4.1 Source Term Concepts: Overview and Readers Guide to this Chapter
The Source Term is the release to the atmosphere of radioactive matter, like noble gases, iodine and cesium, that can occur as a result of an accident.
In this chapter the reader is assisted by explaining central concepts of source term technology, namely the fuel inventory of radioactivity, confinement barriers, leak paths and releases to the atmosphere, i.e. Source Terms.
The complete range of possible outcomes of an accident is mapped up by pointing out those plant, core and containment states that strongly affect releases to the atmosphere, Table 4.1.
The Source Terms can vary in size over at least seven orders of magnitude and are accordingly sorted into seven Classes of severity: from small foreseen Design Basis Accident (Class 1) through to extreme events (Class 7) on the International Nuclear Event Scale (INES) [8]. The INES scale is given in Appendix 3. This scale was originated for use in retrospect to record post factum the outcome of particular event and to communicate its severity via mass media to the general public. The outcome of the two well known historical accidents in 1979 at
Three Mile Island (unit 2, Harrisburg) and in 1986 at Chernobyl (unit 4) is benchmarked.
A “rule of thumb” formalism is suggested for use in the emergency organization in order to provide as early as possible in an acute emergency a rough order of magnitude prediction of an imminent Source Term. This early prediction will later in time be modified and substantiated by real data from monitor observations when they eventually become available.