TNAG-1776-FCO40-2530-Daya-Bay-nuclear-power-station-project-safety-concerns-in-Ho-1988 — Page 165

Having obtained these data the assessor should review his or her earlier calculations which were based upon a predetermined isotopic mix. Other information on gamma dose rates throughout Hong Kong should by now be being collected and a picture of the magnitude and extent of the release can be built up and used to decide which, if any, plume countermeasures are appropriate. When plotting these data a number of different methods can be used, for example:

(i)

(ii)

(iii)

gamma dose rates alone;

dose to an individual based upon external gamma dose and the internal dose due to inhalation, assuming a four hour linear release (unless other information on the duration of the release is available). This dose may be expressed either in millisieverts or in emergency reference levels for a particular countermeasure, say, sheltering;

time to reach the emergency reference level for a particular countermeasure, say sheltering, based upon both external and internal dose assuming the release continues steadily.

Each of these plotting systems has its own advantages and disadvantages, however it is likely that (iii) would be most useful to the MAC Controller and the CCC.

Fourth Stage

After the passage of the plume, ground deposition can be assessed. It is possible to assess ground deposition whilst the plume is still passing if samples are taken, however it is difficult to do this by direct probe measurement.

Early samples of grass, etc., will allow deposition to be measured and gamma spectrometry will allow the isotopic mix of the deposition to be assessed. From these results the level of deposition in terms of the appropriate derived emergency reference levels (DERL) may be obtained and plotted. By comparing this with the gamma dose rate at the sampling point after the plume has passed, deposition can be quickly assessed by gamma dose rate measurements at other locations. Care should be taken however as some of the deposited gamma emitters are likely to be short lived and the ratio of gamma dose rate to DERL values are likely to change with the passage of time.

As time progresses more and more data will become available to the incident assessor. By interpolation and extrapolation he or she will be able to build up a complete picture of the potential radiological hazard to the people of Hong Kong and will therefore be able to recommend the appropriate countermeasure actions. Care should be taken when interpolating or extrapolating deposition values if there is any rainfall at the time of the release, since the amount of rainfall whilst the plume is passing is likely to dominate the deposition mechanisms. The use of the Royal Observatory's system of real-time rainfall gauges will greatly aid the incident assessor when trying to estimate levels of ground deposition.

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