Nuclear fuel extraction hinges on robot surveys

A still image of a video taken by a small cord-controlled robot, shows inside the reactor vessel of the No. 1 reactor building at TEPCO's tsunami-crippled Fukushima Daiichi nuclear power plant in Fukushima prefecture.

A robot entered Tokyo Electric Power Co.'s No. 1 reactor at its Fukushima No. 1 nuclear power plant for the first time last month.

Undeterred by radiation, robots are indispensable assets that can shed light on the conditions of such sites normally inaccessible to humans. TEPCO, the government and other parties are continuing to develop robots to deal with melted fuel inside reactors.

A big headache

The Great East Japan Earthquake of March 2011 cut off power and disrupted cooling functions at reactor Nos. 1-3. Nuclear fuel inside the pressure vessels subsequently melted after reaching critical temperatures.

Constantly emitting deadly radiation, melted nuclear fuel presents the biggest headache for decommissioning work. Moreover, each of the reactors has its own unique problems.

At the No. 1 reactor, computer simulations and other assessments suggest that most if not all nuclear fuel inside its pressure vessel has melted through and pooled at the bottom of the containment vessel underneath. As for the other two reactors, some melted fuel is still thought to remain in their pressure vessels.

A clear picture is needed to be able to safely process melted nuclear fuel. Robots are therefore being developed to fit each of the reactor's conditions.

In April, two small robot probes were sent into reactor No. 1 using a pipe 10 centimeters in diameter. Once the units reached the floor of the containment vessel, they transformed into a "C" shape for enhanced stability. The robots successfully photographed contaminated water pooled underground, and were able to determine the condition of debris.

"The images tell us that the underground entrance isn't blocked by debris," TEPCO Nuclear Power & Plant Siting Division Acting General Manager Teruaki Kobayashi said. "Now we know that we can send a robot down there next time."

The robots were developed by the International Research Institute for Nuclear Decommissioning (IRID) and Hitachi-GE Nuclear Energy Ltd. The team is now working on an underwater robot that can survey the floor of the No. 1 reactor's containment vessel, which is flooded with contaminated water, sometime this fiscal year.

Lifespan: 10 hours

The planned survey of the No. 2 reactor is also making progress. Economy, Trade and Industry Minister Yoichi Miyazawa told reporters on May 7 that a "scorpion" robot is set to be deployed there in August.

An entry rail will bring the robot directly underneath the pressure vessel, where melted fuel is thought to remain. The unit's "scorpion" tail will then flip up, armed with a camera that will provide a view of the fuel from below.

To prepare for a robot survey at the No. 3 reactor, an array of tools including endoscopes and thermometers will be used to examine the state of its interior.

According to ultrasound readings, water levels at the No. 3 reactor are around 5.5 meters to 6.5 meters - more than twice as deep as the roughly 2.8 meters seen at the No. 1 reactor. This indicates the floor that robots were able to navigate at the No. 1 reactor would be submerged.

Therefore, underwater robots could be developed. But concerns linger over the possibility that water visibility could be poor, hampering the ability to take photographs.

Radiation levels inside the containment vessel are so strong that they cause cameras and electric circuitry to malfunction. Robots are being built to tolerate radiation exposure up to 1,000 sieverts, but their lifespan is capped at approximately 10 hours.

"The world has never seen a case like Fukushima's, where nuclear fuel melted and fell through," IRID technology advisory committee member and University of Tokyo School of Engineering Prof. Hajime Asama said.

"With an eye to the harsh interior environment as well as the daunting tasks involved, the specifications of the robots need to be ironed out. Looking to the future, we also need to think about training engineers."

Extraction start set for 2021

The Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) announced a "battle plan" for decommissioning work in April, proposing three ways to extract melted nuclear fuel.

The first approach involves the containment vessel being flooded with water to mitigate the effects of radiation, allowing fuel to be extracted from above. The second also suggests extraction from above but without the use of water, while the last stipulates that a hole be drilled in the side of the containment vessel.

Robot-led surveys are needed to determine the position and state of melted nuclear fuel before a feasible approach could be chosen.

Robots are also an essential aspect of the extraction process, which for example could involve long remote-control arms. With TEPCO aiming to start extracting melted nuclear fuel by 2021, expectations are mounting for the survey robots to bring home results.Speech