Regional Basic Professional Training Course in Korea

❙ 937 ❙
16. In‐Plant Accident Management
16.5. SEVERE ACCIDENT SOURCE TERM
16.5.1. Introduction
The fission product source term is defined by the quantity, timing, chemical species and
physical forms of the radioactive material released from a degrading reactor core into the
environment. While the containment building in severe accidents acts as an attenuator of
the gas borne debris, mainly by aerosol agglomeration and sedimentation, the primary
circuit modifies the physicochemical form and timing of the radioactive release. These
modifications can occur via condensation of vapours onto structural surfaces and
aerosols, chemical reactions with other species in the system, and aerosol formation and
attenuation processes, such as agglomeration, deposition, etc.
16.5.2. Radioactive releases associated with severe accidents
16.5.2.1. Source of radioactivity
With regard to the source formed by the core, the release of fission products mainly
occurs at the beginning of the degradation stage (clad rupture and core meltdown), which
follows its dewatering, while the fuel temperature follows a range from 1300 to 1800°C.
Chart one gives, per fission product group and starting with the most volatile, the order of
size of the fraction of the core inventory that is released in this way. Over a greater time
scale, i.e., until the vessel has failed, a part of the fuel and the structures is vaporized. At