Surface water may get contaminated through direct discharges to water bodies or through fallout from radioactive clouds. All contaminated waste water from the operation of a NPP is collected and treated in the liquid waste treatment system. This normally includes a high efficiency filtration system to retain particulate materials, followed by ion exchange to retain dissolved ions and also by a residual water evaporation system. At the end of these treatments pure distilled water is obtained; it can be recycled into the plant but any surplus of water has to be released to a nearby water body under strict control. Although of high efficiency, all these waste water treat­ment processes cannot retain all radioactive materials; moreover tritium substitutes hydrogen in the water molecule and it cannot be separated by any of the three processes mentioned above.

Tritium is a fission product — about one tritium atom is produced every 10,000 fissions. In PWRs tritium is also produced by activation of boron added to the coolant to control reactor core reactivity that is used in such reactors and with lithium also used in PWRs to control water pH. Tritium (half-life 12.26 years) is continuously produced in the stratosphere by cosmic radiation reacting with oxygen and nitrogen, from the stratosphere it gradually descends to the lower parts of the atmosphere by natural dif­fusion, and it ends in the ocean and terrestrial waters. As it is generated and decays constantly, it has reached an equilibrium estimated at about one million curies. It is against this natural background that tritium generated in NPPs and reprocessing facilities has to be measured.

The behaviour of the released nuclides to surface water, especially tritium, has to be predicted to measure the potential effects of such releases on the health and safety of the affected people and on the environment. Surface water dispersion and dilution parameters have to be measured by dedicated processes to determine water flows, and the transfer mechanism by which nuclides may reach humans. With all these parameters dispersion models are developed which also take into account all potential phenomena that control the behaviour of the different contaminants. These studies are con­ducted at least one year before starting plant construction and are main­tained during the whole operating life of the NPP.

As in the case of meteorological data, surface water dispersion and dilu­tion data and models are essential at the time and during an accidental release of radioactive products to properly manage the emergency use of such waters. In these cases dispersion and dilution data have to be supple­mented with the data and dispersion models of the country’s central hydro­logical agency. The 2011 Fukushima event released substantial amounts of contaminated water with radioactive nuclides to the Pacific Ocean with the potential of becoming concentrated in the bodies of fish and marine food products, thus requiring the definition of accepted concentration limits for human consumption.

The IAEA safety guide already mentioned (IAEA, 2002b) indicates the ways and means to obtain surface water data and develop the site water body’s dispersion models. The guide describes how to select and display a data gathering system appropriate to the hydrology of the region where the plant is located. Differences between river, open coastal, estuarine and artificial lake receivers are marked. The guide also describes methods to develop appropriate dispersion and dilution models to clearly determine the different pathways through which contamination can reach humans. Such models are also an essential part of the theoretical estimation of the potential radiation doses that the affected population may receive from surface water pathways. The model is also essential in the epidemiological studies generally conducted around nuclear sites.

Dispersion of radioactive material through ground water

Ground water may be contaminated by leakages from buried pipes carrying contaminated fluids, through seepage and infiltration of surface water that has been contaminated and from interactions with contaminated surface waters. Several instances of tritium presence in ground water from NPP underground leaking pipes have been recently reported. Ground water uses include human consumption and irrigation, pathways that may bring tritium into human contact. Therefore the protection of aquifers from such events should be prevented and a geological barrier should be considered.

A description of the ground water hydrology at the local and regional level is then required to assess the behaviour of any contaminant, its poten­tial migration and dilution, the retention characteristics of the soil and the physicochemical properties of the materials, mainly its retention properties. From this information a model is developed to estimate the radionuclide pathways during normal operation and under accident conditions.

As in the case of surface waters, IAEA (2002b) describes which data should be collected, that may require drilling boreholes for geophysical and tracer studies. The models will serve to estimate the expected contamination of ground waters at the point of use, and to assess the doses received by the exposed population and for the management of such waters in case of accident.