ENEA Bologna Research Centre, May 23, 2014:
This Report presents the results of the application of the fast-running US-NRC direct code RASCAL 4.2 to the estimation of the Fukushima Source Term. [...] it is plausible that the ventings that TEPCO announced during the accident as being conducted from the wetwell were, as a matter of fact and because of the degraded conditions of the plants, conducted actually from the drywell. […] wetwell properties imply releases which can be several oder of magnitudes lower than those from the drywell […] it can clearly be seen that the most probable path is the combination of Drywell+Direct option […] the true venting path, i.e. from Drywell instead of from Wetwell, is an extremely important issue. […] in several instances when TEPCO tried to operate venting, in order to release pressure outside the building through the stack, it proved impossible […] there are many indications that probably the radioactive material escaped from the drywell; this may have occurred without TEPCO’s immediate knowledge and because of several factors; for example: structural damages to the pipings connecting drywell to torus room (vent piping bellows), due either to the earthquake, and/or to the too violent pressure and temperature increase in the D/W; leakages through the top head manhole, the top head flange, the piping penetrations, the electrical wiring penetrations, the personal airlocks, the S/C manholes, the machine hatches, etc. [...] The value of 1%/h was chosen by ENEA because of the possible highly damaged conditions of the Fukushima NPPs due to the BDB [Beyond Design Basis] earthquake.
> Table 7. Cumulative Source Term (Bq) TEPCO MELCOR
Pu-241 Total = 1.2E+12 (1,200,000,000,000 Bq)
> Table 7. Cumulative Source Term (Bq) ENEA RASCAL 4.2
Pu-241 Unit 1 = 6.52E+13 (65,200,000,000,000 Bq)
Pu-241 Unit 2 = 1.86E+14 (186,000,000,000,000 Bq)
Pu-241 Unit 3 = 2.67E+13 (26,700,000,000,000 Bq)
Pu-241 Total = 2.78E+14 (278,000,000,000,000 Bq)
Source: Evaluation of the Fukushima Accident Source Term through the fast running code RASCAL 4.2 (pdf) http://openarchive.enea.it/bitstream/handle/10840/4840/UTFISSM-P000-017_rev.1.pdf?sequence=4
Environmental Science & Technology (American Chemical Society), July 11, 2014: [S]tandard alpha spectrometry techniques [...] are not able [...] to measure 241Pu.
Source: Environmental Science & Technology (American Chemical Society) http://pubs.acs.org/doi/abs/10.1021/es501890n
Boreal Environment Research , Feb. 28, 2014:
The 241Pu isotope was introduced into the environment from [...] accidents that released nuclear reactor fuel, such as [...] the 2011 Fukushima catastrophe. As compared with other Pu isotopes in the environment that are alpha-emitting and long-lived, 241Pu is a short-lived isotope with a half-life of 14.35 years [...] 241Pu decays to the alpha emitter 241Am, that has a much longer half-life (432.2 years) and is highly radiotoxic when it is incorporated into either the human or animal body. [...] The 241Pu isotope has been studied less extensively than the α-emitting Pu isotopes for several reasons. Activity concentration of 241Pu cannot be determined from the same alpha spectrum as the Pu isotopes 238, 239, and 240, and extra effort is needed in order to analyze 241Pu concentration of a sample. Actually, 241Pu emits alpha particles, but they have so low probability (0.002%) that 241Pu cannot be measured directly by α-spectrometry [...]
Source: Boreal Environment Research (pdf) http://www.borenv.net/BER/pdfs/ber19/ber19-051.pdf