Tokyo Electric Power Company (Tepco) has completed a five-day investigation of the primary containment vessel (PCV) of the damaged unit 1 of the Fukushima Daiichi plant using a robot. The results of the survey will help Tepco determine the location of fuel debris within the vessel.
On 18 March, Tepco inserted the PMORPH robot - developed by Hitachi-GE Nuclear Energy and the International Research Institute for Nuclear Decommissioning - into the basement area of unit 1's primary containment vessel (PCV) around the pedestal, on which the reactor pressure vessel sits.
The robot made several excursions into the PCV over a five-day period, completing its mission on March 22. Equipped with a dosimeter and waterproof camera, it took radiation readings and digital images at ten different measurement points.
Discussing the results at a press conference yesterday, Tepco said that deposits had been found at the bottom of the PCV and on piping. It added these deposits will be analysed by examining the images obtained by the robot and by taking samples.
It noted that radiation dose levels measured by the robot were found to decrease upon submerging into the water in the lower part of the vessel and to rise again when nearing the floor.
Tepco said there had been little change in dose levels above the grating compared to the previous survey, made in April 2015, and no significant damage had been found to existing structures within the PCV. The latest measurements taken above the grating varied between 3.6 and 12.0 Sieverts per hour. Dose levels taken at various depths within the water in the lower parts of the PCV varied between 1.5 and 11.0 Sv/h.
"It will take more time to conclude whether fuel debris exists at the measured points or how far it is spread over the PCV basement," Tepco said. It will continue to examine the results and announce the findings as soon as they are available.
One challenge in gathering data from the bottom of unit 1's PCV is the floating deposits in the standing water that obscure photography, Tepco said. For that reason, the next step in the unit 1 exploration will be the extraction of a water sample to help engineers determine the precise nature of the floating deposits and devise a strategy to obtain clear photos.
Researched and written
by World Nuclear News