Particle Resuspension and its Relation to Radiological Risk

The IAEA's Code of Conduct on the Safety and Security of Radioactive Sources has, as its basis, activity thresholds corresponding to source categories representing particular levels of risk. These are based both on internal and external exposure scenarios, the limiting values of which are designated as "dangerous" or D-values from which the category limits are obtained. Those categories 1-3 represent sources which, if not safely managed or securely protected, would or could cause permanent injury upon handling for a few minutes, minutes to hours, or some hours respectively. Scenarios are grouped by whether the material was dispersed or not. Four dispersed scenarios are considered: inhalation of airborne material, ingestion of contamination from handling a leaking source or of a poisoned water supply, contamination of the skin, and immersion in a cloud of noble gas. The first two of these represent internal exposure, the other two being external exposure. Of the 371 radionuclides listed in the Code, 84 of those are limited by external exposure, the other radionuclides being limited by external exposure or, in a few cases, having no limit. In a dispersal event where a source has been purposely scattered over an extended area, the dose is received over four pathways, somewhat different than what was used in determination of the D values. If the material is dispersed energetically or in such a way that airborne radioactivity is generated, external exposure from "cloudshine" directly to the individual from radiation in the plume and internal exposure from the airborne radioactivity itself will provide the primary hazards. After the plume has passed or deposited, external exposure from "groundshine," radiation emitted from the deposited material, and internal exposure from "resuspension," airborne radioactivity from deposited material environmentally or electrostatically returned to the air, are the relevant exposure components. While only the plume exposure was considered in development of D-values, the groundshine and resuspension pathways remain important considerations in proactively estimating consequence from a dispersal, and, in some cases, actually represent source protection actions identified by some states. For beta/gamma emitting radionuclides post plume passage, groundshine tends to be the limiting dose parameter, and in the case of such radionuclides of greatest concern – 60Co, 137Cs, and 192Ir – represents over 90% of a potential dose. This is not the case for alpha-particle-emitting radionuclides. Internal dose from 241Am represents over 90% of a potential dose from a distributed source. This is mostly due to a radiation weighting factor of 20 representing the additional hazard present by the alpha particle depositing all of its energy in a very small volume. In this case, it is of extreme importance that the understanding of the expected resuspension be as accurate as possible. There have been several papers attempting to create an empirical model of resuspension using similar or identical data sets. These data are from nuclear weapons tests, some of which include a time-varying component, and some representing single measurements of serendipitously detected contamination. Additional research has been conducted to attempt to understand the mechanisms under which material resuspends and how representative current and previous models are of particular radionuclides and their associated chemical and physical properties. (author)