After an eventful recent few days at the Fukushima Daichii nuclear power plant, which I’ve reported on here, and here last night —see “Radioactivity spreads in Japan”, today has seen few major developments, and the situation remains very serious. The detection of radioactive hotspots outside of the evacuation zone is a growing concern, with the International Atomic Energy Agency (IAEA) tonight confirming hotspots Northwest of the plant where radioactivity is so high as to require urgent evacuation of people in the area.
Meanwhile, authorities trying to bring the reactors at the Fukushima Daichii plant under control are still battling with the same problems that since the weekend have prevented them from even beginning the critical task of reestablishing electrical supplies to the coolant systems of the plant’s reactors 1 to 3. The basements of the reactors have been flooded with radioactive water, which in reactor 2 is so radioactive – with a dose rate of 1000mSv/h – that a few hours exposure would deliver a lethal dose. The problem is that while electricity has been restored to the control rooms, the cabling that needs to be connected to get electricity to, and restart, the reactors coolant systems, is in these basements. So workers are first having to pump out and clean out the basements before work on trying to stabilize the reactors can proceed.
Efforts are also being hampered by problems on other new fronts at the plant. As I reported yesterday in Nature, radioactive water has been found in trenches, outside of the reactor buildings, and less than 70 metres from the sea shore, raising the spectre of serious contamination of the sea and groundwater in the area. The dose rates at the trench adjacent to reactor 2 is also at a potentially lethal 1000mSv/h. The trenches in question are not what most people might think of as trenches; they are several metres wide and up to 15m deep, and they are filled to the brim. Workers are trying to pump the contaminated water from the trenches, and from the basement, into holding tanks, but may soon run out of space, and so the idea is being floated of digging a new containment vessel. To add to their worries, very low levels of plutonium have been found in the soil.
In terms of the impact of fallout from the plant, the US Department of Energy last night released updated data from its aerial monitoring of the zone. Its maps show a continuing decline of ground radioactivity since its first survey on 17–19 March due to radioactive decay of short-lived isotopes such as I-131 — they also suggest that there has not been a significant additional dump of radioisotopes since, despite continued emissions from the plant (see maps below). Its surveys are increasingly flying further outside of the 20km compulsory evacuation zone around the plant. It’s picking up levels far higher than background more than 40 km from the plant, but assessed that these levels did not warrant evacuation, but did call for greater monitoring of agriculture and food in these regions.
IAEA tonight, however, reported results from new soil monitoring in localized hotspots outside of the zone which exceeded its evacuation criteria with iodine-131 levels as high as 25 megaBq/m2, and cesium-137 levels as high as 3.7 megaBq/m2 — these hotspot readings are very high, as evacuation starts being considered at around a few hundred thousand Bq/m2 (and even a few thousand Bqs/m2 exceeds recommended contamination levels for growing green leafy vegetables). This emphasizes the need for extensive local soil sampling in addition to getting the big picture of the overall zone from aerial monitoring.
The high Bq/m2 readings above are obviously well above levels requiring evacuation. Formally, limits for evacuation are based on dose rates — measured in Sieverts — and not on radioactivity units of Bq/m2 or Bq/kg, though obviously dose is related to both the latter. IAEA guidelines recommend temporary evacuation from zones where people risk a dose of 30mSv/month, and permanent resettlement if the lifetime dose people would be exposed to is 1Sv. For more on the relation between dose and health effects see this table from the UK Health Protection Agency).
Hotspots result from the patchy distribution of fallout, which reflects the role of wind patterns and rainfall in washing out radioisotopes to the ground, and similar hotspots were observed after the Chernobyl accident. IAEA confirmed that the same is being observed at Fukushima:
“Based on measurements of I-131 and Cs-137 in soil, sampled from 18 to 26 March in 9 municipalities at distances of 25 to 58 km from the Fukushima Nuclear Power Plant, the total deposition of iodine-131 and cesium-137 has been calculated. The results indicate a pronounced spatial variability of the total deposition of iodine-131 and cesium-137. The average total deposition determined at these locations for iodine-131 range from 0.2 to 25 Megabecquerel per square metre and for cesium-137 from 0.02-3.7 Megabecquerel per square metre.”
On the coastal front, increased radioactivity near sea discharge points from the reactors was reported earlier this week by IAEA, citing Japanese data showing 74,000 Bq/l of I-131 and 12,000 Bq/l of Cs-134. Yesterday it reported,however, that levels had fallen to 11,000 and 1,900 respectively. Recommended maximum coastal discharges from nuclear power plants are typically lower than 4,000 Bq/l. Press reports today suggested that levels had increased again, though no concentrations in Bequerels were given. As IAEA rightly points out, however, reported levels could fluctuate considerably depending on discharge rates and the prevailing currents, so short-term changes in marine data should perhaps be taken with a pinch of salt.
For full coverage of the Fukushima disaster, go to Nature’s news special.
US DOE maps of aerial monitoring data
For an explanation of the units used in the legend, and how these dose rates translate into health effects see my article from last night.
And from a wider zone, 27-28 March