not present a potential for release of radioiodine. NPPs in the United States are designed and built to withstand severe accidents and to prevent the escape of radionuclides that could threaten public health. Severe damage to the reactor core coupled with extreme damage to the reactor facility is required for radioiodine to be released to the environment. US NPP design features ensure that in the worst-case situations there will be a delay, usually days, before radionuclides can escape to the environment in amounts that would be of a public-health significance. Additional steps are taken in reactor design to ensure that such unexpected events as earthquakes and tornadoes do not lead to damage of nuclear fuel or release of radionuclides to the environment. These rigid design features coupled with heightened security measures make radioiodine releases from terrorist activities highly unlikely.
The approximate inventory of 131I and other iodine radioisotopes in the core of a NPP depends on the size of the reactor core (megawatt-electrical (MWe)) and on how long the reactor has been in operation. The table in Appendix B can be used to estimate the approximate inventory (actual inventory will depend on how long the reactor has been in operation) of the predominant radionuclides of public-health concern that are present in a reactor core. By multiplying the “Ci/MWe” value indicated in the appendix by the operational power level of the plant, one can obtain an approximation of the total activity (in curies) of each radionuclide. For example, in the case of Three Mile Island, a 740 MWe reactor, the 131I inventory at the time of the accident was equal to about 740 MWe × 85,000 Ci/MWe or 63 megacuries (MCi). Less than one-millionth of the inventory of radioiodine in the Three Mile Island core was postulated to have escaped to the environment, that is, about 15 Ci (Kemeny et al., 1979). In the United States, nuclear power plants are designed to isolate the reactor building in the event of an accident. In the case of Three Mile Island, initial isolation of the reactor building was not totally successful. As accident-generated water began to flood the lower levels of the reactor building, sump pumps began pumping this