Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition
Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition
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DOI：
10.5194/acp-12-2313-2012
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发表时间：
2012-01-01
影响因子：
6.3
通讯作者：
Yasunari, T. J.
中科院分区：
文献类型：
--
作者：
Stohl, A.;Seibert, P.;Yasunari, T. J.
2011年3月11日，日本本州岛太平洋沿岸约130公里处发生地震，随后发生大海啸。由此造成的福岛第一核电站电力损失发展成一场灾难，导致大量放射性物质释放到大气中。在这项研究中，我们确定了两种同位素，惰性气体氙-133（Xe-133）和气溶胶结合的铯-137（Cs-137），它们在大气中具有非常不同的释放特性和行为。为了确定4月20日之前的放射性核素排放量与高度和时间的关系，我们根据燃料库存和现场记录的事故事件对释放率进行了初步猜测。这一初步猜测随后通过逆建模得到了改进，逆建模将其与大气传输模型FLEXPART的结果以及来自日本、北美和其他地区几十个台站的测量数据相结合。我们既使用了大气活动浓度测量，以及为Cs-137，测量散装沉积。关于钚-133，我们发现总释放量为15.3 EBq（不确定性范围12.2-18.3），是切尔诺贝利总释放量的两倍多，可能是历史上最大的放射性惰性气体释放量。2011年3月11日至15日，1-3号反应堆单元的全部惰性气体存量被释放到大气中。事实上，我们的估计释放量高于福岛第一核电站的全部估计碘-133库存，我们用碘-133（半衰期为20.8小时）衰变为碘-133来解释。有强有力的证据表明，在第一次主动排气之前就已经开始释放氚-133，这可能表明反应堆部件的结构性损坏和（或）由于超压而导致的泄漏，而超压本来会使惰性气体提前释放。对于Cs-137，反演结果给出的总排放量为36.6（20.1-53.1）PBq，约为估计的切尔诺贝利排放量的43%。我们的研究结果表明，Cs-137排放量在3月14日至15日达到峰值，但从12日到3月19日，当4号机组乏燃料池开始喷水时，排放量突然下降了几个数量级。这表明，排放物可能不仅来自受损的反应堆堆芯，还来自4号机组的乏燃料池。这也将证实喷洒是一种有效的对策。我们探索的主要分散和沉积模式的放射性云，无论是区域性的日本以及整个北方半球。虽然乍一看似乎幸运的是，西风盛行的大部分时间在事故中，一个不同的图片出现从我们的详细分析。在3月14日和15日Cs-137最强排放期间和之后，以及3月19日的另一个强排放期之后，放射性羽流在本州岛东部平流，降水将大部分Cs-137沉积在陆地表面。放射性云于3月15日到达北美，3月22日到达欧洲。到4月中旬，铯-133已相当均匀地分布在整个北方半球的中纬度地区，并首次在南半球（澳大利亚的达尔文站）进行了观测。总的来说，我们估计，截至4月20日，6.4 PBq的Cs-137或总沉降物的18%沉积在日本陆地地区，而其余大部分落在北太平洋。只有0.7 PBq，即总沉降物的1.9%沉积在日本以外的陆地地区。
On 11 March 2011, an earthquake occurred about 130 km off the Pacific coast of Japan's main island Honshu, followed by a large tsunami. The resulting loss of electric power at the Fukushima Dai-ichi nuclear power plant developed into a disaster causing massive release of radioactivity into the atmosphere. In this study, we determine the emissions into the atmosphere of two isotopes, the noble gas xenon-133 (Xe-133) and the aerosol-bound caesium-137 (Cs-137), which have very different release characteristics as well as behavior in the atmosphere. To determine radionuclide emissions as a function of height and time until 20 April, we made a first guess of release rates based on fuel inventories and documented accident events at the site. This first guess was subsequently improved by inverse modeling, which combined it with the results of an atmospheric transport model, FLEXPART, and measurement data from several dozen stations in Japan, North America and other regions. We used both atmospheric activity concentration measurements as well as, for Cs-137, measurements of bulk deposition. Regarding Xe-133, we find a total release of 15.3 (uncertainty range 12.2-18.3) EBq, which is more than twice as high as the total release from Chernobyl and likely the largest radioactive noble gas release in history. The entire noble gas inventory of reactor units 1-3 was set free into the atmosphere between 11 and 15 March 2011. In fact, our release estimate is higher than the entire estimated Xe-133 inventory of the Fukushima Dai-ichi nuclear power plant, which we explain with the decay of iodine-133 (half-life of 20.8 h) into Xe-133. There is strong evidence that the Xe-133 release started before the first active venting was made, possibly indicating structural damage to reactor components and/or leaks due to overpressure which would have allowed early release of noble gases. For Cs-137, the inversion results give a total emission of 36.6 (20.1-53.1) PBq, or about 43% of the estimated Chernobyl emission. Our results indicate that Cs-137 emissions peaked on 14-15 March but were generally high from 12 until 19 March, when they suddenly dropped by orders of magnitude at the time when spraying of water on the spent-fuel pool of unit 4 started. This indicates that emissions may not have originated only from the damaged reactor cores, but also from the spent-fuel pool of unit 4. This would also confirm that the spraying was an effective countermeasure. We explore the main dispersion and deposition patterns of the radioactive cloud, both regionally for Japan as well as for the entire Northern Hemisphere. While at first sight it seemed fortunate that westerly winds prevailed most of the time during the accident, a different picture emerges from our detailed analysis. Exactly during and following the period of the strongest Cs-137 emissions on 14 and 15 March as well as after another period with strong emissions on 19 March, the radioactive plume was advected over Eastern Honshu Island, where precipitation deposited a large fraction of Cs-137 on land surfaces. Radioactive clouds reached North America on 15 March and Europe on 22 March. By middle of April, Xe-133 was fairly uniformly distributed in the middle latitudes of the entire Northern Hemisphere and was for the first time also measured in the Southern Hemisphere (Darwin station, Australia).In general, simulated and observed concentrations of Xe-133 and Cs-137 both a Japanese as well as at remote sites were in good quantitative agreement. Altogether, we estimate that 6.4 PBq of Cs-137, or 18% of the total fallout until 20 April, were deposited over Japanese land areas, while most of the rest fell over the North Pacific Ocean. Only 0.7 PBq, or 1.9% of the total fallout were deposited on land areas other than Japan.