South Korea has idled two nuclear power plants after finding that test results for crucial control cables were falsified in a new blow to an industry mired in a graft scandal and safety lapses.

South Korea’s trade and energy ministry said Tuesday a company contracted to conduct tests fabricated the results for cables that failed to meet international standards for capacity to withstand changes in voltage and pressure. It warned that the plant shutdowns would result in summer power shortages.

The cables control valves that are responsible for cooling nuclear fuel or preventing the release of radioactive materials during an emergency. Another four nuclear reactors that were either shut down for scheduled maintenance or under construction were also using cables that had failed the tests.

“If these control cables do not operate well during an emergency, we viewed that it would not guarantee to cool nuclear fuels or to shut off radioactive materials,” South Korea’s Nuclear Safety and Security Commission said in a statement.

It said the cables, which were in use since December 2011, failed nine of 12 tests pertinent to their operation in a “loss of coolant accident.”

Han Jinhyun, vice trade and energy minister, declined to name the company while the government’s investigation is ongoing. The ministry will sue the company and also ask prosecutors to launch a probe, he told a press conference.

The revelations add to public worries about nuclear safety and power shortages during the summer when demand is at its peak. They are a new blow to South Korea’s ambitions to export its nuclear technology.

With the shutdown of the Shin-Kori No. 2 and Shin-Wolsong No. 1 reactors to replace cables, a total of 10 nuclear plants are now offline.

The minister said it would take around four months to replace the cables and warned “unprecedented power shortages” are expected in coming months.

“There is no means to increase power supply in the short term, so we expect we need to lower demand considerably to weather the crisis,” he said.

Last year, the South Korean nuclear industry was rocked by revelations that thousands of components used in nuclear plants had falsified quality certificates. Dozens of employees at state owned nuclear power plant operator, Korea Hydro & Nuclear Power Co., were prosecuted for taking bribes from contractors to accept substandard parts and machinery.

The investigation into the cable problems began after the nuclear safety commission received tips through a whistleblowing channel that was set up in the wake of last year’s scandal.

“This incident is more serious than previous scandals because it is wrongdoing by a company that is supposed to oversee products,” said Kim Ik-jung, a medical professor at Dongguk University who has become prominent as an anti-nuclear activist since the government decided to build a nuclear waste dump in Gyeongju city where he lives.

“Corruption is widespread in the nuclear industry because there is no agency that can truly regulate Korea Hydro & Nuclear Power,” he said.

South Korea has 23 nuclear power plants which supply about 30 percent of its energy and plans to add another 11 reactors by 2025.