History Of the Disaster in Japan

Events at Fukushima Daiichi 1-3 & 4
It appears that no serious damage was done to the reactors by the earthquake, and the operating units 1-3 were automatically shut down in response to it, as designed. At the same time all six external power supply sources were lost due to earthquake damage, so the emergency diesel generators located in the basements of the turbine buildings started up. Initially cooling would have been maintained through the main steam circuit bypassing the turbine and going through the condensers.
Then 41 minutes later, at 3:42 pm, the first tsunami wave hit, followed by a second 8 minutes later. These submerged and damaged the seawater pumps for both the main condenser circuits and the auxiliary cooling circuits, notably the Residual Heat Removal (RHR) cooling system. They also drowned the diesel generators and inundated the electrical switchgear and batteries, all located in the basements of the turbine buildings (the one surviving air-cooled generator was serving units 5 & 6). So there was a station blackout, and the reactors were isolated from their ultimate heat sink. The tsunamis also damaged and obstructed roads, making outside access difficult.
All this put those reactors 1-3 in a dire situation and led the authorities to order, and subsequently extend, an evacuation while engineers worked to restore power and cooling. The 125-volt DC batteries for units 1 & 2 were flooded and failed, leaving them without instrumentation, control or lighting. Unit 3 had battery power for about 30 hours.
At 7.03 pm Friday 11 March a Nuclear Emergency was declared, and at 8.50pm the Fukushima Prefecture issued an evacuation order for people within 2 km of the plant. At 9.23 pm the Prime Minister extended this to 3 km, and at 5.44 am on 12th he extended it to 10 km. He visited the plant soon after. On Saturday 12th he extended the evacuation zone to 20 km.
Inside the Fukushima Daiichi reactors
The Fukushima Daiichi reactors are GE boiling water reactors (BWR) of an early (1960s) design supplied by GE, Toshiba and Hitachi, with what is known as a Mark I containment. Reactors 1-3 came into commercial operation 1971-75. Reactor power is 460 MWe for unit 1, 784 MWe for units 2-5, and 1100 MWe for unit 6.

On 4/10/2015, Tepco announced their first robot for Reactor 1 stopped moving after traveling approx. 10m.



In order to investigate the possibly overflowed molten fuel, Tepco put a robot into PCV (Primary Containment Vessel) to Reactor 1.

This robot forms itself like a train (60cm, 7cm, 9.5cm) to go through a penetration pipe. It deploys itself in the shape of “C” when it reaches the ground floor of PCV.

It was around 9AM of 4/10/2015 when Tepco started the investigation. They managed to land the robot onto the ground floor of PCV, however it stopped around 2PM. It moved only for approx. 10m.



They planned to collect the images, temperature and dose data of Reactor 1. They reported 2/3 of planned data was collected, however did not announce what the robot found.

The specific cause to have stopped the robot is not identified. Tepco is commenting some cable could have been stuck somewhere.