Radon activity

Since radon is a colorless, odorless, and tasteless gas, the only way to detect its presence is to sample and analyze an area s air using a conventional radon measurement test. If the test reveals elevated radon levels, the homeowner will have to decide what steps to take to reduce the levels.7 The higher the level of radon present in a home, the more likely an active radon reduction system such as subslab depressurization (SSD)8 may be required. Lower radon levels may require only a passive reduction system, such as simple sealing. 

Noble gas collection. The radioactive argon, krypton, and xenon radioisotopes are monitored near nuclear power plants and related facilities to determine the magnitude of releases of gases generated by fission or neutron activation. Radon radioisotopes and their particulate progeny are measured in homes and mines to determine whether their airborne concentrations are below radiation protection limits. 

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

Static sampling systems are defined as those that do not have an active air-moving component, such as the pump, to pull a sample to the collection medium. This type of sampling system has been used for over 100 years. Examples include the lead peroxide candle used to detect the presence of SO2 in the atmosphere and the dust-fall bucket and trays or slides coated with a viscous material used to detect particulate matter. This type of system suffers from inability to quantify the amount of pollutant present over a short period of time, i.e., less than 1 week. The potentially desirable characteristics of a static sampling system have led to further developments in this type of technology to provide quantitative information on pollutant concentrations over a fked period of time. Static sampling systems have been developed for use in the occupational environment and are also used to measure the exposure levels in the general community, e.g., radon gas in residences. 

An activity of 20 picocuries (20 X 10-12 Ci) of radon-222 per liter of air in a house constitutes a health hazard to anyone living there. The half-life of radon-222 is 3.82 days. Calculate the concentration of radon in air (moles per liter) that corresponds to a 20-picocurie activity level. 

Total exposures vary considerably with human activities as well. Frequent flyers, for example, receive higher doses of radiation because the intensity of cosmic radiation is significantly greater at high altitude than it is at ground level. Residents in locations such as Montana and Idaho, where there are uranium deposits, receive higher doses of radiation from radon, one of the radioactive decay products of uranium. 

In theory, the application of radon barriers should be adequate to avoid elevated radon levels in houses. In practice, however, a backup radon mitigation system has been found essential for maintaining indoor radon concentrations below 4 pCi/L in most homes studied. In the recent radon-resistant residential construction projects conducted by U.S. EPA and/or private builders, several of the homes designed to be radon resistant have contained radon concentrations above 4 pCi/L. In each of those houses, a backup system consisting of an active (fan-assisted), or passive (wind-and-stack-effect-assisted), SSD system was installed at the time of construction. When mechanical barriers failed to adequately control radon, the soil depressurization methods were made operational. 

Of the study homes mentioned in the previous section, some passive systems seemed sufficient to lower the radon concentrations, while in all cases, active systems resulted in significantly lower concentrations. Table 31.1 summarizes the findings of these particular projects.9... 

Some builders prefer laying perforated PVC piping in the gravel before the slab is poured and connecting the perforated pipe to the exhaust pipe of the system. The use of perforated pipe may not be necessary in active systems but probably will assist a passive system. Membranes beneath the slab help us to keep a continuous radon barrier in the event of slab cracking. 

As can be seen in Figure 31.13, active SSD systems consist chiefly of a pipe system and a fan. There are several other components that should be included in a good system, but are not necessary to make the system reduce radon concentrations. 

Additional materials and components that are normally included in a system satisfy safety needs, system performance indications, and common sense. Service switches should be placed within view of the fan to ensure that the system will not be activated while maintenance is in progress. Systems should be clearly marked as a radon reduction device to ensure that future owners of the building do not remove or destroy the system. An operation manual describing the system and its purpose should be made available. 

A passive system is much the same as an active system with the exception of the fan. A passive system relies only on stack and wind effects to produce the pressure field. As can be seen in Table 31.1, passive systems do not always reduce radon concentrations to acceptable levels, but careful design and installation may improve the effectiveness of a passive system. 

Weep holes are used to drain water from the block cores into the subslab area when surface waterproofing barriers fail. Such a connection between the exterior and interior subslab areas is an obvious channel for radon entry, allowing soil gas to pass from the subslab to the interior of the block wall. Openings from the subslab into the block wall would also make it difficult to apply active SSD at a later date. If the block tops are sealed and the interior of the block wall is sealed, then weep holes would be much less of a problem as radon entry points or as barriers to SSD. 

The physico-chemical properties of radon and its decay products are presented in a series of reports primarily focusing on the decay products. However, Stein (1987) presents a review of his pioneering studies of radon chemistry and the reactions of radon with strong oxidizing agents. Although radon is not chemically active in indoor air, it is interesting to note that radon is not an "inert gas. 

There are a series of papers that focus on the behavior of the radon decay products and their interactions with the indoor atmosphere. Previous studies (Goldstein and Hopke, 1983) have elucidated the mechanisms of neutralization of the Po-218 ionic species in air. Wilkening (1987) reviews the physics of small ions in the air. It now appears that the initially formed polonium ion is rapidly neutralized, but can become associated with other ions present. Reports by Jonassen (1984) and Jonassen and McLaughlin (1985) suggest that only 5 to 10% of the decay products are associated with highly mobile ions and that much of the activity is on large particles that have a bipolar charge distribution. 

Bocanegra, R. and P.K. Hopke, The Feasibility of Using Activated Charcoal for Indoor Radon Control, this volume (1987). 

A general transport equation describing the rate of change of the radon activity concentration in the pore space results from combining the effects of diffusion and convection ... 

As part of the radon program at EML to develop or improve and field test radon monitors, a modified activated carbon device (Warner, 1986) was developed to obtain higher measurement sensitivity. As a result, we have surveyed 380 buildings in six states in the eastern United States. The purpose of the measurements reported in this paper was to test the feasibility of the new version of the passive activated carbon device and to obtain data on indoor radon levels in different geographical locations. 

The detector used to measure indoor radon was the latest version of the passive activated carbon device developed at EML (George, 1984 Warner, 1986), which consists of a thin-walled aluminum canister with a screen cover to expose 80 g of carbon to the test atmosphere. Although not as physically rugged as earlier models, properly packed this monitoring device was as successful in conducting the surveys through the mail. 

George, A. C., Passive Integrated Measurement of Indoor Radon Using Activated Carbon, Health Phvs. 46 867 (1984). 

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