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Some Reactions Between Thorium Oxide And Inhibited Heavy Liquid Metals
J. Nucl. Energy, Part B: Reactor Technology, 1961, Vol. I, pp. 221
to
228. Pergamon Press Ltd. Printed in Northern Ireland
SOME REACTIONS BETWEEN THORIUM OXIDE AND INHIBITED HEAVY LIQUID METALS G. H. BROOMFIELD, J. M. MATTHEWS and A. BARTLETT Atomic Energy Research Establishment, Harwell, Berks. (First received 27 Apri/1960 and in revised form 25 August 1960)
Abstract-As part of a study of liquid metal slurries as blanket materials for liquid metal fuelled reactors, thoria prepared by calcination of the oxalate has been mixed vigorously with inhibited lead, lead-bismuth eutectic and bismuth at temperatures in the range 500-600°C to form slurries containing up to 18 per cent (by volume) solids. Solidified samples have been examined for reactions between the corrosion inhibitors (zirconium and magnesium) and thoria, immediately after preparation and after circulation in thermal convection loops and in a pumped loop. Magnesium at eutectic concentration in lead and bismuth reacted with thoria to produce intermetallic compounds which plugged thermal convection loops. Magnesium at normal inhibitor levels did not reduce thoria to give a thorium concentration in excess of the solubility limit in bismuth at 400°C. Thoria concentrations of up to 7 per cent in bismuth were not found to effect the rheological properties of the liquids in these experiments, nor to alter corrosion inhibition effects by the ZrN film technique. The rheology of lead slurries was influenced principally by thoria settling upwards. Apart from settling effects the slurries were physically stable.
INTRODUCTION
MATERIALS
SOME of the considerations governing the choice of Cans in which slurries were prepared were made slurry blankets for liquid metal fuelled reactors have from mild steel to B.S. 970, En 3a. been outlined in an earlier paper (BROOMFIELD et a!., Loops were made from Samuel Fox Ltd., CML 1959). Such blankets may contain 238 U as the fertile quality steel tube, the batch analysis of which was: source of 239 Pu; or 232 Th as a source of 232 U. The early work in this field was concerned with the suitacarbon 0·095% bility of suspensions of the fertile elements as intermanganese 0·525% metallic compounds in lead and in bismuth. These silicon 0·245% slurries were shown to be unsuitable due to the small chromium 0·9% solubility, varying with temperature, of the fertile nickel 0·12% elements in the liquid metals which led to plugging of molybdenum 0·64% thermal convection systems and pumped systems sulphur 0·034% (HAWES eta!., 1958) and also to segregation in closed phosphorus 0·026%. can experiments both out-of-pile (GREENWOOD and SHARPE, 1956) and in-pile (SHARPE, 1957). Bismuth was pharmaceutical grade material with a Any tendency for fertile material, or mixture of fertile and fissile material to segregation, or other purity of better than 99·98 per cent and further refined uncontrolled alteration in concentration in the by bubbling hydrogen through and filtering with a blanket or cooling system of a reactor is patently No. 2 glass filter, to reduce chloride and remove intolerable from the aspects of reactor control and undissolved oxide (HORSLEY, 1957). Lead was Tadanac brand with a purity of 99·99 per safety, consequently later studies have been directed towards slurries of fertile element compounds that cent; it was further refined in the same way as the may be more stable in heavy liquid metals containing bismuth. Zirconium was approximately 97 per cent pure with steel corrosion inhibitors, zirconium and magnesium (WEEKS et a!., 1955). The present paper is concerned hafnium, iron and carbon as principal impurities. Magnesium, supplied by Magnesium Electron Ltd. with preparation of slurries of thoria in lead, bismuth, and lead-bismuth alloys and observations on reactions was 99·95 per cent pure. Thorium oxide was prepared by Thorium Ltd., by between thoria and inhibited liquid metals in small precipitating and calcining the oxalate. Its particle loop experiments. 221
222
G. H.
BROOMFIELD,
J.
M. MATTHEWS
shape was cubic, a pseudomorph of the oxalate, and its size range was typically 100 % less than 30 t-t 78 % less than 20 t-t 58% less than 15 t-t 36% less than 10 f1, 21% less than 5 t-t 9 % less than 2 f1, with a crystallite size of approximately 700 A (BROWN and CHITTY, 1960). Impurities were principally chloride ( <1000 p.p.m.), sulphate (450-1500 p.p.m.) and carbon compounds giving a total loss on ignition of 0·1 per cent. The highest metal impurity was calcium at 160 p.p.m. EXPERIMENTAL
and A.
BARTLETT
30
HEATER
~ENGTH
Experiments carried out may be divided into three groups: slurry preparation, thermal convection loop T C. 3. circulation of slurries, and the operation of a forced TC =THERMOCOUPLE. circulation loop. FIG. I.-Assembly of thermal convection loop with filling can and permanent magnet. In the slurry preparation experiments approximately 80 grammes of materials for each slurry sample were at approximately outgassed at 150°C in small mild steel or silica bismuth-thoria slurry was circulated of 150°C for gradient temperature a through em/sec 28 containers, provided with an argon atmosphere, sealed pump and loop, the of and shaken at a rate of 1000 strokes per minute 2730 hours. The arrangement preslurry of type the and device sampling pot, head through an amplitude of 1t" for 2 hours at 500 or used tube steel The 2. Fig. in shown are can paration 600°C. The resulting slurries were subjected to for the loop and pump channel was of the same commetallographic examination. loops, but the Thermal convection loop experiments were made by position as that used for the convection by heating to use before nitrided was loop assembled preparing a slurry in a similar manner to that desfor four through ammonia passing and 500°C to 400 cribed above, running the slurry into a small CML in the prepared was loop this for slurry The hours. steel loop under vacuum, and allowing the loop experiments previous the for used that as manner same contents to circulate under convection for a predetermined period or until the loops plugged. In two and the loop was evacuated to lOt-t Hg pressure for head pot was about instances convection circulation proved difficult to filling. The depth of slurry in the filled with purified normally was space free the and em 5 initiate and a simple electromagnetic pumping arwhen a sample evacuated was space free The argon. rangement was provided. The thermal convection of liquid metal layers surface the when and taken was loop assembly with magnet and connexions for E.M. A complete stream. flowing the into plunged were pumping is shown in Fig. 1. be found in can accessories and loop the of description Chromel-alumel thermocouples were spot welded and (BROOMFIELD subject this on Report A.E.R.E. an to each loop in three positions. Heat input to the 1959). BARTLETT, heaters was controlled by an instrument set to 550°C RESULTS and connected to No.1 thermocouple. Readings from preparation Slurry 1. thermocouples 2 and 3 were recorded and provided an These experiments in slurry preparation extend the indication of the state of circulation in the loops. Measurements of heat transported by the flowing work carried out by CHITTY (1957) in which he showed liquid metal in these loops, and the temperature that thoria produced by calcining the oxalate was not gradient around the loops showed that the rate of flow readily wetted by bismuth unless an element capable of of the bismuth slurries was approximately 1·5 em/sec. reducing the thoria was dissolved in the bismuth. It The rate of flow of the lead slurries was probably about was found that provided thoria is outgassed at a temperature of 150°C until a vacuum of about 20 t-t Hg half this value. The thermal convection loop experiments were obtains, and the oxide and metal are sufficiently well followed by the pumped loop in which an inhibited agitated then iron dissolving in the bismuth from the
Some reactions between thorium oxide and inhibited heavy liquid metals
SAMPLER-----.JI
SLURRYj CAN.
SLUICE VALVE
JOINTING FLANGE
FLANGE JOINT
-VACUUM
TC.1
TC.7.
PUMP
TC.b.
TC.
=
the solidified slurries showed that the cubic particles were to some extent broken down. This breakdown was more marked in slurries containing magnesium and calcium than in those made in an iron container, as shown in Fig. 3(a, band c). The structures of solidified slurries of thoria in lead and in lead-bismuth eutectic, as indicated by light microscopy, were similar to those ofthoria in bismuth, but preparation of slurries in lead in an iron container necessitated periods of agitation of up to 8 hours before satisfactory wetting occurred, indicating a considerable difference in interface energies in the thoriabismuth and thoria-lead systems. In nine experiments where wetting did not appear to have occurred microscopical examination and radiation counting of washed beads of metal showed them to be free from thoria. A further experiment to examine slurry stability was made by preparing a slurry of 10 per cent wt. thoria in bismuth containing 0·05 per cent wt. magnesium and and 0·025 per cent wt. zirconium and maintaining it at S00°C for 2160 hours under vacuum. The slurry-free surface had a bright metallic appearance after this time and its micro-structure when solidified was similar to that of freshly prepared slurries of similar composition.
THERMOCOUPLE
M
1FT SCALE FIG.
223
2(a).-Arrangement of loop, pump and head pot.
APPROX. FULL SIZE.
FIG. 2(b).-Arrangement of lower end of sampler.
steel container enables wetting to take place. If a silica container is used some other addition to the bismuth is necessary. Slurries containing up to 18 per cent Th0 2 were made with the addition of 1 per cent magnesium, with no indication that the concentration of thoria could not be considerably exceeded. Microscopical examination of prepared sections of
2. Thermal convection loops The thermal convection loop experiments were designed to show whether the changing thermal conditions that may be encountered in a reactor would promote reactions between the constituents of the slurries to produce detectable products. The maximum temperature chosen, 550°C is that proposed for some reactor systems (HosEGOOD and CHILTON, 1956) whilst the temperature gradient in the loops was approximately similar to that proposed for the reactor heat exchanger cycles. Principal details of the experiments are given in Table 1 and the lower loop temperature variations are shown in Fig. 4. The results showed that tho ria,in a slurry containing magnesium at the probable maximum concentration for corrosion inhibition purposes, is not reduced in sufficient quantity for any indication to be found of thorium in excess of its solubility in the liquid metal. Higher concentrations of magnesium reduced sufficient thoria to form intermetallic compounds of thorium and the liquid metal. Together with a careful search for products of reactions between the thoria and inhibited liquid metals, microscopy of the loops included an examination of the steel tubes for corrosion and mass transfer
G. H. BROOMFIELD, J. M. MATTHEWS and A. BARTLETT
224
TABLE 1.-THERMAL CONVECTION LOOPS Loop
Slurry composition
Circulation time (hr)
Plug
A. Thoriainhibited bismuth
5% Th0 2 , 0·1% Mg, 0·025% Zr. Remainder Bi
1376
B. Thoriamagnesium-bismuth eutectic
5% Th0 2 , 0·54% Mg, 0·025% Zr. Remainder Bi
152
ThBi 2 in cold part of loop
C. Thoriainhibited lead
5% Th0 2 , 0·1% Mg, 0·01% Zr. Remainder Pb
78
Settled Th0 2
D. Thoria-leadmagnesium eutectic
5% Th0 2 , 2·2% Mg, 0·01% Zr. Remainder Pb
89
ThPb 3 in cold part of loop
E. Thoria-
5% Th0 2 , 44·5% Bi, 0·1% Mg, 0·01% Zr. Remainder Pb
2136
inhibited leadbismuth eutectic
2
6
No plug
Settled Th0 2
10,000
hr
FIG. 4.-Variation of lower loop temperature with log time.
No evidence for pitting corrosion was found; whilst the duration of most of the experiments was too short for this to be of major significance, the absence of corrosion in loops A and E indicates that corrosion inhibition by the ZrN film technique may be equally as effective when thoria is present, as in the single phase heavy metal systems. The detailed results of individual experiments are as follows. Loop A. Microscopic examination of prepared sections showed no pitting of the heated tube wall, but some small dendrites of iron were present in the cold part of the loop. Thoria was distributed in a fairly uniform manner in the cold limb of the loop except for segregation due to bismuth crystal growth on solidification, but apart from some particles apparently adhering to the tube wall there was no thoria in the hot limb. It appeared probable that thoria had been filtered out of the bismuth in later stages of circulation by bismuth dendrites
growing in the cold part of the loop, since this region contained a high concentration of thoria in interdendritic positions. Associated with the higher concentration of thoria and the iron dendrites in the cold part of the loop were some particles of a brown coloured phase having the appearance of newly prepared ThBi 2 (BARTON and GREENWOOD, 1957 and FERRO, 1957) and other particles having the more characteristic appearance of rapidly oxidized ThBi 2 (Fig. 5). The brown particles did not oxidize over a long period, hence it may have been some material as yet unidentified, or a fine dispersion of ThBi 2 in bismuth which on oxidation produced oxide particles of insufficient thickness to have the characteristic black appearance. Circulation in the loop did not slow down until the heater power was reduced indicating that there was insufficient iron or thorium in solution to exceed the solubility in the liquid bismuth at the lower loop temperature of about 400°C. Loop B. Crystals of ThBi 2 (identified by the colour and oxidation rate) had grown from the tube wall at the cold point, trapping circulating Th0 2 particles, and as in loop A the bismuth in the heated tube contained little thoria.
Some reactions between thorium oxide and inhibited heavy liquid metals
Loop C. This slurry proved difficult to circulate, and movement could only be maintained under the added impetus of the electromagnetic pumping effect using the arrangement shown in Fig. 1. At the rate of circulation required to give a temperature drop round the loop of less than 50°C (i.e. > 1 em/sec) turbulence was insufficient to maintain the suspension, and thoria settled out in the small angle bend at the top of the loop and on the top side of the lower horizontal section of tube. Microscopic examination of the loop after solidification revealed no product of reaction between the constituents of the slurry, and gave no evidence of steel corrosion. Solidified lead was easily pulled away from the steel tube in the region of the d.c. current contacts; this may have been due to poor wetting, but more probably to solidification contraction of the lead. Loop D. Circulation in this loop started readily under convection forces. As in the loops containing thoria-bismuth slurries the rate of circulation, as indicated by the cold point temperatures, appeared to increase for a number of hours before it began to fall to the freezing point of the slurry. Micro examination showed that although the thoria particles in the slurry had not undergone any detectable change in shape or size, a dendritic growth from the walls in the cold part of the loop had, by trapping thoria, formed a plug. The dendritic growth was probably of ThPb 3 formed by the reaction Th0 2
+ 2Mg + 3Pb --.. ThPb + 2Mg0. 3
Loop E. Circulation of the slurry in this loop did not start under convection forces but started readily under the added impetus of the E.M. pump. After a few hours convection circulation was maintained under the conditions shown in Fig. 4, increasing for 350 hours, and then decreasing in an unsteady fashion until it ceased after 2136 hours. Micro examination of sections of the loop showed that the impediment to circulation was settled thoria. No product of reaction between thoria and the inhibited eutectic was detected.
A common feature of all the convection loop experiments was that there was no break-down of the oxalate pseudomorph thoria particles and no change in particle size after circulation, nor did light or electron microscopy reveal any rounding of particles that could be definitely attributed to loop circulation. 3. The pumped loop
The loop was filled with a slurry of 7 per cent wt. thoria, 0·1 per cent magnesium and 0·5 per cent wt. zirconium, in bismuth, under a vacuum of 10 fl Hg with the loop heated to a minimum temperature of 400oC and the electromagnetic pump power full on. Circulation started immediately on filling and after some hesitation continued with a maximum temperature controlled at 480°C and a minimum of 400°C for five days until analysis reports showed that the inhibitor concentration levels were satisfactory. The loop controls were adjusted on the sixth day to give the operating conditions required for the experimental run. These were: control temperature 550°C, temperature gradient around loop 150°C, flow rate 28·5 em/sec. During the run samples were taken from the stream for analysis for inhibitor and thoria concentration, these are shown in Table 2. Small additions of magnesium and zirconium were
225
made after 1130 hours. Subsequent analysis showed no corresponding increase in inhibitor concentrations, but they did not appear to have been lost by oxidation at the free surface. A reduction of thoria concentration in the slurry of about 1 per cent was expected, due to dilution by bismuth left in the loop from a test run, but analysis of samples 5 and later (taken from the flowing stream) showed reductions in concentration which were proved by the plunging operation, carried out before taking sample 7, to be due to settling towards the free surface. After this period no further settling was detected. As the analysis figures show, lead was an appreciable impurity in the slurry. This was due to lead remaining in the loop after an initial test with lead-bismuth eutectic. After the seventh sampling, circulation was terminated by slowly reducing the pump power whilst maintaining a constant maximum temperature, a process lasting 35 min. An attempt to establish a thermal gradient up to the head pot was made by adjustment of heaters; this was not entirely successful as the pump entry tube was split by solidifying bismuth. During the run and afterward the loop was radiographed using an iridium 192 source of about 0·4 curies at 18 in. source to specimen distance. No changes were detected in the t in. bore tube but a considerable deposit of material with lower density than the slurry was detected in the pump tube. This material was, by micro examination, identified as iron but there appeared to be no pitting corrosion of the heated tube. The iron deposit, both in the pump and elsewhere had grown round thoria particles. Micro examination of the t in. bore tube in the cold point showed further deposits of iron, in the form of a rough layer about 0·3 mm thick. Fourteen sections of the loop, pump tube and head pot were prepared for micro examination. These showed evidence of a small degree of mass transfer without pitting corrosion of the heated tube, and that the thoria in the suspension in the bismuth had not undergone any change of physical form detectable by micro examination. There was, however, a constituent associated with the thoria that had one of the characteristics of ThBi 2 in that it corroded in air, but at a slower rate. This constituent immediately on preparation had a grey colour (ThBi 2 is brown) and resembled, with its irregular but smooth outline, the intermetallic compound Mg 3Bi 2 which would have been expected in an alloy of bismuth with 0·1 per cent wt. magnesium. However, its oxidation behaviour was not typical of Mg 3Bi 2 observed in other magnesium-zirconium inhibited loops. TABLE 2.-ANALYSIS OF CIRCULATING SLURRY
Hours Sample I Th0 2 Mg i Zr Pb I run ! (% wt.) I (% wt.) I (% wt.) (% wt.) number - - - - ______ I 1----·----!---0·085 0·60 0·045 Proving 0·085 0·035 6·4 Proving 2 0·090 0·040 670 3 0·140 0·140 0·040 0·60 815 4 0·120 0·040 1150 3-4 5 0·120 0·042 1250 3-4 6 0·100 5·7 0·039 1415 7 5·7 0·035 0·11 1845 8 5·7 0·035 0·11 1870 9 5·7 0·033 0·13 2255 10 5·7 0·033 0·14 2730 11
226
G. H. BROOMFIELD, J. M. MATTHEWS and A. BARTLETT TABLE 3.-ANALYSIS OF SLURRY AT THE END OF THE EXPERIMENT Position in loop
I Top of slurry in head pot Tube joint level in head pot Bottom of head pot Centre of heated tube
Th0 2 (% wt.)
I
10·9
I I
5·5
0·26 6·3
Further attempts to identify the oxidizing constituent were made by electron microscopy, micro-beam electron diffraction and by use of a scanning electron microscope with X-ray and electron recording. Because of the difficulty of isolating the particles and the type of X-radiation excited from bismuth the only useful information obtained from the employment of these techniques was that the constituent contained no concentration of lead above that of the surrounding slurry. (As stated earlier lead became an appreciable impurity in the slurry since the loop was first tested with lead-bismuth eutectic). As in the small thermal convection loops, thoria had segregated on solidification of the slurry to form intergranular concentrations; it had also settled upward in the head pot during cooling leaving the region below the level of the pipe joints almost free from thoria, except for a layer of thoria particles at the bismuth steel interfaces. The analysis of the solidified slurry from various positions in the loop is given in Table 3; it can be seen that there is a tendency for both magnesium and zirconium to segregate with the thoria. The solubility of iron in bismuth at 550°C is approximately 0·0025 per cent wt. (WEEKS et al., 1955), but the levels found by analysis were considerably in excess of this. No particulate iron was found in the slurry by micro examination. The presence of a zirconium nitride film on the inside of the heated tube was confirmed by electron diffraction. A few tests were carried out to check the physical properties of the steel tube after cooling. The material was ductile in a notched fracture test, had no hardness variations attributable to the nitriding treatment (the nitride case may have diffused to give an almost uniform composition in the time at the running temperature) and the tensile properties were typical of the type of steel with the thermal history of the loop tubing. DISCUSSION
1. Wetting Measurements of the contact angle of bismuth on thoria by the sessile drop method at temperatures up to 500°C have given results of over 90° (FROST et al., 1958), and the first experiments by CHITTY indicated that bismuth would only wet thoria if a metal capable of reducing thoria was present in the bismuth. Results from the present series showed that thoria reduction was not a pre-requisite of wetting, but the presence may be required of an element soluble in bismuth having a free energy of oxide formation greater than bismuth. (Iron appeared to act as a wetting agent, and 0 -1:1G° Fe~ 2 Feo > -1:1G Bi~iBi 2 o 3). This may be partly true, but further work by BROWN and CHITTY (1960) confirmed the existance of a removable barrier to
!
Mg (% wt.)
Zr (% wt.)
Fe (% wt.)
0·190 0·150 0·085 0·140
0·060 0·039 0·020 0·033
0·005 0·018 0·013 0·015
I
wetting and showed that this was carbon dioxide adsorbed on the thoria. The stability of the slurries, on standing and when circulating indicates that in the presence of corrosion inhibitors, and possibly without them, the contact angle of bismuth on thoria is less than 90°. Probably the contact angle measurements made so far have been on incompletely outgassed thoria. The thoria, derived from thorium oxalate as used in these experiments, has a large specific surface and may be suitable for calorimetric experiments to determine the strengths of bonds formed between thoria and pure or alloyed bismuth. 2. Chemical reactions in the slurries Results of the thermal convection loop experiments on slurries of thoria in bismuth-magnesium and eutectic (0·54 per cent wt. Mg) and in lead-magnesium eutectic (2·2 per cent wt. Mg) show that the thorium activity in the liquid metal due to the reaction Th0 2 + 2Mg0 = 2Mg0 + Th (1) at 550°C is sufficient to allow the reactions Th 2Bi = ThBi 2 (2) and Th 3Pb = ThPb 3 to occur at about 400°C. In loop experiments (including the pumped loop) in which magnesium was at a concentration of 0·1 per cent wt. the thorium activity was insufficient to allow formation of the intermetallic compounds. It is clear that the activity of thorium will be depressed considerably by reduction of magnesium content, since the equilibrium constant for reaction (1) [Th] b . . f . . 1 re duces to [Mg] 2 ut estimatiOn o quantitative va ues
+ +
for activities is difficult, because of compound formation in the metal systems and lack of experimental data. However it may be argued that the maximum thorium activity in bismuth, due to the presence of 0·54 per cent wt. Mg is close to that required to allow formation of ThBi 2 at 550°C, in which case the Th concentration in equilibrium with 0·1 per cent wt. Mg would be about 0·0032 per cent wt., assuming Henry's law to hold. This value is considerably below the
Some reactions between thorium oxide and inhibited heavy liquid metals
solubility limit at 400°C (BRYNER and BRODSKY, 1958) but is still well above the level that should have been detected as ThBi 2 crystals formed on cooling of the pumped loop. A possible explanation of the fact that ThBi 2 was not detected is that the MgO formed in reaction (I) was in a fine state of division and allowed the reaction to go to the left at such a rate that the activity of thorium did not become high enough at the lower temperature to allow reaction (2) to occur. An alternative possibility is the presence of a eutectic near to zero thorium in the Bi-Th-Mg system. 3. Irradiation effects Fission effects may constitute a severe test of the stability of these slurries. It may be assumed that several effects with possible deleterious results would occur. (a) Fission energy imparted to a small tho ria particle may be sufficient to free it from a liquid surface, to which it may return after a period as a dust particle in the blanket atmosphere; if such particles do not re-wet, free surfaces will become covered with thoria dust. (b) Neutron or fission fragment-thorium atom collision may detach thorium atoms from the thoria matrix. These detached atoms may go into solution, effectively accelerating the rate of reduction of thoria by corrosion inhibitors. (c) Break-down of thoria particles is to be expected due to fission effects, internal inert gas pressures and changes in structure on transmutation, and this may be sufficient to influence the rheology of a dense slurry. (d) The apparent density of tho ria will change due to occluded inert gas leading to possible segregation difficulties. An irradiation experiment in progress is designed to give some indication of the magnitude of these effects. 4. Steel corrosion Corrosion of the steel by bismuth, bismuth alloys and lead has been inhibited with varying degrees of success in all the experiments. The rate of corrosion did not exceed that which may have been expected in thoria free systems, and zirconium nitride films were identified on steel exposed to inhibited slurry. These results indicate that steel corrosion by a slurry is not necessarily a more difficult problem than corrosion by a solution fuel, but control of inhibitor concentration is more difficult, judging by results obtained with the pumped loop. Normally magnesium and zirconium in circulating bismuth are slowly depleted, the magnesium by oxidation and removal from the liquid
227
by settling out (or de-wetting and adhesion to tube walls etc.), and the zirconium by formation of nitride and possibly oxide at the tube walls; the depletion is revealed by analysis of the liquid. In the case of the pumped slurry loop the zirconium concentration showed a steady downward trend in the later 1500 hours of operation but the magnesium did not. It is certain that some oxidation of the magnesium did occur, consequently it must be assumed that MgO was entrained in the slurry. This is substantiated by the segregation of magnesium with thoria shown in Table 3. In this respect the inhibitor level may be difficult to control; a remedy may be to allow a period of settling of a liquid sample and to determine inhibitor level in the 'clear' portion of the liquid. CONCLUSIONS
1. Slurries of thoria in bismuth or lead with physical stability that may be good enough for reactor purposes can be prepared provided the liquid metal has a suitable wetting agent in solution, and that mixing of the constituents is sufficiently vigorous. Suitable wetting agents are those metals with a free energy of oxide formation appreciably above that of the liquid metal, but not necessarily high enough to reduce thoria. Magnesium and zirconium at concentrations normally used for corrosion inhibition are effective. 2. Magnesium in solution in a lead or bismuth slurry with thoria, at eutectic concentration reduces the thoria, allowing formation of solid intermetallic compound, ThPb 3 or ThBi 2 , but the concentration of thorium in solution does not appear to exceed the solubility at 400°C when the magnesium does not exceed the maximum concentration that is normally used for de-oxidation purposes (0·1 per cent wt. Mg). 3. Within the limitations of the experiments made, the presence of thoria suspended in lead, lead-bismuth eutectic, or bismuth does not effect the inhibition of steel corrosion by the ZrN film technique, nor has mechanical erosion by the slurries been detected. However, the normal analysis methods for control of inhibitors needs some modification due apparently to entrainment of oxidized inhibiting elements in the slurry. 4. It has been demonstrated that a thoria-bismuth slurry can be circulated in an electromagnetically pumped loop for 2370 hours with frequent sampling and without serious erosion or corrosion of the steel. Acknowledgements-The authors' thanks are due to colleagues in the UKAEA for electron diffraction and analysis work, for help and advice, and in particular to Dr. B. R. T. FROST for guidance and encouragement.
G. H. BROOMFIELD, J. M. MATIHEWS and A. BARTLETI
228
REFERENC ES BARTON P. J. and GREENWOOD G. W. (1958) J. Inst. Metal. 86, 12, 504. BROOMFIELD G. H. et al. (1959) J. Nucl. Energy 1, 42. BROOMFIELD G. H. and BARTLETT A. F. (1959) Some Reactions Between Thoria and Inhibited Heavy Liquid Metals. A.E.R.E./ R3131. BROWN A. and CmTTY A. (1960) J. Nucl. Energy Part B: Reactor Tech. 1, 145. BRYMER J. S. and BRODSKY M. B. (1958)Proceedingsof the Second
International Conference on the Peaceful Uses of Atomic Energy, Geneva, Vol. 7, P/460, p. 208. United Nations, N.Y.
CHITTY A. (1957) Unpublished.
FERRO R. (1957) Acta Crystallog. Vol. 10, 7, 476. FROST B. R. T. (1958) Proceedings of the Second International
Conference on the Peaceful Uses of Atomic Energy, Geneva,
P/270. United Nations, N.Y. GREENWOOD G. W. and SHARPE B. (1956) J. Nucl. Energy. 3, 1. HAWES R. 1., HORSLEY G. W. and SHELDON R. (1958) A.E.R.E. M &R/R2529. HoRSLEY G. W. (1957) J. Nucl. Energy 6, 41. HOSEGOOD S. and CHILTON H. (1956) Unpublished. SHARP B. (1957) Unpublished. WEEKS J. R. et al. (1955) Proceedings of the First International
Conference on the Peaceful Uses of Atomic Energy, Geneva,
Vol. 9. P/118, p. 341. United Nations, N.Y.
"'
~
FIG. 3(a).-Bismuth -l per cent calcium 10 per cent thoria slurry showing partly dispersed thoria particles in a matrix of bismuth and bismuth-calcium compound. ( X 150). FIG. 3(b).-Bismuth -l per cent magnesium 10 per cent thoria slurry showing similarly dispersed thoria particles in a hypereutectic bismuth-magne sium structure. ( X 600). FIG. 3(c).-Bismuth -l0 per cent thoria slurry prepared in an iron container, showing almost intact thoria particles in bismuth. ( x 1000).
FIG. 5.- A section of the cold part of loop A, identified as an iron dendrite, particles of tho ria segregated at bismuth grain boundaries rapidly oxidized ThBi 2 and a brown particle with the appearance of unoxidized ThBi 2 •
to
228. Pergamon Press Ltd. Printed in Northern Ireland
SOME REACTIONS BETWEEN THORIUM OXIDE AND INHIBITED HEAVY LIQUID METALS G. H. BROOMFIELD, J. M. MATTHEWS and A. BARTLETT Atomic Energy Research Establishment, Harwell, Berks. (First received 27 Apri/1960 and in revised form 25 August 1960)
Abstract-As part of a study of liquid metal slurries as blanket materials for liquid metal fuelled reactors, thoria prepared by calcination of the oxalate has been mixed vigorously with inhibited lead, lead-bismuth eutectic and bismuth at temperatures in the range 500-600°C to form slurries containing up to 18 per cent (by volume) solids. Solidified samples have been examined for reactions between the corrosion inhibitors (zirconium and magnesium) and thoria, immediately after preparation and after circulation in thermal convection loops and in a pumped loop. Magnesium at eutectic concentration in lead and bismuth reacted with thoria to produce intermetallic compounds which plugged thermal convection loops. Magnesium at normal inhibitor levels did not reduce thoria to give a thorium concentration in excess of the solubility limit in bismuth at 400°C. Thoria concentrations of up to 7 per cent in bismuth were not found to effect the rheological properties of the liquids in these experiments, nor to alter corrosion inhibition effects by the ZrN film technique. The rheology of lead slurries was influenced principally by thoria settling upwards. Apart from settling effects the slurries were physically stable.
INTRODUCTION
MATERIALS
SOME of the considerations governing the choice of Cans in which slurries were prepared were made slurry blankets for liquid metal fuelled reactors have from mild steel to B.S. 970, En 3a. been outlined in an earlier paper (BROOMFIELD et a!., Loops were made from Samuel Fox Ltd., CML 1959). Such blankets may contain 238 U as the fertile quality steel tube, the batch analysis of which was: source of 239 Pu; or 232 Th as a source of 232 U. The early work in this field was concerned with the suitacarbon 0·095% bility of suspensions of the fertile elements as intermanganese 0·525% metallic compounds in lead and in bismuth. These silicon 0·245% slurries were shown to be unsuitable due to the small chromium 0·9% solubility, varying with temperature, of the fertile nickel 0·12% elements in the liquid metals which led to plugging of molybdenum 0·64% thermal convection systems and pumped systems sulphur 0·034% (HAWES eta!., 1958) and also to segregation in closed phosphorus 0·026%. can experiments both out-of-pile (GREENWOOD and SHARPE, 1956) and in-pile (SHARPE, 1957). Bismuth was pharmaceutical grade material with a Any tendency for fertile material, or mixture of fertile and fissile material to segregation, or other purity of better than 99·98 per cent and further refined uncontrolled alteration in concentration in the by bubbling hydrogen through and filtering with a blanket or cooling system of a reactor is patently No. 2 glass filter, to reduce chloride and remove intolerable from the aspects of reactor control and undissolved oxide (HORSLEY, 1957). Lead was Tadanac brand with a purity of 99·99 per safety, consequently later studies have been directed towards slurries of fertile element compounds that cent; it was further refined in the same way as the may be more stable in heavy liquid metals containing bismuth. Zirconium was approximately 97 per cent pure with steel corrosion inhibitors, zirconium and magnesium (WEEKS et a!., 1955). The present paper is concerned hafnium, iron and carbon as principal impurities. Magnesium, supplied by Magnesium Electron Ltd. with preparation of slurries of thoria in lead, bismuth, and lead-bismuth alloys and observations on reactions was 99·95 per cent pure. Thorium oxide was prepared by Thorium Ltd., by between thoria and inhibited liquid metals in small precipitating and calcining the oxalate. Its particle loop experiments. 221
222
G. H.
BROOMFIELD,
J.
M. MATTHEWS
shape was cubic, a pseudomorph of the oxalate, and its size range was typically 100 % less than 30 t-t 78 % less than 20 t-t 58% less than 15 t-t 36% less than 10 f1, 21% less than 5 t-t 9 % less than 2 f1, with a crystallite size of approximately 700 A (BROWN and CHITTY, 1960). Impurities were principally chloride ( <1000 p.p.m.), sulphate (450-1500 p.p.m.) and carbon compounds giving a total loss on ignition of 0·1 per cent. The highest metal impurity was calcium at 160 p.p.m. EXPERIMENTAL
and A.
BARTLETT
30
HEATER
~ENGTH
Experiments carried out may be divided into three groups: slurry preparation, thermal convection loop T C. 3. circulation of slurries, and the operation of a forced TC =THERMOCOUPLE. circulation loop. FIG. I.-Assembly of thermal convection loop with filling can and permanent magnet. In the slurry preparation experiments approximately 80 grammes of materials for each slurry sample were at approximately outgassed at 150°C in small mild steel or silica bismuth-thoria slurry was circulated of 150°C for gradient temperature a through em/sec 28 containers, provided with an argon atmosphere, sealed pump and loop, the of and shaken at a rate of 1000 strokes per minute 2730 hours. The arrangement preslurry of type the and device sampling pot, head through an amplitude of 1t" for 2 hours at 500 or used tube steel The 2. Fig. in shown are can paration 600°C. The resulting slurries were subjected to for the loop and pump channel was of the same commetallographic examination. loops, but the Thermal convection loop experiments were made by position as that used for the convection by heating to use before nitrided was loop assembled preparing a slurry in a similar manner to that desfor four through ammonia passing and 500°C to 400 cribed above, running the slurry into a small CML in the prepared was loop this for slurry The hours. steel loop under vacuum, and allowing the loop experiments previous the for used that as manner same contents to circulate under convection for a predetermined period or until the loops plugged. In two and the loop was evacuated to lOt-t Hg pressure for head pot was about instances convection circulation proved difficult to filling. The depth of slurry in the filled with purified normally was space free the and em 5 initiate and a simple electromagnetic pumping arwhen a sample evacuated was space free The argon. rangement was provided. The thermal convection of liquid metal layers surface the when and taken was loop assembly with magnet and connexions for E.M. A complete stream. flowing the into plunged were pumping is shown in Fig. 1. be found in can accessories and loop the of description Chromel-alumel thermocouples were spot welded and (BROOMFIELD subject this on Report A.E.R.E. an to each loop in three positions. Heat input to the 1959). BARTLETT, heaters was controlled by an instrument set to 550°C RESULTS and connected to No.1 thermocouple. Readings from preparation Slurry 1. thermocouples 2 and 3 were recorded and provided an These experiments in slurry preparation extend the indication of the state of circulation in the loops. Measurements of heat transported by the flowing work carried out by CHITTY (1957) in which he showed liquid metal in these loops, and the temperature that thoria produced by calcining the oxalate was not gradient around the loops showed that the rate of flow readily wetted by bismuth unless an element capable of of the bismuth slurries was approximately 1·5 em/sec. reducing the thoria was dissolved in the bismuth. It The rate of flow of the lead slurries was probably about was found that provided thoria is outgassed at a temperature of 150°C until a vacuum of about 20 t-t Hg half this value. The thermal convection loop experiments were obtains, and the oxide and metal are sufficiently well followed by the pumped loop in which an inhibited agitated then iron dissolving in the bismuth from the
Some reactions between thorium oxide and inhibited heavy liquid metals
SAMPLER-----.JI
SLURRYj CAN.
SLUICE VALVE
JOINTING FLANGE
FLANGE JOINT
-VACUUM
TC.1
TC.7.
PUMP
TC.b.
TC.
=
the solidified slurries showed that the cubic particles were to some extent broken down. This breakdown was more marked in slurries containing magnesium and calcium than in those made in an iron container, as shown in Fig. 3(a, band c). The structures of solidified slurries of thoria in lead and in lead-bismuth eutectic, as indicated by light microscopy, were similar to those ofthoria in bismuth, but preparation of slurries in lead in an iron container necessitated periods of agitation of up to 8 hours before satisfactory wetting occurred, indicating a considerable difference in interface energies in the thoriabismuth and thoria-lead systems. In nine experiments where wetting did not appear to have occurred microscopical examination and radiation counting of washed beads of metal showed them to be free from thoria. A further experiment to examine slurry stability was made by preparing a slurry of 10 per cent wt. thoria in bismuth containing 0·05 per cent wt. magnesium and and 0·025 per cent wt. zirconium and maintaining it at S00°C for 2160 hours under vacuum. The slurry-free surface had a bright metallic appearance after this time and its micro-structure when solidified was similar to that of freshly prepared slurries of similar composition.
THERMOCOUPLE
M
1FT SCALE FIG.
223
2(a).-Arrangement of loop, pump and head pot.
APPROX. FULL SIZE.
FIG. 2(b).-Arrangement of lower end of sampler.
steel container enables wetting to take place. If a silica container is used some other addition to the bismuth is necessary. Slurries containing up to 18 per cent Th0 2 were made with the addition of 1 per cent magnesium, with no indication that the concentration of thoria could not be considerably exceeded. Microscopical examination of prepared sections of
2. Thermal convection loops The thermal convection loop experiments were designed to show whether the changing thermal conditions that may be encountered in a reactor would promote reactions between the constituents of the slurries to produce detectable products. The maximum temperature chosen, 550°C is that proposed for some reactor systems (HosEGOOD and CHILTON, 1956) whilst the temperature gradient in the loops was approximately similar to that proposed for the reactor heat exchanger cycles. Principal details of the experiments are given in Table 1 and the lower loop temperature variations are shown in Fig. 4. The results showed that tho ria,in a slurry containing magnesium at the probable maximum concentration for corrosion inhibition purposes, is not reduced in sufficient quantity for any indication to be found of thorium in excess of its solubility in the liquid metal. Higher concentrations of magnesium reduced sufficient thoria to form intermetallic compounds of thorium and the liquid metal. Together with a careful search for products of reactions between the thoria and inhibited liquid metals, microscopy of the loops included an examination of the steel tubes for corrosion and mass transfer
G. H. BROOMFIELD, J. M. MATTHEWS and A. BARTLETT
224
TABLE 1.-THERMAL CONVECTION LOOPS Loop
Slurry composition
Circulation time (hr)
Plug
A. Thoriainhibited bismuth
5% Th0 2 , 0·1% Mg, 0·025% Zr. Remainder Bi
1376
B. Thoriamagnesium-bismuth eutectic
5% Th0 2 , 0·54% Mg, 0·025% Zr. Remainder Bi
152
ThBi 2 in cold part of loop
C. Thoriainhibited lead
5% Th0 2 , 0·1% Mg, 0·01% Zr. Remainder Pb
78
Settled Th0 2
D. Thoria-leadmagnesium eutectic
5% Th0 2 , 2·2% Mg, 0·01% Zr. Remainder Pb
89
ThPb 3 in cold part of loop
E. Thoria-
5% Th0 2 , 44·5% Bi, 0·1% Mg, 0·01% Zr. Remainder Pb
2136
inhibited leadbismuth eutectic
2
6
No plug
Settled Th0 2
10,000
hr
FIG. 4.-Variation of lower loop temperature with log time.
No evidence for pitting corrosion was found; whilst the duration of most of the experiments was too short for this to be of major significance, the absence of corrosion in loops A and E indicates that corrosion inhibition by the ZrN film technique may be equally as effective when thoria is present, as in the single phase heavy metal systems. The detailed results of individual experiments are as follows. Loop A. Microscopic examination of prepared sections showed no pitting of the heated tube wall, but some small dendrites of iron were present in the cold part of the loop. Thoria was distributed in a fairly uniform manner in the cold limb of the loop except for segregation due to bismuth crystal growth on solidification, but apart from some particles apparently adhering to the tube wall there was no thoria in the hot limb. It appeared probable that thoria had been filtered out of the bismuth in later stages of circulation by bismuth dendrites
growing in the cold part of the loop, since this region contained a high concentration of thoria in interdendritic positions. Associated with the higher concentration of thoria and the iron dendrites in the cold part of the loop were some particles of a brown coloured phase having the appearance of newly prepared ThBi 2 (BARTON and GREENWOOD, 1957 and FERRO, 1957) and other particles having the more characteristic appearance of rapidly oxidized ThBi 2 (Fig. 5). The brown particles did not oxidize over a long period, hence it may have been some material as yet unidentified, or a fine dispersion of ThBi 2 in bismuth which on oxidation produced oxide particles of insufficient thickness to have the characteristic black appearance. Circulation in the loop did not slow down until the heater power was reduced indicating that there was insufficient iron or thorium in solution to exceed the solubility in the liquid bismuth at the lower loop temperature of about 400°C. Loop B. Crystals of ThBi 2 (identified by the colour and oxidation rate) had grown from the tube wall at the cold point, trapping circulating Th0 2 particles, and as in loop A the bismuth in the heated tube contained little thoria.
Some reactions between thorium oxide and inhibited heavy liquid metals
Loop C. This slurry proved difficult to circulate, and movement could only be maintained under the added impetus of the electromagnetic pumping effect using the arrangement shown in Fig. 1. At the rate of circulation required to give a temperature drop round the loop of less than 50°C (i.e. > 1 em/sec) turbulence was insufficient to maintain the suspension, and thoria settled out in the small angle bend at the top of the loop and on the top side of the lower horizontal section of tube. Microscopic examination of the loop after solidification revealed no product of reaction between the constituents of the slurry, and gave no evidence of steel corrosion. Solidified lead was easily pulled away from the steel tube in the region of the d.c. current contacts; this may have been due to poor wetting, but more probably to solidification contraction of the lead. Loop D. Circulation in this loop started readily under convection forces. As in the loops containing thoria-bismuth slurries the rate of circulation, as indicated by the cold point temperatures, appeared to increase for a number of hours before it began to fall to the freezing point of the slurry. Micro examination showed that although the thoria particles in the slurry had not undergone any detectable change in shape or size, a dendritic growth from the walls in the cold part of the loop had, by trapping thoria, formed a plug. The dendritic growth was probably of ThPb 3 formed by the reaction Th0 2
+ 2Mg + 3Pb --.. ThPb + 2Mg0. 3
Loop E. Circulation of the slurry in this loop did not start under convection forces but started readily under the added impetus of the E.M. pump. After a few hours convection circulation was maintained under the conditions shown in Fig. 4, increasing for 350 hours, and then decreasing in an unsteady fashion until it ceased after 2136 hours. Micro examination of sections of the loop showed that the impediment to circulation was settled thoria. No product of reaction between thoria and the inhibited eutectic was detected.
A common feature of all the convection loop experiments was that there was no break-down of the oxalate pseudomorph thoria particles and no change in particle size after circulation, nor did light or electron microscopy reveal any rounding of particles that could be definitely attributed to loop circulation. 3. The pumped loop
The loop was filled with a slurry of 7 per cent wt. thoria, 0·1 per cent magnesium and 0·5 per cent wt. zirconium, in bismuth, under a vacuum of 10 fl Hg with the loop heated to a minimum temperature of 400oC and the electromagnetic pump power full on. Circulation started immediately on filling and after some hesitation continued with a maximum temperature controlled at 480°C and a minimum of 400°C for five days until analysis reports showed that the inhibitor concentration levels were satisfactory. The loop controls were adjusted on the sixth day to give the operating conditions required for the experimental run. These were: control temperature 550°C, temperature gradient around loop 150°C, flow rate 28·5 em/sec. During the run samples were taken from the stream for analysis for inhibitor and thoria concentration, these are shown in Table 2. Small additions of magnesium and zirconium were
225
made after 1130 hours. Subsequent analysis showed no corresponding increase in inhibitor concentrations, but they did not appear to have been lost by oxidation at the free surface. A reduction of thoria concentration in the slurry of about 1 per cent was expected, due to dilution by bismuth left in the loop from a test run, but analysis of samples 5 and later (taken from the flowing stream) showed reductions in concentration which were proved by the plunging operation, carried out before taking sample 7, to be due to settling towards the free surface. After this period no further settling was detected. As the analysis figures show, lead was an appreciable impurity in the slurry. This was due to lead remaining in the loop after an initial test with lead-bismuth eutectic. After the seventh sampling, circulation was terminated by slowly reducing the pump power whilst maintaining a constant maximum temperature, a process lasting 35 min. An attempt to establish a thermal gradient up to the head pot was made by adjustment of heaters; this was not entirely successful as the pump entry tube was split by solidifying bismuth. During the run and afterward the loop was radiographed using an iridium 192 source of about 0·4 curies at 18 in. source to specimen distance. No changes were detected in the t in. bore tube but a considerable deposit of material with lower density than the slurry was detected in the pump tube. This material was, by micro examination, identified as iron but there appeared to be no pitting corrosion of the heated tube. The iron deposit, both in the pump and elsewhere had grown round thoria particles. Micro examination of the t in. bore tube in the cold point showed further deposits of iron, in the form of a rough layer about 0·3 mm thick. Fourteen sections of the loop, pump tube and head pot were prepared for micro examination. These showed evidence of a small degree of mass transfer without pitting corrosion of the heated tube, and that the thoria in the suspension in the bismuth had not undergone any change of physical form detectable by micro examination. There was, however, a constituent associated with the thoria that had one of the characteristics of ThBi 2 in that it corroded in air, but at a slower rate. This constituent immediately on preparation had a grey colour (ThBi 2 is brown) and resembled, with its irregular but smooth outline, the intermetallic compound Mg 3Bi 2 which would have been expected in an alloy of bismuth with 0·1 per cent wt. magnesium. However, its oxidation behaviour was not typical of Mg 3Bi 2 observed in other magnesium-zirconium inhibited loops. TABLE 2.-ANALYSIS OF CIRCULATING SLURRY
Hours Sample I Th0 2 Mg i Zr Pb I run ! (% wt.) I (% wt.) I (% wt.) (% wt.) number - - - - ______ I 1----·----!---0·085 0·60 0·045 Proving 0·085 0·035 6·4 Proving 2 0·090 0·040 670 3 0·140 0·140 0·040 0·60 815 4 0·120 0·040 1150 3-4 5 0·120 0·042 1250 3-4 6 0·100 5·7 0·039 1415 7 5·7 0·035 0·11 1845 8 5·7 0·035 0·11 1870 9 5·7 0·033 0·13 2255 10 5·7 0·033 0·14 2730 11
226
G. H. BROOMFIELD, J. M. MATTHEWS and A. BARTLETT TABLE 3.-ANALYSIS OF SLURRY AT THE END OF THE EXPERIMENT Position in loop
I Top of slurry in head pot Tube joint level in head pot Bottom of head pot Centre of heated tube
Th0 2 (% wt.)
I
10·9
I I
5·5
0·26 6·3
Further attempts to identify the oxidizing constituent were made by electron microscopy, micro-beam electron diffraction and by use of a scanning electron microscope with X-ray and electron recording. Because of the difficulty of isolating the particles and the type of X-radiation excited from bismuth the only useful information obtained from the employment of these techniques was that the constituent contained no concentration of lead above that of the surrounding slurry. (As stated earlier lead became an appreciable impurity in the slurry since the loop was first tested with lead-bismuth eutectic). As in the small thermal convection loops, thoria had segregated on solidification of the slurry to form intergranular concentrations; it had also settled upward in the head pot during cooling leaving the region below the level of the pipe joints almost free from thoria, except for a layer of thoria particles at the bismuth steel interfaces. The analysis of the solidified slurry from various positions in the loop is given in Table 3; it can be seen that there is a tendency for both magnesium and zirconium to segregate with the thoria. The solubility of iron in bismuth at 550°C is approximately 0·0025 per cent wt. (WEEKS et al., 1955), but the levels found by analysis were considerably in excess of this. No particulate iron was found in the slurry by micro examination. The presence of a zirconium nitride film on the inside of the heated tube was confirmed by electron diffraction. A few tests were carried out to check the physical properties of the steel tube after cooling. The material was ductile in a notched fracture test, had no hardness variations attributable to the nitriding treatment (the nitride case may have diffused to give an almost uniform composition in the time at the running temperature) and the tensile properties were typical of the type of steel with the thermal history of the loop tubing. DISCUSSION
1. Wetting Measurements of the contact angle of bismuth on thoria by the sessile drop method at temperatures up to 500°C have given results of over 90° (FROST et al., 1958), and the first experiments by CHITTY indicated that bismuth would only wet thoria if a metal capable of reducing thoria was present in the bismuth. Results from the present series showed that thoria reduction was not a pre-requisite of wetting, but the presence may be required of an element soluble in bismuth having a free energy of oxide formation greater than bismuth. (Iron appeared to act as a wetting agent, and 0 -1:1G° Fe~ 2 Feo > -1:1G Bi~iBi 2 o 3). This may be partly true, but further work by BROWN and CHITTY (1960) confirmed the existance of a removable barrier to
!
Mg (% wt.)
Zr (% wt.)
Fe (% wt.)
0·190 0·150 0·085 0·140
0·060 0·039 0·020 0·033
0·005 0·018 0·013 0·015
I
wetting and showed that this was carbon dioxide adsorbed on the thoria. The stability of the slurries, on standing and when circulating indicates that in the presence of corrosion inhibitors, and possibly without them, the contact angle of bismuth on thoria is less than 90°. Probably the contact angle measurements made so far have been on incompletely outgassed thoria. The thoria, derived from thorium oxalate as used in these experiments, has a large specific surface and may be suitable for calorimetric experiments to determine the strengths of bonds formed between thoria and pure or alloyed bismuth. 2. Chemical reactions in the slurries Results of the thermal convection loop experiments on slurries of thoria in bismuth-magnesium and eutectic (0·54 per cent wt. Mg) and in lead-magnesium eutectic (2·2 per cent wt. Mg) show that the thorium activity in the liquid metal due to the reaction Th0 2 + 2Mg0 = 2Mg0 + Th (1) at 550°C is sufficient to allow the reactions Th 2Bi = ThBi 2 (2) and Th 3Pb = ThPb 3 to occur at about 400°C. In loop experiments (including the pumped loop) in which magnesium was at a concentration of 0·1 per cent wt. the thorium activity was insufficient to allow formation of the intermetallic compounds. It is clear that the activity of thorium will be depressed considerably by reduction of magnesium content, since the equilibrium constant for reaction (1) [Th] b . . f . . 1 re duces to [Mg] 2 ut estimatiOn o quantitative va ues
+ +
for activities is difficult, because of compound formation in the metal systems and lack of experimental data. However it may be argued that the maximum thorium activity in bismuth, due to the presence of 0·54 per cent wt. Mg is close to that required to allow formation of ThBi 2 at 550°C, in which case the Th concentration in equilibrium with 0·1 per cent wt. Mg would be about 0·0032 per cent wt., assuming Henry's law to hold. This value is considerably below the
Some reactions between thorium oxide and inhibited heavy liquid metals
solubility limit at 400°C (BRYNER and BRODSKY, 1958) but is still well above the level that should have been detected as ThBi 2 crystals formed on cooling of the pumped loop. A possible explanation of the fact that ThBi 2 was not detected is that the MgO formed in reaction (I) was in a fine state of division and allowed the reaction to go to the left at such a rate that the activity of thorium did not become high enough at the lower temperature to allow reaction (2) to occur. An alternative possibility is the presence of a eutectic near to zero thorium in the Bi-Th-Mg system. 3. Irradiation effects Fission effects may constitute a severe test of the stability of these slurries. It may be assumed that several effects with possible deleterious results would occur. (a) Fission energy imparted to a small tho ria particle may be sufficient to free it from a liquid surface, to which it may return after a period as a dust particle in the blanket atmosphere; if such particles do not re-wet, free surfaces will become covered with thoria dust. (b) Neutron or fission fragment-thorium atom collision may detach thorium atoms from the thoria matrix. These detached atoms may go into solution, effectively accelerating the rate of reduction of thoria by corrosion inhibitors. (c) Break-down of thoria particles is to be expected due to fission effects, internal inert gas pressures and changes in structure on transmutation, and this may be sufficient to influence the rheology of a dense slurry. (d) The apparent density of tho ria will change due to occluded inert gas leading to possible segregation difficulties. An irradiation experiment in progress is designed to give some indication of the magnitude of these effects. 4. Steel corrosion Corrosion of the steel by bismuth, bismuth alloys and lead has been inhibited with varying degrees of success in all the experiments. The rate of corrosion did not exceed that which may have been expected in thoria free systems, and zirconium nitride films were identified on steel exposed to inhibited slurry. These results indicate that steel corrosion by a slurry is not necessarily a more difficult problem than corrosion by a solution fuel, but control of inhibitor concentration is more difficult, judging by results obtained with the pumped loop. Normally magnesium and zirconium in circulating bismuth are slowly depleted, the magnesium by oxidation and removal from the liquid
227
by settling out (or de-wetting and adhesion to tube walls etc.), and the zirconium by formation of nitride and possibly oxide at the tube walls; the depletion is revealed by analysis of the liquid. In the case of the pumped slurry loop the zirconium concentration showed a steady downward trend in the later 1500 hours of operation but the magnesium did not. It is certain that some oxidation of the magnesium did occur, consequently it must be assumed that MgO was entrained in the slurry. This is substantiated by the segregation of magnesium with thoria shown in Table 3. In this respect the inhibitor level may be difficult to control; a remedy may be to allow a period of settling of a liquid sample and to determine inhibitor level in the 'clear' portion of the liquid. CONCLUSIONS
1. Slurries of thoria in bismuth or lead with physical stability that may be good enough for reactor purposes can be prepared provided the liquid metal has a suitable wetting agent in solution, and that mixing of the constituents is sufficiently vigorous. Suitable wetting agents are those metals with a free energy of oxide formation appreciably above that of the liquid metal, but not necessarily high enough to reduce thoria. Magnesium and zirconium at concentrations normally used for corrosion inhibition are effective. 2. Magnesium in solution in a lead or bismuth slurry with thoria, at eutectic concentration reduces the thoria, allowing formation of solid intermetallic compound, ThPb 3 or ThBi 2 , but the concentration of thorium in solution does not appear to exceed the solubility at 400°C when the magnesium does not exceed the maximum concentration that is normally used for de-oxidation purposes (0·1 per cent wt. Mg). 3. Within the limitations of the experiments made, the presence of thoria suspended in lead, lead-bismuth eutectic, or bismuth does not effect the inhibition of steel corrosion by the ZrN film technique, nor has mechanical erosion by the slurries been detected. However, the normal analysis methods for control of inhibitors needs some modification due apparently to entrainment of oxidized inhibiting elements in the slurry. 4. It has been demonstrated that a thoria-bismuth slurry can be circulated in an electromagnetically pumped loop for 2370 hours with frequent sampling and without serious erosion or corrosion of the steel. Acknowledgements-The authors' thanks are due to colleagues in the UKAEA for electron diffraction and analysis work, for help and advice, and in particular to Dr. B. R. T. FROST for guidance and encouragement.
G. H. BROOMFIELD, J. M. MATIHEWS and A. BARTLETI
228
REFERENC ES BARTON P. J. and GREENWOOD G. W. (1958) J. Inst. Metal. 86, 12, 504. BROOMFIELD G. H. et al. (1959) J. Nucl. Energy 1, 42. BROOMFIELD G. H. and BARTLETT A. F. (1959) Some Reactions Between Thoria and Inhibited Heavy Liquid Metals. A.E.R.E./ R3131. BROWN A. and CmTTY A. (1960) J. Nucl. Energy Part B: Reactor Tech. 1, 145. BRYMER J. S. and BRODSKY M. B. (1958)Proceedingsof the Second
International Conference on the Peaceful Uses of Atomic Energy, Geneva, Vol. 7, P/460, p. 208. United Nations, N.Y.
CHITTY A. (1957) Unpublished.
FERRO R. (1957) Acta Crystallog. Vol. 10, 7, 476. FROST B. R. T. (1958) Proceedings of the Second International
Conference on the Peaceful Uses of Atomic Energy, Geneva,
P/270. United Nations, N.Y. GREENWOOD G. W. and SHARPE B. (1956) J. Nucl. Energy. 3, 1. HAWES R. 1., HORSLEY G. W. and SHELDON R. (1958) A.E.R.E. M &R/R2529. HoRSLEY G. W. (1957) J. Nucl. Energy 6, 41. HOSEGOOD S. and CHILTON H. (1956) Unpublished. SHARP B. (1957) Unpublished. WEEKS J. R. et al. (1955) Proceedings of the First International
Conference on the Peaceful Uses of Atomic Energy, Geneva,
Vol. 9. P/118, p. 341. United Nations, N.Y.
"'
~
FIG. 3(a).-Bismuth -l per cent calcium 10 per cent thoria slurry showing partly dispersed thoria particles in a matrix of bismuth and bismuth-calcium compound. ( X 150). FIG. 3(b).-Bismuth -l per cent magnesium 10 per cent thoria slurry showing similarly dispersed thoria particles in a hypereutectic bismuth-magne sium structure. ( X 600). FIG. 3(c).-Bismuth -l0 per cent thoria slurry prepared in an iron container, showing almost intact thoria particles in bismuth. ( x 1000).
FIG. 5.- A section of the cold part of loop A, identified as an iron dendrite, particles of tho ria segregated at bismuth grain boundaries rapidly oxidized ThBi 2 and a brown particle with the appearance of unoxidized ThBi 2 •