Method for Monitoring the Content of Radionuclides in Radioactive Waste

Abstract

The present invention relates to atomic engineering. A method for monitoring the content of radionuclides in radioactive waste includes a preparatory stage, in which the flow of radioactive waste is determined and its radionuclide carrier is determined. Sampling is carried out to determine the radionuclide vector in the flow. Their specific activities and the standard uncertainties of the activities are determined relative to the activity of the reference radionuclide. Lead coolant 207 The activation product of Bi is used as the reference radionuclide, and its specific activity is determined using a γ - spectroscopy complex with a resolution of at least 8% along the 0.662 keV line, 210 Pb, 202 Pb and 205 Pb are included in the radionuclide carrier. When processing the readings of the liquid scintillation spectrometer, data on the activity of γ - emitting radionuclides in the counted samples are used. This processing is carried out by the generalized least squares method, taking into account the specific activity of the reference radionuclide and the uncertainty values of the measurements of the hardly detectable radionuclides as weights of the measured values. The present invention makes it possible to increase the selectivity, accuracy, and sensitivity in determining the content of hardly detectable radionuclides in radioactive waste.

Description

Technical Field

[0001] The present invention relates to nuclear technology and a method for controlling the content of radionuclides in radioactive waste generated during the processing of spent nuclear fuel.

[0002] A specific application area of the present invention is a new generation energy complex, which consists of a power unit of a fast neutron reactor with a liquid lead coolant and a closed nuclear fuel cycle unit. Background Art

[0003] The radioactive hazard of radioactive waste and thus the methods for handling and disposing of radioactive waste are determined by the composition of radionuclides and the specific activities of individual radionuclides, and the specific activities of radionuclides and the categories of radioactive waste should be determined during certification. Most of these radionuclides belong to the so-called difficult-to-detect radionuclides, and the direct measurement of their activities is related to the complex procedures of sampling, sample preparation, measurement, and analysis.

[0004] A promising solution to the problem of controlling the content of difficult-to-detect radionuclides in radioactive waste is a method called the "nuclide-vector method" or "scaling-factor method" [IAEA Nuclear Energy Series NW-T-1.18. Determination and use of scaling factors for waste characterization in NPP. IAEA, Vienna, Austria, 2009].

[0005] At the same time, the radionuclide vector is understood as the value of the relative fraction of the activity of each radionuclide in the total activity of an object, which is determined by the results specifically representing spectral and radiation measurements and is reasonably attributed to a number of objects of the same type, combined by a common material composition and formation source.

[0006] This method is based on establishing a correlation between the activities of radionuclides in an object and applying the established correlation during the radiation monitoring of the object and the measurement of the activities of only reference radionuclides.

[0007] This method involves separating the radioactive waste of a nuclear power plant into types according to the formation source (stream) and establishing a correlation for each stream. The correlation relationship between the specific activities of radionuclides is established within the stream and consists of a set of ratios of the partial specific activities of standardized difficult-to-detect radionuclides to the specific activities of reference radionuclides.

[0008] This method enables current control to be restricted to measuring the activity of reference γ-emitting radionuclides that are readily detectable and have significant energy and decay rates.

[0009] Currently, various methods for monitoring radionuclides in radioactive waste have been proposed based on methods using radionuclide carriers.

[0010] In particular, a method for monitoring radionuclides based on the results of gamma-ray spectrometry using the correlation ratio of isotope 244Cm is known [Japanese Patent No. 5546174].

[0011] This method cannot be directly applied to the specific composition of radioactive waste generated during the operation of a reactor facility with a lead coolant, and does not allow control of the difficult-to-detect radionuclides characteristic of them, especially the activation products of the lead coolant.

[0012] The feasibility of the radionuclide vector method for the power unit of a lead-cooled reactor has also been confirmed, provided that it is improved and adapted to this particular type of facility.

[0013] [A method for estimating the content of α- and β-emitting radionuclides in radioactive waste from the radioactive chemical industry based on the matrix of γ-emitting radionuclides present. Nuclear and Radiation Safety, No. 3(85)-2017, UDC621.039.75].

[0014] The method for monitoring the content of radionuclides in the radioactive waste of the power unit of a fast neutron reactor with a liquid lead coolant proposed in the cited work was chosen as the prototype of the claimed method, including a preparatory stage of determining the radioactive waste stream and determining its radionuclide carrier, and a subsequent stage of routinely characterizing the radioactive waste in the stream and taking samples to determine the radionuclide carrier in the stream, from which counting samples are prepared.

[0015] With the help of gamma energy spectrum analysis, the specific activity of radionuclides is determined when counting the samples, whose decay is accompanied by gamma radiation, reference radionuclides are selected, target radionuclides that are difficult to detect are selected, and their specific activity and the standard uncertainty of the activity related to the activity of the reference radionuclides are determined. In the routine characterization stage of the radioactive waste, only the specific activity of the reference radionuclides in the radioactive waste is measured by non-destructive gamma energy spectrum methods. Then, based on the characteristics of the established radionuclide carrier obtained in the preparatory stage, the activity of the target complex detectable radionuclides in the radioactive waste stream is calculated.

[0016] The disadvantages of known methods for controlling radioactive waste are that it is presented only in a conceptual form and is not sufficiently developed. In particular, it does not provide justifications for the accuracy and sensitivity of the measurement methods and technical means proposed therein and provides acceptable errors in determining the specific activity of radionuclides that are difficult to detect.

[0017] In addition, the known methods do not provide for tracking radionuclides 210 Pb, 202 Pb, 205 the content of Pb, the monitoring of which is necessary when processing radioactive waste from power plants of the type under consideration. Summary of the Invention

[0018] The present invention solves the problem of improving the technical and economic characteristics of monitoring the content of radionuclides in radioactive waste from a power plant using a fast neutron reactor with a liquid lead coolant, including increasing the reliability of measurements and solving this problem by expanding the scope of application and improving non-destructive methods for controlling radioactive waste using carriers of radionuclides and their adaptation to these types of power plants.

[0019] The technical results achieved in this case include a significant increase in selectivity, accuracy, and sensitivity in determining the content of radionuclides that are difficult to detect in radioactive waste, making it possible to extend non-destructive control methods to such difficult-to-detect radionuclides whose decay is accompanied by α and β radiation, which were previously not identifiable by such methods.

[0020] Such a technical result is achieved due to the use of the isotope 207 Bi as a reference radionuclide, while using gamma energy spectroscopy and liquid scintillation spectrometry for measurements, as well as using a new method for jointly processing measurement results independently obtained from these two experimental data sources, which significantly improves the reliability of control, reduces uncertainty and errors. Due to the high selectivity of the method, it becomes possible to control the content of α- and β-emitting radionuclides 210 Pb, 202 Pb, 205 Pb, the monitoring of which is necessary when processing radioactive waste from power plants of the type under consideration.

[0021] At the same time, in the claimed invention, an additional increase in the sensitivity and accuracy of measurements is achieved by using the physical characteristics of the selected reference radionuclide ( 207 Bi). Composed in the presence of two separate gamma-ray peaks.

[0022] In the present invention, the said technical result is achieved due to the fact that the method for monitoring the content of radionuclides in radioactive waste generated during the processing of spent nuclear fuel in a fast neutron reactor with a liquid lead coolant includes a preparatory stage of determining the radioactive waste stream and determining its radionuclide vector, and a subsequent stage of routinely characterizing the radioactive waste in the stream, with sampling to determine the radionuclide vector in the stream. Using gamma energy spectrometry, the activity of radionuclides is determined when counting samples that decay with gamma radiation. Reference radionuclides are selected, target radionuclides that are difficult to detect are selected, and their specific activities and the standard uncertainties of the activities related to the activities of the reference radionuclides are determined. In the routine characterization stage of radioactive waste, only the specific activity of the reference radionuclide in the radioactive waste is measured by a non-destructive gamma spectrometry method. Then, based on the characteristics of the established radionuclide carriers obtained in the preparatory stage, the activity of the detectable radionuclides of the target complex in the radioactive waste stream is calculated, and the lead coolant 207 The activation product of Bi is used as the reference radionuclide, and along the 0.662 keV line, 210 Pb, 202 Pb and 205 The specific activity of Pb elements is determined using a gamma spectral complex with a resolution of at least 8%. Their decay is accompanied by α- and β radiation, and their activities are measured using a liquid scintillation spectrometer. When processing the readings of the liquid scintillation spectrometer, data on the activity of γ-emitting radionuclides in the counted samples are used and processed by the generalized least squares method, taking into account the uncertainty values of the reference radionuclide specific activity measurements and the weights of the difficult-to-detect radionuclides as measured values.

[0023] The technical result of the present invention is also due to the fact that when using a gamma spectrometer to determine the specific activity of the reference radionuclide 207Bi, the spectrum is recorded and two total absorption peaks corresponding to the gamma radiation energies of 569.7 keV and 1063.7 keV are obtained.

[0024] The required method can also be implemented, where 60 Co is used as a reference radionuclide to additionally monitor the content of 55 Fe in structural steel; 59 Ni; 63 The content of the activation product of Ni, and the results are processed by the generalized least squares method, taking into account the uncertainty values of the reference radionuclide specific activity measurements and the weights of the difficult-to-detect radionuclides as measured values.

[0025] In addition, an implementation scheme can also be adopted, where nuclear fuel 90 Sr; 99 Tc; 93 Мо; 129 I; 135The fission product content of Cs is also monitored, using a sub- 137m Ba or 137 Cs ( 137m Ba) of 137 Cs as a reference radionuclide, and the results are processed using the generalized least squares method, taking into account the uncertainty values of the reference measurements and the hard-to-detect radionuclides as weights for the measured values. Detailed implementation manner

[0026] The proposed method is carried out as follows.

[0027] During the operation of the power unit of a fast neutron reactor with a liquid lead coolant, the current monitoring of the radionuclide content in radioactive waste is carried out using the radionuclide vector technique within the framework of the routine characterization stage. Before this main stage, there is a preparatory stage, during which a set of preliminary studies is carried out, including identifying the parameters of the radioactive waste stream and the radionuclide carriers, justifying the reasons, and preparing for the routine characterization stage.

[0028] The identification of the waste stream means grouping the waste according to its chemical and physical properties and determining the list of reference radionuclides and radioactively significant composite detectable radionuclides for each waste stream. The target control object of the present invention is the radioactive waste stream related to the compacted incineration waste of the structural materials of the primary circuit equipment and other solid radioactive wastes contaminated by the activation products of the lead coolant, a group of hard-to-detect radionuclides, consisting of 210 Pb and 202 Pb and 205 Pb, whose decay is accompanied by β radiation.

[0029] In addition, for this radioactive waste stream, the radionuclide carriers are established under the current rules in the field of monitoring radionuclide activity, including the guidance of international regulations [IAEA Nuclear Energy Series NW-T-1.18. Determination and use of scaling factors for waste characterization in NPP. IAEA, Vienna, Austria, 2009], [ISO 21238-2007 Scaling factor method to determine the radioactivity of low- and intermediate-level radioactive waste packages generated at nuclear power plants]. The relationship between the activities of the radionuclides is described by a function obtained by mathematically processing the measurement results.

[0030] The correlation between the specific activities of radionuclides is established in the stream, and a set of ratios of the partial specific activity of the detectable radionuclide by the normalized complex to the specific activity of the reference radionuclide is obtained. Therefore, it is possible to limit oneself to measuring the activity of a reference easily detectable γ-emitting radionuclide with significant energy and decay yield during the routine characterization stage ( 60 Co, 137 Cs, 207 Bi, etc.).

[0031] The specific activity of a radionuclide in the waste at the time of its formation can be represented as the sum of a significant number of independent random variables. At the same time, according to the central limit theorem of probability theory, it will have a normal distribution. Experimental data show that for radioactive waste from a single formation source, the distribution of the specific activity of a single radionuclide has a normal or log-normal form. For statistical analysis, when calculating the total uncertainty with a confidence probability of 0.95, measurements of 20 - 30 samples should be used in order to achieve a swelling coefficient approximately equal to 2.

[0032] Therefore, according to the present invention, for a sample from which a counting sample is prepared, the specific activity of the radionuclide in the counting sample is determined using gamma energy spectrometry, the decay of which is accompanied by gamma radiation, and then a reference radionuclide and a target difficult-to-detect radionuclide are selected. In the claimed method, 207Bi is designated as the reference radionuclide for a group of lead coolant activation products, 210 Pb, the decay of which is accompanied by α-radiation, 202 Pb and 205 Pb, the decay of which is accompanied by β-radiation, are taken as the target difficult-to-detect isotopes 207 bi is assigned to the reference radionuclide is possible because liquid lead is used as a coolant in the reactor, and when activated, the radionuclides 210 Рo, 207 Рo, 205 Рo, 203 Рo, 206 Bi, 207 Bi, 208 Bi, 210 Bi, 210 Pb, 202 Pb, 205Pb, etc. are formed. [G.L. Khorasanov, A.I. Blokhin, Fundamental principles for creating a low-activation lead coolant with isotope enrichment for promising nuclear power plants, Problems of Atomic Science and Technology, Nuclear Constants Series, 2001, No. 1]. A characteristic of the radionuclide 207bi is a half-life of 32 years, a boundary energy of β-radiation of 765 keV, and its presence in the gamma-ray spectra of two main lines at 569.7 keV and 1063.7 keV. When determining the specific activity of the reference radionuclide 207Bi, the pulse count rate is measured at the peaks of the total absorption of the gamma-radiation energies corresponding to 569.7 keV and 1063.7 keV, Ni and N2, C 1 The use of two peaks makes it possible to reduce the 207 uncertainty in the measurement of the activity of Bi, Bc by reducing the relative standard uncertainty of type A, due to the random nature of the radiation, and is calculated by the formula:

[0033]

[0034] In the case of using the statistics on two peaks:

[0035] The measurement of the gamma-radiation spectrum of the reference radionuclide is performed using a gamma spectrometer with a resolution of at least 8% along the 0.662 keV line. This is due to the presence in the radioactive waste of some radionuclides with similar gamma-radiation energies - 137 Co (662 keV), 134 Co (569 keV, 605 keV), 207 Bi (569.7 keV), etc.

[0036] A significant feature of the claimed method is that at the stage of determining the ratio of the specific activities of radionuclides in a sample, the activities of detectable radionuclides of the α- and β-emission complexes are measured using a liquid scintillation spectrometer as prior information on the activities measured using a gamma spectrometer for reference radionuclide samples, whose decay is accompanied by α or β and γ radiation. This makes it possible to achieve a significant increase in accuracy through the normalization of the spectra of the mixed radiation and the mutual verification of the two measurement methods.

[0037] Another significant feature of the claimed method is that during the sample study, not only the uncertainty in the measurement of the activity of the reference radionuclide is evaluated, but also the uncertainty in the measurement of the activity of hardly detectable radionuclides, so that their values are used as "weights" when calculating the average ratio of the sample activities and in the correlation and regression analysis of the experimental data. If the correlation coefficient RA is greater than or equal to the standard value, the correlation between the specific activity of the hardly detectable radionuclide and the reference radionuclide is confirmed:

[0038]

[0039] where A i,k is the activity of the i-th radionuclide difficult to detect in the k-th sample; A r,k is the activity of the reference radionuclide in the k-th sample; n is the number of samples.

[0040] This criterion value is set based on the required precision for determining the specific activity of radionuclides that are difficult to detect. In most countries using similar methods, it is assumed to be 0.7 [Determination and Use of Scaling Factors for Waste Characterization in NPP. IAEA Nuclear Energy Series NW-T-1.18. Vienna, IAEA, 2009].

[0041] To establish the correlation between the specific activity of radionuclides that are difficult to detect and the reference radionuclide, the geometric mean is used. Since individual values in the experimental data summary are significantly removed from the others. Within the framework of the proposed method, the geometrically weighted average ratio of the specific activities of the radionuclides that are difficult to detect and the reference radionuclide is used to establish a numerical ratio. This method enables us to consider the uncertainty of each measured value as a "weight". For the i-th radionuclide, the weighted geometric mean of the ratio is determined by the expression:

[0042]

[0043] where К i,k = А i,k / А r,k is the ratio of the activity of the i-th radionuclide to the reference radionuclide in the i-th sample;

[0044] is the calculated relative standard uncertainty of the ratio of the S-th radionuclide in the k-th sample;

[0045] are the relative standard uncertainties of the specific activities of the i-th radionuclide difficult to detect and the reference radionuclide in the k-th sample, respectively. They are used as the weights of the measured values.

[0046] If there is no significant correlation (R4 is less than the standard value) between the specific activity of the i-th radionuclide difficult to detect and the reference radionuclide in the controlled waste, then the ratio between their logarithms is proposed to be determined:

[0047] ln(A i ) = ln(a) + b ln(A r ) (4)

[0048] where a and b are constants.

[0049] For the correlation and regression analysis of experimental data representing the logarithm of the specific activity of a radionuclide that is difficult to detect and a reference radionuclide, the generalized least squares method [Seidel R.M. Estimation of the error of regression parameters in the least squares method - Atomic Energy, 1994, Vol. 77, No. 6, M, 1994, pp. 463 - 466] is used. Since during the construction of the radionuclide carrier. In this case, the solution (the values of In(a) and b with the corresponding uncertainties) is found using iterative calculations of the Newton - Raphson method.

[0050] When using the proposed new method for constructing a radionuclide vector, the accuracy and sensitivity of monitoring radionuclides that are difficult to detect in radioactive waste generated during the processing of spent nuclear fuel in the power unit of a fast - neutron reactor with a liquid - lead coolant are significantly increased. Within the framework of the claimed method, it is possible to additionally control the content of radionuclides in other radioactive waste streams using the proposed scheme for calculating the radionuclide vector. In particular, if it is necessary to characterize the activation products of structural steel 35 Fe; 54 Ni; 63 Ni, 60 Co are assigned as reference radionuclides. And the processing of the results is carried out by the generalized least - squares method, taking into account the uncertainty values of the measurements of the specific activities of the reference radionuclides and the radionuclides that are difficult to detect as the squares of the weights of the measured values.

[0051] Another embodiment of the method is that the fission products of nuclear fuel 90 Sr; 99 Tc; 93 Mo, 129 I, 135 Cs content is determined using 137 Cs as a sub - 137m Ba or 137 cs( 137m Ba) as a reference radionuclide, and the processing of the results is also carried out by the generalized least - squares method, taking into account the measurement uncertainty values as the weights of the reference and the radionuclides that are difficult to detect as the measured values.

Claims

1. A method for monitoring the content of radionuclides in radioactive waste generated during the treatment of spent nuclear fuel of a power plant with a liquid lead coolant in use, including a preparatory stage of determining the radioactive waste stream and determining its radionuclide vector, and a subsequent stage of conventional characterization of the radioactive waste in the stream, and taking samples to determine the radionuclide vector in the stream, thereby preparing counting samples, determining the specific activity of the radionuclides during counting of the counting samples with the help of gamma energy spectrum analysis, whose decay is accompanied by gamma radiation, selecting reference radionuclides, selecting target radionuclides that are difficult to detect, and determining their specific activity and standard uncertainty of their activity by determining their activity relative to the reference radionuclides, during the conventional characterization stage of the radioactive waste, only the activity of the reference radionuclides in the radioactive waste was measured by a non-destructive gamma energy spectrum method, and then, according to the characteristics of the established radionuclide vector obtained in the preparatory stage, the activity of the target radionuclides that are difficult to detect in the radioactive waste stream was calculated, characterized in that Lead coolant 207 The activation product of Bi is used as a reference radionuclide, and its specific activity is determined using a gamma-ray spectrometry device having a resolution of at least 8% along the 0.662 keV line. The list of radionuclides that are difficult to detect includes 210 Pb, 202 Pb and 205 Pb, whose decay is accompanied by α- and β radiation, and their activities are measured using a liquid scintillation spectrometer. When processing the readings of the liquid scintillation spectrometer, data on the activities of γ-emitting radionuclides in the counting sample are used and processed by the generalized least squares method, taking into account both the uncertainty value of the reference radionuclide specific activity measurement result and the uncertainty value of the specific activity measurement result of the target radionuclide that is difficult to detect as weights for the measured values.

2. The method according to claim 1, characterized in that, When using a gamma spectrometer to determine the specific activity of the reference radionuclide Bi-207, record the spectrum and measure the pulse count rate in two total absorption peaks corresponding to the gamma radiation energies of 569.7 keV and 1063.7 keV.

3. The method according to claim 1, characterized in that, Monitoring the content of activated products of structural steel 55 Fe; 39 Ni; 63 Ni, using 60 Co as a reference radionuclide, and processing the results by the generalized least squares method, while considering the measurement uncertainties of the specific activities of the reference radionuclide and the radionuclides that are difficult to detect as the weights of the measured values.

4. The method according to claim 1, characterized in that, The content of nuclear fuel fission products is monitored as 90 Sr. 99 Tc, 93 Mo, 129 I. 135 Cs, using a child relationship 137m Ba or 137 Cs( 137m Ba) 137 Cs was used as the reference radionuclide, and the results were processed by the generalized least squares method, taking into account the uncertainty values ​​of the specific activity measurement of the reference radionuclide and the uncertainty values ​​of the difficult-to-detect radionuclides as weights for the measured values.

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