Cash-strapped Japanese power utilities, which reported Y1.3 trillion ($13.3 billion) in annual pretax losses in the past week, will now likely face the extra burden of shutting down aging nuclear power plants that don't meet new regulations from the country's revamped nuclear industry regulator.

In a detailed draft of reactor safety regulations presented in April, the Nuclear Regulation Authority requires all reactors to adopt the latest safety measures. This means reactors with designs dating back to 1975 or earlier will need costly overhauls before restarting.

"We are not going to be lenient on old reactors," NRA Chairman Shunichi Tanaka said at a news conferences in April.

"It's the decision of the utilities," Mr. Tanaka added, but: "I imagine it wouldn't be easy cost-wise" to make them safe enough to restart.

The three reactors that suffered the most severe damage in the 2011 Fukushima Daiichi accident all had designs that dated back to before 1975. Nationwide, the requirement is expected to mean at least 10 of 50 licensed reactors will have to shut down.

The hardest-hit would be Kansai Electric Power Co. (9503.TO) in western Japan , with seven of its 11 reactors' using pre-1975 designs. The utility previously generated more than half of its electricity from nuclear power, and currently operates the country's only two units that have been restarted since the Fukushima accident.

Like other utilities, Kansai Electric Power has had to make up for idle nuclear capacity by burning expensive fossil fuels. As a result, it posted a record Y243 billion net loss for the fiscal year ended March, following a Y242 billion net loss the previous year.

Without further restarts the red ink is expected to keep flowing, potentially threatening the company's ability to tap banks for new loans.

The costs of permanently shutting down the facilities would add to these operating losses. Utilities haven't provided any cost figures, but the 2008 decision by Chubu Electric Power Co. (9502.TO) in central Japan to shut down two units at the Hamaoka nuclear power plant due to safety upgrade costs gives some idea.

Based on their financial results, the cost of shuttering the two units came to around Y194 billion. But even those figures exclude the potentially massive long-term costs of tending spent nuclear fuel, which must be looked after for centuries.

Asked in a late April about possible decommissioning, Kansai Electric President Makoto Yagi said only: "We are hoping to find ways to keep them in use," while not ruling out decommissioning.

Other than Kansai Electric, at least three more reactors have designs approved by the government in or before 1975. All are in western Japan .

The new regulations will take effect July 18 with any review taking at least six months, according to the agency.

For the utilities, time is pressing. In the two years after the Fukushima accident, Japan 's annual imports of liquefied natural gas rose 23% to 86.9 million metric tons.

The government is also clearly worried about the costs of the fuel and the impact on the trade balance, which now shows monthly deficits--largely unknown before the Fukushima accident. The merchandise-trade balance logged a deficit of Y362.4 billion in March, extending the run of deficits to nine months.

At the same time, the government doesn't want to appear to undermine the NRA, which was created after the previous agency was disbanded over complaints it was too close to the industry.

Minister of Economy, Trade and Industry Toshimitsu Motegi said last week he expects some reactors to go back online in autumn. But he quickly downplayed his own comments later in the week, saying only: "The NRA makes its decisions independently."

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