The accident at the Fukushima No. 1 nuclear power plant has brought to light the cascading problem of spent nuclear fuel that threatens to overwhelm Japan's nuclear power plants.
Local governments are demanding that electric power companies remove the spent nuclear fuel from nuclear power plants, but plans for a reprocessing facility and an off-site storage facility are on hold.
According to a survey by The Asahi Shimbun, while the nation's 17 nuclear power plants are capable of holding 83,000 spent nuclear fuel assemblies in storage pools, 70 percent of the combined storage capacity has already been used.
Tokyo Electric Power Co.'s Fukushima No. 1 nuclear power plant has the highest ratio of 93 percent, followed by Japan Atomic Power Co.'s Tokai No. 2 nuclear power plant in Ibaraki Prefecture with 86 percent.
The figure exceeds 60 percent at 10 other nuclear power plants.
If storage pools are filled, a nuclear power reactor cannot continue to operate because newly created spent nuclear fuel cannot be removed from the reactor.
At the Fukushima No. 1 and Tokai No. 2 nuclear power plants, operators have put some spent nuclear fuel in containers and stored them in a separate building on the plant premises.
Spent nuclear fuel needs to be cooled in storage pools because it emits large amounts of heat and radiation even after it is removed from a reactor.
But fuel in storage pools, mainly located in reactor buildings, is separated from the outside only by the buildings' concrete walls, while a pressure vessel within a container vessel holds nuclear fuel in a reactor.
In the accident at the Fukushima No. 1 nuclear power plant, storage pools lost cooling functions, raising the possibility that spent nuclear fuel is damaged and large amounts of nuclear materials are released.
Electric power companies are resorting to a last-ditch measure known as "re-racking."
In storage pools, spent nuclear fuel is usually put into grid-like containers called racks and placed at certain intervals.
Re-racking means that racks are repositioned and intervals are narrowed so that more spent nuclear fuel is stored in a given space.
A survey by The Asahi Shimbun found that electric power companies have taken this approach at 29 nuclear power reactors.
TEPCO repositioned spent nuclear fuel at the Fukushima No. 1 and No. 2 nuclear power plants in the past.
At the Hamaoka nuclear power plant in Shizuoka Prefecture, Chubu Electric Power Co. more than doubled the capacity of the No. 2 reactor's storage pool to 1,820 spent nuclear fuel assemblies from the initial 840.
Currently, the pool contains 1,164 assemblies. The No. 2 reactor, which started operations in 1978, has been shut down for decommissioning.
But experts say narrowing intervals between spent nuclear fuel could increase risks.
Tadahiro Katsuta, associate professor of nuclear engineering at Meiji University, said re-racking is a stopgap measure.
"Nuclear fuel assemblies will be located closer to each other than initially designed," Katsuta said. "At least theoretically, the possibility will increase that criticality (self-sustaining nuclear chain reaction) occurs when assemblies are damaged in a severe accident."
To lower such risk, boron is added to rack materials because the chemical element absorbs neutrons, which cause nuclear fission.
Electric power companies are also considering plans to build intermediate facilities to store spent nuclear fuel in steel containers that can shield radiation.
An official of Kansai Electric Power Co., which operates 11 nuclear power reactors in Fukui Prefecture, said, "We are aware of the need of building an intermediate storage facility and are considering it."
Japan aims to recover plutonium from spent nuclear fuel at a reprocessing facility for use as nuclear fuel again.
The spent nuclear fuel reprocessing facility is scheduled to begin operations in Rokkasho, Aomori Prefecture, in October 2012.
But the plant was suspended during trial runs due to a series of technical problems, and no date has been set for resuming trial operations.
The reprocessing plant has accepted spent nuclear fuel from nuclear power plants, but there is little room left because the amount has exceeded 90 percent of its capacity.
TEPCO and Japan Atomic Power are planning to build an intermediate storage facility in Mutsu, Aomori Prefecture, to accept 5,000 tons of spent nuclear fuel that cannot be held at their nuclear power plants for 50 years.
But construction was suspended after the Great East Japan Earthquake.
"I think the government is considering reviewing safety guidelines," Mutsu Mayor Junichiro Miyashita told a news conference in April. "With such a move in mind, we have to take a cautious stance to the intermediate storage facility."
(This article was written by Jin Nishikawa and Nobuyoshi Nakamura.)
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