Photo/IllutrationA photograph taken from an Asahi Shimbun helicopter shows the No. 7, right, No. 6, center, and No. 5 reactors of Tokyo Electric Power Co.’s Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture. (Asahi Shimbun file photo)

The Nuclear Regulation Authority on Oct. 18 decided to require utilities to install an emergency circular cooling water system to prevent containment vessels of boiling-water reactors from bursting in accidents in which nuclear fuel has melted.

This requirement will lead to increased safety costs and could force some nuclear plant operators into deciding to decommission their reactors.

It may also prolong the NRA’s screening process of reactors that utilities are seeking to bring back online.

In addition, the NRA will require utilities employing pressurized-water reactors in their nuclear plants, which are receiving permission to restart ahead of ones utilizing boiling-water reactors, to install filtered equipment for vents that could lower the pressure if the size of their containment vessels are small.

Pressurized-water reactors are considered more resistant to critical accidents because they have larger containment vessels compared with ones for boiling-water reactors, which are of the same type used at the crippled Fukushima No. 1 nuclear power plant.

Smaller containment vessels may aggravate the situation in the event of a nuclear accident. The authority will revise the new regulation that was put in place in 2013.

The new circular cooling water system was proposed by Tokyo Electric Power Co., operator of the Fukushima No. 1 plant, during the screening process toward the restart of its Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture.

The system would draw water from the bottom of the containment vessel if the water is heated due to melted fuel.

After the water is cooled in the outside cooling equipment, it will be returned to the containment vessel to again cool the nuclear fuel.

With this new system, utilities can avoid filtered vents that would release vapor containing radioactive substances into the atmosphere.

The NRA also will require other nuclear power plants to install the circular cooling water system including Tohoku Electric Power Co.’s Onagawa nuclear power plant in Miyagi Prefecture; Chubu Electric Power Co.’s Hamaoka nuclear power plant in Shizuoka Prefecture; and Chugoku Electric Power Co.’s Shimane plant in Shimane Prefecture.

Japan Atomic Power Co. decided to introduce the system to its Tokai No. 2 plant in Ibaraki Prefecture during an Oct. 17 screening meeting.

Concerning countermeasures to prevent the rupturing of containment vessels, the NRA decided to allow some pressurized-water reactors such as the No. 1 and No. 2 reactors at Kansai Electric Power Co.'s Oi nuclear power plant in Fukui Prefecture to install filtered equipment for vents.

Those reactors are more at risk of a containment vessel rupture because the size of their containment vessels is only half the size of conventional containment vessels, which would likely mean an increase of internal pressure during an accident.