It's been a year now since the Great TÅhoku Earthquake and tsunami which struck Japan, leaving over twenty thousand dead and tens of thousands more homeless and displaced. On top of this was the crisis at the Fukushima Daiichi nuclear facility brought about by the tsunami itself.
This blog too is celebrating its anniversary - it began as a trial by fire, attempting to communicate what was going on to the general public in a way free of the sensationalism and wild inaccuracy which was taking hold in much of the mainstream press. Our effort began with a simple note I'd put up on Facebook - "Fukushima in Layman's Terms" - my attempt to break down what was happening in accurate but understandable terms. The note proved surprisingly popular among my friends - so popular that originally I had briefly moved it to its own web page. With the help of my colleagues Alan and Cyrus, we continued to try to update the FAQ as events unfolded but realized that trying to update a static page was proving too overwhelming with the rapid pace of events. Thus, this blog was born.
As I look back, I realize that we didn't get everything perfect, given the incomplete information we had as it was emerging. But by and large our objective was always focused on getting out the most accurate information directly from official sources (including much translation directly from Japanese sources via Alan) as well as trying to explain each of the relevant concepts involved, which I hope at least that we succeeded at. At the time, it was a bit of a Herculean task; information from Western media sources was shoddy at best (I still recall NPR reporting the Air Force making deliveries of "coolant" to the stricken reactors, leaving me puzzled - these reactors are cooled by ordinary water, after all). Most of the time we relied on the hourly updates coming directly from sources like TEPCO (the utility which owned the Fukushima facility), NISA (Japan's NRC), and the JAIF (Japan Atomic Industrial Forum). Trying to follow and distill the crisis as it unfolded in slow motion consumed most of our lives for the few weeks which followed the quake and tsunami - which is of course nothing compared to those who were actually impacted by these events.
One year later, Japan is still picking up the pieces. As we've seen, the crisis became much larger than any of us had originally anticipated, based on the information we had - the damage proved to be far more extensive, with partial fuel melting at the three active reactors at Fukushima Daiichi (Unit 4 was offline, and contrary to some reports, melting of fuel in the spent fuel pools appears to be extremely unlikely). As of now, only two of Japan's fifty-four reactors are online - at great expense to Japan's economy (which is now running its first trade deficit - of $540 billion - in thirty years). Many of those reactors may never be turned on again - regardless of whether or not there exists a realistic chance of the same sequence of events unfolding at these units. (One of the main objections is that these reactors are on a fault line - something which is true for most of Japan itself. It is worthwhile to remember however that it was not the earthquake which resulted in the crisis at Fukushima Daiichi, but rather the tsunami which swept away backup power generators, resulting in the "total station blackout" condition which prevented cooling of the still-hot fuel.)
Much of what will determine whether these reactors restart goes beyond the safety and stress tests being conducted - instead, it will depend upon local approval. Yet in the meantime, Japan is down to less than 5% of its existing nuclear generating capacity - which as of 2009, made up about 27% of Japan's electricity sector. The result is a net loss of nearly 47 GW of electricity - made up for with a combination of rolling blackouts and dramatic spikes in fossil fuel imports (natural gas imports alone are reported to have increased by 27% in the last year). Such a shutdown, if made permanent, is projected to increase Japan's annual carbon dioxide output by 60 million tons per year - a more than 5% increase.
Like all big stories today, this one too had global reach - including forcing a politically opportunistic reversal of course in Germany, which is now phasing out nuclear energy entirely. (How they will replace this power and meet carbon-cutting targets remains to be seen). The "nuclear renaissance" has continued in the U.S. - albeit at a much slower pace. In other words, this is just the beginning of what will remain a very long story. On that note, over the next few days, I hope to cover some of the recent media retrospectives on the issue - some accurate, some more of the "vulture journalism" variety - both deserving to be carefully scrutinized.
On a separate note, I was having a conversation recently with the students in a class I teach, emphasizing that part of what needs to come out of Fukushima is, similar to the case of Three-Mile Island, a thorough assessment of the improvements to human factors in planning for emergencies like this one. One attitude which is perhaps still prevalent - even understandable - among the nuclear is that everything possible was done in the case of Fukushima as well as possible. Rod Adams goes further, criticizing the academic prose of the recent report by the ANS, arguing strongly that nuclear professionals need to "get a backbone" in defending the performance of the Fukushima plant and its operators.
To some degree, Rod is right - the plants survived a magnitude 8.9 earthquake (well beyond design basis) and failed not because of structural failure but because of the loss offsite power. Further, Adams is correct in the assertion that not one single individual has died from radiation exposure from the plant.
Without getting into the murky issue of radiation epidemiology (i.e., the relationship between excess cancers and radiation), there is a certain degree of dangerous complacency to this attitude, in my opinion; not only exhibited here but in my students' arguably defensive reaction to the idea that anything was done less than perfectly. Even though Three Mile Island resulted in only tiny levels of radioactive release, it was a watershed moment for evaluating human factors in reactor operation. The way control rooms were designed and operated - and the way operators were trained - changed dramatically as a result of this incident - one in which again, no one died. Indeed, Three Mile Island was so minor that no evacuation was even required - period.
Yet as a result this incident, a great deal of study has been committed to factors such as looking at the complexity of instrumentation within control rooms and training operators to respond correctly to abnormal circumstances. And as a result, the U.S. nuclear industry has one of the safest track records of any industry - bar none. But these kind of improvements can only come about through critical self-reflection in the face of events such as these. In my mind - and what I tried to convey to my students - the onus is upon nuclear professionals to demonstrate their commitment to learning how we can improve our ability to respond to conditions such as natural disasters and worst-case conditions such as total station blackout.
There are many who complain this burden is unfair - after all, smaller-scale disasters which do result in fatalities never seem to put entire industries in jeopardy - one need look no further than the natural gas industry for this. And even enormous-scale environmental disasters such as the BP oil spill don't seem to bring these industries to a halt - while the expansion of off-shore drilling is on temporary hiatus, we are still pumping oil from off-shore rigs. Even when industrial accidents are of unthinkable magnitude - think of the Bhopal disaster in India, which killed nearly four thousand people - we still don't talk of shutting down entire industries. So why the special scrutiny for nuclear?
This of course isn't a question that can be answered in this space alone; but the main takeaway is that it also does not matter. Like it or not, this is the environment nuclear professionals must work under - the nuclear industry as a whole essentially serves at the pleasure of the public (with all of its attendant consequences for the risk put up for capital-intensive projects). Until that factor changes, it is the obligation of the nuclear professional to constantly maintain that public trust. Rightly or wrongly, it means being held to a higher standard; it means constantly learning how to better respond to (extremely rare) crisis events like Fukushima.
Meanwhile, I take is as a personal obligation to continue to provide accurate, objective information about developments in nuclear technology and the nuclear fuel cycle. I will never deny having a personal preference for nuclear technology - namely given its enormous potential to solve social ills including global warming and energy poverty - but ultimately it is the job of nuclear professionals like myself to provide accurate, understandable information such that the public can make informed assessments of energy choices - especially given the fact that other agenda-driven groups are not as beholden to the truth. For me, this began with attempting to relay timely and accurate information as the events unfolded during last year's crisis, but it is a continuing obligation that I hope to diligently maintain.
Showing posts with label Japan quake. Show all posts
Showing posts with label Japan quake. Show all posts
Monday, March 12, 2012
Tuesday, January 24, 2012
What to Expect from Japan's Nuclear Fleet and 2011 in Context
A late Happy New Years to all our readers! And the new year, of course, means we have new exciting full sets of data for 2011!
The EIA hosts a useful online data browser for international energy statistics. However, if one was interested in the nuclear electric generation in Japan, for instance, they would find the data only goes through the year 2009. Although that is still decent, it certainly won't give up-to-date information about how the 3/11/2011 earthquake-tsunami event off Japan's Tohoku coast affected the energy market. For that, I have turned to statistics from the Federation of Electric Power Companies of Japan (FEPC). It was just last week, 1/17, that they released the data for December 2011, wrapping up the year. As pdf files, it wasn't the most convenient format to work with, but it's the only place I know to find this. I couldn't resist plotting this data to compare what the nuclear generation in 2011 versus 2010 (as a reference) was. Here is what I found:
What is this graph showing? The Earthquake automatically tripped a number of reactors, so there was a instantaneous decrease in generation at that second, so for the month of March 2011 the time before the earthquake averages with the time after the earthquake to give a number in-between what you would have if the fleet was running normally versus when crippled by the earthquake. One would expect the number for April would be a better representation of the generating capability of the post-earthquake fleet.
A funny thing happens after April 2011. The generation from nuclear in Japan continues to decline almost every month. This is the political and regulatory backlash from the event. Reactors that were not immediately affected were shut down due to safety concerns. Others had trouble getting approval to restart after scheduled outages. By December, the total generation is at a disturbingly low level. Just for a sanity check, the reported capacity factor for Dec 2011 is just 15.2%, and this is excluding the Fukushima plants.
How does this compare to past troubles Japan's nuclear industry has faced? And what are reasonable expectations for the future? Glad you asked. Firstly, I'm going to explain how I made up a guesstimate for future generation. I made up 3 different values.
For this graph, keep in mind that my number for 2011 includes the first 2 months and 10 days when everything was working normally. Because of that, I didn't find it representative to stop with that year. Let me address each of the historical dips in this graph so we can look at the current state of affairs in context.
I'll take a shot at what's predicted and set my own reasonable expectations for nuclear generation in the United States, somewhat like I've done for Japan here.
Extra Links
Here are some thing I ran across after the fact:
Summary of these two: Japan is now importing much more fossil fuels. Again, not surprising.
The EIA hosts a useful online data browser for international energy statistics. However, if one was interested in the nuclear electric generation in Japan, for instance, they would find the data only goes through the year 2009. Although that is still decent, it certainly won't give up-to-date information about how the 3/11/2011 earthquake-tsunami event off Japan's Tohoku coast affected the energy market. For that, I have turned to statistics from the Federation of Electric Power Companies of Japan (FEPC). It was just last week, 1/17, that they released the data for December 2011, wrapping up the year. As pdf files, it wasn't the most convenient format to work with, but it's the only place I know to find this. I couldn't resist plotting this data to compare what the nuclear generation in 2011 versus 2010 (as a reference) was. Here is what I found:
What is this graph showing? The Earthquake automatically tripped a number of reactors, so there was a instantaneous decrease in generation at that second, so for the month of March 2011 the time before the earthquake averages with the time after the earthquake to give a number in-between what you would have if the fleet was running normally versus when crippled by the earthquake. One would expect the number for April would be a better representation of the generating capability of the post-earthquake fleet.
A funny thing happens after April 2011. The generation from nuclear in Japan continues to decline almost every month. This is the political and regulatory backlash from the event. Reactors that were not immediately affected were shut down due to safety concerns. Others had trouble getting approval to restart after scheduled outages. By December, the total generation is at a disturbingly low level. Just for a sanity check, the reported capacity factor for Dec 2011 is just 15.2%, and this is excluding the Fukushima plants.
How does this compare to past troubles Japan's nuclear industry has faced? And what are reasonable expectations for the future? Glad you asked. Firstly, I'm going to explain how I made up a guesstimate for future generation. I made up 3 different values.
- For 2012, I multiplied the average of Oct, Nov, and Dec 2011 by 12 (months per year). This is how much energy the fleet with produce in 2012 if they continue at the same performance as the end of 2011.
- For 2014 and beyond I took the generation in April 2011 (see my prior arguments) and multiplied that by 12 (mo/year).
- For 2013 I just made up a number.
For this graph, keep in mind that my number for 2011 includes the first 2 months and 10 days when everything was working normally. Because of that, I didn't find it representative to stop with that year. Let me address each of the historical dips in this graph so we can look at the current state of affairs in context.
- Chernobyl - This event obviously did not happen in Japan but it obviously affected their safety regulations and the nuclear plants (even though in the middle of a growth period) saw reduced availability as a result.
- TEPCO Scandals - Around 2003 there were numerous data fabrication scandals by TEPCO coming to light. Before 2011, this was one of the most costly events in the worldwide nuclear industry. You can see that the event was more gradual than abrupt, contrary to the natural disasters. It takes time for investigations to proceed and the regulator starts with incomplete information.
- Kashiwazaki-Kariwa earthquake - This was a major event impacting the largest nuclear plant in the world owned by TEPCO. Record breaking shaking occurred in many of the buildings on site, up to 20 m/s^2 in one turbine hall. The severity of the earthquake exceeded design specs by a large margin, although the reactors safely shut down. Because this kicked up new concerns about seismic safety, the plants were completely out of commission in 2008 and slowly began starting back up in 2009.
- 2011 Tohoku Earthquake-Tsunami event - Obviously the greatest nuclear disaster since Chernobyl, this was domestic to Japan. Similar to the K-K earthquake, it was found that a natural disaster far exceeding the design specifications not only posed a risk, but actually caused a full scale nuclear disaster. Similar to the anatomy of prior scandals and regulatory revisions, however, plants all over the nation were gradually shut down over safety concerns. This event, no doubt, caused more loss of capacity than all other events combined and today even threatens to bring Japan's entire nuclear fleet to a halt if the recent months are any indication.
What has made up for this shut down of nuclear plants? Fossil fuels exclusively. Hydroelectric or any renewables don't have the ability to adjust. Well, we need to mention that demand reduction was also a large factor.
Japan's Generation (TW-h) by all Fuel Type
(only large utilities) for 2010 and 2011
(only large utilities) for 2010 and 2011
One would imagine that this could have an impact on Japan's economy. I am very interested to know myself. The fact that fossil fuel imports to Japan have increased and will stay high is obvious. The impact on the Uranium market also can't be understated.
Topic for my Next Post
I'll take a shot at what's predicted and set my own reasonable expectations for nuclear generation in the United States, somewhat like I've done for Japan here.
Extra Links
Here are some thing I ran across after the fact:
Summary of these two: Japan is now importing much more fossil fuels. Again, not surprising.
Monday, January 16, 2012
When the cure proves more harmful than the disease
Recently, the Boston Globe published a series of haunting images taken from the evacuation zone surrounding the stricken Fukushima plants. Perhaps what is most striking is the unearthly nature of the abandoned towns, now turned feral, their only inhabitants being abandoned pets and livestock. These are more snapshots from a post-apocalyptic wasteland; it is difficult to imagine these as bustling, populated places less than a year ago.
Several news agencies have taken up the issue of the evacuation zone, referring to it as "poisoned" and "uninhabitable for decades to come." And of course, taking a look at maps such as that put out by the Washington Post, indeed the situation does look quite grim. (For an excellent, authoritative view of the data, the NNSA has continued to publish updated radiation measurements).
The evacuation zone - and the surrounding dire rhetoric - is founded on the idea of maintain a "safe" level of annual radiation exposure, set as 20 milliSieverts (mSv) per year, or 2 rads/year. (Note that the NRC limits for the general public are 1 mSv/year for the general public, and 50 mSv/year for radiation workers). By contrast, an average member of the public would receive on average 4 mSv/year simply from natural and synthetic sources of radiation each year (including x-rays, CT scans, and the like).
Yet without getting into the issue of radiation hormesis (i.e., the theory that low doses of radiation may show an overall benefit), the epidemiological understanding of radiation risk from protracted exposures at levels under 50-100 mSv is extremely murky. (This is not to assert no long-term risk of excess cancers exists for this level, but simply that our currently understanding of this risk is extremely limited). Conversely, long-term doses above 100 mSv have been shown to contribute to an excess risk of cancers.
Two critical questions should arise at this point. First, what risk exactly is the 20 mSv/year limit based upon? For reference, estimates of radiation doses from smokers put their annual doses around 150 mSv/year (due to polonium-210 deposited into smokers' lungs). While clearly no one should endorse smoking as a healthy life choice, it is regardless a risk that individuals freely make each day at that we as a society (reluctantly) accept. In this sense then, why is the government of Japan making a choices of risk for thousands of displaced individuals at thresholds far lower than individuals themselves accept?
Taking the linear-no-threshold model (LNT) at its face, a common value for excess fatal cancers is assumed to be 0.005 fatal cancers / Sv (i.e., 0.00005 fatal cancers / mSv; the value established by NCRP 115). Assuming a dose of 20 mSv/year, this leads to an overall increase in risk of fatal cancer of 0.03 over 30 years - in other words, 1 in 33 individuals can be expected to develop an excess fatal cancer over a period of 30 years. That comes down to 1 fatal cancer per year per 1,000 individuals exposed - a rate approaching statistical noise.
Still sound scary? Let's put this into the context of other common everyday risks:
In other words, the absolute limit on exposure established by the Japanese government corresponds to roughly equal that of the mortality risk of an average smoker (assuming conservative projections of low-dose risk). Beyond this, residents are to be permanently exiled from their homes.
Several news agencies have taken up the issue of the evacuation zone, referring to it as "poisoned" and "uninhabitable for decades to come." And of course, taking a look at maps such as that put out by the Washington Post, indeed the situation does look quite grim. (For an excellent, authoritative view of the data, the NNSA has continued to publish updated radiation measurements).
The evacuation zone - and the surrounding dire rhetoric - is founded on the idea of maintain a "safe" level of annual radiation exposure, set as 20 milliSieverts (mSv) per year, or 2 rads/year. (Note that the NRC limits for the general public are 1 mSv/year for the general public, and 50 mSv/year for radiation workers). By contrast, an average member of the public would receive on average 4 mSv/year simply from natural and synthetic sources of radiation each year (including x-rays, CT scans, and the like).
Yet without getting into the issue of radiation hormesis (i.e., the theory that low doses of radiation may show an overall benefit), the epidemiological understanding of radiation risk from protracted exposures at levels under 50-100 mSv is extremely murky. (This is not to assert no long-term risk of excess cancers exists for this level, but simply that our currently understanding of this risk is extremely limited). Conversely, long-term doses above 100 mSv have been shown to contribute to an excess risk of cancers.
Two critical questions should arise at this point. First, what risk exactly is the 20 mSv/year limit based upon? For reference, estimates of radiation doses from smokers put their annual doses around 150 mSv/year (due to polonium-210 deposited into smokers' lungs). While clearly no one should endorse smoking as a healthy life choice, it is regardless a risk that individuals freely make each day at that we as a society (reluctantly) accept. In this sense then, why is the government of Japan making a choices of risk for thousands of displaced individuals at thresholds far lower than individuals themselves accept?
Taking the linear-no-threshold model (LNT) at its face, a common value for excess fatal cancers is assumed to be 0.005 fatal cancers / Sv (i.e., 0.00005 fatal cancers / mSv; the value established by NCRP 115). Assuming a dose of 20 mSv/year, this leads to an overall increase in risk of fatal cancer of 0.03 over 30 years - in other words, 1 in 33 individuals can be expected to develop an excess fatal cancer over a period of 30 years. That comes down to 1 fatal cancer per year per 1,000 individuals exposed - a rate approaching statistical noise.
Still sound scary? Let's put this into the context of other common everyday risks:
| Risk | Order (death/year) |
| Meteorite impacts | ~0.000000001 |
| Radiation risk to U.S. population from nuclear power plant | ~0.00000001 |
| Radiation from high-altitude flight | ~0.0000001 |
| Lightning | ~0.0000001 |
| Radiation death from consumer products | ~0.0000001 |
| Flood, tornadoes, or earthquakes | ~0.000001 |
| Death from cholera or whooping cough | ~0.000001 |
| Train passenger death | ~0.000001 |
| Natural background radiation | ~0.00001 |
| Passenger deaths in aircraft accidents | ~0.00001 |
| Death from leukemia or tuberculosis | ~0.00001 |
| Brain damage from whooping cough vaccination | ~0.0001 |
| Overall road accident deaths | ~0.0001 |
| Death from bronchitis or influenza | ~0.0001 |
| Death from childbirth | 0.00024 |
| Lung cancer risk from smoking (20 cigarettes/day) | ~0.001 |
| Death from all causes at age 55 | ~0.001 |
| Death from cancer, stroke, or heath disease | ~0.001 |
| Deaths from 20 mSv exposure (Fukushima exposure limit) | ~0.001 |
In other words, the absolute limit on exposure established by the Japanese government corresponds to roughly equal that of the mortality risk of an average smoker (assuming conservative projections of low-dose risk). Beyond this, residents are to be permanently exiled from their homes.
Now conceive of a relatively conservative limit of even 50 mSv/year - a little over double the current exposure limit. From the above exposure maps, this would roughly halve the area requiring permanent evacuation ("permanent" in the sense of on the order of 30 years given the half-life of Cs-137, assuming no further decontamination). This would lead to a risk of about 0.0025 cancer deaths per year - 1 in 400.
While such a certainly not trivial (although based in conservative projections), the question which should be asked is, "Compared to what?" Those evacuated have been deprived entirely of their livehoods - homes, farms, businesses, and ancestral lands. Clearly, they should be made whole by TEPCO, and failing this, the Japanese government. Yet the additional risk of death from cancer should be balanced against the very real physical and emotional costs of a mass involuntary exodus (not even taking into account the economic consequences for both those impacted and the country as a whole). What is the toll in terms of premature death from stress and associated mental health disorders (including alcoholism, depression, etc.) In essence, what is the cost in terms of human life from a forced exile?
Nor are forced evacuations without cost - recently, Japan's blanket 20 km evacuation order recently came under fire due to its generic nature (ignoring real-time radiation exposure data, showing that contamination was spreading in a northwestern direction due to seasonal winds). As a result, many individuals received a higher dose than that if they'd simply stayed in place (due to evacuating to the northwest). Likewise, these evacuations themselves necessitated leaving homes (areas where radiation exposure is lower) and taking up residence in less sanitary conditions with greater exposure to the outdoors (increasing overall exposure). Thus, evacuation itself is not without risk. Each alternative must be evaluated for its own inherent risks - there is no free lunch.
I am not about to assert that there is zero risk from allowing residents to return - namely because the data regarding prolonged low-dose exposures is too poor to make a definitive assessment of risk. However, what should happen is that residents themselves should be allowed to make this choice from themselves - rather than the government. Perhaps one of the most disturbing images of the Fukushima evacuation zone is the constant vigil of police forces enforcing the no-entry zone; an act done ostensibly for residents' "own good," despite the legions of family pets and livestock abandoned to starvation or a return to ferity.
Within reason, these are risks residents should be allowed to evaluate on their own in an informed manner, rather than having them imposed upon them.
Sunday, March 13, 2011
A Little More Information About Boiling Water Reactors
First off, there are two distinct sites that are owned by Tokyo Electric Power Company TEPCO in the Fukushima Prefecture. Fukushima Daiichi (literally One) and Daiini (Two):
Daiichi is the home to six separate reactor cores while Daiini has four. Currently, the reactors in question are units 1 and 3 of Daiichi. Unit 1 is a General Electric BWR/3 design while Unit 3 is a BWR/4 design. The major difference is an increased power density of 264 MWe (Mega Watts electric).
Japanese power plants take advantage of using the ocean as their natural heat sink. All power plants require a large supply of water to disperse unusable heat. Typically either the ocean, a lake, river or a picturesque cooling tower is used for this purpose.
At a glance, a nuclear power plant is similar in many ways to its fossil fuel cousins. A heat source, whether it be the burning of coal, oil, natural gas or the fissions of uranium from a nuclear plant, provide energy to increase the temperature of water.
The energy density though of uranium is orders of magnitude higher than any fossil fuels. This means that a few tons of fuel material (uranium dioxide and zirconium cladding) can supply the same amount of energy as massive heaps of coal.
The concept is not unlike that of a pressure cooker in yours grandmother's kitchen. We can use the pressure differential of the built up steam to turn the blades of a turbine. This turbine is connected by a huge shaft to a generator; basically a massive rotating magnet. Using good ole' Faraday's Law we can induce a movement of electrons which is currently powering that glowing box you are staring into.
One, a great many of the ~12,000 or so fuel rods would have to be pretty fully uncovered for a decent amount of time before getting up to the 5189 degrees Fahrenheit needed to begin to melt. Even then, it would need to become a molten ball of what is called 'corium' that then slumps to the bottom of the pressure vessel. This massive structure would then dissipate a tremendous amount of heat due to conduction with the molten fuel. A typical pressure vessel is around eight inches thick made of steel with an outer liner on top of that. Just think about that. That is a lot of steel to go through. Even if it gets that far, it then gets to dissipate its heat through the pedestal and eventually the concrete base-mat that's around 20 feet thick (to stabilize against earthquakes mind you).
On top of that, there are multiple systems within containment that are designed to keep the gaseous fission products in. There are series of HEPA and charcoal filters that catch quite a bit.
On top of that, there are perimeters established at nuclear facilities to keep the public at a safe distance in case of a release. On top of that, the Japanese government has take the precaution of moving everyone a LONG ways away.
The bottom line:
Is this a significant occurrence: yes; one I will certainly always remember in my career.
Is there going to be a catastrophic event where the deaths of many people are involved with the nuclear facilities: there is a finite probability that tends to nil.
Should the news agencies be focusing on the reactors instead of other events: no, let the engineers do their job and we will let the journalists do theirs.
Should the engineers keep everyone informed: yes, and they are but with the internet media these days its a little hard to find Waldo (TEPCO, NEI, IAEA) in this sea of ramblings and misinformation.
Daiichi is the home to six separate reactor cores while Daiini has four. Currently, the reactors in question are units 1 and 3 of Daiichi. Unit 1 is a General Electric BWR/3 design while Unit 3 is a BWR/4 design. The major difference is an increased power density of 264 MWe (Mega Watts electric).Japanese power plants take advantage of using the ocean as their natural heat sink. All power plants require a large supply of water to disperse unusable heat. Typically either the ocean, a lake, river or a picturesque cooling tower is used for this purpose.
At a glance, a nuclear power plant is similar in many ways to its fossil fuel cousins. A heat source, whether it be the burning of coal, oil, natural gas or the fissions of uranium from a nuclear plant, provide energy to increase the temperature of water.
The energy density though of uranium is orders of magnitude higher than any fossil fuels. This means that a few tons of fuel material (uranium dioxide and zirconium cladding) can supply the same amount of energy as massive heaps of coal.
The concept is not unlike that of a pressure cooker in yours grandmother's kitchen. We can use the pressure differential of the built up steam to turn the blades of a turbine. This turbine is connected by a huge shaft to a generator; basically a massive rotating magnet. Using good ole' Faraday's Law we can induce a movement of electrons which is currently powering that glowing box you are staring into.
One, a great many of the ~12,000 or so fuel rods would have to be pretty fully uncovered for a decent amount of time before getting up to the 5189 degrees Fahrenheit needed to begin to melt. Even then, it would need to become a molten ball of what is called 'corium' that then slumps to the bottom of the pressure vessel. This massive structure would then dissipate a tremendous amount of heat due to conduction with the molten fuel. A typical pressure vessel is around eight inches thick made of steel with an outer liner on top of that. Just think about that. That is a lot of steel to go through. Even if it gets that far, it then gets to dissipate its heat through the pedestal and eventually the concrete base-mat that's around 20 feet thick (to stabilize against earthquakes mind you).On top of that, there are multiple systems within containment that are designed to keep the gaseous fission products in. There are series of HEPA and charcoal filters that catch quite a bit.
On top of that, there are perimeters established at nuclear facilities to keep the public at a safe distance in case of a release. On top of that, the Japanese government has take the precaution of moving everyone a LONG ways away.
The bottom line:
Is this a significant occurrence: yes; one I will certainly always remember in my career.
Is there going to be a catastrophic event where the deaths of many people are involved with the nuclear facilities: there is a finite probability that tends to nil.
Should the news agencies be focusing on the reactors instead of other events: no, let the engineers do their job and we will let the journalists do theirs.
Should the engineers keep everyone informed: yes, and they are but with the internet media these days its a little hard to find Waldo (TEPCO, NEI, IAEA) in this sea of ramblings and misinformation.
Fukushima in layman's terms
This post started out as a facebook note to try and educate and inform my non-technical friends and associates about what's been going on in the situation with Japan post-quake with respect to the reactors at Fukushima Daiichi. I was surprised at how many people appreciated having the relevant issues explained to them in both a way that was factual (rather than sensationalist) and in a way they could understand.
With the help of my colleagues Alan and Cyrus, this post began evolving into a platform for communicating what's been going on since the initial response and digesting much of the information for a more general audience. Given that, it made sense to branch these updates out into a more flexible, updated format - in other words, a blog.
This post will cover some of the frequently-asked questions I posted on facebook, and following from this, we'll be covering the situation as it has evolved from there.
First, the most authoritative place for news would be the Tokyo Electric Power Company, which is releasing regular press releases on the situation. Likewise, ANS nuclear cafe is featuring constant media updates on the situation. Rod Adams at Atomic Insights also has a good summary of the situation. NEI also has a good fact sheet on the events at Fukushima that have occurred up until now and details as they unfold.
What happened in the earthquake?
When the earthquake struck, Japan's reactors were immediately shut down by a quick insertion of control rods, stopping the chain reaction responsible for producing fission (known as a "scram.") This was successful in all of the reactors. However, the reactor still remains "hot" for a short time after shutdown, because of very short-lived radioactive fission products which are in the fuel. As these fission products decay, they produce heat - this heat still must be removed from the reactor in order to keep the fuel cool. As my colleague Cyrus points out, this can be 6-7% of full power at shutdown - in a reactor like Fukushima, this can be 60-70 MW. However, the rate of heat produced decays exponentially; after 24 hours, it would be around 10 MW and falling. In other words, the first few days are the most crucial - keeping the fuel submerged over the next week is the highest safety priority.
Normally, when the power is cut off in an emergency such as this, a diesel generator serves as a backup system, which powers pumps in order to circulate coolant (much like in your car's radiator). However, it appears that these diesel systems were damaged during the earthquake - thus, the pumps had to operate on limited battery power until this was exhausted. Contrary to early reports, the USAF wasn't "flying in coolant" (as this just consists of ordinary water); however, a backup diesel generator was flown in and installed to get the pumps working again. (Cyrus points out that the diesel generator was working for about an hour until the subsequent tsunami struck, which is what disabled the diesel backup system.)
What is the big concern?
The biggest concern in this case is keeping the fuel cool - even though the reactor is "off" (e.g., not producing any more fissions), heat is still being produced which needs to be "wicked" away. Without the water circulating, what will happen is similar to in your car's radiator if the car's water pump fails - i.e., the coolant will continue to get hotter and hotter until it boils. This increases pressure inside containment (or your radiator). In each case, the pressure build-up can eventually cause a blow-out, where the containment (or your radiator) is breached. This is obviously undesirable.
To prevent this, some of the steam is being vented, to "bleed off" the pressure. The downside of this of course is that letting out steam means there's less water available for cooling now. Likewise, a very small amount of radiation may be released in the process (carried with the steam). However, the amounts are generally incredibly small - the largest dose indicated has been in the reactor building, where one worker received 106.3 mSv - elevated beyond regulatory limits, but far from fatal. Current estimates of the control room put the dose around 70 microsieverts/h, or about 7 mrem/h - elevated, but quite small. One would have to be exposed continuously for nearly an entire work year for it to begin to hit regulatory limits, which are themselves conservative.
The big concern about keeping the fuel cool is to keep the fuel intact. When fissions occur, almost all of the radioactive isotopes created are trapped in the ceramic fuel itself - this is a safety feature. Thus, the main concern about keeping the fuel cool isn't a "China Syndrome" type of situation (which itself is physically impossible), but rather a matter of keeping the radioactivity safely confined.
As water is boiled away from the reactor, there is a chance that the fuel can be "uncovered," which is where the risk of partial melting of the fuel exists. (i.e., nothing is left to wick away heat from the element itself). However, the fuel itself is only the first radiation containment barrier - the containment building itself is also designed to prevent the release of radiation to the environment, specifically under these types of circumstances.
Has there been a meltdown? (Is this like Three-Mile Island or Chernobyl?)
Basically, no. First, it's helpful to define the term "meltdown." Were the reactor completely devoid of coolant, eventually the entire core assembly would heat and melt - producing a large, very hot radioactive pool of metal on the floor of the containment building. (Rod Adams helpfully points out that it's unlikely it would even get this far - in Three Mile Island's case, a substantial portion of the core melted, however it cooled into a lump of metal - "corium" - at the bottom of the pressure vessel.) This is not what is happening, nor is it the danger. The risk is in "uncovering" fuel from coolant, where the top portion of the fuel may melt and release radioactive fission products.
What has happened is that the fuel in Unit 1 may have been exposed for some time due to loss of coolant, which may have resulted in some loss of radioactivity.
Three-Mile Island was a partial fuel melt due in part to operator errors - operators incorrectly believed the reactor was being flooded with coolant (when in fact a pump was stuck closed), turning off coolant to the reactor. While the core itself was rendered unusable and the unit shut down, the actual dose received by the public was extremely minimal.
Chernobyl was a reactor different than the kind operated in Japan or the United States (and in fact would be illegal to build in the U.S. for technical reasons). Chernobyl operators were conducting tests with poor communication and had bypassed several safety devices. This was a full "meltdown" in the true sense, resulting in an explosion in thecontainment reactor building and a release of radioactivity. However, it should be noted that the death toll was relatively small, and most of the dose received (and subsequent casualties) were in the first responders to the accident.
Update: Reader David helpfully points out the following about Chernobyl's lack of safety systems which are present in all U.S. and Japanese reactors:
In the case of Japan, the operators have been doing things correctly - the fuel has been kept as cool as possible to prevent any possible overheating of the fuel. Everything they've done so far has been to minimize the risk of damage to the core or accidental release of radiation to the public.
Wasn't there a radiation leak at one of the reactors?
Fukushima Daiichi Unit 1 appears to have released a small amount of radioactivity when the containment was vented in order to relieve pressue (due to the buildup of boiling water). However, this release appears to have been very small and of no real danger to the public. The measured radiological levels near the plant have been reported to have been elevated from 0.007 rem/hr to 0.67 rem/hr. While this is elevated beyond normal acceptable limits, this is far below the levels of Three Mile Island (itself quite small) or Chernobyl (much larger). The IAEA has given the incident a 4 on its International Nuclear and Radiological Event Scale (on a scale of 1-7; TMI was a "5", and Chernobyl was a "7").
TEPCO has announced that it is venting containment in Unit 3 as the high-pressure coolant injection system - an emergency system designed to inject coolant into the core - has stopped and cannot be restarted. Like Unit 1, venting is a safety precaution to release pressure from containment.
To clarify: when the containment is vented, a very small amount of radioactivity is released into the environment. This primarily consists of nitrogen-16, which is created when atmospheric nitrogen is bombarded by neutrons. N-16 has an extremely short half-life (7.16 seconds), meaning that by the time any release would reach any member of the public, it would have already decayed back away into stable oxygen; thus, the exposure from nitrogen would be nearly non-existent.
One of the other major isotopes released would be tritium, an isotope of hydrogen with two neutrons (instead of none) which exists in trace quantities in nature, and is created in small quantities in reactors when hydrogen in water molecules absorbs neutrons. Tritium is weakly radioactive - as a beta radiation emitter, it only poses a real issue if it is ingested into the body (and even then, in large quantities). (This is unlike gamma emitters, which are deeply penetrating, or alpha emitters, which can be stopped by the surface of the skin or a sheet of paper). Tritium actually is present in a lot of everyday applications, from exit signs (yes, really!) to glow-in-the-dark watch dials. Overall, this represents a very minor amount of radiation compared to that present in the fission products contained by the clad and fuel material itself.
Why are they evacuating the area?
This is done as a preventative precaution to protect the public. While so far there has been no known escape of radioactivity (save for what may have been released when the containment was vented), the evacuation is to ensure that this can be verified without putting anyone at risk.
My colleague Alan disagrees with my characterization somewhat, indicating that he thinks the evacuation may be more of a preventative measure against possibly larger failures. While I remain more optimistic, I don't fundamentally disagree with the idea that any such evaluation is fundamentally preventative in nature. My point here is more emphasize that no significant public danger currently exists (i.e., no known significant radiological hazard at the present time). However, Alan's contention is that he believes that a potential, undetermined radiological hazard would not warrant such an extreme evacuation alone (especially under these conditions), and thus the evacuation is a precautionary measure against potential larger failures.
What caused the explosion? (Is this a meltown?)
There was an explosion in the reactor building (not the reactor or the containment building). Official sources speculate that this was due to an ignition of hydrogen gas. Hydrogen gas can build up due to the extreme heating of the water and dissolution into hydrogen and oxygen. When the cladding which encases the fuel gets very hot, it can dissolve water into its components of oxygen and hydrogen. (This may have been caused by the fuel becoming "uncovered"). This hydrogen may have ignited as containment was vented, causing an explosion. However, this is not a meltdown - so far, there has been no good indication that any kind of catastrophic fuel failure (melt) has occurred.
Some further explanation: for this reactor, there are two "nested" containment buildings - "primary" containment, which houses the core itself and is the chief barrier against a release of radioactive materials if the fuel, clad, and pressure vessel fail. Then there is a larger building around this, "secondary" containment, which is the reactor building itself, including industrial equipment like cranes, etc. used to service the core. The explosion occurred outside of primary containment, in secondary containment. This diagram from NEI shows a diagram of the building, including the core and where the explosion occurred. You can see what happened from this photo to give you some perspective of what these buildings normally looked like, see here (you're looking at the tall, rectangular-looking buildings - light blue). Basically, an explosion is serious, but not an indication that primary containment has failed or that radioactive materials are being released from the fuel/core itself.
Why are they flooding the containment building with seawater?
Basically, they need to keep the temperature down in the reactor. Because they've been losing water to boiling, they need to quickly cool the reactor. In order to do this, the operators have made the decision to flood the containment building with seawater containing boron (boron is used to "quench" nuclear reactions by absorbing neutrons - the point here is as an added safety precaution). What this ultimately means is that Unit 1 is likely a total loss - i.e., it will never operate again. However, this appear to be the only reactor which was so significantly impacted. Again, the other reactors on the same site (along with other reactors in the general area) have shut down and been cooled normally.
Doesn't this prove nuclear power is fundamentally unsafe?
No no no. A thousand times no. First, bear in mind that earthquakes are part of the design basis for every single reactor built today. Second, after an magnitude 8.9 earthquake - the largest in Japan's recorded history, and the fifth largest earthquake in human history - and a subsequent tsunami, the integrity of the reactor (and all of the other units at this same site) are intact. While breaking reports indicate that this reactor may have been "ruined" by the catastrophe, the danger to the public has been extremely minimal - namely, because engineered safety systems worked as planned.
Let me re-emphasize: nuclear reactors are often over-designed for the point of safety. The very first safety system to kick in was to turn off the reactor. This worked for every single unit. The second safety system, a diesel generator, worked in most cases - it would appear that the severity of the quake / tsnunami damaged the diesel backup in the case of Unit 1. However, battery backup systems gave operators time to provide a contingency. Second, the physical containment itself has operated as designed in providing a means of containing potential releases. (As Cyrus points out, nuclear systems are designed with a "defense in depth" - with the fuel, clad, pressure vessel, and containment building providing multiple layers against a radioactive release. At the moment, the main concern is at the level of the fuel / clad - not beyond this.)
What this proves is that in the very worst scenario - a once-in-a-lifetime earthquake beyond the design basis - that the systems can safely contain the integrity of the reactor, particularly with well-trained personnel.
To put a further point to it, this is what is going on right now at a liquified natural gas facility in the same area. (More images of the devastation here.) Basically, no system out there is going to stand up favorably to a disaster like this, but nuclear systems are specifically engineered against situations like this - again, unlike natural gas.
The BWR/6 design manual gives a more detailed explanation of the emergency cooling systems being employed here, in pages 57-68 of the PDF (pages 4-4 to 4-15 of the manual). While the BWR/3 at Unit 1 is slightly different these details are discussed on page 11 (page 1-2).
Hopefully this clears up some of people's questions, but if you have more, by all means ask, and I'll update this post accordingly. (And of course, if my NE friends want to add anything, please do!)
And of course, a special thanks to my colleagues Alan and Cyrus who have been helping me compile information for these updates.
What happened in the earthquake?
When the earthquake struck, Japan's reactors were immediately shut down by a quick insertion of control rods, stopping the chain reaction responsible for producing fission (known as a "scram.") This was successful in all of the reactors. However, the reactor still remains "hot" for a short time after shutdown, because of very short-lived radioactive fission products which are in the fuel. As these fission products decay, they produce heat - this heat still must be removed from the reactor in order to keep the fuel cool. As my colleague Cyrus points out, this can be 6-7% of full power at shutdown - in a reactor like Fukushima, this can be 60-70 MW. However, the rate of heat produced decays exponentially; after 24 hours, it would be around 10 MW and falling. In other words, the first few days are the most crucial - keeping the fuel submerged over the next week is the highest safety priority.
Normally, when the power is cut off in an emergency such as this, a diesel generator serves as a backup system, which powers pumps in order to circulate coolant (much like in your car's radiator). However, it appears that these diesel systems were damaged during the earthquake - thus, the pumps had to operate on limited battery power until this was exhausted. Contrary to early reports, the USAF wasn't "flying in coolant" (as this just consists of ordinary water); however, a backup diesel generator was flown in and installed to get the pumps working again. (Cyrus points out that the diesel generator was working for about an hour until the subsequent tsunami struck, which is what disabled the diesel backup system.)
What is the big concern?
The biggest concern in this case is keeping the fuel cool - even though the reactor is "off" (e.g., not producing any more fissions), heat is still being produced which needs to be "wicked" away. Without the water circulating, what will happen is similar to in your car's radiator if the car's water pump fails - i.e., the coolant will continue to get hotter and hotter until it boils. This increases pressure inside containment (or your radiator). In each case, the pressure build-up can eventually cause a blow-out, where the containment (or your radiator) is breached. This is obviously undesirable.
To prevent this, some of the steam is being vented, to "bleed off" the pressure. The downside of this of course is that letting out steam means there's less water available for cooling now. Likewise, a very small amount of radiation may be released in the process (carried with the steam). However, the amounts are generally incredibly small - the largest dose indicated has been in the reactor building, where one worker received 106.3 mSv - elevated beyond regulatory limits, but far from fatal. Current estimates of the control room put the dose around 70 microsieverts/h, or about 7 mrem/h - elevated, but quite small. One would have to be exposed continuously for nearly an entire work year for it to begin to hit regulatory limits, which are themselves conservative.
The big concern about keeping the fuel cool is to keep the fuel intact. When fissions occur, almost all of the radioactive isotopes created are trapped in the ceramic fuel itself - this is a safety feature. Thus, the main concern about keeping the fuel cool isn't a "China Syndrome" type of situation (which itself is physically impossible), but rather a matter of keeping the radioactivity safely confined.
As water is boiled away from the reactor, there is a chance that the fuel can be "uncovered," which is where the risk of partial melting of the fuel exists. (i.e., nothing is left to wick away heat from the element itself). However, the fuel itself is only the first radiation containment barrier - the containment building itself is also designed to prevent the release of radiation to the environment, specifically under these types of circumstances.
Has there been a meltdown? (Is this like Three-Mile Island or Chernobyl?)
Basically, no. First, it's helpful to define the term "meltdown." Were the reactor completely devoid of coolant, eventually the entire core assembly would heat and melt - producing a large, very hot radioactive pool of metal on the floor of the containment building. (Rod Adams helpfully points out that it's unlikely it would even get this far - in Three Mile Island's case, a substantial portion of the core melted, however it cooled into a lump of metal - "corium" - at the bottom of the pressure vessel.) This is not what is happening, nor is it the danger. The risk is in "uncovering" fuel from coolant, where the top portion of the fuel may melt and release radioactive fission products.
What has happened is that the fuel in Unit 1 may have been exposed for some time due to loss of coolant, which may have resulted in some loss of radioactivity.
Three-Mile Island was a partial fuel melt due in part to operator errors - operators incorrectly believed the reactor was being flooded with coolant (when in fact a pump was stuck closed), turning off coolant to the reactor. While the core itself was rendered unusable and the unit shut down, the actual dose received by the public was extremely minimal.
Chernobyl was a reactor different than the kind operated in Japan or the United States (and in fact would be illegal to build in the U.S. for technical reasons). Chernobyl operators were conducting tests with poor communication and had bypassed several safety devices. This was a full "meltdown" in the true sense, resulting in an explosion in the
Update: Reader David helpfully points out the following about Chernobyl's lack of safety systems which are present in all U.S. and Japanese reactors:
Chernobyl didn't have a containment building. It was not designed to withstand internal pressure from the reactor, and relied on continuous atmospheric ventilation. Chernobyl was also graphite moderated, the graphite reacting with superheated steam to create the large explosion internal to the core that blew the roof off the place. No commercial reactors, in the US or Japan, make use of graphite, and all have containment buildings designed to withstand significant internal pressures. The GE Mark I suppression-pool containment design, their first, is arguably the weakest design (structurally) still in service. As design basis accidents evolved, Mark I containments were retrofitted with a venting system to limit containment overpressurization, and prevent catastrophic containment failureIn that sense, Fukushima could not be more different than the Chernobyl design; it has a containment designed to withstand high internal pressures, and does not rely upon air cooling with the outside. Likewise, it is a water-moderated reactor, rather than graphite, which has several key safety advantages, including a design which automatically "shuts down" the reaction as temperatures increase. This is known in technical parlance as a "void coefficient," which means how the reactor responds to an increasing fraction of stream-to-liquid, or in other words, "void." Unlike Chernobyl, all reactors licensed in the U.S. must have a negative void coefficient, meaning they "slow down" as the fraction of steam in the coolant increases, preventing a runaway acceleration of the core reaction rate as it heats up. Because the core is already shut down, any Chernobyl type of scenario is already off the table - the issue now is simply one of keeping the fuel cool and intact until the decay heat comes down over the course of a few days. (End update).
In the case of Japan, the operators have been doing things correctly - the fuel has been kept as cool as possible to prevent any possible overheating of the fuel. Everything they've done so far has been to minimize the risk of damage to the core or accidental release of radiation to the public.
Wasn't there a radiation leak at one of the reactors?
Fukushima Daiichi Unit 1 appears to have released a small amount of radioactivity when the containment was vented in order to relieve pressue (due to the buildup of boiling water). However, this release appears to have been very small and of no real danger to the public. The measured radiological levels near the plant have been reported to have been elevated from 0.007 rem/hr to 0.67 rem/hr. While this is elevated beyond normal acceptable limits, this is far below the levels of Three Mile Island (itself quite small) or Chernobyl (much larger). The IAEA has given the incident a 4 on its International Nuclear and Radiological Event Scale (on a scale of 1-7; TMI was a "5", and Chernobyl was a "7").
TEPCO has announced that it is venting containment in Unit 3 as the high-pressure coolant injection system - an emergency system designed to inject coolant into the core - has stopped and cannot be restarted. Like Unit 1, venting is a safety precaution to release pressure from containment.
To clarify: when the containment is vented, a very small amount of radioactivity is released into the environment. This primarily consists of nitrogen-16, which is created when atmospheric nitrogen is bombarded by neutrons. N-16 has an extremely short half-life (7.16 seconds), meaning that by the time any release would reach any member of the public, it would have already decayed back away into stable oxygen; thus, the exposure from nitrogen would be nearly non-existent.
One of the other major isotopes released would be tritium, an isotope of hydrogen with two neutrons (instead of none) which exists in trace quantities in nature, and is created in small quantities in reactors when hydrogen in water molecules absorbs neutrons. Tritium is weakly radioactive - as a beta radiation emitter, it only poses a real issue if it is ingested into the body (and even then, in large quantities). (This is unlike gamma emitters, which are deeply penetrating, or alpha emitters, which can be stopped by the surface of the skin or a sheet of paper). Tritium actually is present in a lot of everyday applications, from exit signs (yes, really!) to glow-in-the-dark watch dials. Overall, this represents a very minor amount of radiation compared to that present in the fission products contained by the clad and fuel material itself.
Why are they evacuating the area?
This is done as a preventative precaution to protect the public. While so far there has been no known escape of radioactivity (save for what may have been released when the containment was vented), the evacuation is to ensure that this can be verified without putting anyone at risk.
My colleague Alan disagrees with my characterization somewhat, indicating that he thinks the evacuation may be more of a preventative measure against possibly larger failures. While I remain more optimistic, I don't fundamentally disagree with the idea that any such evaluation is fundamentally preventative in nature. My point here is more emphasize that no significant public danger currently exists (i.e., no known significant radiological hazard at the present time). However, Alan's contention is that he believes that a potential, undetermined radiological hazard would not warrant such an extreme evacuation alone (especially under these conditions), and thus the evacuation is a precautionary measure against potential larger failures.
What caused the explosion? (Is this a meltown?)
There was an explosion in the reactor building (not the reactor or the containment building). Official sources speculate that this was due to an ignition of hydrogen gas. Hydrogen gas can build up due to the extreme heating of the water and dissolution into hydrogen and oxygen. When the cladding which encases the fuel gets very hot, it can dissolve water into its components of oxygen and hydrogen. (This may have been caused by the fuel becoming "uncovered"). This hydrogen may have ignited as containment was vented, causing an explosion. However, this is not a meltdown - so far, there has been no good indication that any kind of catastrophic fuel failure (melt) has occurred.
Some further explanation: for this reactor, there are two "nested" containment buildings - "primary" containment, which houses the core itself and is the chief barrier against a release of radioactive materials if the fuel, clad, and pressure vessel fail. Then there is a larger building around this, "secondary" containment, which is the reactor building itself, including industrial equipment like cranes, etc. used to service the core. The explosion occurred outside of primary containment, in secondary containment. This diagram from NEI shows a diagram of the building, including the core and where the explosion occurred. You can see what happened from this photo to give you some perspective of what these buildings normally looked like, see here (you're looking at the tall, rectangular-looking buildings - light blue). Basically, an explosion is serious, but not an indication that primary containment has failed or that radioactive materials are being released from the fuel/core itself.
Why are they flooding the containment building with seawater?
Basically, they need to keep the temperature down in the reactor. Because they've been losing water to boiling, they need to quickly cool the reactor. In order to do this, the operators have made the decision to flood the containment building with seawater containing boron (boron is used to "quench" nuclear reactions by absorbing neutrons - the point here is as an added safety precaution). What this ultimately means is that Unit 1 is likely a total loss - i.e., it will never operate again. However, this appear to be the only reactor which was so significantly impacted. Again, the other reactors on the same site (along with other reactors in the general area) have shut down and been cooled normally.
Doesn't this prove nuclear power is fundamentally unsafe?
No no no. A thousand times no. First, bear in mind that earthquakes are part of the design basis for every single reactor built today. Second, after an magnitude 8.9 earthquake - the largest in Japan's recorded history, and the fifth largest earthquake in human history - and a subsequent tsunami, the integrity of the reactor (and all of the other units at this same site) are intact. While breaking reports indicate that this reactor may have been "ruined" by the catastrophe, the danger to the public has been extremely minimal - namely, because engineered safety systems worked as planned.
Let me re-emphasize: nuclear reactors are often over-designed for the point of safety. The very first safety system to kick in was to turn off the reactor. This worked for every single unit. The second safety system, a diesel generator, worked in most cases - it would appear that the severity of the quake / tsnunami damaged the diesel backup in the case of Unit 1. However, battery backup systems gave operators time to provide a contingency. Second, the physical containment itself has operated as designed in providing a means of containing potential releases. (As Cyrus points out, nuclear systems are designed with a "defense in depth" - with the fuel, clad, pressure vessel, and containment building providing multiple layers against a radioactive release. At the moment, the main concern is at the level of the fuel / clad - not beyond this.)
What this proves is that in the very worst scenario - a once-in-a-lifetime earthquake beyond the design basis - that the systems can safely contain the integrity of the reactor, particularly with well-trained personnel.
To put a further point to it, this is what is going on right now at a liquified natural gas facility in the same area. (More images of the devastation here.) Basically, no system out there is going to stand up favorably to a disaster like this, but nuclear systems are specifically engineered against situations like this - again, unlike natural gas.
The BWR/6 design manual gives a more detailed explanation of the emergency cooling systems being employed here, in pages 57-68 of the PDF (pages 4-4 to 4-15 of the manual). While the BWR/3 at Unit 1 is slightly different these details are discussed on page 11 (page 1-2).
Hopefully this clears up some of people's questions, but if you have more, by all means ask, and I'll update this post accordingly. (And of course, if my NE friends want to add anything, please do!)
And of course, a special thanks to my colleagues Alan and Cyrus who have been helping me compile information for these updates.
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