Fukushima I from 1975 (Wikimedia Commons)via houseoffoust.com / October 31, 2011

[Note: A very rough translation into English of the IRSN French study on Fukushima ocean discharges. The original document in French can be found here.]

Summary of knowledge on the impact on the marine environment of radioactive discharges from nuclear site rugged Fukushima Dai-ichi

October 26, 2011

This information note presents and discusses the latest information collected by IRSN, since the previous briefing note of 11 July on the same subject.

A strong radioactive contamination of the marine environment occurred after the accident in the plant Nuclear Fukushima Dai-ichi March 11, 2011. It had the main source direct discharge contaminated water from the plant, which lasted until about April 8, and to a lesser extent, impact in the ocean part of radionuclides discharged into the atmosphere between 12 and 22 March. A immediate vicinity of the plant, the concentrations in seawater reached the end of March and early April up to several tens of thousands of becquerels per liter (Bq / L) for cesium-134 and 137 and even exceeded 100 000 Bq / L for iodine-131. Iodine 131 has declined rapidly because of its half-life short (8 days) and the measurement results came under the detection limit in late May. The concentrations of cesium-137 and 134 began to decline in this area from April 11 and since mid-July, are placed below the detection limits (5 Bq / L) measurement techniques used for monitoring.

In interpreting the results of measurement of cesium-137 in seawater, IRSN has updated its estimate of the total amount of cesium-137 released directly into the sea from 21 March until mid-July. The value thus obtained was 27.1015 Bq, the majority (82%) was rejected by 8 April. The radioactive release at sea
is the largest one-time contribution of artificial radionuclides to the marine environment ever observed. However, the location of the site of Fukushima has a dispersion of radionuclides exceptional, with one of the most important currents of the world that far to the contaminated water wide in the Pacific Ocean. Thus, the measurement results obtained in the seawater and coastal sediments suggest that the consequences of the accident, in terms of radiation protection, to become weak
pelagic species from the fall of 2011 (low concentrations in seawater and storage sediment limited).

However, significant pollution of sea water on the coast near the central hilly may persist over time because of continuous intake of radioactive substances transported to the sea ​​by runoff of surface waters of contaminated soils. Also, some coastal areas, not yet identified, could show the conditions of dilution or sedimentation less favorable than those observed so far. Finally, the possible presence of other radionuclides persistent, as strontium 90, plutonium, has not been sufficiently characterized by the measurements. The measurement results show the persistence of recent contamination of marine species (fish mostly) caught off the coast of Fukushima Prefecture. Benthic organisms and filter feeders and fish at the top of the food chain are, over time, the most sensitive to pollution
cesium. It is therefore justified to continue monitoring of marine species harvested in the waters coast of Fukushima.

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