16 Août 2013
August 11, 2013
At Chubu Electric Power's Hamaoka nuclear power plant in Omaezaki, Shizuoka Prefecture, preparations are being made to dismantle and remove the first and second of its five reactors.
"Move the apparatus!" "You got it!" The voices of workers echo around the inside of the structure that houses the second reactor. An 88-ton carbon steel cylindrical capsule more than six meters long is suspended from a crane and slowly lifted 26 meters to the fifth floor.
Twenty-two spent nuclear fuel units held in a pool on the fifth floor are collected into the capsule. Then the capsule is moved in a purpose-built trailer to the pool for the fifth reactor, which is located within the same site. The capsule has screw-like grooves carved into them to allow the heat produced by the fuel to escape.
Yoshiaki Shimizu, 55, the decommissioning project's group chief, says no radiation would leak out if a capsule fell to the floor during the operation.
"What's more, we have devices on standby just in case that will prevent radiation from leaking outside," he says.
If spent nuclear fuel is not extracted from the inside of a reactor, it cannot be taken apart. The extraction process is fraught with danger due to the potential of radiation exposure to workers and radiation leaks. Large capsules are used, and the process is carried out cautiously.
When the second reactor ceased operating in 2009, there were 1,312 nuclear fuel units held in the fifth floor pool. The transfer of just one took several days, and 724 were left as of late June. The operation will end by late February 2014 at the earliest. The pool for the first reactor has already been emptied.
A number of workers measured the radiation level inside the structure housing the second reactor. They spent several months checking 10,000 spots to create a contamination map of the entire facility. The careless destruction of a structure in a radiation controlled area would be disastrous.
To date, five of Japan's key nuclear reactors have ceased operations in order to be decommissioned, including the two reactors at Hamaoka. The dismantling and removal of the 12,500-kilowatt Japan Power Demonstration Reactor at Tokaimura, Ibaraki Prefecture, was completed in 1996. Currently, the Japan Atomic Power Co. (JAPC)'s 166,000-kilowatt Tokai Power Station at Tokaimura and the Japan Atomic Energy Agency (JAEA)'s 165,000-kilowatt Fugen advanced thermal reactor prototype in Tsuruga, Fukui Prefecture, are in the midst of being decommissioned.
The Hamaoka plant's first (540,000 kilowatts) and second (840,000 kilowatts) reactors are much larger than its other three. Additionally, the reactors that are being decommissioned and those that are scheduled to continue operating are on the same site. Such a case is the first for Japan.
"Our staff will be able to experience running a reactor at the same time as being involved in shutting another down," says Motonori Nakagami, 55, a group chief in Chubu Electric Power's nuclear power division.
The decommissioning process is running slightly behind schedule. This is because many of the approximately 2,300 staff that were to be involved in shutting down the first and second reactors have been preoccupied with inspections of the third, fourth and fifth reactors, which have been stopped since the accident at the Fukushima No. 1 nuclear power plant.
The combined cost of decommissioning the first and second reactors has been estimated at 84.1 billion yen ($840.8 million). It will be a lengthy endeavor, taking nearly 30 years. Throughout that period, their functions that prevent radiation leaks will continue to be maintained, and the structure itself will be constantly repaired as it is used.
The decommissioning of the Tokai reactor, which ceased operating in 1998, is at a more advanced stage. All of its spent nuclear fuel has been removed, and the surrounding apparatus is being dismantled as the JAPC waits for the radiation level inside to go down.
The disassembly of the reactor's four heat exchangers began in 2006. These cylinders, 25 meters tall and six meters in diameter, are being taken apart by slicing them into rings from the bottom up and removing them one at a time. Workers observe from a separate room as a French-made robot cuts into the cylinders. The dismantling of the reactor will begin from fiscal 2014, and the entire process is scheduled for completion in fiscal 2020.
Power companies want to minimize decommissioning costs as much as possible. "Rather than developing new apparatus, we're using a combination of technology that's already available to us to rationalize the process," says JAPC division manager Toyoaki Yamauchi.
Cost reduction is also a priority for the decommissioning of the Fugen reactor. According to the JAEA's Shigetoshi Iwanaga, 59, "We're drawing on foreign expertise as we consider the cheapest way of taking it apart."
(The first part of this article was written by GLOBE writers Norito Kunisue and Hitoki Nakagawa.)
UKRAINE: CAN MELTED NUCLEAR FUEL BE EXTRACTED?
The worst nuclear disaster in history occurred in April 1986 at the Chernobyl plant in Ukraine. Its fourth reactor exploded during a test run, releasing 5,200 quadrillion becquerels of radioactive material in the first 10 days, roughly sixfold that of the accident at the Fukushima No. 1 nuclear power plant.
Just as with ordinary nuclear plants, the disassembly and removal of the reactors that caused these catastrophes cannot begin until their nuclear fuel is extracted.
Even today, 27 years after the Chernobyl accident, the fourth reactor still contains nuclear fuel that has melted and solidified. It produces a level of radiation that would instantly kill anyone who approached it, and cannot be touched until a suitable robot is developed. Extraction will be impossible for quite some time, and the reinforced concrete “stone coffin" that entombs the structure and prevents radiation from leaking out is the only measure that can be taken at present.
Today, crevices and cracks can be found all over the stone coffin. Rainwater that has flowed inside in large amounts is thought to have become contaminated and started leaking into the earth. A major repair effort was carried out from 2004 to 2008, reducing the threat of collapse for the time being. Construction is proceeding on a massive shelter that will cover the stone coffin in order to carry out more permanent countermeasures. There are plans to tear down the stone coffin and remove the nuclear fuel when the shelter is completed in 2015, but there is no concrete schedule.
* * *
Tokyo Electric Power Co. has made a decision to decommission reactors No. 1 through No. 4 at its Fukushima No. 1 nuclear power plant.
TEPCO and the Japanese government are planning to extract the nuclear fuel that is thought to have burned through pressure vessels within reactors No. 1 through No. 3 and remove the structures surrounding them. Thirty to 40 years later, the site will be turned into a vacant lot.
On June 27, the government and TEPCO revised their mid- to long-term decommissioning road map. With regard to when the fuel in reactors No. 1 through No. 3 would be extracted, it states that work on reactors No. 1 and No. 2 will begin in the first half of fiscal 2020 at the earliest, and on reactor No. 3 in the latter half of fiscal 2021. This meant that the extraction process for reactors No. 1 and No. 2 will commence around 18 months earlier than was initially planned.
In reality, there is no technical evidence to support these projections. As a form of “insurance," the plan also states that the commencement date will be shifted back to the first half of fiscal 2024 in the event that little technological progress is made.
To extract nuclear fuel, it is necessary to fill a reactor with water to block the radiation. For that purpose, the damage the reactor has sustained must first be ascertained and then repaired.
It is not yet known where and to what degree it is damaged, or the condition of the fuel that is thought to have burned through its vessels. Surveys were attempted using a remotely controlled robot, but to little success.
Some melted fuel escaped from its pressure vessel and burned through into the containment vessel. There are three reactors where this has occurred, and a wide range of experts will have to pool their knowledge and devise countermeasures in order for the decommissioning process to move forward.
However, more than two years on from the disaster, the system in place for developing fuel extraction technology is still inadequate.
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The second reactor at the Three Mile Island nuclear plant near Middletown, Pa., where a partial meltdown occurred in March 1979, has not yet been dismantled and removed. Even so, the process is at a more advanced stage than at the Chernobyl and Fukushima No. 1 plants.
Several years after the accident, camera inspections of the reactor core commenced, and a work program was carefully devised with input from the nuclear industry, the government and academia. Fuel extraction began in 1985, and by 1990, almost all of it was successively removed. Around half of the fuel was said to have melted through.
Meanwhile, the station's first reactor, located on the same site, has been approved for operation until 2034 and continues to run. To efficiently decommission the reactors, the second will be disassembled at around the same time as the first. The process itself will not begin in earnest for some time down the road.
(The Ukraine portion of this article was written by Naoatsu Aoyama, GLOBE staff writer, and Hisashi Hattori, senior staff writer of The Asahi Shimbun.)