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Vent pipe not reaching top caused high radiation level in Fukushima reactors

  • February 28, 2021
  • , Asahi , Lead
  • JMH Translation

By Toshio Kawada, staff writer, and Eisuke Sasaki, senior staff writer


The Nuclear Regulation Agency (NRA) reported in January on a previously missed design flaw in no. 1 and no. 2 reactors at Tokyo Electric Power’s Fukushima Daiichi Nuclear Power Plant. Inside a 120-meter exhaust stack, a ventilation pipe from the reactors extended no further than the bottom of the stack instead of reaching the opening at its top.


The pipe extended from no. 1 and no. 2 reactors, which experienced meltdowns in the nuclear accident 10 years ago. It was designed to move and release radioactive steam and hydrogen gas, reducing the pressure inside containment vessels around the reactors that were boiling dry. 


The level of radiation at the bottom of the common exhaust stack between no.1 and no. 2 reactors exceeded 10 sieverts per hour right after the accident, making the area inaccessible. The unusually high level of radiation in comparison with the exhaust stack that serviced reactors no. 3 and no. 4. could be explained by the vent pipe that stopped at the bottom part of the stack, according to the NRA, which renewed efforts to shed light on unsolved questions surrounding the accident.


The NRA report concluded that because the pipe didn’t reach the top of the exhaust stack, part of radioactive materials remained inside and accumulated at the bottom of the stack.


NRA Chairman Toyoshi Fuketa warned that the design, which allows the release of hydrogen inside the exhaust stack, could have triggered a hydrogen explosion if the high concentration of hydrogen had been allowed to mix with air, destroying the stack and further complicating the situation.


Installing a vent pipe was recommended as a countermeasure against severe accidents. It is left to each utility, however, and the government was not involved in supervising design details or the installment.


Following the announcement of the report, Fuketa didn’t hide his mistrust of the company at a press conference, saying, “At the time [the reactors were built], we were already aware of the danger of hydrogen. I don’t understand how [TEPCO] could come up with such a design, and I must wonder how serious it had been [about safety],” adding, “If [TEPCO] wants to recover trust, it should disclose what it actually did in the past after reassuring it had everything under control.”


The reason for the pipe not extending to the top is unclear. The NRA will inspect other reactors of the same type. (Abridged)

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