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Because all of the explosions that occurred at Fukushima were hydrogen/oxygen explosions caused by bleeding the built up hydrogen from the pressure vessel. The hydrogen was being thermally separated from the oxygen because of the high temperatures in the core. Rather than let pressure build up in the pressure vessel (and exposing the core) emergency relief valves bled it into the environment -- the environment in this case being the mostly sealed building around the reactor. Hydrogen built up near the roof until an electrical spark -- or just something hot enough -- caused it to ignite.

The result was the "explosion" (rapid deflagration) of the hydrogen/air mix which rapidly disassembled the thin steel building around the reactor vessel. The reactor itself was never open to the air, and no one has ever claimed it was.

Educate yourself on what happened before making wild claims.

This site used to have a complete, day-by-day discussion of everything going on at Fukushima.

Severe slip of the mind, I shouldn't post in the morning after working a double shift. I meant Tohoku Denryoku's Onagawa NPS that was far closer to the epicenter, see Japan's Atomic Industrial Forum map of situation of NPS's in Japan:
http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS02_1330597193P.pdf

Now, Chuden's Hamaoka NPS is in the process to being reinforced against tsunami and quakes, at least it is what the company says.
http://hamaoka.chuden.jp/english/provision/index.html

Aside the dunes, the operator plans to build a 18 m tall sea wall behind the dunes, and increase the eight of dunes to 20 m; they claim that the station is designed to withstand a quake of 1000 gal with the reinforcement work that ended in march 2008, well above the japanese standard of 800 gal. Certainly, all this work is not done by the goodness of the owner's hearts, but doesn't make sense to the company to not try to quell the claims of Hamaoka being called the most dangerous nuclear power plant in Japan, better try to convince citizens of the safety and security of the station and restart operation instead of keeping it in cold shutdown.

Look up the definition of cold shutdown. It doesn't matter if reactor containment is breached.

Not in this case. Here, "cold shutdown" has been redefined somewhat, to "below boiling if we can keep cooling water going in." Normally, in a cold shutdown, you can take the lid off the reactor, look inside, and replace fuel rods. They're a long way from that point.

More like: "the molten corium has burrowed deep enough to be cooled by groundwater and we are only reading 90 degrees at the twisted, melted reactor because the radioactive steam coming from below ground is dissipating the heat"

But not that bad, either. These reactors were built on bedrock. That placed them lower than would have been desirable for flood protection, but if they leak, they leak sideways, not down. There's been plenty of sideways leakage, but by now most of that water is being collected. There's now a cleaning plant in place to run the water through zeolites and catch the radioactive salts and solids. (Water itself doesn't become radioactive from exposure to gamma radiation; the longest lived radioactive isotope of oxygen has a half-life of 122 seconds.)

Now they have to figure out how to do the tough job - safely dismantling the radioactive mess in the melted core into small bits for disposal. That may take decades.

The daily reactor status updates from Japan Atomic Industrial Forum have stated core and fuel damage (or in some cases unknown status) for all three reactors since within days of the accident.

Now the company is worried that the molten pool of radioactive fuel may have burned a hole through the bottom of the containment vessel, causing water to leak.

They're saying there "may have" been a breach - not that there "was" a breach.
What's really strange, is a lot of reputable sources are reporting this wrong.

In fact, a note from the Japan Atomic Industrial Forum (JAIF) quotes Banri Kaieda, the nation's Economy, Trade and Industry Minister, as saying that it is "a fact" that there were holes created by the meltdown. That would likely mean at least some of the uranium fuel is now lying on the basemat below, or perhaps even outside the concrete containment.

But nowhere in their linked report does it say anything about a breach in the containment vessel.

Economy, Trade and Industry Minister Banri Kaieda said it is a fact that the
water injected into the No.1 reactor leaked away because of a hole or holes
created by the meltdown.
[...]
The operator, TEPCO, said on Thursday that most of the fuel rods in the reactor are believed to have melted and sunk to the bottom of the reactor's pressure vessel.
TEPCO says the melted fuel has apparently cooled, even though much of the injected water is leaking through holes at the bottom of the vessel.
Under a plan decided last month, the utility was to fill up the containment vessel with water and set up a system to circulate the water through a heat exchanger.

Not that I can really blame them too much for mixing up some of the terms, considering how many different "vessels" there are.

Though, it does seem TIME got it right:

It's important to note, however, that the worst has not come to pass, nor do experts believe that it will. In that scenario, all of the rods would have fully melted, collapsed, and burned through the pressure and containment vessels, causing a large radioactive leak outside.

WSJ too:

Within 16 hours, the reactor core melted, dropped to the bottom of the pressure vessel and created a hole there. By then, an operation to pump water into the reactor was under way. This prevented the worst-case scenario, in which the overheating fuel would melt its way through the vessels and discharge large volumes of radiation outside.

The nuclear industry lacks a technical definition for a full meltdown, but the term is generally understood to mean that radioactive fuel has breached containment measures, resulting in a massive release of fuel.

I'd like to read more about your second link, but it says the NRC report is "confidential". Got a closer-to-the-source link? Or at least a newer one? (the report is from March)

JAIF has published in 13/may/2011 a map of the radiation surveys by MEXT and US's DOE, with the radiation lecture in air 1m above ground, cesium 137 deposition in soil and marine contamination readings:
http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1305269890P.pdf

That's just the very beginning - hook up an air filtration system so humans can briefly enter the containment. Then try to hook up a water level gauge for the reactor pressure vessel, so they can actually tell how much of the core is uncovered. Then they can think about what to do next.

All this work is taking place in partially collapsed buildings where explosions have destroyed the structure. Ordinarily, one would bring in big cranes with grabs and start removing debris. But they can't do that.

The situation remains dangerous as long as there are still many red blocks on the JAIF's status chart. Note that reactors 1,2, and 3 still have not reached cold shutdown, where the reactor core is below the boiling point of water, all steam has condensed to water, and pressure in the reactor vessel is down to one atmosphere. All the ad-hoc cooling measures aren't enough to get the core temperature down. Normal time to cold shutdown for a GE Mark I reactor is about a day. Even at Three Mile Island, it took only about two days to reach cold shutdown.

The Japan Earthquake thread in the nuclear engineering forum at physicsforums.com has become a more reliable and timely source of information on the stricken reactors at Fukushima than mainstream news sources, according to commenters posting from Japan. The latest news:Japan's Nuclear and Industrial Safety Agency says air may be leaking from theNo 2 and No 3 reactors at the Fukushima Daiichi power plant.Another example, as of March 30, 11 AM JST: Radioactive iodine 3,355 times legal limit found in seawater near plant. Another from March 30: IAEA Confirms Very High Levels of Radiation Far From Reactors.

April 11, 2011. The Japanese government's nuclear safety agency has decided to raise the crisis level of the Fukushima Daiichi power plant accident from 5 to 7, the worst on the international scale. Also, see this post from the physics forum. In each case, the news was available on physicsforums.com before publication in the mainstream press.

Let's hope that the Japanese government does not suppress this essential source of information.

Normally, cold shutdown takes a few days. At Three Mile Island, it took two weeks. Six months is worrisome. Too many more things can go wrong during that period.

They still have so little information about what's going on inside the reactors. Check the latest JAIF status report. Pressure is unknown. Temperature is unknown. Water level is unknown. "Fuel rods exposed partially or fully". Reactors 1 and 3 are buried under piles of rubble. And they have to fix the plumbing under that debris.

Japan Atomic Industrial Forum has also been posting updates. The latest one includes robot exploration results. Here:
Robot measures radiation
The operator of the damaged Fukushima Daiichi nuclear power plant says the maximum radiation level inside the No. 3 reactor building is 57 millisieverts per hour.
Tokyo Electric Power Company used US-made remote-controlled robots on the 1st floor of the No. 1 and No. 3 reactor buildings on Sunday to measure radiation levels, temperatures and oxygen densities.
It announced on Monday that radiation readings were 10 to 49 millisieverts per hour in the No.1 building, and 28 to 57millisieverts per hour in the No. 3 building.
Exposure to the maximum reading in the No. 3 building for 4 and a half hours would exceed the emergency safety limit for nuclear power plant workers, set at 250 millisieverts.
Oxygen densities in both buildings were around 21 percent, high enough for workers to enter the buildings.
On Monday, TEPCO plans to use the robots to take measurements inside the No. 2 reactor.
Based on the collected data, the company will study what kind of work can be done inside the reactor buildings. ...
Monday, April 18, 2011 12:44 +0900 (JST)

There's also info on the (rising) water levels in the tunnels of reactor 2, the 6-9 month containment plan

Get daily updates about Fukushima in PDF format.

best information I have found.

http://www.jaif.or.jp/english/

While not terribly cheap, the technology for separating dissolved compounds from water(to fairly extreme degrees of purity, in the case of water for lab/analytic use) is very much off-the-shelf.

Right. That was done at Three Mile Island. Bear in mind that you can't make water itself radioactive; hydrogen and oxygen don't have any radioactive isotopes with long half-lives. (The longest, 15O has a half-life of 122 seconds, so it's gone within an hour.) All the radioactivity is in dissolved solids. So the process looks a lot like desalinization - the water is forced through membranes that catch all the solids. Eventually, you have dry salts, which you put in casks and bury in some desert or hard-rock cave.

That's the easy part of the problem, though. Remember that the reactor buildings are wrecked from the hydrogen explosions. All the fuel rods in the spent fuel pools have to be carefully moved to some other location, probably newly built spent fuel pools nearby. In 3-5 years, they'll have decayed enough for dry storage, and they'll be put into casks. They can then be moved off site.

This leaves the reactors themselves. Units 1,2, and 3 still haven't reached cold shutdown. Until that's achieved, cleanup can't even start. The situation isn't even close to safe until all three reactors are in cold shutdown, not leaking, and have redundant cooling. Look at the status reports at the Japan Industrial Atomic Forum. Until all the red squares turn yellow, there's a sizable risk of things getting worse.

Decommissioning the damaged reactors will be really tough. They're too damaged to de-fuel, and they need constant cooling, so they can't just be encased in steel and concrete. I don't know what will be done.

This is much, much worse than Three Mile Island. At TMI, the control room was up and running through the whole episode, they reached cold shutdown in a few days, they never had an explosion, and radioactivity was confined to the containment vessel.

And they probably don't know either.

The reactor may have melted through the base of its pressure vessel, but it's hard to tell. The high radiation levels could either be from a melt-through or from a leak as attempts are made to force water into the reactor pressure vessel. The latest JAIF status report contains almost all the hard data that's coming out. Everything else is secondary speculation based on that limited data.

No data seems to be available about pressure or temperature inside the reactor. That's listed as "unknown" for unit 2. The sensors involved were probably destroyed in one of the fires, explosions, or building collapses. Pressure in the containment vessel for unit 2 is listed as "low", whatever that means.

A full meltdown is now a real possibility. The JAIF chart has been showing "Fuel rods exposed partially or fully" for units 1, 2, and 3 for ten days now. Reactor pressure vessels are tough, as are containment structures, but ten days of no core cooling is well beyond design limits.

Understand that the water spraying operation refers to the containment structure, which is normally dry. Inside the containment is the reactor pressure vessel, which is a boiler. Getting water inside there, which is needed to cover the core and achieve cold shutdown, requires forcing it in against steam pressure. This has to be done in a highly radioactive environment, in a fire-ruined building where the walls and beams have collapsed, the pumps are damaged, and valves which are usually operated remotely have to be operated by people turning handwheels. Some people are trying very hard to do that. Some of them will probably die. If they succeed, there will be a local mess, but it will be manageable. If they fail, there will be a meltdown.

The best reports on reactor status are at Japan Atomic Industrial Forum, which publishes a status table every day. This is addressed to people in the industry. They just list the facts, without explanation.

The good news for March 28 is that Unit 3's containment is now listed as "undamaged" instead of "possibly damaged". Unit 2 is listed as "damaged and leakage suspected", and that's now the most worrisome unit.

There's finally a fresh water supply for cooling. That's a big relief. Sea water cooling in a boil-off situation leaves tons of salt behind, and there was a real worry that the seawater cooling would stop working once too much salt accumulated. Fresh water cooling can continue indefinitely. It's not clear where the water is coming from. Hopefully they have a water line to a reliable source by now, and aren't just bringing in tanker trucks.

The cores in units 1,2, and 3 still have exposed fuel rods. Until water injection into the core is working again, the reactor can't be brought to cold shutdown. Remember, the reactor vessel is pressurized and contains a mixture of water and steam. Injecting water into a boiler is inherently difficult. Injecting water into a damaged boiler in a ruined structure in a highly radioactive area is very tough.

The spent fuel pool situation on reactors 3 and 4 is marginally under control. Seawater spray continues, but if they have to keep putting water in, the situation is still bad.

They're weeks from a stable emergency shutdown.

That's just the beginning. The situation isn't safe until there are again redundant closed loop cooling systems working. The current cooling hacks dump radioactive water into the ocean.

Then comes decommissioning. The spent fuel pools have to be cooled for three years or so, and then the fuel rods transferred from the wrecked buildings to dry casks. It will probably be necessary to build another containment building around unit 2, at least. Units 1,2, and 3 are all too damaged to ever de-fuel normally. It's not clear what will be done there. Unit 4 wasn't fueled, but it had a hydrogen explosion while cooling was lost, and will probably never be restarted. Units 5 and 6 can potentially be restarted, but it's doubtful that they will be.

The interesting thing to me is how completely inaccurate all of the media has been in this entire "nuclear crisis". I work for a very large energy company with some of the guys that go visit those nuclear plants every year, most of them with PHDs in Nuclear Physics. Their concerns right now focus mainly on the nuclear fuel rod storage and how they are going to handle the excess amount of heating and unspent fuel rods sitting in empty cooling pools. There are absolutely no major concerns around the radiation levels past the power plants property lines. There has so far been ONE casualty to this accident, and people think that nuclear is unsafe? People in California are taking Potassium Iodide and several of them have gone to the hospital for their stupidity. If you are interested in the information about the nuclear event, and information about the actual power plants and exposure levels? Here's some reading, enjoy :)
Things it would be nice for the news media to have read before they started talking...
GE BWR Manual
http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf
GE ESBWR - Latest Design: Unbuilt.
http://www.gepower.com/prod_serv/products/nuclear_energy/en/downloads/gea14429g_esbwr.pdf
Wiki Concerning Accident
http://en.wikipedia.org/wiki/Fukushima_I_nuclear_accidents
Wiki BWR
http://en.wikipedia.org/wiki/BWR
Spent Nuclear Fuel Calculations
http://repository.lib.ncsu.edu/ir/bitstream/1840.16/2309/1/etd.pdf
Graphic: Plant Status
http://news.nationalpost.com/photo_gallery/japan-earthquake-graphic-nuclear-reactor-status/
Earthquake/ Radiation Levels/ No.2 / Status
http://news.nationalpost.com/2011/03/16/graphics-explaining-japans-nuclear-reactor-disaster/
Tsunami
http://news.nationalpost.com/photo_gallery/japan-earthquake-graphic-where-the-wave-hit/#more-52826
Inside Reactor 2
http://news.nationalpost.com/photo_gallery/japan-earthquake-graphic-inside-fukushima-daiichis-most-worrisome-reactor/
Meltdown Dynamics
http://news.nationalpost.com/photo_gallery/graphic-meltdown-fears/
Exposure Levels
http://news.nationalpost.com/photo_gallery/japan-earthquake-graphic-how-fast-will-radation-kill-you/#more-52930
Earthquake Data/ H2 Blast/ Radiation Spread
http://news.nationalpost.com/photo_gallery/japan-earthquake-graphic-nuclear-plant-blasts/
Nuclear Fission product Decay
http://en.wikipedia.org/wiki/Nuclear_fission_product
NRC: Zirconium Cladding Fire
http://www.irss-usa.org/pages/documents/SGS_213-223_response.pdf
Reactor Status: Excel Spreadsheet
http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_13002

Well, at least they did place it on Tokyo. And the rest are actual nuclear plants but they missed a few.

An accurate map is on the last page of this report. 16 nuclear plants total, 12 of them active and unaffected. That's 40 nuclear reactors working safely, 8 safe even after the quake, and 6 at Fukushma Daiichi giving them trouble.

Fukushma Daiichi is ONE plant, which has 6 reactors. Plant != Reactor.

Well, at least they did place it on Tokyo. And the rest are actual nuclear plants but they missed a few.

An accurate map is on the last page of this report. 16 nuclear plants total, 12 of them active and unaffected. That's 40 nuclear reactors working safely, 8 safe even after the quake, and 6 at Fukushma Daiichi giving them trouble.

See the Japan Atomic Industrial Forum site for reactor status reports that have real information. The latest status report (22:00, March 17) Their table for the spent fuel pool in #4 says "Level low, preparing water injection, damage to fuel rods suspected."

Reactors 1,2 and 3 still have water levels below the top of the core, and seawater injection continues. So they still haven't reached cold shutdown.

Desperate attempts are being made to fill the spent fuel pool. which, incidentally, is well above ground level in a severely damaged building that has just had a fire. Radiation levels are too high for anyone to approach. Helicopter water drops have been tried. Military fire trucks, ones that can spray water without the operator getting out of the truck, are being used. "The effect of these operations is under evaluation".

See the Japan Atomic Industrial Forum site for reactor status reports that have real information. The latest status report (22:00, March 17) Their table for the spent fuel pool in #4 says "Level low, preparing water injection, damage to fuel rods suspected."

Reactors 1,2 and 3 still have water levels below the top of the core, and seawater injection continues. So they still haven't reached cold shutdown.

Desperate attempts are being made to fill the spent fuel pool. which, incidentally, is well above ground level in a severely damaged building that has just had a fire. Radiation levels are too high for anyone to approach. Helicopter water drops have been tried. Military fire trucks, ones that can spray water without the operator getting out of the truck, are being used. "The effect of these operations is under evaluation".

Yeah, that's right, hearsay and rumour from TEPN:
http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1300273535P.pdf

Ooooh look, it reports containment integrity at each unit at each of the two stations! And ooh look, it reports that at Units 2 and 3 at Station 1 the status is not "Not damaged" but "Damage suspected".

So oooh look, you've made yourself look like an even bigger twat -- in public

Sorry to reply myself, but this is the last report from JAIF:
http://www.jaif.or.jp/english/aij/member/2011/2011-03-11earthquake4.pdf

Still, almost 3 days without new info and Tepco's website down, this will be a PR nightmare.

This is a perfect case of trying to be better safe than sorry. There is no radiation infection. There is radiation contamination, and this is what this 4 people had. I'm more sad about the crane operator that died yesterday. I hope that they can manage to control both units today. Oddly enough, the Japan Atomic Industrial Forum as not issued a new press release since this http://www.jaif.or.jp/english/aij/member/2011/2011-03-11emergency.pdf at March 11, if they had been more careful with PR they could have stopped the wild rumors that are going at the moment. On the plus side, it appears that the great majority of the nuclear sites in earthquake area withstood the earthquake without problems. This is bad, but if the design troubles were true, this could have been far worse.

Correction - no leak, and the fire is not in the reactor building:
http://www.neimagazine.com/story.asp?sectioncode=132&storyCode=2059127

See also http://www.jaif.or.jp/english/ for updates.

Product PDF :: http://www.ihi.co.jp/ihi/file/technologygihou2/100 04_6.pdf which mentions this interesting phrase:

The Dynajet 2.6 is also undergoing development of mobile dry toilets featuring its Merit (3).

Originally built for military and civilian use, IHI's Dynajet 2.6 KW microturbine genset is selling commercially in Japan is 1.2 million Yen (about $9000) "for use in Japan only" (kerosene fuel). There are no plans for export. They don't have a natural gas version. Very little information is available, though I do have a 2-page product description and spec sheet (*available). The unit measures 30"x10"x11" and weighs 140 lb. [The contact at IHI prefers not to be listed.]

Last year, about 90,000 small power generators were sold in Japan. Japanese manufacturers are now working hard to expand their sales and add new models to their product lines. IHI Aerospace, for example, has released a portable model, the Dynajet 2.6, driven by a microgas turbine. Fueled by kerosene, which is easier to store than gasoline, the generator retails for ¥1.2-1.3 million ($10,100-10,900). It is the lightest gas turbine-driven model on the market. The company is also developing a cogeneration system that utilizes waste heat from gas turbines.