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Radiochemistry in Nuclear Power Reactors Appendix C GAMMA-RAY SPECTROMETRIC ANALYSIS C.1 INTRODUCTION The measurement of radioactivities by gamma-ray spectrometry using a high resolution solid state detector is the most frequently used technique in a nuclear power plant to monitor the radioactivity concentration in the reactor coolant as well as in the process effluents. The powerful computer has made the gamma-ray spectrometric analysis easier and faster, but some fundamentals which cannot be done by a machine should be carefully exercised. In this section some applications of gamma-ray spectrometric analysis in a nuclear power plant will be presented. More fundamental aspects of gamma-ray spectrometry can be found in the literature(1,2) C.2 DETECTOR CALIBRATION C.2.1 Standard Sources Certified Radioactivity Source—A NIST (National Institute of Standard and Technology), formerly National Bureau of Standards (NBS) standard source, or a calibrated radioactive source, with stated accuracy, whose calibration is certified by the source supplier as traceable to NIST, should be used in energy and efficiency calibrations. The standard sources to be used in energy and efficiency calibrations should contain gamma-ray energies ranging from ~60 to ~2000 keV. A standard source may be a single nuclide with a single gamma-ray (e.g., Cs-137, Co-57) or multiple gamma rays (e.g., Co-60, Ba-133, Eu-152), or a mixture of many nuclides covering a good range of gamma-ray energies (e.g., NIST Standard SRM 4215-B). It is also important to note that the half-lives of the standards selected for routine calibration should be ~5 years or longer to minimize the error in decay correction. A list of commonly used standards is given in Appendix A, Nuclear Data.
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Radiochemistry in Nuclear Power Reactors Radioactivity Check Source—a radioactive source, not necessarily calibrated but preferred to contain long-lived isotopes with both low and high-energy gamma rays (e.g., Eu-152), which should be used to confirm the continuing satisfactory operation of the counting instrument. Radioactivity Source Strength—a radioactive source selected as either a check source, or a calibration standard, should contain radioactivity high enough to produce a statistically reliable count rate in the full-energy gamma-ray peak of interest. However, it should be cautioned that when a source is counted at any distance from the detector, or in any geometry, the count rates should be low enough to reduce the effect of random summing of gamma rays to a level where it may be neglected. A better criterion may be the indication of counting system dead time when the source is placed at the counting position. Any source with ≥5% dead time should not be used in calibration at that position. Physical Form of Standard Source—the radioactive source can be in any physical form or size. However, the result of the calibration is applied to the same physical form and size of the standard used only (more discussion on source geometry in Section C.4.3). C.2.2 Calibration Procedure Energy Calibration—Determine the energy calibration (channel number versus gamma-ray energy) of the detector system at a fixed gain by determining the full energy peak channel numbers from gamma rays emitted from one or more known energy standard sources. A linear correlation is normally obtained. Efficiency Calibration – Accumulate an energy spectrum using calibrated radioactivity standards at a desired and reproducible source-to-detector distance. At least 20,000 net counts should be accumulated in each full-energy gamma-ray peak of interest using Certified Radioactivity Standard Sources.
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Radiochemistry in Nuclear Power Reactors Obtain the net count rate (total count rate of region of interest minus the Compton continuum count rate and, if applicable, the ambient background count rate within the same region) in the full energy gamma-ray peak, or peaks. Correct the peak net count rate for random summing, decay during counting and coincidence summing (see Section C.4) if applicable. Correct the standard source emission rate for decay to the count time. Calculate the full-energy peak efficiency Eγ as follows: where Eγ = full energy peak efficiency (counts per gamma ray emitted) Cγ = net gamma ray count in the full energy peak (c/s) Nγ = gamma ray emission rate (gamma rays per second) If the standard source is calibrated as to activity, the gamma ray emission rate is given by where A = number of nuclear decays per second Pγ = probability or intensity per decay for the gamma ray After obtaining the full energy peak efficiency for at least 10 gamma rays, spreading from ~60 to ~2000 keV, plot the values for full energy
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Radiochemistry in Nuclear Power Reactors peak efficiency versus gamma ray energy, or fit to an appropriate mathematical function. The expression or curve showing the variation of efficiency with energy must be determined for a particular detector with a particular source geometry, and must be checked for changes with time. The form of the calibration curve or function will be better defined by using more gamma ray standards, particularly in the ~60–300 keV region. C.3 MEASUREMENT OF GAMMA RAY EMISSION RATE OF THE SAMPLE Place the sample to be measured at the source-to-detector distance for efficiency calibration. Accumulate the gamma-ray spectrum, recording the count duration. Determine the energy of the gamma rays present by use of the energy calibration curve. Obtain the net count rate in each full-energy gamma ray peak of interest as described in Efficiency Calibration. Determine the full-energy peak efficiency for each energy of interest from the curve or function obtained in Efficiency Calibration. Apply any sample geometric correction factors, if the sample is not in the same geometry as the calibrating standard. Calculate the number of gamma rays emitted per unit time for each full energy peak as follows:
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Radiochemistry in Nuclear Power Reactors When calculating a nuclear disintegration rate from a gamma ray emission rate determined for a specific radionuclide, a knowledge of the gamma ray intensity per decay is required. That is, C.4 CORRECTIONS OF COUNTING DATA C.4.1 Decay Corrections Simple Decay Correction The measured activities are corrected for the decay between the times of sampling and counting by using where A = measured activity Ao = activity at t=0 t = time between counting and t=0 λ = decay constant Generally, the midpoint of the sampling and counting is used for decay correction if the sampling or counting duration is less than 1/3 of the decay half-life. When the sampling or counting duration is longer than 1/3 of the decay half-life, correction for the decay during counting or sampling should be applied (see below), and the end of sampling and the beginning of counting are used in decay correction.
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Radiochemistry in Nuclear Power Reactors Correction for Decay During Counting and Sampling If the value of a net count rate is determined by a measurement that spans a significant fraction of a half-life, and the value is assigned to the beginning of the counting period, a multiplicative correction factor, F, must be applied. where F = correction factor A = activity or ‘average’ counting rate during the counting period Ac = activity at beginning of count (T=0) T = duration of counting λ = decay constant The same correction factor is also used in correction for decay during sampling. In this case where AT = total activity collected in the sample Am = activity measured at end of sampling T = duration of sampling λ = decay constant
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Radiochemistry in Nuclear Power Reactors C.4.2 Coincident Photon Summing Correction When another gamma ray or x-ray is emitted in cascade with the gamma ray being measured, in many cases a multiplicative coincidence summing correction C must be applied to the net full-energy-peak count rate if the sample-to-detector distance is 10 cm or less. Coincident summing correction factors for the primary gamma rays of Co-60 and Y-88 are approximately 1.09, 1.04, and 1.01 for a 65 cm3 detector at 1 cm, 4 cm, and 10 cm sample-to-detector distances, respectively. The data for cascade-summing corrections for some major nuclides can be found in the literature(3). Similarly, when a weak gamma ray occurs in a decay scheme as an alternate decay mode to two strong cascade gamma rays with energies that total to that of the weak gamma ray, a negative correction would be applied to the weak gamma ray. However, the correction factors may be negligible for most of the radionuclides observed in nuclear power plants. C.4.3 Sample Geometry Calibration The gamma-ray full energy peak efficiency is sensitive to the following counting geometry factors: (1) source-to-detector distance; (2) physical form of the source (gas, liquid, solid); and (3) size and shape of the source or source container (point source, filter papers of various sizes, charcoal cartridge, liquid bottles of various sizes, gas vials of various sizes, Marinelli beakers of various sizes). For most accurate results, the source to be measured must duplicate, as closely as possible, the calibration standards in all aspects. If this is not practical, appropriate corrections must be determined and applied. The methods of determining geometric correction factors and the preparation of standard source for various geometric calibration are described in Appendix D. C.5 RESOLUTION OF A COMPOSITE DECAY CURVE For a mixture of several independent activities, the result of plotting log A versus t is always a curve concave upward (convex toward the original). This curvature results because the shorter-lived components become relatively less significant as time passes. In fact, after sufficient time, the longest-lived activity will entirely predominate, and its half-life may be read from this late portion of the decay curve. Now, if this last portion, which
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Radiochemistry in Nuclear Power Reactors is a straight line, is extrapolated back to t=0 and the extrapolated line subtracted from the original curve, the residual curve represents the decay of all components except the longest-lived. This curve may be treated again in the same way, and in principle any complex decay curve may be analyzed into its components. In actual practice, experimental uncertainties in the observed data may be expected to make it difficult to handle systems of more than three components, and even two-component curves may not be satisfactorily resolved if the two half-lives differ by less than about a factor of two. An example of reactor water sample containing N-13, F-18 and Cu-64 is given in Section C.9. C.6 ACTIVITY DECAY-GROWTH CALCULATIONS The parent-daughter decay-growth relationship, described in the following equations, is used in correction for activities which may grow into the sample from the decay of its parent nuclide: or where = parent activity at time, 0, A1 = parent activity at time, t, = daughter activity at time, 0, A2 = daughter activity at time, t.
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Radiochemistry in Nuclear Power Reactors If the sample is purified for the parent activity free from the daughter activity, then =0 at separation time. If the sample contains both daughter activities at sampling time, the activity is calculated by the following equation: The frequently observed activities requiring decay-growth calculations include Mo-99(Tc-99), Zr-95(Nb-95), and Cs-139(Ba-139). C.7 IODINE ACTIVITY MEASUREMENTS* Place the sample on a shelf position at which the system dead time should be less than 5%. In order to obtain the best result, a sample should be counted several times similar to the following schedule: Time After Sampling (hrs) Count Time** (min) 0.5 10 1.0 10 2.0 20 5–10.0 20 10–24.0 20–60 It is important to count the sample as soon as the sample is prepared to measure the shorter-lived isotopes, I-134 and I-132, and to count the sample after most of the shorter-lived isotopes have decayed so that the lower intensity longer-lived isotope, I-131 can be measured accurately. * The procedure for sample preparation is given in Appendix B. ** The count time may vary depending on the level of activity in the sample.
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Radiochemistry in Nuclear Power Reactors The following gamma ray energies and intensities for five major iodine nuclides should be used in gamma ray spectrometric analysis: Isotope T1/2 Gamma Ray 1 keV (%) Gamma Ray 2 keV (%) I-131 8.04 d 364.5 (81.2) I-132 2.28 h 667.7 (98.7) 772.6 (76.2) I-133 20.8 h 529.9 (86.3) I-134 52.6 m 847.0 (95.4) 884.1 (65.3) I-135 6.57 h 1260.4 (28.9) 1131.5 (22.5) More complete decay characteristics of iodine isotopes and other nuclides are given in Appendix A. C.8 OFFGAS SAMPLE ANALYSIS* Place the sample vial on a shelf position where the counting geometric factor, or the vial sample calibration, is available. The system dead time should not be higher than 5%. In order to obtain the best result, a sample should be counted several times similar to the following schedule. Time After Sampling Count Time** (min) <10 min 5 20 min 5 1–2 hrs 10–20 2–5 hrs 10–20 10–24 hrs 20–60 * The sampling procedure is described in Appendix B. ** The count time may vary depending on the level of activity in the sample.
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Radiochemistry in Nuclear Power Reactors It is important to count the sample as soon as the sample is taken to measure the shorter-lived isotopes (Xe-138 and others), and to count the sample after most of the shorter-lived isotopes have decayed so that the lower intensity longer-lived isotope, Xe-133, can be measured accurately. The following gamma-ray energies and intensities for major nuclides in the offgas sample should be used in gamma ray spectrometric analysis: Isotope T1/2 Gamma Ray 1 keV (%) Gamma Ray 2 keV (%) Gamma Ray 3 keV (%) *Kr-85 m 4.48 h 151.2(75.1) 304.9(13.7) *Kr-87 1.37 h 402.6(49.6) *Kr-88 2.84 h 196.3(26.0) 834.8(13.0) 1530(10.9) Kr-89 3.15 h 220.9(20.0) 585.8(16.6) *Xe-133 5.24 d 81.0(38.3) Xe-133 m 2.19 d 233.2(10.3) *Xe-135 9.10 h 249.8(90.2) Xe-135 m 15.30 m 526.7(80.5) Xe-137 3.82 m 455.5(31.0) *Xe-138 14.10 m 258.3(31.5) 434.5(20.3) 1768.3(16.7) N-13 9.97 m 511.0(199.6) More complete decay characteristics of noble gases and other nuclides are given in Appendix A. * Nuclides used in fuel performance evaluations.
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Radiochemistry in Nuclear Power Reactors C.9 SPECTROMETRIC ANALYSIS OF THE NUCLIDES EMITTING 511 keV PHOTONS The following nuclides have been identified to decay by emitting 511 keV photons (annihilation radiation) in a reactor coolant sample: Nuclide Half-Life Photon Intensity Associated Major Gamma-rays (keV) and Intensity N-13 9.97 m 200% — F-18 109.8 m 193.5% — Na-24 15.0 m ~2% 1368.5 (100%) Co-58 70.8 d 30.0% 810.8 (99.4%) Cu-64 12.7 h 35.7% 1345.9 (0.5%) Zn-65 243.8 d 2.8% 1115.5 (50.7%) Among these nuclides, Zn-65, Co-58 and Na-24 are normally determined by their major gamma-rays at 1115 keV, 810 keV and 1368 keV, respectively. Their contribution to the 511 keV peak is generally small and can be easily estimated. The easiest way to accurately measure the other three nuclides is to perform a simple cation-anion separation by using ion-exchange membranes (see Appendix B, Sampling Practices and Sample Preparation). F-18 is in the anion fraction, Cu-64 is on the cation fraction, and N-13 can be in either the cation or anion fraction (see Section 5.4). Both cation and anion fractions are counted separately for several times to follow the decay of activities. The sample should be counted as soon as possible so that the 10 min N-13 can be measured. The decay curves obtained from the 511 keV photopeak in each fraction can be easily constructed and graphically resolved into two components: 10 min N-13 and 110 min F-18 in the anion fraction, and 10 min N-13 and 12.7 hr Cu-64 in the cation fraction. Any tailing from the contribution from Zn-65, Co-58 and/or Na-24 in the cation fraction should be subtracted before analysis. It should be pointed out that although there is a low intensity Cu-64 gamma ray at 1346 keV, it cannot be accurately measured in a gamma spectrum containing a mixture of numerous nuclides.
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Radiochemistry in Nuclear Power Reactors C.10 GOOD PRACTICES IN GAMMA RAY SPECTROMETRIC ANALYSIS By using a high resolution solid-state detector with associated electronics and computer system, a complex spectrum may be analyzed. However, in many cases some nuclides could be misidentified if the sample contains two or more nuclides which emit photons of identical or similar energies. In other cases, the activity levels of some nuclides may be too low to be detected in a spectrum which is dominated by other high level activities. For the best results of analysis, the counting schedules for the offgas and iodine (anion fraction) have been recommended in Sections C.7 and C.8. Similar counting schedules may be developed for the insoluble and cation fractions. A well-trained radiochemist should be able to exercise the following practices to obtain the best analysis of the sample (more nuclides are measured accurately): Make sure the updated nuclear data (Appendix A) are used in gamma-ray spectrometric analysis. Always count the sample at an appropriate distance from the detector so that good counting statistics are obtained at the lowest counting dead time. Always apply the geometric correction factors, if needed. Always examine the spectrum to see for any energy shift and/or poor gamma peak resolution, which may lead to misidentification of nuclides. Search for the expected gamma rays which may be missing due to energy shift or interference from high intensity gamma rays, particularly in high-energy regions. For a nuclide with more than one major gamma ray, compare the activities calculated from two or three different gamma rays. Determine what can be done in order to obtain the best analysis of the sample (more nuclides are measured accurately).
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Radiochemistry in Nuclear Power Reactors The decay of activities in the sample should be followed by counting the sample several times over a period of a few hours or approximately a week, depending on the half-lives of the activities, so that (a) activities of the nuclides with identical gamma energy, but different half-lives, can be calculated by resolving a composite decay curve (Section C.5), and (b) the longer-lived and low intensity activities can be measured after most of the shorter-lived nuclides have decayed away. Chemical separation in sample preparation (Appendix B) should always be considered, if necessary, as an ultimate method for measuring the low-level activities. C.11 DATA PROCESSING FOR FISSION PRODUCT RELEASE CHARACTERISTICS Calculate the measured individual nuclide concentrations at the sampling time. Use the concentration units recommended in Section 2.2. Correct the concentration values for counting geometry and (see Appendix D) and sample line delay, if necessary. Calculate the activity release rate by: multiplying the off-gas flow rate for the off-gas sample, or Using Equation 3–21 for the iodine activities in the BWR coolant, or Equation 3–34 for the iodine activities in the PWR coolant. Convert the activity release rate values in μCi/s to fission/s and characterize the fission product release patterns according to the procedures described in Section 3. For a step by step instruction, the reader is referred to a procedure described by H.R.Helmholz(4).
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Radiochemistry in Nuclear Power Reactors C.12 REFERENCES (1) American National Standard, “Calibration and Usage of Germanium Detectors for Measurement of Gamma-Ray Emission of Radionuclides,” ANSI N41.14–1978. (2) “A Handbook of Radioactivity Measurements Procedures,” second edition NCRP Report No. 58, NCRP (1985). (3) F.J.Schima and D.D.Hoppes, Int. J. Appl. Rad. Isot., 34, 1109 (1983). (4) H.R.Helmholz, “Chemistry Measurement Methods and Data Interpretation”, Appendix A to “Failed Fuel Action Plant Guidelines”, EPRI NP-5521-SR (November 1987).
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