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Measurement of void volume of a fuel rod and the exchange of occluded gases from mixed carbide fuel with filling gas helium
ELSEVIER
Journal of Nuclear Materials 218 (1995) 231-235
jnurnalof nuclear materials
Measurement of void volume of a fuel rod and the exchange of occluded gases from mixed carbide fuel with filling gas helium G.A. Rama Rao a, S.G. Kulkarni a, V. Venugopal a, V.K. Manchanda b, G.L. Goswami e a Fuel Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India b Radiochemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India cAtomic Fuels Division, Bhabha Atomic Research Centre, Bombay 400 085, India
Received 11 April 1994; accepted 9 September 1994
Abstract
The presence of gaseous impurities in the filling gas of a fuel pin is detrimental to the thermal performance of a nuclear reactor fuel. The composition of the filling gas does not remain constant throughout the life of the fuel pin. The gas exchange phenomena that occur between the cover gas and impurity gases affect the fuel performance more severely in (U, Pu)O 2 fuel pin due to its inherently poor thermal conductivity than in advanced fuels such as mixed carbides and nitrides. In the present study the exchange phenomenon of the occluded gases present in our Fast Breeder Test Reactor (FBTR) fuel pellets [(Uo.30, Puo.70)C with 6500 ppm O] with the cover gas helium was observed as a function of time and temperature. Quantitative analysis of the released gases namely H2, 0 2 + Ar, N2, CH 4 and CO was carried out at subambient pressure by gas chromatography. The void volume of the fuel element is determined experimentally by gas equilibration with known volume.
1. Introduction
The mixed carbide fuel of composition (U0.3Pu0.7)C with 5 to 15 wt% sesquicarbide and less than 7000 ppm of (O + N) has been designed and fabricated for the fast breeder test reactor (FBTR) [1] in India. Helium gas, due to its chemical inertness, radiation stability and good thermal conductivity, finds extensive application as a bonding material [2,3] inside the fuel elements and provides the heat transport medium between the fuel pellet and the clad tube. 1.1. Importance o f filling gas purity
Apart from the composition of fuel and the cladding, the composition of the bonding material (filling or cover gas) influences the thermal behaviour of the fuel pin. Contamination of the filling gas influences the thermal behaviour of a fuel pin only at the beginning of operation through the decreasing heat conductance through the gap between fuel pellets and clad. Accord-
ingly the surface and the central temperature of the fuel pellet increases. It is reported [6] that the decrease in helium concentration in cover gas from 100 to 90% in the oxide fuel pins in fast reactors results in an increase of 30 K at the centre line and surface temperatures. The enhanced centre line temperature would result in centre line melting and increased transport of material down the temperature gradient leading to clad attack/corrosion. In case of carbides the increased partial pressure of CO and its flux may increase the carburization of stainless steel. Specifications for helium purity are more stringent for carbide fuels ( > 99%) than those for oxide fuels ( > 95%) since the carbide fuel is very reactive in presence of traces of oxygen or moisture which produce undesirable impurities like CO, H2, and CH 4 etc. These gases along with the sintering/storing atmospheric gases are retained in the low density (85% TD) fuel pellets. However, the carbon transfer from fuel to the clad occurs through gas media
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-31 15(94)00404-8
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G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
2. Experimental
such as CH4(g) ~-~ C(s) + 2H2(g) and CO(g) ~- CO2(g) + C(s) and would influence the carburization of the clad [4]. The data on the total volume of gases released for mixed carbide fuels has been reported up to 1273 K by Evered et al. [5]. Vollath [6], based on model calculations, suggested a mechanism for the exchange of occluded gases with helium blanket. The contribution from gas exchange thus becomes the principal source of contamination of the cover gas during storing after encapsulation. The (U,Pu)C fuel pin for fast reactors has to have lower smear density ( ~ 85%) compared to mixed oxides ( ~ 90%) to allow release of all fission gases in order to minimize swelling and hence carbide fuel pellets are expected to contain enough open porosity so as to contribute for the exchange phenomenon. The aim of the present work was to study the change in the composition of filling gas helium in FBTR fuel pins during storage and in isothermal heating conditions. The influence of time and temperature on the exchange of major impurities such as H 2, CH 4 and CO with helium in encapsulated fuel pellets was determined. The degree of contamination of the filling gas helium by the exchange of occluded gases is determined primarily by the ratio of the open porosity of the pellets to the void volume in the fuel pin. Therefore, an attempt was also made to measure experimentally the void volume of the fuel pin by a successive partitioning method as well as by exchange in cover gas pressures inside the fuel pin.
2.1. Exchange phenomenon 2.1.1. R o o m temperature The gas analyzer consists of three parts: the puncturing system to puncture the fuel pin, the gas sampling tube which is evacuated to better than 10 -3 Pa to collect the filling gas after the puncture and a commercial gas chromatograph (Varian 3700), which was modified to carry out the injections at subambient pressures. The assembly is shown in Fig. 1. Typical FBTR fuel pins of length 0.51 m containing pellets of (U0.3Pu0.7)C with 5-15 wt% M2C 3 and < 7000 ppm [(O) + (N)] were used to study the effect of storage time on exchange. The pellets used in this study were prepared under similar conditions and sintered in Ar + H 2 gas at 1873 K. The pellets were encapsulated on the same day by TIG welding of the end cap in an evacuable chamber filled with helium. The changes in the helium purity were monitored by storing the pellets in encapsulated condition for 1501500 h. The encapsulated fuel pins were punctured under vacuum after various lengths of storage time. The procedure for the puncturing of the fuel pin, collection of gas and the analysis by gas chromatography was described in detail in our earlier work [7]. 2.1.2. Elevated temperature Four fuel pins encapsulated on the same day were chosen for studying the effect of elevated temperature
GAS SAMPLING TUBE
TO GAS CHROMATOGRAPH
5
PUNCTURING UNIT
4 I
~
T
~-
GLOVE BOX
1:S S. TUBE FILLED WITH HELIUM, 2:DRILL BIT, 3;PARTICULATE FILTER, 4,5,6,7:BELLOW SEALED VALVES, 8:DIAPHRAGM VALVE, 9:MERCURY MANOMETER, 10:DIFFUSION PUMP, 11 : ROTARY VACUUM PUMP.
Fig. 1. Facility to measure the void volume of the FBTR fuel pin.
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
on the release of occluded gases. Fresh pellets were used in each pin. The pins were heated for 1 h at 623, 773 and 923 K cooled for another 1 h and then punctured to collect the gas for the analysis. All the pins were analyzed on the same day. 2.2. Void volume
The volume of the gas sampling tube and the combined volumes of the tube and the fuel rod were determined. The same procedure was employed in both cases. The system was evacuated to a very low pressure (better than 10 -3 Pa). It was connected to another cell with well known volume and pressure. The connecting valve was opened to enable the pressure to be uniform throughout the system. The unknown volume was calculated assuming the ideal gas law i.e. at constant temperature PIV1 = PzV2 . Monitoring P1, P2 and knowing V1, volume V2 can be calculated. The average value determined for the void volume of the fuel pin is (1.86 _+0.06) ml. This void volume corresponds to 20% volume of FBTR fuel pin of 0.51 m long and 4.8 mm diameter. This is in excellent agreement with the one calculated from design considerations taking into account the volumes of plenum and gas. The pressure of helium gas (99.99% pure) filled inside the pin was determined to be (111.5 _+ 10.1) kPa (average of 15 determinations).
3. Results and discussion The impurities present in the fuel pellets (50 g per pin) are much more important than those in the filling gas (3 × 10 -4 g per pin). As the sintering atmosphere of pellets is Ar + H2, H 2 gas is invariably retained in the pores of the pellets. The formation of methane, carbon monoxide and hydrogen are shown in the following possible reactions involving the mono and sesquicarbides [8]. MC(s) + 2 H 2 0 ( g )
, MO2(s ) + CHa(g),
(1)
MzC3(s) + 2Hz(g)
, 2 M C ( s ) + CH4(g),
(2)
MC(s) + x H 2 0 ( g ) , M(CI_y,Ox_y)(s ) + x H 2 ( g ) + y C O ( g ) ,
(3) MO2(s ) + 3M2C3(s )
, 7 M C ( s ) + 2CO(g).
(4)
The release of methane gas is instantaneous, for reaction 1 even at room temperature, if any moisture comes in contact with the pellets during loading into the pin. CH 4 also gets released according to reaction 2, as sesquicarbide is maintained between 5 to 15 wt% during the sintering process in Ar + H 2 mixture at
233
elevated temperatures. The presence of CH 4 and H 2 gases may act as carburising media by the reaction: C(s) + 2H2(g ) ,
' CH4(g ) .
(5)
The carburization of stainless steel clad may decrease its strength leading to rupture and coolant sodium ingress into the pin. Fortunately the hydrogen diffusion studies through stainless steel at elevated temperatures carried out in our laboratory [9] indicated that the rate of diffusion through the clad is sufficiently high to preclude any detrimental role of hydrogen with respect to carbon transfer to the clad. Carbon monoxide is the major cause of carburization in fast reactors with carbide fuels. It vaponr transports carbon by the reaction (6). An exact flux calculation of CO from fuel to clad is not feasible. However, it has been reported that noticeable carburization of clad would occur above a threshold pressure of Pco > 0.1 kPa with mixed carbides having centre line temperatures of 1650 K and surface temperature of 1100 K for a residence time of six months in the reactor [10]. 2CO(g) ,
' C(solid solution in clad) + CO2(g ).
(6) Hence reactions (3) and (4) may contribute to CO and would be possible at temperatures above 1273 K [10]. The release of CO gas generated by the mechanism given in Eq. (4) will continue as long as oxygen is available in MO 2 form in the pellets. Depending on temperature, CO can also be released from the monooxycarbide solid solution as MCxOy
' MCx zOy-z + zCO.
(7)
It is known that oxygen in the pellet exists in the form of MCxOy. The extent of release of CO gas depends upon the equilibrium pressure of CO over MCxOy [11]. Based on the equilibrium studies of Agarwal et al. [8] and Saibaba et al. [12], it can be explained that the release rate of CO gas would be higher for (U, Pu)Cla than that of (U, Pu)C1. 0. Potter [13] has calculated the influence of oxygen, nitrogen impurities in U0.v0Pu0.30C(1-y z) (Y represents nitrogen and z represents oxygen content) in Pco values. For example at 1700 K, the P c o values for y = 0.05 and z = 0.02 and 0.10 are respectively 5.8 × 10 -2 and 0.4 Pa. In case of higher plutonium carbide concentration as in FBTR fuel, with 20% sesquicarbide and 500 ppm nitrogen, the Pco measured [14] at 1650 K for 6000 ppm oxygen is around 7.5 × 10 - 4 Pa. Typical concentration levels of the gases observed in helium after storing of the pellets fabricated under identical conditions are shown in Table 1. The original purity of the helium to be filled was routinely checked and a cylinder with impurity content greater than 100 ppm was not used as cover gas. It is observed from the
234
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
Table 1 The levels of concentrations of impurities observed in the helium cover gas after various storage times at room temperature
Table 3 Concentration of the occluded gases released at room temperature and after heating at different temperatures S.
Temper-
Concentration in ppm
S. no.
Duration of storage (days)
Concentration in p p m / %
no.
H2
O 2 + Ar
N2
CH 4
CO
H2
02 + A r
Ne
CH 4
CO
ature (K)
1 2 3 4 5
07 21 28 32 62
5700 6350 6700 3.6% 17.8%
80 50 100 150 50
400 800 900 1000 1800
1450 1660 1900 2200 7.6%
120 70 < 10 200 200
1 2 3 4
298 623 773 923
< 100 8060 < 100 < 100
1290 960 640 70
5960 1815 1470 350
470 1290 4800 120
< 10 50 290 320
table t h a t H 2 a n d C H 4 c o n t e n t s in the h e l i u m cover gas increase with time at r o o m t e m p e r a t u r e . T h e gradual increase in m e t h a n e c o n t e n t suggests t h a t it was r e l e a s e d from the fuel r a t h e r t h a n from the reaction (1). T h e large c h a n g e in C H 4 c o n c e n t r a t i o n after prolonged storing indicates t h a t C H 4 is b e i n g r e l e a s e d from the pellet a n d not due to c o n t i n u o u s p r e s e n c e of moisture. It is k n o w n that C H 4 / H 2 c a n n o t carburize stainless steel due to faster diffusion of h y d r o g e n t h r o u g h the clad t u b e at the r e a c t o r o p e r a t i n g t e m p e r ature. T h e absolute a m o u n t of the impurities p r e s e n t in the filling gas h e l i u m is several orders of m a g n i t u d e lower t h a n t h a t of the fuel. T h e r e f o r e the role of h e l i u m c o m p o s i t i o n in the fuel p e r f o r m a n c e is of little c o n s e q u e n c e . Similarly the p r e s s u r e of fission gas as r e l e a s e d after few days of r e a c t o r o p e r a t i o n is an o r d e r of m a g n i t u d e larger t h a n the initial p r e s s u r e of helium. As a result, the c o m p o s i t i o n a n d pressure of filling gas affects the t h e r m a l b e h a v i o u r of the pin only during the initial stages of operation. In o r d e r to u n d e r s t a n d the effect of t h e exchange of gases from the pellets with b l a n k e t helium, several exposures with fresh helium were given to the same pellets by r e p e a t e d filling a n d encapsulation. T h e a m o u n t s of gases r e l e a s e d on f u r t h e r storing for different periods are given in T a b l e 2. It is clear from the table t h a t the occluded H 2 a n d C H 4 c o n t i n u e to get e x c h a n g e d with h e l i u m with each
filling a n d t h e r e f o r e t h e i r c o n c e n t r a t i o n s reduce with each filling of fresh h e l i u m gas. T h e increase in the a m o u n t s of n i t r o g e n could be d u e to the storage of pellets in n i t r o g e n e n v i r o n m e n t before encapsulation. T h e release of gases from pellets was f o u n d to b e faster on h e a t i n g of the pellets. T h e s e e x p e r i m e n t s were carried out o n duplicate samples at each t e m p e r ature. T h e data on h e a t i n g studies are shown in T a b l e 3. T h e e n c a p s u l a t i o n of the pellets, heating a n d analysis were carried o u t o n the same day without any storing period. T h e first pin was not h e a t e d a n d the gas was analysed only for c o m p a r i s o n of the impurity levels. T h e release of hydrogen a p p e a r e d to be complete in 1 h of h e a t i n g at 623 K. F u r t h e r increase in t e m p e r a t u r e up to 773 K resulted in the d r o p of hydrogen c o n c e n t r a t i o n a n d increase in the c o n c e n t r a t i o n of m e t h a n e . T h e d e c r e a s e is due to its reaction with the sesquicarbide thus increasing the m e t h a n e c o n t e n t proportionally as shown in r e a c t i o n (2). T h e s u b s e q u e n t r e d u c t i o n in m e t h a n e c o n c e n t r a t i o n at 923 K is att r i b u t e d to its d e c o m p o s i t i o n a n d the slow diffusion of the r e s u l t a n t hydrogen out of the stainless steel clad. T h e d e c o m p o s i t i o n of m e t h a n e at this t e m p e r a t u r e is possible from the calculations b a s e d on its free energy of f o r m a t i o n [15] according to the r e a c t i o n (8). C(s) + 2Hz(g )
, C H a ( g ),
(8)
A G e ' ( J / t o o l ) = - 16520 + 12.25T log T - 15.62T
( 2 9 8 - 1 2 0 0 K).
Table 2 Release of occluded gases on repeated exchange with helium S. n o .
Storage time (days)
1 2 3 4 5 6 7 8
09 15 19 15 21 02 62 09
Remarks
Concentration in p p m / % H2
02 + Ar
N2
CH 4
CO
8200 7700 7000 6500 5300 3770 17.8% 1.4%
70 100 80 50 50 130 50 10
350 1100 30 800 810 560 1800 330
9000 390(I 3500 1750 1660 1090 7.6% 1.8%
< 10 < 10 100 80 70 < 10 200 240
Remake of 1 Remake of 3 Remake of 4 Remake of 5 Remake of 7
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
AG ° becomes positive from 825 K onwards and the decomposition is more favourable at that temperature in the event of hydrogen diffusing out through the stainless steel clad.
4. Total gas in the pellet The total gas content in the pellet as determined by vacuum extraction method at 1273 K consists of 8 0 95% hydrogen followed by 3 - 4 % methane and 1 - 3 % carbonmonoxide. These data indicate the retention of significant quantities of hydrogen and m e t h a n e by the pellets during fabrication and sintering in Ar + H 2 mixture. The gas composition of the occluded gases after exchange with helium on storage corresponds to the total gas composition of the pellet if it is assumed that 1 mole of C H 4 decomposes at that temperature thus increasing the concentration of hydrogen by 2 moles and 1 mole of oxygen corresponds to 2 moles of carbonmonoxide following its reaction with the carbide fuel.
5. Conclusion 1. The exchange of occluded gases from the uran i u m - p l u t o n i u m mixed carbide fuel pellets with the filling gas helium at room temperature is the principal source of contamination of cover gas during the initial stages. Presence of impurities up to 25% has been observed in few cases. 2. R e p e a t e d exchange of the pellets with pure helium during storage even at room temperature lowers the contamination due to m e t h a n e as well as hydrogen. 3. The release of occluded gases was found to be faster at elevated temperatures. The release of hydrogen and m e t h a n e is quantitative on heating the fuel pin for 1 h at 623 and 773 K respectively. 4. Preheating of the fuel pellets at 923 K in helium atmosphere prior to the encapsulation step may ensure that the level of impurities in the filling gas is within 0.1%. 5. The reduction in the concentration of H 2 during
235
heating of the pins from 623 K onwards is attributed to the diffusion p h e n o m e n o n through S.S. clad. Acknowledgements The authors are grateful to Dr. D.D. Sood, Director, Radiochemistry & Isotope group, Dr. H.C. Jain, H e a d Fuel Chemistry Division, Dr. R.H. Iyer, Head, Radiochemistry Division, for their keen interest in this work. References [1] C. Ganguly, P.V. Hegde, G.C. Jain, U. Basak, R.S. Mehrotra, S. Majumdar and P.R. Roy, Nucl. Technol. 72 (1986) 59. [2] D.L. West, AEC Report DP-582 (1961). [3] B.I. Spinrad, Ann. Rev. Energy 3 (1978) 147. [4] S.P. Garg, G.L. Goswami, R. Prasad and D.D. Sood, Proc. Seminar on Fast Reactor Fuel Cycle, IGCAR, Kalpakkam, (1986). [5] S. Evered, M.J. Moreton-Smith, R.G. Sowden, AERE M-1593 (1965). [6] H. Elbel and D. Vollath, J. Nucl. Mater. 106 (1982) 243. [7] S.G. Kulkarni, G.A. Rama Rao, V.K. Manchanda and P.R. Natarajan, J. Chrom. Sci. 23 (1985) 68. [8] R. Agarwal, V. Venugopal and D.D. Sood, Thermodynamic Analysis of U - P u - C - N - O System, BARC Report E/022 (1992). [9] V. Venugopal, K.N. Roy, Z. Singh, R. Prasad, D. Kumar and G.L. Goswami, Seminar on Fast Reactor Fuel Cycle, February 10-12 (IGCAR, Kalpakkam, India, 1986) p. 38. [10] J.M. Horspool, N. Parkinson, J.R. Findlay, P.E. Potter, M.H. Rand, L.E. Russel and W. Batey, Fuel and Fuel Elements for Fast Reactors, Proc. IAEA Symp., Brussels, vol. 1 (1974) 3. [11] T. Iwai, I. Takahashi and M. Handa, J. Nucl. Sci. Technol. 25(5) (1988) 456. [12] M. Saibaba, S. Vanaraman and C.K. Mathews, J. Nucl. Mater. 144 (1987) 56. [13] P.E. Potter, Plutonium 1975 and other Actinides, (North-Holland, Amsterdam, 1976)p. 211. [14] Y.S. Sayi, C.S. Yadav, P.S. Shankaran, G.C. Chapru, V. Venugopal and R. Prasad, Proc. Symp. on Nuclear and Radiochemistry, Visakhapatnam (1992) p. 194. [15] O. Kubaschewski, C.B. Alcock, Metallurgical Thermochemistry (Pergamon, New York, 1989) p. 378.
Journal of Nuclear Materials 218 (1995) 231-235
jnurnalof nuclear materials
Measurement of void volume of a fuel rod and the exchange of occluded gases from mixed carbide fuel with filling gas helium G.A. Rama Rao a, S.G. Kulkarni a, V. Venugopal a, V.K. Manchanda b, G.L. Goswami e a Fuel Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India b Radiochemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India cAtomic Fuels Division, Bhabha Atomic Research Centre, Bombay 400 085, India
Received 11 April 1994; accepted 9 September 1994
Abstract
The presence of gaseous impurities in the filling gas of a fuel pin is detrimental to the thermal performance of a nuclear reactor fuel. The composition of the filling gas does not remain constant throughout the life of the fuel pin. The gas exchange phenomena that occur between the cover gas and impurity gases affect the fuel performance more severely in (U, Pu)O 2 fuel pin due to its inherently poor thermal conductivity than in advanced fuels such as mixed carbides and nitrides. In the present study the exchange phenomenon of the occluded gases present in our Fast Breeder Test Reactor (FBTR) fuel pellets [(Uo.30, Puo.70)C with 6500 ppm O] with the cover gas helium was observed as a function of time and temperature. Quantitative analysis of the released gases namely H2, 0 2 + Ar, N2, CH 4 and CO was carried out at subambient pressure by gas chromatography. The void volume of the fuel element is determined experimentally by gas equilibration with known volume.
1. Introduction
The mixed carbide fuel of composition (U0.3Pu0.7)C with 5 to 15 wt% sesquicarbide and less than 7000 ppm of (O + N) has been designed and fabricated for the fast breeder test reactor (FBTR) [1] in India. Helium gas, due to its chemical inertness, radiation stability and good thermal conductivity, finds extensive application as a bonding material [2,3] inside the fuel elements and provides the heat transport medium between the fuel pellet and the clad tube. 1.1. Importance o f filling gas purity
Apart from the composition of fuel and the cladding, the composition of the bonding material (filling or cover gas) influences the thermal behaviour of the fuel pin. Contamination of the filling gas influences the thermal behaviour of a fuel pin only at the beginning of operation through the decreasing heat conductance through the gap between fuel pellets and clad. Accord-
ingly the surface and the central temperature of the fuel pellet increases. It is reported [6] that the decrease in helium concentration in cover gas from 100 to 90% in the oxide fuel pins in fast reactors results in an increase of 30 K at the centre line and surface temperatures. The enhanced centre line temperature would result in centre line melting and increased transport of material down the temperature gradient leading to clad attack/corrosion. In case of carbides the increased partial pressure of CO and its flux may increase the carburization of stainless steel. Specifications for helium purity are more stringent for carbide fuels ( > 99%) than those for oxide fuels ( > 95%) since the carbide fuel is very reactive in presence of traces of oxygen or moisture which produce undesirable impurities like CO, H2, and CH 4 etc. These gases along with the sintering/storing atmospheric gases are retained in the low density (85% TD) fuel pellets. However, the carbon transfer from fuel to the clad occurs through gas media
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-31 15(94)00404-8
232
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
2. Experimental
such as CH4(g) ~-~ C(s) + 2H2(g) and CO(g) ~- CO2(g) + C(s) and would influence the carburization of the clad [4]. The data on the total volume of gases released for mixed carbide fuels has been reported up to 1273 K by Evered et al. [5]. Vollath [6], based on model calculations, suggested a mechanism for the exchange of occluded gases with helium blanket. The contribution from gas exchange thus becomes the principal source of contamination of the cover gas during storing after encapsulation. The (U,Pu)C fuel pin for fast reactors has to have lower smear density ( ~ 85%) compared to mixed oxides ( ~ 90%) to allow release of all fission gases in order to minimize swelling and hence carbide fuel pellets are expected to contain enough open porosity so as to contribute for the exchange phenomenon. The aim of the present work was to study the change in the composition of filling gas helium in FBTR fuel pins during storage and in isothermal heating conditions. The influence of time and temperature on the exchange of major impurities such as H 2, CH 4 and CO with helium in encapsulated fuel pellets was determined. The degree of contamination of the filling gas helium by the exchange of occluded gases is determined primarily by the ratio of the open porosity of the pellets to the void volume in the fuel pin. Therefore, an attempt was also made to measure experimentally the void volume of the fuel pin by a successive partitioning method as well as by exchange in cover gas pressures inside the fuel pin.
2.1. Exchange phenomenon 2.1.1. R o o m temperature The gas analyzer consists of three parts: the puncturing system to puncture the fuel pin, the gas sampling tube which is evacuated to better than 10 -3 Pa to collect the filling gas after the puncture and a commercial gas chromatograph (Varian 3700), which was modified to carry out the injections at subambient pressures. The assembly is shown in Fig. 1. Typical FBTR fuel pins of length 0.51 m containing pellets of (U0.3Pu0.7)C with 5-15 wt% M2C 3 and < 7000 ppm [(O) + (N)] were used to study the effect of storage time on exchange. The pellets used in this study were prepared under similar conditions and sintered in Ar + H 2 gas at 1873 K. The pellets were encapsulated on the same day by TIG welding of the end cap in an evacuable chamber filled with helium. The changes in the helium purity were monitored by storing the pellets in encapsulated condition for 1501500 h. The encapsulated fuel pins were punctured under vacuum after various lengths of storage time. The procedure for the puncturing of the fuel pin, collection of gas and the analysis by gas chromatography was described in detail in our earlier work [7]. 2.1.2. Elevated temperature Four fuel pins encapsulated on the same day were chosen for studying the effect of elevated temperature
GAS SAMPLING TUBE
TO GAS CHROMATOGRAPH
5
PUNCTURING UNIT
4 I
~
T
~-
GLOVE BOX
1:S S. TUBE FILLED WITH HELIUM, 2:DRILL BIT, 3;PARTICULATE FILTER, 4,5,6,7:BELLOW SEALED VALVES, 8:DIAPHRAGM VALVE, 9:MERCURY MANOMETER, 10:DIFFUSION PUMP, 11 : ROTARY VACUUM PUMP.
Fig. 1. Facility to measure the void volume of the FBTR fuel pin.
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
on the release of occluded gases. Fresh pellets were used in each pin. The pins were heated for 1 h at 623, 773 and 923 K cooled for another 1 h and then punctured to collect the gas for the analysis. All the pins were analyzed on the same day. 2.2. Void volume
The volume of the gas sampling tube and the combined volumes of the tube and the fuel rod were determined. The same procedure was employed in both cases. The system was evacuated to a very low pressure (better than 10 -3 Pa). It was connected to another cell with well known volume and pressure. The connecting valve was opened to enable the pressure to be uniform throughout the system. The unknown volume was calculated assuming the ideal gas law i.e. at constant temperature PIV1 = PzV2 . Monitoring P1, P2 and knowing V1, volume V2 can be calculated. The average value determined for the void volume of the fuel pin is (1.86 _+0.06) ml. This void volume corresponds to 20% volume of FBTR fuel pin of 0.51 m long and 4.8 mm diameter. This is in excellent agreement with the one calculated from design considerations taking into account the volumes of plenum and gas. The pressure of helium gas (99.99% pure) filled inside the pin was determined to be (111.5 _+ 10.1) kPa (average of 15 determinations).
3. Results and discussion The impurities present in the fuel pellets (50 g per pin) are much more important than those in the filling gas (3 × 10 -4 g per pin). As the sintering atmosphere of pellets is Ar + H2, H 2 gas is invariably retained in the pores of the pellets. The formation of methane, carbon monoxide and hydrogen are shown in the following possible reactions involving the mono and sesquicarbides [8]. MC(s) + 2 H 2 0 ( g )
, MO2(s ) + CHa(g),
(1)
MzC3(s) + 2Hz(g)
, 2 M C ( s ) + CH4(g),
(2)
MC(s) + x H 2 0 ( g ) , M(CI_y,Ox_y)(s ) + x H 2 ( g ) + y C O ( g ) ,
(3) MO2(s ) + 3M2C3(s )
, 7 M C ( s ) + 2CO(g).
(4)
The release of methane gas is instantaneous, for reaction 1 even at room temperature, if any moisture comes in contact with the pellets during loading into the pin. CH 4 also gets released according to reaction 2, as sesquicarbide is maintained between 5 to 15 wt% during the sintering process in Ar + H 2 mixture at
233
elevated temperatures. The presence of CH 4 and H 2 gases may act as carburising media by the reaction: C(s) + 2H2(g ) ,
' CH4(g ) .
(5)
The carburization of stainless steel clad may decrease its strength leading to rupture and coolant sodium ingress into the pin. Fortunately the hydrogen diffusion studies through stainless steel at elevated temperatures carried out in our laboratory [9] indicated that the rate of diffusion through the clad is sufficiently high to preclude any detrimental role of hydrogen with respect to carbon transfer to the clad. Carbon monoxide is the major cause of carburization in fast reactors with carbide fuels. It vaponr transports carbon by the reaction (6). An exact flux calculation of CO from fuel to clad is not feasible. However, it has been reported that noticeable carburization of clad would occur above a threshold pressure of Pco > 0.1 kPa with mixed carbides having centre line temperatures of 1650 K and surface temperature of 1100 K for a residence time of six months in the reactor [10]. 2CO(g) ,
' C(solid solution in clad) + CO2(g ).
(6) Hence reactions (3) and (4) may contribute to CO and would be possible at temperatures above 1273 K [10]. The release of CO gas generated by the mechanism given in Eq. (4) will continue as long as oxygen is available in MO 2 form in the pellets. Depending on temperature, CO can also be released from the monooxycarbide solid solution as MCxOy
' MCx zOy-z + zCO.
(7)
It is known that oxygen in the pellet exists in the form of MCxOy. The extent of release of CO gas depends upon the equilibrium pressure of CO over MCxOy [11]. Based on the equilibrium studies of Agarwal et al. [8] and Saibaba et al. [12], it can be explained that the release rate of CO gas would be higher for (U, Pu)Cla than that of (U, Pu)C1. 0. Potter [13] has calculated the influence of oxygen, nitrogen impurities in U0.v0Pu0.30C(1-y z) (Y represents nitrogen and z represents oxygen content) in Pco values. For example at 1700 K, the P c o values for y = 0.05 and z = 0.02 and 0.10 are respectively 5.8 × 10 -2 and 0.4 Pa. In case of higher plutonium carbide concentration as in FBTR fuel, with 20% sesquicarbide and 500 ppm nitrogen, the Pco measured [14] at 1650 K for 6000 ppm oxygen is around 7.5 × 10 - 4 Pa. Typical concentration levels of the gases observed in helium after storing of the pellets fabricated under identical conditions are shown in Table 1. The original purity of the helium to be filled was routinely checked and a cylinder with impurity content greater than 100 ppm was not used as cover gas. It is observed from the
234
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
Table 1 The levels of concentrations of impurities observed in the helium cover gas after various storage times at room temperature
Table 3 Concentration of the occluded gases released at room temperature and after heating at different temperatures S.
Temper-
Concentration in ppm
S. no.
Duration of storage (days)
Concentration in p p m / %
no.
H2
O 2 + Ar
N2
CH 4
CO
H2
02 + A r
Ne
CH 4
CO
ature (K)
1 2 3 4 5
07 21 28 32 62
5700 6350 6700 3.6% 17.8%
80 50 100 150 50
400 800 900 1000 1800
1450 1660 1900 2200 7.6%
120 70 < 10 200 200
1 2 3 4
298 623 773 923
< 100 8060 < 100 < 100
1290 960 640 70
5960 1815 1470 350
470 1290 4800 120
< 10 50 290 320
table t h a t H 2 a n d C H 4 c o n t e n t s in the h e l i u m cover gas increase with time at r o o m t e m p e r a t u r e . T h e gradual increase in m e t h a n e c o n t e n t suggests t h a t it was r e l e a s e d from the fuel r a t h e r t h a n from the reaction (1). T h e large c h a n g e in C H 4 c o n c e n t r a t i o n after prolonged storing indicates t h a t C H 4 is b e i n g r e l e a s e d from the pellet a n d not due to c o n t i n u o u s p r e s e n c e of moisture. It is k n o w n that C H 4 / H 2 c a n n o t carburize stainless steel due to faster diffusion of h y d r o g e n t h r o u g h the clad t u b e at the r e a c t o r o p e r a t i n g t e m p e r ature. T h e absolute a m o u n t of the impurities p r e s e n t in the filling gas h e l i u m is several orders of m a g n i t u d e lower t h a n t h a t of the fuel. T h e r e f o r e the role of h e l i u m c o m p o s i t i o n in the fuel p e r f o r m a n c e is of little c o n s e q u e n c e . Similarly the p r e s s u r e of fission gas as r e l e a s e d after few days of r e a c t o r o p e r a t i o n is an o r d e r of m a g n i t u d e larger t h a n the initial p r e s s u r e of helium. As a result, the c o m p o s i t i o n a n d pressure of filling gas affects the t h e r m a l b e h a v i o u r of the pin only during the initial stages of operation. In o r d e r to u n d e r s t a n d the effect of t h e exchange of gases from the pellets with b l a n k e t helium, several exposures with fresh helium were given to the same pellets by r e p e a t e d filling a n d encapsulation. T h e a m o u n t s of gases r e l e a s e d on f u r t h e r storing for different periods are given in T a b l e 2. It is clear from the table t h a t the occluded H 2 a n d C H 4 c o n t i n u e to get e x c h a n g e d with h e l i u m with each
filling a n d t h e r e f o r e t h e i r c o n c e n t r a t i o n s reduce with each filling of fresh h e l i u m gas. T h e increase in the a m o u n t s of n i t r o g e n could be d u e to the storage of pellets in n i t r o g e n e n v i r o n m e n t before encapsulation. T h e release of gases from pellets was f o u n d to b e faster on h e a t i n g of the pellets. T h e s e e x p e r i m e n t s were carried out o n duplicate samples at each t e m p e r ature. T h e data on h e a t i n g studies are shown in T a b l e 3. T h e e n c a p s u l a t i o n of the pellets, heating a n d analysis were carried o u t o n the same day without any storing period. T h e first pin was not h e a t e d a n d the gas was analysed only for c o m p a r i s o n of the impurity levels. T h e release of hydrogen a p p e a r e d to be complete in 1 h of h e a t i n g at 623 K. F u r t h e r increase in t e m p e r a t u r e up to 773 K resulted in the d r o p of hydrogen c o n c e n t r a t i o n a n d increase in the c o n c e n t r a t i o n of m e t h a n e . T h e d e c r e a s e is due to its reaction with the sesquicarbide thus increasing the m e t h a n e c o n t e n t proportionally as shown in r e a c t i o n (2). T h e s u b s e q u e n t r e d u c t i o n in m e t h a n e c o n c e n t r a t i o n at 923 K is att r i b u t e d to its d e c o m p o s i t i o n a n d the slow diffusion of the r e s u l t a n t hydrogen out of the stainless steel clad. T h e d e c o m p o s i t i o n of m e t h a n e at this t e m p e r a t u r e is possible from the calculations b a s e d on its free energy of f o r m a t i o n [15] according to the r e a c t i o n (8). C(s) + 2Hz(g )
, C H a ( g ),
(8)
A G e ' ( J / t o o l ) = - 16520 + 12.25T log T - 15.62T
( 2 9 8 - 1 2 0 0 K).
Table 2 Release of occluded gases on repeated exchange with helium S. n o .
Storage time (days)
1 2 3 4 5 6 7 8
09 15 19 15 21 02 62 09
Remarks
Concentration in p p m / % H2
02 + Ar
N2
CH 4
CO
8200 7700 7000 6500 5300 3770 17.8% 1.4%
70 100 80 50 50 130 50 10
350 1100 30 800 810 560 1800 330
9000 390(I 3500 1750 1660 1090 7.6% 1.8%
< 10 < 10 100 80 70 < 10 200 240
Remake of 1 Remake of 3 Remake of 4 Remake of 5 Remake of 7
G.A. Rama Rao et al. /Journal of Nuclear Materials 218 (1995) 231-235
AG ° becomes positive from 825 K onwards and the decomposition is more favourable at that temperature in the event of hydrogen diffusing out through the stainless steel clad.
4. Total gas in the pellet The total gas content in the pellet as determined by vacuum extraction method at 1273 K consists of 8 0 95% hydrogen followed by 3 - 4 % methane and 1 - 3 % carbonmonoxide. These data indicate the retention of significant quantities of hydrogen and m e t h a n e by the pellets during fabrication and sintering in Ar + H 2 mixture. The gas composition of the occluded gases after exchange with helium on storage corresponds to the total gas composition of the pellet if it is assumed that 1 mole of C H 4 decomposes at that temperature thus increasing the concentration of hydrogen by 2 moles and 1 mole of oxygen corresponds to 2 moles of carbonmonoxide following its reaction with the carbide fuel.
5. Conclusion 1. The exchange of occluded gases from the uran i u m - p l u t o n i u m mixed carbide fuel pellets with the filling gas helium at room temperature is the principal source of contamination of cover gas during the initial stages. Presence of impurities up to 25% has been observed in few cases. 2. R e p e a t e d exchange of the pellets with pure helium during storage even at room temperature lowers the contamination due to m e t h a n e as well as hydrogen. 3. The release of occluded gases was found to be faster at elevated temperatures. The release of hydrogen and m e t h a n e is quantitative on heating the fuel pin for 1 h at 623 and 773 K respectively. 4. Preheating of the fuel pellets at 923 K in helium atmosphere prior to the encapsulation step may ensure that the level of impurities in the filling gas is within 0.1%. 5. The reduction in the concentration of H 2 during
235
heating of the pins from 623 K onwards is attributed to the diffusion p h e n o m e n o n through S.S. clad. Acknowledgements The authors are grateful to Dr. D.D. Sood, Director, Radiochemistry & Isotope group, Dr. H.C. Jain, H e a d Fuel Chemistry Division, Dr. R.H. Iyer, Head, Radiochemistry Division, for their keen interest in this work. References [1] C. Ganguly, P.V. Hegde, G.C. Jain, U. Basak, R.S. Mehrotra, S. Majumdar and P.R. Roy, Nucl. Technol. 72 (1986) 59. [2] D.L. West, AEC Report DP-582 (1961). [3] B.I. Spinrad, Ann. Rev. Energy 3 (1978) 147. [4] S.P. Garg, G.L. Goswami, R. Prasad and D.D. Sood, Proc. Seminar on Fast Reactor Fuel Cycle, IGCAR, Kalpakkam, (1986). [5] S. Evered, M.J. Moreton-Smith, R.G. Sowden, AERE M-1593 (1965). [6] H. Elbel and D. Vollath, J. Nucl. Mater. 106 (1982) 243. [7] S.G. Kulkarni, G.A. Rama Rao, V.K. Manchanda and P.R. Natarajan, J. Chrom. Sci. 23 (1985) 68. [8] R. Agarwal, V. Venugopal and D.D. Sood, Thermodynamic Analysis of U - P u - C - N - O System, BARC Report E/022 (1992). [9] V. Venugopal, K.N. Roy, Z. Singh, R. Prasad, D. Kumar and G.L. Goswami, Seminar on Fast Reactor Fuel Cycle, February 10-12 (IGCAR, Kalpakkam, India, 1986) p. 38. [10] J.M. Horspool, N. Parkinson, J.R. Findlay, P.E. Potter, M.H. Rand, L.E. Russel and W. Batey, Fuel and Fuel Elements for Fast Reactors, Proc. IAEA Symp., Brussels, vol. 1 (1974) 3. [11] T. Iwai, I. Takahashi and M. Handa, J. Nucl. Sci. Technol. 25(5) (1988) 456. [12] M. Saibaba, S. Vanaraman and C.K. Mathews, J. Nucl. Mater. 144 (1987) 56. [13] P.E. Potter, Plutonium 1975 and other Actinides, (North-Holland, Amsterdam, 1976)p. 211. [14] Y.S. Sayi, C.S. Yadav, P.S. Shankaran, G.C. Chapru, V. Venugopal and R. Prasad, Proc. Symp. on Nuclear and Radiochemistry, Visakhapatnam (1992) p. 194. [15] O. Kubaschewski, C.B. Alcock, Metallurgical Thermochemistry (Pergamon, New York, 1989) p. 378.