Explainer: Is Japan’s Fukushima reactor melting down?

An aerial view shows the quake-damaged Fukushima nuclear power plant on March 12, 2011.

Editor's note: this article was updated at 11:45 a.m. with evidence that "the reactor core may be undergoing a melt down" and "could result in a very large release of radioactivity," according to a nuclear expert. See below for details. Another update at 12:30 p.m., with further analysis from nuclear experts.

BOSTON — The situation appears to be deteriorating at Japan’s Fukushima nuclear power station, on the coast near where the earthquake struck Friday, and experts are increasingly concerned that a major accident is imminent.

On Saturday afternoon, an enormous cloud of smoke rose over the facility. The explosion blew the roof off the building, but it did not occur inside the reactor container at its stricken Daichi (meaning, Number 1) reactor, the Kyodo News Service cited Japan’s Chief Cabinet Secretary Yukio Edano as saying. “Edano said the 3:36 p.m. explosion resulted in the roof and the walls of the building housing the reactor's container being blown away,” Kyodo reported.

In its last detailed press release, Tepco, the utility that operates the plant, stated: “A vertical earthquake hit the site and big explosion has happened near the Unit 1 and smoke breaks out around 3:36 p.m. Our two employees and two cooperation workers who had been working for the foundation of safety are suffered and they are all sent to the hospital.”

Tepco also indicated that radioactive releases were increasing, according to a monitoring car outside the site. “The amount of radiation at the boundary of the site exceeds the limits,” a 5:00 p.m. press release indicated, requiring the company to escalate its emergency measures. Tepco had been issuing releases almost every hour, but the latest releases have become brief, and the 5:00 p.m. release was the last to include an hourly time stamp.

The most recent press release was notably brief. “We are presently checking on the site situation of each plant and effect of discharged radioactive materials. We will endeavor to restore the units and continue monitoring the environment of the site periphery,” it stated. This release — titled “White smoke around the Fukushima Daiichi Nuclear Power Station Unit 1 — was the latest as of 10:00 a.m. Eastern time on Saturday, but was not marked with a time.

While information from the stricken facility is increasingly difficult to obtain, the possibility of a meltdown appears to growing.

“There are a series of events that have to occur before a large release from a core melt can happen,” explained Robert Alvarez, a senior policy adviser to the Energy Department's secretary under the Clinton administration and a senior scholar at the Institute for Policy Studies, in an email to GlobalPost.

“If cooling isn't restored within 24 hours, the reactor core will likely boil away enough water to expose the fuel.” When this happens, there’s a possibility that the zirconium fuel cladding could catch fire and hydrogen could be generated. “Radiation levels from gases and fuel particulates inside the containment will soar. Then the question is whether or not the secondary containment will be breached.”

“The only real world example we have is the 1979 accident at TMI, which fortunately did not result in a catastrophic release. The Fukashima 2 reactor is about 40 years old, and has a secondary containment that's less robust than later designs.”

UPDATE: Alvarez informed GlobalPost via email: "The presence of cesium-137 is clear evidence of radioactive fuel debris is being detected outside of the plant. This indicates that the efforts to control the heat in the reactor are failing and that the reactor core may be undergoing a melt down. If the primary containment of the reactor has failed this could result in a very large release of radioactivity."

The threat stems from the failure — after the massive earthquake — of several redundant electric systems that had been set up to power the emergency cooling system. Yesterday, the Japanese Air Force raced to fly in backup diesel generators, and the U.S. Air Force provided cooling water for the reactor, according to an article Alvarez published in Counter Punch.

UPDATE, Saturday at 12:30 p.m ET: Arnold Gundersen, a former senior vice president with Nuclear Energy Services and a nuclear industry whistleblower, adds:

"Even if the reactor remains intact, the Fukushima explosion indicates that the containment has failed and there is now a direct path for radioactive releases directly into the environment. Events over the last day indicate that volatile radioactive elements such as xenon, krypton, cesium, iodine, and strontium are already being released from the Fufushima nuclear reactor. The fuel rods have lost their integrity and, EVEN IF the reactor maintains its integrity, are being released into the environment through open relief valves on top of the reactor.

"Whether or not there is a meltdown," he concludes, "enormous quantities of radioactive gases will continue to be released through the failed nuclear fuel."

Meanwhile, Tepco has not updated its press releases in as many as 7 hours. It's Sunday, 2:30 a.m. in Japan; the company issued releases the previous day (Saturday) every hour between 2:00 a.m. and 7:00 a.m.

The NY Times is reporting that "Officials said late Saturday that leaks of radioactive material from the plant, which began before the explosion, were receding and that a major meltdown was not imminent. The Times also indicated that Tepco "plans to fill the reactor with seawater to cool it down and reduce pressure. The process would take five to 10 hours, Mr. Edano said, expressing confidence that the operation could 'prevent criticality.'"

Follow David Case on Twitter: @DavidCaseReport

Russia Today posted video of the explosion at the Fukushima Daiichi plant: