Search for building materials as sources of elevated radiation dose.

Abstract

A systematic approach to finding materials that cause elevated gamma radiation exposure rates or 222Rn progeny working levels in buildings was tested in the Atlanta area. With the proposed procedure, exposure rates in planned structures would be derived from a radiation survey of the material performed with a NaI(T1) detector. Working level values under specified conditions would be inferred from an additional measurement of 226Ra concentration in the material. To quantify the procedure, surveyed building materials were analyzed for radionuclide content and categorized according to an exposure rate index related both to the survey meter count rate and the radionuclide concentration. An available calculational model was used to predict from the index the exposure rate in a room totally enclosed by radioactive material. An additional model was developed for application to structures where the radioactive material is only in the floor or walls. 222Radon concentrations in an enclosed space were also predicted according to an available model. Exposure rates were measured in newly constructed buildings to test the approach. Measurements in older buildings found elevated gamma-ray emission from concrete blocks made with phosphate slag from a phosphorus producer in Alabama. Buildings with walls of these blocks were used to test the prediction of 222Rn concentrations in building air due to 226Ra in construction material. The calculational models predicted that the 226Ra concentration of approx. 20 pCi/g in these blocks would result in increases of the average exposure rates by 10 mu R/hr and of 222Rn concentrations by 0.2 pCi/l. under specified conditions. Observed levels were consistent with these predictions, but the major fraction of 222Rn in room air was attributed to inflow from the ground beneath the building.