NRPB-W2: Non-technical summary

Non-technical summary

The Atmospheric Dispersion Modelling Liaison Committee (ADMLC) consists of representatives of government departments, government agencies and organisations in the nuclear industry. It was established mainly to review current understanding of atmospheric dispersion and to identify suitable models for application primarily in the authorisation or licensing processes for the nuclear industry. Contracts are placed by ADMLC to carry out pieces of work on topics of interest. This report presents the work done during the 1999/2000 financial year.

The dispersion of plumes travelling over land has been studied for a number of years and a large number of models have been developed. The models make a number of assumptions which mean that they are not strictly valid for flow over water, so ADMLC placed a contract to review those models that are appropriate to flow over water and to recommend ways in which the models might be improved.

The first study described in this report showed that significant advances have been made recently in understanding some aspects of air flow over water. It identified some simple models that consider some of the features of dispersion over water. It concluded that no single model satisfactorily combines all the features required, and suggested ways in which the available models might be improved.

Information on estimated concentrations in air of material discharged over long periods can be obtained in one of two ways. The first way uses data on the atmospheric conditions for every hour of the release period, calculates the concentrations in each hour and obtains the required quantity (which may be the average concentration or a value which is only likely to be exceeded on a few occasions) from the results for each hour. This requires a very large number of calculations and so considerable amounts of computer time. The second way is to consider a number of atmospheric conditions which represent the conditions occurring over the release period. The quantities of interest can then be derived from the predicted concentration in each of the representative conditions, and the frequency of those conditions. This requires less computation than the first method. The second study described in this report compared concentrations predicted by the two methods. The authors conclude that the differences in the results are generally small, and that, in many cases, the use of representative conditions is appropriate for such calculations.

Last reviewed: 1 September 2009