Workers wearing protective suits conduct training to secure electricity with power source car in the nighttime at the tsunami-crippled Fukushima Daiichi nuclear power plant in Fukushima prefecture, in this handout picture taken on December 7, 2011.??[Photo/Agencies]

TOKYO - The Japanese government may inject about $13 billion into Tokyo Electric Power Co. as early as next summer in a de facto nationalisation of the operator of the crippled Fukushima nuclear power plant, sources said on Thursday.

Tepco's future as an independent firm has been in doubt since an earthquake and tsunami wrecked the plant in March, triggering the world's worst nuclear crisis in 25 years and leaving it with huge compensation payments and clean-up costs.

In addition to public capital, the government and Tepco will also seek additional loans from banks, sources said, but the full scale of any Tepco bailout remains unclear.

Some analysts expressed doubt that the government would take the drastic step of taking control of the giant monopoly, which still has political clout, but the idea has proponents in some sections of Japan's ruling party.

"You have an essentially bankrupt operation, and if you are going to save it, it's going to cost a lot," said Andrew Dewit, a Rikkyo University professor who writes about energy policy.

"You've got a very bad picture getting worse, and dithering just ups the cost."

Tepco President Toshio Nishizawa was mentioned as saying a public fund injection could not be ruled out. "It is better to keep all options, so I don't deny it," Kyodo news agency quoted him as saying in an interview on Thursday.

Tepco has made progress in bringing the Fukushima plant under control and is on track to declare a "cold shutdown" -- when water used to cool the reactors is stable below boiling point -- before the end of the year.

But decommissioning four reactors at the plant is set to cost at least 1.2 trillion yen ($15.4 billion), a sum that would render Tepco insolvent if drastic measures to shore up its financial base were not taken, media reports said.

Energy Policy Reform

The Mainichi newspaper reported earlier on Thursday that the government planned to inject at least $13 billion and perhaps as much as $27 billion, while Kyodo news agency said the total bailout could reach 3 trillion yen ($38.5 billion) over four years, with half coming from private borrowings.

A government-run bailout fund would buy new stocks such as preferred shares to be issued by the utility, sources said.

Shares in Tepco slid as much as 17 percent before regaining some ground to end down 11 percent at 244 yen.

"The report got investors worried that Tepco could possibly become insolvent," said Hiroyuki Fukunaga, CEO of Investrust. ?"If they need 1 trillion yen to avoid that, then the money is not coming from anyone but the government."

Tepco would need to get shareholder approval to raise the ceiling on the number of shares it can issue at its next annual meeting in June.

To cover costs, Asia's biggest utility is pushing for hikes in electricity charges. It also wants permission to restart nuclear reactors, particularly those that have been idled at its biggest plant, Kashiwazaki-Kariwa.

Five of the seven reactors at that plant are off-line for checks or repairs, and two more are scheduled for planned shutdowns before May 2012.

But the ruling party has concluded that the public would be unwilling to accept higher electricity fees, particularly at a time when it is being asked to accept a hike in the sales tax to cover social security costs, Mainichi said, while restarting idled reactors is difficult due to public fears about safety.

Pushing Tepco to accept capital would also allow the government to pursue drastic reform of energy policy, including separating power generation from distribution, the paper said.

But experts questioned whether the two would necessarily go hand in hand. The government aims to finish a review of national energy policy, including nuclear power, by next summer.

"Nationalisation could be a first step toward such reforms as splitting generation and distribution," said Rikkyo University's Dewit. "But whether this is a trial balloon and gets shot down in the short run, who's to say?"

The Mainichi report said a government panel led by Chief Cabinet Secretary Osamu Fujimura could, in the new year, announce plans to inject public funds, though Fujimura told reporters the issue of public funds was not now on the table.

Tepco is due to announce new steps in the coming days, which include an increase in its planned cost cuts over 10 years by 100 billion yen to 2.64 trillion yen as well as the sale of a thermal power plant, a source has said. ?($1 = 77.7300 Japanese yen)