The Isotopes of the Fukushima Disaster

On March 11, 2011, a magnitude 8.9 earthquake struck the coast of Japan, causing the shutdown of 11 of Japan’s nuclear power reactors, including reactors 1, 2, and 3 at the Fukushima Daiichi power plant. Shortly after the shutdown, the pressure within reactor 1 built up to twice that of normal levels. In an attempt to relieve pressure, steam-containing radiation was vented from the reactors. Over the next several days, the plant continued to have problems and the levels of radiation in the surrounding areas continued to rise.Shortly thereafter, Japanese officials formed a 10-kilometer (km) evacuation zone, which, over the next few days, was increased to 20 km, resulting in the relocation of close to 390,000 people.

Fukushima reactor following the tsunamis.

Fukushima reactor following the tsunamis.

The Fukushima disaster released a number of radioactive isotopes into the atmosphere and the surrounding ocean. These included isotopes of iodine, cesium and xenon. Each of the isotopes listed below are a byproduct of the nuclear fission reactions that occur within a nuclear power plant. As the uranium in these plants breaks down, the uranium atoms release energy (such as gamma radiation) and particles (alpha and beta particles). This is called radioactive decay, and the loss of particles effectively transforms the original uranium atom into isotopes of another element. In many cases the resulting isotopes are also radioactive, and thus undergo additional radioactive decay.

Three of the more common isotopes released by the Fukushima disaster are listed below. These isotopes differ considerably in the types of radiation that they release, their half-lives, and their potential adverse effects on living organisms, such as humans.

Cesium-134 and Cesium-137

Perhaps the biggest concern following the Fukushima disaster was the release of radioactive cesium. Naturally, cesium (Cs) is a metal with an atomic number of 55 and an atomic mass of approximately 132.9.  However, within a nuclear reactor two isotopes of cesium (Cs-134 and Cs-137) are formed. Cesium 134 and Cesium 137 have half lives of 2 years and 30 years, respectively. Both of these isotopes release both beta particles and gamma radiation as they decay.  Both beta particles and gamma radiation have the ability to penetrate tissues and damage cells and their DNA. Exposure to beta particles and gamma radiation, either internally or externally, can cause burns and greatly increases the chances of cancer.

Following the Fukushima disaster there were significant concerns of Cs-137 entering the atmosphere and the Pacific Ocean. Because of the winds at the time, most of the Cs-137 was carried back over Japan, forcing evacuations up to 35 kilometers (22 miles) inland. Large regions of this evacuation zone still remain uninhabitable due to soil contamination by Cs-137. Small amounts of atmospheric Cs-137 were detected on the west coast of the United States, and some Cs-137 has been detected in the waters off the coast of California, but in neither case at levels that are considered hazardous to humans.

Iodine -131

Radioactive iodine, I-131, was also released by the reactors. Iodine is a water-soluble element that has an atomic number of 53 and an atomic mass of approximately 126.9. There are several forms of iodine isotopes, with I-131 being formed mainly by nuclear fission reactions. Like Cs-137, I-131 emits beta particles which may damage tissues. It also releases small amounts of gamma radiation.

Radioactive iodine only has a half life of about 8 days, so its long-term effect on the environment is minimal. In our bodies, iodine is mainly used by the thyroid gland to manufacture the thyroid hormones associated with metabolic functions, including overall metabolic rate. Intense exposure to I-131 may cause thyroid problems, including thyroid cancer. Interestingly, isotopes of iodine at low doses are used by the medical profession to diagnose problems with the thyroid gland.


Xenon (Xe) is a noble gas with an atomic number of 54 and an atomic weight of 131.2. One isotope of xenon , Xe-133 has a half-life of only 5 days. Like I-131 and Cs-137, Xe-133 is emits beta particles which may damage tissue, but the short half-life of the isotope limits the chances of severe problems.  Further reducing the risk is the fact that, as a noble gas, xenon does not react with other elements, and thus is not easily introduced into the chemical compounds within cells.

Like radioactive iodine,Xe-133 is used by the medical profession to diagnose disease. Since it is a gas, it is frequently used in the diagnosis of lung disorders. Studies are now underway to use Xe-133 as a form of treatment for certain types of lung cancer, since it easily enters the lungs and the emission of beta particles can help destroy lung cancer cells.

Additional Resources

Image Sources:

  • Fukushima reactor photos: By Digital Globe [CC-BY-SA-3.0 (], via Wikimedia Commons


Michael Windelspecht is an author and instructor of introductory biology. A version of this article will appear in the Mader/Windelspecht Inquiry Into Life (15th edition) text (publication date January 2016).

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