The consequences from liquid pathways after a reactor meltdown accident

The potential radiological impact of a core-melt accident on the human population has been investigated. In particular, the radiation dose received from radioactivity which could reach the population via liquid pathways has been considered. Radioactivity could be released directly to the hydrosphere after a core-melt accident as a result of melt-through of the containment basemat followed by any of three processes: (1) leaching of the melt debris; 2 escape of sumpwater through the hole formed by the melt (or from passage out of the containment by an alternate route); and 3) depressurization of the containment atmosphere through the melt hole. The three types of releases would differ primarily in their rates, their magnitudes and their radioactive compositions. Both the containment atmosphere and the sumpwater releases would occur relatively rapidly. However, most of the radionuclides present in these two releases in substantial quantities would be expected to be rather short-lived. Therefore, such releases could have a significant impact at a specific site only if the travel times of the important radionuclides to the human population were small. In contrast, leaching of radionuclides from the melt debris would be expected to occur relatively slowly. Most of the long-lived isotopes would be expected to be found primarily in the melt debris. Consequently, even though this release occurred relatively slowly, the impact could still be significant. In contrast to the situation for releases to the atmosphere, accidents corresponding to the most probable RSS (Reactor Safety Study) meltdown categories would result in the largest releases to the hydrosphere. Furthermore, substantial amounts of radioactivity would generally be expected to be released to the hydrosphere during any meltdown accident involving complete melt-through of the containment basemat. On the basis of subsurface hydrologies alone, sites range from those that essentially preclude any impacts to the human population via liquid pathways to those that can allow significant impacts to occur. Sites with very slow groundwater movement and/or with long distances to nearby surface water tend to be in the former category; sites with relatively rapid groundwater movement and with short distances to nearby surface water or wells usually are in the latter category. On the basis of surface-water characteristics and the related populations-at-risk, sites range from those having no significant exposure pathways to the human population to sites having several potentially important exposure pathways. Sites with surface water bodies having rapid initial dilution and fast flushing and/or small potentially exposed human populations would tend to have little net human exposure and, therefore, few adverse impacts; sites with water bodies having slow dilution and flushing and large potentially-exposed populations would tend to have much more human exposure and, therefore, more impacts. In order to bound the possible radiation doses to humans that could result from release to the hydrosphere following basemat melt-through, calculations were made under assumptions that tend to maximize population doses. Among the conservative assumptions made were the following: 1. Instantaneous release to groundwater of all radioactivity from both sumpwater and melt debris. 2. One-day transit time from the release point to a surface water body. 3. No interdiction measures taken to prevent exposure of humans. The doses reported in the next two paragraphs are believed to be upper bounds on exposures that would occur under realistic conditions. At approximately 35% of all the reactors considered, the radiation dose to the entire human population as a result of releases to the hydrosphere via the hole in the containment basemat formed by the melt is estimated to be relatively small (in general, much less than 104 to 105 person-rem). At another 25% of the reactors, the resultant radiation dose is estimated to be no more than 106 to 107 person-rem, while at the remaining 40%, the potential dose is no larger than 107 to 108 person-rem for such releases. The somewhat artificial construct of an 'average individual' was used to estimate the individual doses which correspond to the population doses quoted above. It is assumed that this individual is average with respect to usage habits for all the liquid pathways, that all of the individuals usage for the liquid pathways involves the contaminated water body, and that the individual lives long enough to be exposed to all the dominant radionuclides. Under these conditions, the to an average individual over an entire lifetime as a result of the releases to the hydrosphere is estimated to be less than 1 rem at approximately 60% of all the reactors under considered. Furthermore, the total dose to such an individual is estimated to be no more than 100 rem at the remaining 20% of the reactors