天然放射性元素化學(xué)(已修改)
2025-01-25 20:20 本頁面
【正文】 Chapter4 Radionuclides in nature第四章 自然界中的放射性核素楊金玲v Practically all natural materials contain radioactive nuclides, though usually in such low concentrations that they can be detected only by very sensitive analysis. This is true for water (rain water, rivers, lakes, sea), rocks and soil, and all living matter, as well as for structures on natural raw materials except where great care has resulted in use of anonradioactive material. The radionuclides in our environment can be divided into (i) those formed from cosmic radiation, (ii) those with lifetimes parable to the age of the earth, (iii) those that are part of the natural decay chains beginning with thorium and uranium, and (iv)those introduced in nature by modern techniques. v Sources can be categorized as: (i) cosmogenic宇生放射性的 , (ii) and (iii) primordial原生放射性的 , and (iv) anthropogenic 人類活動產(chǎn)生的 .Cosmogenic radionuclidesv Cosmic irradiation of the atmosphere produces neutrons and protons which react with N2, O2, Ar, etc. resulting in the production of radioactive nuclides. These nuclides are produced at constant rates and brought to the earth surface by rain water. Equilibrium is assumed to be established between the production rate and the mean residence time of these radionuclides interrestrial reservoirs (the atmosphere, the sea, lakes, soil, plants, etc) leading to constant specific radioactivities of the elements in each reservoir. Cosmogenic radionuclidesv If a reservoir is closed from the environment, its specific radioactivity decreases. This can be used to determine exposure times of meteorites to cosmic radiation (and the constancy of the cosmic radiation field, using 81Kr), dating marine sediments (using 10Be, 26Al), groundwater (36Cl), glacial ice (10Be), dead biological materials (14C), etc. The shorterlived cosmogenic radionuclides have been used as natural tracers for atmospheric mixing and precipitation processes (. 39Cl or 38S). Only T and 14C are of sufficient importance to deserve further discussion.Tritiumv Satellite measurements have shown that the earth receives some of the tritium ejected from the sun. Much larger amounts are formed in the atmosphere through nuclear reactions。 ., between fast neutrons and nitrogen atomsv The yield for this reaction is about 2 500 atoms tritium per second per square meter of the earth39。s surface。 the global inventory is therefore about 1018 Bq. Tritium has a halflife of y, decaying by weak β emission to 3He.Tritiumv It is rapidly incorporated in water, entering the global hydrological cycle. The average residence time in the atmosphere is about 2 y which is a small fraction of the halflife, as once the tritiated water reaches the lower troposphere, it rains out in 5 20 days. If we define 1 TU (Tritium Unit) as 1 tritium atom per 1018 hydrogen atoms, 1 TU corresponds to 118 Bq/m3. Before the advent of nuclear energy, surface waters contained 2 8 TU (an average value of TU is monly used). The tritium content in water now monly is of the order 20 40 TU. Rainwater contains between 4 and 25 TU, lower at the equatorial zone and increasing with latitude.Tritiumv Tritium is also a product in the nuclear energy cycle, some of which is released to the atmosphere and some to the hydrosphere. The emissions differ between reactor types (usually in the order HWR PWR BWR) and is a function of the energy production. Assuming the annual releases to be 40 TBq/GWe (Giga Watt electricity) from an average power plant and 600 TBq/GWe from a typical reprocessing plant, the annual global injection of tritium in the environment is estimated to ~10 PBq in 1992. Though this is a small fraction of the natural production, it cause
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