TNAG-1776-FCO40-2530-Daya-Bay-nuclear-power-station-project-safety-concerns-in-Ho-1988 — Page 180

Application of Countermeasures for Water Supplies

In the event of an accident at Daya Bay the Incident Assessor in the MAC would assess whether or not the release was likely to affect water supplies and if so make an initial estimate of the likely degree of contamination. This information would be passed to the CCC Controller who would in turn pass it on to the Director of Water Supplies' Liaison Officer in the CCC. The Water Department would then initiate the prearranged procedure for sampling of the water entering the reservoirs, in the reservoirs (at a variety of depths), and the water both entering and leaving the water treatment plants. If appropriate, suitable countermeasures, discussed below, will be actioned by the Water Department. The Water Department will keep the CCC controller briefed on the actions they are taking via their liaison officer in the CCC. The CCC Controller will ensure that the public are informed of any actions they need to take.

Calculations have illustrated that even a very large release of radioactivity from Daya Bay could have only a relatively small effect upon water supplies. As such it is highly unlikely that levels of contamination in even the reservoirs closest to Daya Bay could ever reach the lower DERL for the interdiction of water supplies. It is however a fundamental principle of radiological protection that all doses should be kept as low as reasonably practical (ALARP). Even the small amount of radioactivity in the water supply could be significantly reduced by a number of measures:

(i)

(ii)

(iii)

Use water from non contaminated, or the least contaminated, source first. This will include taking water from the most appropriate depth in a reservoir.

Restrict the non-essential use of water. This will allow more time for settlement in the reservoirs and in the treatment works, and physical decay of the radioisotopes.

Increase the amount of coagulant used in the water treatment process and increase the time allowed for settlement.

The use of these measures could have a large reduction in collective dose to the population of Hong Kong. The use of water from the least contaminated supply first is an obvious method by which doses to the public may be reduced. As time passes the level of radioactivity in any reservoir will reduce by the processes of sedimentation and exchange (ref 29) and physical decay of the radionuclides. It therefore also follows that reducing the demand for water and hence extending the time period taken for the water to reach the consumer will also decrease the resultant dose to the consumer.

Increasing the amount of coagulant used in the water treatment process and the use of additional chemicals can result in a worthwhile reduction in radioactivity. Reductions of greater than 90% for I-131 can be achieved by such methods. Details are given in reference 31. Increasing the settlement time can also result in a significant reduction in radioactivity in the water.

The radioactivity removed from the water will mainly appear in the sludge. Volume for volume the concentration of radioactivity in the sludge could be up to about 100 times the concentration in the untreated water. Consideration must be given to the radiological hazard presented by this sludge. It is difficult to postulate a realistic accident scenario where the gamma dose rate at the surface of any tank or container holding sludge could exceed 0.01 mSv h ̄1. As such it is highly unlikely that the water treatment plant workers would be exposed to any significant radiological hazard.

Notwithstanding this, following any major accident in which radioactivity is measured in the untreated water supply, consideration should be given to the potential hazard presented by water treatment sludges. In particular this will involve sampling the sludges for radioactivity and, if appropriate, monitoring the gamma dose rate from

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