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Science

Is there life after 40 for aging reactors?

TOKYO -- Across Japan, nuclear power plants that went online around the mid-1970s are approaching their government-mandated 40-year service period.

Osaka-based Kansai Electric wants to extend the service period of its Takahama Nos. 1 and 2 reactors in Fukui Prefecture beyond the 40-year limit.

     While power companies have decided to decommission some of their aging facilities, the Osaka-based Kansai Electric Power has applied to extend the legal life span to up to 60 years for three reactors. 

     But squeezing another two decades of use out of the facilities is an ambitious goal. Just like with machines, nuclear plants can become rickety and prone to breakdowns after years of regular use. But the stakes are far higher when things go wrong in a facility containing hazardous radioactive material.

Game changer     

The March 2011 disaster at Tokyo Electric Power Co.'s Fukushima Daiichi nuclear plant prompted the government to introduce a 40-year cap on the service period of nuclear reactors. The change came as part of revised rules -- introduced in July 2013 -- on nuclear source material, nuclear fuel material and reactors.

     In line with the new guidelines, several power companies decided in March to decommission five reactors, including Japan Atomic Power's Tsuruga No. 1 reactor, in Fukui Prefecture. The reactor was built when the country introduced commercial-use nuclear plants nearly half a century ago.

     However, reactors do not necessarily have to be decommissioned immediately upon turning 40. If approved by the Nuclear Regulation Authority (NRA), power companies can extend the service period to up to 60 years. That is what Kansai Electric is hoping to do with the Takahama No. 1 and No. 2 reactors and the Mihama No. 3 reactor, all in Fukui Prefecture.

     To get the extensions approved, Kansai Electric must prove that the reactors meet all the safety standards. The first step is showing that the facilities are structurally sound. Because reactor containment vessels play host to nuclear fission to generate massive amounts of energy, they are by necessity made of strong materials. Still, concerns about deterioration arise when the reactors have been in use for decades.

     For example, metals used in reactor chambers can become brittle after years of being exposed to neutrons generated during fission.

Up to scratch?     

To prove that their older reactors are still structurally sound, power companies must carry out a set of special checks, such as ultrasound examinations, and undergo an assessment by the NRA. The nuclear watchdog probes containment vessels and concrete structures for signs of weakness or corrosion.

     Also, older reactors may not meet the standards introduced after the Fukushima accident. For instance, power cables in reactors that went online in the 1970s are not made of flame-resistant materials, a requirement under the new rules.

     Power companies around the world have stepped up the use of flame-resistant cables after a fire broke out in 1975 at the Browns Ferry Nuclear Plant in the U.S. state of Alabama. In Japan, the practice became standard in the 1980s, but nuclear plants built before then were not subject to this rule. The work-around for aging reactors has been to use fireproof agents and other methods to keep them online.

     The containment vessels at Kansai Electric's Takahama Nos. 1 and 2 reactors are older models. Because they are not as structurally robust as newer types, they would be less effective at preventing radiation from being spewed into the air should an accident occur. "As a countermeasure, we will install dome-shaped roofs made of ferroconcrete on top," said a Kansai Electric official.

     Experts say a single reactor uses several hundred kilometers' worth of power cables. That means it is virtually impossible to replace them, and the containment vessel, wholesale.

     Power companies claim they can meet all the safety standards by taking certain extra measures. But NRA Chairman Shunichi Tanaka has his doubts, calling such retrofitting "quite difficult."

      In addition to Takahama Nos. 1 and 2 and Mihama No. 3, Japan has four more reactors that will reach 40 in a few years. Receiving NRA approval before that time comes will require clearing some high technical hurdles. Their ability to keep operating is far from guaranteed.

"Best" energy mix     

And the hurdles are not just technical. How long Japan's nuclear plants will remain in operation is closely tied to the government's blueprint for the future power supply. Under the plans for the country's "best" energy mix for fiscal 2030, released June 1, the government envisions nuclear power making up 20-22% of Japan's energy needs. To achieve that goal, some of the older reactors would need to stay online.

     Industry experts argue that there is little scientific basis for capping the operation period at 40 years. "Ensuring safety is quite important, but the majority of nuclear reactors around the world continue to operate beyond 40 years," said Hokkaido University professor Tadashi Narabayashi.

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