Reactor № 4 of the Fukushima Daiichi Nuclear Power Station on the northeast coast of Japan roughly 150 miles north of Tokyo was shut down for service past November, and according to most sources, e.g. this Areva report with the title "The Fukushima Daiichi Incident"dated Mar. 27, 2011, the fuel was moved into the spent fuel pool (SPF) adjacent to the reactor vessel on the service floor of the reactor building.
The strong earthquake and more than 10-foot high tsunami on the afternoon of Mar. 11 disrupted water cooling to the SFP. In the aftermath, according to the first TEPCO press release on the issue with the title "Damage to the Unit 4 Nuclear Reactor Building at Fukushima Dai-ichi Nuclear Power Station" dated Mar. 15 a loud explosion destroyed the superstructure of the reactor building on that day. Hydrogen produced by radiolysis or the burn of fuel rod cladding were suspected causes. However, video footage taken with an underwater camera submersed in the SFP of reactor № 4 released by TEPCO on May 8, 2011, showed no major damage to the rod assemblies stored in the pool.
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| Fuel rod assemblies stored in spent fuel pool of reactor #4 (courtesy TEPCO). |
Hypothetically, the hydrogen might have found its way from adjacent reactor № 3, which had exploded violently on Mar. 14 [TEPCO press release with the title "White smoke around the Fukushima Daiichi Nuclear Power Station Unit 3" dated Mar. 14, 2011]. However, the exact path the gas possibly could have taken has not been established yet. Radioactive fission products have been found at elevated concentrations in the pool water since the first sampling after the quake on Apr. 13 [TEPCO press release with the title "Results of analysis of spent fuel pool water of Fukushima Daiichi Nuclear Power Plant Unit 4 (2nd continued report)" dated May 9, 2011], and radiation dose levels at the inland side of the reactor building were measured at 100 milliSv/h (TEPCO press release with the title "Plant Status of Fukushima Daiichi Nuclear Power Station" as of 2:00 PM" dated Mar. 23). People spending 10 hours at that location would suffer from acute radiation syndrome.
Intriguingly, thermographs taken by Japanese Self-Defense Forces helicopter overflights consistently show a higher temperature over the reactor well than over the SFP. The example below was recorded on Mar. 24.
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| Thermograph of the surface of the building of reactor № 4 taken from JSDF helicopter. The seaside is at the bottom. The reactor well co-localizes with the red spot, whereas the spent fuel pool is situated under the green area to the left (courtesy JSDF). |
Taken together, the above observations suggest that nuclear fission has been proceeding at low levels in the building of reactor № 4. However, it is not yet established how and where precisely this fission occurs, and what constituted the source of the hydrogen accumulation leading to the March 15 explosion.
This TEPCO document with the title "Start of regular inspection Fukushima Daiichi Nuclear Power Station Unit 4" dated Nov. 30, 2010, details the work to be accomplished in reactor № 4 until Sep. 24, 2011. It mentions only fuel assembly replacement.
Addenda
- Andrew Gilligan and Robert Mendick report an intriguing observation by a worker at the Fukushima I nuclear power station immediately after the earthquake. According to their post with the title "Japan tsunami: Fukushima Fifty, the first interview" published online by The Telegraph on Mar. 27, 2011, “(a) ghastly boom was heard in the suppression chamber of reactor 4” The suppression chamber is the torus under the reactor pressure vessel, constituting an instrumental part of the primary containment system. It remains difficult to understand why this boom occurred, since reactor № 4 was shutdown (06/05/11).
- An instructive brief on the efforts of constructing a new closed-loop cooling water system and filtration for the spent fuel pool of reactor № 4 is shown in the video below (06/17/2011):
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