TNAG-1776-FCO40-2530-Daya-Bay-nuclear-power-station-project-safety-concerns-in-Ho-1988 — Page 185

Delay Time (hours)

2

4

6

I 2

24

Effectiveness (%)

80

60

50

25

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Stable iodine tablets will only provide protection from radioiodine and indirectly from tellurium-132 which decays to iodine-132. The use of stable iodine as a prophylactic is not universally advocated. In the case of Hong Kong making contingency plans for potential accidents at the Daya Bay plant, it is difficult to postulate any accident, even a large beyond design-basis accident, where general distribution of stable iodine tablets would be of any significant benefit over and above sheltering alone. Planning for their general distribution in Hong Kong is therefore not recommended. Small stocks of potassium iodate tablets should, however, be held to give to anyone who had been close to the Daya Bay site at the time of a major accident and has subsequently returned to Hong Kong within 12 hours, and also to give to any emergency staff who need to be outside when the public in the area are being advised to shelter.

Ground Contamination

The advice in the previous paragraphs will ensure that the public will be adequately protected from the immediate effects of the radioactive plume even in the event of a very large beyond design-basis accident. Such an accident would leave ground contamination in the area over which the plume has passed. In some circumstances, for example local heavy rainfall during the passage of the plume, particularly high levels of contamination could occur. It is possible that the residual ground contamination in such 'hot spots' could be such as to warrant consideration of relocating people from these areas.

The possibility of this contingency arising is so remote that detailed planning for relocation of large groups of people is not justifiable. However, even if such a situation were to arise the necessary monitoring and assessment systems will be in existence. Relocation, as opposed to early evacuation, is designed to prevent the accumulation of undesirably high doses of radiation over a period of days, weeks or even years. As such, time would be available to organise any necessary relocation after the catastrophic accident has occurred.

Hong Kong Airport

In the event of a major release of radioactivity, following a beyond design-basis accident, which affects Hong Kong, consideration should be given to diverting or curtailing flights during the short period of the plume passage.

9.8 The Recovery Phase

The recovery phase is the time-period during which any countermeasures which may have been imposed during the early or intermediate phases will be reviewed and lifted at the appropriate time.

Of the range of conceivable accidents which could occur at the Daya Bay plant the vast majority would have little or no physical effect upon Hong Kong. In a small fraction of these accidents it is possible that radioactive contamination will have affected some agricultural produce, in particular milk. In such cases, action in the recovery phase will consist of continued monitoring of the food chains with any interdictions on agricultural produce being

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