MATSUYAMA, EHIME PREF. – During recent maintenance work at the Ikata nuclear power plant that was to include the country’s first removal of spent mixed-oxide (MOX) fuel following full-scale “pluthermal” power generation, a control rod was removed from the reactor by mistake, according to Shikoku Electric Power Co.
The incident involving reactor No. 3 at the plant in Ikata, Ehime Prefecture, caused no loss of control of the nuclear reaction in the unit and no radioactive materials were released, the company said Sunday. The reactor has been shut down since Dec. 26 to perform the maintenance work.
To remove fuel assemblies during the inspection, it was necessary to raise the apparatus at the top of the reactor from which fuel is suspended. While raising the apparatus, a control rod was also lifted out of the reactor along with the fuel assemblies, Shikoku Electric stated.
Control rods are used to suppress nuclear fission, and are inserted in order to halt a nuclear reactor. According to the power company, for fission to be allowed to occur within the reactor, it is necessary to adjust the concentration of boron within the reactor coolant in addition to removing the control rods.
According to Shikoku Electric and prefectural authorities, one of the 48 control rods in the unit was lifted out of the reactor containment vessel accidentally at around 1:20 p.m. Sunday together with the upper part of the apparatus that holds fuel assemblies in place from above and below.
As the apparatus was lifted by crane, a worker noticed that the control rod had also been raised out of the containment vessel. Around seven hours later, the control rod was reinserted into the reactor.
Because the crane’s weight sensor had shown a value typically seen when the control rods are disconnected, the control rods were determined to have been detached from the apparatus and the crane was raised, Shikoku Electric said.
The company had planned to start removing the spent MOX fuel assemblies, which contain a blend of plutonium extracted from spent nuclear fuel and uranium, from the reactor at 12 a.m. Monday. The work is now expected to be delayed due to the company’s investigation into the incident involving the control rod.
Shikoku Electric also announced last Tuesday that in October 2017 it mistakenly conducted an inspection of an emergency air supply filter in the main control room of reactor No. 3 without removing all the fuel from the reactor as stipulated in the safety regulations.
The operation to extract the MOX fuel assemblies is the first such removal to be performed at any commercial nuclear power plant in Japan since an initiative on pluthermal power generation using the mixed fuel was announced in 1997, according to Shikoku Electric.
The government and power firms are promoting pluthermal power generation as part of the nuclear fuel cycle featuring the extraction of plutonium from spent nuclear fuel for reuse. Shikoku Electric started pluthermal power generation in 2010 using 16 MOX fuel assemblies installed in the Ikata No. 3 reactor.
The company had been planning to remove all of them during the reactor maintenance, which is to continue through April 27. The firm will consider reusing the spent MOX fuel, which is expected to be stored at the power plant for the foreseeable future due to a lack of reprocessing facilities in this country.