TNAG-1776-FCO40-2530-Daya-Bay-nuclear-power-station-project-safety-concerns-in-Ho-1988 — Page 140

Weather conditions are often classified into stability categories using a scheme originally devised by Pasquill (ref 41). This divides atmospheric stability into six categories designated A to F which correspond to variations in atmospheric stability ranging from extremely unstable (Category A) to very stable (Category F). The stability category can have a major effect on the potential dose received by a person standing downwind of a release. In stable conditions, say category F, there will be very little turbulence in the atmosphere and hence the plume will disperse less than in average weather conditions, typically category D. In general, very stable conditions are associated with low wind speeds and therefore more time will be available to prepare for the arrival of the plume than would be available in average weather conditions. These effects are illustrated in Table 4.1 where for a given very large release of radioactivity (as could only arise following a major beyond design-basis-accident-typically a core melt followed by basement failure) potential doses have been calculated for various distances downwind of the plume in various weather stability categories. The table also illustrates the frequency of occurrence of each weather category in Hong Kong.

The rate at which radioactivity is deposited will depend upon a number of factors but the most important will be whether or not it is raining. Rainfall will greatly enhance the deposition rate and this is one mechanism by which ground contamination 'hot spots' can occur.

4.4 The Radiological Hazards

Anyone downwind of the reactor may be subject to the effects of the release of radioactivity through a number of mechanisms, the more important of which are:

(i)

direct irradiation from the plume;

(ii)

irradiation from deposited radioactive materials;

(iii)

inhalation of radioactive materials;

(iv)

ingestion of contaminated foodstuffs, especially milk. (See Figure 4.1).

FIG. 4.1 PRINCIPAL RADIOLOGICAL HAZARDS

FOLLOWING AN ACCIDENTAL RELEASE OF RADIOACTIVITY

WIND DIRECTION

RELEASE OF RADIOACTIVITY FROM THE REACTOR

INCREASED DEPOSITION RATE DUE TO RAINFALL

DIRECT IRRADIATION FROM THE PLUME

INHALATION OF RADIOACTIVITY

DEPOSITION ON CROPS, PASTURE, WATER COURSES, ETC.

DIRECT RADIATION FROM REACTOR CONTAINMENT

IRRADIATION FROM

DEPOSITED RADIOACTIVITY

алин миллать

UPTAKE BY GRAZING

ANIMALS AND CONCENTRATION IN MILK

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