TEPCO: 8 Months out, Reactor may have reached Criticality again

November 3, 2011

Fukushima, 8 months later:  the accident is still not over, much less the clean up, which has hardly begun.

Asahi Shimbun -November 02, 2011

Officials of Tokyo Electric Power Co. (TEPCO) pumped water mixed with boric acid into the No. 2 reactor at the Fukushima No. 1 nuclear power plant early on Nov. 2 after finding traces of xenon, a radioactive gas that might indicate nuclear fission has taken place.

Officials of the company said that some parts of the reactor may have reached criticality, a state of self-sustaining nuclear fission. Fuel believed to have melted in the accident triggered by the March 11 earthquake may have caused the fission. The boric acid was pumped into the reactor to suppress the reactions.

Radiation levels near the Fukushima plant have not shown any unusual surge, but the latest development may hinder plans by the central government to bring the Fukushima nuclear accident under control by the end of the year.

TEPCO officials said a test on Nov. 1 of the gases processed found traces of what appeared to be radioactive xenon 133 and xenon 135.

The identification of the gas as xenon is now being checked by another research institute. Radioactive xenon is produced when uranium in fuel in a reactor core undergoes nuclear fission.

Xenon 133 has a half-life of about five days while xenon 135 has a half-life of about nine hours.

If the gas found in the reactor was xenon, it would mean nuclear fission is continuing in the reactor.

Because of such results, TEPCO officials mixed boric acid into water pumped into the reactor for cooling purposes from 2:48 a.m. on Nov. 2.

However, no radioactive iodine, which is also produced during nuclear fission with xenon, was detected in the gas checked at the No. 2 reactor.

Moreover, neutron rays, which are emitted during nuclear fission, were not detected in the vicinity of the reactor building.

“We cannot deny the possibility of a temporary, small-scale state of criticality,” said TEPCO official Junichi Matsumoto. “However, because there was no sudden increase in the temperature or pressure of the reactor core, we concluded there was no major state of criticality.”

Large scale criticality will not usually occur unless, as is found in a normal reactor core, nuclear fuel is carefully positioned and surrounded by water to stimulate nuclear fission.

Through a constant pumping of water into the cores of the No. 1 to No. 3 reactors, temperatures at all three reactors had fallen under 100 degrees by late September, six months after the nuclear accident, indicating stable cooling of the reactors

UPDATE: TEPCO Denies Criticality Generated Xenon at Fukushima Plant

Tokyo Electric Power Co. said Nov. 3 that naturally occurring “spontaneous nuclear fission”–not a state of criticality–created the xenon detected at the Fukushima No. 1 nuclear power plant.

The company said it reached that conclusion after evaluating the state of nuclear fission based on the concentration of xenon detected and temperatures and pressure levels at the No. 2 reactor of the crippled plant.

Instead of being caused by self-sustaining nuclear fission, the xenon was likely generated by spontaneous fission of curium and other radioactive substances derived from the reactor fuel, TEPCO said. Spontaneous fission takes place in suspended nuclear reactors under normal conditions.

The company also said the xenon discovery will not affect its schedule of having all of the damaged reactors reach a stable “cold shutdown” state by the end of this year.


2 Responses to “TEPCO: 8 Months out, Reactor may have reached Criticality again”

  1. BlueRock Says:

    Elsewhere: Belgium and Mexico latest to announce end of the nuclear dream. And:

    * Areva suspends uranium mine project. French nuclear giant Areva says it is suspending a mining project in the Central African Republic for “a year or two” because of a drop in uranium prices since the Fukushima disaster. http://news.smh.com.au/breaking-news-business/areva-suspends-uranium-mine-project-20111103-1mw4h.html


    * French Solar PV Installations Now Rival Those in California. http://www.renewableenergyworld.com/rea/news/article/2011/11/french-solar-pv-installations-now-rival-those-in-california?cmpid=rss

    Death by a thousand cuts….

  2. mrsircharles Says:

    Thanks for that article, Peter, and for guiding us to that extraordinary web page => Fukushima no. 1 nuclear power plant

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