TEPCO estimates extent of contamination from unit 2 trench leak

21 April 2011

The amount of radiation leaked out of a trench near Fukushima Daiichi unit 2 that was found to have a leak on 2 April, and which was successfully plugged on 6 April, has been estimated by utility TEPCO.

Reactor-by-reactor, system-by-system summary from JAIF as of 21 April
Reactor-by-reactor, system-by-system summary from JAIF as of 21 April; yellow indicates abnormal/unstable; red means damaged/nonfunctional

It concluded that a total of 520 cu m of water leaked into the breakwater, with an estimated total of 4.7x10^15 Bq, from 1-6 April. Utility engineers estimated the volume based on the geometry taken by the angle of water spouting from the crack (1.17x10^-3 m3/sec).

Leaking trench at Fukushima Daiichi
Leaking trench at Fukushima Daiichi

Only on 21 April has TEPCO stopped grout injection operations. In addition to installation of steel plates in the screen rooms of units 1-4, silt fences in front of the screen rooms of units 1-4, sandbags around the south pier of the power station, and 10 sandbags with radiation absorber zeolite in front of the screen rooms, TEPCO is planning to install sheet piles in front of the south pier to wall off the water.

In other news, TEPCO has released a report about its radionuclide detection analysis. On 1 April, Japanese regulator the Nuclear and Industrial Safety Agency (NISA) verbally warned the company about its measurement of some relatively short-lived radionuclides after the Tellurium-129 results of an analysis of water taken from the trench and groundwater sample was found to be suspect. As a result, the company stopped publishing results of these surveys except for Iodine-131 and Caesium-134 and Caesium-137, and reanalysed the data by consulting third-party experts, checked with the equipment manufacturer and utility peers.

TEPCO reports one cause of the problem with the results was that a Germanium detector did not pick up Tellurium-129 because its parent, Tellurium-129m, was not registered in the library.

Now, TEPCO has published a reassessment of late March analysis of water samples in a new set of documents; none of the revised analyses resulted in higher density readings. Of the four reactor turbine building basements sampled, the unit 2 basement was generally the most contaminated, with I-131 levels of 1.3x10^7 (see attached file). Reevaluations of airborne samples from the site and seawater samples are still to be published.

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FilesReactor-by-reactor, system-by-system summary from JAIF as of 26 April
Fukushima-Daiichi parameters 20 April as of 25 April

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