As the situation at Japan's 40-year-old Fukushima Daiichi nuclear plant goes from bad to worse—four of the plant's six boiling-water reactors have been damaged by explosions or fire, and radiation has begun leaking into the atmosphere—officials there continue to pump seawater into the reactors in a desperate attempt to cool down fuel rods and avoid a complete meltdown that could release radioactive fallout across much of the country and beyond. The move by Tokyo Electric Power Co. (TEPCO), which operates Daiichi, to use seawater doped with neutron-absorbing boron in the reactors' pressure vessels all but ensures that they will never function properly again, permanently damaging one of the world's 25 largest nuclear power stations.

Such extreme measures were necessary because the normal and auxiliary cooling systems, which circulate purified water to keep the fuel rods from melting down, failed. Last week's tsunami not only cut normal electrical power to Daiichi, it also flooded and disabled the backup diesel generators. Three of the plant's six reactors were already shut down for maintenance when the magnitude 9.0 earthquake-induced tsunami struck, but TEPCO has struggled to cool the fuel rods used in the operational reactors. The lack of sufficient water to cover these rods has led to explosions at all three active reactors, likely due to a build-up of hydrogen gas.

A fourth reactor, one of those that had been shut down prior to March 11, experienced a fire Tuesday that threatened to evaporate water in a storage pool for spent nuclear fuel. That fire may have been caused by hydrogen seeping from the pool, reported.

The prospect of ruining a half dozen nuclear reactors pales in comparison with the alternative—a complete meltdown that would contaminate the ground below the complex with radioactive material that could be spread by wind, rain and groundwater, potentially causing radiation sickness in thousands of people.

Scientific American
spoke with Pavel Tsvetkov, an assistant nuclear engineering professor at Texas A&M University in College Station, about why seawater is a last resort for cooling compromised nuclear reactors and TEPCO's options moving forward.

[An edited transcript of the interview follows.]

Why does a nuclear facility normally rely on purified water to cool its reactors? How is it purified?
I'll give you an example: If you have a boiling pot and your water has too many minerals, then condensation will collect inside your boiling pot. When this happens in a reactor, it interrupts the properties of the fuel elements. Energy companies don't want to jeopardize the performance of their reactor materials, so they use purified water, usually from a special water purification plant on site. Purifying the water removes most of the salts and anything that could accumulate on the fuel elements.

Under what circumstances would a nuclear power plant use seawater to cool its reactors?
Using unpurified water is not a normal practice—it's never done. Plants don't take water from the river or the sea to supplement their own internal water, which is in completely closed-loop systems. Of course, they take in some amount of new water periodically to make up for evaporation and other losses like that, but this water is purified before it is used.

TEPCO's reactors lost the water below the normal operating condition, so they had to provide additional water for that. Salt-water obviously has a lot of minerals in it, and if it's taken directly from the sea, it has all sorts of other materials floating in it as well. Even if these things were filtered out, the chemistry of salt-water is not really compatible with what normally goes through the reactor. It's too corrosive for fuel elements. I would guess that after this water was introduced into the reactor cores, those cores would become completely unusable. This is because any materials in the water will attach to the surface of the fuel rods and make heat transfer unpredictable.

What role does the boron in the seawater play?
Boron can be injected into water-coolant systems to control the activity of a reactor core because it is a strong neutron absorber, especially for thermal neutrons. But boron is not usually used in boiling-water reactors such as those at Fukushima Daiichi because boron is also corrosive on fuel elements. In cases of emergency, however, boron and seawater can be used to suppress fission chain reactions in the fuel elements.

So the use of seawater and boron is a last-resort effort to cool a reactor?
Probably if they had more time they would have tried to restore the diesel generators that ran the backup cooling system and circulate the water they already had. But with the water in the core evaporating due to the high temperatures, they needed to add more and more water so they could quickly suppress boiling conditions.

Given that it can cost several billion dollars to build a new reactor, isn't there any way they could clean the fuel elements?
They may try to explore whether the reactor can be restored because it's a significant investment. But I'm not aware of anyone ever using salt-water for any prolonged period of time in their reactors like they are doing now.