Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 217
Radiochemistry in Nuclear Power Reactors Appendix B SAMPLING PRACTICES AND SAMPLE PREPARATION FOR RADIOCHEMICAL ANALYSES B.1 INTRODUCTION In a nuclear power plant, one of the most frequent mistakes and largest errors introduced in radiochemical analysis is sampling and sample preparation. Sampling the reactor coolant, offgas, liquid waste, and airborne samples, etc., may present different problems in each case for an inexperienced radiochemist. Some problems can be avoided, but some can only be minimized. The procedures described in this and following appendices reflect nearly twenty years of experience in sampling and measuring the radioactivities in various types of samples in nuclear power plants. Some techniques are standard, some modified to suit the needs, and still others developed at GE Vallecitos Nuclear Center. The author has attempted to present the technologies in a simple and concise manner but in sufficient detail to make them readily usable. The commercially available ion-exchange membranes have been successfully used to concentrate the trace metallic impurities and radioactive species in reactor coolant and feedwater. The speed and simplicity of filtration make the sampling techniques almost ideally suited for concentrating solutions containing trace amounts of materials. B.2 SAMPLING BWR PRIMARY COOLANT—LABORATORY FILTRATION B.2.1 Sampling Method A large sample of reactor water is preferred for accurate chemical and/or radiochemical analysis. In many cases, however, the radiation level from reactor water is high enough to be hazardous to personnel. Therefore, an appropriate sample size is one liter for the insoluble species and 100 mL for the soluble species. In no case should the sample size be less than 10 mL because such small samples may not be representative. Solubles and
OCR for page 218
Radiochemistry in Nuclear Power Reactors insolubles are separated by filtration in a radiochemistry laboratory. If a sample larger than one liter is needed for elemental analysis or for detection of low intensity nuclides such as Ni-63 and Fe-55, an in-line filtration technique to concentrate the activities is recommended. B.2.2 Laboratory Filtration Set up a filtration apparatus in a radiochemistry laboratory before the coolant sample is collected. An appropriate filtration apparatus includes a Millipore “hydrosal” 47-mm diameter stainless steel filter holder or equivalent, a 2-liter filtering suction flask and a vacuum pump. Place one 47-mm diameter 0.45 μm Millipore filter or equivalent above two cation exchange membranes.* Then place the filter and cation membranes on two anion exchange membranes.* The vacuum must be sufficient to pull the water sample through the filter set at 50 to 100 mL per minute. Follow the steps below to obtain the water sample from the sample line: Flush the sample line at a flow rate greater than 1 kg per minute for at least 10 minutes or at least 3 sample line lengths of water. Operate the sample line valve carefully and slowly to prevent disturbing any corrosion deposit which may have accumulated in the sample line. With the water continuing to flow after flushing the line, collect a 1 liter sample in a new or clean polyethylene bottle. Record the sample time. Turn off the sample flow. Cap the sample bottle and return the sample to the laboratory for processing as soon as possible to minimize the amount of insolubles that settle on the wall of the bottle. * The number of membranes used in filtration depends on the ion-exchange capacities and efficiencies of the membranes at the desirable filtration flow rate. Generally >95% efficiency should be expected for most radioactive species.
OCR for page 219
Radiochemistry in Nuclear Power Reactors Shake the bottle vigorously. Then measure a 100 mL water sample using a clean graduated cylinder. Filter the 100 mL sample through the filters. Adjust the vacuum so that the filtering flow rate is approximately 50 mL per minute to complete the filtration in approximately 2 minutes. (Note: If radiation levels permit, use the entire liter sample for both particulates and solubles.) Turn off the vacuum, remove the cation and anion exchange membranes from the holder and place each in a separate petri dish for activity measurement. Return the Millipore (or equivalent) filter to the filtration apparatus. Filter the remaining 900 mL of water sample to collect the insolubles on the filter. When the sample is completely filtered, rinse the sample bottle and filtration funnel with approximately 200 mL of deionized water. Then filter the rinse water through the same filter. When the rinse water is completely filtered, remove the filter from the holder and place it in a petri dish for activity measurement. B.2.3 Major Nuclides Commonly Observed in Reactor Coolant The major nuclides separately collected in each fraction are as follows: Insoluble Cation Anion Ag-110 m Rh-105 Ba-129 Rb-89 As-76 As-76 Ru-103 Ba-140 Rb-90 Br-84 Ce-141 Ru-105 Ba-141 Rb-90 m Cl-38 Ce-144 Ru(Rh)-106 Ba-142 Sr-87 Cr-51 Co-57 Sb-122 Co-57 Sr-90 F-18
OCR for page 220
Radiochemistry in Nuclear Power Reactors Insoluble Cation Anion Co-58 Sb-124 Co-58 SR-91 I-131 Co-60 Sb-125 Co-60 SR-92 I-132 Cr-51 Sn(IN)-113 Cs-134 Zn-63 I-133 Fe-55 Te-129 m Cs-136 Zn-65 I-134 Fe-59 Te-129 m Cs(Ba)-137 Zn-69 m I-135 Hf-181 Te-132 Cs-138 Mo-99 La-140 W-187 Cs-139 N-13 La-141 Y-91 Cu-64 Tc-99 m La-142 Y-92 Mn-54 Tc-101 Mn-54 Y-93 Mn-56 Tc-104 Nb-95 Zn-65 N-13 Nb-97 Zr-95 Na-24 Nd-147 Zr-97 Ni-63 Ni-63 Ni-65 Ni-65 Np-239 Transuranic Isotopes In some cases, one nuclide can be found in both the soluble and insoluble fractions (e.g., N-13, Co-58, Co-60). In other cases, the insoluble species (e.g., Ce-141, Ce-144, Y-91, Y-92, Y-93) are frequently found in the soluble cation fraction after the soluble parent nuclides have decayed in the cation membranes. B.2.4 Further Chemical Separation Among the radioactive nuclides collected by filtration, only a few α, β, and low-energy gamma emitting nuclides require further chemical separation before their activities can be measured. After the gamma spectrometric analysis is completed, the following procedures are commonly used in further separation: Millipore Filter—When the crud level is low the filter can be counted directly for the gross α activity and α-spectrometric analysis for the transuranic isotopes, Pu-238, Pu-239, Pu-240, Pu-241, Cm-242, Cm-244, Am-241, etc. The particulate collected on the filter can be easily dissolved in ~10 mL of warm 6N HCl. A few drops of concentrated HNO3 may be helpful to
OCR for page 221
Radiochemistry in Nuclear Power Reactors accelerate dissolution. The Millipore filter is removed from the solution and rinsed with ~5 mL of fresh 6N HCl. Combine the rinse with the solution for further chemical separation. The solution may contain β-emitting Fe-55 and Ni-63, in addition to the transuranic isotopes and other gamma-emitting nuclides. Cation Membranes—Cationic species can be readily removed from the ion exchange membranes by digesting the membranes in a beaker containing ~10 mL of warm 3N HNO3. After ~2 minutes, the membranes are removed from the solution and repeat the process with a fresh acid solution. Combine the solutions for further chemical separation. The solution contains the β-emitting nuclides, Ni-63, Sr-89 and Sr-90, and all other cationic gamma-emitting nuclides. Anion Membranes—For a routine radiochemical analysis, there is no anionic species which cannot be measured by gamma spectrometric analysis directly from the anion membranes. B.3 SAMPLING HIGH PURITY WATER CONTAINING LOW-LEVEL RADIOACTIVITIES—IN-LINE FILTRATION The water sample is collected by using a standard design of corrosion product sampler connected directly to the sample line. A schematic of a typical sampler is shown in Figure B-1. Flash the sample line through a bypass line in the sampler at a water flow rate ≥1 kg/min for at least 10 min, or 3 sample line lengths of water flow. Place one 0.45 μm Millipore filter (on top), 3 cation exchange membranes (in middle) and 3 anion exchange membranes (at bottom) in a high pressure Millipore filter holder to collect and concentrate the insoluble, cationic and anionic activities separately.
OCR for page 222
Radiochemistry in Nuclear Power Reactors Figure B-1. A Typical In-line Filtration Sampling Arrangement
OCR for page 223
Radiochemistry in Nuclear Power Reactors While maintaining the same bypass flow rate, carefully/slowly open the inlet valve to allow water to pass through the filter holder at a flow rate of 0.1–0.2 kg/min. Record the time, flow rate and the reading on the water flow accumulator. Collect the desirable amount (at least 1 kg) of water sample at a constant flow rate. Stop the water flow and record the time and meter reading again. Remove the filters from the holder and the Millipore, cation and anion membranes are placed separately in petri dishes for activity measurements. B.4 PWR PRIMARY COOLANT B.4.1 General Comments and Precautions Problems in withdrawing representative samples from a PWR primary coolant through long sample lines are well known(1,2). Unlike the high purity water in the BWR coolant, the PWR coolant chemistry is rather complex, and it presents a special environment for some radiochemical species which may easily change their solubilities and undergo certain interactions with the oxide film on the sample line when the coolant is cooled down and water pH is changed. Experience at Ringhals(3) suggests that, in general, the measured concentrations of some activated corrosion products do not bear any useful relationship to the concentrations of corrosion products in the coolant at operating temperatures. The measured concentrations of some corrosion products (e.g., soluble Co-60 and Mn-54) are strongly dependent on sampling flow rate and the boron concentration. The Cs-137 concentration was also found to correlate directly with boron concentration. In order to minimize the sample error and to maintain a consistency of sampling for radioactivities, the sample line purge at a constant flow rate is necessary and should not be compromised for reducing the liquid radwaste generation.
OCR for page 224
Radiochemistry in Nuclear Power Reactors B.4.2 Sampling Procedure If the volatile species is not desired, the grab sample can be withdrawn from the sample line using the sampling procedures described earlier for the BWR coolant. Separation of radioactivities in a grab sample by filtration through a particulate filter and cation/anion change membrane has been reported by Polley and Andersson(3). If the volatile species is also to be measured, a pressurized sample cylinder (“bomb”) is used to collect the sample for subsequent analyses of liquid and gaseous constituents after appropriate manipulation in the laboratory. The following steps are followed at many plants(4): The reactor coolant sample is collected at primary system pressure employing a sample cylinder at the primary coolant sampling sink. The pressurized cylinder is transferred to the laboratory and installed in the sample manipulation panel. (See below for panel operation procedures.) The cylinder is vented, and the liquid and gas equilibrated at approximately atmospheric pressure by recirculating the gas. The volume of the total gas evolved is measured and employed to calculate total gas concentration in the sample. Aliquots of the gas and liquid are withdrawn for analysis. No standard sample manipulation panel design presently exists, and a wide variety of designs are encountered in the industry. A schematic of a representative design is given in Figure B-2. The operating procedure for this sample panel is described below.
OCR for page 225
Radiochemistry in Nuclear Power Reactors Figure B-2. Primary Coolant Sample Manipulation Apparatus (Ref. 4)
OCR for page 226
Radiochemistry in Nuclear Power Reactors Before connecting the sample cylinder, close valve #1 and introduce water through the sample cylinder connection upstream of valve #2. Fill the portions of the system downstream of valve #7. Establish a starting level in the buret by draining as necessary through the pinch clamp (valve #8). Record the initial buret level. Attach the sample cylinder to the apparatus with valve #3 and valve #4 closed. With valves #2, #7 and #9 open and valve #1 closed, allow the sample to expand slowly through valve #3 into the measuring buret. Record the new buret level. Close valve #7 and open valves #1, #2, #3, #4, #5, and #6. Start the Rollflex pump. Periodically, stop the pump, close valve #1, and open valve #7 to allow expansion of the system gases. Record all changes in level in the measuring buret. When no changes in buret level are noted, the stripping of gases from the sample is complete. Aliquots of gas sample can now be taken through the surge volume septum cap and transferred to a calibrated sample vial for radio activity measurement. Break a connection downstream of the surge volume to allow air into the system. Aliquots of water sample are taken from the cylinder for chemical separation and analysis.
OCR for page 227
Radiochemistry in Nuclear Power Reactors B.5 SAMPLING PROCEDURE FOR THE BWR OFFGAS Prepare three standard 14.1 mL glass sample vials with rubber caps (septum). Clearly mark sample I.D. Record the sample date, reactor power level, offgas monitor readings, stack monitor readings, the offgas flow rate, and the offgas monitor flow rate. Follow the operation instruction at the sample station to obtain the samples: Purge the sample system for 2 min. Carefully insert the sample vial to sampling position with the vial holder, making sure that the hypodermic needle is inserted in the center of the rubber cap to evacuate the sample vial. Check the vacuum gauge reading to be sure it is approaching 29.5 in. and remains constant for at least 1 min. Allow the system to purge again for 30 seconds. Fill the sample vial with sample gas and keep the vial at position for 10s. Record the sample time and remove the sample vial. Repeat sampling with a new vial starting with Step (a). Properly timed samples should have been collected exactly every 3 min. Transfer the samples to the counting room as soon as the last sample is taken. Immediately after the samples are brought back to the counting room, place the first sample in an upright position on a NaI(T1) detector and count for 1 min.
OCR for page 228
Radiochemistry in Nuclear Power Reactors Repeat counting for samples 2 and 3 as in Step (5), making sure that each sample is counted exactly 3 min apart, or exactly the time between each sample is taken. On the basis of gross gamma counts, if all three sample counts are within ±5%, the last sample is selected for further detailed gamma-ray spectrometric analysis (see Section 9). If all the sample count rates are within ±10%, the highest activity sample is chosen for detailed analysis. If all the sample count rates are not within ±10%, a new set of samples should be taken for analysis. If the sample vial is not properly evacuated, or the sample vial after filling is still under partial vacuum, the activity concentration in the final analysis should be corrected for the actual sample gas volume (at 760 mm). B.6 REFERENCES (1) C.A.Bergmann and J.Roesmer, “Coolant Chemistry Effects on Radioactivity at Two Pressurized Water Reactor Plants,” EPRI NP-3463 (March 1984). (2) N.R.Large, et al., “Studies of Problems of Corrosion Product Sampling from PWR Primary Coolant,” Water Chemistry of Nuclear Reactor System 5, Vol. 1, p. 63, BNES, London (1984). (3) M.V.Polley and P.O.Andersson, “Study of the Integrity of Radioisotope Sampling From the Primary Coolant of Ringhals 3 PWR,” ibid, p. 71, (1989). (4) PWR Primary Water Chemistry Guidelines Committee, “PWR Primary Water Chemistry Guidelines: Revision 2,” EPRI NP-7077, Final Report (November 1990).
Representative terms from entire chapter: