How did plutonium get carried far away?


Question:


We know that plutonium found in soil across Japan come from nuclear bomb tests because they don’t occur in nature in Japan. Doesn’t that contradict what TEPCO claims about plutonium being too heavy to get carried away very far? (40s, Tokyo)

Answer:

It does sound contradicting to say that plutonium too heavy to be carried away had traveled across Japan in the past. However, both claims are actually correct, and that’s because of characteristic differences between fuels used in nuclear bombs and nuclear plants.

Nuclear bombs are made so as to produce huge explosions when detonated. The nuclear test carried out in 1954 around Bikini Atoll, for example, created a mushroom cloud 30 kilometers in height. Plutonium and other radioactive substances in nuclear bombs are generally shot up with the dust in the mushroom cloud into the stratosphere (altitude 11 kilometers to 50 kilometers), and remain there for about a year before coming down to the surface of the Earth.

On the other hand, the fuels used at nuclear plants are designed to induce nuclear reactions little by little, bit by bit, and to moderate the slightest excess of energy production by the use of control rods. Plutonium did get out at the Fukushima Daiichi, but that was a small leakage from the damaged fuel rods. The plutonium travels as a heavy molecule of several micrometers in size, and gets deposited on ground in the vicinity of the source.

Let us see how that has worked in the past. Meteorological Research Institute in Tsukuba, Japan, has been conducting measurements of plutonium and other radioactive substances deposited in soil over 50 years. Take a look at the figure below:



There were equally large amount of plutonium (red) and cesium (blue) fallouts during the 60s and 70s when many nuclear tests were carried out. Note the year 1986. That is when the Chernobyl accident occurred, and there is a noticeable difference between the two plots: the amount of cesium fallout shot up while the amount of plutonium fallout stayed the same.

Furthermore, scientists can reliably separate the plutonium of plant-origin from the plutonium of bomb-origin by analyzing the ratios of plutonium isotopes found in the fallout. Officials at Fukushima found plutonium that was plant origin, but the amount was no different from the background plutonium. Please note, just 50 years ago the background level was 1,000 times more than that reported at Fukushima.


Science Communicator: Misato Hayashida