As I look back over the news coverage and reports I have read over past 11 months—and I admit I do benefit from hindsight here—there are still many things I do not understand about the Fukushima Daiichi accident and response. Below I have put the top four things I?would like to know. By Will Dalrymple, editor.
Tsunami preparation. I appreciate that immediately after the station was hit by the earthquake, all attention would have focused on post-earthquake recovery, especially after the many issues that electrical utility TEPCO’s Kashiwazaki-Kariwa station faced following the Chuetsu earthquake of 2004. Apparently, the unusual behaviour of waves at the seashore indicate an impending tsunami. I remember that an English schoolgirl had learned this in a geography class a few weeks before her parents took her to Southeast Asia on vacation over Christmas 2004; she saw some weird waves and warned her parents to get to high ground (www.tinyurl.com/6bwtf). At Fukushima, did no-one think of the risk of a tsunami, and watch what was happening to the sea? Were there no functioning regional or national tsunami warning systems that could have alerted the station to for example begin emergency cooling operations, one early countermeasure that Béla Lipták has suggested (NEI January 2012, pp31-3)?
Delay of water injection. The lack of reactor cooling is at the heart of the accident, and ultimately accounted for explosions at unit 1 and unit 3. At unit 1, the failure of a vital low-pressure makeup reactor cooling system, the isolation condenser, made the situation worse. The system is essentially a two-loop heat exchanger that circulates steam/water from the reactor and/or primary containment vessel, and cools it in low-pressure water that absorbs heat through evaporation. Unit 1 had two systems, A and B. In a November report, TEPCO said that IC A may have failed because of a stuck valve, or because high hydrogen concentrations blocked flow. IC B did not fail, because it had been turned off. TEPCO’s report said that operators switched off IC B after the earthquake but before the tsunami. Once site power was lost post-tsunami, the motor-operated valves could not be reopened. But why did the operators switch off IC B in the first place? A later investigation found that the cooling tank of IC B was mostly (80%) full. Had it not been switched it off, it might have prevented the explosion of unit 1.
Mysterious noise. Unit 2 was the only unit of the four whose top did not blow, although at some stage its blow-out panel was opened. However, TEPCO staff reported hearing a loud noise from the torus, the water-filled, donut-shaped tunnel that holds water for emergency cooling, at about the same time as the explosion at unit 4 on 15 March. Reports at the time suggested that it was the sound of the failure of the torus, because a pressure indicator fell to atmospheric pressure. But that reading was later found to be incorrect, according to the Institute of Nuclear Power Operation’s report 11-005. It is now clear that the unit 2 primary containment vessel is leaking. But what caused the loud noise?
Public acceptance. How will TEPCO be able to restart Fukushima Daiini, and Daiichi 5&6? TEPCO officially shut down Fukushima Daiichi units 1-4 in May 2011. This pragmatic decision would have been made easier by the injection of corrosive salt water into the reactors and spent fuel pools into those units. But I suspect that it eventually intends to restart the others. Fukushima Daiichi 5&6 are slightly more modern reactors than units 1-4, set a few hundred meters apart. They were also flooded, but not so dramatically, and they did not lose all power, so control was reestablished more quickly and easily. Down the coast, Fukushima Daiini was also flooded (and quite dramatically), but a station blackout was avoided, and the reactor cores were kept covered, so the scale of the accident was much less. Restart requires both national regulator and local government approval. The question is, politically speaking, will TEPCO ever be able to operate any reactor again with the word ‘Fukushima’ in its name?
This article was published in the March 2012 issue of Nuclear Engineering International magazine.
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