Survey under way of Fukushima Daiichi 1 vessel

A robot has entered into the primary containment vessel of the damaged unit 1 of the Fukushima Daiichi plant and provided Tokyo Electric Power Company with radiation and temperature measurements within it. The company hopes the data, together with video footage, will enable it to locate the molten fuel in the unit.

A robot has entered into the primary containment vessel of the damaged unit 1 of the Fukushima Daiichi plant and provided Tokyo Electric Power Company (Tepco) with radiation and temperature measurements within it. The company hopes the data, together with video footage, will enable it to locate the molten fuel in the unit.

The PMORPH robot within unit 1's PCV (Image: Tepco)

On 18 March, Tepco inserted the PMORPH robot into unit 1 in the first of a series of four planned robot explorations of the basement area of its primary containment vessel (PCV) around the pedestal, on which the reactor pressure vessel sits. The investigation is part of preparatory work for the eventual removal of fuel debris.

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.

Court says government partly liable for accident

Negligence by Tepco and the Japanese government contributed to the March 2011 accident at the Fukushima Daiichi plant, the Maebashi District Court ruled on 17 March.

In a case brought by 137 people evacuated from Fukushima Prefecture - 61 of whom left on their own accord - the court said the operator and the government were negligent in preparing anti-tsunami measures at the plant site.

The court awarded a total of JPY 38.55 million ($342 million) in damages to the plaintiffs, who had sought a combined JPY1.5 billion in damages for emotional stress.

The ruling marks the first time a court has recognized that the government was liable for negligence.

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.

In the latest investigation, the robot travelled along a section of the first floor grating, on which it measured a radiation dose of 7.8 Sieverts per hour. The robot also lowered its sensor unit into the water that has collected at the bottom of the primary containment vessel. At a height of about 1 metre above the PCV basement floor, Tepco recorded a dose level of 1.5 Sv/h. The robot also recorded temperature measurements within the PCV of 14-23°C.

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." Readings indicate the temperature within the area of the containment vessel where the robot stopped was around 16.5°C and the dose rate was about 210 Sv/h, significantly higher than those measured in unit 1.

Tepco said the latest reading and images obtained from unit 1 will now be examined in greater detail. "The conditions of the PCV basement floor will be examined later," it noted.

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.

Tepco is preparing to conduct similar investigations using a robot in unit 3 at the plant.

Researched and written
by World Nuclear News