Cesium-137: A Deadly Hazard Colin Wessells

March 20, 2012 Submitted as coursework for PH241, Stanford University, Winter 2012

Introduction

Among the many fission product nuclides, cesium 137 deserves attention because it possesses a unique combination of physical properties and historical notoriety. It is readily produced in large quantities during fission, has an intermediate half-life, decays by high-energy pathways, and is chemically reactive and highly soluble. These physical properties have made cesium 137 a dangerous legacy of major nuclear accidents such as Chernobyl, but it has also caused relatively small incidents as well.

The Dangers of Cesium-137

Cesium-137 is among the most common heavy fission products. Fission of various isotopes of thorium, uranium, and plutonium all yield about 6% cesium-137. [1] This high fission yield results in an abundance of cesium-137 in spent nuclear fuel, as well as in regions contaminated by fission byproducts after nuclear accidents. [2] The large quantities of cesium-137 produced during fission events pose a persistent hazard. Its half-life of about 30 years is long enough that objects and regions contaminated by cesium-137 remain dangerous to humans for a generation or more, but it is short enough to ensure that even relatively small quantities of cesium-137 release dangerous doses of radiation (its specific radioactivity is 3.2 × 1012 Bq/g). [2-4]

Along with its intermediate half-life, a combination of high-energy radioactivity and chemical reactivity makes cesium-137 a particularly dangerous fission product. Cesium-137 undergoes high-energy beta decay, primarily to an excited nuclear isomer of Barium 137, which in turn undergoes gamma decay with a half-life of about 150 seconds. [4] The energies of both the beta decay of cesium-137 and the subsequent gamma decay of the excited barium 137 are 512 keV and 662 keV, respectively. [4] In addition, cesium is much more chemically reactive than many of the transition metal fission products. As a group 1 alkaline metal, elemental cesium is quite electropositive, and is readily oxidized by water, forming highly soluble Cs+. [5] For this reason, elemental cesium-137 may contaminate large volumes of water during nuclear accidents, which are difficult to contain or process. [6]

Despite its prevalence in spent nuclear fuel and nuclear waste, cesium-137 is actually extremely rare. Its half-life is too short for it to persist from natural fission sources, and on earth it is a synthetic isotope only. Should further nuclear accidents be avoided, the dangers of cesium-137 will eventually cease.

The Legacy of Cesium-137 After Nuclear Accidents

The Chernobyl nuclear power plant accident and the less well-known Goiânia scrap metal accident illustrate the true dangers of cesium-137. During the Chernobyl explosion, about 27 kg of cesium-137 were expelled into the atmosphere. [2] After the rapid decay of iodine-131, cesium-137 was the predominant source of radiation in fallout from the Chernobyl explosion. Particles of the reactor fuel settled densely within about 100 km or their release, and within a 30 km radius of the facility, cesium radioactivity due to ground deposition of fallout particles was over 1.5 × 106 Bq/m2. [2] Fallout hotspots to the northeast in Belarus, much farther from the disaster site, were found to have cesium-137 radioactivities of up to 5 × 106 Bq/m2. [2] In comparison, measurements in southern Sweden, several hundred kilometers northwest (and upwind) of the disaster found that the ground surface radioactivity due to cesium-137 was only 8 × 104/m2 Bq, and total radiation doses peaked at only about twice the typical background rate. [7] In 2002, sixteen years (about one half of a cesium 137 half life) after the Chernobyl disaster, a 4,000 km2 area still contained too much cesium-137 to be inhabited or used for agricultural purposes. [2] Much of this area must remain unpopulated for decades to come, until several more half-lives of the released cesium-137 have elapsed.

A much smaller nuclear accident involving cesium-137 occurred in Goiânia, Brazil, in 1987. A thorough account of the entire affair was published by the International Atomic Energy Agency in 1988. [8] The accident was initiated when two men who were looking for old equipment to sell for scrap broke in to an abandoned medical clinic. There, they found a radiation therapy device left behind when the clinic closed. Upon ripping apart the device, the men discovered about 30 g of 137CsCl. The men were immediately attracted to it because of its glowing blue color. The owner of a local junkyard purchased the device from the men, and proceeded to show off the 137CsCl to friends and neighbors. After several people involved with the looting of the device and the release of the 137CsCl fell morbidly ill with radiation sickness, Brazilian authorities declared a local state of emergency, and within days, the vast majority of the cesium-137 had been contained. In contrast to the large disaster at Chernobyl, only a few people were killed or sickened by cesium-137 in Goiânia. Without the prompt and well-executed response of the Brazilian government, this incident could have harmed many more people. The Goiânia incident shows that failure to properly account for seemingly small quantities of cesium-137 can be deadly.

Treatment of Cesium-137 Ingestion With Prussian Blue

The Goiânia incident also provides insight about one treatment method for treatment of cesium-137 that achieved some success. Ferric ferrocyanide, better known as Prussian Blue, is a metal organic framework material that has large interstices in its structure. [9] Electrochemists and battery scientists have long exploited the structure of Prussian Blue in electrochromic devices because alkaline ions such as potassium and cesium rapidly intercalate the structure. [10] Prussian Blue has such a strong affinity for these ions that it was administered as an antidote for cesium-137 exposure during and after the Goiânia incident. [8] Up to 10 g of Prussian Blue were administered daily to patients who had suffered large amounts of cesium-137 exposure. Later analysis of Goiânia survivors found that the administration of Prussian Blue resulted in a decrease in cesium-137 exposure by about 70%. [11] Cesium-137 remains an extremely toxic radioisotope, but Prussian Blue provides some help to those who have ingested it.

Final Summary

Cesium-137 is an especially dangerous fission product because of its high yield during fission, moderate half-life, high-energy decay pathway, and chemical reactivity. Because of these properties, cesium-137 is a major contributor to the total radiation released during nuclear accidents. Finally, a discussion of its practical applications is beyond the scope of this report, but cesium-137 has received some use as a medical radioisotope for cancer therapies [6,8]. Future research of effective methods for cesium-137 containment and capture may someday alleviate future nuclear crises.

© Colin Wessells. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] N. Kocherov, M. Lammer, and O. Schwerer, "Handbook of Nuclear Data for Safeguards," International Atomic Energy Agency, INDC(NDS)-376, December 1997.

[2] Chernobyl: Assessment of Radiological and Health Impacts (OECD Nuclear Energy Agency, 2002).

[3] M. P. Unterweger, D. D. Hoppes, and F. J. Schima, "New and Revised Half-Life Measurements Results," Nucl. Instrum. Meth. Phys. Res. A312, 349 (1992).

[4] R. L. Bunting, "Nuclear Data Sheets for A=137," Nuclear Data Sheets 15, 335 (1975).

[5] A. F. Holleman and E. Wiberg, Inorganic Chemistry (Academic Press, 2001), Ch. 28.

[6] K. Buesseler, M. Aoyama, and M. Fukasawa, "Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity," Environ. Sci. Technol. 45, 9931 (2011).

[7] L. Devell et al., "Initial Observations of Fallout From the Reactor Accident at Chernobyl," Nature 321, 192 (1986).

[8] The Radiological Accident in Goiânia (International Atomic Energy Agency, 1988).

[9] H. J. Buser et al., "The Crystal Structure of Prussian Blue: Fe 4 [Fe(CN) 6 ] 3 - H 2 O," Inorg. Chem. 16, 2704 (1977).

[10] J. W. McCargar and V. D. Neff, "Thermodynamics of Mixed-Valence Intercalation Reactions: The Electrochemical Reduction of Prussian Blue," J. Phys. Chem. 92, 3598 (1988).

[11] D. R. Melo et al., "Cs-137 Internal Contamination Involving A Brazilian Accident, and the Efficacy of Prussian-Blue Treatment," Health Phys. 66, 245 (1994).