Environ. Sci. Technol, 1989, 23.89-95
Characteristics of Radioactive Particles Released from the Chernobyl Nuclear Reactor R. G. Cuddlhy," 0. L. Finch, G. J. Newton, F. F. Hahn, J. A. Mewhlnney, S. J. Rothenberg, and D. A. Powersf Lovelace Biomedical and Envlronrnental Research Institute, Inhalation Toxicology Research Institute, P.O. Box 5890, Albuquerque, New Mexico 87185
Particles from the Chernobyl nuclear reactor accident that deposited on a nearby surface were analyzed to determine their radioactive and chemical compositions. They were mainly composed of uranium, carbon, cerium, and lanthanum, or zirconium. The latter two types of particles also contained aluminum, silicon, calcium, phosphorus,and chlorine that probably derived from high-temperature interactions between fuel and the reactor structures or soil that was deposited on the core. The major y-emitting radionuclides associated with the particles were 95Zr-g5Nb, lmRu,106Ru-106Rh,lUCs, 1S7Cs-137Ba,14Teand '%e-lGPr. They did not include l3II or 1MBa-140La,radionuclides that were frequently reported as being abundant in samples collected at more distant locations throughout Europe. When selected particle samples were placed in aqueous media similar in ionic composition to lung fluids, all of the radionuclides were slow to dissolve; the average dissolution half-time was 160 days. We concluded that if similar particles were inhaled by people near the reactor site, the largest portion of the related radiation doses would be delivered to lung tissue.
Introduction The nuclear reactor accident at Chernobyl Unit 4 led to the release of a large quantity of radioactive material to the environment between April 26 and June 1,1986 (1, 2). The reactor accident first became apparent to outside observers when two explosions occurred at 1:24 a.m. on April 26. Four phases of radionuclide discharges were described. The first phase resulted from the explosions which caused mechanical discharges of fuel material, vapors, and gases. The second phase, April 26 through May 2, resulted from high fuel temperatures caused by the initial reactivity excursion, radioactive decay energy, and burning graphite. Daily releases of radioactivity decreased during this phase because the core was smothered with soil, lead, and boron carbide in efforts to contain the accident. However, finely dispersed fuel particles and volatile fission products still escaped in the flow of hot combustion products. During the third phase, May 3 through May 6, fuel temperatures increased again due to the release of residual heat. This caused more fission products to migrate through the covering material and into the atmosphere. The fourth phase, after May 6, was characterized by a rapid decline in radionuclide releases (1). Although 23aUwas the major constituent of the core, it constituted a negligible fraction of the total radioactivity because of its very low specific activity. Amounts of other radionuclides present depended upon their fission yields, the yields of radioactive precursors in their decay chains, and their radioactive half-lives. Estimated discharges are highest for the most volatile radionuclides and lowest for the most refractory materials. Virtually all of the core contents of inert radioactive gases, xenon and krypton, D.A.P. is in the Department of Reactor Safety Research of Sandia National Laboratories in Albuquerque, NM. 0013-936X/89/0923-0089$01.50/0
were released and dispersed widely in the atmosphere. Approximately 10-20% of the more volatile elements (iodine, tellurium, and cesium) and 2-5% of the fuel material were also released (1). Most of these were in particulate form or became associated with particles that have now settled back to earth, mainly in the northern hemisphere. The highest concentrations of radioactivity deposited in eastern Europe; probably 30% of the deposited radioactivity stayed within 50 km of the Chernobyl plant. This report is based upon analyses of particles that deposited on a wood surface soon after the reactor accident. The wood was part of a shipping crate that was on a railroad car probably within 80 km of Chernobyl at the time of the reactor accident, but a precise exposure history of the shipping crate is not known. The railroad car arrived in Munich, Germany, toward the end of May where the radioactive crate was removed. In June, a sample of the wood was provided for the analyses we describe. We studied particle characteristics that (1)indicate how the radionuclides were released during the accident and (2) influence the deposition, retention, and long-term radiation doses to body organs if the particles are inhaled or ingested by people. These include particle size, shape, chemical composition, and rate of dissolution in aqueous media. This information is important for evaluating the long-term exposure risks to people who live near the Chernobyl reactor and to others who may be exposed to similar material in the event that serious nuclear reactor accidents occur in the future. Because our analyses began in July 1986, no information was gained related to very short-lived radionuclides.
Experimental Section Methods. A lithium-drifted germanium detector was used to obtain a y-ray spectrum of the wood sample to identify radionuclides present. Autoradiographs of the intact wood surface were done using Kodak standard X-ray film to locate radioactive particles. Small areas of the wood surface were then microdissected under a stereo light microscope to obtain individual particle samples, Some of these were embedded in paraffin and cut with a microtome into 7-pm-thick sections to produce microautoradiographs. Sections were dipped in Kodak NTB-3 emulsion, exposed for 20 min, and developed. Other particle samples were mounted on carbon stubs and coated with carbon or gold for scanning electron microscopy (SEM; JEOL Model JSM-35, operated at 20-25 keV) and energy dispersive X-ray analysis (EDXA; Kevex Model 5100 spectrometer). Particles that were imaged by standard secondary mode SEM were first located by using backscattered electron images of large areas of the samples (Figure 1). Those composed of elements with high atomic numbers, such as uranium or fission products, appeared bright against the darker background. These were individually analyzed by placing the electron beam on the particle in reduced raster mode (typically
