Formation mechanisms of insoluble Cs particles observed in Kanto district four days after Fukushima Daiichi NPP accident

ABSTRACT

Insoluble Cs particles (Type A) were first observed in Tsukuba city in the morning of 15 March 2011. The particles were considered to have been formed in the reactor pressure vessel of Unit 2 by evaporation/condensation based on the measured ¹³⁴Cs/¹³⁷Cs ratio and the accident progression of each Unit. However, the particles were covered by almost pure silicate glass and showed a trace of quenching. This can be explained by other mechanisms, that is, the particles were formed by the melting of glass fibers of the high efficiency particulate air filter in the Standby Gas Treatment System owing to the fire due to hydrogen detonation in Unit 3 at 11:01 on March 14 and atomization due to the explosion, followed by quenching of the molten materials. Although the particles formed in this way were mostly dispersed to the sea by wind at that time, some of them were deposited on the lower elevation of the reactor building of Unit 3, and they could have been resuspended and released into the environment, by the flow owing to the generation of a large amount of steam as result of a restart of core cooling water injection at 02:30 on March 15.