Plutonium Chemistry - American Chemical Society

8 Thermodynamic Aspects of the Plutonium-OxygenSystem MARVIN TETENBAUM

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch008

Argonne National Laboratory, Chemical Technology Division, Argonne, IL 60439

This paper reviews data on certain thermodynamic aspects of the nonstoichiometric Pu-O system, which may serve as a basis for use in reactor safety analysis. Emphasis is placed on phase relationships, vaporization behavior, oxygen-potential measurements, and evaluation of pertinent thermodynamic quantities. Limited high temperature oxygen potential data obtained above the fluorite, diphasic, and sesquioxide phases in the Pu-O system are presented.

R e l i a b l e data on the thermodynamic and phase r e l a t i o n s h i p s of a c t i n i d e oxide systems are e s s e n t i a l f o r reactor s a f e t y a n a l y s i s . This paper reviews c e r t a i n aspects of thermodynamic data c u r r e n t l y a v a i l a b l e on the nonstoichiometric Pu-0 system, which may serve as a b a s i s f o r use i n reactor s a f e t y a n a l y s i s . Emphasis i s placed on phase r e l a t i o n s h i p s , v a p o r i z a t i o n behavior, oxygen-potential measurements , and e v a l u a t i o n of pertinent thermodynamic q u a n t i t i e s . I t should be emphasized that very l i t t l e i s q u a n t i t a t i v e l y known about how the t o t a l pressures o f plutonium-bearing species vary with oxygen p o t e n t i a l , stoichiometry, and temperature i n the b i v a r i a n t region of oxygen-deficient p l u t o n i a between the phase l i m i t s at very high temperatures. New (though l i m i t e d ) oxygen p o t e n t i a l data have been obtained i n our laboratory above the f l u o r i t e , d i p h a s i c , and sesquioxide phases i n the Pu-0 system a t 1750, 2050, and 2250 K. Phase R e l a t i o n s h i p s I t i s g e n e r a l l y agreed that the four known oxide phases i n the Pu-0 system are PU2O3 (hexagonal, La203 type), PuOx.52 (bcc), 1 . 6 1 ( f > t e n t a t i v e ) , and Pu02 ( f e e , f l u o r i t e type). Above

P u 0

c c

0097-6156/83/0216-0109$06.00/0 © 1983 American Chemical Society

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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PLUTONIUM CHEMISTRY

*925 K the P u Û 2 - phase e x h i b i t s a wide range of composition homogeneity, v a r y i n g from the s t o i c h i o m e t r i c composition, O/Pu * 2.00 to 0/Pu 1.61. The t e n t a t i v e phase diagram of the Pu-0 system reported by the 1966 IAEA Panel (1) i s shown i n Figure 1; i t i s based p r i m a r i l y on the high temperature X-ray s t u d i e s of Gardner et a l . (2) and C h i k a l l a et a l . ( 3 ) , the g a l v a n i c c e l l study of Markin and Rand ( 4 ) , as w e l l as v i a e l e c t r i c a l r e s i s t i v i t y and thermal expansion measurements. (5, 6^, 7) According to a recent assessment of P o t t e r and Rand (8) which was based on suggestions by S a r i et a l . (9) and X-ray s t u d i e s by Boivenau (10), the lower phase boundary of the Pu02- f l u o r i t e phase below ^1000 Κ has a composition of 0/Pu * 1.72. Around t h i s composition there i s a narrow d i p h a s i c region with fee Pu02- and a bec phase which e x i s t s from * 0/Pu = 1.70 down to 0/Pu » 1.61. I t should be emphasized that lower phase boundary compositions shown as l i n e compounds even at high temperatures i n Figure 1 have not been c r i t i c a l l y defined as a f u n c t i o n of temperature. For example, the hexagonal sesquioxlde phase shown as a l i n e compound on the phase diagram has been found to e x h i b i t a s i g n i f i c a n t range of homogeniety (11) (see b e l o w ) . F i n a l l y , i t should be noted that u n l i k e the u r a n i a system, h y p e r s t o i c h i o m e t r i c p l u t o n i a (Pu02+x) has not been found to e x i s t . x


s

X

X

V a p o r i z a t i o n Behavior The v a p o r i z a t i o n behavior of the b i v a r i a n t Pu02- phase has been i n v e s t i g a t e d by means of mass e f f u s i o n , mass spectrometry, and t r a n s p i r a t i o n . With tungsten, rhenium, and i r i d i u m e f f u s i o n c e l l s , the agreement between the vapor pressure measurements ( i . e . , t o t a l pressure of plutonium-bearing species) of Mulford and Lamar (12) (2000-2400 K ) , Pascard (13) (1670-2270 K), Messier (14_) (2070-2380 K ) , B a t t l e s et a l . (15) (1937-2342 Κ ) , and Ackermann et a l . (16) (1650-2150 Κ ) i s w i t h i n a f a c t o r of two. The measure ments of Ohse and C i a n i (17) (1800-2200 K) gave vapor pressure values that were somewhat higher than those obtained by the above i n v e s t i g a t o r s . The r e s u l t s of t r a n s p i r a t i o n measurements of Pardue and K e l l e r (18) (1730-2000 Κ ) , which were obtained with c a r r i e r gas streams of oxygen, a i r , and argon, were not of s u f f i c i e n t accuracy to be q u a n t i t a t i v e . With tantalum e f f u s i o n c e l l s , the r e s u l t s of Ackermann et a l . (16) are v i r t u a l l y i d e n t i c a l with the e a r l i e r work of Phipps et a l . (19) (1593-2063 Κ ) and y i e l d e d t o t a l vapor pressure values of plutonium-bearing species which were c o n s i d e r a b l y higher (by approximately a f a c t o r of ten) than those obtained with the l e s s reducing tungsten (as w e l l as Re and I r ) c e l l s . Ackermann et a l . (16) c o r r e l a t e d the vapor pressure r e s u l t s obtained with tantalum e f f u s i o n c e l l s with the u n i v a r i a n t d i p h a s i c system c o n s i s t i n g of Pu203(s)-PuOi,6l(s) as the condensed phases and Pu0(g) as the primary gaseous s p e c i e s . X


Thermodynamic Aspects of the Pu-O System

TETENBAUM

2500 Η


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1900 •

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119

120

PLUTONIUM CHEMISTRY

20 30

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ALEXANDER 2100 Κ

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70 l/mo


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80

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2050 Κ - - /

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Plutonium Chemistry - American Chemical Society

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