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The hydrolysis of uranium-plutonium carbide after neutron irradiation to high burn-up
J. inorg,nucl.Chem., 1969,Vol.3I, pp. 3437to 3446. PergamonPress. Printedin Great Britain
THE HYDROLYSIS OF URANIUM-PLUTONIUM CARBIDE AFTER NEUTRON IRRADIATION HIGH BURN-UP
TO
J. R. F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS* Applied Chemistry Division, Atomic Energy Research Establishment, Harwell, Didcot, Berks., England
(Received 20 February 1969) Abstract-The hydrolytic reactions of uranium-plutonium carbide have been studied with the unirradiated material and with material irradiated with neutrons up to 10 atom per cent burn-up of the heavy metal atoms. The unirradiated carbide is hydrolysed readily by water and hydrochloric acid and yields hydrogen and methane with higher alkanes and alkenes in minor yield. Up to 86 per cent of the original carbon is recovered in the gas phase from the monocarbide; lower recoveries are found for preparations containing sesquicarbide. Neutron irradiation profoundly changes the hydrolytic properties of the carbide. The material becomes inert to water at 80°C, although still reacting readily with hydrochloric acid. The yield of methane from acid hydrolysis decreases with increasing burn-up; irradiation at high temperature or post-irradiation heating reduces the magnitude of this effect. The absence of acetylene among the reaction products is consistent with the assumption that alkaline earth and rare earth fission products are accommodated in the monocarbide lattice as a solid solution. INTRODUCTION
TIlE HYDROLYTIC reactions of uranium carbide have been studied in detail by several workers[I-5]. The reaction is important technologically because of interest in carbide as a nuclear reactor fuel, but in this context some areas of importance have received little attention. Uranium-plutonium carbide is an important reactor fuel whose hydrolytic properties are little known. Furthermore, the effect of neutron irradiation on carbide hydrolysis has not been examined after extensive transmutation of the fissile atoms. The hydrolytic behaviour of uranium-plutonium carbide has been examined in the unirradiated state at this laboratory by Evered et al.[6]; further extensions to higher plutonium contents are reported in the present work. The effect of neutron irradiation has been investigated at low irradiation levels. Bradley e t al. have studied the hydrolysis of uranium carbide irradiated to 1.6 per cent burnupt [7] and Evered included experiments on uranium-plutonium carbide irradiated *Present address: c/o Southampton University, Southampton, England. t 1 per cent bum-up is defined throughout as being that burn-up achieved when 1 per cent of the heavy metal atoms initially present are destroyed by fission. 1. M.J. Bradley and L. M. Ferris, Inorg. Chem. 1,683 (1963). 2. M.J. Bradley and L. M. Ferris, lnorg. Chem. 3, 750 (1964). 3. C. P. Kempter, J. less-common Metals 4, 419 (1962). 4. J. Spitz, Rep. Commissariat a l' energie atomique, CEA-2350 (1964). 5. F. H. Pollard, G. Nickless and S. Evered, J. Chromat. 15, 223 (1964). 6. S. Evered, M. J. Moreton-Smith and R. G. Sowden, J. inorg, nucl. Chem. 27, 1867 (1965). 7. M. J. Bradley, J. H. Goode, L. M. Ferris, J. R. Flanary and J. W. Ullman, Nucl. Sci. Eng. 21, 159 (1965). 3437
3438
J.R.
F I N D L A Y , M. J. MORETON-SMITH and R. MOSS
to 2.0 per cent burn-up [6]. In the present work the effect of neutron irradiation on uranium-plutonium carbide has been investigated up to 10 per cent burn-up; some results have been obtained also on the effect of irradiation temperature. EXPERIMENTAL
Specimen preparation and irradiation Uranium oxide containing 30 mole per cent plutonium oxide was converted to carbide by reduction with carbon at 1600°C under vacuum. The resultant product was ball milled to a fine powder and cold pressed into pellets 1.25 mm diameter by 0.6 -+ 0.1 mm length which were sintered at 1600°C for several hours under argon to a density greater than 95 per cent theoretical. Two batches of pellets A and B of differing carbon contents were prepared, and were characterised as detailed below in Table I. A third material C was obtained from a relatively large specimen of irradiated carbide; the analyses of the initial carbide preparation are given. Table 1. Characterisation of uranium-plutonium carbide Composition (Weight %) Material
Microstructure U
Pu
C
O
Carbon equivalent C+O
A
66.7
28.5
4.5
0-34
4.75
B
66-3
28.4
4.9
0-45
5.24
C
80.8
14.3
4.6
0.3
4.82
Monocarbide + trace metal Monocarbide + sesquicarbide Single phase monocarbide
Pellets of materials A and B were irradiated for various periods upto 10 per cent burn-up and at temperatures of either 350 or 1050°C. Irradiations at 350°C were performed on material A in a flux of 1014 neutrons cm -2 sec -1 in a materials testing reactor. Pellets were placed between copper discs to aid heat dispersal and were sealed in metal cans under a helium atmosphere. For irradiations at 1050°C the specimens were contained in molybdenum and were irradiated in a flux of 3 x 10 TM neutrons cm -2 sec -1 using a reactor position containing an electrically heated high temperature furnace[8]. Material C was irradiated to 5.3 per cent burn-up at a temperature of 1200°C: Temperatures in the vicinity of the specimens were measured directly by thermocouples, but the temperatures of the specimens themselves were higher due to the generation of fission heat. A correction for this effect is included in the values quoted but, due to uncertainties of the calculation, the temperatures are subject to an overall uncertainty of_+ 30°C. In the case of the larger specimen C, the uncertainties are much greater and the error can not be reduced below + 200°C.
Experimental method The presence of plutonium and the high levels of radioactivity induced in the irradiated specimens imposed restrictions upon the experimental methods and led to the decision to use only small specimens. The experiments on unirradiated material were performed in an argon atmosphere glove box where operations were performed manually. The irradiated material, however, could only be handled remotely and experiments were conducted in a lead-shielded cell, which also had a purified argon atmosphere. The hydrolysis apparatus is shown in Fig. 1. It consists of a glass reaction tube fitted with a capillary side-arm connected via polythene tube and a silica wool filter to two 10 ml syringes. In the top of the apparatus, an extended ground glass cone, sealed above by a ground glass joint and pulled 8. J. R. Findlay, F. Mason, E. Wait and G. S. Wright, Nucl. Energy, 130 (1968).
Hydrolysis of irradiated (U, Pu)C
• f/.
HYDROLYSIS FLUID
GROUND GLASS
~/
/ SILICA WOOL FILTER BREAK
HYDROLYSIS SAMPLE _
~
3439
POLYTHENE CAPILLARY TO SYRINGES
SEAL
dr3
_
HEATER Fig. 1. Hydrolysis apparatus. off to a breaker point below, contained the hydrolysing fluid and was fitted into the uppermost ground glass joint. The apparatus, less syringes, had a volume of 5 ml and was set in a copper block containing a heater and a thermocouple. The specimen was contacted with the hydrolysing fluid by fracturing the breaker point; as the reaction proceeded the gaseous products of hydrolysis were drawn into the two syringes, which were filled simultaneously to provide two equivalent samples. Their volume was such that several volumes of gas could be swept through the apparatus to ensure complete collection of the reaction products. Finally, the syringes were detached and their contents analysed by gas chromatographic techniques. Hydrogen and methane were determined from one syringe using an alumina column and a thermal conductivity detector under an argon carrier gas; methane and higher hydrocarbons were determined from the other syringe using an alumina column under helium and a flame ionisation detector. Hydrocarbons from C1 to C3 were determined individually; those from C4 to Ca were not completely separated and were assessed in groups. RESULTS
Water hydrolysis The rate of hydrolysis of control and irradiated material was determined in water at 80°C. Preparations o f the unirradiated carbide were hydrolysed rapidly with c o m p l e t e reaction within 4 hr; the products o f hydrolysis are given in Table 2. N o differences in rate were observed b e t w e e n the materials A and B, nor was any difference detectable on c o m p a r i s o n with earlier results on uranium carbide or (U0.9, Pu0.1)C [6]. T h e behaviour o f irradiated material w a s very different. Material A irradiated to 10 per cent burn-up at 350°C was essentially inert to water and had reacted to less than 0.01 per cent after 24 hr at 80°C. Material B irradiated to 3 per cent burn-up at 1050°C was similarly unreactive.
Hydrochloric acid hydro&sis The reaction of both control and irradiated material with 4N hydrochloric acid at 80°C was rapid, being complete within a f e w hours in all cases. A s a
3440
J.R.
F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS
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3441
Hydrolysis of irradiated (U, Pu)C
result, this reaction was studied extensively to compare the hydrolysis products of control and irradiated material; the analytical results are given in Table 2 for the various experimental conditions employed. The yields of hydrogen and gaseous hydrocarbons are quoted in ml of gas (S.T.P.) per gram of carbide specimen hydrolysed. The cumulative yields of both species are then expressed as a percentage of the theoretical yield, on the assumption that uranium becomes fourvalent and plutonium three-valent in the solution after hydrolysis; allowance is made also for metal already combined with the initial oxygen impurity. The carbon and hydrogen not recovered are assumed to remain as a non-volatile wax or as a wax and free carbon. The carbon content of the residue expressed as a percentage of the total carbon is given in Table 2. The existence of a waxy residue was demonstrated in some instances by extraction with petroleum ether and subsequent evaporation. Due to the small size of the hydrolysis sample, the amounts of residue were very small and could not be estimated quantitatively. DISCUSSION
Unirradiated carbide The results in Table 2 show no significant differences between the products from water or acid hydrolysis. Experiments on uranium monocarbide and sesquicarbide by Bradley-Sears[9] also indicate little difference between the two media. The measurements of the rate of hydrolysis of uranium-plutonium carbide were rather imprecise, but were in qualitative agreement with those reported by other workers for uranium carbide and therefore suggest that no significant rate effect exists that can be attributed to the presence of plutonium. There are changes in the products of hydrolysis however, and the plutonium-containing carbides yield less hydrogen (see Table 3). Table 3. Yields of free hydrogen ml g-1 UC Ref. [6] Water HC1
15. l --
(Uo.gPUo.1)C (Uo.rPuo,a)C (Uo.TPUo.3)C Ref. [6] A B 10.8 11.0
7-0 4-0
-4.8
This can be attributed to the final oxidation states of the cations in solution. After the hydrolysis of uranium carbide by water or hydrochloric acid, uranium was found by several workers to exist in the IV-valent state. Although in our experiments no analysis of the final solutions was possible, their colour suggested that plutonium was in the III-valent state; this is supported by the work of Drummond et al., who found that plutonium carbide when hydrolysed by water yielded plutonium (III) hydroxide[10]. Compared with 100 per cent uranium carbide, the hydrolysis of carbide containing 10 per cent and 30 per cent plutonium to give 9. M. Bradley-Sears and L. M. Ferris,J. inorg, nucl. Chem. 29, 1548 0967). 10. J. L. Drummond, B. J. MacDonald, M. M. Ockenden and G. A. Welch, J. chem. Soc. 4785 (1957).
3442
J.R.
F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS
uranium (IV) and plutonium (III) ions results in a reduction of the available hydrogen by 5-5 and 16 ml (STP) respectively. The volume changes observed in Table 3 are slightly lower than these predictions, perhaps due to a small increase in the hydrogen yield from the increased hydrocarbon condensation occurring in the presence of plutonium. Other evidence of the dependence of hydrogen yield upon valency has been obtained by Bradley-Sears et al., who found increased yields of hydrogen from the hydrolysis of uranium carbide by 2 to 18 M N a O H , where the final solution contained uranium (VI) ions [11]. The composition of the gaseous hydrocarbons has been the subject of much conjecture concerning the mechanism of the reaction and its relation to the carbide structure. It is generally accepted that when the carbon atoms exist singly in the carbide lattice surrounded by metal atoms, as they do in uranium monocarbide, the principal hydrolysis product is methane. The methane is accompanied, however, in all cases by higher hydrocarbons, which probably arise from condensation of hydrocarbon free radicals. In the sesquicarbide or dicarbide the hydrolysis products are more complex and depend upon the carbon-carbon bond length. In uranium dicarbide and sesquicarbide the carboncarbon distance approximates to that of a double bond and the hydrolysis products are principally alkanes, with some alkenes. The bond length in calcium carbide is equal to a triple bond, giving acetylene almost quantitatively; in the rare earth carbides the bonds are of intermediate length and several hydrocarbons are formed, with acetylene predominating. The carbides used in this investigation were composed principally of the monocarbide phase. Preparations A and C were essentially single phase, but preparation B contained approximately 15 per cent of second phase sesquicarbide. The principal hydrolysis product in all cases was methane. Comparison with previous studies on uranium carbide indicated that there were no significant differences in the yield of ethane, ethylene and higher hydrocarbons that could be attributed to the presence of plutonium. However, in the case of the singlephase preparation A the yields of methane were lower than those from uranium carbide and, as seen from Table 2, the carbon originally present was not recovered quantitatively from the gas phase. Bradley and Ferris[1], and Spitz[4] found a quantitative recovery of carbon from uranium monocarbide, and it is concluded that the presence of plutonium precludes the complete recovery of carbon from t h e monocarbide, probably by the formation of non-volatile condensation products. The hydrolysis products from preparation B, incorporating a sesquicarbide phase, contained less methane than the corresponding monocarbide preparation and showed an increase in the residual carbon. The hydrolysis of uranium sesquicarbideuranium monocarbide mixtures has been studied by Bradley and Ferris [2], who report an increased formation of wax and hydrogen at the expense of the methane yield as the sesquicarbide fraction is increased. This was accompanied also by some increase in the yield of higher gaseous hydrocarbons. These observations are consistent with ours on the carbon-rich uranium-plutonium carbide, except that the increase in hydrogen yield was small and no increase in the yield of the C~-C8 hydrocarbons could be detected. 11. M.J. Bradley-Sears, M. D. Pattengill and L. M. Ferris,J. inorg, nucl. Chem. 30, 2111 (1968).
Hydrolysis of irradiated (U, Pu)C
3443
Irradiated carbide: hydrolysis by water Irradiated carbide is essentailly inert to water at 80°C, whilst reaction proceeds readily in 4N hydrochloric acid at the same temperature. Similar findings for irradiated uranium carbide are reported by Bradley et a/.[7]; they found also that the material is inert to caustic soda and that the rate of reaction in hydrochloric acid is dependent upon the acid concentration. This behaviour differs fundamentally from that of the unirradiated control material, which is hydrolysed readily in all three media. The reasons for these changes in reactivity remain obscure but some discussion of them is worthwhile. The inactivity of the irradiated material could arise from spurious oxidation of the carbide during irradiation. Oxidation might result in a surface film which would protect the carbide from attack by water whilst permitting reaction with acid. Oxidation of the bulk carbide may be ruled out, since hydrolysis in hydrochloric acid liberates an extensive yield of hydrogen which would not be formed if much of the specimen existed as oxide. The effects of surface oxidation have been studied by Bradley and Ferris by abrading irradiated carbide in contact with water [7]. They demonstrated that the material is inert even when the surface has been freshly exposed. From this evidence it is concluded that the passivity of irradiated carbide is not due to spurious oxidation. The possibility still remains, however, that the reaction of water with irradiated material is inhibited by a coherent film of oxide product. The work of Colby [ 12], and the ready hydrolysis of the unirradiated carbide by acid, alkali and neutral media suggest that the attacking species is the water molecule; a coherent film of oxide product would thus be impenetrable to water. Dell has explained the different hydrolysis rates of uranium and plutonium nitride by the adherence of the oxide film, which in uranium is structurally compatible with an underlying layer of a higher nitride[13]. Plutonium does not form the higher nitride and the oxide is not adherent. In the case of carbides however, it is not obvious why the layer should be coherent only on irradiated carbide, and in the absence of further evidence this hypothesis can be adv~inced only tentatively. Other reasons that could account for the inertness of the irradiated carbide are associated with the presence of the fission products. Evered et al. investigated the influence of internal fl radiation upon the reaction by hydrolysing uranium carbide ground and sintered with 17°thulium oxide of high specific activity [6]. No inhibiting action was observed, but the specific activity of the thulium oxide was a factor of ten lower than that expected from uranium carbide specimens irradiated to 1 per cent burn-up even when the cooling period is prolonged. However, even at the rather low levels of/3 radiation that were employed, the complete absence of any effect implies that/3 radiation is unimportant. A chemical effect could be exerted by the fission product atoms themselves, either segregated at boundaries or dispersed homogeneously throughout the bulk. At 1.0 per cent burn-up, where effects are first seen, sufficient fission product atoms are produced to cover the grain boundary area, but if inhibition resulted 12. L.J. Colby., Jr, J. less-common Metals, 10, 429 (1966). 13. N . J . Bridger, R. M. Dell and V. J. Wheeler, 6th Intern. Syrup. Reactivity in Solids. Schenectady, New York (1968).
3444
J.R. FINDLAY, M. J. MORETON-SMITH and R. MOSS
from this cause, some reaction would be expected on grinding, when some grain fracture must occur. Fission product effects must be restricted therefore to their presence in the bulk. At the level of 1 per cent burn-up individual fission products are separated on a surface by several atom spacings and at this low concentration it is difficult to conceive how they produce such a drastic inhibition of the hydrolysis reaction.
Irradiated carbide: hydrolysis by acid There are significaft differences in the products of hydrolysis which are related to burn-up and irradiation temperature. The yield of methane from specimens irradiated at 350°C decreases with increasing burn-up whilst the yield of free hydrogen increases. The yields of other gaseous hydrocarbons are unchanged, but the lower recoveries of carbon in the gas phase suggest an increased formation of wax residues. The increased yield of hydrogen is consistent with the formation of the non-volatile hydrocarbon residues by condensation reactions, which probably have a free radical origin. Increasing the irradiation temperature lessens the effects of irradiation; materials A and B irradiated to 3.0 p e r c e n t burn-up and material C at 5.3 per cent burn-up exhibit higher methane and lower hydrogen yields than was the case in the comparable low temperature irradiations. The results in Table 2 demonstrate that the effects of irradiation can be influenced also by post-irradiation annealing. A specimen of preparation A, irradiated at 350°C to 10 per cent burn-up, was annealed for 4 hr at 1500°C, and on hydrolysis gave yields of hydrogen and methane comparable to those experienced at 3 per cent burn-up at an irradiation temperature of 1050°C. It can be shown clearly that this behaviour is not attributable to an effect of irradiation field. The decay times between the end of irradiation and the hydrolysis experiments were widely different and were such that the specific activity of specimens irradiated to 10 per cent burn-up was sometimes lower than that of the specimens at lower burn-up. The experiments show that the yields of hydrocarbon were strictly dependent upon burn-up, and no correlation with specific activity existed. An explanation must be sought therefore in the structural changes brought about by irradiation. These could arise either from the displacement of lattice atoms or from the injection of foreign fission product species. Which of these effects is important cannot be resolved on present evidence. The influence of temperature in reducing the effects of irradiation can be interpreted either as an annealing effect resulting in a lowering of the irradiation damage or as a thermal effect tending to segregate and hence reduce the influence of fission products that are insoluble in the host lattice. The effect of irradiation damage is perhaps a more likely explanation, since the hydrolysis products are influenced by the relative positions of the carbon atoms which are likely to be changed by irradiation. Throughout all the experiments, the virtual absence of acetylene as a hydrolysis product is of interest. The atomic concentrations of the principal components Of uranium-plutonium carbide irradiated to 10 per cent burn-up have been calculated by Wait[14]. The principal metallic constituents expressed in groups are given in the table below:
Hydrolysis of irradiated (U, Pu)C
3445
Table 4. Composition of uranium-plutonium carbide at 10 per cent burn-up in atom % Uranium and plutonium Alkali metals Alkaline earths Yttrium and rare earths Zirconium
43.0 1-0 0.5 2-1 0.9
Niobium Molybdenum Technetium Platinummetals
0.03 1.0 0-3 1.7
At this level of irradiation, fission product species are present in significant amounts, and if they are able to form discrete carbide phases it should be possible to identify their characteristic hydrolysis products. T h e carbides of the transition metals are refractory and inert to water but the alkaline earth and rare earth carbides, either as dicarbide or sesquicarbide, react readily with water to yield acetylene as the principal hydrolysis product[15-17]. T h e absence of acetylene in all these experiments suggests that these carbide phases are not being formed and that the fission products which are soluble are incorporated in the monocarbide lattice as a solid solution. Constitutional studies of uraniumplutonium carbide and fission product species by Potter[18] have shown that the rare earth carbides are completely soluble in the monocarbide lattice at least up to the concentration present at 10 per cent burn-up. Zirconium is similarly soluble and, although nothing is known of the behaviour of the alkaline earths, our experiments indicate that they too must be in solid solution with the monocarbide phase. Although the alkali metal carbides would yield acetylene on hydrolysis [17], they would not be expected to form under these conditions of irradiation. CONCLUSIONS Unirradiated uranium-plutonium carbide is readily hydrolysed in water and hydrochloric acid at 80°C. T h e principal hydrolysis product is methane; other higher alkanes and alkenes are formed in minor yield. It is not possible to recover all the initial carbon in the gas phase due to the formation of a nonvolatile hydrocarbon residue. U p to 86 per cent of the carbon may be recovered from the monocarbide; the presence of a sesquicarbide phase reduces the recovery figure still further. Uranium-plutonium carbide hydrolyses in a similar manner to that reported for uranium carbide, but differences exist in the lower yield of hydrocarbons and the decreased yield of hydrogen from the plutonium-containing system. T h e latter behaviour is thought to be associated with the valencies adopted in the final hydrolysis solution. N e u t r o n irradiation profoundly changes the hydrolytic behaviour of the carbides, and these effects extend beyond the proportion of the carbide phase 14. 15. 16. 17.
E. Wait, Private communicationat A.E.R.E. F.H. Pollard, G. Nickless and S. Evered, J. Chromat. 15, 211 (1964). G.J. Palenik andJ. C. Waft, lnorg. Chem. 1,345 (1962). H.J. Emeleus and J. S. Anderson, Modern Aspects of Inorganic Chemistry, Routledge & Kegan Paul (1963). 18. P. E. Potter, Private Communicationat A.E.R.E.
3446
J.R.
F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS
that undergoes fission. The irradiated material is inert to water at 80°C, but it is still hydrolysed readily by hydrochloric acid. The yield of methane from acid hydrolysis decreases with increasing burn-up but that of hydrogen is increased. These effects are diminished by a higher irradiation temperature or by postirradiation annealing, probably due to decreased irradiation damage or to increased segregation of insoluble fission products. The absence of acetylene in the hydrolysis products throughout the experiments suggests that the alkaline earth and rare earth carbides in the irradiated carbide are accommodated in the monocarbide lattice and do not form discrete phases of higher carbides. Acknowledgement-The authors wish to thank Mr. J. W. Isaacs, Process Technology Division, A.E.R.E., for his help in preparing the carbide samples.
THE HYDROLYSIS OF URANIUM-PLUTONIUM CARBIDE AFTER NEUTRON IRRADIATION HIGH BURN-UP
TO
J. R. F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS* Applied Chemistry Division, Atomic Energy Research Establishment, Harwell, Didcot, Berks., England
(Received 20 February 1969) Abstract-The hydrolytic reactions of uranium-plutonium carbide have been studied with the unirradiated material and with material irradiated with neutrons up to 10 atom per cent burn-up of the heavy metal atoms. The unirradiated carbide is hydrolysed readily by water and hydrochloric acid and yields hydrogen and methane with higher alkanes and alkenes in minor yield. Up to 86 per cent of the original carbon is recovered in the gas phase from the monocarbide; lower recoveries are found for preparations containing sesquicarbide. Neutron irradiation profoundly changes the hydrolytic properties of the carbide. The material becomes inert to water at 80°C, although still reacting readily with hydrochloric acid. The yield of methane from acid hydrolysis decreases with increasing burn-up; irradiation at high temperature or post-irradiation heating reduces the magnitude of this effect. The absence of acetylene among the reaction products is consistent with the assumption that alkaline earth and rare earth fission products are accommodated in the monocarbide lattice as a solid solution. INTRODUCTION
TIlE HYDROLYTIC reactions of uranium carbide have been studied in detail by several workers[I-5]. The reaction is important technologically because of interest in carbide as a nuclear reactor fuel, but in this context some areas of importance have received little attention. Uranium-plutonium carbide is an important reactor fuel whose hydrolytic properties are little known. Furthermore, the effect of neutron irradiation on carbide hydrolysis has not been examined after extensive transmutation of the fissile atoms. The hydrolytic behaviour of uranium-plutonium carbide has been examined in the unirradiated state at this laboratory by Evered et al.[6]; further extensions to higher plutonium contents are reported in the present work. The effect of neutron irradiation has been investigated at low irradiation levels. Bradley e t al. have studied the hydrolysis of uranium carbide irradiated to 1.6 per cent burnupt [7] and Evered included experiments on uranium-plutonium carbide irradiated *Present address: c/o Southampton University, Southampton, England. t 1 per cent bum-up is defined throughout as being that burn-up achieved when 1 per cent of the heavy metal atoms initially present are destroyed by fission. 1. M.J. Bradley and L. M. Ferris, Inorg. Chem. 1,683 (1963). 2. M.J. Bradley and L. M. Ferris, lnorg. Chem. 3, 750 (1964). 3. C. P. Kempter, J. less-common Metals 4, 419 (1962). 4. J. Spitz, Rep. Commissariat a l' energie atomique, CEA-2350 (1964). 5. F. H. Pollard, G. Nickless and S. Evered, J. Chromat. 15, 223 (1964). 6. S. Evered, M. J. Moreton-Smith and R. G. Sowden, J. inorg, nucl. Chem. 27, 1867 (1965). 7. M. J. Bradley, J. H. Goode, L. M. Ferris, J. R. Flanary and J. W. Ullman, Nucl. Sci. Eng. 21, 159 (1965). 3437
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J.R.
F I N D L A Y , M. J. MORETON-SMITH and R. MOSS
to 2.0 per cent burn-up [6]. In the present work the effect of neutron irradiation on uranium-plutonium carbide has been investigated up to 10 per cent burn-up; some results have been obtained also on the effect of irradiation temperature. EXPERIMENTAL
Specimen preparation and irradiation Uranium oxide containing 30 mole per cent plutonium oxide was converted to carbide by reduction with carbon at 1600°C under vacuum. The resultant product was ball milled to a fine powder and cold pressed into pellets 1.25 mm diameter by 0.6 -+ 0.1 mm length which were sintered at 1600°C for several hours under argon to a density greater than 95 per cent theoretical. Two batches of pellets A and B of differing carbon contents were prepared, and were characterised as detailed below in Table I. A third material C was obtained from a relatively large specimen of irradiated carbide; the analyses of the initial carbide preparation are given. Table 1. Characterisation of uranium-plutonium carbide Composition (Weight %) Material
Microstructure U
Pu
C
O
Carbon equivalent C+O
A
66.7
28.5
4.5
0-34
4.75
B
66-3
28.4
4.9
0-45
5.24
C
80.8
14.3
4.6
0.3
4.82
Monocarbide + trace metal Monocarbide + sesquicarbide Single phase monocarbide
Pellets of materials A and B were irradiated for various periods upto 10 per cent burn-up and at temperatures of either 350 or 1050°C. Irradiations at 350°C were performed on material A in a flux of 1014 neutrons cm -2 sec -1 in a materials testing reactor. Pellets were placed between copper discs to aid heat dispersal and were sealed in metal cans under a helium atmosphere. For irradiations at 1050°C the specimens were contained in molybdenum and were irradiated in a flux of 3 x 10 TM neutrons cm -2 sec -1 using a reactor position containing an electrically heated high temperature furnace[8]. Material C was irradiated to 5.3 per cent burn-up at a temperature of 1200°C: Temperatures in the vicinity of the specimens were measured directly by thermocouples, but the temperatures of the specimens themselves were higher due to the generation of fission heat. A correction for this effect is included in the values quoted but, due to uncertainties of the calculation, the temperatures are subject to an overall uncertainty of_+ 30°C. In the case of the larger specimen C, the uncertainties are much greater and the error can not be reduced below + 200°C.
Experimental method The presence of plutonium and the high levels of radioactivity induced in the irradiated specimens imposed restrictions upon the experimental methods and led to the decision to use only small specimens. The experiments on unirradiated material were performed in an argon atmosphere glove box where operations were performed manually. The irradiated material, however, could only be handled remotely and experiments were conducted in a lead-shielded cell, which also had a purified argon atmosphere. The hydrolysis apparatus is shown in Fig. 1. It consists of a glass reaction tube fitted with a capillary side-arm connected via polythene tube and a silica wool filter to two 10 ml syringes. In the top of the apparatus, an extended ground glass cone, sealed above by a ground glass joint and pulled 8. J. R. Findlay, F. Mason, E. Wait and G. S. Wright, Nucl. Energy, 130 (1968).
Hydrolysis of irradiated (U, Pu)C
• f/.
HYDROLYSIS FLUID
GROUND GLASS
~/
/ SILICA WOOL FILTER BREAK
HYDROLYSIS SAMPLE _
~
3439
POLYTHENE CAPILLARY TO SYRINGES
SEAL
dr3
_
HEATER Fig. 1. Hydrolysis apparatus. off to a breaker point below, contained the hydrolysing fluid and was fitted into the uppermost ground glass joint. The apparatus, less syringes, had a volume of 5 ml and was set in a copper block containing a heater and a thermocouple. The specimen was contacted with the hydrolysing fluid by fracturing the breaker point; as the reaction proceeded the gaseous products of hydrolysis were drawn into the two syringes, which were filled simultaneously to provide two equivalent samples. Their volume was such that several volumes of gas could be swept through the apparatus to ensure complete collection of the reaction products. Finally, the syringes were detached and their contents analysed by gas chromatographic techniques. Hydrogen and methane were determined from one syringe using an alumina column and a thermal conductivity detector under an argon carrier gas; methane and higher hydrocarbons were determined from the other syringe using an alumina column under helium and a flame ionisation detector. Hydrocarbons from C1 to C3 were determined individually; those from C4 to Ca were not completely separated and were assessed in groups. RESULTS
Water hydrolysis The rate of hydrolysis of control and irradiated material was determined in water at 80°C. Preparations o f the unirradiated carbide were hydrolysed rapidly with c o m p l e t e reaction within 4 hr; the products o f hydrolysis are given in Table 2. N o differences in rate were observed b e t w e e n the materials A and B, nor was any difference detectable on c o m p a r i s o n with earlier results on uranium carbide or (U0.9, Pu0.1)C [6]. T h e behaviour o f irradiated material w a s very different. Material A irradiated to 10 per cent burn-up at 350°C was essentially inert to water and had reacted to less than 0.01 per cent after 24 hr at 80°C. Material B irradiated to 3 per cent burn-up at 1050°C was similarly unreactive.
Hydrochloric acid hydro&sis The reaction of both control and irradiated material with 4N hydrochloric acid at 80°C was rapid, being complete within a f e w hours in all cases. A s a
3440
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F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS
t9
_0 t9 t.
p
,A r~
.o e~
o
o V,:.i _o
t~
e*
-6.~ .t .o ot .
3441
Hydrolysis of irradiated (U, Pu)C
result, this reaction was studied extensively to compare the hydrolysis products of control and irradiated material; the analytical results are given in Table 2 for the various experimental conditions employed. The yields of hydrogen and gaseous hydrocarbons are quoted in ml of gas (S.T.P.) per gram of carbide specimen hydrolysed. The cumulative yields of both species are then expressed as a percentage of the theoretical yield, on the assumption that uranium becomes fourvalent and plutonium three-valent in the solution after hydrolysis; allowance is made also for metal already combined with the initial oxygen impurity. The carbon and hydrogen not recovered are assumed to remain as a non-volatile wax or as a wax and free carbon. The carbon content of the residue expressed as a percentage of the total carbon is given in Table 2. The existence of a waxy residue was demonstrated in some instances by extraction with petroleum ether and subsequent evaporation. Due to the small size of the hydrolysis sample, the amounts of residue were very small and could not be estimated quantitatively. DISCUSSION
Unirradiated carbide The results in Table 2 show no significant differences between the products from water or acid hydrolysis. Experiments on uranium monocarbide and sesquicarbide by Bradley-Sears[9] also indicate little difference between the two media. The measurements of the rate of hydrolysis of uranium-plutonium carbide were rather imprecise, but were in qualitative agreement with those reported by other workers for uranium carbide and therefore suggest that no significant rate effect exists that can be attributed to the presence of plutonium. There are changes in the products of hydrolysis however, and the plutonium-containing carbides yield less hydrogen (see Table 3). Table 3. Yields of free hydrogen ml g-1 UC Ref. [6] Water HC1
15. l --
(Uo.gPUo.1)C (Uo.rPuo,a)C (Uo.TPUo.3)C Ref. [6] A B 10.8 11.0
7-0 4-0
-4.8
This can be attributed to the final oxidation states of the cations in solution. After the hydrolysis of uranium carbide by water or hydrochloric acid, uranium was found by several workers to exist in the IV-valent state. Although in our experiments no analysis of the final solutions was possible, their colour suggested that plutonium was in the III-valent state; this is supported by the work of Drummond et al., who found that plutonium carbide when hydrolysed by water yielded plutonium (III) hydroxide[10]. Compared with 100 per cent uranium carbide, the hydrolysis of carbide containing 10 per cent and 30 per cent plutonium to give 9. M. Bradley-Sears and L. M. Ferris,J. inorg, nucl. Chem. 29, 1548 0967). 10. J. L. Drummond, B. J. MacDonald, M. M. Ockenden and G. A. Welch, J. chem. Soc. 4785 (1957).
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F I N D L A Y , M. J. M O R E T O N - S M I T H and R. MOSS
uranium (IV) and plutonium (III) ions results in a reduction of the available hydrogen by 5-5 and 16 ml (STP) respectively. The volume changes observed in Table 3 are slightly lower than these predictions, perhaps due to a small increase in the hydrogen yield from the increased hydrocarbon condensation occurring in the presence of plutonium. Other evidence of the dependence of hydrogen yield upon valency has been obtained by Bradley-Sears et al., who found increased yields of hydrogen from the hydrolysis of uranium carbide by 2 to 18 M N a O H , where the final solution contained uranium (VI) ions [11]. The composition of the gaseous hydrocarbons has been the subject of much conjecture concerning the mechanism of the reaction and its relation to the carbide structure. It is generally accepted that when the carbon atoms exist singly in the carbide lattice surrounded by metal atoms, as they do in uranium monocarbide, the principal hydrolysis product is methane. The methane is accompanied, however, in all cases by higher hydrocarbons, which probably arise from condensation of hydrocarbon free radicals. In the sesquicarbide or dicarbide the hydrolysis products are more complex and depend upon the carbon-carbon bond length. In uranium dicarbide and sesquicarbide the carboncarbon distance approximates to that of a double bond and the hydrolysis products are principally alkanes, with some alkenes. The bond length in calcium carbide is equal to a triple bond, giving acetylene almost quantitatively; in the rare earth carbides the bonds are of intermediate length and several hydrocarbons are formed, with acetylene predominating. The carbides used in this investigation were composed principally of the monocarbide phase. Preparations A and C were essentially single phase, but preparation B contained approximately 15 per cent of second phase sesquicarbide. The principal hydrolysis product in all cases was methane. Comparison with previous studies on uranium carbide indicated that there were no significant differences in the yield of ethane, ethylene and higher hydrocarbons that could be attributed to the presence of plutonium. However, in the case of the singlephase preparation A the yields of methane were lower than those from uranium carbide and, as seen from Table 2, the carbon originally present was not recovered quantitatively from the gas phase. Bradley and Ferris[1], and Spitz[4] found a quantitative recovery of carbon from uranium monocarbide, and it is concluded that the presence of plutonium precludes the complete recovery of carbon from t h e monocarbide, probably by the formation of non-volatile condensation products. The hydrolysis products from preparation B, incorporating a sesquicarbide phase, contained less methane than the corresponding monocarbide preparation and showed an increase in the residual carbon. The hydrolysis of uranium sesquicarbideuranium monocarbide mixtures has been studied by Bradley and Ferris [2], who report an increased formation of wax and hydrogen at the expense of the methane yield as the sesquicarbide fraction is increased. This was accompanied also by some increase in the yield of higher gaseous hydrocarbons. These observations are consistent with ours on the carbon-rich uranium-plutonium carbide, except that the increase in hydrogen yield was small and no increase in the yield of the C~-C8 hydrocarbons could be detected. 11. M.J. Bradley-Sears, M. D. Pattengill and L. M. Ferris,J. inorg, nucl. Chem. 30, 2111 (1968).
Hydrolysis of irradiated (U, Pu)C
3443
Irradiated carbide: hydrolysis by water Irradiated carbide is essentailly inert to water at 80°C, whilst reaction proceeds readily in 4N hydrochloric acid at the same temperature. Similar findings for irradiated uranium carbide are reported by Bradley et a/.[7]; they found also that the material is inert to caustic soda and that the rate of reaction in hydrochloric acid is dependent upon the acid concentration. This behaviour differs fundamentally from that of the unirradiated control material, which is hydrolysed readily in all three media. The reasons for these changes in reactivity remain obscure but some discussion of them is worthwhile. The inactivity of the irradiated material could arise from spurious oxidation of the carbide during irradiation. Oxidation might result in a surface film which would protect the carbide from attack by water whilst permitting reaction with acid. Oxidation of the bulk carbide may be ruled out, since hydrolysis in hydrochloric acid liberates an extensive yield of hydrogen which would not be formed if much of the specimen existed as oxide. The effects of surface oxidation have been studied by Bradley and Ferris by abrading irradiated carbide in contact with water [7]. They demonstrated that the material is inert even when the surface has been freshly exposed. From this evidence it is concluded that the passivity of irradiated carbide is not due to spurious oxidation. The possibility still remains, however, that the reaction of water with irradiated material is inhibited by a coherent film of oxide product. The work of Colby [ 12], and the ready hydrolysis of the unirradiated carbide by acid, alkali and neutral media suggest that the attacking species is the water molecule; a coherent film of oxide product would thus be impenetrable to water. Dell has explained the different hydrolysis rates of uranium and plutonium nitride by the adherence of the oxide film, which in uranium is structurally compatible with an underlying layer of a higher nitride[13]. Plutonium does not form the higher nitride and the oxide is not adherent. In the case of carbides however, it is not obvious why the layer should be coherent only on irradiated carbide, and in the absence of further evidence this hypothesis can be adv~inced only tentatively. Other reasons that could account for the inertness of the irradiated carbide are associated with the presence of the fission products. Evered et al. investigated the influence of internal fl radiation upon the reaction by hydrolysing uranium carbide ground and sintered with 17°thulium oxide of high specific activity [6]. No inhibiting action was observed, but the specific activity of the thulium oxide was a factor of ten lower than that expected from uranium carbide specimens irradiated to 1 per cent burn-up even when the cooling period is prolonged. However, even at the rather low levels of/3 radiation that were employed, the complete absence of any effect implies that/3 radiation is unimportant. A chemical effect could be exerted by the fission product atoms themselves, either segregated at boundaries or dispersed homogeneously throughout the bulk. At 1.0 per cent burn-up, where effects are first seen, sufficient fission product atoms are produced to cover the grain boundary area, but if inhibition resulted 12. L.J. Colby., Jr, J. less-common Metals, 10, 429 (1966). 13. N . J . Bridger, R. M. Dell and V. J. Wheeler, 6th Intern. Syrup. Reactivity in Solids. Schenectady, New York (1968).
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J.R. FINDLAY, M. J. MORETON-SMITH and R. MOSS
from this cause, some reaction would be expected on grinding, when some grain fracture must occur. Fission product effects must be restricted therefore to their presence in the bulk. At the level of 1 per cent burn-up individual fission products are separated on a surface by several atom spacings and at this low concentration it is difficult to conceive how they produce such a drastic inhibition of the hydrolysis reaction.
Irradiated carbide: hydrolysis by acid There are significaft differences in the products of hydrolysis which are related to burn-up and irradiation temperature. The yield of methane from specimens irradiated at 350°C decreases with increasing burn-up whilst the yield of free hydrogen increases. The yields of other gaseous hydrocarbons are unchanged, but the lower recoveries of carbon in the gas phase suggest an increased formation of wax residues. The increased yield of hydrogen is consistent with the formation of the non-volatile hydrocarbon residues by condensation reactions, which probably have a free radical origin. Increasing the irradiation temperature lessens the effects of irradiation; materials A and B irradiated to 3.0 p e r c e n t burn-up and material C at 5.3 per cent burn-up exhibit higher methane and lower hydrogen yields than was the case in the comparable low temperature irradiations. The results in Table 2 demonstrate that the effects of irradiation can be influenced also by post-irradiation annealing. A specimen of preparation A, irradiated at 350°C to 10 per cent burn-up, was annealed for 4 hr at 1500°C, and on hydrolysis gave yields of hydrogen and methane comparable to those experienced at 3 per cent burn-up at an irradiation temperature of 1050°C. It can be shown clearly that this behaviour is not attributable to an effect of irradiation field. The decay times between the end of irradiation and the hydrolysis experiments were widely different and were such that the specific activity of specimens irradiated to 10 per cent burn-up was sometimes lower than that of the specimens at lower burn-up. The experiments show that the yields of hydrocarbon were strictly dependent upon burn-up, and no correlation with specific activity existed. An explanation must be sought therefore in the structural changes brought about by irradiation. These could arise either from the displacement of lattice atoms or from the injection of foreign fission product species. Which of these effects is important cannot be resolved on present evidence. The influence of temperature in reducing the effects of irradiation can be interpreted either as an annealing effect resulting in a lowering of the irradiation damage or as a thermal effect tending to segregate and hence reduce the influence of fission products that are insoluble in the host lattice. The effect of irradiation damage is perhaps a more likely explanation, since the hydrolysis products are influenced by the relative positions of the carbon atoms which are likely to be changed by irradiation. Throughout all the experiments, the virtual absence of acetylene as a hydrolysis product is of interest. The atomic concentrations of the principal components Of uranium-plutonium carbide irradiated to 10 per cent burn-up have been calculated by Wait[14]. The principal metallic constituents expressed in groups are given in the table below:
Hydrolysis of irradiated (U, Pu)C
3445
Table 4. Composition of uranium-plutonium carbide at 10 per cent burn-up in atom % Uranium and plutonium Alkali metals Alkaline earths Yttrium and rare earths Zirconium
43.0 1-0 0.5 2-1 0.9
Niobium Molybdenum Technetium Platinummetals
0.03 1.0 0-3 1.7
At this level of irradiation, fission product species are present in significant amounts, and if they are able to form discrete carbide phases it should be possible to identify their characteristic hydrolysis products. T h e carbides of the transition metals are refractory and inert to water but the alkaline earth and rare earth carbides, either as dicarbide or sesquicarbide, react readily with water to yield acetylene as the principal hydrolysis product[15-17]. T h e absence of acetylene in all these experiments suggests that these carbide phases are not being formed and that the fission products which are soluble are incorporated in the monocarbide lattice as a solid solution. Constitutional studies of uraniumplutonium carbide and fission product species by Potter[18] have shown that the rare earth carbides are completely soluble in the monocarbide lattice at least up to the concentration present at 10 per cent burn-up. Zirconium is similarly soluble and, although nothing is known of the behaviour of the alkaline earths, our experiments indicate that they too must be in solid solution with the monocarbide phase. Although the alkali metal carbides would yield acetylene on hydrolysis [17], they would not be expected to form under these conditions of irradiation. CONCLUSIONS Unirradiated uranium-plutonium carbide is readily hydrolysed in water and hydrochloric acid at 80°C. T h e principal hydrolysis product is methane; other higher alkanes and alkenes are formed in minor yield. It is not possible to recover all the initial carbon in the gas phase due to the formation of a nonvolatile hydrocarbon residue. U p to 86 per cent of the carbon may be recovered from the monocarbide; the presence of a sesquicarbide phase reduces the recovery figure still further. Uranium-plutonium carbide hydrolyses in a similar manner to that reported for uranium carbide, but differences exist in the lower yield of hydrocarbons and the decreased yield of hydrogen from the plutonium-containing system. T h e latter behaviour is thought to be associated with the valencies adopted in the final hydrolysis solution. N e u t r o n irradiation profoundly changes the hydrolytic behaviour of the carbides, and these effects extend beyond the proportion of the carbide phase 14. 15. 16. 17.
E. Wait, Private communicationat A.E.R.E. F.H. Pollard, G. Nickless and S. Evered, J. Chromat. 15, 211 (1964). G.J. Palenik andJ. C. Waft, lnorg. Chem. 1,345 (1962). H.J. Emeleus and J. S. Anderson, Modern Aspects of Inorganic Chemistry, Routledge & Kegan Paul (1963). 18. P. E. Potter, Private Communicationat A.E.R.E.
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that undergoes fission. The irradiated material is inert to water at 80°C, but it is still hydrolysed readily by hydrochloric acid. The yield of methane from acid hydrolysis decreases with increasing burn-up but that of hydrogen is increased. These effects are diminished by a higher irradiation temperature or by postirradiation annealing, probably due to decreased irradiation damage or to increased segregation of insoluble fission products. The absence of acetylene in the hydrolysis products throughout the experiments suggests that the alkaline earth and rare earth carbides in the irradiated carbide are accommodated in the monocarbide lattice and do not form discrete phases of higher carbides. Acknowledgement-The authors wish to thank Mr. J. W. Isaacs, Process Technology Division, A.E.R.E., for his help in preparing the carbide samples.