Civil Engineering Reference
In-Depth Information
function of the quantitative relationships between the liquid effluents and the
ambient water volume.
The individual radioactive nuclides can enter the human body on various
pathways, where they again may have different radiological impacts.
4.1.1 Exposure Pathways of Significant Radionuclides
4.1.1.1 Tritium, Carbon-14 and Krypton
Tritium (half-life 12.4 year) is produced in the reactor core mainly by ternary
fission; on the average, about 1 in 10
4
fissions of U-235 is accompanied by the
formation of tritium. About twice as many tritium nuclei are formed in the fission of
Pu-239. In addition, tritium is generated in the coolant by neutron capture in
deuterium atoms (deuterium has an abundance in natural water of 0.015 at.%),
and by the interaction of neutrons with the boron control material. It is released
from nuclear reactors and reprocessing plants as HT gas or as tritiated water (HTO),
either into the atmosphere or into, e.g., a river or lake or into the ocean. Gaseous
tritium, HT, is very soon oxidized into HTO. Ultimately, any tritium escaping or
being released in a controlled manner thus will be present as tritiated water. Plants
and animals may contain HTO/H
2
O ratios close to those existing in the environ-
ment. Radioactive exposure of the human body then occurs as a result of the
ingestion of food and drinking water. Moreover, tritiated water (HTO) can be
absorbed by inhalation and through the skin of the human body. In this way, the
β
-radiation (maximum energy 18 keV) of tritium causes a whole body exposure.
C-14 (half-life 5,730 year) is built up in the reactor core by (n,p)-reactions with
N-14, (n,
α
γ
)-reactions with O-17, and (n,
)-reactions with C-13. C-14 emits
-radiation (maximum energy 156 keV). In plants and animals,
14
CO
2
/
12
CO
2
ratios
may be established which are very close to those in the atmosphere. Radioactive
exposure of the human body then occurs mainly as a result of the ingestion of food
(milk, vegetables, meat). Direct inhalation and exposure from the ambient atmo-
sphere only play minor roles.
Kr-85 (half-life 10.7 year) is a fission product. Kr-85 emissions from a LWR are
diluted in the atmosphere. Approximately 99.6 % of the Kr-85 nuclei decay by
emitting
β
-particles with a maximum energy of 0.67 MeV. Only 0.4 % of the Kr-85
nuclei decay by emitting a
β
-radiation
(0.51 MeV). There is no reduction in the airborne concentration as a result of
deposition or washout. Kr-85 is only sparingly soluble in water. Its main radiolog-
ical impact on the human body is due to the exposure to the skin. The inhalation of
Kr-85 plays a smaller role.
β
-particle (maximum energy, 0.16 MeV) and
γ
