morning update

via cnn--Tests showed that levels of radioactive iodine in seawater just offshore
of the embattled Fukushima Daiichi nuclear plant are more than 1,250
times higher than normal, Japan's nuclear and industrial safety agency
said Saturday.Its potential effect on Japan's fishing industry -- even if consumers
stay away, for simple fear of contamination -- remains a major concern.
So, too, is the fact that authorities have yet to pinpoint the exact
source of the radiation, and thus to determine if it's stopped.

via nature -- Core of reactor 3 possibly breached. Japan's Nuclear and Industrial Safety Agency today said this was highly likely, NHK TV reported,
and that the highly-contaminated water at the plant seems likely to
have come from the core, not spent fuel ponds. Reactor 3 is where three
workers were injured yesterday by exposure to high levels of radiation,
when water overtopped their boots while wading through floodwater - now
known to be highly contaminated - in a basement floor.

Reactor 3 also is the only reactor at the plant to use plutonium as
part of a mixed oxide fuel with uranium, increasing the hazards of
leaked radiation. The mix of radionuclides found in the water and published today
by TEPCO, the plant operator, are novel compared to other escapes over
the past two weeks (see image). Fission products so far detected in
escaped radiation have been volatiles such as Cs-137 and I-131 released
during ventings to reduce reactor pressure. The new mix contains
elements that are not volatile, and so must have leaked from broken fuel
rods into the water.

Highly contaminated water pool found today at reactor 1, and pools of contaminated water also found at reactors 2 and 4, Kyodo news reports.
The water in underground parts of the buildings is thought to have come
from either the cores or spent fuel pools – reactor 4 contained no fuel
at the time of the accident, and its fuel rods were being stored in the
pools.

Evacuation zones around plant extended. People
living in the 10km radius beyond the current 20 km evacuation zone, were
today encouraged by the government to leave voluntarily.

TEPCO, the plant operator, is starting to switch from
seawater to freshwater to cool the reactors as salt crusts on the rods
were hampering flow. The New York Times had an interesting piece earlier this week on the risk of salt crusts resulting in fuel melt.

Japan's science ministry has started posting data from radiation monitoring of coastal waters.
The first samples from 30 km off-shore had surface seawater
concentrations of 24.9 to 76.8 Bq/l of I-131, and 11.2 to 24.1 Bq/l of
Cs-137. For a discussion of what these units mean, see here, and here.

A dilute radioactive cloud has already crossed much of the Northern Hemisphere.
Austria's weather service, the Central Institute for Meteorology and
Geodynamics in Vienna, reported today that 24 radionuclide detecting
stations worldwide run by the Comprehensive Nuclear-Test-Ban Treaty
Organization are now detecting Fukushima fallout as far away as Western
Europe. See report in English here, and the maps here.

Since the Fukushima accident, the CTBTO network, designed to pick up
nuclear tests, has also proven itself to be the key (and indeed unique)
system for detecting worldwide fallout from nuclear accidents, as it emphasized tonight. For my recent article on its role here see "Radiation data from Japanese disaster starts to filter out." Meanwhile, the German Federal Office for Radiation Protection has followed Austria's lead in making its findings from the CTBTO's classified data public.

another explanation of sieverts from nature.