19 may 2011

Doubt over meltdown dispelled

By Jonathan Soble in Tokyo, Financial Times, May 18 2011

In the first days after the Fukushima Daiichi nuclear power station began spewing radiation in mid-March, the plant’s operator and Japanese safety regulators studiously avoided the word “meltdown”.

Yes, they said, uranium fuel rods in the tsunami-hit facility’s reactors might have been damaged after cooling systems failed. But the official view was that the rods were still mostly intact – and radioactive material was safely contained inside their zirconium sheaths.

Now, a little over two months later, new information on the state of Fukushima Daiichi’s three overheated reactors is making the m-word impossible to avoid. Fuel inside the cores, it is now understood, melted far more quickly and extensively than was initially believed – disintegrating just a few hours after the tsunami knocked out the plants electricity and cooling systems.

Tokyo Electric Power, Fukushima’s operator, says there may be little left of the rods at all – just clumps of uranium at the bottom of the reactors’ innermost steel containers. Some of the melted fuel may have leaked into the concrete vessels that form the next layer of protective containment, making for a meltdown by even the narrowest industry standards.

On Wednesday Naoto Kan, prime minister, said Tepco was working on the assumption that some fuel from Fukushima Daiichi’s No 1 reactor core had leaked out. On Monday Haruki Madarame, chairman of the Nuclear Safety Commission, said: “Our understanding is that the No 2 reactor melted down. We surmise that the No 3 reactor is in the same situation.” The darkening picture of conditions inside the cores – which has emerged since Tepco began sending workers into the reactor buildings for the first time last week – has added to doubts about whether Tepco and the government disclosed all they knew in the early days of the crisis.

“Why did it take two months to get to this point?” the Nikkei newspaper asked in an editorial on Wednesday. “Even a rough calculation of conditions inside the reactors would have helped in choosing the best response in the shortest time.” One person close to the government’s response effort said Tepco and bureaucrats who oversaw nuclear safety used the fact such calculations were inherently speculative as an excuse to withhold bad news. “Everyone was trying to avoid responsibility.”

Tepco says it was able to reach its new conclusions only after engineers managed to get close enough to the cores to read water-level gauges and other instruments. But independent experts in Japan and abroad had reached similar conclusions much earlier, using computer models and contamination readings from around the plant.

It is unclear whether a quicker understanding of the situation would have changed the course of events. Within one and three days of the tsunami, hydrogen given off by the melting of the fuel rods’ zirconium housings caused radiation-releasing explosions at each of the overheated units. Experts have criticised Tepco for being too slow to vent pent-up gas outside the plant. But the utility says it worked as fast as it could, given the lack of electrical power, and it is still not understood exactly when and how the hydrogen escaped – some believe the venting effort contributed to the leaks.

Japan is standing by previous assessments of the environmental damage caused by the accident, which are based on contamination readings in areas around the plant rather than the amount of fuel melting. Fukushima Daiichi is believed to have released about a 10th of the radioactive material the Soviet Union’s Chernobyl facility did in 1986. Most of the contamination occurred in the first few days after the tsunami.

Technicians have also discovered thousands more tonnes of radioactive water in basement rooms and tunnels at the plant than they believed were there a week ago. Tepco insists it can still achieve a safe “cold shutdown” by January, but has been forced to make changes to its strategy for cooling the reactors.

The possibility fuel has leaked into the units’ containment vessels has forced it to abandon plans to fill the normally dry areas with water, a process known as “water entombment”. Instead, it will focus on decontaminating the flood water and cycling it as coolant through the cores.

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