Fukushima update: did nuclear chain reactions continue after shut-down? - April 01, 2011

[n2doc]

There is growing evidence that uranium and plutonium fuel at the Fukushima nuclear plant may have continued nuclear fission chain reactions long after the reactors were shut down almost three weeks ago. This worrying development may explain the continued release of some shorter-lived radioisotopes from the stricken site.

Tepco, the plant operator, said earlier this week that it had – on 13 occasions – detected beams of neutrons near the reactors. Neutrons are produced during fission of nuclear fuel, and are the key driver of the chain reaction that sustains continuous fission reactions in a reactor.

Japan Today reports that "the neutron beam was measured about 1.5 kilometers southwest of the plant’s No. 1 and 2 reactors over three days from March 13."

The neutron beam didn't pack much of a punch – if anyone got in its way, it would likely deliver a dose of just 0.01 to 0.02 microsieverts per hour. But the finding tallies with a recent analysis of other isotopes found at the plant, published in the Asia-Pacific Journal: Japan Focus

Ferenc Dalnoki-Veress, at the James Martin Center for Non-Proliferation Studies of the Monterey Institute of International Studies in California, hones in on the significance of a very short-lived radioisotope, chlorine-38, in the water in the turbine building of reactor 1.


more

http://blogs.nature.com/news/thegreatbeyond/2011/04/fukushima_update_did_nuclear_c.html