Fukushima Units 1, 2, And 3 Were Completely Exposed, Melting Was Extensive If Not Complete

Utility: Fukushima Cores More Damaged Than Thought (Science Insider, May 17, 2011):

TOKYO—Over the last several days, evidence has emerged indicating that the situation at the Fukushima Daiichi Nuclear Power Plant was far more dire than previously recognized. The main evidence is extensive—rather than partial—melting of the nuclear fuel in three reactors in the hours after the 11 March earthquake and tsunami. Despite that bad news, however, today plant owner Tokyo Electric Power Co. pledged it would still meet the target set 17 April to stabilize the situation by January 2012 so 100,000 residents evacuated from around the plant can return to their homes and the decade-long process of demolishing the reactors can get started.

At first, analysts from Tokyo Electric and the government believed there was only limited damage to the fuel cores. But over the last week, a combination of robotic and human inspections has led to the conclusion that the fuel assemblies in units 1, 2, and 3 were completely exposed to the air for from over 6 hours to over 14 hours and that melting was extensive if not complete. Much of the fuel is now likely at the bottom of the reactor pressure vessels.

Despite extensive melting of the fuel, “we do not believe there is massive damage to the reactor pressure vessel,” Sakae Muto, Tokyo Electric’s chief nuclear officer told reporters this evening. Last week workers found that an estimated 3000 tons of water has leaked from the unit 1 containment vessel into a basement. In its 17 April roadmap, Tokyo Electric envisioned flooding the containment vessel and building a new cooling system to lower the temperature of the core. But the containment vessel now appears to be too leaky for that scheme to work. Instead they will collect water from the basement, purify it, and inject it back into the reactor pressure vessel, from where it will leak back into the basement. It is simpler than a new cooling system, but it will also require additional measures to watch for and counter leaks of contaminated water into the environment, Muto said. They may still build new cooling systems to supplement or replace the water injection scheme.

The 17 April roadmap for containing radiation also called for wrapping the wrecked buildings in tentlike structures of polyester sheets supported on a steel framework. That part of the work is proceeding as planned.

At the same press briefing, Goshi Hosono, a member of the Japanese parliament and a special advisor to the Prime Minister Naoto Kan on the crisis, said the government had decided to establish a mechanism to track the radiation doses and manage the long-term health care of the hundreds of workers battling to bring the crippled reactors under control. “This is not only in the interests of Tokyo Electric and the government but of all the people of Japan,” he said.

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