New Fukushima Containment Investigation Plan Confirmed
Details of the new upcoming effort to investigate the containment structures of the melted reactors at Fukushima Daiichi have been published. Information was given out in bits from various involved parties. We have been able to assemble this into a more coherent format to show the proposed plan. This work is extremely important as it should give clues about the meltdowns themselves and the damage to the containment structures.
Hitachi has been working on a shape changing robot to use in containment inspections. The robot includes a camera but is constrained by the need for a physical tether cable.
This bot can traverse pipes as small as 100mm diameter and travel over uneven surfaces. Hitachi hopes this will be enough to allow the robot to enter the containment structure and move around.
The first demonstration test of the robot will be in unit 1 in the latter half of 2014.
The insertion of the robot will take place in slightly different locations for units 1, 2 and 3 due to slight design differences and possible water levels. The control rod hatch is the initial proposed entry point for each unit. This would allow the robot to traverse down the CRD rail if it is still in place to reach the higher floor grate.
Unit 1 would use the X-6 penetration entrance into the containment structure. Using this access point (control rod hatch) could give radiation levels as high as 100Gy/h as was estimated for unit 2.
This diagram below shows where the X-6 port and CRD rail are located. The arrow points north, the 180 mark faces the turbine building.
This diagram shows the slightly different location of the CRD rail and X-6 port at units 2 and 3.
The illustration below shows the process for gaining entry into containment. It will involve removing parts of the original penetration structure and installing a temporary closure for the pipe.
At unit 2 this route will also have to overcome a break in the CRD rail near the pedestal opening in order to allow the robot to look inside the pedestal. A shield wall will be installed in the area around the hatch opening to try to block some of the radiation levels. These images show a mock up of the type of shield wall they are considering using to help protect workers. The blocks would be installed using a modified remote control forklift.
After completing an inspection of the floor grate level and pedestal, further work would be done to investigate the actual floor of the containment structure. Some specific details have been given for the proposed work in each reactor unit. At unit 1 the work will focus on looking at the area outside the pedestal and to see if fuel has left the pedestal.
Work at units 2 and 3 will attempt to look inside the pedestal itself first to determine the extent of the damage. Unit 3 will require some modification of the plan after they confirm exactly where the water level inside is. It does appear that Hitachi is heavily depending on TEPCO’s initial estimation of fuel melt extent at each reactor. There has been considerable criticism that TEPCO’s estimates may be overly optimistic. If this is the case there may be a need for more adjustment to the investigation efforts as they go.
All of these incremental steps to investigate the containment structures make take until 2017 to complete. The illustration below shows the series of investigation routes as they would be conducted in unit 2.
The investigation work will involve additional scope inspections into the containment structures in an effort to gather more details ahead of time. This would lower the risk of losing a robot in the containment structure and allow pre-planning for at least some potential problems.
The rough plan is established and may be subject to repeated revision as they incrementally discover new information. There is at least a good idea how they will approach this difficult task and some preparation for unanticipated challenges.
METI: (machine translation)
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