A water cooling pump at Fukushima.Jorge Rodriguez
- The chemistry of seawater mixing with nuclear fuel is little known.
- Previous studies haven't accounted for the conditions in a core.
- Many reactors worldwide are near coasts and vulnerable to flooding or tsunamis.
A year after the Fukushima nuclear disaster, questions remain just what happens when you mix seawater with nuclear fuel.
Since this mixing produces chemicals that can then be released during an accident, some experts are calling for experiments to be done in case something like Fukushima happens again.
When the Fukushima Daiichi nuclear power plant was hit by a tsunami, it lost the diesel generators that powered the cooling system.
Nuclear fuel generates a lot of heat, and without cooling, it will melt, and quickly. The workers at the plant, in a desperate attempt to lower the temperature of the reactor core, used seawater -- spraying tons of it into the reactor building. A lot of seawater ended up leaking back into the ocean and the ground.
In a paper published today in Science, Peter Burns, a professor of civil engineering at Notre Dame, and his co-authors note that it is not clear exactly what kinds of chemicals were released.
There have been studies of the effect of water on nuclear fuel, Burns said, and on what kinds of elements get released when a reactor core melts.
"They either focused on the gaseous products -- you melt the fuel, and see what gets released -- or they focused on the interaction of used fuel with the geologic environment. In our study we're pointing out that neither of these gives us the insight we need," they wrote.
Nuclear fuel at Fukushima was largely uranium oxide (UO₂), with a small portion of the fuel containing plutonium. The cladding of the fuel rods is made of zirconium alloy.
When the water pumps failed, the temperature in the core got hot enough for the zirconium alloy to react with the water, which released hydrogen. That caused the reactor buildings to explode, because hydrogen reacted violently with the oxygen in the air.
In addition, several radioactive byproducts were released into the air and groundwater, such as iodine and cesium.
When the seawater hit the reactor core, it heated up and evaporated. That likely left salt deposits. Another factor was the extreme heat near the fuel rods. Burns said at those temperatures and in the presence of radioactivity seawater can form peroxides, which are even more likely to react with the elements in the core and do so differently from water.
Burns said there is a need for more experiments that would tell us about the complex chemistry that happens in disaster scenarios like that at Fukushima. He added that there are a number of ways to do this kind of research.
Preliminary work can be done with non-radioactive isotopes of cesium, iodine and technetium, and there are analogues of the radioactive elements that can be studied before experimenting with real nuclear fuel.
While this may sound like planning for exceedingly rare accidents, when you add up the sheer number of reactors it becomes a lot more urgent. Since the first reactors went online, there have been about 20 meltdowns, though only three were big enough to garner much public attention and others might have been kept secret (it's possible that there were some on old Soviet nuclear submarines).
Allison Macfarlane, associate professor of environmental science and policy at George Mason University, chairs the Bulletin of Atomic Scientists Science and Security Board, and was a member of the Blue Ribbon Commission on America's Nuclear Future. If nuclear power plants are going to be built near coastlines, then seawater, she said, should probably be studied if we want to understand the risks of meltdowns.
Engineers often use past events to guide risk assessments. That didn't work at Fukushima because the earthquake was more powerful than any they had experienced.
"No one thought about what happens if, 'Hey we need to put seawater on the reactor to cool it down'," Macfarlane said.
She added that in hindsight, it seems odd that the effects of seawater haven't been studied more deeply, given that earthquakes aren't that uncommon in Japan -- one of the most geologically active regions of the world.
"People said, 'well, it was the tsunami that was the problem,' but what caused the tsunami? An earthquake," she said.
According to the European Nuclear Society there are 435 reactors operating in the world today, with 63 more under construction.
"If you plan for a once in 10,000-year event, that means you get one every 20 years," Burns said.