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Extracting and transporting the particulates for the CPAM to measure in such a way that the measurement is representative of what is being released from the facility is challenging. 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.

For this application the CPAM may be physically placed directly in the occupied compartment, or it may extract sampled air from the HVAC system that serves that compartment. Radiation monitors in general have a number of process-control applications in nuclear power plants; [9] a major CPAM application in this area is the monitoring of the air intake for the plant control room. In the event of an accident, high levels of airborne radioactivity could be brought into the control room by its HVAC system; the CPAM monitors this air and is intended to detect high concentrations of radioactivity and shut down the HVAC flow when necessary.

This defines a requirement for monitoring the air intake for the control room, such that the exposure limits, including for inhalation exposure, shall not be exceeded. CPAMs are often used for this. Leakage from the so-called "reactor coolant pressure boundary" is required to be monitored in USA nuclear power plants. It is the case that when primary coolant escapes into the containment structure, certain noble gas nuclides become airborne, and subsequently decay into particulate nuclides. Relating the observed CPAM response to the 88 Rb back to a leakage rate from the primary system is far from trivial.

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.

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The specific attributes of the reactor coolant leakage detection systems are outlined in Regulatory Positions 1 through 9 of Regulatory Guide 1. This instrumentation is required by Specification 3. Step changes in reactor coolant leakage can be detected with moving filter media to satisfy the quantitative requirements of USNRC Regulatory Guide 1. The method was first put into use in the s at a nuclear power plant in the United States. Further refinements to the mathematical methodology have been made by the inventor; these developments obviate the described patented collimator apparatus for making quantitative assessment of leak rate step change when rectangular collection grids are employed.

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Airborne particulate radioactivity monitoring - Wikipedia

The response of the monitor is sensitive to the half-life of the nuclide being collected and measured. It is useful to define a "long-lived" LL nuclide to have negligible decay during the measurement interval.

On the other hand, if the decay cannot be ignored, the nuclide is considered "short-lived" SL. In general, for the monitor response models discussed below, the LL response can be obtained from the SL response by taking limits of the SL equation as the decay constant approaches zero. If there is any question about which response model to use, the SL expressions will always apply; however, the LL equations are considerably simpler and so should be used when there is no question about the half-life e.

The output of the radiation detector is a random sequence of pulses, usually processed by some form of "ratemeter," which continuously estimates the rate at which the detector is responding to the radioactivity deposited on the filter medium. There are two fundamental types of ratemeters, analog and digital.

The ratemeter output is called the countrate , and it varies with time. Ratemeters of both types have the additional function of "smoothing" the output countrate estimate, i. This process is more correctly termed "filtering. Even when the filter medium is clean, that is, before the pump is started that pulls the air through the filter, the detector will respond to the ambient "background" radiation in the vicinity of the monitor.

The countrate that results from deposited radioactivity is called the "net" countrate, and is obtained by subtracting this background countrate from the dynamically-varying countrate that is observed once the pump is started. The background is usually assumed to be constant. The countrate of the monitor varies dynamically, so that a measurement time interval must be specified. Also, these are integrating devices, meaning that some finite time is required to accumulate radioactivity onto the filter medium.

The input to the monitor is, in general, a time-dependent concentration in air of the specified nuclide. However, for the calculations given below, this concentration will be held constant over that interval. Thus, measurement intervals on the order of several hours are not plausible for the purposes of these calculations. There are situations in which a nuclide deposited on the CPAM filter decays into another nuclide, and that second nuclide remains on the filter.

This "parent-progeny" or decay chain situation is especially relevant to so-called "radon-thoron" RnTn or natural airborne radioactivity. The mathematical treatment described in this article does not consider this situation, but it can be treated using matrix methods see Ref [11]. Another issue is the fact that in a power reactor context it would be unusual for a CPAM to be collecting only a single particulate nuclide; more likely there would be a mixture of fission product and activation product nuclides.

The modeling discussed in this article considers only one nuclide at a time. However, since the radiation emitted by each nuclide is independent of the others, so that the nuclides present on the filter medium do not interact with each other, the monitor response is the linear combination of the individual responses. Thus the overall CPAM response to a mixture is just the superposition i. CPAMs use either a Geiger tube , for "gross beta - gamma " counting, or a NaI Tl crystal, often for simple single-channel gamma spectroscopy. In this context, "gross" means a measurement that does not attempt to find the specific nuclides in the sample.

Plastic scintillators are also popular. Essentially, in power reactor applications, beta and gamma are the radiations of interest for particulate monitoring. In other fuel-cycle applications, such as nuclear reprocessing , alpha detection is of interest. 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 I 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. Single-channel spectroscopy is usually specified for iodine monitors. 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.

For the purpose of this article, a few useful results from that paper will be summarized. The objective is to predict the net countrate of a CPAM for a single, specific manmade nuclide, for a given set of conditions. The response predictions can also be used to calculate alarm setpoints that correspond to appropriate limits such as those in 10CFR20 on the concentration of airborne radioactivity in the sampled air. This factor is meant to compensate for these losses. Sampling lines are specifically designed to minimize these losses, for example, by making bends gradual as opposed to right-angled.


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Thus, overall there will be some fraction of the disintegrations that emit the radiation of interest e. The response models are based on the consideration of the sources and losses of the deposited radioactivity on the filter medium. Taking the simplest case, the FF monitor, this leads to a differential equation which expresses the rate of change of the monitor countrate: The first term accounts for the source of radioactivity from the sampled air, and the second term is the loss due to the decay of that radioactivity.

A convenient way to express the solution to this equation uses the scalar convolution integral, which results in. The last term accounts for any initial activity on the filter medium, and is usually set to zero clean filter at time zero. The initial countrate of the monitor, before the concentration transient begins, is only that due to ambient background. Various solutions for the time-dependent FF countrate follow directly, once a concentration time-dependence Q t has been specified.

Note that the monitor flowrate F m is assumed constant; if it isn't, and its time-dependence is known, then that F m t would need to be placed inside the integral.

Also note that the time variable in all the models is measured from the instant the concentration in the sampled air begins to increase. For the moving-filter CPAMs, the above expression is a starting point, but the models are considerably more complicated, due to 1 the loss of material as the filter medium moves away from the detector's field of view and 2 the differing lengths of time that parts of the filter medium have been exposed to the sampled air. The basic modeling approach is to break down the deposition regions into small differential areas and then consider how long each such area receives radioactive material from the air.

The resulting expressions are integrated across the deposition region to find the overall response. The RW solution consists of two double integrals, while the CW response solution consists of three triple integrals. A very important consideration in these models is the "transit time," which is the time required for a differential area to traverse the window along its longest dimension. As a practical matter, the transit time is the time required for all differential elements that were in the deposition window at time zero to leave the window.

This figure shows contours of constant activity on a CW deposition area, after the transit time has expired.

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The filter moves from left to right, and the activity increases from left to right. The differential areas on the diameter have been in the deposition window the longest, and at the far right, have been in the window, accumulating activity, for the full transit time. Finally, to illustrate the complexity of these models, the RW response for time less than the transit time is [17]. In these equations, k is a conversion constant for units reconciliation. These plots show the predicted CPAM countrate responses for these parameter settings: The concentration instantly steps up to its constant value when the time reaches 30 minutes, and there is a count per minute cpm constant background.

In the LL plot, note that the FF countrate continues to increase. This is because there is no significant loss of radioactivity from the filter medium. The RW and CW monitors, on the other hand, approach a limiting countrate and the monitor response remains constant as long as the input concentration remains constant. For the SL plot, all three monitor responses approach a constant level. For the FF monitor, this is due to the source and loss terms becoming equal; since 88 Rb has a half-life of about 18 minutes, the loss of radioactive material from the filter medium is significant.

To present a wide picture of the field, it covers the international and national standards that guide the quality of air sampling measurements and equipment. With coverage of fundamental air sampling techniques and practical knowledge, the book provides insight into the contemporary thinking of experts, the maturity of the field, and its deep literature base.

Airborne particulate radioactivity monitoring

Building a bridge between the science behind air sampling and its practice, it supplies the know-how required to achieve technically rigorous air sampling data. Maiello The Physics of Aerosols, E. Generic Calibration Procedure, J. A Graded Approach, M.

First Responder Radiological Monitoring, T. Clendenin Monitoring Nuclear Fallout, H. Determination of Carbon in Air Method 6: Determination of the Iodine Content of the Atmosphere Method 7: Determination of the Rn Content of the Atmosphere Method 9: We provide complimentary e-inspection copies of primary textbooks to instructors considering our books for course adoption.

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