Civil Engineering Reference
In-Depth Information
6.4.3 Releases of Radioactivity
Radioactivity may be released from the reactor core in the following events
[
10
-
13
]:
- In cladding tube failures the gaseous and highly volatile fission products are
released.
- As the fuel is heated to melting temperature and melts, fission products as well as
chemical compounds with lower melting points will be released as aerosols.
- During interaction of the melt with the concrete, aerosols are generated.
In a pipe leak of the primary circuit, or if the reactor pressure vessel has been
penetrated by molten core material, the gaseous and volatile fission products will
enter the containment. They can be retained there by active removal (e.g. spray)
systems and by diffusion, coagulation, condensation, sedimentation and
thermophoretic processes of the aerosols. Radioactive decay makes the retention
by the containment more effective, the longer its integrity can be ensured. Studies
(German Risk Study, Phase A [
2
]) showed that the time after which the maximum
pressure would be exceeded and the containment fails would be like 5-12 days
(depending on the concrete composition). Within such a time period of about 5 days
or more, the concentration of airborne aerosols decreases already by orders of
magnitude [
11
]. This is shown by Fig.
6.3
.
6.4.4 Distribution of the Spread of Radioactivity After
a Reactor Accident in the Environment
In reactor risk studies, releases of radioactivity are determined for various accident
categories and all kinds of meteorological conditions at all the different reactor
sites. This is used to determine a mean value for the consequences of radioactive
exposure (early deaths, late health consequences, soil contamination). However, an
accident at a specific reactor site is determined only by the weather conditions
prevailing at that time.
Computer codes, such as COSYMA [
14
] and RODOS [
15
], were written to
describe this situation on the basis of the release of radioactive gases and aerosols
during the accident and further spreading of this radioactivity in the atmosphere as a
function of weather conditions and wind direction (Fig.
6.4
). For each point at a
certain distance from the reactor site it is possible in this way to determine the
radioactivity in the atmosphere and contamination of the soil. The radioactive
exposure of the population and the environment is determined on this basis.
The radioactive cloud causes external and internal radiation exposures of per-
sons. The external radiation exposure is the result of
- cloud-borne radiation (radioactive nuclides),
