Intercomparison of T W O Systems for Continuous Measurement of Radon - Flux Density

The ability to monitor radon-flux density continuously is of great value in elucidating radon-transport mechanisms. Intercomparisons of two radon-flux measurement systems were conducted in one of the two test structures operated by Colorado State University (CSU). Systems used by the U.S. Department of Energy Grand Junction Projects Office (GJPO) Radon Laboratory and the CSU investigators are similar in that each uses a flowthrough collector sealed to the measurement surface with the collector outflow connected to a continuous radon monitor. Configuration of the collectors is the principal difference between the two systems. The collector for the GJPO system is cylindrical with a relatively large volume and a low ventilation rate (0.5 h l ) . The CSU system uses a thin plate-like collector with a small volume and a high ventilation rate (8 h l ) . Because of the high ventilation rate, the CSU system is sensitive to the presence of thoron (̂ ~n). Results from the two systems exhibit good agreement when the CSU system incorporates a thoron filter (a delay line between the collector and the CRM). INTRODUCTION The ability to monitor radon-flux density continuously from a surface within a structure has proven to be a valuable tool in understanding the radon transport mechanisms operating in a given situation. As an example, Kendrick and Langner (1991) found a strong relationship between differential pressure across the structure boundary and the radon-flux density for several houses studied in the Grand Junction, Colorado, area. An interesting result of the data analysis was that the so called pressure-driven flow component of the radon-flux accounted for only a small fraction (20%) of the average flux. The radon research group led by Dr. T. B. Borak of Colorado State University (CSU) evaluated factors affecting radon entry into the two radon test structures located near the CSU campus (Ward et al., 1993). Ward el al. separated the radon entry into two terms: one that correlated with changing differential pressure and a second that persisted even when the structure was overpressurized. This persistent term accounted for more than 80% of the radon entering the test structures under average wind conditions. CSU has the ability to measure not only the radon-flux density over small selected portions of the structure boundary but also the global radon entry rate for (lie structure. The measurements made by Ward et al. exhibited little dependence on pressure differentials induced by (lie stack effect. The most significant variations in radon entry rates correlate with wind events. In the work of Kendrick and Langner (1991), the measured radon-flux density exhibited significant fluctuations attributable to stack-effect-induced pressure differentials. Less frequent wind-induced * This work was supported by the Program Support Research and Development Program of the US. Department of Energy Office of Environmental Restoration and Waste Management at the Grand Junction Projects Office (GJPO) under DOE Contract No. DE-AC04-86ID12584. The 1993 International Radon Conference V 19 pressure differentials also were observed to affect the radon-flux density. In addition, these wind events affected the radon concentrations indoors and in the soil gas. Because of the similarities and differences the two groups observed in separate investigations, a direct intercomparison of the two measurement techniques was conducted to ensure the comparability of both group's results.