Design, calibration, and operation of 220Rn stack effluent monitoring systems at Argonne National Laboratory

A group of stack effluent monitoring systems have been developed to monitor discharges of 220Rn from a hot cell facility at Argonne National Laboratory. The stack monitors use flow-through scintillation cells and are completely microprocessor-based systems. A method for calibrating the stack monitors in the laboratory and in the field is described. A nominal calibration factor for the stack monitoring systems in use is 15.0 cts min-1 per kBq m-3 (0.56 cts min-1 per pCi L-1) +/- 26% at the 95% confidence level. The plate-out fraction of decay products in the stack monitor scintillation cells, without any pre-filtering, was found to be nominally 25% under normal operating conditions. When the sample was pre-filtered upstream of the scintillation cell, the observed cell plate-out fraction ranged from 16-22%, depending on the specific sampling conditions. The instantaneous 220Rn stack concentration can be underestimated or overestimated when the steady state condition established between 220Rn and its decay products in the scintillation cell is disrupted by sudden changes in the monitored 220Rn concentration. For long-term measurements, however, the time-averaged response of the monitor represents the steady state condition and leads to a reasonable estimate of the average 220Rn concentration during the monitoring period.