Federal emergency management agency fema rep-2, rev. 2 / June 1990

2.2Release Composition

The most important radionuclides available for accidental release from a reactor in terms of potential health effects will probably be the radio-iodines^xxii , of which I-131 is the critical component. However, the health effects of releasing other fission-product radionuclides cannot be ignored. Important among these are the isotopes of tellurium, strontium, cesium, cerium, ruthenium, and the noble gases, krypton and xenon.

In spite of the fairly definitive information available regarding the inventories of radioactive material in reactor cores for different types of reactors with different operating histories, there are many important factors which influence the quantities and relative amounts of those materials which may contaminate the environs during an accident. Included are preferential releases of the more volatile components, such as noble gases and radioiodine. Radioactivity plate out from a release, and the effectiveness of engineered safeguards such as filtering systems and building containment are important in retarding releases. It should also be clearly recognized that releases calculated for accidents are only estimates. At best, they provide an order of magnitude indication of the extent of the emergency environmental monitoring which might be required at the time of an accident. Guidance on the anticipated radionuclides which may be accidentally released to the atmosphere is given in NUREG-0396 pp 22-23.^xxiii

2.3Dispersal of Radioactivity to the Environs

Comprehensive planning is required for emergency environmental radiation monitoring following a nuclear accident because the combination of a large fission product inventory in the reactor and the possibility of atmospheric dispersal could result in widespread contamination of the environs. In general, emergency environmental radiation monitoring plans that are based on atmospheric dispersal of radioiodine as the principal radionuclides of concern, should also provide adequate preparedness to measure other radionuclides dispersed by the atmosphere or other routes. Emergency monitoring plans for a specific site must include emergency instrumentation appropriate for measuring the radionuclides that may be released.

Radioactive materials released to the atmosphere from a nuclear accident in the form of gases or aerosols are mostly dispersed by turbulent diffusion in the air. The released material, in the form of a radioactive cloud, will move downwind under the existing meteorological conditions. Material initially in the form of aerosols may be deposited from the cloud on the ground or may be carried down by rain or snow. It is feasible, by the application of theories concerning turbulent diffusion and on the basis of a number of plausible assumptions, to estimate the radioactivity concentration pattern and exposure rate pattern resulting from the passing cloud. The projected dose can be calculated from the time integral, the concentration of airborne radioactive material, and appropriate dose conversion factors. It is also feasible to calculate the estimated concentrations of deposited radioactive material by applying estimated deposition velocities to the airborne concentration and the time interval for exposure to the plume. Predictions of exposure rate patterns are essential for making initial protective action decisions. Also, these predictions are needed in the planning and implementation of the offsite emergency radiation surveys to confirm the presence or passage of the radioactive plume.

2.4Radiation Exposure

1. 
   Whole body exposure to external radiation from:
   
   1. 
      the radioactive cloud,
      
   2. 
      radioactive materials deposited on the ground or other surfaces, and
      
   3. 
      radioactive materials deposited on the skin or clothes.
      
2. 
   Internal exposure to radiation following the inhalation of airborne radioactive material or resuspended deposited material; and
   
3. 
   Internal exposure to radiation following ingestion of contaminated food or water.
   

Since radioactive materials may be rapidly dispersed over substantial distances following a release to the atmosphere, emergency monitoring over a large area may be required to determine the concentrations of airborne and deposited material. It should be recognized, however, that some human exposure to the radioactive cloud cannot be avoided if the decision to implement protective action is based only on measurements at the exposure site, because exposure will occur during the period of measurements, analyses, and protective action implementation. Furthermore, continued environmental measurements will be required over an extended period of time after the release has ended to follow the movement of contaminants through food-chain pathways.

The objectives of any emergency monitoring program are to supply the information needed: 1) to confirm or modify protective actions taken to limit radiation exposure to members of the public, 2) to indicate need for further actions, and 3) to determine when actions should be terminated. Moreover, at the planning stage, the consequences of the types of accidental releases to the environs as described in NUREG-0396^xxiv must be assessed in order to determine the extent of the emergency monitoring required. The information from emergency monitoring must be used in conjunction with the PAGs, to establish a basis upon which to take protective actions. Appropriate PAGs are discussed in the EPA Manual (EPA-520/1-75-001).^xxv