Presentation Transcript

1. Problems Encountered with the Analysis of Fresh Mixed Fission Products

   Guy Backstrom ASP 2012 Workshop Idaho Falls, Id 9/18/2012
2. DISCLAIMER The slides in this presentation contain a lot of information and all of it can’t be discussed in the allotted time. However, these slides were intentionally left in the presentation for later review by the attendee.
3. Background: Fresh Mixed Fission Product Proficiency Test Sample. U.S. Environmental Protection Agency provided a simulated water sample that might be encountered following an improvised nuclear device event. Second Round of irradiated uranium proficiency testing. The sample was proposed to be a surface water from an estuary that has been affected by the event. Fission and Activation Products from a uranium foil, resulting from a one hour irradiation. EMS, a contractor for the EPA provided coordination. Eckert & Ziegler Analytics prepared and shipped the samples.
4. Initial Screening – Unknown Activity Initial screening results of the sample were the same as the background count rate for the entire sample bottle using a 2A scalar, approximately 50 CPM. The activity was not detectable and the entire sample bottle was placed in a plastic bag and put on the gamma detector to assess the dead time and evaluate how much sample to use. As the dead time was less than 1%, 390 mL was used in RESL standard counting geometry. For the actinide screening analyses, 0.1 mL of the sample was evaporated directly onto a mirror finished stainless steel disk, flamed to cherry red heat over a blast lamp, cooled and counted in an alpha spectrometer; which gave no detectable activity in a 10 minute count.
5. Radionuclides found in the MFP sample Instructions: gamma count Nd-147 (t1/2=11.1 day) so that the relative combined standard uncertainty(CSU) is less than or equal to 13%. After achieving the relative CSU for Nd-147 then report all gamma peaks in the sample having less than 50% uncertainty (k=1). Two reports – First within one week of receiving the sample, Second one month after receiving the sample. Nb-95/Zr-95 I-132/Te-132 Tc-99m/Mo-99 Ba-140/La-140 Ru-103 Ru-106 I-131 Nd-147 Ce-144 Ce-141 Cs-137 Np-239 Sr-89/Sr-90 Am-241 U-238/U-235
6. Main issues found analyzing a MFP sample Bateman Equations Transient Equilibrium when initial daughter activity is zero Ba-140/La-140 Transient Equilibrium when initial daughter is not zero Zr-95/Nb-95 Inadequate Software
7. Ba-140/La-140 (t1/2= 12.8d/40.3h) Main Gammas(keV,%BR): Ba-140 (537.4,19.9),(162.6,5.07), La-140(1596.2, 95.47),(815.9,22.32),(487.03,43.0)…. For the determination of Ba-140/La-140, the Ba-140 activity from the first count was decay corrected to the sample’s time of irradiation to establish a theoretical La-140 ingrowth relationship using the Bateman equations assuming the initial activity of La-140 was zero. This assumption only holds because the time between irradiation and measurement was about ten half lifes of the La-140. If the measurement were made earlier, the La-140 activity from the irradiation would have to be accounted for. The measured La-140 activity, of the first and four subsequent counts, were compared to the theoretical ingrowth relationship. All of the measured activities for both Ba-140 and La-140, over 38 days, were within 5% of the theoretical relationship. The reported activities for both the Ba-140 and La-140, at the reference date, were derived from the Bateman relationship curve. ~20 days post irradiation
8. Zr-95/Nb-95 (t1/2= 64.4d/35.2d) Main Gammas(keV,%BR): Zr-95 (724.18, 44.2), (756.7,54.8), Nb-95 (765.8,99.0) For the determination of Zr-95/Nb-95, the Zr-95 was measured and decay corrected to the reference date. The Nb-95 activity was not in equilibrium and was not zero, because of the time difference between the irradiation and the measurement, The Nb-95 activity at the time of irradiation was approximately 210 +/- 10 pCi/L. The activity of the Nb-95 at the time of irradiation was calculated using the initial measured values of Zr-95/Nb-95 and the Bateman equation on the next slide. The activities of the Zr-95 and Nb-95 at the time of irradiation were entered into the Bateman equation and subsequent verification measurements were made over 38 days. ~20 days post irradiation
9. Zr-95/Nb-95 (t1/2= 64.4d/35.2d) The subsequent measurements did not follow the expected decay curves better than about +/- 15%. This is not due to inaccuracy of the analytical measurement but due to inadequate preservation of the sample. For example, water samples with a pH of 2 and a density of less than 1 do not contain enough acid to keep +4 and +5 elements from precipitating from hydrolysis. Iodate forms extremely insoluble compounds with these same elements. Iodate was added to the solution as a carrier. ~20 days post irradiation
10. Nb-95/Zr-95 Continued On July 11th 2012 the sample was acidified to 1% with concentrated nitric acid and 0.1 mL of hydrofluoric acid was added with 1 mg, of each, stable Zr and Nb carriers. The sample was swirled vigorously then recounted. The subsequent measured values for both Zr-95 and Nb-95 agreed with the theoretical Bateman curves to within a few percent. Ado=Ad*exp(B*T)-(B/(B-C))*Apc*[exp((B-C)T)-1] Where: Ado = Activity of Nb-95 at reference date Ad = Activity of Nb-95 at midpoint of measurement Apc = Activity of Zr-95 (decay corrected) from midpoint of measurement to reference date. B= decay constant Nb-95 = ln(2)/34.991 days, units (1/days) C=decay constant Zr-95 = ln(2)/64.02 days, units (1/days) T=elapsed time between midpoint of measurement to reference date, units (days)
11. Nd-147 (t1/2 = 11.1d) Main Gammas(keV,%BR): (91.1,28.3), (275.4,1.0), (319.4,2.2), (531.0, 13.5) Instructions: gamma count Nd-147 (t1/2=11.1 day) so that the relative combined standard uncertainty(CSU) is less than or equal to 13%. Had to remove the 275.4 and 319.4 keV gammas as Np-239 277.6 and 315.9 keV gammas interfered with the analysis. The initial 72 hour count of 4/5ths of the entire sample resulted in an uncertainties of approximately had 7% using the 91.1 keV gamma and about 12% using the 531 keV gamma. First round, lots of activity and this wasn’t a problem. ~20 days post irradiation ~50 days post irradiation
12. Nd-147 (t1/2 = 11.1d) 30 days later Nd-147 was listed as not detected even though the 531 keV Peak is clearly identified. ~20 days post irradiation ~50 days post irradiation
13. Te-132/I-132 (t1/2= 77.9h/142.8min) Main Gammas(keV,%BR): Te-132 (228.2, 88.5), I-132 (667.7, 98.7), (772.6,76.2)…. Modifications to the library included removing interfering peaks in the weighted mean calculations. Np-239 has a 228.19 (10.72%BR) keV and Te-132 has a 228.16(88.5%BR) keV gamma. These energies were removed as the interference report was not capable of completely correcting the activities. Te-132 activity was determined by decay correcting the measured I-132 activity, using the Te-132 half life and the known transient equilibrium ratio of 1.031. ~20 days post irradiation ~50 days post irradiation
14. Te-132/I-132 (t1/2= 77.9h/142.8min) Main Gammas(keV,%BR): Te-132 (228.2, 88.5), I-132 (667.7, 98.7), (772.6,76.2)…. Modifications to the library included removing interfering peaks in the weighted mean calculations. Np-239 has a 228.19 (10.72%BR) keV and Te-132 has a 228.16(88.5%BR) keV gamma. These energies were removed as the interference report was not capable of completely correcting the activities. Te-132 activity was determined by decay correcting the measured I-132 activity, using the Te-132 half life and the known transient equilibrium ratio of 1.031.
15. Mo-99/Tc-99m (t1/2= 66.2h/6.1h) Main Gammas(keV,%BR): Mo-99 (140.51,89.3),(739.47,13.0),(181.09,6.0)…,TC-99M(140.51, 90.9) A similar problem was encountered with the determination of Mo-99/Tc-99m and the respective activities were calculated using the methodology described for Te-132/I-132 with a transient equilibrium ratio of 1.100. ~20 days post irradiation ~20 days post irradiation
16. I-131 (t1/2= 8.0d) Main Gammas(keV,%BR): (354.48,81.24)……… ~20 days post irradiation
17. Ru-103 (t1/2= 39.4d) Ru-106 (t1/2= 368.2d) Ru-103 Main Gammas(keV,%BR): (497.08, 86.4), (610.3, 5.3),……… No problems Ru-106 – No Gammas – Rh-106 (t1/2= 29.90 sec) Main Gammas(keV,%BR): (621.8,9.8),(1050.1,1.46), (661.2,0.015 (potential interference with Cs-137)…… Ru-106/Rh-106 was a statistical zero ~20 days post irradiation
18. Ce-144 (t1/2= 284.1d) Ce-141 (t1/2= 32.4d) Ce-141 – Main Gamma(keV,%BR): (145.45, 48.0 (potential interference Mo-99/Tc99m 140.51, 90.9) Ce-144 Main Gamma (keV,%BR): (133.53, 10.8) (interference Ba-140 (132.84,0.154)) The activity of the Ce-144, for the first count, was corrected by subtracting the activity of the Ba-140 interference that also has a gamma emission at ~133 keV and was not resolvable. ~20 days post irradiation
19. Np-239 (t1/2= 56.5h) Main Gammas(keV,%BR): (99.5,15.0), (103.7,24.0), (106.13,22.7), (117.7,8.4), (120.7,3.2), (209.75,3.24), (228.19,10.72), (277.6,14.1), (315.88,1.59), (334.3,2.03) The low energy gammas were fairly difficult to resolve due to peak over lap (99.5-120.7) and were not used. The 228.19 and 277.6 keV energies were chosen. Np-239 is also the daughter of Am-243 and a calibration using Am-243 was made prior to the measurement. ~20 days post irradiation
20. Cs-137 (t1/2= 30.1y) Cs-137 – No Gammas – Ba-137m (t1/2= 153.12 sec) Main Gammas(keV,%BR): (661.62, 84.62) The (661.2,0.015) from Ru-106 would interference with Cs-137 but was a statistical zero. I-132 (667.7, 98.7) also caused interference with Cs-137 as the elevated peak on the high side over corrected the continuum background subtraction. 5/30/12 5+/2 pCi/L (k=1) while on 7/2/12 7.2+/-1.1 (k=1) pCi/L after the Te-132/I-132 had decayed away. The I-132 on 5/30/12 was about 230 pCi/L. ~20 days post irradiation ~50 days post irradiation
21. Sr-89/Sr-90 Approximately 100 mL of the initial sample and adding the applicable actinide tracers with 100 mg of Strontium carrier. The spiked solution was acidified to 5% with nitric acid and permanganate was added to oxidize the Plutonium, Uranium and Neptunium so that they would not coprecipitate with the Strontium Sulfate. Americium and Curium will partially be coprecipitated under these conditions. Strontium Sulfate was reprecipitated, from EDTA, and the supernates were recombined with the main actinide fraction. A Barium Sulfate precipitations were performed to remove Ba-140. The final Strontium Sulfate was dissolved in 10 mL of 0.2 M alkaline EDTA and Cerenkov counted. The regions on the liquid scintillation analyzer were set to the end point of the Sr-89 Cerenkov spectrum and from the end point of the Sr-89 spectrum to the end point of the Y-90 spectrum.
22. Sr-89/Sr-90 The Cerenkov count of a freshly purified Strontium fraction will only detect Sr-89 and not Sr-90. If high activities of Sr-90 are present, corrections due to the ingrowth of Y-90 must be made. Subsequent counts of the purified Strontium fraction detect the Sr-89 and the Sr-90 daughter Y-90. It is important to note, under the conditions of 10 mL of 0.2 M alkaline EDTA in a glass liquid scintillation vial, Sr-90 is not detected by Cerenkov Counting; where the use of plastic vials, acidic solutions and wavelength shifters may result in a Sr-90 Cerenkov signal. The purified Strontium fraction can be further processed after the initial Cerenkov count to immediately determine a total Strontium value by normal liquid scintillation counting.
23. Sr-89/Sr-90 The Cerenkov count of the Sr-89 can then be subtracted from the total Strontium value to determine the Sr-90 value, with corrections made for the Y-90 ingrowth. ICP-Emission Spectrometry was used to determine the recovery of Strontium. This Sr-89 analysis takes approximately 2 hours, including the count time, to generate an analytical result. Fission yields of Sr-90 can be several orders of magnitude below those of Sr-89. From the sample size that was analyzed it was determined that there would be less than 1 pCi of Sr-90 present in the final fraction. Because of sensitivity limitations it was decided to wait for the Y-90 ingrowth rather than perform an immediate count by liquid scintillation analyses.
24. Sr-89/Sr-90 Sr-89 Sample Result: 1170 +/- 60 (k=1) pCi/L as of 5/25/2012 12:00 EST Sr-89 Sample gross counts: 4387.2 Sr-89 Sample background counts: 409.2 Sr-89 Cerenkov Efficiency: 0.359+/-0.002 CPM/DPM Sr-90 from Y-90 Sample Result: -12+/-18 (k=1) pCi/L as of 5/25/2012 12:00 EST Y-90 Sample gross counts: 84.6 Y-90 Sample background counts: 93.6 Y-90 Cerenkov Efficiency: 0.0690+/-0.0014 CPM/DPM Sr Yield: 83+/-2% Quench level: Cerenkov Counting Sample volume: 100 mL Count time: 60 minutes
25. Actinides Even though actinide analyses were not specifically requested, RESL chose to perform the full suite of analyses on the sample provided. Pu-238, Pu-239/240, U-234, U238 and Am-241 results were reported. It is interesting to note that the Uranium present in this sample was Natural and not enriched as would be expected from a normal fission product sample. Natural Uranium and Pu-239/240 were the only actinides detected in the sample. The sample was also analyzed for Pu-238, Am-241 and Cm-244, however these radionuclides activities were statistically zero. U-238 4.1+/-0.3 pCi/L (k=1) U-234 3.8+/-0.3 pCi/L (k=1) Pu-239/240 0.69+/-0.05 pCi/L (k=1) Activity reported as of 5/25/2012 12:00 EST
26. Other corrections The Pm-149 286 keV, I-131 284 keV, Ce-144 80 keV and I-131 80 keV gammas were removed from the library as they interfered with the analysis and caused erroneous results at those peak energies. The analytical result from the 859.4 keV gamma for Pm-149 was a statistical zero and was not reported. Modifications made to routine parameters included half life, library and coincidence summing corrections. The half lifes of the short lived progeny were changed to the parent’s half lifes for I-132, Tc-99m and La-140. The decay algorithms in the gamma spectrometry software were inadequate for the complex parent daughter relationships encountered in this sample related to transient equilibrium.
27. Questions
28. DOELAP Assessor Training First Count of MFP 5/30/2012
29. DOELAP Assessor Training Last Count MFP 7/11/2012