TNAG-1776-FCO40-2530-Daya-Bay-nuclear-power-station-project-safety-concerns-in-Ho-1988 — Page 184

Sheltering

As discussed earlier in this report, the dose received by a person standing downwind of a typical release of radioactive gases and volatiles is likely to be due primarily to inhalation of radioactive material and, to a lesser extent, due to direct radiation from the passing cloud. If people stay indoors, with the doors and windows closed and ventilation systems switched off, the amount of radioactive material they will inhale will be reduced. In addition the structure of the building will provide some shielding from the direct radiation from the cloud. The amount of dose that sheltering would prevent depends upon the construction of the building and where a person is located in a building, however, typically 50-90% of the direct radiation component of the dose which would have been received may be avoided and up to about 90% of the dose due to inhalation, especially for short duration releases (ref 10). It is clear therefore that sheltering, provided it is carried out correctly and at the appropriate time, can greatly reduce doses to the public.

Sheltering may be implemented within a reasonably short timescale, provided pre-planned arrangements have been made to issue instructions to the public at the time of the accident. These may include the use of radio and television, and mobile public address systems on police vehicles or helicopters, (see Section 11.3). The time-scale would be particularly important if no warning of a release were received until radioactivity has been detected at the monitoring stations in Hong Kong. Prior familiarity of the public with the relevant parts of the contingency plan may therefore be of great benefit. It is worth noting that actions advised in the event of a tropical cyclone following the hoisting of a No. 9 or 10 signal are similar to the correct actions to be taken in the event of a very large release of radioactive material affecting Hong Kong.

If sheltering is to be implemented, the public should be advised to stay indoors, close all doors and windows, and switch off ventilation and air conditioning systems. Any obvious gaps under doors, etc., should be plugged using, say, damp towels. Occupants should then stay in the centre of the building (or in the basement if the building has one) away from windows. It is important to ensure that once the plume has passed the public are advised to open windows and switch on any ventilation or air conditioning systems.

Provided it may be carried out in a timely and orderly manner, the introduction of sheltering for a limited period of time is regarded as a highly effective, low risk, low social cost, countermeasure. It is therefore suggested that consideration is given to its implementation if projected doses are likely to exceed the lower ERL for sheltering (see Chapter 10). A serious problem would arise if there was little notice of the arrival of a large plume. In these circumstances asking the public to shelter at short notice would undoubtedly cause great concern together with chaos on the roads and transportation systems. Fortunately such an eventuality is extremely unlikely. Asking the public to take shelter at short notice should only be initiated if there has been a confirmed very large release of radioactive material from Daya Bay, and the potential doses to the public are well in excess of the lower ERL.

Issue of Stable Iodine Tablets

One of the volatile components of a release of radioactive material which is likely to be of concern is radioactive iodine, particularly iodine-131. If iodine is inhaled it will concentrate in the thyroid of the individual concerned. This can, however, be reduced by administering stable iodine tablets either before inhaling the radioiodine, or shortly afterwards. The effect of taking the stable iodine tablets is to greatly dilute the radioiodine taken into the thyroid, (Ref 37). Taking the tablets before the arrival of any plume would reduce the subsequent dose by about 95%. The approximate effectiveness of delayed administration, in terms of averted thyroid dose from iodine-131, is as follows:

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