We have investigated the status of the nuclear fuel assemblies in Unit-1 reactor of the Fukushima Daiichi Nuclear Power plant by the method called Cosmic Muon Radiography. In this study, muon tracking detectors were placed outside of the reactor building. We succeeded in identifying the inner structure of the reactor complex such as the reactor containment vessel, pressure vessel, and other structures of the reactor building, through the concrete wall of the reactor building. We found that a large amount of fuel assemblies was missing in the original fuel loading zone inside the pressure vessel. It can be naturally interpreted that most of the nuclear fuel was melt and dropped down to the bottom of the pressure vessel or even below.

中文翻译：

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宇宙μ子射线照相法研究福岛第一核电站1号反应堆

我们通过称为宇宙介子射线照相术的方法调查了福岛第一核电站 1 号机组反应堆中核燃料组件的状态。在这项研究中，μ 子跟踪探测器被放置在反应堆建筑物的外面。通过反应堆厂房的混凝土墙，我们成功地识别了反应堆综合体的内部结构，如反应堆安全壳、压力容器和反应堆厂房的其他结构。我们发现在压力容器内的原始燃料装载区丢失了大量燃料组件。可以很自然地解释为，大部分核燃料都熔化了，掉到了压力容器的底部，甚至下面。