ONE DAY AT A TIME...
As intimated almost a week ago (see SAT Briefing #7 of March the 23rd), the chain of
events at Fukushima seems to consolidate on a daily basis the textbook definition of a nuclear
accident 'Late Phase':
The Late Phase begins after the source of a major release is under control. However, for some
accidents (e.g. a reactor core damage accident) there may be ongoing small releases even after the
threat of a major release has passed. These small releases should not prevent assessments
concerning the Late Phase. (U.S. NRC. International RTM-95 Response Technical Manual. May, 1995,
Washington, D.C. p. E-2.)
1. While, as again indicated, the theatre of action has, to a large extent, moved off-site, the
situation at the power plant itself 'remains very serious' (IAEA Briefing on Fukushima Nuclear Accident
(29 March 2011, 16:30 UTC). This persistent characterization rests mainly on two facts: (a) there is no
certainty as yet about the cause, source, contents and radiological threat posed by the 'grayish' smoke
erratically emanating from plant Units 2 and 3; and (b) new abnormal occurences, some of which fed by
stress, fatigue and sheer human error pop up to further compound the situation.
As we wait for clarifications about (a), we have turned to aspect (b), briefly addressing two recent
concerns: the puddles, ditches and ponds of contminated water and the unfortunate, severe contamination
of a small group of workers that did their drill in a flooded space of Unit 3; and the occurence of
Plutonium in the environmental measurements (see this notes' Appendix).
2. The key aspect targeted about the contaminated water was the chance to link the dose reported
to have been acquired by the workers - between 2 and 6 Sv, to water origin - a normal coolant, or a fuel
gap release-affected coolant; or a core melt-affected coolant - this (hopefully) feeding an opinion on how
serious the respective reactor core was being damaged. When such a determination will be made and
consolidated by appropriate PSA-level-1-oriented and competent people, the entire battery of analyses of
the Fukushima accident will be reset for a more accurate appraisal.
Our current estimations based on upper bounds for the nuclides that would contaminate the
coolant in a BWR-type reactor would place the diagnostic of the 'puddle water' that have irradiated the
workers between less-than-1Sv/h for a normal coolant and more than 1.0e5 Sv/h for a severe affectation
of the fuel in the core, which opens the following fan of possibilities:
a) the water does not originate directly from the core if, as generally accepted by now, the core
has undergone some degree of melting; a possible source may then be, inter alia, a punched wet
well, where a degree of dilution inevitably occured;
b) the core damage was not as severe as originally believed;
c) the 'puddle water' is a mixture of coolant of a certain, original degree of contamination, with
the additional, injected cooling water, and/or sprayed cooling (sea/fresh) water - which of course
would scale down the relevance of, or at least complicate the backward analysis geared to
inferring the core damage degree.
3. The occurence, in the environmental measurements, of Plutonium isotopes of allegedly
Fukushima origin has stirred up new concerns about (i) the degree of damage of the reactor cores - the
presence of actinides being loosely associated with higher damage levels; and (ii) the long-term health-
and environmental impact of the accident - given the long halflives of the respective nuclides. Without
engaging in premature speculations, the briefing offers a SAT-generated standard list of features of more
than 100 fission products expected to be considered in the assessment of nuclear accidents, thus allowing
a comparative reading of several values of relevance as far as effects, via doses. Of special interest are
the entries #4; 7 through 20; 30 through 38; 42 through 44.
The figures meaning is unequivocal: if indeed confirmed as additional to the legacy of the atomic
weapon tests of the '50s, Plutonium adds to the local long-term issues entailed by the Fukushima
mishap.
__________
Appendix
International Atomic Energy Agency
(IAEA)IAEA Home
Fukushima Nuclear Accident Update Log
- excerpts by this briefing authors
Updates of 29 March 2011:
http://www.iaea.org/newscenter/news/tsunamiupdate01.html
Staff Report
IAEA Briefing on Fukushima Nuclear Accident (29 March 2011, 16:30 UTC)
On Tuesday, 29 March 2011, the IAEA provided the following briefing on the current status of
nuclear safety in Japan:
1. Current Situation
The situation at the Fukushima Daiichi plant remains very serious.
Accumulated contaminated water was found in trenches located close to the turbine buildings of
Units 1 to 3. Dose rates at the surface of this water were 0.4 millisieverts/hour for Unit 1 and
over 1 000 millisieverts/hour for Unit 2 as of 18:30 UTC on 26 March. The Nuclear Safety
Commission of Japan suggests that higher activity in the water discovered in the Unit 2 turbine
building is supposed to be caused by the water, which has been in contact with molten fuel rods
for a time and directly released into the turbine building via some, as yet unidentified path. An
investigation is underway as to how the water accumulated in the trenches. Measurements could
not be carried out at Unit 3 because of the presence of debris.
[...]
2. Radiation Monitoring
On 28 March, deposition of iodine-131 was detected in 12 prefectures, and deposition of cesium137 in 9 prefectures. The highest values were observed in the prefecture of Fukushima with
23 000 becquerel per square metre for iodine-131 and 790 becquerel per square metre for
caesium-137. In the other prefectures where deposition of iodine-131 was reported, the range
was from 1.8 to 280 becquerel per square metre. For caesium-137, the range was from 5.5 to 52
becquerel per square metre. In the Shinjyuku district of Tokyo, the daily deposition of both
iodine-131 and cesium-137 was below 50 becquerel per square metre. No significant changes
were reported in the 45 prefectures in gamma dose rates compared to yesterday.
Five soil samples, collected at distances between 500 and 1 000 metres from the exhaust stack of
Unit 1 and 2 of the Fukushima Nuclear Power Plant on 21 and 22 March, were analysed for
plutonium-238 and for the sum of plutonium-239 and plutonium-240. (Due to analytical reasons,
the isotopes plutonium-239 and plutonium-240 cannot be measured separately). Plutonium-238
was detected in 2 of the 5 samples, while plutonium-239/240 was detected in all samples as
expected.
Concentrations reported for both, plutonium-238 and plutonium-239/240 are similar to those
deposited in Japan as a result of the testing of nuclear weapons. The ratio of the concentrations of
plutonium-238 and plutonium-239/240 in two of the samples indicate that very small amounts of
plutonium might have been released during the Fukushima accident, but this requires to be
further clarified.
[...]
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