A review of radon emanation and mobilization in minerals and rocks
Cameron, Christopher P.
Miscellaneous PaperAbstract: Determination of the natural radiation environment nnd its variation should receive high priority during screening, characterization, construction, and operational phases of underground facilities. Radon-222, and Radon-220 (Thoron) occur widely throughout the rocks of the earth's crust and are present in almost all geological environments. In terms of human exposure to natural radiation, the best materials for the siting of deep excavations are limestones, gabbros, and serpentinites (and their metamorphic equivalents) all of which have low levels of uranium concentration and radon escape. Because other factors often preclude the siting of deep facilities in radiologically optimum lithologic settings, there is a need for enhanced awareness with respect to natural radiation environments. Radon daughters are natural radioactive substances which exist everywhere, but their concentration may be anomalously high in parts of some deep excavations, particularly those in granitic rocks . Anomalous accumulations of radioactive minerals will generate radon gas in above-background quantities. In deep unlined excavations, radon gas can contaminate the air as it is easily adsorbed onto dust particles, soot (from motors), and water droplets, as the gas emanates from rock faces, broken rock, faults, joints, fractures, and underground waters entering the excavation. Radon concentrations above atmospheric levels have been found in such protected places as underground low-level radiation counting facilities, which normally are constructed with heavy walls of low activity concrete. On the basis of laboratory evidence and field measurements, the mobilization of radon and its precursor isotope, radium, is often more extensive than originally predicted by earlier studies and conceptual thought; albeit, that the mechanisms controlling such mobilizations are, as yet, incompletely understood. The geochemical behavior of daughter isotopes in the radioactive decay series of uranium and thorium is generally not well under stood. It would be advantageous to know more about the emanating power of different types of uranium and thorium minerals particularly with respect to mineralology, grain size, porosity, and permeability. Considerable work is needed on gr ound-water geochemistry and the geochemical behavior of radon precursors in relation to anomalous concentrations of radon in the underground environment. The extent of radon mobility and transportation at depth by various convective mechanisms including atmospheric and other "pumping" effects, with respect to large underground excavations, is speculative as are the details of the effect of subsurface temperature variations and zones of high heat flow. Efforts to develop better instrumentation and passive radon detection systems need continued support. A significant number of scientists feel that too little is known about the long-term health and genetic effects of prolonged exposure to low-level radiation environments. Radon has been recognized as playing a central role in the low-level radiation environment. Numerous research programs have been undertaken to measure radon concentration in human habitations and dwellings and to determine the impact of variable radon concentrations on health safety.
Geotechnical Laboratory (U.S.)Engineer Research and Development Center (U.S.)
Radiation; Rocks; Minerals; Radon; Deep facilities; Underground structures
Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; GL-87-27.
Approved for public release; distribution is unlimited.