1. National Air and Radiation Environmental Lab
Trial and Error…In Search of a Sequential Analysis for U, Th, and Ra-228
Shane Knockemus
National Air and Radiation Environmental Lab
Montgomery, AL

2. Isolate and quantify Ra-228 with Ln resin.
Goal: A reliable method for the separation and quantification of U, Th, and Ra-228 for use in low level applications
Utilize Diphonix® Resin to concentrate actinides and radium (strontium).
Separate radium (strontium) from actinides by manipulating characteristics of Diphonix Resin.
Isolate and quantify Ra-228 with Ln resin.
Separate and quantify U, and Th (Pu) by TEVA/TRU separation.

3. Behavior of target analytes on diphonix

4. Behavior of actinides on TEVA and TRU

5. Basic scope of separation
Everything (Am, Pu, U, Th, Ra, and Sr) is retained on diphonix in 0.1 M HNO3
Ra (and Sr) are stripped from Diphonix with 6 M HNO3.
Ra-228 quantified by use of Ln resin. Ra-228 yield will be determined by batch method (no Ba-133 on hand). The recovery of the Ra-228 standard will be applied to that of control samples, which were run simultaneously.
Diphonix Resin, with actinides still retained, is digested and brought up in load solution for actinides separation.
U and Th separated by TEVA/TRU tandem arrangement. Th (and Pu+4) sticks on TEVA—Th eluted with 9 and 6 M HCl, U sticks on TRU—eluted with 0.1 M (NH4)2C2O4, after precautionary stripping of Am and Pu.

6. Separation schematic (part 1)
Sample (0.1 M HNO3)
6 M HNO3
Diphonix
Actinides
and
Radium
retained
Diphonix
Actinides
retained
Waste
Radium stripped

7. Separation schematic (part 2)
Diphonix digested, and brought up in load
solution of 2.5 M HNO3/Al(NO3)3, ascorbic
(1) 9 M, 4 M HCl
(2) 1 M HCl/0.1 M H2C2O4
(3) 0.1 M (NH4)2C2O4
(1) 9 M, 6 M HCl
(2) 1M HCl+TiCl3
TEVA Th
(Pu+4)
TRU
TEVA
TRU U
(Am?)
(Pu?)
(1) Th
(1) Am*
Pu* (2)
(2) Pu*
U (3)
waste

8. Separation schematic (part 3)
Sample in M HNO3
(after Ac-228 in-growth)
0.35 M HNO3 Ln
Ac-228
retained Ln
Ra isotopes stripped
(saved for Ra by alpha spec)
Ac-228 stripped
(counted on proportional)

9. Trial 1
Goal Separation and quantification of uranium, thorium, and radium-228
Separation Condition
*Control Sample spiked with ~2 pCi U-238 (U-232), ~3.6 pCi Th-230 (Th-234), ~94 pCi Ra-228 for standard and ~19 pCi Ra-228 for control
*Sample loaded onto 0.5 g of Diphonix in 0.1 M HNO3
*Ra (and Sr) stripped from Diphonix with 20 mL of 6 M HNO3
*Diphonix digested and brought up in load solution of 2.5 M / 0.5 M Al(NO3)3, and ascorbic acid for actinides separation on TEVA / TRU
Results
*Simple verification of Ra-228 separation confirmed by 10 minute count (after 36 hour in-growth period) on proportional counter, which yielded 115 cpm.
* U and Th results were not good: Th-234 yield 17%, U-232 yield 292%. Uranium spectrum showed obvious Th contamination

10. Trial 1: U spectrum with Th contamination

11. Probable Causes for poor separation
The effect of the matrix constituents contributed from the digested Diphonix had an adverse affect on the retention of Th on TEVA
How to fix: Increase the [HNO3] of the load sol’n of the load prior to TEVA/TRU

12. Trial 2 66% (Th-234) 52% 29% 93% (Th-230) 100% 102% 65% (U-232) 65%
Separation Conditions
*Initial load of sample onto Diphonix was kept the same.
*The [HNO3] was increased to 5 M HNO3 /0.5 M Al(NO3)3
*Pu-242 was added to check for interference. Pu stripped from TEVA with 1 M HCl + TiCl3
*Radium separation performed with Ln resin (Burnett).
Results
66% (Th-234)
52%
29%
93% (Th-230)
100%
102%
65% (U-232)
65%
54%
103% (U-238)
97%
109%
84% (Ra-228 standard)
102% (Ra-228)
85%
92%

13. Trial 2: Good separation of U and Th (and Pu)

14. Trial 2: Pu Spectrum
Thorium and uranium contamination in Pu spectrum

15. Possible explanation for problems
*0.5 g of digested Diphonix produces enough matrix constituents to interfere with retention properties of Th on TEVA.
*Not enough 6 M HCl is being used to strip all of the Th from TEVA.
*The higher [HNO3] in load sol’n (5 M) increased the retention factor of U+6 on TEVA.

16. Trial 3 Separation conditions Results
Diphonix column packed with 0.3 g, rather than 0.5 g
Increased the volume of 6 M HCl from 20 to 25 mLs
Am-243 added to check for interferences. The trail of Am was not followed.
All other conditions were the same as in trial 2.
Results
Tracer/standard recovery Spike recovery
72% (Th-234)
78%
94% (Th-230)
100%
76% (U-232)
74%
102% (U-238)
109%
59% (Ra-228 standard)
120% (Ra-228)
75%

17. Trial 3 (continued) U and Th yields improved.
No evidence of Am in U, Th, or Pu spectra. I suspect that the Am was either washed off of the Diphonix with the 6 M HNO3, or passed through to TRU and was stripped with HCl wash.
Th and U spectra show clean separation from Pu.
Pu spectrum still shows some Th and U contamination.
Did the smaller amount of Diphonix used hinder the retention of Ra, or was the lower Ra yield contributed to analyst error?
Why is there still U contamination in the Pu spectrum?

18. TEVA behavior in HNO3
At 5 M HNO3 U has
weak retention

19. Trial 3: U and Th spectra
U and Th spectra free of Pu and Am contamination

20. Trial 3: contaminated Pu spectrum
U and Th still present in Pu spectrum, although Th contamination seems to be less than in trial 2

21. Trial 4 Separation conditions Results
Diphonix column packed with 0.35 g diphonix resin
Th strip solution changed to 6 M HCl / 0.25 M H2C2O4 with the hope that the oxalate will help remove more of the Th from TEVA.
All other conditions remained the same as in trial 3.
Results
Th yield 0%!! What happened?
U-232 yield 70%, U-238 recovery 105%
Ra-228 standard yield 76%, Ra-228 spike recovery 108%
Pu spectrum shows that Th was not stripped as planned.

22. Trial 4: U and Pu spectra

23. Trial 4 (continued)
Was the poor Th yield a result of another analyst error?
There is still a small amount of U showing up in the Pu spectrum (from TEVA). Test to see if additional washing with 5 M HNO3 will bring down the remaining U from TEVA.

24. Conclusion
With the exception of trial 4, results showed considerable improvement from the onset of the experiment. Ra-228 analysis was fairly consistent throughout, while there is still room for improvement in regards to U and Th recoveries. I also hope to improve the quantification of Pu and Am (if possible).
Future work will include Sr analysis. What would be the best point to separate Sr from Ra? I would guess that either a MnO2 coprecipitation to separate Ra from Sr prior to loading the sample on Ln resin, or putting the Diphonix strip solution containing Ra and Sr directly onto Sr resin and save the rinse for Ra analysis on Ln resin would both be effective modes for separation.
Future work will also include incorporating Ra-226 by alpha spec into the scenario.