Examination of the Reason Why Hydrogen Explosion did not Occur at Fukushima Daiichi NPP Unit-2

Abstract

During the Fukushima Daiichi NPP accident, hydrogen explosions occurred in the reactor buildings of Units-1, 3, and 4, but didn’t happen in the reactor building of Unit-2. One potential reason why Unit-2 avoided the explosion can be attributed to the mitigation of hydrogen concentration inside the reactor building, which occurred due to the accidental opening of the BOP on the operating floor. This hypothesis was quantitatively examined by assessing the flammability of the environment inside the reactor building using GOTHIC code. The hydrogen was assumed to enter the reactor building through the reverse flow of the vent gas from Unit-1 and leakage from the drywell of Unit-2. The conditions of the reverse flow and leakage were evaluated according to the predicted accident scenario based on the onsite measurement data. Gas composition on each floor was assessed by simulating its distribution throughout the entire reactor building via staircases and hatches. This information was subsequently employed to evaluate the gas's flammability using H2-Air-Steam ternary diagram. The results showed that gas exchange through the BOP opening effectively reduced the hydrogen concentration in the reactor building and only the 5th floor environment became flammable for a short period. On the other hand, assuming the BOP were closed, the entire reactor building would become flammable for an extended duration. It is suggested that the main reason for the absence of a hydrogen explosion in the reactor building of Unit-2 might be the opening of the BOP, which corroborated the conventional estimate.