PWR incorporate a pressure relief system in the containment such that if, following a major accident, there was a high pressure inside the containment building, some gases and volatiles could be vented to atmosphere via filters. In this, and a number of other ways, a relatively small quantity of radioactivity could be released into the environment in the event of a major accident at the reactor. It should be emphasised that in such a major accident scenario, which is itself extremely unlikely, the majority of the fission products would remain in the fuel pins and of the fission products which escape into the containment only a small fraction is ever likely to be released into the atmosphere.
4.3 Behaviour of Fission Products in the Atmosphere
The behaviour of fission products released into the atmosphere is complex. Where they go, and hence the areas which may be affected, depends upon a number of factors, including wind direction and speed, the weather generally, topography, etc.
In general the cloud or 'plume' of fission products will travel downwind. The inert gases such as xenon and krypton will simply travel downwind becoming more and more dilute. Some of the volatiles, such as iodine, caesium and tellurium, will have already condensed into particles by the time they reach the atmosphere. These particles will be deposited on areas over which the plume passes with the amount of deposition generally decreasing with increasing distance from the reactor.
The geometry of the plume will depend upon a number of factors especially the weather. It will also depend on the amount of heat that the plume contains, since a hot plume will tend to rise. For planning purposes it is generally assumed that the plume would occupy the 30o sector downwind of the damaged reactor, however weather conditions on the day of the accident could mean that the plume angle was as little as 10° or as wide as 60°.
TABLE 4.1 ILLUSTRATION OF THE EFFECTS OF WEATHER AND DISTANCE ON POTENTIAL DOSE (ASSUMING NO COUNTERMEASURES)
Approximate
Weather Category
% occurrence in Hong Kong
Distance (km)
Typical time for plume to Arrive
External Dose from Plume (millisieverts)
Thyroid Dose due to Inhalation
(millisieverts)
A
1%
B
8%
C
11%
Ꭰ
50%
E
10%
25
50
F
17%
10
25
50
900 900 ano ano ano eno
10
2 hrs 46 min
0.03
1.3
25
6 hrs 56 min
0.01
0.5
50
13 hrs 53 min
0.005
0.25
10
1 hr 23 min
0.15
2.3
25
3 hrs 28 min
0.01
0.52
50
6 hrs 56 min
0.005
0.26
10
40 min
0.17
2.5
25
1 hr 41 min
0.04
0.62
50
3 hrs 23 min
0.004
0.20
10
33 min
0.28
4.3
25
1 hr 23 min
0.06
1.1
50
2 hrs 46 min
0.006
0.31
10
49 min
0.67
11.0
2 hrs 02 min
0.15
2.7
4 hrs 05 min
0.015
0.77
1 hr 23 min
0.96
33.0
3 hrs 28 min
0.10
6.8
6 hrs 56 min
0.06
2.7
Source term as given in Table 7.1
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