12.108. During the course of the Three Mile Island accident in 1979 (§12.178), a surprising small amount of iodine, about 18 Ci, escaped to the environment. Since this was so much less than the 6 million curies of the inert gas, xenon, that escaped, studies were instigated which showed that as a result of the reducing chemical state in the reactor vessel, the iodine was converted to cesium iodide, which is soluble in water and hence was not released. Therefore, it was recognized that chemical and physical phenomena during the course of an accident have a great deal to do with reactor safety, and major attention was devoted to source term studies.

12.109. In assessing the hazards associated with fission product release from the fuel, we must consider in a stepwise manner how each nuclide of radiological importance makes its way from the point of release, through barriers that may have failed, into the coolant, and then into the contain­ment. Some release from the containment to the environment by some means would then constitute the hazard. An analysis of this sequence depends very much on the nature of the accident. Types of accidents are considered in §12.94 et seq. However, in all cases, the fission product transport depends first on chemical changes that affect their characteristics, then on their behavior when they are in the form of particles or aerosols. Therefore, we will examine in a general way the chemistry of the fission products. Then, an introduction to fission product particulate mechanics will be given.