Excessive cesium-134 at Daiini. Evidence of criticality there April 2012 or after.

This table was recently released by Tepco. The first row shows radioactive cesium measurements made of fallout at Fukushima Daiichi on December 25, 2013. Note that the amount of cesium-134 is about 39% that of cesium-137. At 3/11 the ratio of Cs-134 to Cs-137 was around 100%. Cs-134 has a half-life of 2 years, and has now decayed to around 35-40% of Cs-137.

The second row shows radioactive cesium measurements made of fallout at Fukushima Daiini on December 26, 2013. The amount of Cs-134 is about 57% of Cs-137.

From Wikipedia,

Caesium-134 has a half-life of 2.0652 years. It is produced both directly (at a very small yield because 134Xe is stable) as a fission product and via neutron capture from nonradioactive Cs-133 (neutron capture cross section 29 barns), which is a common fission product. Caesium 134 is not produced via beta decay of other fission product nuclides of mass 134 since beta decay stops at stable 134Xe. It is also not produced by nuclear weapons because 133Cs is created by beta decay of original fission products only long after the nuclear explosion is over.

So cesium-134 is mainly an activation product, which means it needs neutrons flying around and interacting with nonradioactive cesium-133 to be produced in significant quantities.

I used a radioactive decay application to figure how far back in time Cs-134 would have to have been produced to get the current ratio of 57% of Cs-137. The Cs-134 would have been produced around April 1, 2012.

This means there has been a criticality at Daiini sometime after April 1, 2012. How long after this date is unknown.