The Nuclear Regulation Authority may require utilities to introduce a new defense against ruptures in containment vessels of boiling-water reactors--the same type used in the crippled Fukushima No. 1 nuclear plant.

Such an obligation would not only lead to additional safety costs for the nuclear plant operators, but it could also prolong the NRA’s screening process of boiling-water reactors that utilities want to bring online.

The measure, consisting of a circulating cooling water system, would reduce the risk of a containment vessel rupturing under enormous internal pressure during an accident. It would also help to prevent the escape of radioactive substances into the atmosphere in such emergencies, NRA officials said.

Under new safety regulations established after the triple meltdown at the Fukushima No. 1 plant, utilities are required to install filtered vents that can release into the atmosphere gases that are produced as a result of overheated nuclear fuel. The release of the gases eases the pressure inside the reactors and averts a rupture in the containment vessel.

But the NRA is considering going further to contain the effects of a grave accident involving melting nuclear fuel.

The system under consideration would use a pipe to draw water from the bottom of the containment vessel if the water is heated by melted fuel.

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

The circulating system can maintain a continued cooling of melted nuclear fuel, which would prevent gases from being produced and pressure from rising within the containment vessel.

Boiling-water reactors are considered more vulnerable to serious accidents because their containment vessels are smaller than the ones for pressurized-water reactors.

If nuclear fuel overheats, an operator of a boiling-water reactor has few options other than to use the filtered vent in hopes of decreasing pressure and avoiding a blowout in the containment vessel.

Tokyo Electric Power Co., operator of the Fukushima No. 1 nuclear plant, developed the circulating cooling water system. It has been installed at the utility’s Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture, which TEPCO hopes to restart soon.

TEPCO says the system is “more effective than a filtered vent.” The NRA agrees.

When the NRA’s new regulations were being compiled, a filtered vent was regarded as the “only effective defense” to prevent a rupture, according to a senior official at the NRA.

However, the new regulations, put in place in 2013, revolve around a “back-fitting” system that requires utilities to install the latest technology on their licensed facilities.

After the Fukushima No. 1 nuclear plant lost power in the Great East Japan Earthquake and tsunami in March 2011, nuclear fuel overheated and evaporated the cooling water inside the reactors. Pressure within the containment vessels soared from the accumulation of highly radioactive gases produced by the fuel.

To prevent the containment vessels from rupturing, TEPCO released the gases into the atmosphere, resulting in contamination of a broad area around the Fukushima plant.

Although venting through a filter is regarded as a last resort to keep the containment vessel intact, it cannot completely stop the release of radioactive substances to the outside.

Proponents of the new defense mechanism say that plant operators would not have to use the venting system because pressure inside a reactor would be kept low--even if nuclear fuel melts inside--under the circulating cooling water system.

The Onagawa plant in Miyagi Prefecture, the Hamaoka plant in Shizuoka Prefecture and the Shimane plant in Shimane Prefecture are among the nuclear power facilities in Japan that have boiling-water reactors.