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March 18, 2011 

UCS Analysis: Possible Explanation for Hydrogen Leaks and Explosions at Fukushima Daiichi Reactors 1 and 3

Experts have speculated that the detonations in two of the reactors at Fukushima Daiichi nuclear power plant last Saturday were explosions from hydrogen that accumulated in the reactor building. The likely source of that hydrogen was damaged fuel in the reactor cores.

The reactor core, however, is surrounded by a steel reactor vessel and the reactor’s primary containment. A key question is how a significant amount of hydrogen escaped from the primary containment into the reactor building, and how this low-probability event would have happened at two reactors at the site.

The answer to this question is critical to prevent hydrogen explosions at the other reactors at Fukushima. David Lochbaum, the director of the Union of Concerned Scientists’ Nuclear Safety Project, has a possible explanation based  on the results of a test in the 1970s at the Brunswick nuclear power plant in North Carolina, which had a  containment system that was similar to the ones at the Fukushima plant.

That test at Brunswick showed that if the pressure in the primary containment increases about 12 percent above its design pressure (up to 70 pounds per square inch (psi)), it can loosen seals and allow gas to flow from the containment into the reactor building. Since this test was used to check the structure of a new containment structure and was not repeated, very few people, either inside or outside the nuclear industry, knew about it. Moreover, since the high-pressure leak was incidental to the purpose of the test and was not relevant to the reactor under operating conditions, it did not raise any concerns at the time.

Here’s why the Brunswick test may be relevant to the explosions at the two Japanese reactors: The earthquake and tsunami shut off cooling to the reactors, causing fuel damage in the reactor cores that led to the production of hydrogen. When high pressure due to steam and hydrogen built up in the reactor vessel, plant staff  eventually vented it into the primary containment. As workers pumped water into the reactor for cooling, it produced more steam and higher pressure, so they continued venting to the containment.

As the resulting pressure in the containment vessel grew, workers vented gas—including hydrogen—outside of the plant into the atmosphere to reduce that pressure. But to reduce the number of ventings and the amount of radioactive gas released, workers allowed the pressure in the containment to grow to levels well above normal operating levels. If this pressure grew high enough, the result of the Brunswick test suggests it could have caused a high-pressure leak from containment into the reactor building.

Lochbaum notes that if this is the cause of the leak, it could be averted easily and effectively simply by changing the venting procedures so that workers vent the containment pressure to the atmosphere more frequently and do not let it build up to such high level. Taking such action might moderately increase the amount of radioactive gases vented into the atmosphere, but it could eliminate a source of hydrogen inside the reactor buildings that could cause another explosion.

Authorities should launch an  investigation to pinpoint the source of the hydrogen leak to eliminate this risk in the future. But in the meantime, since the Brunswick test showed that this containment is vulnerable to high-pressure leaking, Tokyo Electric Power Co. can and should take immediate steps to avoid creating such a leak by changing its procedures to vent the containment before it builds up to such high pressure (70 psi).

For more information and diagrams, go to UCS’s All Things Nuclear blog.


The Union of Concerned Scientists puts rigorous, independent science to work to solve our planet's most pressing problems. Joining with citizens across the country, we combine technical analysis and effective advocacy to create innovative, practical solutions for a healthy, safe, and sustainable future.

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