficult task it would seem…so it is no wonder that those on the “outside” have a much vaguer idea of what is happening…so instead they can only speculate!

Anyway, can someone explain to me why it would be that the plant operators in Fukushima are not able to re-establish some sort of reliable power supply that would enable them to in turn re-establish normal & emergency cooling functions for the reactors and most importantly now, the spent fuel pools? After all, it seems that the cooling & shutdown systems were working OK when on battery power at first.

Could it be that the emergency diesel generators are still “wet” from the flooding and therefore won’t run? Was their fuel supply system damaged/lost?

If they can’t get the diesels to work, why not bring in many more “portable” units of some kind? Or is it that the piping/pumps/valves etc. in the cooling water system are damaged from the hydrogen explosions?

What I mean is that instead of trying to focus exclusively on applying all these “improvised” tactics, why not simultaneously work hard to put the originally designed system back in some sort of working order and thereby bring the plant back under some semblance of control? Of course maybe they are doing just this, but I have not seen it reported anywhere, and it seemed like such an obvious thing to me that I had to ask.

I am hoping that one of you experts out there will deign to comment on my thoughts…Thank you.


I wouldn’t want to underplay the seriousness of this terrible accident in Fukushima, but predictions that it will spell the end of nuclear power are basically wishful thinking. If it was true then why do we still drive motor cars?

The World Health Organization reports that about 3,000 people die in car crashes worldwide EACH AND EVERY DAY! This adds up to an awful lot of human trauma. Three people in my immediate extended family have died horrendously in car smashes, and that’s not an unusual statistic. Most people in the world have either suffered similar or have lost close friends and associates in car accidents.

Yet, amazingly, nearly all of us still drive cars. We choose to do that even when we can use much safer public transport. Car accidents haven’t spelled the end of the car.

In short, if there is a strong enough demand for a technology then society generally says yes to it, despite horrendous risks. Much though nuclear power is not a panacea for society’s future I have absolutely no doubt that 7 billion humans, mostly in crowded cities, will place such enormous demands on energy systems that multiple jurisdictions will opt for it.

I certainly don’t make that claim as an advocate, just as a realist. (The collapsed wind generators on top of my city’s high rise look very forlorn but that spectre won’t kill off the growing wind power industry either.)


Hello Barry and thank you a lot for the clear explanations on this type of reactors.
I’d like to ask something wich appears confusing to me in the late evolution of the situation. On the MIT site you relay the info that fires, possibly due to defective pumps, broke out on reactor 4, actually twice. But, the pictures which are broadcasted on screens since are showing huge damages, especially of what seems to be the secondary confinement shell. Can you confirm or infirm? Would this account for the very high level of radioactivity on site ?
Thank you for very seriuosly covering this tragedy.


Can someone please say what will happen when this “Meltdown” happens.
Does anyone have experience or are we all in the dark.


This is new territory in the Nuclear power field!
What happens at this point happens!
I have said before Cool it to acceptable levels
And encase it in a Lead first then encasement in concrete! Monitor it forever!
This may be a stupid Idea but!
Experiment with a spent fuel rod and drop it in a live volcano and see what happens if the molten lava can consume it!
Then cut the containment cell out and drop it in the same molten lava and stand back!


The concept of nuclear energy may be safe but it’s the implementation that worries me. If I were the engineering firm on this, there is no way I depend on anything but gravity for my last ditch water cooling system. There would be sufficient water and boron above the sites to safely shut down regardless of the circumstance (and yes go ahead and throw in a 100 ft tsunami and a 9.0 quake). So to continue this diatribe stating all is well is asinine. Call it like it is…misengineered and let’s hope that the winds don’t blow inland because those highly radioactive, exposed spent rods are the ones that concern me. You know the ones that the don’t haul off toYucca …instead they just sit in a minimal confinement structure (now damaged) on the same grounds.


Greg, sorry, I misread what you were saying. I thought you were saying that Alvarez had said that Fukushima had released 6 million curies of material.

Helps if you link your sources. Here is a link: Anyway, I think I found your reference. I think you missed something there. Here is the quote:

“On average, spent fuel ponds hold five-to-ten times more long-lived radioactivity than a reactor core. Particularly worrisome is the large amount of cesium-137 in fuel ponds, which contain anywhere from 20 to 50 million curies of this dangerous radioactive isotope. With a half-life of 30 years, cesium-137 gives off highly penetrating radiation and is absorbed in the food chain as if it were potassium.

In comparison, the 1986 Chernobyl accident released about 40 percent of the reactor core’s 6 million curies.”

He is specifically talking about how much cesium-137 was released, not the total value of material that was released at Chernobyl, as described in your source. That fits in with your statement, where you say that Chernobyl “released 300 million curies, half of which were xenon 133, inert gas, much of the rest the more dangerous iodine 131 and Cesium 137”

I can see why it is confusing because he mentions that 40% of core material was released, but reviewing that statement in the context of the previous paragraph, he is really only talking about Cesium, which unlike Iodine gets into the food chain and imitates potassium. Iodine has a biological life of 70 days, or so, from what I understand.

The precise figure of the overall release from Chernobyl is another one of those hot debates, here is an broad sweep accounting of the various estimations, from various reports:

I found this statement interesting:

‘Geneticist Valery N. Soyfer, founder of the former Soviet Union’s first molecular biology laboratory, analyzed the 1986 report to the IAEA, which has since been condemned as a cover-up. Dr. Soyfer says that if only 100 million curies were vented, then world “background radiation doubled at once.”[10] This claim was unsupported by accompanying evidence, but if “background” was doubled by 100 million curies, then it was multiplied 180 times by the release of Chernobyl’s “full inventory.” Nineteen months after the disaster, in Nov. 1987, the U.S. government officially doubled its estimate of the “background” radiation to which we are exposed every year.’

If correct this then would lead to the conclusion that a total of 200,000 curies was released, half gas, half radioactive material. In other words that the original report underestimated the amount of expelled material by half, but that more extreme estimates were also incorrect, the am I right?


Thanks for posting this really excellent summary. However, I think parts of it can still be dumbed down. My wife who is a science communicator sent me this, which sums up the events in terms that everyone can understand.

Apart from the horrible japanese cartoon characters, it is excellent. Could something like this be re-made for a western audience?


I am very disappointed !
In the cooling effort. A tempory pipeline could have been in place to the ocean there flooding these sites.
Cooling efforts could have been already in place for at least 4 days now!
Even now it could be implemented as they are using sea water.
Why has no engineer implemented this option?
They had prob ten fire trucks attempting to spray to no avail.
However to use these pumper trucks to fill a pipe- line to the containment pool would work.


In Canada, the proponents of nuclear electricity generation are all over the media saying, “this cannot happnen in Canada because we do not have such severe earthquakes or tsnamis”. Obviously the folks who designed the Fukushima Daiichi reactors knew Japan has severe earthquakes and tsnamis, so what was wrong with their engineering and regulatory process that these facts were not properly taken into account. Saying the event was more severe than those projected just says to me that the engineering process was worng. I would also like to know why NRC plans for American reactors requires evacuation plans to 10 miles, but the head of the NRC has suggested Japanese evacuate to 50 miles. What design process in the Canadian and AMerican reactors was so different that we can rest assured a similar set of events will not cause a similar problem in North America. For instance, yesterday the head of the Nuclear operators in a senate briefing admited the portable back-up diesel pumps in American reactors are not seismically rated.


My correction, it was Dr. Lyman who said the protable diesels are not seismically rated. That statement was left unchallenged.


What part of Tsunami did you not understand?
The reactors were fine until a 11ft wall of water
knocked out the back up generators and power supply!
Their major design flaw was the location so close to the sea.
If they would have located inland about 20 miles the current situation would not be a factor.


Dr. Oehmen’s explanation makes the situation in Japan much more clear, but he has omitted the danger of the exposure of the spent fuel rods. These appear to be stored outside the containment vessel. I assume they contain a mix of uranium and plutonium. I also assume each of the 6 reactors has its own storage facility. What are the implications now that we know the water is boiling off?


The explanation of the design and purpose of the reactor building is simply wrong. First off, in a BWR plant, the “coolant” is also the steam. Therefore you have radioactive fluid moving throughout the piping in the reactor building and the turbine building.
The reactor building is designed as a containment barrier and it is in fact the secondary containment that you described as being within the reactor building. There are various high rad and contaminated areas throughout the reactor building. It is maintained at a lower pressure than atmosphere and the turbine building to contain contamination and the radiation it gives off.

Don’t you think there is a reason they would vent to the reactor building and not directly to the atmosphere? That reason would be they hoped to contain the release. Obviously the explosion ruined that idea.

Aside from all that, a huge concern is the spent fuel pool, which is contained in the reactor building. “Spent” fuel is still highly radioactive and the reactor building is the only containment it has.

I’m not against with nuclear power in any way. I have worked in the industry for 10 years. However, I think it is wrong to downplay a very significant event. [personal opinion unsubstantiated]


I see where there is Seawater being brought in to cool the “hot” site. Where will the water being used go to after is is sprayed/dumped on the reactor? Will it flow to surrounding areas that are below the hot site or back to the sea, and if so how contaminated will this water be? The addition of the sea water may spread contaminates to ground water or streams and back to the ocean.


in the case of a nuclear melt down how good of an idea is it to send a robot to pull the rods? i know that the chernobyl incident they sent humans who gave their lives to pull those rods, and after reaserching the incident i felt terrable for those who died. its a bad way to go. they made themselves heroes however. im actually quite worried about the situation in japan. im quite worried it might turn out just as badly as chernobyl. its still quite a mess over there in chernobyl. i heard that the concrete they poured over that mess is breaking down and getting cracks. i love japan (partly for its technology and partly for anime and its awesome art) and i dont want to see those people hurt any more….


Thank you for providing easier to understand information and analysis. Please accept my apologies in advance for my ignorance, but could someone please explain (in simple terms) what the significance / likely impact of the “Nuclear Plume” that the US media is reporting should reach the west coast on Friday? I had taken comfort in the analysis above (quoted below) so am trying to understand what level of radiation may still exist in the “plume”? Thanks in advance for your assistance

“As mentioned previously, steam and other gases are vented. Some of these gases are radioactive fission products, but they exist in small quantities. Therefore, when the operators started venting the system, some radioactive gases were released to the environment in a controlled manner (ie in small quantities through filters and scrubbers). While some of these gases are radioactive, they did not pose a significant risk to public safety to even the workers on site. This procedure is justified as its consequences are very low, especially when compared to the potential consequences of not venting and risking the containment structures’ integrity”


A question is bugging me for some time. Was it possible to restart a reactor, may be no.1 or no.3 at low power levels and produce some electricity from it’s turbine, when off-site power was lost, backup diesel generators gone and the emergency battery power was running low? Wouldn’t it have restarted the reactor’s original coolant flow, eliminating the need of ECCS, where inability to provide power for ECCS caused further problems which lead to current condition? And also, if one reactor was restarted, was it possible to use its electricity to provide power to other reactors’ ECCS?


@Swapnil: My understanding is that all nuclear plants were SCRAMed when the earthquake hit (control rods fully lowered, cores effectively shut down). Starting a nuclear plant is a non-trivial task, and takes some time. Indeed, restarting a nuclear plant after an earthquake and without thorough inspection is merely inviting a new disaster to the mix. Think of how much more fun this situation would be if the cores were generating 100MW each rather than the single-digit MW’s from decay heat.


Very nice explanatory article. i wish to see many more explanator articles like this. I wathed aproximated 10 to 15 hours of CNN program. surpirsingly none of these show showed a schematic of the plant showing which wall has been busted. Rather they like to say all buzzing words like melt down, blast, radiantion dose etc. etc.
ashim banik


Someone asked why the heck it was built there? Plate theory was not well understood. Even among believers, many plates were not yet even identified when the nuclear plants were built. The ocean was largely a mystery until very recently. Continental drift theory is very young- there are still many mistakes to make!


To add to Snord’s entry, the theory of plate tectonics was not fully appreciated until the 1960s – even Albert Einstein was sceptical of the theory in the early 1950s.

The Fukushima facility was built in 1971.

What does this mean? Well perhaps that we could excuse those who built the Fukushima plant back then, but what’s around the corner that we don’t know about yet?


I’ve been reading an article on the Herald tribune march 16, europe, from Thomas R. Zeller, that there were so many concerns about the Mark I GE BWR reactor, including specific issues on safety. It is told in particular how explosive is the process of an emergency depressurisation of the containment vessel. Several commissions in US have recognized it, early in the late 70s, but would not recommend to ban the Mark I reactors, for fear of the industrial reversal costs would “be the end of nuclear power”. Now that this has happened in Fukushima, this statement is ironically reversed. It is also told that the PWR are known to be better built, especially the sealed thick steel-concrete tomb surrounding the core and its vessel.
But one must also still recall that the conditions that put the GE mark I into awful troubles was a natural catastrophe of rare magnitude. I’m not sure any other nuclear power companies would try to play it smart with their own older designs( generationII).
Another critic though is about the waste pool: how in hell aren’t there at least as actively secured as the core? why is it so easy to have all this threatening material in contact with the outer air? This seems on the other hand a real flaw in the design, which could have today dramatic incidence. Hopefully not.
Can anyone comment on this?
And also on the fact that the so called fires of reactor 4 seem however to have blasted off the second containment wall just as much as on reactor 3, on satellite pictures?


I would really like to hear the whole story of nuclear reactors/power plants…what happens to the waste? How can this be considered sustainable?

Honestly, we can do a whole lot better than this!


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