San Onofre nuclear power plant
San Onofre nuclear generating station, as seen from the beach. The plant was closed down in 2012, after tubes carry radioactive water were found to be severely damaged. D Ramey Logan

The cost of decommissioning the faulty San Onofre nuclear power in California is expected to reach $4.4 billion (£2.6bn), and will take at least 20 years.

The power plant in San Clemente, southern California, was closed down in 2012 after an inspection revealed significant damage to pipes that carried radioactive water. A failure of the plant's steam generators led to a small amount of radioactive waste leaking.

The Orange County Register reports that a full investigation into the accident has yet to be completed by the California Public Utilities Commission (CPUC).

After the accident the CPUC commissioned a report into why the generators failed, while those at a similar plant – Diablo Canyon Power Plant, San Luis Obispo County – did not.

The report, carried out by Lawrence Berkeley National Lab researcher Robert Budnitz, showed the plant's generators were made by different companies, and San Onofre's two generators provided more power.

However, the parameters of the CPUC's report mean further questions could not be answered.

In the report Budnitz wrote: "'What error(s) led to the tube failure(s)?' or 'At what stage were those errors made?' or 'Who made those errors?' or 'What might have been done, and by whom, and at what stage, to have averted those errors?'"

According to CPUC spokesperson Andrew Kotch, answering these questions has been ruled out because Budnitz's report "identified no basis to think similar problems existed at the Diablo Canyon facility steam generators."

According to a report by U-T San Diego dismantling San Onofre could be the most expensive process ever undertaken by the nuclear power industry, since it began 70 years ago.

Southern California Edison announced that the dismantling work will begin in 2016. The company's chief executive, Ted Craver, said there was enough money to pay for the work.

Edison plans to store the twin-reactor plant's spent nuclear fuel on site, until the US government can provide a permanent storage for the radioactive waste.

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