Tokyo Electric Power Company (Tepco) is preparing to insert a Hitachi-developed robot into the primary containment vessel of the damaged unit 1 of the Fukushima Daiichi plant. Last month, a Toshiba-developed robot was used to survey the vessel of unit 2.
|The PMORPH robot (Image: IRID)
The company wants to insert the PMORPH robot into unit 2 on 14-17 March to explore the basement area of the primary containment vessel around the pedestal, on which the reactor pressure vessel sits. The robot will take digital images and collect radiation data. The investigation is part of preparatory work for the eventual removal of fuel debris.
Tepco has replaced the guide pipe through which the robot will access the containment vessel. It is also taking various steps to ensure radiation does not escape from the vessel during the investigation, including tight air seals around the guiding pipe and continuous dust monitoring at the work site.
The PMORPH robot was developed by Hitachi-GE Nuclear Energy and the International Research Institute for Nuclear Decommissioning (IRID). It can assume a long, straight shape for passing through narrow spaces, such as pipes. Alternatively, it can rotate its crawlers by 90 degrees in relation to its central body to assume a U-shape, with the crawlers providing better stability when travelling over flat surfaces.
The robot features a combined total of five cameras and also includes a winch used for lowering and raising a sensor unit that incorporates an underwater radiation-resistant camera, LED and a dosimeter.
The investigation will see PMORPH changing its shape to avoid internal structures of the containment vessel whilst travelling across the first floor grating. It will lower the sensor unit at five points on the grating through to the basement floor some 3.5 meters below.
The insertion of the PMORPH robot follows an investigation of the unit's containment vessel by another shape-changing robot in April 2015. That was the first time a robot had entered the containment vessel of any of the damaged units. However, after taking several images and measurements, that robot got stuck in the grating and stopped working.
"Now armed with that information, the [PMORPH] robot will carry a dosimeter and waterproof camera to measure the distribution of radiation rates in the water that has been retained inside the reactor, specifically around the pedestal opening of the basement area of the primary containment vessel, to estimate the location and spreading of fuel debris." The new robot has also been fitted with laser guides to improve its "spatial ability as well as to enable the device to travel whilst checking obstacles and openings", Tepco said.
The company noted that engineers believe deposits seen floating in the water inside the reactor during the earlier investigation may pose an obstacle for future investigations and fuel debris removal. Tepco said that once investigations using the robot have concluded, it will use a hose to extract some of the water and analyse the deposits using X-ray fluorescence in order to identify them and determine how to remove or treat them.
Last month a "scorpion-shaped" robot developed by Toshiba and IRID was sent into the primary containment vessel of unit 2 of the Fukushima Daiichi plant. "In that case," Tepco said, "although the robot was obstructed from reaching all the way into the pedestal area, important information was obtained about the conditions at the base of the reactor."
Researched and written
by World Nuclear News