Full system decontamination applied before core internals replacement at the Fukushima-Daiichi Nuclear Power Station

As a preventive maintenance measure to prepare for stress corrosion cracking of the shroud at Tokyo Electric Power's Fukushima-Daiichi Nuclear Power Station, work to successively replace the reactor core shrouds, jet pumps and other structures in the reactor is being conducted. Because this work requires that human beings enter the reactor core to perform tasks, full system decontamination was first executed on the core interior and recirculation piping as measures to protect workers from radioactivity and reduce doses, prior to the start of work in the core interior. The decontamination was conducted using the CORD (chemical oxidation reduction decontamination) method. During decontamination, the concentrations of radioactive components, metal irons, and decontamination reagent were monitored at appropriate intervals to control the decontamination process. The efficacy of decontamination was evaluated in terms of changes in the dose within the reactor, changes in the dose equivalent rate in the recirculation pipes, and the amount of radioactivity removed by decontamination. The initial chemical decontamination was performed in July 1997 on unit No. 3 at the Fukushima-Daiichi Nuclear Power Station, and in the following September on the unit No. 2 as well. The critical decontamination process required 168 hours at the unit No. 3 and 179 hours for the unit No. 2. By means of this chemical decontamination, 10 TBq of radioactivity for both units, the equivalent of 72 kg of Co-60 (63 kg for the No. 2 unit), was removed. As a result, the dose equivalent rate DF in the reactor bottom water was raised to the target 20 or higher. (author)