The requirements also consider a rather long list of local phenomena that may produce harm to the safety of the NPP, such as volcanism, sandstorms and subsurface freezing of sub-cooled water. There are also phenomena which may have an impact on the long-term removal of decay heat; in this respect consideration should be given to air temperature and humidity, water temperature, available flow of water and natural and human-induced phenomena which could impair the loss of the heat removal function, such as insufficient river flow, loss of the available water reservoir, water intake blockage by marine organisms or freezing of cooling towers, among others, All these aspects have to be considered to include appropriate preventative and mitigation devices, equipment and procedures.

The IAEA has not yet developed any safety guide to measure the hazards associated with these varied concerns. Only a safety guide on volcanic hazards in site evaluation for nuclear installations is in preparation (IAEA, 2009). The guide, based on a previous 1997 document, includes the knowl­edge gained in the science of volcanology and associated risks mainly due to the enormous amount of volcanic ashes that are injected into the upper atmospheric levels, which is already evident in the effects produced on air traffic. The fallout of a large amount of ashes on the plant premises, water intakes and roads could impair the safe operation of a nuclear power plant. As in the past, there could also be mega-volcanic eruptions with the poten­tial of blocking sunlight, which may have a serious impact on all types of installations and on life on earth. These extreme effects are generally outside the scope of the design.