Methodological questions concerning the application of remote gamma spectrometry for radiation monitoring of air emissions from nuclear power plants and detecting trans-boundary transport of products of an accident are examined for the example of the development and experimental operation of the components of the Spaider data-acquisition system produced by the KonverSia company. A system of collimated γ spectrometers arranged along the monitoring contour at the site boundary of a nuclear power plant makes it possible to increase the accuracy of the computational predictions and to obtain and accumulate, during normal operation of a nuclear power plant, emissions data for radioactive inert gases and iodine in a continual monitoring regime.Integral dosimeters are used to monitor radioactive emissions in most automated radiation monitoring systems operating in nuclear power plants. The low dose sensitivity (~600 µR −1 ) and impossibility of autonomous classification of any anomalies detected are forcing developers of such systems to switch to more sensitive γ-spectrometric means for performing measurements (55000 µR −1 and more) [1]. Assessments of the predicted irradiation dose in the case of accidental emission are based on the accepted model of impurity transport and dissemination in the atmosphere near the ground, making it possible to assess the radionuclide concentration field near the ground. Such calculations require as initial information data on the amount and composition of technogenic radionuclides which have entered the environment. Today, many spectrometers are installed in the technological equipment of nuclear power plants to monitor emissions [2]. More accurate assessments of the actual emissions as well as monitoring of the radiation parameters of the emission stream in an on-line regime for use in models can be performed by using the results of remote spectrometry of streams [1]. This makes it possible to increase the accuracy of the computed forecast, obtain and accumulate data during normal operation of a nuclear power plant on the emission of radioactive inert gases and iodine in a continual regime, and to monitor the time and radiation parameters of legally allowed emissions, radiation incidents, and accidents.The present article examines the methodological questions concerning the use of remote gamma spectrometry for radiation monitoring of emissions from nuclear power plants and detecting trans-boundary transport of the products generated by an accident. This method has been tested during development and experimental operation of the components of the Spaider data-acquisition system produced by the KonverSia company [1].Requirements for the Main Characteristics of Gamma Spectrometers. In remote gamma spectrometry performed for the purpose of monitoring emissions, the radiation characteristics of the γ field generated by the radionuclides in a stream, which at short distances (up to 1.5 km) from the exhaust stack of a nuclear power plant has well-defined transverse dimensions, which are only ...
Methodological questions concerning the use of gamma spectrometers for measuring gas-aerosol emissions from nuclear power plants and radiation monitoring of the environment by remote gamma spectrometry are examined. A system of intercoupled programs was developed to solve this problem. This system makes it possible to calculate the energy-angular distribution of the gamma ray flux created at the monitoring point from a uniform extended volume source with user-prescribed characteristics and measuring geometry. The computational result is used as a source for direct simulation of the line shape in the NaI(Tl) crystal spectrometer used.Determining the emission intensity in real time for normal operation and in accident situations requires developing a gamma-spectrometric channel for performing remote measurements of the volume activity of radionuclides in the atmospheric layer near the ground. Gamma spectrometers based on semiconductor detectors, high-pressure xenon chambers, or inorganic scintillators can be used as detectors in such a channel. When such a spectrometer is inserted in a channel of an automated system for monitoring radiation conditions, it becomes necessary to measure the main metrological characteristics -the sensitivity and the minimum measurable volume activity of radionuclides. The fact that it is impossible to create artificial extended volume sources with spectral characteristics similar to those of real sources and to calibrate them for diverse geometries complicates the determination of these characteristics.Similar problems arise with the use of direct continuous gamma spectrometric methods for monitoring the atmospheric layer near the ground.Radionuclides in a gas-aerosol emissions stream can be regarded as an extended volume source, creating in the environment a radiation field whose main parameters are the energy flux density, the gamma ray flux density, and the exposure rate or absorbed dose. Switching to gamma spectrometric monitoring methods, the gamma ray flux density and its angular distribution must be determined at the location of the spectrometer. Knowing these quantities makes it possible to calculate the volume activity of radionuclides in the stream at the monitoring site and, using an appropriate model of their transport, to estimate the magnitude of the emission.A special system of intercoupled programs was developed at the KonverSia JSC to solve these problems. This system makes it possible to perform a Monte Carlo calculation of the energy-angular distribution of the gamma ray flux at the monitoring point from a uniform extended volume source with a prescribed radionuclide composition, volume activity, and measuring geometry. This distribution is then used as a source for direct simulation of the line shape in the scintillation detector of the spectrometer [1]. The CLOUD computer program calculates the energy-angular distribution of the gamma ray flux and the absorbed dose rate in air from an ellipsoidal source. These same characteristics can be calculated using as a volum...
The application of statistical algorithms in the problem of early detection of technogenic, nuclear power plant emissions in the atmosphere near the ground by means of the spectrometric channel of ARMS (Automatic Radiation Monitoring System) is examined. It is noted that the changes in the radon progeny content which are associated with the diurnal behavior of the intensity of turbulent mixing influence the variance of the readings in the 131 I and 137 Cs detection ranges. As a result, when a stationary Poisson model is used in the algorithm the number of false alarms increases and the algorithm becomes ineffective. Regional corrections lowering the probability of false alarms to ≤10 -3 with correct detection probability ≥0.7 were introduced into the detection and classification algorithm on the basis of the results of monitoring performed over the course of a year for intervals with stationary γ background.When the ARMS (Automatic Radiation Monitoring System) spectrometric channel is used for early detection of nuclear power plant emissions, technogenic radionuclides at low concentrations must be detected with a definite reliability in real-time. Ordinarily, the lowest measurable activity, determined by the background conditions in the monitoring zone and the method and means for performing the measurements, is taken as the conventional boundary between the detection and measurement processes. Our assessments have shown that a gamma spectrometer with a 63 × 63 mm NaI(Tl) crystal with exposure time 15 min can be used for quantitative measurements of 131 I and 137 Cs provided that their volume activity ≥8 Bq/m 3 , i.e., starting at values 2-3 times lower than the admissible volume activity AVA PUBLIC according to . Statistical algorithms for processing the spectrometric information make it possible to detect 131 I and 137 Cs with probability ≥0.7 at concentrations concentration 2-3 Bq/m 3 , which expands the possibility of using the ARMS spectrometric channel for monitoring nuclear power plant emissions. In what follows, the term "detection" is used precisely in this probabilistic sense, since, for example, according to Rosgidromet normative documents the detection of a radionuclide not present in the global radiation background is taken as a level of detection two times above the minimum measurable activity of the radionuclide for the apparatus being using [2].The concept of the minimum detectable volume activity can be introduced as a characteristic of the spectrometric channel for detecting technogenic radionuclides at low concentrations under background conditions in the ARMS monitoring zone. It can be determined as the volume activity which is detected by the algorithm used for processing the spectrometric information with probability of correct detection P c.d and false-alarm probability P f.a established for the given ARMS.
The metrological support for monitoring of ground layer of the atmosphere with ASKRO gamma spectrometers requires determining their sensitivity in the measurement geometry of technogenic radionuclides. The application of numerical modeling, the method of similarity of radiation fields, and a combined method in evaluating sensitivity of a spectrometer is examined. It is shown that the model method overstates the sensitivity and does not permit checking the spectrometer. To decrease the error of estimation, a combination of measurements of the efficiency of the spectrometer for a point source and a Monte Carlo calculation of the incident flux is used. An estimate of the total relative error of determination of the sensitivity is given and it is shown that it can be verified. The estimates obtained for the sensitivity increase the minimum measured volume activity of the spectrometer at energies less than 1 MeV.Metrological support for measurements of the volume activity of technogenic radionuclides in the atmospheric layer at the ground by means of the spectrometric channel ASKRO includes certification of the gamma spectrometer and methods for checking its main measuring characteristics, development of methods for performing measurements and running monitoring of the data, collection of control sources and standard measure of activity. The sensitivity and limit of admissibility of the relative main error are, according to GOST 21496-89 [1], the main metrological characteristics of the means of measurement of the volume activity of radionuclides in a gas. The need to determine them in the geometry of a semi-infinite source (referred to below as a 2π source) with respect to all technogenic radionuclides raises difficulties in the certification of the spectrometer and verifying it with respect these parameters under experimental conditions. The information of the spectrometric channel ASKRO serves for establishing a correspondence between the volume activity of the ith technogenic radionuclide q i and control levels (control correspondence), its variation in time (monitoring), the probability of evaluating the presence in the ground layer of the atmosphere (discovery). The δq i error in evaluating the concentration is related with, first and foremost, the accuracy δS ij with which the sensitivity of the spectrometer with respect to the jth line of the ith radionuclide is evaluated in the geometry used for the measurements:where ΔX ij is the excess count rate above the background in the region of the total absorption peak of the jth characteristic line of the radionuclide [2].If the concentration q i of a radionuclide is less than the minimum measured value, the relative estimate of the excess of the count in the jth energy range N ij above the a posteriori average count N ij , calculated on the stationariness interval, (N ij -N ij )/N ij 1/2 is used to calculate the probability of its presence in the ground layer of the atmosphere [3]. The sensitivity of the spectrometer with respect to the ith radionuclide in explicit...
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