New highly radioactive particles derived from Fukushima Daiichi Reactor Unit 1: Properties and environmental impacts

摘要

A contaminated zone elongated toward Futaba Town,north-northwest of the Fukushima Daiichi Nuclear Power Plant (FDNPP),contains highly radioactive particles released from reactor Unit 1.There are uncertainties associated with the physio-chemical properties and environmental impacts of these particles.In this study,31 radioactive particles were isolated from surface soils collected 3.9 km north-northwest of the FDNPP.Two of these particles have the highest particle-associated~(134+137)Cs activity ever reported for Fukushima (6.1 ×10~5 and 2.5 × 10~6 Bq per particle after decay-correction to March 2011).The new,highly-radioactive particle labeled FTB1 is an aggregate of flaky silicate nanoparticles with an amorphous structure containing ～0.8 wt% Cs,occasionally associated with SiO_2 and TiO_2 inclusions.FTB1 likely originates from the reactor building,which was damaged by a H_2 explosion,after adsorbing volatilized Cs.The~(134+137)Cs activity in the other highly radioactive particle labeled FTB26 exceeded 10~6 Bq.FTB26 has a glassy carbon core and a surface that is embedded with numerous micro-particles: Pb-Sn alloy,fibrous Al-silicate,Ca-carbonate or hydroxide,and quartz.The isotopic signatures of the micro-particles indicate neutron capture by B,Cs volatilization,and adsorption of natural Ba.The composition of the micro-particles on FTB26 reflects the composition of airborne particles at the moment of the H_2 explosion.Owing to their large size,the health effects of the highly radioactive particles are likely limited to external radiation during static contact with skin; the highly radioactive particles are thus expected to have negligible health impacts for humans.By investigating the mobility of the highly radioactive particles,we can better understand how the radiation dose transfers through environments imparted by Unit 1.The highly radioactive particles also provide insights into the atmospheric conditions at the time of the Unit 1 explosion and the physio-chemical phenomena that occurred during reactor meltdown.