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EXCLUSIVE: Fire in Fukushima unit 4 may have been caused by unit 3

Fukushima 16 May.jpgA preliminary analysis suggests that a fire inside the unit 4 reactor building at the Fukushima Daiichi nuclear power plant may have been sparked by hydrogen gas from unit 3, Nature News has learned. The new findings contradict previous reports of serious fuel damage at the unit 4 reactor and may mean that clean up of its fuel will be less difficult than previously feared.

A common ventilation system shared by the two reactors could have allowed explosive hydrogen gas to seep from unit 3 to unit 4, a new analysis from the Tokyo Electric Power Company (TEPCO) concludes. The work is preliminary and could still turn out to be wrong, the company emphasized in a note to industry insiders seen by Nature. But it could explain why fuel inside unit 4 appears mostly intact, contrary to early reports of heavy damage.

Shortly after a massive earthquake and tsunami on 11 March, three operating reactors at Fukushima Daiichi found themselves in serious trouble. The tsunami knocked out the unit 1, 2, and 3 reactors’ backup cooling systems, causing their nuclear fuel to heat up and melt. As it did so, it released volatile hydrogen gas, which built up inside the reactors. When the gas was vented to relieve the pressure, it triggered a series of massive explosions that destroyed their outer buildings.

Just hours after unit 3 exploded, a fire was reported at the unit 4 reactor. Unlike the other reactors, unit 4 was in shutdown at the time of the emergency. Its fuel was being temporarily stored in the spent fuel pool above the reactor itself. When the fire broke out, most observers (including Nature) concluded that the water inside unit 4 had dropped to dangerously low levels, sparking a meltdown and release of hydrogen in a manner similar to the operating reactors.

Because the fuel was not inside any sort of containment, some feared that it was at risk of spreading radiation far and wide. A recent analysis by one physicist suggested that unit 4 may have been home to short bursts of nuclear activity.

But those sorts of dire warnings don’t square with the latest video of the spent fuel pool at unit 4 (see below). The video seems to show the fuel from the reactor largely intact, aside from some debris (such as a ladder that likely fell into the pool as a result of the quake or fire).

With so much fuel unmelted, the obvious question is: how did the fire start? According to TEPCO, the cause may have been hydrogen gas from unit 3. Units 3 and 4 apparently share a common Standby Gas Treatment System (SGTS)—a system of pipes and filters designed to pump air out of the buildings holding the reactors.

After the accident at unit 3, hydrogen gas would have flowed through the SGTS. Much of the hydrogen is likely to have left through one of the iconic, pylon-like chimneys at the power plant. But the new analysis shows that a significant portion could have flowed from unit 3 to the unit 4 building. When enough gas had built up, it could have started the fire.

It is an interesting theory, though questions remain. TEPCO says they’re still not sure whether key vents and valves between the reactors were open at the time of the accident. Nor does the theory mean that the fuel in unit 4 didn’t heat up or sustain damage. But it could help explain why the building has been so badly crippled, while the fuel appears largely intact. If most of the fuel is undamaged, that would make clean up much easier.

Unit 4 spent fuel pool on 8 May

For full coverage of the Fukushima disaster, go to Nature’s news special.

For a selection of our coverage in Japanese, see Nature Asia Pacific.

Image/Video: TEPCO


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