On March 26, the Fukushima No. 1 nuclear power plant's No. 1 reactor would have celebrated its 40th birthday, making it a senior citizen, where once, utilities estimated reactors' life spans to be 30 to 40 years.
On March 26, the Fukushima No. 1 nuclear power plant's No. 1 reactor would have celebrated its 40th birthday, making it a senior citizen, where once, utilities estimated reactors' life spans to be 30 to 40 years.
Unfortunately, the events of March 11 following the Great East Japan Earthquake ensured the No. 1 reactor, operated by Tokyo Electric Power Co. (TEPCO), would not live to a happy old age.
Of the three reactors at the crippled plant that were struck by a hydrogen explosion (No. 1 - 3), the aged No. 1 suffered the fastest core damage and an eventual core meltdown--fuel melted and slumped to the bottom of the reactor pressure vessel.
In contrast, the Fukushima No. 2 nuclear power plant, which was hit by the same massive tsunami, was shut down safely. This plant, also run by TEPCO, started generating electricity in the 1980s.
Some experts say age-related problems led to such a serious accident at the older Fukushima No. 1 plant and has raised fresh concerns about the safety of Japan's geriatric reactors.
Many of the operating nuclear reactors in Japan are long past their primes and reaching old age. The badly damaged No. 1 reactor of the wrecked nuclear power plant was one of the oldest operating reactors in this country.
During the next 10 years, 30 percent of the reactors in Japan will be over 40.
While experts are warning about age-related problems of these old facilities, such as reactor embrittlement, policymakers and regulators are working to increase their longevity in the face of increased difficulty of finding locations to construct new atomic plants.
No. 1 reactor of Japan Atomic Power Co.'s Tsuruga power plant in Fukui Prefecture began commercial operations in 1970, making it the oldest among the operating reactors in Japan.
Soon after, No. 1 reactor of the Mihama nuclear power plant, operated by Kansai Electric Power Co., came online, ushering in the nuclear energy era in Japan.
Of the 17 commercial nuclear power plants in Japan, 10 started operations in the 1970s. All the six reactors at the Fukushima No. 1 plant also began operating in the 1970s.
Initially, utilities expected their nuclear reactors to go out of service after 30 to 40 years. But much longer service life is now assumed for the operating reactors.
Of the 54 reactors in service, 19 are over 30 years old, while only five have been in operation for less than 10 years. The average age is 25.
Fukui Prefecture is home to 13 commercial nuclear reactors, including Tsuruga No. 1 and Mihama No. 1, more than in any other prefecture.
Six of them are offline, currently undergoing regular safety inspections. However, Fukui Governor Issei Nishikawa is unwilling to allow them to start running again.
When Economy, Trade and Industry Minister Banri Kaieda visited the prefecture in May, Nishikawa explained his opposition to the resumption of their operations.
"In the case of (the reactors in) Fukui, there are aging problems," he told Kaieda. "We have yet to see clear plans to solve these problems."
Eight of the 13 reactors in the central Japanese prefecture along the Sea of Japan coast have already turned 30.
Tsuruga No. 1, the oldest, is built upon the General Electric Mark 1 boiling water reactor design. Japan Atomic Power planned to shut down the reactor and start the process of decommissioning in 2010.
After it became clear that the planned construction of No. 3 and No. 4 will be delayed, however, the company decided in September 2009 to keep the reactor in service until 2016.
The ill-fated Fukushima No. 1 plant's No. 1 reactor is the same type of reactor as the Tsuruga No. 1. In 1996, the government said nuclear reactors can be operated for 60 years if necessary measures to secure their safety are implemented. The announcement was fully in line with an earlier U.S. decision to stretch the legal life span of atomic plants to 60 years, a move prompted by the fact that no new plants have been ordered since the Three Mile Island accident in 1979.
The government admits that aging causes such safety problems as neutron embrittlement of reactor pressure vessels and wear and corrosion of pipes. But no reactor pressure vessel or the containment vessel of any reactor in Japan has ever been replaced. The Tsuruga No. 1's vessels are still those that were installed when the reactor was built.
The date for decommissioning and dismantling Mihama No. 1 has not been set, either. Last year, Kansai Electric Power decided to eventually scrap the reactor and replace it with a new one under a detailed plan that was to be announced this fall. But the March 11 disaster has forced the company to put the project on hold.
There is growing concern about the safety of the aged reactor in the town of Miahama in Fukui Prefecture, where the facility is located.
During a June 24 meeting of the municipal assembly members, officials of the Nuclear and Industrial Safety Agency stressed the safety of No. 1. But they were bombarded with questions from assembly members who were concerned about the fact that the safety design concept of the reactor differs from those of newer reactors.
They doubt that the reactor is designed appropriately to withstand a huge earthquake and tsunami because when the reactor was built four decades ago, much less was known about the possible effects of a major earthquake and tsunami on nuclear facilities.
This is a serious question for the 280 residents of the some 90 households in the Nyu district of Mihama, which is just one kilometer from the Mihama plant.
Kiyokazu Nakamura, a 71-year-old member of the municipal assembly living in the district, is calling for the decommissioning of the reactor.
"We approved the extension of the reactor's service life to over 40 years on condition that a new reactor to replace it should be built," says Nakamura. "If the successor reactor is not built, the Unit 1 should be scrapped immediately."
The most dangerous problem facing aging atomic plants is the tendency of reactors to become brittle. The steel walls of reactor vessels that cradle tons of radioactive fuel become increasingly brittle over time as a result of absorbing high-energy neutrons released during nuclear reactions.
This radiation damage, known as neutron embrittlement, is measured by a rise in the so-called ductile-to-brittle transition temperature. The higher the temperature, the weaker the material.
A pressure vessel with a higher transition temperature is more likely to crack or fracture when it is cooled down quickly from a high operation temperature. It is a phenomenon similar to the shattering of heated glass when it is cooled quickly.
The 36-year-old No. 1 reactor at the Genkai plant, operated by Kyushu Electric Power Co., is showing alarming signs of growing dangerously brittle. The reactor's transition temperature has been rising steadily from 35 degrees in 1976, reaching 56 degrees in 1993 and jumping to 98 degrees in 2009, according to data compiled by the utility.
Kyushu Electric claims it has confirmed that the reactor will run safely for more than 60 years. The utility also pledged to take appropriate safety measures if it operated the reactor for more than three decades.
But Hiromitsu Ino, a professor emeritus of the University of Tokyo, is sounding the alarm about the reactor's safety.
"If the reactor is hit by a major quake and subjected to emergency cooling, its pressure vessel is likely to break," Ino warns.
Public attention is focused on whether the reactors currently closed for safety checks at the Genkai nuclear power plant in Saga Prefecture will be restarted. The closed reactors are No. 2, which started operations in 1981, and No. 3, which began running in 1994. The two other reactors, No. 4, which came online in 1997, and the oldest, No. 1, are in operation.
On June 29, Saga Governor Yasushi Furukawa indicated his willingness to green light the resumption of the operation of the two reactors after a meeting with industry minister Kaieda.
But Furukawa nevertheless said he was "rather dissatisfied" with the policy debate over the safety measures for aging reactors.
At a July 1 session of the Saga prefectural assembly, Furukawa suggested that he will start discussions on the fate of the No. 1 reactor without ruling out possible decommissioning, saying, "It is time to consider seriously the future (of the reactor)."
Most researchers believe aging weakens nuclear reactors. But they don't agree on how long the service life of a reactor should be.
According to the Japan Nuclear Technology Institute, there are 13 reactors in the world that have been operating for more than 40 years, in such countries as the United States, Britain and India.
Despite this, it is now clear that the government needs to make a sweeping review of its nuclear safety standards, which claim 40-year-old reactors are safe to operate.
(This article was compiled from reports by Takaaki Nishiyama, Toru Saito and Takuya Sumikawa.)