Tepco study of unit 3 containment vessel under way

UPDATED Tokyo Electric Power Company has confirmed the presence of what could be melted fuel and fallen debris within the lower part of the flooded primary containment vessel of unit 3 at the damaged Fukushima Daiichi nuclear power plant in Japan. Similar surveys using robots have already been carried out at units 1 and 2 of the plant.

UPDATED: This article has been updated to include details of Tepco's findings from a survey of unit 3's PCV on 22 July. 

Tokyo Electric Power Company (Tepco) has confirmed the presence of what could be melted fuel and fallen debris within the lower part of the flooded primary containment vessel (PCV) of unit 3 at the damaged Fukushima Daiichi nuclear power plant in Japan. Similar surveys using robots have already been carried out at units 1 and 2 of the plant.

A fallen piece of grating can be seen in the right of this photo taken within the lower area of the pedestal area in the PCV (Image: IRID)

A survey conducted by Tepco in October 2015 found that the PCV of unit 3 was flooded with coolant to a depth of about 6 meters. A photographic inspection of the reactor suggested that, during the March 2011 accident, fuel assemblies melted from the excess heat, dropping from their original position down to the pedestal area. In this process, it is believed the pedestal area may have experienced some damage.

Tepco inserted a screw-driven submersible robot developed by Toshiba and the International Research Institute for Nuclear Decommissioning (IRID) into the unit's PCV on 19 July. The company said an initial exploration of the vessel revealed damage to multiple structures inside the pedestal and also that some of the support fittings for the control rod drive (CRD) housing had fallen.

Tepco announced on 21 July that a survey of the pedestal area by the robot had confirmed the presence of what could be melted fuel as well as other fallen debris, including parts of the CRD housing support fittings. It also said it has not been able to identify some of the grating on the platform in the pedestal area.

The company carried out an investigation on 22 July of the lower part of the pedestal area. "As a result, we could examine likely melted materials that are consolidated," Tepco said. It added that it had also found "some fallen substances such as grating and sediment".

The conditions inside the pedestal will be examined in greater detail based on the images obtained from the consecutive surveys, Tepco said.

In February, Tepco sent a "scorpion-shaped" robot - also developed jointly by Toshiba and IRID - into the PCV of Fukushima Daiichi unit 2. About 54cm long, 9cm high and 9cm wide, and weighing about 5kg, that robot was designed to enter the unit's PCV along a pipe about 10cm in diameter. It too was remotely operated by a wired cable. 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.

The following month, 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.

Researched and written
by World Nuclear News