Tohoku Quake and Tsunami Monitoring: "Internal Combustion"

Tohoku Quake and Tsunami Monitoring: "Internal Combustion"

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At 6:14 a.m. a loud thud rocked Reactor 2 of Fukushima 1 nuclear power plant, quite unlike the previous two blasts that blew apart the surrounding structures. This time, the explosion occurred inside the steel containment chamber that surrounds the core reactor. The containment chamber captures and holds run-off water and condensed steam that escapes from the core reactor.

Tokyo Electric Power Company (TEPCO) and the nuclear safety agency have not explained the process that led to the blast.

In the first two blasts, these volatile gases recombined outside the reactor unit and blew apart the concrete rooms. This third and latest blast is one of internal combustion.

Inside the core reactor, "dry" fuel rods cause a chain reaction, which releases radioactive ions. The high energy of fission, in turn, splits the chemical bond in water, producing hydrogen and oxygen, which flow with the steam into the outer containment chamber.

In this case, hydrogen and oxygen radicals re-bonded inside the containment chamber with sudden vigor, expressed as an explosion. This explains why the air pressure in the surrounding room suddenly decreased. The chemical reaction that re-creates water also happens to suck in air.

Following the re-formation of water, the core reactor's heat again ramped up pressure inside the containment chamber, forcing the release of radiation-contaminated steam into the surrounding room. Inside that concrete room, the radiation level quickly rose to 3,000 micro-sieverts (a unit used to measure the threat to human health) to a level of either 4 times or 6 times higher than safety limits, depending on which of two difficult guidelines one chooses from.

This process began on the afternoon of March 14, when the water level inside Reactor 2 sharply dropped, exposing 2.4 meters of the upper section of the fuel rods to air -- or about half their length. These half-rod sections began the fission process.

The unexpected drop in the water level is due to the fact that TEPCO nuclear engineers underestimated the speed of the chemical reaction that splits water into volatile gases.

Seawater was pumped into the core reactor, but by then the build-up of gases inside the containment chamber had reached a point of no return.

That's the bad news.

The even worse news is that the explosive force of the internal blast reflected off the outer surface of the container chamber, thus focusing a surge of massive pressure against the core reactor vessel. There is a strong likelihood, yet to be determined by TEPCO, of damage to the steel alloy of the reactor vessel. So far, any micro-fissures in the core reaction have apparently not yet leaked heavily.

For the foreseeable future, there is no way to prevent this process of gas creation, which means combustion will recur again and again. The only procedure to prevent repeated internal combustion from eventually cracking or collapsing the core reactors is much faster release of steam into the atmosphere.

Since the filter equipment is apparently destroyed, much larger releases of radioactivity will start to pose a wider public-health threat. To partially lower the amount of particles released, TEPCO engineers should quickly jerry-rig an improvised steam-condensation unit using something simple like a 40-foot refrigerated container (and put some water sprinklers inside to trap particles) and then pump the wastewater through a long hose into the ocean floor.

Instead of moping and apologizing, TEPCO has to think outside the box, find fast solutions for every challenge and in the meantime, speed up worst-case preparations for the entombment of the reactors. The public’s safety is in their hands.

Several minutes earlier than the blast inside Unit 2, a fire broke out in Unit 4, where some nuclear materials were being held in storage. The fire releases a signficant amount of radioactive particles that would be harmful to human health.

While comparatively little radiation was released from Reactor 2, the situation is more ominous due to the potential for more internal blasts that could weaken the reactor chamber.


Yoichi Shimatsu, former editor of the Japan Times Weekly, has covered the earthquakes in San Francisco and Kobe, participated in the rescue operation immediately after the Indian Ocean tsunami in 2004 and led the field research for an architectural report on structural design flaws that led to the tsunami death toll in Thailand.