TEPCO Data Shows Ongoing Criticalities Inside Leaking Fukushima Daiichi Unit 2
- Press Release (Apr 28,2011) Detection of Radioactive Materials from Subsurface Water near the Turbine Building
- www.tepco.co.jp
Summary
Data released on April 28, 2011 by TEPCO is now unequivocal in showing ongoing criticalities at Unit 2, with a peak on April 13. TEPCO graphs of radioactivity-versus-time in water under each of the six reactors show an ongoing nuclear chain reaction creating high levels of “fresh” I-131 in Unit 2, the same reactor pressure vessel (RPV) with a leak path to reactor floor, aux building, and outdoor trenches, that is uncontrollably leaking high levels of I-131, Cs-134, Cs-137 into the Pacific Ocean.
Analysis
When a nuclear reactor goes “critical” it means that the fissioning of U-235 or Pu-239 becomes a self-sustaining process, called a chain reaction, where fissile material hit by a neutron then spilts (or fissions) into two atoms with atomic numbers between ~90 and ~140 while “throwing off” a few neutrons which then hit other fissile atoms and so the reaction then continues until it’s stopped, usually by dropping the control rods, or reactor scram.
During normal reactor operation, short-lived nuclides like I-131 (8 day) that pose high radiological hazard decay as quickly as they are created, because its halflife is much shorter than the refueling cycle, so I-131 reaches an equilibrium value quickly. In contrast, because the cesiums decay slower than they are created, reactor inventories of Cs-134 (2 year) and Cs-137 (30 year) gradually rise during the cycle, reaching a maximum at end of cycle.
When Units 1-3 were all scrammed on March 11, 2011 from earthquake-caused station blackout, the chain reaction of splitting fissile U-235 and Pu-239 into numerous fission products came to an immediate stop. Reactor scram means that neutron-absorbing control rods are dropped into the reactor core to absorb enough neutrons that the chain reaction ceases. Because I-131 has no long-lived “parent” to “feed it” by parent decay, the levels of I-131 in scrammed reactors with intact geometry will decrease exponentially with an 8-day halflife, meaning that after 5 halflives (40 days) the I-131 levels are only 3% of what they were at scram.
But instead of seeing that expected decrease in I-131 levels relative to Cs-134 and Cs-137 in the regular TEPCO press releases, I-131 was seen to be increasing, instead of decreasing as the physics said it should.
Until the April 28 press release with accompanying graphs and table, I discerned that something strange was happening with the elevated I-131 levels, but until this latest news, it was impossible to know where, exactly, was the source of the high I-131 levels.
The answer is clear if you look at the graphs of groundwater radioactivity measurements from all six reactors. “Outlier” Unit 2 has I-131 levels roughly 20 times its levels of Cs-134/137. The only possible source of I-131 would be “pockets” of molten core in the Unit 2 RPV settled in such a way that the boron in the injected water is insufficient to stop the localized criticalities.
April 28, 2011
Source: Gerson Lehrman Group