Airborne particulate radioactivity monitoring
Radionuclides that occur in the gaseous form (e.g., 85Kr) are not collected on the CPAM filter to any appreciable extent, so that a separate monitoring system is needed to assess these nuclide concentrations in the sampled air.
In monitoring, the region of deposition of this material onto the filter medium is continuously viewed by a radiation detector, concurrent with the collection.
This is as opposed to a sampling system, in which the airborne material is collected by pumping air, usually at a much higher volumetric flowrate than a CPAM, through a collection medium for some period of time, but there is no continuous radiation detection; the filter medium is removed periodically from the sampler and taken to a separate radiation detection system for analysis.
In general, sampling has better detection sensitivity for low levels of airborne radioactivity, due to the much larger total volume of air passing through the filter medium over the sampling interval (which may be on the order of hours), and also due to the more sophisticated forms of quantitative analysis available once the filter medium is removed from the sampler.
In both types of CPAM the sampled air is pulled (not pushed) by a pump through the piping of the monitor up to the structure that holds the filter medium.
The entire deposition area, regardless of its geometric shape, is assumed to be viewed by a radiation detector of a type appropriate for the nuclide in question.
For occupational exposure (inhalation) assessment, CPAMs may be used to monitor the air in some volume, such as a compartment in a nuclear facility where personnel are working.
The following portions of 10CFR20[8] are relevant to the requirement for occupational exposure CPAM applications in the USA: 10CFR20.1003 (definition of Airborne Radioactivity Area), 1201, 1204, 1501, 1502, 2103.
Leakage from the so-called "reactor coolant pressure boundary" is required to be monitored in USA nuclear power plants.
[10] Monitoring the airborne particulate radioactivity in the reactor containment structure is an acceptable method to meet this requirement, and so CPAMs are used.
[11] The regulatory basis for this CPAM application is found in 10CFR50:[12] For use in the USA, standard 10 CFR 50, Appendix A, "General Design Criteria for Nuclear Power Plants," Criterion 30, "Quality of reactor coolant pressure boundary," requires that means be provided for detecting and, to the extent practical, identifying the location of the source of reactor coolant leakage.
For use in the USA, standard 10 CFR 50.36, "Technical Specifications," paragraph (c)(2)(ii)(A), specifies that a Limiting Condition for Operation be established for installed instrumentation that is used to detect and indicate in the control room a significant abnormal degradation of the reactor coolant pressure boundary.
Step changes in reactor coolant leakage can be detected with moving filter media to satisfy the quantitative requirements of USNRC Regulatory Guide 1.45.
Further refinements to mathematical methodologies have been made by the inventor; these set aside the patented collimator apparatus for making the quantitative assessment of leak rate step change when rectangular OR circular collection grids are employed.
30-second YouTube video examples: search ‘airborne particulate radioactivity moving filter.’ The response of the monitor is sensitive to the half-life of the nuclide being collected and measured.
CPAMs use either a Geiger tube, for "gross beta-gamma" counting, or a NaI(Tl) crystal, often for simple single-channel gamma spectroscopy.
In those cases, the interference from other isotopes such as RnTn is a major problem, and more sophisticated analysis, such as the use of HPGe detectors and multichannel analyzers, are used where spectral information, such as is used for Radon compensation, is required.
Radioiodine (especially 131I) monitoring is often done using a particulate-monitor setup, but with an activated charcoal collection medium, which can adsorb some iodine vapors as well as particulate forms.
Detailed mathematical models that describe the dynamic, time-dependent countrate response of these monitors in a very general manner are presented in[14] and will not be repeated here.
That predicted response can be compared to the expected background and/or interferences (nuclides other than the one sought), to assess the monitor’s detection capability.
Again, a very important parameter for moving-filter monitors is the “transit time” (T), which is the window length (or diameter) divided by the filter tape speed v. The countrate is denoted by
In both plots, Poisson "noise" is added and a constant-gain digital filter is applied, emulating the countrate responses as they would be observed on a modern CPAM.
Having mathematical models that can predict the CPAM response, i.e., the monitor's output, for a defined input (airborne radioactive material concentration), it is natural to ask whether the process can be "inverted."
For example, in power reactor leak detection applications, as mentioned in the first section of this article, CPAMs are used, and a primary nuclide of interest is 88Rb, which is far from long-lived (half-life 18 minutes).
Also, in the dynamic environment of a reactor containment building the 88Rb concentration would not be expected to remain constant on a time scale of hours, as required by this measurement method.
Estimating the rate of change (time derivative) of the countrate is difficult to do with any reasonable precision, but modern digital signal processing methods can be used to good effect.
This approach was implemented at the SM-1 Nuclear Power Plant in the late 1960s, for estimating the releases of episodic containment purges, with a predominant, and strongly time-varying, nuclide of 88Rb.
An interesting subtlety to these calculations is that the time in the CPAM response equations is measured from the start of a concentration transient, so that some method of detecting the resulting change in a noisy countrate must be developed.
The calibration of a CPAM usually includes: (1) choosing a quantitative method; (2) estimating the parameters needed to implement that method, notably the detection efficiency for specified nuclides, as well as the sampling line loss and collection efficiency factors; (3) estimating, under specified conditions, the background response of the instrument, which is needed for calculating the detection sensitivity.
This variability is measured using the standard deviation; care must be taken to account for bias in this estimate due to the autocorrelation of the sequential monitor readings.
