NRPB - W16 Atmospheric Dispersion from Releases in the Vicinity of Buildings

Synopsis

A non-technical summary  is also available.

In the United Kingdom, the Food Standards Agency (FSA) has statutory responsibility for protection of the food supply from radioactive contamination. One potential source of such contamination is the routine discharge of radioactivity to air from licensed sites. Air flows around buildings on a site have the potential to influence the local air concentrations and resulting deposition (building wake effects). FSA carries out routine measurements of activity concentrations in foods around such sites, but also relies on the use of mathematical models to estimate the likely impact of airborne discharges on the foodchain, including descriptions of the impact of buildings. These models are continually reviewed, and the present study forms part of this process. The purpose of this study is to explore whether there are circumstances in which it is necessary to model the influence of building wakes explicitly in order to obtain reliable estimates of activity concentrations in food from continuous releases, and how to model dispersion from sites with several buildings. The study was intended to provide advice to FSA on the extent to which its current models are adequate, to indicate whether there are circumstances for which the explicit modelling of building wake effects is required, and, if so, to recommend an appropriate model for this.

In order to carry out this study, three atmospheric dispersion models were selected, namely:

Results from the models are presented and compared, for a range of on-site building configurations and release locations. In addition, the extent to which details of the buildings on the site are required in the complex model is considered; results are presented for several different descriptions of the building complex and conclusions are drawn. In particular, results are presented for the models which show the sensitivity of the predicted air concentrations and depositions to:

• the ways in which particular features of a nuclear site are described in the models.
• the extent to which small buildings should be included in the calculations.
• whether the models are sensitive to the precise location of the release point relative to buildings.

The main findings of this project can be summarised as follows. The predictions of NRPB-R91 and ADMS for air concentration within about 10 km of the release if buildings are not considered are generally within a factor of 2 to 3. This is similar to the difference which the UK Atmospheric Dispersion Modelling Liaison Committee  felt could be expected between observed concentrations and predictions of Gaussian plume models. The assumptions used by FSA as to where locally produced food may be grown mean that FSA is concerned with levels of deposition close to the site, and therefore potentially subject to building wake effects. Building effects only modify predicted concentrations by more than a factor of 2 at distances within less than about 1 km of the release point. For the Amersham site, where the buildings are small (less than 10 m tall) the effects are confined to within 100 or 200 m of the release point. At Dungeness, the effects extend to larger distances, but they are still confined to distances less than 1 km from the stack. It is reasonable to conclude that building effects in general are only important at distances of a few hundred metres from a site, and so buildings need not be considered in assessing doses at larger distances. Within this distance range, some predictions are sensitive to particular features of the model, and results of any calculations in this region should be treated with caution. ADMS is considered to be a more appropriate model than a simple Gaussian model if building effects must be considered as it treats a number of effects that cannot easily be considered in simpler models. In particular, the practice of using NRPB-R91 with the stack height divided by 3 is not judged to be an adequate alternative model; it does not approximate the relevant physical processes in the building wake and overestimates the concentration at larger distances out to at least 10 km.

This study was funded by the Food Standard Agency (reference numbers RR42, RO1048).

Last reviewed: 29 July 2009