Modeling the Substoichiometric Behavior of 238 PuO 2 and 241 AmO 2 in the Low Oxygen Potential Envrionments Found in Radioisotope Power Systems

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1 Modeling the Substoichiometric Behavior of 238 PuO 2 and 241 AmO 2 in the Low Oxygen Potential Envrionments Found in Radioisotope Power Systems C. E. Whiting, E. J. Watkinson, C. D. Barklay, D. P. Kramer, H. R. Williams, and R. M. Ambrosi University of Dayton Research Institute (937) Chris.Whiting@udri.udayton.edu NETS 2015 Conference Albuquerque, NM 2/24/2015 University of Dayton Research Institute Shaping the technology of tomorrow

2 Why is PuO 2 Important? 238 Pu 0.54 W T per g Convert W T to W E using thermoelectric technology Ceramic PuO 2 used as fuel

3 Is PuO 2 Really PuO 2? PuO 2?

4 Is PuO 2 Really PuO 2? CO 2 + C 2 CO K = P CC 2 P CC2 CO can be a strong reducing agent C + O 2 CO 2 and 2 C + O 2 2 CO PuO 2?

5 Is PuO 2 Really PuO 2? Carbon is found in several places but where is the O 2 coming from? C + O 2 CO 2 and 2 C + O 2 2 CO PuO 2?

6 Is PuO 2 Really PuO 2? Carbon is found in several places but where is the O 2 coming from? C + O 2 CO 2 and 2 C + O 2 2 CO PuO 2-x

7 PuO 2 Can Be Reduced + Relationship between x, P O2, and T is very well studied PuO 2-x O 2 Partial Pressure

8 PuO 2 Can Be Reduced + Relationship between x, P O2, and T is very well studied PuO 2-x Directly relates to oxidation potential

9 PuO 2 Can Be Reduced + Since we know: P O2 can be found CO 2 + C 2 CO K = P CC 2 P CC2 2 CO + O 2 2 CO 2 K = P 2 CC 2 P CC2 P O2

10 Relationship Between x, T, and P O2 Once P O2 is known, x can be determined using the semi-empirical method described by Lindemer RT lnp O2 = T 3 RT ln 1.5 x 1 x 2 ( T) 1 2x x2 12x x 2 2 Some terms based off of the thermodynamics of the reduction of PuO 2 to Pu 2 O 3, so limited to x values between 0 and 0.5 Other terms are empirically fit to hundreds of data points in the literature Very good empirical fit even in miscibility gap T.M. Besmann and T.B. Lindemer J. Nucl. Mater., 1985, 130,

11 The Catch CO 2 CO As the reduction occurs, P CO and P CO2 are changing O 2 Which means P O2 is also changing

12 Iterative Process Was Developed Need to Know: Temperature Initial pressure of O 2 n / V Ratio Initial O 2 reacts T (K) = 1273 Initial PO2 (atm) = 1.00E-02 mol PuO2 / Liters (n/v) = 1.E+03 1st Iteration "x" = PCO from initial O2 (atm) = 2.00E-02 Stoichiometry = PCO2 from initial O2 (atm) = 2.97E-06 Total P (atm) = PO2 after rxn with C (atm) = 1.60E-22 New P O2 (atm) = 5.223E+01 Quadratic a = 1 C + CO2 --> 2 CO Quadratic b = 67 dg (J/mol) = Quadratic c = -7.04E+03 K eq = P CO after rxn with C (atm) = 5.673E+01 P CO2 after rxn with C (atm) = 2.387E+01 2 CO + O2 --> 2 CO2 P O2 after rxn with C (atm) = 1.291E-15 dg (J/mol) = K eq = E+14 K obtained from G G calculated from JANAF tables With P O2, x can be determined With new x, new P O2 can be determined x converges at equilibrium value

13 Assumptions Stoichiometry can be theoretically determined under any set of starting conditions, assuming: Maximum reduction is at x = 0.5 Closed system All O 2 reacts to form CO and CO 2 Excess graphite Lindemer s relationships can be extrapolated past the data used to create the experimental fits - x fit between: 0 < x < T fit between: 680 o C < T < 1780 o C

14 CeO 2 and AmO 2 Calculations Relationships derived using Lindemer s method exist for CeO 2 and AmO 2 CeO 2 - Based on over a thousand data points from 10 studies - Fit between 0 < x < 0.33 and 540 o C < T < 1550 o C - Help determine if CeO 2 is a good surrogate AmO 2 - Only 1 study with ~100 data points (need more data) - Narrow fit range: 0 < x < 5 and 780 o C < T < 1390 o C - Evaluate behavior of AmO 2 in the presence of carbon

15 CeO 2 and AmO 2 Calculations CeO 2 Equation RT lnp O2 = T + 4 RT lnn 2 5 RT lnn T 4N 1 1 N 2 5N 2 1 N T {4N 1 2N 2 + N 1 + 2N 2 N 2 N 1 5N 2 2N 1 N 2 2N 1 N 1 N 2 } Where: N 1 = 2 * x and N 2 = 1 N 1 AmO 2 Equation RT lnp O2 = T T 4 RT ln T (1 4x) 2x 1 2x

16 Variables Temperature to 2000 o C Initial Quantity of O 2 - Air (0.21 atm) - 1% O 2 (0.01 atm) - Ultra-High Purity inert gas (1x10-5 atm) - High Vacuum pulled from air (1x10-9 atm) - High Vacuum pulled from UHP (1x10-13 atm) n / V ratio ratio of mol of fuel to open volume - 1x10-5 (equal to ~270 g of 238 PuO 2 in 100 m 3 ) - 1x10 5 (equal to ~270 g of 238 PuO 2 in 0.05 ml)

17 Realistic RTG Realistic RTG: n / V between to 10 - For 5 kg of 238 PuO 2 - = 1.85 L open volume - 10 = 185 L open volume Atmosphere usually UHP inert or better Temperature varies considerably

18 Effect of Initial Atmosphere on x log x o C Air 1% UHP HV HV UHP log x o C log x n / V 1500 o C Air 1% UHP HV log x n / V 1800 o C n / V HV UHP n/v

19 Effect of Initial Atmosphere on x log x log x o C n / V 1500 o C n / V Air 1% UHP HV HV UHP Air 1% UHP HV HV UHP Initial O 2 results converge: When n / V > As T increases Initial O 2 < 1x10-5 atm Initial O 2 appears to have minimal effect Only UHP considered for future calculations

20 Effect of n / V on x for PuO 2 x n / V log x n / V Order of magnitude changes in volume can impact x General Rule: 10x change in n / V 100 o C

21 Effect of n / V on x for PuO 2 x Experimental Fit n / V log x n / V Order of magnitude changes in volume can impact x General Rule: 10x change in n / V 100 o C

22 Effect of T on x for PuO 2 x Temperature ( o C) x < 0.5 under all studied conditions As n / V increases x decreases x appears to be large for 0.05 < x < 0.40 log x /T (K -1 )

23 Effect of T on x for PuO 2 x Experimental Fit Temperature ( o C) log x x < 0.5 under all studied conditions As n / V increases x decreases x appears to be large for 0.05 < x < /T (K -1 )

24 CeO 2 as a Surrogate x CeO n / V x PuO n / V CeO 2 starts reduction at lower T x for PuO 2 is larger at low T and smaller at high T

25 CeO 2 as a Surrogate CeO PuO 2 x x Temperature ( o C) Temperature ( o C) CeO 2 appears to be an ok as a first-order surrogate for PuO 2, but very limited predictive power

26 AmO 2 Calculations x Experimental Fit Temperature ( o C) Why does AmO 2 become MORE stable at higher temps? Flaws in the empirical fit

27 AmO 2 Calculations x Temperature ( o C) Equation doesn t fit as well at larger x P O2 values predicted converge! when x ~ This result is completely irrational

28 AmO 2 Calculations AmO 2 is very easy to reduce x CO/CO 2 from graphite generates huge reduction potential Massive reduction expected Temperature ( o C) Perhaps complete reduction to Am 2 O 3? Even though numerical results from the AmO 2 equations are not reliable, they still tell an important story

29 AmO 2 Calculations o C and x = PuO 2 has P O2 = 7.0x10-27 x CeO 2 has P O2 = 1.4x AmO 2 has P O2 = 1.0x Temperature ( o C) PuO 2 < CeO 2 << AmO 2 Hardest to Reduce Easiest to Reduce All three models fit the empirical data well in this range

30 AmO 2 Calculations x Temperature ( o C) o C and x = 0 PuO 2 has P O2 = -587 kj / mol CeO 2 has P O2 = -401 kj / mol AmO 2 has P O2 = -135 kj / mol PuO 2 < CeO 2 << AmO 2 Hardest to Reduce Easiest to Reduce AmO 2 will experience A LOT MORE reduction in the presence of graphite than CeO 2 or PuO 2 CeO 2 is not likely to be a good surrogate for AmO 2

31 Conclusions Graphite creates a significant reduction potential that can affect the stoichiometry of PuO 2, CeO 2, and AmO 2 - Contact with graphite is not required - In order of reduction severity: PuO 2 < CeO 2 << AmO 2 - AmO 2 is reduced very easily suggesting massive reduction - If graphite is in the system, you won t end up with the dioxide Model can determine degree of reduction CeO 2 is an ok surrogate for PuO 2 reduction behavior CeO 2 is not likely to be a good surrogate for AmO 2 - AmO 2 may exhibit similar trends, but under very different conditions

32 Conclusions Initial O 2 pressures to not impact reaction much - Reduction is driven by the O 2 released from the fuel - In a realistic RTG initial O 2 pressure is irrelevant Small changes in volume will not impact reduction - Factor of 10 change in n / V ~ 100 o change in temperature Need more data on the AmO 2-x, P O2, and T relationship - Current models do not behave well at large x values Graphite will be consumed in this reduction reaction

33 Acknowledgements DOE Contract #: DE-NE Prof. Howie Knachel Students Bethany Cremeans (Barklay) and Emily Kaufman

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

35 PuO 2 Can Be Reduced + Pu(IV) is the preferred state Reducing conditions create Pu(III) and O 2 O 2 reacts with C to form CO/CO 2

36 Known Limitations Use of Lindemer s relationships is limited under certain circumstances: When x falls in the miscibility gap iterative method fails and a trial-and-error method was required At very small x values (i.e. < 0.05), Lindemer notes that his fits can have a high degree of error

37 Solid Condensate