Diesel generators are a critical failsafe against loss of coolant during a catastrophic reactor accident, as bitterly illustrated by Fukushima, where a tsunami knocked out back-up and diesel generators, resulting in a triple meltdown.

The shutdown of the reactors comes as unwelcome news to OKG, the utility that runs the plant as Sweden marshals its energy resources with the onset of the Scandinavian country’s coldest months yet to come.

Bellona general manager and nuclear physicist Nils Bøhmer said the discoveries “show that Sweden’s ageing nuclear industry is coming to a point where it can no longer reliably supply power.”

The Oskarshamn plant was commissioned in 1972, and Bøhmer said the engineered lifespan for Sweden’s reactors is 32 years, so they are pushing their limits.

“Sweden therefore has to have a serious debate about its power future – whether it will continue with nuclear, or make up the difference with renewable sources,” he said.

“Cost overruns at the Olkiluoto nuclear power plant in Finalnd for the construction of the European Pressurized Reactor are already at 100 percent of the original projected cost,”said Bøhmer, adding, “Sweden needs to consider whether it is willing to burden itself with such costs should it build new nuclear reactors.”

OKG hopes to have Oskarshamn reactor No. 1 running by January 15, World Nuclear News reported. The fate of Oskarshamn 2, or O2, however, remain unclear, however, after one of the back-up diesel generator units failed a 48-hour safety test.

“We decided that Oskarshamn nuclear power plant should take nuclear reactor O2 offline immediately,” Sweden’s Radiation Safety Authority told Bellona in an emailed statement.

While stressing there was “no immediate danger” as a result of the failed generator tests, safety inspection chief Leif Karlsson told the TT Swedish news agency that: “The power supply to the reactors is extremely important. This was one of the main problems at Fukushima.”

“If you don’t have this system running, you cannot add water to the reactor,” Karlsson said.

He said OKG would be allowed to switch on the reactor again before repeating the test run and also checking to ensure the second generator undergoes a maintenance check, the TT agency reported.

Oskarshamn 1 had only recently restarted after over a year out of service following problems with vibrations in the turbine system and the detection of damage to reactor vessel internals during the plant’s regular maintenance outage, said WWN.

The unit was resynchronised to the grid on November 29 and undergoing final tests prior to resuming full operation. Earlier that month, the Oskarshamn plant had anticipated having all three of its reactors online for winter, said the agency. 

In-service tests revealed the problems with the diesel generators, which failed to start in the specified time frame. Emergency back-up generators must be able to start up reliably at a moment’s notice, noted WWN. To address the problem, OKG says it will either replace or reconstruct the generators’ starter motors, but warned that whichever option is followed the work will take at least one month to complete.

Oskarshamn 2 will be kept off line for work on its backup generators. Swedish regulators ordered an immediate shutdown of the unit on December 6 and the Radiation Safety Authority said that it would remain offline, as the nuclear power plant had been unable to show that “equipment essential for ensuring safe operation” fully met requirements for operability and reliability, WNN reported.

According to a statement from the Radiation Safety Authority, one of the diesel generators had not been serviced as scheduled in 2011, preclusing OKG from carrying about additional maintenance tests on the backup generators.