Workers are nowhere close to determining the state of melted fuel at the Fukushima No. 1 nuclear plant, a year after the government declared the damaged reactors were in a “cold shutdown” state.
Storage tanks at the site are nearing capacity for radioactive water. A makeshift system is still being used to cool the nuclear fuel. And leaks of contaminated water and quake-induced collapses of plant facilities remain a threat.
Although progress has been made in clearing rubble and reducing the amount of radioactive substances released from the plant, NRA Chairman Shunichi Tanaka acknowledged that preparations to decommission the reactors are only slowly getting under way.
"Workers have been obliged to respond with highly stopgap measures," Tanaka said. "Many devices, such as a purifier for radioactive water, have been installed with no time for sufficient design considerations and safety screenings.
“The situation surrounding the decommissioning process is volatile, so there is a need for constant reviews in securing safety."
The No. 1, No. 2 and No. 3 reactors at the Fukushima plant melted down after the Great East Japan Earthquake struck off the coast of northeastern Japan on March 11, 2011, spawning a tsunami that devastated coastal communities and knocked out power to the plant.
After a furious battle to bring the situation under control, Prime Minister Yoshihiko Noda on Dec. 16 last year declared that the three reactors at the plant had reached a state of cold shutdown, a term used when a normally operating reactor is taken offline and remains at a cool temperature on a stable basis.
But the decommissioning process, including the No. 4 reactor that contained no fuel at the time of the disaster due to a regular inspection, is expected to take decades to complete.
The decommissioning work also represents an imminent challenge for the Liberal Democratic Party, which will control the government following its victory in the Dec. 16 Lower House election.
The government and Tokyo Electric Power Co., the plant operator, presented a road map on Dec. 21 last year that established landmarks for the eventual decommissioning of the four reactors.
Goals for the period to spring 2013 included endoscopic inspections of the interiors of reactor containment vessels and a reduction in the length of pipes used in the "circulating water cooling" system, which recycles radioactive water to cool down the melted reactors.
Endoscope surveys of the containment vessels at the No. 2 reactor in January and the No. 1 reactor in October found radiation levels high enough to kill a human within one hour. Specifically, up to 73 sieverts per hour was detected inside the No. 2 reactor and 11 sieverts per hour inside the No. 1 reactor.
But TEPCO cannot determine the state of the melted fuel because cameras can only be inserted for a limited time period in the extremely hazardous environment.
One immediate problem facing TEPCO is the accumulation of radioactive water used to cool down the melted fuel. TEPCO says it will mobilize robots and take other measures to locate where the radioactive water is leaking from the reactors.
Storage tanks on the plant's premises have a total capacity of 257,000 tons. As of Dec. 11, the tanks contained 237,000 tons of radioactive water.
TEPCO plans to build additional tanks on deforested land to expand the total capacity to 700,000 tons within three years.
Groundwater flowing into the reactor buildings is exacerbating the radioactive water problem. TEPCO said it will dig wells west of the reactor buildings to pump up the groundwater and reduce the inflow, but little is known about groundwater flow variations, sources said.
The 4 kilometers of pipes in the "circulating water cooling" system were installed on a temporary basis in the frantic battle to keep the melted fuel submerged. They remain in the same state, and the risk of radioactive water leaking from damage on the pipes remains.
TEPCO is preparing full-scale operations of a device that can eliminate 62 varieties of radioactive substances from the contaminated water. But the device is still being tested for durability, and the government's Nuclear Regulation Authority has yet to give the green light for its use.
Rubble has been removed from the No. 4 reactor building, which was severely damaged in a hydrogen explosion in the early stages of the disaster and received relatively light contamination from radioactive substances.
TEPCO removed two nuclear fuel assemblies from the No. 4 reactor building's storage pool on a trial basis in July. The assemblies showed no signs of damage or deformities, and the utility plans to start removing the remaining fuel in November 2013.
Still, about 3,100 nuclear fuel assemblies, including unspent ones, are now sitting in the storage pools of the No. 1 through No. 4 reactor buildings.
The amount of radioactive substances released from the reactor buildings has remained low since February. In November, a maximum of 10 million becquerels were leaking from the No. 1 through No. 3 reactors per hour, only one-sixth the discharge rate in December 2011.
But Fumiya Tanabe, a former chief research scientist at the now-defunct Japan Atomic Energy Research Institute, said persistent danger surrounds the plant's reactors.
"Despite the (officially declared) cold shutdowns of the reactors, the cooling functions have been maintained there with no knowledge of where the melted fuel lies and in what state," Tanabe said. "There is a risk of unforeseen circumstances arising if another major earthquake hits."
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