Once emergency plans have been activated, the operator is responsible for timely and accurate transmission of information about the evolution of the accident within the plant, to ensure that the public authorities receive data they need for managing the situation. Of special importance are data related to the nature and amount of the radioactive releases from the plant, usually called the source term, because the scope and nature of countermeasures to be implemented depend critically on this parameter. The source term can be evaluated by using data from of the radiometers installed in the main discharge channels in the nuclear facility, e. g. chimneys and ventilation exhaust systems. This method can be used when radioactive materials are released through these channels and the corresponding instruments were not affected by the accident. The source term can also be estimated by using mathematical models that reproduce the physical-chemical behaviour of the plant under accident conditions. Some of these have been adapted for use in emergency situations and are available in the emergency coordina­tion centres operated by operators and regulatory authorities (IAEA, 2003b).

In case of maximum severity, the emergency coordinator can decide the implementation of precautionary urgent protective action to prevent severe deterministic health effects by keeping doses below those for which inter­vention would be expected to be undertaken under any circumstances. This situation is extremely unlikely and it is expected that the emergency coor­dinator would have enough time to decide the implementation of counter­measures based on dose estimation.

Estimation of the dose needs detailed meteorological data that can be obtained from the stations existing in every nuclear facility and from regional or national meteorological services. These data are used as input to mathematical models able to predict transport of radioactive materials released in the atmosphere, and to estimate the dose that the people living in the areas affected could receive due to radiation from a contaminated cloud or radioactive aerosols deposited in soil or waters. The dose can also be evaluated by using the radiometric instrumentation that is available in the emergency areas as part of the means and resources arranged during the preparedness phase of the emergency plans. This instrumentation is composed of automatic radiation surveillance networks, mobile units, per­sonal dosimeters, contamination meters, and sampling stations and analyti­cal laboratories and procedures to evaluate contamination of affected pathways, e. g. air, soil, foods and water.

The emergency coordinator can evaluate the radiological situation by using the different methods available to estimate the source term, the spread of contamination and the dose. Use of an adequate technique is a compromise decision between the need for quick or accurate results. The use of mathematical models allows very quick results, even predictive, but can involve some uncertainties. The use of radiometric measures is more accurate but can lead to delay, especially if the results are obtained by sample analysis or with off-line instruments. Automatic radiation surveil­lance networks can reduce the time needed to obtain results, but their accuracy, sensitivity or location could be inadequate for taking decisions. The emergency coordination centres are equipped with systems based on different techniques and their operators are trained in using all of them and taking decisions based on combining the results obtained from all of them.

Upon consulting with the regulatory authority, the emergency coordina­tor decides the implementation of urgent protective action to prevent sto­chastic effects to the extent practicable by averting doses, in accordance with international standards. The decision is based on the dose rate and contamination levels existing in the affected area, and the dose that can be averted by applying appropriate countermeasures. Public authorities can take into consideration other factors influencing the implementation of countermeasures. In this regard, meteorological conditions, seasonal demog­raphy and coincidence with other catastrophic events such as earthquakes are examples of circumstances that have to be taken into account in the decision. Finally, the emergency coordinator transmits his or her decisions about emergency actions to emergency response teams (rescue brigades, radiation protection, health services, police, and civil defence teams) for implementation.

During emergency response, the emergency coordinator has to pay special attention to ensure that easily understandable information about existing hazards, emergency decisions and countermeasures to be imple­mented is properly transmitted:

• Directly to the permanent, transient and special population groups or those responsible for them and to special facilities within the emergency zones, for getting an adequate undertaking of emergency decisions and their full collaboration in implementing emergency measures

• To emergency coordination centres to act cooperatively

• To the media to ensure that all stakeholders have adequate information on emergency operations to act properly if their support is required

• To international partners, international organizations and national sig­natories of bilateral agreements, to facilitate the adoption of adequate emergency actions in their own territories by the relevant emergency coordinator.

During emergency operations, special attention should be paid to protect emergency responders who may undertake intervention in order to save lives or prevent serious injury due to doses that could cause severe deter­ministic health effects, take action to avert a large collective dose, or take action to prevent the development of catastrophic conditions.

Activation and implementation of emergency plans could be especially difficult when response to a nuclear accident has to be given in an area that has been simultaneously affected by an extreme natural or anthropogenic disaster. In this case, the emergency coordinator has to pay special attention to coordinating implementation of radiological and non-radiological coun­termeasures with the relevant authorities.