Determination of subtrace quantities of technogenic plutonium in the environment and living organisms

Abstract

Plutonium, in particular, a-active 239Pu (T1/2 = 24.104 yr), is present in natural samples at least in small quantities. Its concentration has been determined to be (2-6)-10 -16 g/g in granite from super deep wells and (2-3)-10-16 g/g in geothermal waters from the Salton Sea (USA) [1, 2]. In rich uranium ores the ratio of 239pu tO uranium reaches 10-12-10 -11 g/g. It is obvious that in uranium ores 239pu is formed continuously by neutrons generated by the nucleon component of cosmic rays in (o~, n) reactions on light nuclei; these neutrons are then absorbed by 238U. Intense contamination of the environment by technogenic transuranium nuclides, specifically, the longest lived nuclide 239pu, occurred during the last few decades as a result of numerous nuclear weapons test, large accidents on nuclear objects, imperfectness of the technology for chemical reprocessing of reactor fuel elements, dumping and burying at the bottom of the ocean tens of ship-propulsion nuclear plants, as well as storage of nuclear wastes. Technogenic transuranium elements, specifically, 239pu _ the main component of nuclear bombs, are now widely disseminated over the earth's surface, and they create a radiation background. They have already become incorporated in the atmospheric and soil cycles, they have entered the biosphere, they are contained in food products, and they can produce a direct hazard to human health. Estimates show [3, 4] that the content of plutonium in the surface layers of the soil reaches 10 -13 g/g. These estimates agree with measurements of the plutonium content (R. Brandt, private communication, March 27, 1993) in soil samples from Central Europe. The results of investigations of radioactive fallout, resulting from the Chernobyl accident, have shown a significant contamination of extensive territories in Eastern and Central Europe [5, 6] by hot particles containing plutonium isotopes. Our objective in the present paper is to analyze the possibilities of different methods for determining the plutonium concentration in the environment and to search for methods of fast highly sensitive serial analysis with a sensitivity of 10-t3-10 -15 g/g. Determination of Plutonium Concentration by Registration of ~t Particles. The simplest nondestructive methods for determining the plutonium concentration in substances are based on the use of track detectors which are sensitive to aparticles. Soil and plant samples from the region of the Chernobyl nuclear power plant were analyzed using low-sensitivity nuclear emulsions and high-sensitivity plastics CR-39 cellulose nitrate [5-8] exposed for several days in order to record the a-particles from heavy radionuclides. Together with optical microscopes, spark breakdown counters of the type "Aist" and "Iskra," developed at the V. G. Khlopin Radium Institute [7, 8], were used to count the a-particle tracks. To decrease the background, first the track detectors were heated for a long time (up to 200~ or their surface was etched to a depth of 2 0 #m. This method provided simple and reliable registration of hot particles containing transuranium elements. However, the sensitivity of the track method of determining a-active plutonium in the substance being analyzed is low 10 -8 ag/g for 239pu under exposures of up to 10 days. The most favorable conditions for registering a-active radionuclides are obtained by chemical separation of actinide groups from soils followed by the preparation of thin samples and registration of a-particles using semiconductor detectors [1-3]. However, this is a complicated and laborious method, and as a result it is not widely used, especially for low concentrations of technogenic plutonium.