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What Caused the High Cl-38 Radioactivity in the Fukushima Daiichi Reactor #1?

Published online by Cambridge University Press:  07 May 2025

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In its press release of April 20, TEPCO has retracted the Cl-38 radioactivity concentration measurement (1.6 MBq/mL) for the seawater used to cool reactor #1that it had issued on March 25, saying that it was “below minimum detectable density”. Based on this original measurement, we had determined that the value was too high to be explained without invoking the possibility of inadvertent, transient criticalities. We are pleased that TEPCO has retracted this result and has set out to improve its analysis protocol as described in the same press release. But we would appreciate further explanation of why previous results were simply retracted with inadequate categorization and explanation of the errors, as in the TEPCO press release. (The Cl-38 reading was changed on April 20th from 1.6MBq to a value “below detection limit” with the following explanation: “Identification and determination of radioactivity density were conducted based on main peaks.”) For example, the main gamma lines of Cl-38 are at 1.64 MeV and 2.16 MeV. What lines did these interfere with that required a downscaling of 6 orders of magnitude? If the count rate could not be attributed to Cl-38 what isotope had a count rate equivalent to 1.6 MBq/mL?

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Research Article
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivatives licence (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is unaltered and is properly cited. The written permission of Cambridge University Press must be obtained for commercial re-use or in order to create a derivative work.
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Copyright © The Authors 2011

References

Notes

1 Thanks go to Dr. Patricia Lewis (CNS, MIIS) and Arjun Makhijani (IEER) for carefully reviewing this memo, and for thoughtful and stimulating discussions. Dr. Lewis may be contacted at .

2 Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry, News Release, March 26, 2011.

3 Dr. J. Floor Anthoni, The Chemical Composition of Seawater (2000, 2006).

4 Press Release (Mar 26, 2011) TEPCO News, Plant Status of Fukushima Daiichi Nuclear Power Station (as of 8:00 PM Mar 26th): “At approximately 2:30 am on March 23rd, seawater was started to be injected to the nuclear reactor through the feed water system.”