Design guidelines for nuclear plants do not require that utilities take steps against long-term power loss, according to rules published in 1990 and obtained by The Japan Times. The apparent lapse was based on the assumption that in case of an emergency, electricity would be quickly restored or backup power would suffice.

Still in effect, the "Regulatory Guide for Reviewing Safety Design of Light Water Nuclear Power Reactor Facilities," compiled by the Nuclear Safety Commission of Japan in 1990, covers 59 issues, including the safety of reactor facilities, fuel handling systems and radiation control.

An article under Guideline 27, on the reactor cooling system, stipulates that facilities "shall be so designed that safe shutdown and proper cooling of the reactor after shutting down can be ensured in the case of a short-term total AC (alternating current) power loss."

A note to this clause adds that "no particular considerations are necessary against a long-term total AC power loss because the repair of troubled power transmission lines or emergency AC power systems can be expected in such cases."

The March 11 quake and tsunami cut off the power to Tokyo Electric Power Co.'s Fukushima No. 1 plant and knocked out its backup generators, and thus the cooling systems.

A NSCJ spokesman said the rules aren't binding.

However, University of Tokyo professor emeritus Kenzo Miya, an expert on nuclear plants, said utilities must strictly follow the guidelines on matters of safety to receive permission to construct new plants.

"When the guide was compiled, (the NSCJ) did not consider 'severe accidents' in the premise," Miya said, adding that this clause, and the guidelines as a whole, will need to be revised.