Continuous particulate air monitors (CPAMs) have been used for years in nuclear facilities to assess airborne particulate radioactivity (APR). In more recent times they may also be used to monitor people in their homes for the presence of manmade radioactivity. These monitors can be used to trigger alarms, indicating to personnel that they should evacuate an area. This article will focus on CPAM use in nuclear power plants, as opposed to other nuclear fuel-cycle facilities, or laboratories, or public-safety applications.
In nuclear power plants, CPAMs are used for measuring releases of APR from the facility, monitoring levels of APR for protection of plant personnel, monitoring the air in the reactor containment structure to detect leakage from the reactor systems, and to control ventilation fans, when the APR level has exceeded a defined threshold in the ventilation system.
Introduction CPAMs use a pump to draw air through a filter medium to collect airborne particulate matter that carries very small particles of radioactive material; the air itself is not radioactive.For the material in this introductory section, see, e.g., Harrer and Beckerley, Nuclear Power Reactor Instrumentation Systems Handbook, TID-25952-P1, NTIS (1973), Vol. 2 Section 13.6.2, ; Eisenbud, Environmental Radioactivity, Academic (1973), p. 449; Assessment of Airborne Radioactivity, International Atomic Energy Agency (1967), p. 24 The particulate radioactive material might be natural, e.g., radon decay products ("progeny", e.g., 212Pb), or manmade, usually fission or activation products (e.g., 137Cs), or a combination of both. There are also "gas monitors" which pass the air through a sample chamber volume which is viewed continuously by a radiation detector. 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. These gas monitors are often placed downstream of a CPAM so that any particulate matter in the sampled air is collected by the CPAM and thus will not contaminate the gas monitor's sample chamber.
Monitoring vs. sampling
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. On the other hand, monitoring with CPAMs provides nearly real-time airborne radioactivity level indication. It is common practice to refer to "sampled" air even when discussing a CPAM, i.e., as opposed to "monitored" air, which would, strictly, be more correct.
CPAM types
There are two major types of CPAMs, fixed-filter and moving-filter. In the former, the filter medium does not move while the airborne material is collected. The latter type has two main variants, the rectangular deposition area (“window”) and the circular window. 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. CPAM pumps are specially designed to maintain a constant volumetric flowrate.
As the air passes through the collection medium (usually a form of filter paper), particulate matter is deposited onto the filter in either a rectangular or circular pattern, depending on the instrument's design, and then the air continues on its way out of the monitor. 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.
Moving-filter monitors are often used in applications where loading of the filter medium with dust is an issue; this dust loading reduces the air flow over time. The moving-filter collection medium (“tape”) is assumed to move across the deposition area at a constant, known rate. This rate is often established in such a way that a roll of the filter tape will last about one month; a typical filter movement rate is about one inch per hour.
The rectangular-window moving filter monitor will be denoted as RW, and the circular, CW. Fixed filter is FF.
CPAM applications
Effluent monitoring
CPAMs are used to monitor the air effluents from nuclear facilities, notably power reactors. Here the objective is to assess the amount of certain radionuclides released from the facility.ANSI 42.18-2004, Specification and Performance of On-Site Instrumentation for Continuously Monitoring Radioactivity in Effluents Real-time measurement of the very low concentrations released by these facilities is difficult; a more-reliable measurement of the total radioactivity released over some time interval (days, perhaps weeks) may in some cases be an acceptable approach.Evans, W. C., "Quantitative Assessment of Time-Varying Rb-88 Using Continuous Air Monitors", Trans. Am. Nucl. Soc.,24 (1976), p. 129 In effluent monitoring, a sample of the air in the plant stack is withdrawn and pumped (pulled) down to the CPAM location. This sampled air in many cases must travel a considerable distance through piping. 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.ANSI 13.1-1999, Sampling and Monitoring Releases of Airborne Radioactive Substances from the Stacks and Ducts of Nuclear Facilities
In the USA there are effluent monitoring requirements in both 10CFR20 and 10CFR50; Appendix B to the former and Appendix I to the latter are especially important. 10CFR50 Appendix A10CFR50 Appendix A states:
Criterion 64--Monitoring radioactivity releases. Means shall be provided for monitoring the reactor containment atmosphere, spaces containing components for recirculation of loss-of-coolant accident fluids, effluent discharge paths, and the plant environs for radioactivity that may be released from normal operations, including anticipated operational occurrences, and from postulated accidents.
Also in the USA, Regulatory Guide 1.21, Measuring, Evaluating, and Reporting Radioactivity in Solid Wastes and Releases of Radioactive Materials in Liquid and Gaseous Effluents from Light-Water-Cooled Nuclear Power PlantsRegulatory Guide 1.21 is highly relevant to this CPAM application.
Occupational exposure assessment
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.ANSI 42.17B-1989, Performance Specifications for Health Physics Instrumentation- Occupational Airborne Radioactivity Monitoring Instrumentation A difficulty with this is that, unless the air in the compartment is uniformly mixed, the measurement made at the monitor location may not be representative of the concentration of radioactive material in the air that the workers are breathing. 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. The following portions of 10CFR2010CFR20 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.
Process monitoring and control
Radiation monitors in general have a number of process-control applications in nuclear power plants;See, e.g., Harrer and Beckerley, Chapters 13, 16 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.
For use in the USA, standard 10CFR50 Appendix A states: Criterion 19--Control room. A control room shall be provided from which actions can be taken to operate the nuclear power unit safely under normal conditions and to maintain it in a safe condition under accident conditions, including loss-of-coolant accidents. Adequate radiation protection shall be provided to permit access and occupancy of the control room under accident conditions without personnel receiving radiation exposures in excess of 5 rem whole body, or its equivalent to any part of the body, for the duration of the accident. Equipment at appropriate locations outside the control room shall be provided (1) with a design capability for prompt hot shutdown of the reactor, including necessary instrumentation and controls to maintain the unit in a safe condition during hot shutdown, and (2) with a potential capability for subsequent cold shutdown of the reactor through the use of suitable procedures.
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.
Reactor leak detection
Leakage from the so-called "reactor coolant pressure boundary" is required to be monitored in USA nuclear power plants.Regulatory Guide 1.45 Reactor Coolant Pressure Boundary Leakage Detection Systems, USNRC Monitoring the airborne particulate radioactivity in the reactor containment structure is an acceptable method to meet this requirement, and so CPAMs are used. 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. One of the most common of these pairs is 88Kr and 88Rb; the latter is detected by the CPAM. Relating the observed CPAM response to the 88Rb back to a leakage rate from the primary system is far from trivial.Evans, W. C., "Concentration Dynamics Modeling for Continuous Particulate Air Monitor Response Prediction", IEEE Transactions on Nuclear Science, 49, 5, Oct 2002
The regulatory basis for this CPAM application is found in 10CFR50:10CFR50
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. The specific attributes of the reactor coolant leakage detection systems are outlined in Regulatory Positions 1 through 9 of Regulatory Guide 1.45.
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. This instrumentation is required by Specification 3.4.15, "RCS Leakage Detection Instrumentation."