Explosion Rocks Japanese Nuclear Plant

There's been an explosion at a nuclear power plant in Fukushima, Japan — the facility that was seriously damaged during Friday's massive earthquake and where authorities have been trying to cool down the reactor core to prevent a meltdown.

NPR's Jon Hamilton tells us it was NOT a nuclear explosion. Images from the scene show one building was destroyed. The Associated Press reports that the blast "tore down the walls of a building Saturday."

Reuters says that:

"A nuclear industry body official said on Saturday he believed a blast at a Japanese atomic power plant was due to hydrogen igniting, adding it may not necessarily have caused radiation leakage. 'It is obviously an hydrogen explosion ... due to hydrogen igniting,' Ian Hore-Lacy, communications director at the World Nuclear Association, a London-based industry body, told Reuters after reports of the explosion in Japan."

And the AP adds that: " 'meltdown' is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant's systems and its ability to manage temperatures. It is not immediately clear if a meltdown would cause serious radiation risk, and if it did how far the risk would extend. Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely. 'It's not a fast reaction like at Chernobyl,' he said. 'I think that everything will be contained within the grounds, and there will be no big catastrophe.' "

Prime Minister Kaoto Kan, just moments ago, said no Japanese citizens have been affected by any radiation leaks — if there have been any — from his nation's nuclear power plants. As NPR's Hamilton notes, "there are reports that radioactivity has been detected both inside and outside the [Fukushima] plant."

We'll be following this development, and other news from Japan, as the day continues.

Update at 11:25 a.m. ET: We just published a new post with much more on the "last-ditch effort" to prevent a meltdown.

Update at 10 a.m. ET. How It May Have Happened And The Key Question — Is The Core Melting?

On Weekend Edition earlier today, NPR's Jon Hamilton told host Linda Wertheimer that the explosion may have happened "as workers were trying to add water to keep this overheating nuclear core from getting any hotter. When you do that, you can produce hydrogen," which is highly explosive.

The explosion, he said, destroyed "the outer-most building, which is really just a shell housing. All of the radioactive material is still inside that containment structure."

The key unknown at this point, Jon said, is: "What is going on in the core? Are they flooding just because it's hot or has it actually started to melt?" If so, everything inside that containment structure could become highly radioactive and the only thing left to do is "seal it up with concrete. You sort of entomb it."

Update at 8:45 a.m. ET. Seawater To Be Used To Cool Reactor:

Japanese authorities "say seawater — combined with boric acid — will be used to fill the steel container to try to cool the reactor," NPR's Louisa Lim reports. "That process will take 5 to 10 hours."

Update at 7:30 a.m. ET. Pressure In Reactor Said To Be Decreasing:

Government spokesman Yukio Edano, the Associated Press writes, "says the radiation around the plant did not rise after the blast — but instead is decreasing. He added that pressure in the reactor was also decreasing. Pressure and heat have been building at the nuclear reactor since an earthquake and tsunami Friday caused its cooling system to fail."

Update at 7:15 a.m. ET. Blast Did Not Damage Reactor, Government Says:

Japan's Kyodo News writes that "Japanese authorities have confirmed there was no explosion at the troubled No. 1 reactor of the Fukushima No. 1 nuclear power plant, top government spokesman Yukio Edano said. The chief Cabinet secretary also told an urgent press conference that the operator, Tokyo Electric Power Co., has confirmed there is no damage to the steel container housing the reactor."

Update at 7 a.m. ET. Video Of The Blast.

Russia's RT News has posted this video. It's said to show the moment of the blast: