Chapter 6: The Dispersion of Radioactivity Releases

6-1 The most interesting releases for safety evaluations

This chapter deals with some simple and quick methods for the evaluation of the dispersion in the environment of gaseous releases (gases, volatile products, aerosols and particulates). Chapter 7 describes some methods for evaluating the health consequences of releases.

There are three steps in the evaluation of the consequences of accidents:

1. Evaluation of the releases (the source term; amount, chemical physical form, trend with time).

2. Evaluation of the dispersion of releases in the environment.

3. Evaluation of the health consequences (see Chapter 7).

The gaseous releases which are dealt with here, are the most relevant ones for the evaluation of the immediate accident consequences and for the preparation of short-term emergency plans. Solid and liquid releases are less important for a nuclear power station because the radioactive products released to the environment are mainly gaseous and have high velocity (which may cause adverse consequences outside the plant). However, liquid releases have to be taken into account under some circumstances. The situation, then, is very different from many non-nuclear process plants where the prevailing accidental release from the point of view of the consequences may frequently be the release of flammable or toxic liquids.

The radioactive isotopes which could in theory be released during an accident from a nuclear power station are listed in Table 2-1. In practice, however, as recalled in Chapter 2, for order of magnitude evaluations and in order to only evaluate the scale of the accident consequences, it is sufficient to...