should be able to give an initial estimate of the amount of radioactivity released. The accuracy of the first assessment of the source term will depend upon a number of factors, particularly whether or not the release was via a monitored stack. From this initial estimate of the source term and from the available meteorological data, an assessment can be made of which areas, if any, of Hong Kong may be directly affected, together with an approximate calculation of the degree of radiological hazard, if appropriate. In order to facilitate this it may be useful to adopt the concept of a 'nominal source term'. This is a source term which has been defined both in terms of magnitude and isotopic composition. A suggested nominal source term is given in Table 7.1, which represents a very large release towards the upper end of the range of possible source terms. Given this nominal source term the radiological consequences together with the appropriate necessary countermeasures may be calculated for different weather conditions (in Table 4.1 some of the radiological consequences have been calculated for this nominal source term). Such calculations may be made by the methods given in the NRPB Emergency Data Handbook (ref 20) or by the use of appropriate computer codes. The advantage of using the nominal source term concept is that, from the initial data available about the source term, an estimate of the likely radiological consequences may be quickly made by simply scaling the nominal source term consequences. As a rough ‘rule of thumb' a tenfold increase or decrease in the source term will result in about a fourfold increase or decrease in the distance out to which countermeasures are required. When using this method it must always be borne in mind that the prejudged nominal. source term may have a significantly different isotopic composition to the actual source term. As such this method should be used for gaining an early feel for the size of the accident and it is unlikely that countermeasures would be initiated on the basis of this information alone. It must be emphasised that contingency planning must consider a wide range of possible source terms. The term given in Table 7.1 must not be used as the sole basis for contingency planning.
If the release is travelling towards Hong Kong the installed detectors will record the dose rate as the plume passes. In attempting to interpret these data the assessor has two main problems:
(i)
the isotopic mix will have to be assumed;
(ii)
the duration of the release is unlikely to be known.
The difficulty with not knowing the isotopic mix accurately at this stage is that the dose rate measured by the installed equipment may be dominated by the noble gases, whereas the radiological hazard from the plume is likely to be dominated by the volatile fission products in the plume, in particular the iodines. Faced with this uncertainty cautious assumptions must be made until more survey data are available.
It is a common practice in planning for reactor accidents to assume that the radiological hazard will be dominated by iodine-131. This simplification can be useful to the assessor if he or she is relying on simple calculational methods, however until the isotopic composition of the release is known this assumption should be used with. caution. If computer models are available many radionuclides may be considered in the assessment, and this will be preferable. Even if computer models are available, the assessor should always retain the ability to perform simple 1-131 only calculations in case of computer failures.
Not knowing for how long the release will continue clearly presents the assessor with difficulties. A high measured dose rate may present a small hazard if it only continues for a few minutes, but if it is likely to continue for several hours then the hazard may be significant. Information about the likely release duration may be available from the reactor operators but if they are not in a position to give this information it should be assumed that the release will continue for the next four hours. If the release terminates in less than four hours then the assessment of the hazard will prove to be pessimistic. If, however, after two or three hours the release appears to be continuing a reassessment can be made.
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