A public-private organization decommissioning the stricken Fukushima No. 1 nuclear power plant is poised to adopt a "dry" nuclear fuel retrieval method -- extraction without filling the containment vessels with water -- for the three reactors at the station that melted down in March 2011, the Mainichi Shimbun has learned.
Sources close to the Nuclear Damage Compensation and Decommissioning Facilitation Corp. (NDF) say the method will be incorporated into the strategic plan for decommissioning that will soon be announced to the public. The government and Fukushima plant operator Tokyo Electric Power Co. (TEPCO) will establish fuel retrieval plans based on consideration of this method, and will deliberate a revision to the decommissioning road map as early as this summer.
Until now, the NDF had considered employing the submersion fuel retrieval method -- filling the containment vessels with water -- alongside the dry method. In the submersion method, water shields plant workers from radiation. The NDF, however, determined that repairing all damaged areas of the containment vessels in order to be able to fill the reactor wells to the top with water would be too difficult. Instead, for the time being, the NDF decided to prioritize dry removal of the nuclear fuel debris using robotic arms.
"It isn't that we've decided to completely do away with the submersion method, but we have to think about how best to distribute the technological resources we have," said one source closely involved with the NDF.
When using the dry nuclear fuel retrieval method, it is crucial to implement measures to prevent microscopic radioactive substances from spreading in the air. To counter this, the NDF is considering spraying water on the fuel as robotic arms are used to sever and retrieve the fuel debris.
The damage differs from reactor to reactor at the Fukushima No. 1 plant. Probes of the reactor interiors conducted by TEPCO have yet to directly observe the nuclear fuel, meaning that the shape and distribution of the debris remain unknown. Fuel removal methods specific to the state of each reactor must be decided before moving ahead.
In the No. 1 reactor, much of the nuclear fuel is believed to have melted through the pressure vessel onto the floor of the reactor containment vessel. Inserting a robotic arm through the side of the containment vessel to remove the melted fuel is under primary consideration to deal with this situation.