The potential impacts that an operating NPP may bring to the site are related to the atmospheric, surface and ground water dispersion of radioac­tive material affecting the population and the use of land and water in the affected region. Specific requirements and corresponding safety guides follow.

Atmospheric dispersion of radioactive materials

During NPP operation small amounts of radioactive nuclides are released to the atmosphere under strict control. Those nuclides include some noble gases which cannot be retained by any treatment process, as well as some volatile elements and particulate matter which may not be completely retained in the high-efficiency radioactive waste treatment system.

In pressurized water reactors (PWRs) fission and activation gases in the coolant are separated and stored for decay and finally vented to the atmos­phere before refuelling outages; most of these gases have short lives and disappear during the storage period with the exception of krypton-85 (half­life 10.6 years) which becomes the larger contributor of gaseous releases. Iodine-131 (half-life 8.06 days) and hot particles — radioactive particles including fission and activation generated radioactive nuclides — could also be found in the containment atmosphere from coolant leakages. Small amounts of such materials can also be released to the atmosphere after being filtered by activated carbon filters, to retain iodine, and high-efficiency particle air filters. In boiling water reactors (BWRs) radioactive gases gen­erated in the coolant are carried by the steam, separated in the condenser and released to the atmosphere continuously after passing for a few days through a delay system composed of a large activated carbon bed main­tained at low temperature; short-life nuclides decay during transit through the bed with the exception of krypton-85 and xenon-137 (half-life 5.27 days), while most of the iodine nuclides are retained in the activated carbon bed.

The behaviour of the released radioactive nuclides 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. Meteorological dispersion parameters have to be measured by dedicated meteorological towers to determine wind speed and direction, air temperature, precipitation, humid­ity and atmospheric stability parameters. With all these data sophisticated dispersion models are developed which also take into account the topogra­phy of the place and the effects that buildings may have on the dispersion of the released materials. These studies are conducted at least one year before starting plant construction and meteorological towers and data gath­ering are maintained during the whole operating life of the NPP.

The availability of meteorological data at the time and during an acci­dental release of radioactive products is essential to emergency manage­ment. In these cases the site dispersion data have to be supplemented with the data and dispersion models of the country’s central meteorological agency.

An IAEA safety guide has been developed to indicate the ways and means to obtain such data and develop the site dispersion models (IAEA, 2002b). The guide describes how to select and display a meteorological data gathering system appropriate to the topography and physical characteristics of the region where the plant is located. The guide also describes methods to develop an ad hoc dispersion models which will also include the con­tamination of soil and the potential resuspension of radioactive aerosols and hot particles, as well as contamination of vegetables and other food products. Such models are essential parts of the theoretical estimation of the potential radiation doses that the affected population may receive from atmospheric radioactive releases by direct exposure to the radioactive cloud and other pathways. The model is also essential in the epidemiological studies generally conducted around nuclear sites.