While NPPs are designed and operated with a very high level of safety, it is essential that an adequate level of preparedness is still maintained to deal with the highly unlikely situation of a reactor accident. Reactor accidents can be broadly categorized as design basis accidents (DBA) and beyond design basis accidents (BDBA). For a DBA the design provisions, including the engineered safety features such as the emergency core cooling system, and the reactor containment system, together with the actions taken by well-trained operators, should be able to contain or confine the radioactivity released from the reactor core such that there is no significant adverse impact beyond the NPP site. In the case of a BDBA that may be caused by unanticipated failure sequences or by multiple failures occurring simultane­ously or due to a natural phenomenon of intensity greater than that con­sidered in the design, there could be significant impact in the public domain. In modern NPP designs, due consideration is given to BDBAs also and provisions are made to enable the operator to control their progression and to minimize their adverse consequences.

The first step in emergency preparedness is to develop emergency operat­ing procedures for all envisaged situations and to impart intensive training to operators for their execution when required. Extensive use should be made of the training simulator for this purpose. It should, however, be kept in mind that it is not possible to anticipate all possible emergency situations. At times the operators will have to use their ingenuity and take actions that might not have been included in the emergency operating procedures. This is possible only when the operators have a thorough understanding of plant behaviour and a high level of technical competence.

Emergency plans need to be in place for actions that are to be taken in case a reactor accident has a potential for or causes actual release of radio­activity outside the reactor containment. The actions could be in the form of countermeasures such as administration of prophylactics to prevent uptake of radioactive iodine by people, impounding food and milk, barri­cading of radioactively contaminated areas or even evacuation of affected or likely to be affected populations. For deciding on the type of emergency actions, their extent and the zone around the NPP where these need to be implemented, an assessment of the quantum of activity released has to be made. Further, the dispersal of the activity in the atmosphere and its deposi­tion on the ground taking into account the prevailing weather conditions has to be estimated For the longer term the radiation dose to the public by the terrestrial and aquatic routes and through the food chain has to be computed. These assessments have to be made through analysis of a large number of air, water, soil and food samples for their radioactivity content and by using computational models for estimating the dose to the members of the public by direct exposure as also through the inhalation and ingestion routes.

Emergency preparedness involves developing the requisite technical competence for carrying out such assessments in quick time, deciding on the countermeasures to be implemented and finally executing the actions in an organized manner. As implementation of countermeasures will be done by the public authorities, it is essential to have a mechanism in place for proper and timely coordination between the NPP and the public author­ities. Emergency exercises have to be carried out regularly according to the time schedule approved by the regulatory body to test the plans to be in a good state of preparedness to manage emergencies.