There is a controversy currently raging in the Fuku community about spent fuel pool #4. Tepco and the Japanese government claim to have begun removing fuel rods from the Unit 4 SFP, and have released photos of a nice clean spiffy pool, enclosed in a recently constructed building exterior, that has apparently suffered little damage. Warnings have been issued by Harvey Wasserman, Arnie Gundersen and others about the dangers involved in the removal of these rods, especially if they have suffered damage. Dire scenarios about the fate of the northern hemisphere have been talked about.
The other faction claims that the FOIA documents and news reports from the time talk about a fire in SFP#4 (and #3) starting March 15, 2011 or so. This fire supposedly released all the radiation from these pools at the time, and we are now already in the dire scenario that the other side was claiming might happen.
There is a way to settle this controversy. It is by looking at the isotopic ratios of cesium isotopes that were measured in the soil and air at the time, that is, the ratio or cesium-136 to cesium-137 in these samples.
Caesium-136 has a half-life of 13.16 days. It is produced both directly (at a very small yield because 136Xe is stable) as a fission product and via neutron capture from long-lived Cs-135 (neutron capture cross section 8.702 barns), which is a common fission product. Caesium-136 is not produced via beta decay of other fission product nuclides of mass 136 since beta decay stops at stable 136Xe. It is also not produced by nuclear weapons because 135Cs is created by beta decay of original fission products only long after the nuclear explosion is over. (link)
The ORIGEN computer simulation code was used to estimate the releases of radionuclides from the 3 reactors and 4 spent fuel pools. The reactors release about 28% as much Cs-136 as Cs-137 (for a ratio of 0.28 on day zero). But the spent fuel pools release virtually no Cs-136 at all.
Table 1 shows amounts of isotopes measured at the Daiichi plant, as measured by Tepco, and available at its website. The amounts of Cs-136 were corrected for decay, and it was assumed that the radiation releases all occurred on March 11. This is a conservative assumption. Units 2 and 3 didn’t explode till later. So this is an overestimate of the amount of Cs-136 that was on hand.
Cs-136/-137 ratios Cs-136/-137 ratios 0.420 0.202 0.311 0.314 0.176 0.229 0.322 0.198 0.238 0.272 0.321 0.326 average 0.277
This agrees with the ratio of 0.28 generated by the ORIGEN code for day zero. According to Tepco, all the cesium that was released came from the reactors, and none came from the spent fuel pools.
But wait. Researchers from the University of Tokyo also performed measurements of Cs-136 and Cs-137 in locations to the northwest of the plant.
Two locations close to the NPP in the northwest direction were found to be depleted in short-lived (136)Cs. This likely suggested the presence of distinct sources with different (136)Cs/(137)Cs isotopic ratios, although their details were unknown at present.
So some areas had little or no Cs-136 found, even though Cs-137 was there. This could only come from the spent fuel pools. Another article shows the decay-corrected ratios at four different locations (study 1):
Location Cs-136/-137 ratios Soil samples in near-zone (<80 km) 0.22 Air samples in Japan (80–2,000 km) 0.19 Air samples in Pacific Ocean and US (2,000–12,000 km) 0.22 Air samples in EU (>12,000 km) 0.20 average 0.2075
These figures are significantly below the level of 0.28 which would have resulted if all the cesium came from the reactors. Japan had lower ratios than those in the US and Europe, which means that distant regions received a higher proportion of cesium from the reactors than Japan did. This is not surprising, since the Unit 3 reactor blew sky-high, and its contents entered into the jet stream.
Researchers at RIKEN Wako Institute in Wako, Japan (in Saitama prefecture) also conducted analysis of airborne cesium radionuclides. This attachment contains non-decay corrected measurements from Wako. I performed the decay correction back to March 11, for the non-italicized figures (study 2). The italicized ones were deemed to be unreliable.
0.17 0.11 0.18 0.12 0.19 0.12 0.16 0.11 0.14 0.14 average 0.144
These ratios are much lower than the previous, indicating that this area received a higher proportion of cesium which was released from the pools.
Finally, in this comprehensive Fukushima report, Tables 2.3 to 2.5 estimate the Cs-136/-137 ratio in the inventory of the three reactors separately:
Unit 1 0.27 Unit 2 0.32 Unit 3 0.34 average 0.31
This gives a better idea of the actual amount of Cs-136 in the reactors, which is higher on average than the figures from the ORIGEN code. Study 1 has the ratio of observed ratios to inventory ratio at 0.2075/0.31 = 66.9%, or about two-thirds of the cesium coming from the reactors, and one-third coming from the spent fuel pools. Study 2 has the ratio at 0.144/0.31 = 46.5%, or around 54% coming from the pools.
All the ratios in study 2 were below the expected value of 0.31, the ratio of reactor-only releases. A simple binomial test indicated a two-tailed P<.002, rejecting the hypothesis that all the cesium came from the reactors. All Cs-136 measurements besides Tepco’s indicate a significant cesium release from the spent fuel pools. Since the assumptions were conservative, a range of 33%-60% of the cesium was observed to come from the pools, depending on the location of measurement.
Of course, this does not reveal WHICH spent fuel pools were involved. The FOIA documents and photographs indicate fires in SFP#3 and #4. The other two pools may have been involved, also, but it is less clear. Since there are 4 (not 3) reactors involved, and 2 pools, we can think that maybe a similar proportion of the pools’ corium were emitted as from the reactors… maybe a little higher percentage. Since there are still underground coriums from the reactors still releasing radiation, we can say the same thing about residue from the pools. So it is unlikely that ALL the radiation was released from the pools. There is still goop left over, which is extremely radioactive… but considering the amount of cesium released from them, it is hard to imagine that these rods are still intact.