Civil Engineering Reference
In-Depth Information
contamination gets washed off with continuing rain. This process depends on the
rain intensity and is modeled by using data describing the retention capacity of
leaves for different precipitation classes.
20.4.2.3 Cloud Depletion
Due to mass conservation, deposition of radionuclides from the cloud leads to a
depletion of the concentration in the cloud—the material deposited on surfaces is
no longer present in the cloud.
20.4.3 Processes and Models for the Transport of Activity
Through the Human Food Chain
Airborne activity is deposited onto the ground and onto vegetation parts above
ground by dry and wet deposition processes; the latter causing the external con-
tamination of plants, to begin with. A fraction of the deposited activity gets carried
into the interior of the plant and transported to other parts of the plant (transloca-
tion), also to those underground, while the activity remaining on the above-ground
plant surfaces gets removed by weathering processes as time goes on, and will end
up finally on the topmost soil layer. The direct deposition onto plants and the
processes following afterwards depend significantly on the seasonal development
of the plant. This is the reason why the date of an accident plays a crucial role for
the ingestion pathways in regions with pronounced seasonal variations of the
climate.
Because direct deposition can occur only during the passage of the radioactive
cloud, all respective processes act only in the affected plants of the current
vegetation and harvesting cycles, respectively, that is, typically within the first
year after the deposition event. On the other hand, there are the processes of root
uptake (the transport of activity from the soil into the plant via the roots) and
resuspension (the repeated external pollution by raised-up radioactive soil particles)
that lead to a longer term contamination of plants. As a rule of thumb one may say
that the contamination by direct deposition will exceed the contamination by root
uptake by one or more orders of magnitude, provided there are any plant parts
above ground at the time of the accident.
For the human consumption of plant products, only the contamination of the
edible parts plays a role—leaves (salad, vegetables, herbs); fruit and fruit vegeta-
bles; roots (e.g. potatoes, carrots); and cereals.
An uptake of activity by animals followed by biological transport and excretion
processes leads to a contamination of animal food stuffs—milk and dairy products,
meat, and eggs. Activity uptake via feed stuffs is the most important pathway for
animal contamination. However, also inhalation from the cloud may play a role for
