Decommissioning preparation in progress at Fukushima I NPS
Dec. 21, 2012
A reactor cover is being constructed at unit 4 (photo courtesy of TEPCO).
TOKYO --A year has passed on December 16 since the Japanese government and Tokyo Electric Power Company (TEPCO) announced that the Fukushima I nuclear power station (NPS) had reached a "cold shutdown state." Decommissioning work has steadily progressed compared with a year ago. A reactor building cover is being constructed at unit 4 to prepare for spent fuel removal. The design of the unit 3 cover is almost finished. Surveys have been conducted inside the reactor containments of units 1 and 2 to collect data such as radiation and water levels. As regards the issue of contaminated water, introduction of new equipment has provided better perspectives on mid- and long-term treatment plans.
Plans to transfer spent fuel from the reactor buildings damaged by hydrogen explosions to the plant common pool have advanced substantially. At unit 4, which holds the greatest number of fuel assemblies (1,533), workers have completed removal of debris scattered on the top building floor and in the fuel pool. In July, two non-irradiated fuel assemblies were successfully retrieved for testing. A building cover equipped with a crane for removing fuel is being constructed, with the start of fuel removal planned for mid-November next year. TEPCO expects to complete fuel removal at the end of 2014, a year earlier than originally scheduled.
Plans for the design and construction of the unit 3 building cover are mostly set. Once the cover is complete, the company will be ready to remove spent fuel safely and be able to shut off nearly all radioactive releases from the building. In September this year, however, there was an accident of dropping a steel beam into the pool when clearing debris, renewing the workers' awareness of the risks associated with this work. Owing to this trouble, debris removal at unit 3 is still only half done, and safety management is to be conducted more rigorously.
There was also substantial advancement in surveys inside the reactor buildings during the past one year. After sending remote operated robots into the buildings several times, radiation levels on each building floor of units 1 to 3 are now known in detail. Surveys inside the reactor containments of units 1 and 2 revealed high radiation levels, with maximum values of about 11 and 73 Sieverts per hour, respectively. Water levels in the containments are also known now, based on which TEPCO is trying to identify the locations of containment damage. Once damage locations are identified, the utility can advance to the next stage, containment repairs.
Regarding the issue of contaminated water, which has been afflicting Fukushima I since the onset of the accident, introduction of new equipment has provided better mid- and long-term perspectives. The previous approach was to treat contaminated water by the second cesium adsorption apparatus (SARRY) and then store the treated water. Radical solutions have been sought because of the limited space for additional storage tanks, and TEPCO has adopted three main approaches in this regard. One is to prevent the inflow of groundwater by developing groundwater bypaths in order to reduce the amount of contaminated water generated; they are expected to be fully operable after the turn of the year. One other approach is the multi-nuclide removal equipment (ALPS), which can remove 62 nuclides other than cesium down to less than their statutory limits; once the equipment operates, the risks associated with storing contaminated water will be reduced significantly. The company also plans to secure space to store contaminated water by developing large-volume underground storage tanks.