Agriculture Reference
In-Depth Information
I NTRODUCTION
The industrial production of fertilizers uses raw material that contains high
concentrations of naturally occurring radionuclides. Therefore it has been considered as a
NORM (Naturally Occurring Radioactive Material) industry, and its processes, by-products,
end products, and wastes must be controlled due to the radiological hazard to the general
population that they may pose (IAEA, 2003). Phosphate ores are the most important raw
material, which can present enhanced concentrations of uranium and thorium, and their
descendants in various degrees of equilibrium, depending on its origin. During the industrial
treatment of these ores to produce fertilizers, the equilibrium within the natural decay chains
is disrupted and the naturally occurring radionuclides are fractioned according to their
physico-chemical properties and the industrial procedure. As a result, the fertilizers produced
and the by-products and wastes generated are also enriched in naturally occurring
radionuclides, depending on the procedure and the by-product considered. In order to assure
the adequate radiological protection to workers and the general population, the pathways in
which they are exposed to those radiations must be evaluated. External exposure to γ-
radiation, inhalation of radon and their transfer into foodstuff are among the most important
pathways to man. The release of wastes, mainly phosphogypsum, into the environment can
also have a radiological impact, especially in aquatic environments. In terrestrial ecosystems,
the fertilizers can also be used to reduce the transfer of anthropogenic radionuclides to plants,
by supplying them with the stable element chemically analogue.
D ETERMINATION OF R ADIONUCLIDES
Radionuclides are unstable isotopes that usually lose their excess energy by the emission
of ionizing radiation. The emission probability is different for each radionuclide and is
expressed in terms of its half-life. The half-life of a radionuclide, T ½ , is the time it takes for
the activity to decrease by a half. There are three main types of ionizing radiation: α, β, and γ,
whose main properties are the following:
α-particles are helium nuclei (2 protons + 2 neutrons). They have high values of mass
and kinetic energy, the latter within the range 3.95-8.78 MeV 1 . Their interaction with
matter is so strong that even a sheet of paper is enough to stop them, with all their
energy being lost in crossing its thickness. Most of α-emitting radionuclides are
naturally occurring: 238 U, 226 Ra, 210 Po, 232 Th, etc.
β-particles have various origins depending on the characteristics of the nucleus (β - ,
β + , EC or electron conversion, …), but the final result is the emission of electrons or
positrons within a wide range of energies (0.01859-3.54 MeV). Due to their low
mass and charge compared with α-particles, they can penetrate more deeply into
matter, requiring a few centimetres of aluminium to stop them. Some β-radionuclides
are: 3 H, 210 Pb, 90 Sr, 40 K, 137 Cs etc.
1 An electron volt (eV) is a unit of energy equal to the amount of kinetic energy gained by a single unbound
electron when it accelerates through an electric potential difference of 1 V. 1 eV = 1.609·10-9 J.
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