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Preparation of a highly reactive plutonium dioxide powder for plutonium-uranium-carbide and nitride fuel
Journal of Nuclear Materials 61 (1976) 219-220 o North-Holland Publishing Company
LETTERS TO THE EDITORS - LETTRES AUX REDACTEURS
PREPARATION OF A HIGHLY REACTIVE PLUTONIUM DIOXIDE POWDER FOR PLUTONIUM-U~NIUM-CARBIDE AND NITRIDE FUEL K.P. LOUWRIER, K. RICHTER, G. KRAMER and M. LEBRUN Ewopean Institute for ~a~~~a~iurn .&‘lements(EURA TOM& D-75 Karate,
P.P. Box 2266, WestGermany
Received 31 March 1976
A fme powder improves the contact between graphite and metal oxides thus resulting in a higher rate of reaction. Therefore it is possible to obtain a more complete reaction, hence a low oxygen and the correct carbon and nitrogen content, at a lower reaction temperature. Only the influence of the particle size and the surface area is considered in this study. The currently employed plutonium oxalate precipitation for (U, Pu)02 preparation is characterised by a free flowing Pu02 powder of large PuO2 particles. To achieve this a 1 M oxalic acid solution is added slowly to a plutonium (IV) nitrate solution in 1 M free HNO, at 60%. A 0.1 M excess of oxalic acid is ensured-and the precipitate is allowed to digest for a few hours to secure large plutonium oxalate crystals [3,4]. In the newly developed plutonium oxalate precipitation method some parameters have been changed. The procedure aims at small crystals that still fdtrate rapidly and that result in a free flowing PuO, powder with a high surface area after calcination.
1. Introduction One method to prepare Pu-U carbide, carbonitride and nitride is the carboreduction of a mixture of U3Og or U02 and PuO, with graphite under reduced pressure or in a N2 atmosphere at 1300-1600°C according to the general formulas uo2+,
10Torr +*C~UOZ+;COZf 2 0 1o-2 torx
(U, Pll)O,
+
3c v @I,Pu)C + 2cot 1300- 1600°C
(U,Pu)O, + (3 -x)C t+N2 Ar-N, ( ----+(U,
Pu)C, _,N, + 2COt
16OO’C
The rate of these reactions depends besides on the reaction temperature strongly on the contact between oxide and graphite, because the reacting products are solids and the reaction is diffusion controlled at low gas pressures [ 1,2]. The temperature cannot be increased ~d~c~minately since evaporation of Pu becomes a serious draw back at reaction temperatures above 1600°C. A method is developed to produce a fine reactive PuO2 powder by modifying the current plutonium oxalate precipitation method. The surface area of the powder should be as high as possible and should be combined with a small mean size of the agglomerates. Powders that tend to agglomerate too strongly must be avoided in order to ensure a good microhomogeneity of the blend.
2. Experimental 2. I. Plutoniumoxalateprecipitation A 1 L pluto~um nitrate solution containing about 25g plutonium per liter in 1.5 M free HIV03 is poured rapidly into a container with 450 ml saturated oxalic acid in 3 M IWO3 at room temperature. The mixture is stirred for 10 min, allowed to settle for an hour and filtered, The precipitate is washed with a solution of 0.25 M oxalic acid in 1 M HNO,. The supernatant still contains 100 mg plutonium 219
K.P. Louwrier et al. /Preparation of a high/y reactive PuOz powder
220 Table 1 Carboreduction
conditions for carbide and nitride preparation with 12.20% respectively 8.2% C showing influence of particle size _~----Experiment no I
Component U308
PuO2 C
II - _. _..
III
.-..
Specific surface (m’g-‘) 3 3 4 2.6 (I)
Reaction conditions 1650 T (“0 time (h) 3 Analysis in reacted product 6000 0 (mm) 4.86 c (w%o) N (w%) 0.05 -_-._
-._._. 9.8
4 9.8 (2)
IV
.--
-
15 9.8 (2)
V
-. ;:;
15 9.8 (2)
(2)
1650 3
1650 2
1600 4
1500 4
4500 4.15 0.06
2600 4.63 0.05 __..-. _
2400 0.23 5.02
1200 0.09 5.4
-_
~---__
-
(l) Ringsdorf graphite, type RW-A. (2) Kropfmiihle graphite, type A F spezial.
per liter and is recycled directly in a plutonium recovery installation after the oxalic acid is destroyed by oxidation with concentrated nitric acid. The precipitate is dried at 150°C in air and calcined at 600°C. The iesulting powder consists out of approximately cubic particles with linear dimensions between 2 and 3 bm. The surface area measured using the B.E.T. method with Ar at -195°C is around 20 m2/g. The particle size of powder made in the conventional ways is much larger i.e. between 12 and 26 pm. Here the particles are elongated, a result of the crystal form of the oxalate, and the surface area is around 5 m2,‘g. 2.2. Carboreduction The PuO, powder is blended with u30g and graphite and pressed into pellets. These pellets react under reduced pressure (10m2 torr) at temperatures between 800 and 1650°C. CO is monitored continuously; its release is a measure for the reaction rate and for the completeness of reaction. Five experiments are designed to study the general influence of particle size on the carboreduction. Relatively coarse metal oxide powders are used in experiment I, II and IV, fine powders in experiment III and V. The effect of higher surfaces areas of the metal oxide powders is demonstrated in experiment III and
V and of the graphite powder in experiment II. Table 1 shows the conditions and results of these experiments. All data reff ect the results of some series of essays. 3. Results and discussion Preparation of a fine free flowing highly reactive plutonium dioxide powder through an oxalate prkcipitation is a feasible process, providing a plutinitim re; covery facility is at hand to deal with plutonium containing effluents. Applying fine-reactive powder [QI the carboreduction reaction to form (IJ, Pu)C and (U, &)N allows reactions at 1560°C for carbide formation and 1500°C for nitride formation within 2 and 4 h resp. A considerable decrease in oxygen contamination -Isobserved compared to experiments with coarser powders. The reaction conditions reduce the energy-consumption of the carboreduction and minimize- plutonium losses. References [ll S. Pickles, T.R.G. report 1393 (1967). PI T.B. Lindemeyey, M.D. AIlen and J.M. Leitnaker, J. Am. Cer. Sot. 52 (1969) 283. [31 Plutonium Handbook (Gorden and Breach NY 1965) Voi. I; p. 338, 371. 141 H. Wedemeyer, Reaktortagung 1973 (1973) p. 365.
LETTERS TO THE EDITORS - LETTRES AUX REDACTEURS
PREPARATION OF A HIGHLY REACTIVE PLUTONIUM DIOXIDE POWDER FOR PLUTONIUM-U~NIUM-CARBIDE AND NITRIDE FUEL K.P. LOUWRIER, K. RICHTER, G. KRAMER and M. LEBRUN Ewopean Institute for ~a~~~a~iurn .&‘lements(EURA TOM& D-75 Karate,
P.P. Box 2266, WestGermany
Received 31 March 1976
A fme powder improves the contact between graphite and metal oxides thus resulting in a higher rate of reaction. Therefore it is possible to obtain a more complete reaction, hence a low oxygen and the correct carbon and nitrogen content, at a lower reaction temperature. Only the influence of the particle size and the surface area is considered in this study. The currently employed plutonium oxalate precipitation for (U, Pu)02 preparation is characterised by a free flowing Pu02 powder of large PuO2 particles. To achieve this a 1 M oxalic acid solution is added slowly to a plutonium (IV) nitrate solution in 1 M free HNO, at 60%. A 0.1 M excess of oxalic acid is ensured-and the precipitate is allowed to digest for a few hours to secure large plutonium oxalate crystals [3,4]. In the newly developed plutonium oxalate precipitation method some parameters have been changed. The procedure aims at small crystals that still fdtrate rapidly and that result in a free flowing PuO, powder with a high surface area after calcination.
1. Introduction One method to prepare Pu-U carbide, carbonitride and nitride is the carboreduction of a mixture of U3Og or U02 and PuO, with graphite under reduced pressure or in a N2 atmosphere at 1300-1600°C according to the general formulas uo2+,
10Torr +*C~UOZ+;COZf 2 0 1o-2 torx
(U, Pll)O,
+
3c v @I,Pu)C + 2cot 1300- 1600°C
(U,Pu)O, + (3 -x)C t+N2 Ar-N, ( ----+(U,
Pu)C, _,N, + 2COt
16OO’C
The rate of these reactions depends besides on the reaction temperature strongly on the contact between oxide and graphite, because the reacting products are solids and the reaction is diffusion controlled at low gas pressures [ 1,2]. The temperature cannot be increased ~d~c~minately since evaporation of Pu becomes a serious draw back at reaction temperatures above 1600°C. A method is developed to produce a fine reactive PuO2 powder by modifying the current plutonium oxalate precipitation method. The surface area of the powder should be as high as possible and should be combined with a small mean size of the agglomerates. Powders that tend to agglomerate too strongly must be avoided in order to ensure a good microhomogeneity of the blend.
2. Experimental 2. I. Plutoniumoxalateprecipitation A 1 L pluto~um nitrate solution containing about 25g plutonium per liter in 1.5 M free HIV03 is poured rapidly into a container with 450 ml saturated oxalic acid in 3 M IWO3 at room temperature. The mixture is stirred for 10 min, allowed to settle for an hour and filtered, The precipitate is washed with a solution of 0.25 M oxalic acid in 1 M HNO,. The supernatant still contains 100 mg plutonium 219
K.P. Louwrier et al. /Preparation of a high/y reactive PuOz powder
220 Table 1 Carboreduction
conditions for carbide and nitride preparation with 12.20% respectively 8.2% C showing influence of particle size _~----Experiment no I
Component U308
PuO2 C
II - _. _..
III
.-..
Specific surface (m’g-‘) 3 3 4 2.6 (I)
Reaction conditions 1650 T (“0 time (h) 3 Analysis in reacted product 6000 0 (mm) 4.86 c (w%o) N (w%) 0.05 -_-._
-._._. 9.8
4 9.8 (2)
IV
.--
-
15 9.8 (2)
V
-. ;:;
15 9.8 (2)
(2)
1650 3
1650 2
1600 4
1500 4
4500 4.15 0.06
2600 4.63 0.05 __..-. _
2400 0.23 5.02
1200 0.09 5.4
-_
~---__
-
(l) Ringsdorf graphite, type RW-A. (2) Kropfmiihle graphite, type A F spezial.
per liter and is recycled directly in a plutonium recovery installation after the oxalic acid is destroyed by oxidation with concentrated nitric acid. The precipitate is dried at 150°C in air and calcined at 600°C. The iesulting powder consists out of approximately cubic particles with linear dimensions between 2 and 3 bm. The surface area measured using the B.E.T. method with Ar at -195°C is around 20 m2/g. The particle size of powder made in the conventional ways is much larger i.e. between 12 and 26 pm. Here the particles are elongated, a result of the crystal form of the oxalate, and the surface area is around 5 m2,‘g. 2.2. Carboreduction The PuO, powder is blended with u30g and graphite and pressed into pellets. These pellets react under reduced pressure (10m2 torr) at temperatures between 800 and 1650°C. CO is monitored continuously; its release is a measure for the reaction rate and for the completeness of reaction. Five experiments are designed to study the general influence of particle size on the carboreduction. Relatively coarse metal oxide powders are used in experiment I, II and IV, fine powders in experiment III and V. The effect of higher surfaces areas of the metal oxide powders is demonstrated in experiment III and
V and of the graphite powder in experiment II. Table 1 shows the conditions and results of these experiments. All data reff ect the results of some series of essays. 3. Results and discussion Preparation of a fine free flowing highly reactive plutonium dioxide powder through an oxalate prkcipitation is a feasible process, providing a plutinitim re; covery facility is at hand to deal with plutonium containing effluents. Applying fine-reactive powder [QI the carboreduction reaction to form (IJ, Pu)C and (U, &)N allows reactions at 1560°C for carbide formation and 1500°C for nitride formation within 2 and 4 h resp. A considerable decrease in oxygen contamination -Isobserved compared to experiments with coarser powders. The reaction conditions reduce the energy-consumption of the carboreduction and minimize- plutonium losses. References [ll S. Pickles, T.R.G. report 1393 (1967). PI T.B. Lindemeyey, M.D. AIlen and J.M. Leitnaker, J. Am. Cer. Sot. 52 (1969) 283. [31 Plutonium Handbook (Gorden and Breach NY 1965) Voi. I; p. 338, 371. 141 H. Wedemeyer, Reaktortagung 1973 (1973) p. 365.