Civil Engineering Reference
In-Depth Information
20.3.3.1 Measures in the Early Phase
In the early phase, sheltering and evacuation are measures for protecting the
population partially or completely against external irradiation from the passing
cloud, against external irradiation from activity deposited onto the ground, and
against internal irradiation following inhalation of radioactive material from the
cloud. The intake of stable iodine tablets is a measure for blocking the thyroid
against inhaled radioactive iodine in cases where the inhalation itself can or could
not be prevented e.g. by evacuation before cloud arrival. Various measures
addressing the ingestion pathways should also be taken early, for instance, public
recommendations not to consume locally produced milk or vegetables in order to
prevent the uptake of radioactive substances by ingestion of these food stuffs.
If possible, evacuation, or the distribution and intake of iodine tablets, should be
ordered and carried out as a preemptive measure in the pre-release phase. While a
release is going on, it may become impossible to carry them out at all, or makes
little sense due to uncertainties about the future development of the accident and the
weather. The request for staying indoors (with windows closed and ventilation
systems turned off) is meaningful as a preventive measure even when a release has
already begun.
In Germany, the intervention level for evacuation is 100 mSv, to be applied to
the projected effective dose resulting from external cloud gamma exposure during
the cloud passage, ground gamma exposure over the first seven days plus the
projected effective committed dose from inhalation during cloud passage. The
intervention level for sheltering is the projected effective dose of 10 mSv, consid-
ering the same exposure pathways and integration times as for evacuation.
Timely evacuation can entirely avoid any radiation exposure of the population.
On the other hand, evacuation constitutes a major disruption of normal life, and the
risks associated with this measure must not be disregarded in decision making.
For indoor locations, the achievable shielding against external exposure depends
on the type of building and the construction materials. As an example, Table
20.1
shows the mean shielding factors derived for open air without surrounding struc-
tures and for various types of buildings that are used in the RODOS decision
support system; a value of 1 means no shielding effect. Contaminated surfaces in-
and outside buildings and of vegetation—in particular trees—in the vicinity of a
given location can increase the exposure in comparison with the open-air irradia-
tion, thus giving rise to shielding factors greater than 1.
In Germany, the intervention level for the distribution of iodine tablets is 50 mSv
for children under the age of 18 and pregnant women, and 250 mSv for adults, to be
compared with the committed dose to the thyroid resulting from the inhalation of
radioactive iodine nuclides. Preemptive intake before the inhalation takes place
offers 100 % protection; a slightly delayed intake, e.g. 3 h later, still reduces the
thyroid dose commitment by 50 %. In iodine deficiency areas, however, this
measure can lead to health problems in particular for the elderly, therefore, in
