Utilization of 134Cs/137Cs in the environment to identify the reactor units that caused atmospheric releases during the Fukushima Daiichi accident

Abstract

The Fukushima Daiichi nuclear power reactor units that generated large amounts of airborne discharges during the period of March 12–21, 2011 were identified individually by analyzing the combination of measured 134Cs/137Cs depositions on ground surfaces and atmospheric transport and deposition simulations. Because the values of 134Cs/137Cs are different in reactor units owing to fuel burnup differences, the 134Cs/137Cs ratio measured in the environment was used to determine which reactor unit ultimately contaminated a specific area. Atmospheric dispersion model simulations were used for predicting specific areas contaminated by each dominant release. Finally, by comparing the results from both sources, the specific reactor units that yielded the most dominant atmospheric release quantities could be determined. The major source reactor units were Unit 1 in the afternoon of March 12, 2011, Unit 2 during the period from the late night of March 14 to the morning of March 15, 2011. These results corresponded to those assumed in our previous source term estimation studies. Furthermore, new findings suggested that the major source reactors from the evening of March 15, 2011 were Units 2 and 3 and that the dominant source reactor on March 20, 2011 temporally changed from Unit 3 to Unit 2.

Analysis of External Surface Irregularities on Fukushima-Derived Fallout Particles

“…Such a contrasting size and form; combined with (i) differing radiocesium ratios ( 134 Cs/ 137 Cs) determined in previous works (Chino et al, 2016), (ii) the location at which the samples were obtained and (iii) reactor core modeling (Nishihara et al, 2012), has led many to speculate that each particulate type represents emission from a specific reactor at the Fukushima site (Shibahara et al, 2014;Zheng et al, 2014;Snow et al, 2016).…”

Temporal Change in Radiological Environments on Land after the Fukushima Daiichi Nuclear Power Plant Accident

“…A map of 134 Cs: 137 Cs deposition-density ratios over a wide area in eastern Japan indicated regional variation [23], which was analyzed by atmospheric dispersion simulation and using information from reactor analysis [63]. The regional distribution of deposition density ratio obtained from the model simulation showed general agreement with that observed.…”

“…In particular, the ratio of 134 Cs to 137 Cs is an effective indicator of origin, because the chemical behavior is considered to be the same between the isotopes and the releasedradionuclide ratio is maintained in the deposition density ratio. Reactor analysis results indicated that the 134 Cs: 137 Cs release ratios were 0.94, 1.08, and 1.05 for reactors 1, 2, and 3, respectively, as of March 15, 2011 [63,64].…”

“…The analysis facilitated estimation of the dominant reactors for each region's contamination and for different periods' release during the accident. Also, a long, narrow north-northwest-trending area with low ratios was identified in the vicinity of the NPP site and was thought to have been contaminated by a plume from reactor 1 [63,65].…”

Time-series analysis of atmospheric radiocesium at two SPM monitoring sites near the Fukushima Daiichi Nuclear Power Plant just after the Fukushima accident on March 11, 2011

“…These datasets have many data of radionuclides, however, are not available for the investigation of the hourly atmospheric radionuclides concentrations, and might have missed if the polluted air masses passed without precipitation. Hence, the source term identification of plumes still has a large uncertainty, although many studies have been reported (Chino et al, , 2016Katata et al, 2015).…”