Estimation of high-level, rapidly-changing concentrations using moving-filter continuous particulate air monitors

摘要

A previously published mathematical model for the dynamic response of moving-filter continuous particulate air monitors has been enhanced to extend that model to include decay chains. During this work, it was observed that a quantitative relationship appeared to exist between the monitor count rate and the time-dependent particulate airborne radioactive material concentration if, and only if, the filter (tape) speed was much faster than the nominal 2.54 cm h (1 in h). The extended model demonstrated that operating moving-filter monitors at this nominal filter speed does not provide a quantitative measurement of a changing airborne particulate concentration of a fission product or other contaminant. By contrast, at faster filter speeds [e.g., 76.2 or 152.4 cm h (30 or 60 in h)], numerical experimentation with this model showed that the count rate trace has essentially the same shape as the concentration profile. It was then found that a quantitative relationship applies, but only when the filter speed is sufficiently fast so that a Taylor series expansion of the monitor count rate can be reasonably well truncated at the first-order term. This mode of operation, which does not require any new monitor hardware, is capable of tracking rapidly changing concentrations. Since the fast filter speed also reduces the monitor's count rate, all else being equal, the approach will best be used for relatively high-level concentrations, such as may occur in abnormal or "accident" conditions. The count rate suppression may also be useful for reducing the detector saturation that can occur with higher levels of airborne particulate radioactivity in post-accident situations.