Root cause study on hydrogen generation and explosion through radiation-induced electrolysis in the Fukushima Daiichi accident

Highlights: • Reviewed how LWRs have coped with "water radiolysis", during normal operation to severe accidents. • Concluded "water radiolysis" is not likely a route course of the hydrogen explosions at Fukushima. • Performed modeling studies based on "radiation-induced electrolysis" on Unit 1–Unit 4. • Generation of several tens of thousands cubic meters hydrogen gas is predicted before the hydrogen explosions. • Upon SBO, early safe disposal of hydrogen from RPVs is indispensable in BWRs. - Abstract: Since the scientific cause for a series of hydrogen explosions during the Fukushima accident has not been established, the author investigated his basic theory named "radiation-induced electrolysis (RIE)" by applying the estimation of the amounts of H₂ generation during the active phase of the Fukushima accident. The author's theory was originally developed by including Faraday's law of electrolysis into the basic time-dependent material balance equation of radiation-chemical species for his study on accelerated corrosion phenomena which is widely observed in aged plants. As such this theory applies to the early phase of the accident before the loss of water levels in the reactor cores, although the simulations were performed from the time of seismic reactor trip to the hydrogen explosions in this paper. Through this mechanism as much as 29,400 m³-STP of hydrogen gas is estimated to be accumulated inside the PCV just prior to the hydrogen explosion which occurred one day after the reactor trip in 1F1. With this large volume of hydrogen gas the explosion was a viable possibility upon the "venting" operation. In view of this observation, hydrogen generation from the spent fuel pools was also investigated. For the investigation of the 1F4 SFP, the pool water temperature and flow velocity due to natural circulation were changed widely to identify conditions of large hydrogen generation. During the trial calculations it was discovered that SBO induced a rapid initiation of electrolysis when the pool water temperature surpassed 40 °C with a range of low water flow velocity through the spent fuels. With a mix of different levels of radioactivity of spent fuel, a difference in the absorbed dose rate of water through γ-decay heat should have existed. This configuration induced an electrochemical potential difference between the highly radioactive region where there was spent fuel stored by evacuating the core and less radioactive fuels stored for several years. The spent fuel was stored in racks placed at the bottom of the pool where the wall was covered with a stainless steel lining. The metallic contacts enabled electric conduction between the highly radioactive fuel assemblies and the cooled spent fuel.