Page 107 - Radiochemistry and nuclear chemistry

96 Radiochemistry and Nuclear Chemistry

to the earth surface by rain water. Though they are formed in extremely low concentrations,
the global inventory is by no means small (w167 and 5.1.3). Equilibrium is assumed to
be established between the production rate and the mean residence time of these
radionuclides in terrestrial reservoirs (the atmosphere, the sea, lakes, soil, plants, etc)
leading to constant specific radioactivities of the elements in each reservoir. 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 l~ 26A1), groundwater
(36C1), glacial ice (10Be), dead biological materials (14C), etc. The shorter-lived cosmogenie
radionuclides have been used as natural tracers for atmospheric mixing and precipitation
processes (e.g. 39C1 or 38S). Only T and 14C are of sufficient importance to deserve further
discussion.

5.1.2. Tritium

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; e.g.,
between fast neutrons and nitrogen atoms

n(fast) + 14N-', 12C + 3H (5.~)

The yield for this reaction is about 2 500 atoms tritium per second per square meter of the
earth's surface; the global inventory is therefore about 1.3 • 1018 Bq. Tritium has a
half-life of 12.33 y, decaying by weak/3" emission to 3He. 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 half-life, 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/m 3. Before the advent of
nuclear energy, surface waters contained 2 - 8 TU (an average value of 3.5 TU is
commonly used). The tritium content in water now commonly is of the order 20 - 40 TU.
Rainwater contains between 4 and 25 TU, lower at the equatorial zone and increasing with
latitude.
Tritium is also a product in the nuclear energy cycle, some of which is released to the
atmosphere and some to the hydro sphere. The emissions differ between reactor types
(usually in the order HWR > PWR > BWR, see Ch. 19) and is a function of the energy
production. Assuming the annual releases to be 40 TBq/GW e (Giga Watt electricity) from
an average power plant and 600 TBq/GW e 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.
The hydrogen bomb tests conducted in the atmosphere during the decade of the 1950's
and early 1960's injected large amounts of tritium into the geosphere; 2.6 x 102o Bq up to
the end of the tests in 1962. This considerably exceeds the natural production inventory.
Before 1952 (first hydrogen bomb tests) the tritium content could be used to date water
(i.e. determine when it became isolated from contact with the atmosphere). This was very