Kinetics of the High Temperature Oxygen Exchange Reaction on 238 PuO 2 Powder

Transcription

1 Kinetics of the High Temperature Oxygen Exchange Reaction on 238 PuO 2 Powder C. E. Whiting, M. Du, L. K. Felker, R. M. Wham, C. D. Barklay, and D. P. Kramer University of Dayton Research Institute (937) Chris.Whiting@udri.udayton.edu NETS 2015 Conference Albuquerque, NM 2/23/2015 University of Dayton Research Institute Shaping the technology of tomorrow

2 Why is PuO 2 Important? 238 Pu 0.54 W T per g Heat is produced by the high energy α decay of 238 Pu

3 (α,n) Creates Detrimental Radiation Pu U 4 α Ne O 1 n Worker Dose Impact on scientific equipment

4 Isotopic Oxygen Exchange ~ 20,000 n / s*g 2,600 n / s*g

5 CeO 2 as a Surrogate for PuO 2 Same crystallographic space grouping - FF3 m; #222 Similar solid state chemistry Similar oxygen behavior

6 Rate Independent of Several Parameters At 1000 o C Oxygen Exchange on CeO 2 was Independent of All Parameters Studied Parameter Tested Low High Isotopic Effects 16 O 18 O Sample Size (mg) O 2 Partial Pressure (torr) Total Pressure (torr) SSA (m 2 * g -1 ) Binary Gas Composition He, Ne, Ar, and N 2 Not a Reaction at the Material Surface C.E. Whiting, et al., J. Solid State Chem., 2014, 218,

7 Exchange is Governed by 3 Different Mechanisms -4 ln k E a = 114 kj / mol Surface Exchange X -325 mesh nm nm with thermal treatment /T (K -1 ) Literature reports E a ~120 kj / mol for the surface exchange mechanism C.E. Whiting, et al., Solid State Ionics, 2015, 272, 18-23

8 Exchange is Governed by 3 Different Mechanisms -4 ln k E a = 29.1 kj / mol Surface Mobility E a = 31.2 kj / mol Surface Mobility X -325 mesh nm nm with thermal treatment /T (K -1 ) No previous studies of surface mobility on CeO 2 in the literature C.E. Whiting, et al., Solid State Ionics, 2015, 272, 18-23

9 Exchange is Governed by 3 Different Mechanisms -4 ln k E a = 11.0 kj / mol Internal Chemical Reaction X -325 mesh nm nm with thermal treatment /T (K -1 ) No previous studies of this novel internal chemical reaction in the literature C.E. Whiting, et al., Solid State Ionics, 2015, 272, 18-23

10 Exchange is Governed by 3 Different Mechanisms -4 ln k E a = 11.0 kj / mol Internal Chemical Reaction X -325 mesh nm nm with thermal treatment /T (K -1 ) Internal Chemical Reaction Fast Independent of particle surface area Independent of gas phase composition C.E. Whiting, et al., Solid State Ionics, 2015, 272, 18-23

11 Temperature Decreases Specific Surface Area -4 ln k Surface Mobility Faster Surface Mobility Slower X -325 mesh nm nm with thermal treatment /T (K -1 ) 1000 o C Treatment nm m 2 /g -325 mesh m 2 /g None h h C.E. Whiting, et al., Solid State Ionics, 2015, 272, Experiment can cause dynamic change in SSA Results would appear erratic

12 Designing Exchange Experiments for 238 PuO 2 Surrogate Experiments Tell Us to Expect: 3 different exchange mechanisms - Internal chemical reaction (Fastest) - Surface mobility - Surface exchange (Slowest) Internal chemical reaction is independent of SSA Exposure to high temperature will reduce SSA - Surface mobility and exchange are slower after thermal exposure - Thermal treatment at high temperature can stabilize surface and prevent confusing results

13 238 PuO 2 Exchange Apparatus A fresh batch of 238 PuO 2 was synthesized 238 PuO 2 exchange apparatus similar to CeO 2 apparatus Rate monitored via neutron detector

14 Design of Experiments Perform 2 exchanges at 700 o C - Get rate information before significant thermal exposure Perform 4 exchanges at 1000 o C - Obtain replicates to estimate precision - Mechanism should be the internal chemical reaction Perform 2 exchanges each at 950, 900, 850, and 800 o C - Obtain activation energy information - Attempt to see changes in mechanism Perform 2 exchanges at 700 o C - Observe effect of thermal exposure on rate

15 238 PuO 2 Exchange Results Internal Chemical Reaction E a = kj / mol -6.3 ln k /T (K -1 ) Initial 700 o C rate controlled by internal chemical reaction Independent of SSA replicates at 1000 o C were reproducible E a in same ballpark as CeO 2 (11.0 kj / mol )

16 238 PuO 2 Exchange Results ln k Surface Mobility E a = kj / mol /T (K -1 ) Significant change in rate and E a Reduction in SSA appears to have occurred at 1000 o C E a in same ballpark as CeO 2 (29.1 kj / mol )

17 238 PuO 2 Exchange Results ln k /T (K -1 ) Surface Exchange or mix of Surface Mobility and Exchange Significant change in rate compared to pre-1000 o C Reduction in SSA appears to have occurred at 1000 o C Not enough data to extract surface exchange E a

18 238 PuO 2 Exchange Results ln k /T (K -1 ) Surface Exchange or mix of Surface Mobility and Exchange Thanks to surrogate studies, verified the very complex behavior of PuO 2 oxygen exchange using only 16 experiments.

19 Previous Exchange Data on 238 PuO 2 Exchange T ( o C) Sintering T ( o C) PuO 2 Data Oxalate k x10-3 (s -1 ) Hydroxide k x10-3 (s -1 ) Data published by Deaton and Wiedenheft in * ~1 hour >1 hour >1 hour ~2 hours Only previously published data on PuO 2 oxygen exchange Observed exchange behavior was very complex Unable to obtain E a or mechanism information due to complexities Comparison with our data shows strong similarities Deaton, R. L., Wiedenheft, C. J., J. Inorg. Nucl. Chem., 34, (1972)

20 Previous Exchange Data on 238 PuO 2 Exchange T ( o C) Sintering T ( o C) PuO 2 Data Oxalate k x10-3 (s -1 ) Hydroxide k x10-3 (s -1 ) * ~1 hour >1 hour >1 hour ~2 hours Rates appear to be similar Different sintering temperatures produced different SSA SSA independent rate suggests internal chemical reaction Deaton, R. L., Wiedenheft, C. J., J. Inorg. Nucl. Chem., 34, (1972)

21 Previous Exchange Data on 238 PuO 2 Exchange T ( o C) Sintering T ( o C) PuO 2 Data Oxalate k x10-3 (s -1 ) Hydroxide k x10-3 (s -1 ) * ~1 hour >1 hour >1 hour ~2 hours ln k /T (K -1 ) Since a single mechanism was identified, E a can be calculated E a = kj / mol Statistically similar to E a obtained from our PuO 2 studies (17.9 kj / mol ) Deaton, R. L., Wiedenheft, C. J., J. Inorg. Nucl. Chem., 34, (1972)

22 Previous Exchange Data on 238 PuO 2 Exchange T ( o C) Sintering T ( o C) PuO 2 Data Oxalate k x10-3 (s -1 ) Hydroxide k x10-3 (s -1 ) Hydroxide precipitation produced much larger particles (lower SSA) Rate appears similar between replicate runs * ~1 hour >1 hour >1 hour ~2 hours Hydroxide fines (higher SSA) produced a faster rate Dependence on SSA suggests surface mobility mechanism Deaton, R. L., Wiedenheft, C. J., J. Inorg. Nucl. Chem., 34, (1972)

23 Previous Exchange Data on 238 PuO 2 Exchange T ( o C) Sintering T ( o C) PuO 2 Data Oxalate k x10-3 (s -1 ) Hydroxide k x10-3 (s -1 ) * ~1 hour >1 hour >1 hour ~2 hours ~16 minutes to complete reaction at 1550 o C Significantly slower rate > 1300 o C suggests shift towards surface exchange Rate observed to be influenced by 2 different mechanisms Similar results observed on CeO 2 Deaton, R. L., Wiedenheft, C. J., J. Inorg. Nucl. Chem., 34, (1972)

24 Competing Surface Mobility and Exchange Surface Exchange 0-3 Surface Mobility 800 o C CeO 2 Surface Exchange ln F -6 Surface Mobility Time (s) Surface mobility and exchange are competitive at some temperatures Hydroxide precipitated PuO 2 produces exchange behavior similar to CeO 2 Deaton, R. L., Wiedenheft, C. J., J. Inorg. Nucl. Chem., 34, (1972)

25 Low Temperature Initial Phase Sintering 4 Hour Thermal Treatment of CeO o C 800 o C 600 o C 500 o C At 400 o C no change in SSA after 8 weeks Reduction in SSA is most likely due to initial phase sintering - Grains and particles begin to grow and consolidate at a slow rate - Particles will become more spherical (reduce SSA) - Normally begins at ~half the melting point (~1200 o C)

26 Conclusions PuO 2 exhibits exchange behavior very similar to CeO 2 - All 3 mechanisms are observed - Internal chemical reaction is fast and independent of SSA - Surface mobility and exchange are observed as T and SSA decrease - CeO 2 is a good surrogate for PuO 2 exchange Historical results are replicated and clarified Activation energies for PuO 2 are similar but larger Exchange temperature can reduce SSA of PuO 2 - Low temperature initial phase sintering

27 Acknowledgements DOE Contract #: DE-NE Doug McClelland and Dr. Robert Ellefson, Mound Technical Solutions, Inc. John Douglas and Emily Kaufman (Students) Mound Museum Library and Archives

28 238 PuO 2 Synthesis Conditions Plutonium (75 % as 238 Pu) oxalate was precipitated from a 1.3 g/l solution of plutonium nitrate with ~25-fold excess of solid oxalic acid. The solution was agitated and allowed to settle over the course of a week. After filtration of the solid plutonium oxalate, an additional ~12-fold excess of solid oxalic acid was then added to improve the plutonium recovery. This solution was agitated and allowed to settle over the weekend.

29 238 PuO 2 Synthesis Conditions Calcination of the plutonium oxalate was performed at 700 o C for 90 min with a ramp rate of 20 o C/min. A calculated final sample mass of mg PuO 2 (154.7 mg 238 Pu) was then transferred to the quartz sample vessel; calculations were based on the Pu concentrations in the nitrate solution before and after precipitation. All experiments were performed on this sample of PuO 2.

30 Lattice-Interstitial Space Transition Lattice oxygen ions are labile Shift from lattice position to interstitial space is easy Lattice oxygen ions diffuse along oxygen vacancies Shifting to interstitial space creates new channels for ion motion New channels allow diffusing oxygen to exchange with new parts of the lattice Speed that oxygen travels along these channels (i.e. conductivity) is unrelated Modeling suggests that for this shift E a ~0.1 ev or ~10 kj / mol

31 Standard Testing Parameters 20 mg of CeO 2 placed in an Al 2 O 3 boat (1 µg sensitivity) - CeO 2 nominal particle size: nm and -325 mesh Temp set with control thermocouple (Type K) Gases phase: ~760 torr He, ~260 torr 18 O 2, ~780 torr He - 18 O % purity manually mixed with He - 18 O % purity premixed 1:5 premixed with N 2 Bleed gas into vacuum chamber RGA monitored m/z: 4, 7, 18, 28, 32, 34, 36, and 44 - Measurements taken every 5 s

32 Where Diffusion Takes Over Rate of exchange based on diffusion is proportional to so as grain radius increases diffusion slows down 1 r 2 M M t = 6 1 e r π n= 1 n Dn π t Time necessary to reach 99.9% completion 3.76 µm grain radius = 617 s (i.e. same as chemical reaction) 1.19 µm grain radius = 61.7 s 11.9 µm grain radius = 6170 s Diffusion plays a role in rate when grain radius nears 1-10 µm

33 Evidence of Diffusion in Large Grains 0-2 ln F Time (s) 10 µm Spex milled pellets with mean grain radius = 2.8 µm Early diffusion will be fast and rate mostly dominated by polaron hopping As time increases, diffusion will become slower and rate plot will curve k = 7.64 x10-3 s -1 ; Time to completion = 1300 s

34 Diffusion Through a Thick Sample Bed ln F Time (s) Diffusion slows down over time A thick sample bed clearly causes a reduction in the exchange rate Total processing time ~50 min (10 mm thick bed)

35 Deriving the Base Rate Law O Exchange is occurring constantly, but an exchange is only observed when the isotopes are different vs. Forward Reverse Rate f = k*p = k* 18 Rate χ (g) * 16 = χ k*( 16 (s) χ Rate r = k*p = k* 16 χ (g) * 18 (s),init 16 χ (g),init ) * F χ (s) Fraction of reaction remaining; F M t = 1 M

36 Exchange Mechanism R1 Monatomic Exchange R2 Diatomic Exchange Homomolecular Exchange

37 Diatomic and Homomolecular Exchange 0.30 Total 16 O 700 o C 0.30 Total 16 O 800 o C 16 O (mmol) O 2 16 O 18 O 16 O (mmol) O 2 16 O 18 O Time (s) Time (s) Equilibrium Homomolecular Exchange 900 and 1000 o C experiments do not show any 16 O 2 Fast enough to reach equilibrium before detection

38 Previous CeO 2 Exchange Results 600 o C 18 O 2 pulses < 1 s residence Primarily Diatomic Exchange Most likely diatomic exchange with limited homomolecular exchange Homomolecular exchange limited by small residence time Bueno-López, A., et al., Catal. Today, 121, (2007)

39 Backexchange With 16 O % Backexchange 10% 8% 6% 4% 2% Assume self-heating to 400 o C Backexchange in air Dt/r % Time (s) Time (s) ~1.09% backexchange per hour Dt/r 2 curved from 0-5,000 s (surface reaction) Dt/r 2 linear from 5,000-30,000 s (lattice diffusion)

40 Backexchange With 16 O Assume self-heating to 400 o C % Backexchange 6% 5% 4% 3% 2% 1% 0% Backexchange in 37 torr 18 O Time (s) Time (s) ln F ~0.7% backexchange per hour over first 7 hours ln F plot curved from 0-5,000 s (equilibration) ln F plot linear from 5,000-25,000 s (surface reaction)

41 Backexchange of Sintered CeO 2 Pellets Signal Intensity (I.U. x10-10 ) Time (s) No signal after 11 hours LOD ~0.02% backexchange Assume self-heating to 400 o C Backexchange in air 96% Theoretical Density Sintered Pellets

42 Atmosphere Control and Testing Apparatus

43 Chemical Reaction vs. Diffusion Reduced Diffusion Equation * 20 mg of nm particles M M t = 6 Dt 2 πr nm Signal Intensity (AU) 6.E-07 4.E-07 2.E-07 0.E Time (s) Dt/r Not linear Time (s) Intercept 0 * Deaton, R.L. and Wiedenheft, C.D., J. Inorg. Nucl. Chem., 35, 649 (1973)

44 First Order Rate Plots First Order in F Rate = k * F ln F Time (s) Total O Intensity (A.U.) 5.0E E E E E E-06 Matches Reincorporation Region 1 of O Time (s) Low Temp High Pressure High Temp Low Pressure

45 First Order Rate Plots First Order in F Rate = k * F Signal (x10-7 A.U.) Time (s) ln F Time (s) k Time to Completion

46 Oxygen Exchange Mechanism Rate limited by a chemical reaction NOT occurring at the surface of the material O 2(g) O 2(ads) O 2 - (ads) O 2 2- (ads) 2 O - (ads) 2 O - (lattice) Oxygen exchange theory says that the entire exchange process occurs at the surface of the material Must not be observing part of the exchange reaction proper Probably describing isotopic redistribution through the lattice