Five nuclear power plants that have passed safety clearances may be at risk of having their cooling systems crippled during huge eruptions of nearby volcanoes, the nation’s nuclear safety watchdog said Monday.

The five plants are Kyushu Electric Power Co.’s Sendai and Genkai plants in Kagoshima and Saga prefectures, respectively, the Mihama and Oi plants, both in Fukui Prefecture and run by Kansai Electric Power Co., and the Ikata plant in Ehime Prefecture run by Shikoku Electric Power Co.

Additional research and data have revealed that the possible concentration of volcanic ash from huge eruptions could soar up to around 100 times that previously estimated. The findings emerged only after screenings of the plants by the Nuclear Regulation Authority.

According to the Federation of Electric Power Companies of Japan, the concentration of volcanic ash that would be spewed could exceed the limit of the plants’ air filters.

In the event that volcanoes nearby erupt, the five plants and eight of their reactors could lose their external power supply and their emergency diesel generators would be rendered useless, according to the nuclear authority.

It now aims to raise the density level of volcanic ash that can affect nuclear plants by 100 times the current level, while pressing utilities to upgrade their air filters.

Reactor No. 3 at the Ikata plant and reactors Nos. 3 and 4 at the Genkai plant top the list of those most likely to be affected by clogged filters.

News of the findings by the NRA followed Kyushu Electric Power’s move Friday requesting that the regulatory agency perform inspections on the No. 4 reactor at the Genkai plant because it aims to put the reactor back online in early March. Pre-operational checks are the last procedure on the list to be carried out before a nuclear reactor can restart.

Kyushu Electric plans to load 193 fuel assemblies into the reactor in February. After reactivating it in early March, commercial operations scheduled to start in April. If things work out as planned, the No. 4 reactor will be active for the first time since December 2011, when it was halted for routine checkups.

On March 11 of that year, reactor cooling systems at the Fukushima No. 1 power plant were crippled following a powerful earthquake and massive tsunami, resulting in the world’s worst nuclear disaster since Chernobyl. The tsunami inundated the six-reactor plant, located 10 meters above sea level, and flooded power supply facilities there.

Reactor cooling systems were crippled. Reactors Nos. 1 to 3 suffered fuel meltdowns, while hydrogen explosions damaged the buildings housing reactors No. 1, No. 3 and No. 4.

The five nuclear plants passed the tougher safety requirements introduced after the Fukushima meltdowns.