Modelling applications

the site, with obvious implications. Usually, the information will be passed on to the emergency services or the Met Office by voice (see Figure 4 on previous page).

Risk areas display

Displaying an estimated area at risk rather than just wind information has many advantages. Confusion about wind direction is eliminated (assuming, of course, that the raw data used in estimating the risk area is sound). The simplest approach is some sort of overlay on a map that is just marking a downwind area. Some authorities use software to do this, while others use just such a transparent overlay to place on a map. There are several variations on the

theme. The Local Authority and Emergency Services Information (LAESI) regulations specify information provided for the emergency services and local authorities on arrangements to be implemented in the event of an emergency during the transportation of nuclear material by the Ministry of Defence. Advice for several scenarios involves asking the public up to five kilometres downwind in a 45° arc centred on the wind direction to take shelter.

Computer models

Computer models of the spread of atmospheric plumes have been around for several decades and have become increasingly sophisticated and accurate. All need some sort of input of wind data, and their accuracy can be no better than the accuracy of that data. In the emergency response scenario, there will not usually be any information on the amount of material released or the rate of release. In actual fact, there may not even be any

information on what has been released. All that can be computed is the spread of material. This is in contrast to supporting the response to incidents over timescales of several hours, when source terms may become available, or air quality applications, often using the same models. Sometimes, the wind data itself is the output of a computer model. The Met Office and other organisations model the wind over the UK as part of the general forecasting process. These are not a best fit to observations and are based on a meteorological model that has various sources of data. While, in principle, a model wind will be inferior to an actual measurement, this will not be the case if the measurement comes from a poorly installed sensor.

Emergency services

The Met Office offers two ‘free at the point of use’ services. Both are based on computer models, but operate in very different ways. They require some input from the user at the time of an incident, and will need new input as conditions change. The CHEMET scheme is based on running a dispersion model specifically for an actual incident, and is activated by a telephone call to the Met Office. The Met Office has a target response time of

20 minutes to provide a detailed response, but in practice the response time is usually nearer 15 minutes. The model used, ‘NAME’, is capable of predicting plumes over large distances (hundreds of miles) over long periods (days) by taking in data from global meteorological models.

Normally, the model uses wind data taken

from computer simulations of the wind over the UK, but it can ingest meteorological data from the site if available. It is also capable of assessing concentrations of material, given the appropriate ‘source terms’ (what is being released and how much), but this is only going to be available later into an incident. The NAME model runs can take into account variations in terrain and forecast changes in weather. These are essential for its role in providing guidance over longer timescales (see Figure 5 below).

Although the basic CHEMET output does not give information on actual values of material concentration, it is based on the variation in concentration. The display shows lighter and darker colours; the darker


Figure 5: A typical (simulated) Met Office CHEMET plot. DECEMBER 2018/JANUARY 2019 19

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