Japan's nuclear regulator has released draft safety rules aimed at protecting nuclear power facilities against natural disasters and terrorist attacks.

New safety measures will be mandatory under the proposed standards by the Nuclear Regulation Authority, the new government safety agency created last September.

Prior to the March 2011 Fukushima nuclear power plant disaster, utilities could decide themselves whether to take steps against the possibility of a major disaster, based on the assumption that such disasters were extremely unlikely.

NRA Chairman Shunichi Tanaka referred to the new rules as "the beginning of real [nuclear] regulation in Japan," the Financial Times reports.

So far only two of Japan's 50 reactors have been restarted following shutdowns after the Fukushima disaster.

Under the proposed rules, plants must be outfitted with back-up control rooms located away from reactor buildings, thus reducing the risk of plant workers being irradiated in an emergency. Also, protective structures would need to be reinforced to withstand the impact of a jet airliner if it were used to carry out a terrorist attack.

The proposed measures include the installation of vents capable of filtering out radioactive gases, in case the reactors need to undergo emergency venting. While the Fukushima facility had venting systems, they weren't equipped with radiation-screening filters.

The proposed rules also set criteria for evacuating areas around nuclear power plants during an emergency.

Some of the measures, such as mobile back-up electrical generators, have already been implemented by nuclear operators but other measures would involve considerable upgrades.

The country's nuclear power plant operators are expected to apply to restart idled reactors once NRA finalizes the rules, scheduled for release in July.

Tanaka this month said it would take time for operators to complete the anti-disaster construction necessary to clear the safety standards. The authority is expected to give utilities three to five years to upgrade facilities to comply with the new requirements.

"I don't think it will be possible to (restart all of the reactors) in three years, although we will act as swiftly as we can," Tanaka said in a Kyodo report.

In a New Year's message posted on the agency's website, Tanaka admitted that, from the standpoint of international levels, "the safety standards in Japan were insufficient" at the time of the Fukushima disaster.

"To make up for this lag, the NRA will work rationally to reach calm, scientific and strict judgments and to fulfill its responsibility as the regulatory authority," he said.