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Japan breathed a collective sigh of relief when the government declared that the worst of the Fukushima Daiichi nuclear disaster was over – the stricken plant having reached a more-or-less stable state known as “cold shutdown”.
Six months on, a fresh wave of safety concerns, focused on what some experts say are vulnerabilities in a pool housing spent uranium fuel, has put the government and Tokyo Electric Power (Tepco), the plant’s owner, on the defensive. It is also highlighting the daunting challenges that remain before the site, devastated by the March 2011 earthquake and tsunami, can be fully secured.
Tepco allowed journalists inside the wreckage-choked Unit 4 building for the first time at the weekend, in an effort to assuage growing public concern. Unlike Units 1-3, Unit 4’s reactor was offline when the tsunami swamped the plant and knocked out its cooling systems, and it was not initially thought to pose a significant threat.
Within days of the tsunami, however, its lightly protected pool of more than 1,500 fuel rods began to overheat, and hydrogen explosions blew out parts of its upper walls. Had crews not acted quickly to stabilise the pool – along with three less-crowded storage tanks in Units 1-3 – experts say the uranium might have melted, releasing massive amounts of radiation directly into the atmosphere.
Today, a narrow makeshift staircase leads to the exposed upper floor, which overlooks the mouth of the pool, now covered by a white tarpaulin. On the south side of the building are earth-moving equipment and a crane, which workers plan to use to erect a giant cover over the reactor, something they have already done at Unit 1.
The hope is that the cover will keep radioactive material from scattering when the fuel rods are removed from the pool, a task that is to start in the second half of next year and could take up to three years to complete.
Goshi Hosono, the cabinet minister overseeing work at Fukushima Daiichi, endorsed Tepco’s view that the building would withstand another strong earthquake like the one that struck last March. Still, he said he had urged Tepco “not to take an optimistic view, but to deal with it strictly in order to of ensure safety”.
Concerns about the structural integrity of Unit 4 have been expressed almost since the crisis began. The stored fuel rods must be kept cool by constantly cycling fresh water through the pool, and separated from each other to prevent a dangerous atomic chain reaction. If the structure were to give way in an aftershock, even partially, it could become impossible to do either.
Last year Tepco reinforced the pool’s floor, by installing rows of steel columns on the level below, but the issue flared up again a month ago after Ron Wyden, a US senator, visited the plant. In letters to Hillary Clinton, US secretary of state, and the Japanese ambassador to the US, among others, he declared the situation “precarious” and said he feared a loss of containment that would result in an “even greater release of radiation than the initial accident”.
The subsequent discovery that one wall of the Unit 4 building has buckled outward by 3 cm, a result of explosions or the earthquake or both, added to concerns.
This month Tepco took further steps to reinforce the structure, including filling part of the level below the pool with concrete, which it says have improved the building’s integrity by 20 per cent. Still, with the uranium likely to remain in the ravaged building close to the open air for several more years, worries have persisted.
“It’s important to move the fuel rods out and get them to another location as soon as possible,” according to Hiroaki Koide, an expert on nuclear plant construction at Tokyo University.
At the weekend, Mr Hosono said Tepco and the government were examining ways to move more quickly. But he warned that, given the complex nature of the operation, “rushing” could create its own dangers. “We need to look at how quickly we can move without compromising the stability of the process,” Mr Hosono added.