【正文】 dionuclidesv 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。 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 causes local increases.Tritiumv The hydrogen bomb tests conducted in the atmosphere during the decade of the 195039。s injected large amounts of tritium into the geosphere。s surface。 it decays by soft β emission(Emax 158 keV).14carbonv 14C is also formed by reaction in nuclear tests. From these 220 PBq is assumed to have been injected into the atmosphere up to 1990. This 14C es to equilibrium with other atmospheric carbon (CO2) in 1~2 years. Some 14C, about 18 TBq/GWe per year, is also released from nuclear power plants (mainly from HWR). The global atmospheric value is 300 TBq/y.v The bustion of fossil fuel adds CO2, which is almost free of 14C, to the atmosphere, thus reducing the specific activity (the dilution was about 3% for the period 1900 1970). Taking all anthropogenic sources into account, a global average specific activity of modern carbon is now 177。當(dāng)?shù)貧す袒瘯r(shí)，這些核