[Fukushima Nuclear Power Plant] How Bad is the ``Worst-Case Scenario''?

How much Iodine-131 could be leaked at most?

The spent fuel storage at Fukushima Daiichi Nuclear Power Plant was calculated to be 1,760 tons as of March last year, while the total storage capacity is only 2,100 tons. After a year, it is certainly still some distance from being completely full. The maximum capacity of nuclear fuel inside a typical reactor is about 100 tons, while the actual capacity depends on the specific power of each reactor, generally only a few dozen tons. To estimate on the high side, let’s assume a total of 2,600 tons of fuel across the six reactors, consisting of 2,000 tons of spent fuel and 600 tons of nuclear fuel; these are all dioxides of the corresponding radioactive metals.

If a nuclear power plant uses standard low-enriched uranium fuel, the effective fuel (Uranium-235) is generally only about 4%, with the remaining 96% being Uranium-238. If the emerging MOX mixed fuel is used, it consists of 7% Plutonium and 93% Uranium-238. Within that 7% Plutonium, only about two-thirds is fissile (mostly Plutonium-239, with a small amount of Plutonium-241), which equates to approximately 4.5%. For a high-end estimate, let’s use a uniform 5%. The combined Uranium-235 and fissile Plutonium in spent fuel will certainly not exceed 2%. Thus, the maximum amount of fissile uranium and plutonium dioxides would be 70 tons (5\% \times 600 \text{ tons} + 2\% \times 2000 \text{ tons}); subtracting the two oxygen atoms, the actual mass of uranium and plutonium elements is only about 62 tons.

Iodine-131 is an important fission product of Uranium-235 and Plutonium-239, but in terms of the number of atoms, it only accounts for 2.8% of all fission products. Considering that iodine is nearly half as light as uranium and plutonium, its mass accounts for only 1.6%. Furthermore, the spontaneous decay of either uranium or plutonium does not produce iodine; they stop decaying once they reach lead (Lead-207). The Iodine-131 currently being released should theoretically be the fission product from the days before the reactors were shut down. Now that fission has almost completely stopped, only decay remains. However, since we are estimating the worst-case scenario, let’s be ruthless and assume all the nuclear fuel fissions as much as possible. This would yield a maximum of 1 ton of Iodine-131 (1.6\% \times 62 \text{ tons} = 0.99 \text{ tons}; we’ll round up for good measure). Of course, the actual figure is certainly far smaller than this value.

One ton! That sounds terrifying; it could fill a Jiefang truck!

How much impact on seawater?

But as the ancients wrote in their poems, “The ocean, it’s all water...”

The total volume of the Pacific Ocean is estimated at 622 million cubic kilometers. One cubic kilometer of seawater weighs approximately 1.03 \times 10^9 tons, which means the total mass is 6.4 \times 10^{17} tons! If 1 ton of Iodine-131 is thrown in, the mass concentration is less than 10^{-17}. To poison the entire planet is simply beyond its reach.

Well, comparing it to the entire Pacific Ocean might be unfair, so let’s compare it to the main ocean current on Japan’s east coast—the Kuroshio Current. It is the second-largest ocean current in the world, and if pollution spreads, it poses the greatest threat. The flow rate of the Kuroshio is approximately 65 million cubic meters per second, which is 6.7 \times 10^7 tons per second; its thickness is 500–1000 meters, and its width is about 200 kilometers. Let’s assume 1 ton of Iodine-131 is dumped instantly into the very center of the Kuroshio. The fish and shrimp at the entry point would likely perish. But the iodine must diffuse. Assuming the iodine diffuses in a hemispherical manner and stops automatically upon reaching the boundaries of the Kuroshio, by the time it reaches the bottom surface of the current (500 meters), the mass of the iodine-containing water mass would already be 2.6 \times 10^8 tons, and the mass concentration of iodine would dwindle to less than 10^{-8}. By the time it reaches the sides of the current (100 kilometers), the mass of the iodine-containing water mass would be 2 \times 10^{13} tons... uh, then the mass concentration would be less than 10^{-13}...

Another problem is that the half-life of Iodine-131 is a measly 8.1 days, while the average flow velocity of the Kuroshio is only 1–2 meters per second. In a short 8.1 days, it can flow at most 1,400 kilometers. The Kuroshio has to flow all the way to Canada before turning back—a journey that would take at least five or six half-lives. Even if it takes 40 days (5 half-lives), by the time it reaches Canada, only 1/32 of the original amount would survive decay. If it takes the same amount of time to flow back, only 1/1024 would remain...

The most critical issue is that this tiny amount of iodine likely won’t even enter sea salt! Seawater itself contains a very small amount of iodine, with an average concentration of 0.064 ppm, which is a mass concentration of 6.4 \times 10^{-8}. However, the salt we eat requires additional iodine to be added manually because even if we use sea salt, according to current concentrated brine methods, the iodine in seawater cannot reach the concentration required to precipitate into salt. No matter how you calculate it, the concentration of the newly added Iodine-131 will be far lower than the natural iodine in seawater, making it impossible to precipitate. Furthermore, by the time the entire sea salt production process is completed, who knows how many 8-day periods will have passed.

Evidently, while Iodine-131 might cause harm in the vicinity of the Fukushima nuclear plant, once it enters the vast ocean, it truly loses its power.

Some Supplementary Notes

What about radioactive Cesium? Cesium-137 is slightly more abundant, accounting for about 3.7% of fission products—more than double that of iodine. Additionally, its half-life is much longer, at 30.23 years. However, this slight increase in dosage does not change the fact that it is overwhelmed by the power of the seawater. Except for the half-life calculation, the orders of magnitude for the other results remain unchanged. If concentrated, these radioactive elements could indeed cause serious damage to areas within dozens of kilometers of the nuclear plant, but once they fall into the vast ocean, they are truly a drop in the bucket. Moreover, with such low concentrations, it is completely insufficient to enter the salt that precipitates during the salt-making process. In fact, among the many products of fission, there are many other radioactive substances, such as Strontium-90, Technetium-99, and Zirconium-93, which together account for nearly 10% of the mass. Fortunately, they are extremely difficult to volatilize and are unlikely to escape in gaseous form like iodine and cesium. Even if they did enter the sea, plugging them into the formulas above would yield the same negligible results.

Of course, we must not forget the bulk of the material: uranium dioxide and plutonium dioxide. If the most serious core meltdown and total leakage were to occur, uranium and plutonium might enter the sea directly. However, both substances are neither volatile nor soluble in water. Once in the sea, they would quickly sink to the bottom and be buried by sediment. This would harm the local seabed ecology but would be very difficult for the damage to spread, let alone pollute the entire ocean.

Finally, a word on the so-called radioactive “contagion.” Some atoms may change and become radioactive themselves after being exposed to intense radiation. However, unlike a disease, to “infect” new substances requires exposure to extremely intense radiation, and the radioactivity of the “infected” substance itself is much, much smaller than that of the original source. Since iodine and cesium already lack significant radioactivity due to their low concentrations, it is even more impossible for them to infect anything else.

In short, even if all the fuel from Fukushima entered the sea, the possibility of serious nuclear contamination of China’s sea salt and offshore fish is actually not high. Of course, this does not mean we can be complacent. In the near term, the focus along the coast will mainly be on the physical process of nuclear radiation dispersion, such as seawater diffusion caused by ocean hydrodynamics. In the long term, radioactive isotopes with long half-lives have the potential to accumulate through the food chain. We must strictly monitor the current nuclear radiation data in seafood to ensure nothing goes wrong. Everyone can follow the latest nuclear radiation detection data regularly released by the maritime, fishery, and environmental protection departments of neighboring countries.

The end of this article has been modified; special thanks to @biodiversity.

The original version of this article was published on Guokr.com - Dead Rationalist.

This article is reprinted from: http://songshuhui.net/archives/51894

References

[1] http://en.wikipedia.org/wiki/Iodine-131

http://en.wikipedia.org/wiki/Kuroshio_Current

http://www.nirs.org/reactorwatch/accidents/6-1_powerpoint.pdf

http://www.world-nuclear.org/info/inf29.html

http://www.seafriends.org.nz/oceano/seawater.htm#composition