fuel from Unit 1. In February 2017, six years after the disaster, radiation levels inside the Unit 2 containment building were crudely estimated to be about 650 Sv/h. The estimation was revised later to 80 Sv/h. These readings were the highest recorded since the disaster occurred in 2011 and the first recorded in that area of the reactor since the meltdowns. Images showed a hole in metal grating beneath the reactor pressure vessel, suggesting that melted nuclear fuel had escaped the vessel in that area. In February 2017, TEPCO released images taken inside Reactor 2 by a remote-controlled camera that show a wide hole in the metal grating under the pressure vessel in the reactor's primary containment vessel, which could have been caused by fuel escaping the pressure vessel, indicating a meltdown/melt-through had occurred, through this layer of containment. Ionizing radiation levels of about 210 sieverts (Sv) per hour were subsequently detected inside the Unit 2 containment vessel. Undamaged spent fuel typically has values of 270 Sv/h, after ten years of cold shutdown with no shielding. In January 2018, a remote-controlled camera confirmed that nuclear fuel debris was at the bottom of the Unit 2 PCV, showing fuel had escaped the RPV. The handle from the top of a nuclear fuel assembly was also observed, confirming that a considerable amount of the nuclear fuel had melted. Damage to unit 4 Reactor 4 was not operating when the earthquake struck. All fuel rods from Unit 4 had been transferred to the spent fuel pool on an upper floor of the reactor building prior to the tsunami. On 15 March, an explosion damaged the fourth floor rooftop area of Unit 4, creating two large holes in a wall of the outer building. It was reported that water in the spent fuel pool might be boiling. The explosion was later found to be caused by hydrogen passing to unit 4 from unit 3 through shared pipes. As a result, from the explosion, a fire broke out and caused the temperature in the fuel pool to increase to . Radiation inside the Unit 4 control room prevented workers from staying there for long periods. Visual inspection of the spent fuel pool on 30 April revealed no significant damage to the rods. A radiochemical examination of the pond water confirmed that little of the fuel had been damaged. In October 2012, the former Japanese Ambassador