INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE Figure 2.2: The Properties <strong>of</strong> <strong>the</strong> Standard <strong>Atmosphere</strong> Depict<strong>in</strong>g <strong>the</strong> Variation <strong>in</strong> <strong>the</strong> Height <strong>of</strong> <strong>the</strong> Tropopause with Latitude (from Transport Canada, 2004). BACK TO TABLE OF CONTENTS 10
INVESTIGATION OF THE ENVIRONMENTAL FATE OF TRITIUM IN THE ATMOSPHERE Transport <strong>of</strong> air masses and debris from <strong>the</strong> stratosphere to <strong>the</strong> troposphere arises from <strong>the</strong> action <strong>of</strong> turbulent diffusion caused by currents <strong>of</strong> horizontal circulation that lead to seasonal changes <strong>in</strong> <strong>the</strong> tritium content <strong>of</strong> ra<strong>in</strong> (Bibron, 1965). Extremely fast jet currents, particularly <strong>in</strong> sub-tropical reaches, can <strong>in</strong>itiate relatively important exchanges that facilitate quite rapid north-to-south mix<strong>in</strong>g <strong>in</strong> <strong>the</strong> stratosphere (Libby, 1958). Therefore, tritium becomes more homogeneously mixed <strong>in</strong> this way (Begemann, 1958). These jet currents reach <strong>the</strong>ir maximum <strong>in</strong>tensity at <strong>the</strong> end <strong>of</strong> <strong>the</strong> w<strong>in</strong>ter and <strong>in</strong> <strong>the</strong> spr<strong>in</strong>g, lead<strong>in</strong>g to seasonal variations <strong>in</strong> tritium fallout, with maximum values occurr<strong>in</strong>g <strong>in</strong> <strong>the</strong> mid-latitud<strong>in</strong>al areas dur<strong>in</strong>g <strong>the</strong> spr<strong>in</strong>g and early summer as tritium is washed from <strong>the</strong> stratosphere <strong>in</strong>to <strong>the</strong> troposphere (Libby, 1963; Taylor, 1964; Taylor et al., 1963). Specifically, it has been estimated that approximately half <strong>of</strong> <strong>the</strong> tritium orig<strong>in</strong>at<strong>in</strong>g from <strong>the</strong> detonation <strong>of</strong> <strong>the</strong>rmonuclear devices falls <strong>in</strong> <strong>the</strong> zone between 30 o and 50 o latitude (Libby, 1963). Follow<strong>in</strong>g entry <strong>in</strong>to <strong>the</strong> troposphere, vertical mix<strong>in</strong>g becomes quite pronounced, lead<strong>in</strong>g to a rapid flush<strong>in</strong>g out by ra<strong>in</strong>s correspond<strong>in</strong>g to a relatively short tritium residence time <strong>in</strong> <strong>the</strong> troposphere (approximately 21 to 40 days) compared to <strong>the</strong> one to 10 year residence time <strong>in</strong> <strong>the</strong> stratosphere (Barrett and Huebner, 1960, 1961; Begemann and Libby, 1957; Bol<strong>in</strong>, 1964; Brown and Grummitt, 1956; Libby, 1958, 1963; Walton et al., 1962). <strong>Tritium</strong> transport can also occur through eddy diffusion, <strong>the</strong> rate <strong>of</strong> which is dependent upon air motion, <strong>the</strong> rate <strong>of</strong> evaporation over water bodies, as well as <strong>the</strong> relative humidity <strong>of</strong> <strong>the</strong> air (Engelke and Hemis, 1962; Eriksson, 1965). 2.4.2 Nuclear Reactors <strong>Tritium</strong> is produced by a number <strong>of</strong> processes <strong>in</strong> reactors, as depicted <strong>in</strong> Figure 2.3 below and as discussed <strong>in</strong> <strong>the</strong> sections that follow. In general, <strong>the</strong>se reactions can be sub-divided <strong>in</strong>to tritium production by ternary fission (or fission <strong>of</strong> <strong>the</strong> reactor fuel); and neutron activation reactions with lithium and boron isotopes dissolved <strong>in</strong> or <strong>in</strong> contact with <strong>the</strong> primary coolant, or with naturally-occurr<strong>in</strong>g deuterium <strong>in</strong> <strong>the</strong> primary coolant (Estournel, 1962; UN, ILO and WHO, 1983). The relative importance <strong>of</strong> <strong>the</strong>se reactions <strong>in</strong> <strong>the</strong> production <strong>of</strong> tritium is dependent upon <strong>the</strong> type <strong>of</strong> reactor and its design. For <strong>the</strong> purposes <strong>of</strong> this report, four reactor designs were discussed from <strong>the</strong> perspective <strong>of</strong> tritium generation. These <strong>in</strong>cluded pressurized-water reactors (PWRs), boil<strong>in</strong>g water reactors (BWRs), heavy water reactors (HWRs) and gas-cooled reactors (GCRs). The relative tritium contributions <strong>of</strong> each design with respect to tritium generation and tritium release via effluent streams have been summarized <strong>in</strong> Table 2.2. BACK TO TABLE OF CONTENTS 11