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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”

Interesting quote from Rod Adams:

According to the writer and editor who approved th[is] summary, “An explosion at a nuclear power plant on Japan’s devastated coast … made leaking radiation, or even outright meltdown, the central threat menacing a nation.” Apparently aftershocks, fires, broken dams, washed out highways, lack of clean drinking water, damaged sewer systems, destroyed airports, and at least a thousand known fatalities are not as much of a threat to the nation of Japan as the possibility that a few people might be exposed to a radiation dose that is roughly equivalent to the ones administered every day as part of routine medical procedures.


It could be pointed out that hospitals rely on backup diesel generators to keep essential equipment working. Thankfully they are not close to shorelines. I presume the reactors of this type that are not decommissioned will get better safety systems installed. I think we could overlook some extra emissions if the Japanese relied on gas for a few years. In 50 years we won’t have that option.

So far the death toll from either radiation or explosions appears to be zero. You wouldn’t think so reading the Murdoch press which is hysterical.


I just received a response from an executive engineer in an international multidisciplinary consultancy. My close friend for over 40 years, he told me that he had no interest in reading my condensed version of events in Japan or why I consider nuclear power to be an attractive option for at least some of Australia’s future energy needs.

It’s sad to find that he has already made up his mind, and that his answer is not to think.

If his mind is closed, perhaps we shouldn’t be so hard on the journalists who think that their duty is to write what (they think) people want to hear. Nuclear power’s time will not come because people are bludgeoned with facts. It will come when the man in the street decides that nuclear is the way to go.

The journalists will then follow their readers, always pretending that they are the leaders.

So, by all means focus on journalists, but remember that public knowledge is what is ultimately needed.

Dr Oehmen’s article is an excellent step along this path.


Thank you for an easy to understand yet explanation of the event!

I am wondering about this however:

The intermediate radioactive materials (Cesium and Iodine) are also almost gone at this stage, because the Uranium decay was stopped a long time ago.

Isn’t the half life of the radioactive Cesium something like 30 years? How can it be gone just a few days after the reactor shutdown?


The cesium was in trace amounts and dispersed via the prevailing winds over the ocean. It then reacts immediately with water to produce cesium hydroxide (CsOH) and is dissipated.


Two questions, if I may:

1. Why these reactors built along the east coast of Japan – ie facing the subduction zone – and not on the west coast?

2. Will this reactor at Fukushima be able to go online again? If so, when?


1. East coast is near major load centres and transmission infrastructure. There are clearly questions about the preparedness of these plants for a tsunami, which will have to be looked at carefully for future planning.

2. Unit #1 will be decommissioned – it was 40 years old anyway and was due to be shut down. Unit #3 will probably also be written off. Units #2 and #4 will probably be restarted, but not for quite some time, anywhere from 6 months to 3+ years.


Good article, apart from the gratuitous innuendo about Iran “building a nuclear bomb”, a lame warmongering propaganda claim which has been demonstrated false long ago.

If you really want information on the difficulties of building a nuclear arsenal, you’d better ask a nation which actually has one, like Israel, or USA.


Thanks for this from those of us living in Japan. It made it to facebook and is being passed around to those living here. Very reassuring!


Thanks for this! I am not a supporter of nuclear reactors, but think of them as a necessary “evil” until we can devise safer ways to make energy. The recurrent thought running through my head while reading your article was “Thank God these are Japanese designed” since I truly respect the engineering of Japanese products.

I hope that all plays out as you have described.


Moderator rods exist in some types of reactor, but common PWRs and BWRs use light water flowing around the fuel bundles as both moderator and coolant.

The rods that absorb neutrons are control rods, not moderator rods.

A moderator is any material which moderates, slows down, neutrons.

It is not a delicate balancing act to make sure the reactor stays precisely critical, in order that the reaction not quickly run away towards zero or towards infinity(until something breaks or explodes).

Reactor are stable and manageable for several reasons. By stable I mean that the power output responds slowly and injection of a small amount of reactivity(e.g. partially withdrawing a control rod) will cause power to increase slightly before leveling off at a new, higher value without any operator intervention.

There are delayed neutrons from some short-lived fission products. In a nuclear bomb only the prompt neutrons are needed to sustain a super-critical state, the rate of the reaction increases so quickly that an enormous amount of energy can be produced before the fissile core of the bomb blows itself apart. A reactor is not like a bomb; it’s not prompt critical; if it weren’t for these delayed neutrons the reaction would quickly die down(we’re talking miliseconds here) these delayed neutrons come from isotopes with half-lives of between a second and a minute. When you rely on delayed neutrons to maintain criticality the power level can’t change very rapidly; the reaction increases or decreases on time scales appropriate for human intervention.

There is also a whole host of effects that tend to stabilize the power level. These can be anything from thermal expansion of moderator, thermal expansion of fuel pins, doppler broadening, chemical dissassociation(hyperion’s uranium hydride design) formation of voids in the coolant/moderator(e.g. the bubbles of steam in a boiling water reactor).

The purpose of slowing down neutrons in a thermal reactor is that it increases the fission capture cross section for fissile U-235(and Pu-239 if present). You can build these reactors with small fissile inventories or very slight enrichment(or none, as in a CANDU).

Reactors that slow neutrons down to such low energies that most of them are at a simular temperature as the reactor itself are called thermal reactors. Light water reactors are all thermal reactors.

In a fast reactor the neutrons remain very fast because the core contains no good moderators(compounds of light elements with a large collision cross section and a small capture cross section, such as water, heavy water, carbon, beryllium and fluoride salts). The purpose of operating in the fast spectrum is that it enables breeding with the plutonium fuel cycle; although the fission cross section is smaller(necessitating more fissile inventory; about 10 metric tonnes per GW), the capture cross section of fission products is reduced even faster, reducing parasitic losses and thereby improving the neutron economy.

Thorium reactors can breed in the thermal spectrum given some online removal of fission products as in the molten salt reactors.

The moderator can also play an important role in making a reactor stable or unstable against increases in temperature or formation of voids(of steam, typically).

In reactors with light water as coolant/moderator the effect of partially withdrawing a control rod is to inject a certain amount of reactivity; neutrons multiply and power increases. But this is self-limiting; as water heats up, its expands, becoming less dense and a poorer moderator.

In a light water reactor the formation of bubbles of steam reduce moderation, causing the reaction to die down.

In RBMK reactors they used both graphite and light water as moderator; graphite is better moderator, so bubbles of steam in the core would tend to increase moderation, not reduce it. In an RBMK the void coefficient is strongly positive, this means that if ever the water in the core were to begin to boil the reactivity would increase, causing power to increase, causing more water to boil and increasing reactivity further in a vicious circle; this run-away reaction happened at chernobyl and the control rods took too long to be inserted stop it in time before the steam explosion.

[Ed: Thanks for picking that up Solyent, I missed it in my rapid read through. Have updated]


Good write up. Only thing I would say different is that the Reactor Building is designed to maintain a negative pressure relative to the outside atmosphere to prevent the spread of radioactive contamination. Its ventilation exhaust is monitored for Radiation/ Contamination and isolates if sensed. This is the standard GE BWR design and called “secondary containment”.

Part of that purpose is to also address a fuel handling event on the Refueling floor. Unfortunately the spent fuel pool and the Refueling floor area is where the walls were lost during the hydrogen explosion.

That is the top area of the Reactor Building image which shows the crane used for vessel disassembly in the overhead. These walls are not as robust as the balance of the Reactor Building walls which would explain why they failed first.

No real word on that status, but current Radiation levels seem to imply adequate water level remains in the Spent Fuel Pool. The actual pool is steel lined and concrete walled.


The most significant problem is not going to be power shortage, but finding the funds to decommission the wrecked reactors. This will cost far more than it did to construct them in the first place.

Presumably TEPCO will declare itself insolvent and leave the next few generations of the Japanese public paying for it.


The author seems to have missed the vertical shock wave just before the outer walls were torn apart. IMHO the hydrogen explosion took place within the third containment and blew away the concrete shield plug (vertical shock wave).


@ esquilax

Actually it is no and yes.

Regarding the first question, this is what the Japanese experts came up with :
The 500 years event was a 8.6 magnitude quake. The present earthquake was 5 times more powerful !
Hindsight bias anyone ?

Regarding the second question, here is one solution : although there is a good chance that a “simple” EPR, with its air-tight and water tight redundant ECCS would have stood the tsunami test.


Thank you for a clear, well-supported summary of the likely events at Fukushima. We’re in Tokyo and are fairly stressed about the ongoing crisis. I’d be grateful if you could confirm whether the summary you’ve provided can be applied all the reactors at the Fukushima site. Evidently, a number are affected.

Really helpful. I’m a big fan of nuclear energy and can’t frankly understand why folks in the US are so frightened of it. The part about the plugs not fitting sent chills up my spine. Beyond that, the lessons of Fukushima seem to be: build better protection for cooling systems, back-up the back-up, and ensure the plugs match. Things not to do? Continue to rely on obsolete systems or curl into the fetal position.



Wonderful explanation, thank you! I have just one remaining question: if the worst-case scenario brought the core catcher into play — where does the SNF end up? Is that storage pool completely separate & does it remain intact? Or could we have a LOCA up there?

Thanks again for your insights!


Bravo, Barry! A very useful post.

The details will keep on coming for a long time — but your estimates and predictions are right on target. The BWR experts in the US are bringing in their materials regularly, now. The disaster speculators will be disappointed — the crisis is about over now.



em: is it certain that this plant had negative pressure?

and what exactly was negatively pressurized? the containment (third)?

Barry: great job. Wolf Blitzer of CNN could not get the word “Chernobyl” out of his mouth enough.

the japanese ambassador never corrected him.


@ charles

The operator (TEPCO) is blaming the tsunami which was/is completely foreseeable, given the site is on the coast (duh!). Which one do the apologists want?

BTW, earth quake magnitude and the size of (any) tsunami wave are not always correlated.


“the crisis is about over now”

The BBC just reported a State Of Nuclear Emergency for Onagana NPP, confirmed by the UN/IAEA because of “excessive radiation levels”.


“(The) water level gauge in Fukushima nuclear reactor 3 may have failed – ‘We don’t know what’s going on’, says chief cabinet secretary.”


Thank you! I have been pulling my hair out over the garbage the media has been incorrectly and dangerously spewing to the public. It is at relief to find an article that accurately describes the accident, written in a manner that most people should understand. Again, thanks!


Well written, but some references to support your claims would go a long way. Particularly on points regarding the specific construction of THIS reactor and its rated specifications. You’re certainly correct on many points but, without references, you only appear as authoritative as the inaccurate media you are chastising.

Also, the difference between 8.2 and 8.9 on the Richter scale is 10^0.7, which is 5, not 7 (the Richter scale is base 10 logarithmic).


Some easily verifiable objective facts: (not from a ‘scientist’ working for the nuclear lobby)

“Chief Cabinet Secretary Yukio Edano said an explosion could take place in the building housing the No. 3 reactor at the Fukushima Daiichi plant in northeastern Japan.

“There is a possibility that the third reactor may have hydrogen gas that is accumulating in the reactor (that) may potentially cause an explosion,” he said.

An explosion caused by hydrogen buildup Saturday blew the roof off a concrete building housing the plant’s No. 1 reactor, but the reactor and its containment system were not damaged in the explosion.

Edano said the No. 3 reactor would also likely withstand a similar blast, noting that workers had already released gas from the building to try to prevent an explosion.

Hydrogen means at least partial core’s meltdown.

MOX very dangerous nuclear fuel is used at the No. 3 Daiichi reactor, maybe at the No. 2 also.

Zirconium liner fuel is only used in Kashiwazaki-6 and 7 (ABWR 1350MW), not in BWR (500MW or 800MW) plant design of Fukushima Daiichi and Onagawa.

Japanese Nuclear Reactors have a strong design, that’s true, but they have to resist to several small/medium earthquake/year.

Since 11 march, they took level 9 earthquake (designed for max level 7) + tsunami 2-10meters wave + many replica level 5 and more.

These reactors aren’t designed to be cooled by sea water.

No. 1 reactor Fukushima Daiichi is 40+ year old.

At least 69 officially irradiated persons.

Last news say :

1. water level don’t increase in reactor No1 2 and 3, sea water don’t work, they don’t understand why

2. Japanese authorities have informed the IAEA that the first, or lowest, state of emergency at the Onagawa nuclear power plant has been reported by Tohoku Electric Power Company + Onagawa had an fire emergency at 11 march.

Japanese people need support and real infos, i really hope things will go better.


Kaj, there might be some slightly different effects on the void reactivity coefficient and stuff like that because the fission cross section for Pu as a function of the neutron energy is different than U-235.

And of course there’s plutonium phobia to contend with with the MOX fuel, irrespective of the real science and engineering information.

Anyway… in the GE BWR-3 like Fukushima I Unit 1… if there is excessive pressure within the primary containment vessel, where will it be vented to? Will it be vented out into the reactor building, outside the inner containment structure?

What about venting excessive pressure from the torus? Will that be vented out into the reactor building?

I’m trying to better understand the path of the hydrogen from within the torus into the area on top of the reactor building, where the fuel transfer crane is, where the steel walls were blown out by the explosion.

The outermost layer of the multiple layers of containment – the reactor building – has walls and a roof made of solid concrete, and it’s roughly cube-shaped.

On top of the concrete reactor building, however, there is an additional part of the structure – it is not made of concrete, but it is made of steel, with steel sheets over a steel frame. Refer to the drawings posted previously above.

This steel building on top of the reactor building houses the fuel transfer crane, and it is built on top of the concrete roof of the reactor building. I’m referring to the part of the structure above the concrete shield plug and the refueling platform at the top of the concrete reactor building.

It is this relatively weak steel structure on top of the concrete reactor building, which is not really part of the reactor building proper, which seems to have been blown out by a hydrogen explosion.

The explosion does not appear to have occurred within nor does it appear to have breached any of the fundamental layers of containment structure.

It appears that the building has been breached as a result of a hydrogen explosion. It’s probable that excessive hydrogen generation within the reactor core, either radiolytically or chemically by reduction of water in the presence of the zirconium cladding at significantly elevated temperatures, has been vented into the torus, and as temperatures and pressures have began to rise within the torus steam pressure in the torus has been vented out into the reactor building surrounding the torus.

From there, the hydrogen mixed with that steam and water vapor has risen, as hydrogen does, and worked its way through the reactor building, escaping at the top of the reactor building, and accumulating at the top, in the area around the fuel transfer crane. It then appears that the accumulated hydrogen has mixed with air and exploded.



Regarding the used fuel storage pool, it is not in the reactor building, as far as I’m aware. It’s a separate building, which is designed to be very robust and seismically hardened as well.

There is a very small pool in the reactor building near the top of the reactor vessel which is used to temporarily hold the used fuel during its unloading from the reactor.


“The Zircaloy casing is the first containment.”

** Not Correct **

1st containment is the ceramic fuel pellet itself. The fission process produces fission fragments, or radionuclides, which almost entirely remain within the ceramic fuel pellets. Only a very small fraction of fission fragments escape the fuel pellet, unless the pelet integrity is comprimised by melting or some other degredation.

The 2nd barrier is the zircalloy tube; 3rd – primary coolant system; and so on…

This is not quibbling, but rather an important and fundemental reactor design safety feature.


Hmmm. The plugs on trucked in temporary diesel generators did not fit? Did anyone in Japan ever heard of temporary connection rigging. Or is the status quo in Japan entrenched like here in US where electrical inspectors will threaten you with multi million dollar fine if you don’t comply with polished electrical code so the guys who try to help in emergency will spit in disgust and walk away? 8 hours is very long time in which temporary electric power should be connected by any means or batteries should be charged with temporary hook-up to prolong the time. Most electrical engineers will be scratching their heads why this was not done in 8 hours time.
This accident in Japan is another classical case how mother nature wins when she is allowed to run her natural course. Add to it the human status quo and you have a screw up of first magnitude.
I am sure after the investigation is done a lot of heads will be rolling. I would not be surprised if some individuals commit hara kiri.
Nevertheless, after the fact solutions are no consolation to reactor owners who face massive financial losses for clean up and overall further damage to nuclear industry that anti nuclear elements will surely exploit.
This is why only three weeks ago I was pointing to unbeatable natural safety features of Molten Salt Reactors as a preferred method to generate nuclear power.
One thing is certain, this accident will not play well for IFR acceptance.


[…] The hysteria really is ridiculous at this point. The way this tragedy has been used is changing my thinking about certain groups and organizations that I may have previously aligned myself with. Instead of a rational, calm discussion about what is going on in Japan and how we can help the Japanese people. classic opportunist theater is on display. If you want a rational explanation of what is going on, do yourself a favor and head over to Brave New Climate. […]


Frank: An old Naval invocation: “Point the Guns Outboard, Lads!!”

Personally, I cannot understand people who take advantage of any opportunity to further their own limited objectives, even in the midst of a most difficult situation such as exists in Japan.

The next time a 10 meter wave washes over a nuclear power plant of any particular design, I hope all of us will take a moment to recognize that the situation on the ground is somewhat less than ideal.

I think the Japanese are doing rather well in controlling this 40-year-old reactor, which was scheduled for final shutdown a couple of months from now.


I can hardly imagine what people, who work in the nuclear station, are surviving right now. I believe that they are aware of how big responsibility is laying on their shoulders. It´s too late and too early to blame somebody, it has already happened. The major task is to save as many lives as possible.


A very interesting post and great to know given that many friends are still living in Japan – the place I called home for two years.

However, ‘safe’ or not, I am still very much relieved that New Zealand, my home country, is Nuclear free – and long may it stay that way! Thank you David Lange for leaving us that!


Excellent summary, thanks. A small point – it would have helped if the diagram had not labeled as primary and secondary containment what the text refers to as secondary and tertiary.

I certainly hope your assessment of the situation at Fukushima plays out.


Barry, thanks for the explanation.

Just one note – we haven’t heard the last of this. Invariably governments & companies lie. Just think about the gulf drilling disaster.

Next time someone compares costs with PV or Wind these scenarios should be included. Tax payers shouldn’t be the ones funding all these …


Here’s a graphic showing what’s been blown away and what’s still there at the Japanese reactor building. The red brackets indicate the flimsy walls that have been blown away:


This part of Japan is next to a subduction zone. Subduction zones are known for their ability to create magnitude 9+ earthquakes with large tsunamis, such as the 1960 Chile earthquake and the 1964 Alaska earthquake. It’s not rocket science to understand that nuclear plants in this kind of environment need to be designed and operated to handle seismic and tsunami events that are quite large.

BTW, the Richter scale is no longer used much, but its most common replacement the moment-magnitude scale uses the same logarithmic scale, just with a different calibration.


Comment on three areas:

1 – the explosion in the reactor building.

It obviously wasn’t hugely powerful – it seems to have removed cladding, but left the structural steelwork substantially undamaged.

I can’t see an obvious pathway for hydrogen generated in the core to the into the building. Or at least, not one that wouldn’t have radioactive material coming along with it, and a substantially greater level of radioactivity.

There is, however, another source of hydrogen on the plant – that’s the hydrogen coolant for turbine stators. There are some reports that the explosion originated in the turbine hall.

2 – the overall impact. Dependent on the end outcome, it’s obviously not good for the pro-nuclear argument. But, there’s a counter-argument, and that’s “in an unforseeably bad combination of circumstances, a 40-year old plant not only survives the intitial fault, but is then managed to a succesful outcome, then how bad would it have to be to actually cause significant release?”

3 – timing of any action. Just doing some basic sums, and I think the heat generation is probably down to under 10% of what it was immediately post-trip – perhaps 8MW versus 90+, and 1500 or so under full power.

So far as I can see, researching online, it’s usual to open up a BWR RPV about 4-7 days into a refuelling shutdown – heat generation at that point, on a plant of Fukushima 1’s size is 7-8MW. Now, obviously, at that point, there’s no boiling, and unpressurised water circulation is enough to maintain cooling.

Can anyone who knows BWRs reasonably well confirm that? My background is AGRs, rather than light water designs. It strikes me that at that point, we can reasonably assume all’s under control.


. Invariably governments & companies lie.

I wouldn’t go so far as invariably. But that’s a quibble.

What’s more important is the direction in which they lie. Today Japan is full of power plants and fuel reservoirs that have been killing people. All the plants had been burning, and all the reservoirs contained, much more expensive fuels than the Fukushima plants, and the Japanese government is a major beneficiary of that expense.

If it must lie, won’t it lie in the direction of favouring its fossil fuel income?


Shame that this isn’t true

“Some radiation was released when the pressure vessel was vented. All radioactive isotopes from the activated steam have gone (decayed). ”

Japan’s nuclear safety agency said on Sunday there was no problem with the cooling process at Tohoku Electric Power Co’s (9506.T) Onagawa nuclear power plant and that a rise in radiation levels there was due to radiation leakage at another plant in a neighbouring prefecture.

100Km away the leaked radioactivity is being detected


It is time for everyone to look at facts, not emotionally respond, nor ignore the realities of living on the Earth. The recent article in the Times Magazine published yesterday, , demonstrates how poor research, bad writing, and incomplete and or incorrect facts can skew and misinform the reader.
The article suggests a doomsday is approaching, and that the slightest amount of pollution of various materials involved in the nuclear process will be fatal to the existence on Earth. Let us examine the facts.
One common email this past week shows the proliferation of 3000 RADS of Nuclear fallout to the Aleutian Islands in three days, 1500 RADS of Nuclear fallout to the coast of Canada in six days, and 750 RADS of Nuclear fallout to Nebraska and Nortyh Central Mexico in ten days. Pollution would travel in the upper atmosphere if it were to spread, and exposure would only be probably caused by particulates from dust or attachment to rain. So far, there is not a dust cloud from the nuclear plants. There is probably small amounts of steam. The liklihood of exposure over more than a few miles is improbable.
The fact is the “Nuclear Fallout Map” is a hoax based on jet stream maps and little additional facts. See further . A “RAD” is a term associated with radiation and nuclear fallout. It refers to exposure, but is not meaningful in itself. (The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as “that quantity of X rays which when absorbed will cause the destruction of the [malignant mammalian] cells in question…”[1] It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one gram of matter. It was restated in SI units in 1970 as the dose causing 0.01 joule of energy to be absorbed per kilogram of matter. The United States Nuclear Regulatory Commission requires the use of the units curie, rad and rem as part of the Code of Federal Regulations 10CFR20.) Rem is more likely to be used. (The röntgen (roentgen) equivalent in man (or mammal[1]) or rem (symbol rem) is a unit of radiation dose equivalent. It is the product of the absorbed dose in rads and a weighting factor, WR, which accounts for the effectiveness of the radiation to cause biological damage.) .
So what is sieverts? “The sievert (symbol: Sv) is the SI derived unit of dose equivalent. It attempts to reflect the biological effects of radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in gray. It is named after Rolf Sievert, a Swedish medical physicist famous for work on radiation dosage measurement and research into the biological effects of radiation.”
Now, given the definitions, a better understanding of nuclear fallout is appropriate. Is it in the atmosphere or will it be on the ground? If it exists, it will start at very high altitudes with the jet stream. Will it reach the ground? Possibly if there is enough up there. Are the Japanese reactors emitting significant amounts of nuclear waste? Probably not. There are layers of containment. A melt down is the destruction of the fuel rods, but a meltdown is a possibility considered in the construction of nuclear facilities. The probable consequence is that local people will be evacuated for safety after the experience with a far less safe nuclear facility, Chernobyl. Additionally, as a society, we should watch for safety flaws, but not over react.
A more serious problem to Earthlings is the rate we are using up natural resources and not replenishing them, the pollution of fresh and salt water, the pollution of the upper atmosphere, and the rapid population explosion that may make the Earth uninhabitable within 500 years.
[This article is published in Time Magazine comments this week.]


i’ve been looking for a sensible interpretation of the japan events—this is very comprehensive—thank you for posting it!


How can you be sure that in the case of a full meltdown, the nuclear fuel wont melt through the steel and concrete containment vessel? Ive heard the steel there is only 6” thick, whereas newer reactors are much thicker?


The line about “ask Iran ” , did not come across as warmongering, rather it is good shorthand for : Building a Nuke bomb represents a formidable engineering challenge. That’s all


As a former NRC licensed operator of a BWR, the write-up was spot on (minor quibbles with some terms). How would anybody fare is every back-up failed when it was called on?

It has been suggested that the designers failed to consider an earthquake of sufficient magnitude (design 8.2 vs. actual 8.9). My question to those would be this: what would be a a good enough design earthquake? Magnitude 10? 11? Nothing could be built economically if that is your criteria. That is pure wishful fantasy.

It is possible to economically design something to withstand foreseeable casualties. It is possible to design something to withstand every possible casualty, foreseeable or not. It is not possible to economically do both.


“The small amounts of Cesium that were measured told the operators that the first containment on one of the rods somewhere was about to give.” – Que? Surely this would indicate that the casing was broke, not about to break?

” I am not quite sure if they flooded our pressure cooker with it (the second containment), or if they flooded the third containment, immersing the pressure cooker. But that is not relevant for us.” –
Umm… flooding a nuclear reactor with sea water kinda seems desperate to a non-specialist like me.

Also it’s not over yet. But, thank you for staying calm however.

Also what’s the fatality rate for petrochemical power vs nuclear? I’m guessing oil kills more…


I know some people at Australian Radiation Services – the Melbourne-based health physics and radiation safety firm whose logo appears on that extremely dodgy looking plume map.

I do not think they would really turn out crap science. They’re serious professionals in a serious field, and they know what they’re doing. I’m seriously tempted to flick them an email to confirm whether or not their organisation did actually produce that chart or if someone else has mendaciously put their name on it.


Thank you. From someone familiar with BWRs, this is a nice synopsis. Agree that we should be celebrating the engineers who had the forethought to have multiple cooling systems, multiple layers of containment, backup generators and batteries, and, when many of these failed, yet another backup in the seawater. Let us continue to give the situation the attention needed to maintain containment integrity while we focus on the immediate needs of thousands stranded without food or water.


While the radioactive Cs and I isotopes are obviously important and a primary focus in nuclear incidents of the type that is occurring at these plants, I think that it is counterproductive (ultimately, in the same way that all the more egregious media misinformation is counterproductive) to frame the discussion as though those are the ONLY two fission product radioisotopes that have been and will be released. As a minimum an informed perspective requires an understanding of why the other radioisotopes of the fission product yield (e.g., are of less significance in this incident.


“A Japanese official said 22 people have been confirmed to have suffered radiation contamination”

This contradicts your claim that “If you were sitting on top of the plants’ chimney when they were venting, you should probably give up smoking to return to your former life expectancy. The Cesium and Iodine isotopes were carried out to the sea and will never be seen again.”

Or perhaps you are suggesting that 22 people were sitting on the chimney?

Just because the operators are trained for these events, that does not mean they are not serious events.

I don’t think I’ve seen any media reports anywhere that claimed that the chain reaction had not been stopped or some other catastrophic situation remained. You do not need to debunk non-existent stories. The media has done a good job.

However, you have written a good summary.


Thank you Mr Brooks. That was excellent. I would like to think that our news media is just ignorant and trying to catch up to reality. Howver, they surely had experts to call upon that gave this information to them within hours. That means they’ve just been hyping things with their talk of meltdown etc for ratings. That’s not surprising, but no less disgusting! Thank you for the truth!


It very helpful read the account provided here. As a retired nuclear professional, I learned specifics about the plant and the sequence of events that I did not see elsewhere. I came across this account via a citation on Real Clear Politics, so hopefully the article will receive additional attention.

I do have a comment to an assertion by esquilax that it is not possible to design around the problems experienced by the Fukushima plants. In 20-20 hindsight, design solution is clear. The diesel generators for backup power should be in an enclosure protected from the effects of a tsunami (as the reactor itself is).

It is reassuring to learn that despite encountering an earthquake five times as severe as the design basis earthquake (which itself is twice as severe as the worst earthquake ever to hit Japan previously), the plants withstood the earthquake.

Let us hope that the scenario does play out as forecast in the article (i.e no new unpleasant surprises) and that the worst is over.


Great explanation. It is unfortunate, however, that the ignorant left will ignore this and call for the death of nuclear power anyway.


I don’t think a meltdown from loss of coolant in a water reactor is the same thing as a chain reaction. At chernobyl, loss of coolant increased the rate of fission due to the particulars of the graphite moderator. That was a chain reaction.


Very nice summary. I am a health physicist but with no NPP experience. If you update the information here I would find this helpful: (1) regarding the facility design: where are the operators located and what protects them? (2) What is the design and location of the coolant piping systems – they apparently are robust enough to have survived the explosion but still must somehow not be completely inside the protected structures if seawater can be brought in. And (3) NISA reports 40 microSv/h (4 mrem/h for US readers) measured at the site boundary – would this have been a short term measurement while noble gases were being vented, and if not what would explain that?


The Fukishima nuclear reactors are Boiling Water Reactors, BWRs. So they were designed and built by the GE-led, BWR reactor consortium.

It is reassuring to note that the new American “Standard Design ” reactors, so painstakingly being designed, reviewed, modified and certified, ALREADY INCLUDE extra provisions for the remote set of circumstances that affected one of the 53 nuclear reactors in Japan forced by the Japanese Earthquake and subsequent tsunami.

New American “Standard Designs” are re-designed so that they have no problems if commercial power is unavailable; and All not just some of the emergency diesel generators did not start.

The new “Standard Designs” nearing final approval, after almost five exhaustive years, of analsysis, modification, and approval and final certification of every nut and bolt in the design. They were redesigned to not need the power for the pumps, at all. These new “Standard Designs” rely on placing the coolant tank above the reactors and letting the emergency cooling water flow down into the reactor vessel by gravity, without needing any pumps. Secondly, they have been redesigned so that the coolant capacity is much larger inside the reactor vessel, requiring less from outside to be added.

Third, the larger coolant capacity reactors are not so time critical to a meltdown. They extend the time to a meltdown without cooling to several hours instead of 45 minutes, allowing more time to thoughtfully react.

Fourth, they have been redesigned so that natural thermal convection will circulate the coolant water, inside the reactor, thus eliminating the need for power to the pumps, or the pumps at all.

Isn’t it further proof that the new reactors and the new NRC certification scheme of “Standard Design”, makes much more sense. It used to be that letting progress occur by each new plant be a single design, perhaps incorporating new features unique to itself, and much more anticipatory rather than reviews while under construction, or in post-accident design fixes.


As to the question how the hydrogen ended up in the reactor building, my (layman) understanding that it first formed inside the reactor vessel and entered the suppression pool system via approaches to cool the reactor (RCIC/HPCI). As suppression pool temperature increased, pressure was released into the containment. After pressure in the containment subsequently reached a critical state, containment was vented – leading to hydrogen mixing with oxygen in the reactor building.

I may of course be totally off. Based on:

Click to access 03.pdf


Reading the Fukushima thread at The Oil Drum I see many assert that we must replace nuclear with wind and solar. Fair enough but how are we going to also replace coal, oil and gas?

There has been a fatality with the death of a crane operator at the Daini group of reactors.


The main thing that needs to happen if or when they overhaul the reactor plants in Japan, is to move the darn things on the WEST coasts, OPPOSITE of the eastern coasts of the Japanese islands that are in the direct line of exposure to major seismic and tsunamic activity……while no place on earth is completely invulnerable to natural disasters, there are some places that are statistically better than others…..


Wait a minute, has this website not been championing a new generation of nuclear technology (Gen IV) that would largely eliminate most of the multifarious risks that have been associated with traditional nuclear reactors?

If so, does this incident not add weight to the case that the new technology ought to be pursued when building any new nuclear plant?

And if so, should this incident not be viewed as a positive, prescient signal to the world that other technology may soon be available – if not already – and to immediately forsake investments in nuclear plant using older technology?

If Brave New Climate is really certain about its prognostications re the new safer technology, then it should have nothing to fear from this incident. It presents a golden opportunity to put its case for a technology transition, surely?


“Kim in USA” asks some very sensible questions in the above comment. I would also like to know the answer to Q1.


I have engineered six BWRs, Mark I, Mark II, and Mark III containments. This is an early Mark I. I would quibble with some statements but consider this article infinitely better than the errors, both ignorant, and intentional, that have been reported. Japan is not out of the woods yet, but we must acknowledge the skill and courage of their people, particularly those working the emergency. There are a lot of unknowns but this report, and time, indicates the good guys are winning, regaining total control, perhaps at the cost of a modest number of deaths, a statement which may have no scale of meaning.
There will be time for informed assessment, and perhaps malfeasance confronted, but now is the time for prayers and best wishes, to a grievously suffering nation.
IMHO, the next unknown to fear is another probably great quake, an aftershock, and possible tsunami over the next week.


Good post – thank you.

However I have to comment that the attempt to downplay the radiation risk is misguided and will do the nuclear cause no good.

You say:-
By “significant” I mean a level of radiation of more than what you would receive on – say – a long distance flight, or drinking a glass of beer that comes from certain areas with high levels of natural background radiation.
However Tepco (the operator) say
“The radiation exposure of 1 TEPCO employee, who was working inside the reactor building, exceeded 100mSv and was transported to the hospital.

Slightly more than you get from a glass of beer, especially as they do the “more than” bullshit.

The 40 year old plant took extraordinary damage and did most of its job, but the general opinion will further harden against nuclear – folk just don’t like even a tiny chance of great risk.

Given that Spain’s electricity is 35% renewables and Portugal 45% I know where I would put Australia’s money. Build wind and solar as fast as possible and plan a few nuclear installations but don’t start for 10 years. By the time you are ready to build we will know how the renewables are doing.


barry: in that video, while you speak, the picture shows arrows pointing out of the nuclear plant with the words “cancer and genetic defects” underneath.


In an earlier video, you mentioned that newer plants had no problems. could you elaborate? what sort of plants were these to which you refer?


Greg, plants like the AP1000 have passive cooling systems that work by convection and gravity feed, so don’t depend on external power. Plants like EPR have fully isolated and sealed power units for the ECCS so would have not been damaged by the tsunami. Sorry for the short reply, being overwhelmed by media stuff right now.


Much better analysis than the TV but Mr. Brooks curiously congratulates Japanese construction because the facility withstood more than the 8.4 or so that it was designed to take (failing then to criticize the forecasters). But the 9.0 earthquake was centered a couple of hundred miles away, so what was the intensity at Fukushima? I’d think it would be below the design limit. More to the point, what would have happened if the 9.0 earthquake had occurred closer to Fukushima, or if several large tremors had occurred back to back and the control rods or the battery somehow was disturbed?


Bill Brown, hard to say at such energy levels, but it was telling that the critical damage was done by the tsunami, knocking out all of the backup generators and redundant generators in one fell swoop. The earthquake itself simply caused a SCRAM and doesn’t seem to have damaged the reactors, which have some degree of seismic isolation. So an ocean-based incident may have made matters worse. But the time for such analysis will have to wait, too much speculation at this stage.


We all love to read something that confirms our opinions. Most people here seem quick to condemn any report that does not support the view that everything is fine, and then just to praise a report from an “MIT Research Scientist” . [ad hom deleted] I will take this report along with all the others claiming there is likely to be a big release of radioactivity with a huge grain of salt until this is all over in a few weeks/months time.


Excellent, informative article.

Interesting perspective from Sonya terBorg, above about how great it is to be safe in a “nuclear free” New Zealand, given that 29 coal miners were recently killed in NZ’s Pike River coal mine disaster, providing non nuclear fuel. So just how many people have been killed in Japan’s nuclear industry?

For some people, fantasy will always triumph over reality.


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