Tepco surveys interior of unit 2 containment vessel

An internal investigation of the primary containment vessel (PCV) of unit 2 at the damaged Fukushima Daiichi nuclear power plant in Japan was completed as scheduled today, Tokyo Electric Power Company (Tepco) has announced. The survey - using a suspended pan-tilt camera attached to a telescopic guiding pipe - identified deposits and fuel assembly components at the bottom of the pedestal area.

A fuel assembly component lies amongst other debris in the bottom of the pedestal(Image: Tepco)

In February 2017, Tepco sent a "scorpion-shaped" robot - developed jointly by Toshiba and the International Research Institute for Nuclear Decommissioning (IRID) - into the PCV of unit 2. That survey discovered part of the grating of the platform inside its pedestal had dropped. Although the robot was unable to reach the area directly under the reactor pressure vessel, the company said the information it gathered would help it determine how to decommission the unit.

"Robotic explorations of unit 2's interior, especially inside the pedestal area at the base of the reactor, are essential to determine the location and condition of fuel that melted and reformed during the 2011 accident," Tepco said today. "But penetration of the area has been made challenging by deposits from the accident that has blocked the path of robots designed to crawl around it, and high radiation levels that limit the life of electronic equipment."

The company today carried out an investigation of the PCV of unit 2 using a suspended pan-tilt camera attached to a telescopic guiding pipe. The small and radiation-hardened device was also developed by Toshiba and IRID.

The device was introduced into the PCV through a pipe about 12cm in diameter. It comprises a guiding pipe some 13m in length and with a diameter of around 11cm. Attached to this is a further telescopic guiding pipe, about 5m long. A camera module weighing some 2kg is mounted on the end of this. The camera module houses two cameras - a pan-tilt camera and a 'bird's eye camera' - as well as an LED lighting unit, a radiation dosimeter and a thermometer.

After examining the pictures obtained using the device, Tepco said: "The entire bottom of the pedestal was found to be covered with sandy and clay-like deposits. Some fuel assembly components have fallen to the bottom of the pedestal and deposits thought to be fuel debris were identified in the vicinity of these fallen components."

Tepco has also carried out robotic surveys of the PCVs of units 1 and 3 at the Fukushima Daiichi plant. Last March, Tepco carried out an investigation of the PCV of unit 1 at Fukushima Daiichi using the PMORPH robot developed by Hitachi-GE Nuclear Energy and IRID. Equipped with a dosimeter and waterproof camera, it took radiation readings and digital images at ten different measurement points within that unit's PCV.

In July, it inserted a screw-driven submersible robot developed by Toshiba and IRID into unit 3's PCV.

Researched and writtenby World Nuclear News