Answer to Question #5367 Submitted to "Ask the Experts"

What are the major radionuclides of concern immediately after a weapons accident and a nuclear power plant accident?

Both nuclear power plants and nuclear weapons of the fission type produce most of their radioactivity in the form of fission products, the smaller fragments produced when the nuclear fuel, either uranium or plutonium, splits in two parts after absorbing a neutron. In both cases approximately 200 different fission products, many of which are radioactive, are produced; however, the relative distributions of the various radioactive species are typically different because of the differences in time over which they are produced.

In a nuclear reactor, the fission products build up in the fuel over time as the reactor operates; shorter-lived radionuclides, such as ¹³¹I with a half-life of about eight days, build up quickly and reach equilibrium values in the reactor core within relatively short times, whereas longer-lived nuclides, such as ⁹⁰Sr, with a half-life of about 30 years, require much longer time periods to approach their maximum activities.

In a nuclear explosion, typified by a bomb, the major burden of fission products is produced essentially immediately within the very short duration of the nuclear event. When a nuclear explosion occurs in the air atmosphere, the entire inventory of fission products that is produced is, in theory, available to produce external radiation dose from the radiations, especially the gamma radiation, and potential internal dose, following inhalation and ingestion of the material. Naturally, many of the fission products become quickly unavailable because of their very short half-lives; such nuclides disappear by radioactive decay. There are many species that may be significant contributors to external dose from gamma radiation and sometimes beta radiation; these include radioactive isotopes of noble gases, such as radioactive isotopes of krypton and xenon, radioactive isotopes of halogen elements, especially iodine, and radioactive isotopes of barium, lanthanum, cerium, zirconium, niobium, cesium, and many others.

In the case of an accident at a nuclear power plant, a nuclear-bomb-type event is not possible, and the kinds of accidents that might occur are generally incapable of dispersing the entire contents of the core, which contains the radioactivity, into the general environment. Usually, the most volatile radionuclides are the ones most likely to be released when the barriers against dispersal are compromised.

Among the most significant radionuclides in this regard are isotopes of the noble gases, such as xenons (¹³³Xe, ¹³⁵Xe, ¹³⁸Xe, and ¹³⁹Xe) and kryptons (^85mKr, ⁸⁵Kr, ⁸⁷Kr, ⁸⁸Kr, and ⁸⁹Kr). These noble gases are potential contributors to external exposure. ¹³³Xe is often the dominant noble gas with respect to relatively short-term exposure following an incident.

Probably the most significant radionuclides with respect to internal exposure, especially in the short term following either a nuclear power accident or a nuclear explosion, are the radioisotopes of iodine, iodine mass numbers 131 through 135, inclusive, the most notable of these being ¹³¹I. In the longer term radionuclides such as ¹³⁷Cs and possibly ⁹⁰Sr are increasingly more important, especially in regard to internal uptakes through foodstuffs. These same radionuclides are also important with regard to exposure following a nuclear explosion.

If you want more detailed information regarding the availability of radionuclides in the event of nuclear power incidents you can review the U.S. Nuclear Regulatory Commission Regulatory Guide 1.183 - Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors. There is also an online book by C. Lin titled Radiochemistry in Nuclear Power Reactors (1996) that you might find informative; click on the Table of Contents and scan down to see what might be of interest—e.g., Chapters 2 and 3 on radioactivity production and fission products, respectively.

You can also review detailed fission product yield information that provides a comprehensive list of fission products and their respective fission yields at various sites, one such being the Lawrence Berkeley National Laboratory site, where you can select the type of neutron, thermal or fast, promoting the fission and the material being fissioned, e.g., ²³⁵U, and see the lists of fission products, along with their respective half-lives, indirect yields (yields that depend on production through radioactive decay of a precursor radionuclide created in the fission process), and total yields.

George Chabot, PhD, CHP