SafetyNet

The SafetyNet example represents a typical a performance assessment model for a near-surface repository for low-level radioactive waste. The model considers the potential transport of radionuclides in groundwater, and their possible release into a river or well that is used on farmland. It features the main components of the ’repository system’ - the repository and other man-made features, the rock through which radionuclides can travel, and the surface environment. The user can specify some of the characteristics for each of these components.

SafetyNet does not support the Back and Forward buttons found on most browsers. Instead, use the buttons that are provided to navigate between pages.

Many browsers cache pages in order to reduce the download time. The SafetyNet main GUI contains an automatically generated image that shows the system as specified by the user. The image is updated to reflect changes to the rock type and transport pathway. Some browsers may not realise that the image is automatically updated and may, by default, choose to not download the new image and instead display the previously downloaded version. To force browsers download the image every time that the image is updated, the browser setting for how often the page is compared to the cached version must be changed. The setting for the common browsers can be found at:
Internet Explorer: Tools/Internet Options/Temporary Internet Files/Settings
Netscape/Mozilla: Edit/Preferences/Advanced/Cache
Galeon: SettingsPreferences/Advanced/Compare Page
Opera: File/Preferences/Network/Check Modified
In each case the setting should be changed to either ’Automatic’ or ’Every time the page is viewed’. The main SafetyNet GUI is shown in the following figure:


(1) shows the whole repository system, comprising the repository (the barrel with the radioactive symbol), the mass of rock in which it is located (in this example it is sandstone), and the surface including a river, well, soil, plants and animals.

The characteristics of this system can be changed, e.g. to consider a range of different possible sites for the repository.

(2) allows the point of release of contaminated water to be specified by the user. It can either be a well drilled into the rock, or a river. Each is assumed to be a water source for a farmer, his crops and animals.

(3) enables the characteristics of the flow of groundwater to be modified. This can determine the time it takes contaminated water from the repository to reach the river or well. The total distance and flow rate (Darcy velocity) can be changed. The porosity of the rock (i.e. the fraction of the rock that is air- or water-filled voids) can also be changed.

(4) permits the type of rock to be changed to one of three typical types - sandstone, clay and granite. The type of rock can be important for retaining some radionuclides in transit, as they adhere to the rock to varying degrees. Note that some combinations of (3) and (4) may not properly reflect the real characteristics of rocks - for example granite typically has a low porosity, and sandstone generally has a high flow rate.

(5) allows the waste and repository characteristics to be defined. The total amount of radioactivity in the waste can be defined for several important radionuclides in Becquerels (the typical concentration of natural radioactivity in rocks is about 400 Bq per kilogram). The height and volume of the repository can be defined, and it can be assumed to be saturated with water (after closure) or unsaturated (i.e. it is located above the water table). The level of engineering in the repository can also be specified (none, basic, moderate or advanced).

(6) allows the time over which the model is run to be set. Because some important radionuclides move slowly with groundwater, the calculation period can range up to 1,000,000 years. However, the results will become increasingly uncertain with time. It is especially important to note that after a few tens of thousands of years major climate change may occur (e.g. ice ages).

(7) enables the model to be run using the user defined inputs; these are displayed on the right of the model picture. The results are presented in two graphs. These show the calculated total dose to a hypothetical farmer from various pathways including the ingestion of contaminated foods and water, external radiation from the soil and inhalation of contaminated dust. The dose is given in units of Sieverts per year - for reference the typical background radiation dose in the UK is about 0.002 Sv/y. The results are also broken down into the contribution from each radionuclide.