- Email: [email protected]
Chemical reactions of caesium, tellurium and oxygen with fast breeder reactor cladding alloys
319
Journal of Nuclear Materials 171 (1990) 319-326 North-Holland
CHEMICAL REACI-IONS OF CAESIUM, TELLURIUV AND OXYGEN WITH FAST BREEDER REACTOR CLADDING ALLOYS Part I - The corrosion by tellurium R.J. PULHAM and M.W. RICHARDS Chemistry Department, University of Nottingham, Nottingham NG7 ZRD, United Kingdom Received 18 September 1989; accepted 24 January 1990
The corrosion of the ahoys PE16, M316 and FV44g by liquid Te in sealed capsules has been studied after 168 h at p48 K both with and without a buffer of MO/MOO,. This buffer sets the thermodynamic oxygen potential at AGo*- -417 M mol-’ Oa. Ah three alloys were severely corroded and the extent was in the order M316 > PE16 = FV448. Oxygen diminished the extent of corrosion but did not change the order. All three alloys carried two layers of corrosion products on top of a damaged metal surface. The outer layer contained Fe,,sNi,,,Te, for PE16 but Fe 2.25Te, for M316 and FV448. The inner layer contained Cr,Te, for all three alloys. The damaged metal surface was largely CrrTe,. The substrate alloys (PE16 and FV448) were penetrated intergranularly by Te, but M316 exhibited an even band of G-depleted steel. The results are explained by the diffusion of Te into the alloy and the diffusion of the alloy components in the opposite direction. The corrosion is exacerbated by dissolution of alloy into the liquid Te from which the metal tellurides subsequently precipitate. 1. Introduction
- 500 to - 400 W mol-’ 0, for bum-up from 3.8 to 11.2 at% f5]. The Cs : Te ratio, derived from the fission
The economics of the Fast Breeder Nuclear Reactor fuel cycle might be improved by increasing the burn-up of the uranium-plutonium oxide fuel. T&s demands fuel cladding alloys that possess a lower neutron-induced swelling and a greater creep resistance than the M316 steel which was the original reference cladding. Resistance to internal clad corrosion is also important, however, and this paper describes the corrosion of PE16 and FV448 advanced clad alloys at 948 K by elemental tellurium under known constant thermodynamic oxygen potentials. The results are compared with those for M316 steel. Future papers will deal with Cs/Te mixtures. Post-irradiation examination of 316 clad fuel pins has indicated that the fission products Te and Cs accumulate in the gap between the fuel and the clad [I]. These elements, together with oxygen, have been designated as the prime corrodants of the internal steel surface [2]. The consensus of opinion is that the oxygen potential in the gap increases with bum-up [3], and measurements of oxygen potential on simulated high bum-up fuel have supported this 141. Similarly for irradiated fuel (U,,sPu,,O,~,) it has been shown that AGo, increases with burn-up. For example, at 1300 K, the measured value for the fuel increases from about
product yields, is relatively constant (ea. 7 : 1) throughout radiation except during the first few days when Te is in large excess over Cs 161. It was this background that shaped the method and the conditions for the present work which attempts to find the effect on the corrosion of each of the variables alloy type, corrodant composition and oxygen potential.
2. Experimental
procedure
The alloys were supplied by UK Atomic Energy Authority, Windscale, and had the compositions given in table 1. The main features are that Nimonic PE16 contains much more Ni and less Fe than does Austenitic M316, but the Cr content is comparable. The Ferritic FV448 contains less Cr than the other two alloys and possesses very Little Ni. The corrosion experiments were carried out in sealed Ni or 316 steel capsules (40 mm tall, 20 mm wide, 1.5 mm wall thickness) which contained an Also, crucible (20 mm tall, 14 mm wide, 3 mm wall thickness) to hold the Te plus the alloy. The capsules were initially degreased with l,l,l-t~c~or~~~e, washed with water and then degassed at 1123 K under 10T2 N m-a for 12
0022.3115/90/%03.50 6 1990 - Elsevier Science Publishers B.V. (North-Holland)
R.J. Pulham, M. W. Richards / Chemiral reactions of Cs, Te and oxygen. I
320
Table 1 Composition (wt%) of test alloys
PE16 M316 FV448
Cr
Ni
Fe
MO
Mn
Al
Ti
Nb
V
si
c
P
16.5 17.0 10.5
43.5 13.5 0.65
33.7 64.7 86.1
3.3 2.4 0.65
0.1 1.75 0.95
1.3 -
1.3 0.005 -
0.3
0.17
0.2 Q 0.6 G 0.55
0.08 d 0.006 GO.12
d 0.045 -
hours. The alloys were in the form of foils (8 X 24 x 0.38-0.295 mm) which had been cut from clad tubing (FV448) or from sheets (PE16, M316). They were degreased, washed with water followed by boiling acetone, and finally dried at 373 K under vacuum. The capsules were loaded under Ar in a glove-box with the A1,Os crucibles which contained the required weight (0.33 g) of solid Te (99.9% Aldrich) packed round the foil (fig. 1). The Al,O, crucible sat on top of the oxygen buffering mixture which was an equimolar mixture (1.4 g) of MO + MOO* and was prepared by heating Moos with Mostar grade, Hopkin and Wil~ams) in a 1: 2 molar ratio at 973 K for 140 h under Ar. The X-ray diffraction pattern of this mixture showed the lines of MO and MoOz only. This MO/MOO, couple was chosen to give an oxygen potential (AGo>) of - 417 f 25 kJ 1.1 x lo-l8 N m-‘1 at 948 mol-’ Oz [pressure (9)s K [7], The threshold oxygen potential for oxidation of Cr in M316, PE16 and FV448 alloys is estimated from activity data [6] to be - 574, - 573 and - 568 kJ mol-’ 4, respectively, at 948 K and hence all three alloys were expected to be oxidised by the MO/MOO, buffer. The choice of Ni for these capsules was deliberate because NiO is the~od~~~ly unstable with respect to MO and hence the capsule would not remove 0, from the system. In some experiments, the oxygen potential was intentionally reduced below that which was capable of oxidising the alloy foils by using a 316 stainless steel capsule instead of Ni, or by using a getter of Ti sponge (1 g) inside a 316 capsule. The equilibrium 0, pressure of the Cr(in 316)/Cr,O, couple is 2.1 X lo-” N mm2 (Acool= -575 kJ mol-’ 4) at 94% K, and Ti establishes a much lower equilibrium Oz pressure. It was unlikely, however, that there was enough oxygen available to establish the equilibrium, so that the prevailing 0, potential was considerably less than that required to oxidize any of the alloys. Each Ni (or 316) capsule was sealed by welding on a lid in an Ar-filled glove-box. The capsules were heated at 948 K for 168 hours in a furnace which was filled with Ar to prevent external oxidation of the capsules. This is probably typical of the clad temperature in an operating fuel pin [6]. After heating, each capsule was
cooled and was opened with a circumferential cutter in a glove-box filled with Ar. The foils were extracted from the Al24 crucible and it was seen that all of the liquid Te (melting point 723 K) had been drawn up and over the alloy foils. The surfaces were subjected to X-ray diffraction analysis (XRD), and the foils were then mounted in bakelite. They were metallurgically polished and their sections were examined by optical microscopy and by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). In all cases, the buffer remained uncontaminated (as determined by Xray diffraction powder analysis) by Te at the end of the experiments.
3. Results of corrosion
Seven corrosion experiments were carried out. The nature of the corrosion is shown in figs. 2, 3 and 4 and analytical data are presented in table 2. The corrosion of all three alloys occurred with the formation of three zones. There were two distinct corrosion scales (labelled outer and inner layers, respectively, figs. 2, 3 and 4) on top of a damaged metal surface. The outer layer was rich in Fe + Ni + Te for PE16 and M316 (figs. 2 and 3) and rich in Fe i- Te for FV448
1-t 1
NICKEL ----+--ARGON
JM
I
J
Fig. 1. Corrosion capsule.
R.J. Pulham, M. W. Richards / Chemical reaciions of Cs, Te and oxygen. I
Fig. 2. SEM photomicrograph
of the corrosion of PE16 by liquid Te at 948 K after 168 h. A = outer layer, B = inner layer, C = damaged alloy surface, and D = alloy subsurface.
{fig. 4). This is shown by the numerical data (table 2). Tellurium was present as shown by the bigb values of the ratios Te : Fe and Te : Ni (1.2 and 0.8, respectively,
Fig. 3. SEM photo~cro~aph
321
for PEX), but ratios involving Cr were zero (PE16). With M316, the pattern was similar except that the outer layer contained also a small proportion of Cr.
of the corrosion of M316 by liquid Te at 948 K after 168 h. A = outer layer, B = inner layer, C = damaged alloy surface, and D = alloy subsurface.
322
R. J. Pufham, M. W. Richards
Fig. 4. SEM photomicrograph
Table 2 Analysis by SEM/EDAX
Te:Cr
PEI6,/Te Surface Inner layer Outer layer M316
g a) gbb’ gb
g gb
Subsurface Surface Inner layer Outer layer Islands FV448 FV448/Te Surface Inner layer Outer layer a)
of the corrosion of FV448 by liquid Te at 948 K after 168 h. A = outer layer. B = inner la)ier, C = damaged alloy surface, and D = alloy subsurface.
of surfaces of alloys corroded by tellurium
Atom ratios
Alloy
PE16
/ Chemical reacrions of Cs. Te and oxygen. I
g gb gb
_ 0.18 1.92 1.59 _ 1.35 1.54 1.73 35.9 1.43 1.70 1.91 _
Te:Fe
Te:Ni
Cr:Fe
Ni:Fe
Cr:Ni
0.07 1.51 0.81
0.54 0.55 0.37 1.26 13.1 0
1.23 1.24 1.03 1.60 0 I.47
0.44 0.44 0.36 0.79 _ 0
0.32 0.33 0.05 0.98 6.29 0.02 0
0.19 0.14 0.02 0.04 _ 0.42 0.05
1.70 2.33 2.76 -
0.12 0.13 0.41 0.24 2.55 0
-
-
_ 0.07 1.39 20.8 1.2 _ 0.07 1.51 11.0 0.85 0 0.59 0.40 4.88 0.62
g = grain; b, gb = gram boundary;
3.73 27.0 2.0
-
‘) = from XRD.
_
0 -
Comments
Assignment ‘)
Te penetration, Cr/Ni depletion Fe depletion Cr/Te rich Te/Ni/Fe rich
CraTe3 Fer.sNit.sTe2
Te penetration, Cr/Ni depletion Te/Cr rich Cr/Te rich Te/Ni/Fe rich Pure Fe
Te/Cr Te/Cr Cr/Te Te/Fe
rich very rich very rich very rich
CrsTe, Fe,.,sTez
Cr,Te, %..asTea
R.J. Pulham, M. W. Richards / Chemical reactions of Cs, Te and oxygen. I
ratio remained high at 1.91. These data all strongly indicated that the inner layer was predominantly CrzTe,. The damaged alloy surfaces were also rich in Cr and Te; the Cr : Te ratio was about the same as in the inner scale layer (table 2). The damaged surface contained a high proportion of Cr,Te, but the texture, as shown by optical and scanning electron miscroscopy, was completely different (figs. 2, 3 and 4). The analytical data (table 2) showed that Fe had diffused out of the surface of PE16 (cf. Cr : Fe of 1.26 with 0.54, and Ni : Fe of 1.60 with 1.23) to leave an Fe-depleted surface. Similarly for M316 except that the damaged steel surface had been supplemented by the diffusion of Cr from the subsurface (cf. Cr : Fe of 0.98 with 0.05 and 0.23). The analytical data can be used in a similar fashion to show that the substrate metal (PE16 and FV448) was penetrated intergranularly by Te. With PE16, there was depletion in the grain boundaries of Cr and Ni but with FV448 there was Cr-enrichment. The substrate of M316 steel consisted of an even band into which Te had diffused but which was depleted in Cr and Ni.
With FV448, the Cr : Fe ratio was zero so that there was no Cr in the outer layer; this consisted entirely of Fe + Te (ratio Te : Fe = 0.62). The XRD powder pattern of the outer layer of scale showed the characteristic lines of Fe,.,Ni,.,Te, for PE16 but those of Fe,z,Te, [9] for M316 and FV448. This reflected the high concentration of Ni in PE16 and the lack of it in FV448. An obvious feature of the corrosion of M316 was the occurrence of islands of nearly pure Fe (table 2) embedded in the outer layer but adjacent to the inner layer. This might reflect competition by Cr for Te since, in general, the thermodynamic stability of the tellurides, as indicated by the values of their standard Gibbs free energy of formation relative to one mole of Te, increases in the order Fe < Ni < Cr [6,8,10-131. Both layers of scale readily separated from the alloy and from each other. The XRD powder pattern of the inner layer of scale showed good agreement with that of Cr,Te, and ‘CrTe’ [9]. These compositions he in the solid solution region of the phase diagram and both have the NiAs type of structure, but with varying degrees of occupancy by Cr of the octahedral holes which leads to similarity in XRD powder patterns [14]. The analytical data (table 2) showed, however, that this inner layer was Te-rich so that the XRD powder patterns were deemed to indicate the presence of Cr2Te, rather than ‘CrTe’. For PE16 (M316), the high ratios of 20.8 (11.0) and 1.59 (1.73) for Te: Cr and Te: Fe, respectively, showed the wealth of Te but lack of Fe, and the values of 13.1 (6.29) and zero for Cr : Te and Ni : Fe, respectively, showed the wealth of Cr but lack of Ni. Similarly for FV448; the high ratio of 4.88 for Te : Fe showed the high concentration of Te, and the high Cr : Fe ratio of 2.25 indicated a preponderance of Cr. Even though the Cr content was so high, the Te : Cr
3.2. The thickness of the corrosion zones The thickness of each corrosion zone was measured by optical microscopy on sectioned and polished alloy foils mounted in bakelite. The thicknesses are recorded in table 3. The overall depth (surface and subsurface) of damaged metal was less for PE16 and FV448 (58-59 pm) than it was for M316 (73 pm). The probable order is PE16 = FV448 < M316. The thinnest inner layer (CrzTes) occurred with the alloy (FV448) of lowest Cr content. The thinnest outer layer (Fe,,,TQ) occurred with the alloy (FV448) which contained no Ni, and the thickest outer layer (Fe,.SNiI,,Te,) occurred with the
Table 3 The thickness of the corrosion zones Thickness (pm)
MOY
Scale identification
Scale
Alloy Subsurface
Surface
Inner
Outer
a)
20 20
38 38
38 32
230 250
Cr,Te, + Fe,,,Ni,,,Te,
M316 M316/M
=)
35 33
38 40
80 40
200 150
Cr,Te, + Fe,.,,Te,
M316/Ti
a)
43
40
80
180
CrzTe, + Fe,.,STe,
25
34
20
115
CrzTe, + Fe2.25Te, Cr2Te3 + F%.zT~,
PE16 PE16/M
FV448 Fv448/M
a)
*) M denotes Mo/Mo02
buffer; Ti denotes Ti getter.
323
324
R.J. F&am,
M. W. Richards
/ Chemicul reuctions of Cs, Te and oxygen. I
Table 4 Corrosion depths (am) Alloy
Maximum
Minimum
Typical
PE16 PE16/M
a)
145 90
60 33
90 40
M316 M316/M M316/Ti
a> a)
168 142 122
39 21 33
115 95 115
FV448 FWW/M
a)
143 100
40 30
80 60
‘) M denotes MO/MOO,
buffer; Ti denotes Ti getter.
alloy (PE16) which contained the most Ni. With M316 steel, an increase in the oxygen potential (provided by the MO/MOO, buffer) to above that needed to oxidise Cr in the steel caused a decrease in the thickness of the inner and outer layers, but these were unchanged on PE16. Gettering of oxygen by Ti was slightly detrimental overall. 3.3. The extent of corrosion of the steels The approximate depths of corrosion of the alloy were derived from measurements of the thickness of the alloy before and after corrosion. Optical measurements on the corroded alloy were the least accurate due to problems in mounting and polishing the specimens, and in assessing exactly the position of the boundary between undamaged and damaged metal. The results are collected in table 4. The maximum and the typical depths show that the extent of corrosion decreased in the order M316 > PE16 = FV448. This order was unchanged by an increase in the oxygen potential. The effect of oxygen was to decrease the extent of corrosion whether this was expressed as maximum, typical or minimum depth.
4. Discussion The outstanding feature is the broad similarity in the nature of the corrosion of the three alloys. Each alloy carried two layers external to the damaged metal surface. These layers had a very different morphology from the metal surface. The combination of SEM and XRD analysis showed that the outer layer was iron or ironnickel tellurides whereas the inner layer was mainly chromium telluride. These layers had a very different morphology from that of the alloy surface so that the
formation of two scales is attributable to a dissolution mechanism whereby the metals of the alloy react to form tellurides which dissolve in the liquid Te. The binary phase diagrams [l&16,17] show that the solubility of Fe, Ni and Cr are 26, 10 and 6 mol%, respectively, at 948 K so that extensive and probably rapid dissolution of the alloy on an even front is expected initially. Chromium is least soluble so that the damaged metal surface is rich in Cr which reacts with inward diffusing Te and this diminishes the amount of free Te. Chromium tell&de, being the least soluble, is the first to pr~ipitate and does so on the damaged alloy surface which itself is mainly chromium telluride. The phase diagram indicates that the telluride which is formed at 948 K is Cr,Tes but that this transforms to Cr,Te, on cooling. Precipitated iron or iron-nickel tellurides comprise the outer scale. This is confirmed by the SEM/EDAX analysis (table 2) which shows that the inner and outer scales are ea. 60 and ca. 30 at% Te, respectively, and both of these compositions are solid at 948 K. As liquid Te is consumed, the diffusion through solid matrices becomes clearer. Tellurium diffused into all three alloy subsurfaces (this was p~rn~ly via grain boundary penetration with PE16 and FV448) and Cr diffused from grains into grain boundaries to combine preferentially with Te as expected from thermodynamics. Chromium (all three alloys) and Ni (PE16 and M316) diffused outwards. With PE16, the diffusion was via grain boundaries which eventually gave Cr and Ni depletion between the grains, With FV448 there was less movement through grain boundaries and no pronounced Cr-depletion between grains. With M316, the outward diffusion of Cr through the lattice left an Fe-rich subsurface which took the form of an even, broad ferritic band. There was some Cr-depletion also of the subsurface of FV448 ahoy. An increase in the oxygen potential inhibited the extent of corrosion as shown by the reduction in the depth of alloy which was corroded and by a corresponding diminution in the thickness of the corrosion scales. The beneficial effect of oxygen is presumably due to the preferential formation of Cr,O, (instead of Cr,Te,) which hinders diffusion of the relevant elements. The present results are broadly similar to the findings of other workers. Lobb and Robins [18J found that with a 20Cr, 25Ni, Nb-stabilized alloy in contact with Te vapour at 723 and 1123 K, there was the initial formation of an external layer which was rich in Fe, Ni and Te. but subsequently a layer of CrTe formed below this. This duplex layer was eventually converted to a single tayer of CrTe; and Fe or Ni were confined to the
R.J. Pulham, M. W. Richards / Chemical reactions of Cs, Te and oxygen. I
outermost edge. The intergranular penetration by Te increased with increasing temperature and time. Anand and Pruthi f19] conducted a similar study using 316 steel and again short term anneals produced a single layer rich in Fe, Ni and Te but a duplex layer was later formed with the inner zone being rich in Cr and Te. With Te vapour at 773 to 1073 K, Sallach et al. [20] found F~z.~~T~ on 316 steel and Ni,s,Te, on Inconel 600 together with small amounts of CrTe in both cases. The corrosion was inhibited by pre-oxidizing the alloys. With SUS-316 steel, Saito et al. [21) found that Te vapour formed FeTe,,s and NiTe,., outside of an ,inner layer of iron, nickel and chromium tellurides at 973K. The use of an Mo/MoOz buffer reduced the extent of corrosion and the inner layer now contained Cr,Os and MnCr,O,. Liquid Te studies by Batey and Bagley 1221 at 823 K showed the formation of FeTea, on alloys, and the times (h) taken to reach a depth of corrosion of 150 pm were ca. 1000 (PE16), 2000 (FV448) and 4000 (M316). Giitzmann et al. [23] stated that at low temperatures Fe, Cr and Ni diffuse into the Te giving a homogeneous attack on a plane front, but at high temperatures the corrosion occurs preferentially along the grain boundaries of austenitic steels. At high concentrations of Te, nickel tellurides are favoured at the reaction front whereas low concentrations encourage the formation of chro~um tellurides. In the presence of Cr and Ni, no iron telluride is formed. Oxygen again inhibits corrosion. GeneraBy, these data show that the nature of the corrosion depends upon the availability of Te! the annealing time at 723-1073 K and that the inmrgranular penetration by Te increases with increasing time and temperature.
5. Conclusions All three alloys were severely attacked by liquid Te. The depth of overall damage decreased in the order M316) PE16 = FV448. The effect of oxygen (MO/ MOO, buffer) was to miminise the extent of corrosion of each alloy but not change the order. The nature of the corrosion was broadly the same for each of the alloys; there were two layers (an outer and an inner) of corrosion products (transition metal tellurides) on top of a damaged metal surface, and Te penetrated the substrate alloy (PE16 and FV448) intergranularly. The corrosion of M316 differed in that the intergranuhu penetration was replaced by a Cr-depleted even band beneath the damaged surface. For PE16, the outer layer consisted mainly of Fe,,sNi,.sT+. The damaged alloy surface was largely Cr,Te, or CrTe, and the grain
325
boundaries of the substrate metal were enriched in Te + Cr. For M316, the outer layer was mainly F+.,,TQ, and the inner layer was mainly Cr;Te,. The damaged steel surface was largely Cr,Te, and the substrate steel was evenly denuded of Cr. With FV448, the outer layer was nearly pure Fe,,25Tez, and the inner layer contained mainly Cr,Te,. The substrate metal was penetrated intergranularIy by Te which was associated with Cr, and there was some Cr depletion at the corroding front. Overall there was Te diffusion into the alloy and diffusion of Fe, Cr and Ni towards the Te. In the early stages of corrosion this diffusion appeared to have taken the form of dissolution of the alloy into the liquid Te. The results show many facets of previously reported work.
Acknowledgement This work was sponsored by UKAEA under agreement number IIZ55615.
Windscale
References R.F. Hifbert, K.J. Perry, WK. Appleby, WE. Baily and C.N. Craig, General ELecttic Report, GEAP-13538 (April 1973). 121J.E. Anti11 and J.B. Warburton, J. Nucl. Mater. 71 (1977) 134. 131H. Kleykamp, J. Nucl. Mater. 131 (1985) 221. [41 R.E. Woodley, J. Nucl. Mater. 74 (1978) 290. (51 H. Matzke, J. Ottaviani, D. Peflottiero and J. Roualt, J Nucl. Mater. 160 (1988) 142. 161 M.G. Adamson, E.A. Aitken and T.B. Lindemer, J. Nucl. Mater. 130 (1985) 375. 171 0. Kubaschewski and C.B. Alccck, Metalhrrgical Thermochemistry (Pergamon, Oxford, 1979). (81 K.C. Mills, Thermodynamic Data for Inorganic Sulphides, Seienides and Telhnides (Butte~o~hs, London, 19745. 191ASTM Powder Diffraction File: 24-794 for FeI,Ni,,,Ter; 29-729 for Fq,,sTe; 37-134 and 28-458 for Cr,Te,; 2-722 for ‘CrTe’. WI B. Saha, R. Viswanathan, M. Sai Baba, and C.K. Mathews, High Temp.-High Press. 20 (1988) 47. u11 M. Sai Baba, R. Viswanathan, R. Balasubramanian, D. Darwin Albert Raj, B. Saha and C.K. Mathews, J. Chem. Thermodyn. 20 (1988) 1157. [12] R. Viswanathan, M. Sai Baba, D. Darwin Albert Raj, R. Balasubramanian, B. Saha and C.K. Mathews, J. Nucl. Mater. 149 (1987) 302; and 167 (1989) 94. [13] R. Prasad, V.S. Iyer, Z. Singh, V. Venugopal, S. Mohapatra and D.D. Sood, J. Chem. Thermodyn. 20 (1988) 319.
326
R.J. Pulham, M. W. Richardr / Chemical reactions of Cs, Te and oxygen. I
[14] H. HaraIdsen and A. Neuber, Z. Anorg. AIlgem. Chem.
[15] [16] [17] [18] [19]
234 (1937) 363. H. Ipser, K.L. Komarek and H. MikIer, Monatsh. Chemie 105 (1974) 1322. K.O. Klepp and K.L. Komarek, Monatsh. Chemie 103 (1972) 934. H. Ipser, K.L. Komarek and K.O. Klepp, J. Less-Comm. Met. 92 (1983) 265. R.C. Lobb and I.H. Robins, J. Nucl. Mater. 62 (1976) 50. H.S. Anand and D.D. Pruthi, Government of India AEC,
Bhaba Atomic Research Centre, Bombay, BARC-1009 (1979). [20] R.A. SaIlach, C.J. Greenholt and A.R. Taig, USDOE Report, NUREG/CR-2921 (1984). [21] M. Saito, H. Furuya and M. Sugisaki, Mater. Sci. and Eng. 87 (1987) 211. [22] W. Batey and K.Q. Bagley, J. Brit. Nucl. Energy Sot. 13 (1974) 49. 12310. Gtitzmann, P. Hoffman and F. Thiimmler, J. Nucl. Mater. 52 (1974) 33.
Journal of Nuclear Materials 171 (1990) 319-326 North-Holland
CHEMICAL REACI-IONS OF CAESIUM, TELLURIUV AND OXYGEN WITH FAST BREEDER REACTOR CLADDING ALLOYS Part I - The corrosion by tellurium R.J. PULHAM and M.W. RICHARDS Chemistry Department, University of Nottingham, Nottingham NG7 ZRD, United Kingdom Received 18 September 1989; accepted 24 January 1990
The corrosion of the ahoys PE16, M316 and FV44g by liquid Te in sealed capsules has been studied after 168 h at p48 K both with and without a buffer of MO/MOO,. This buffer sets the thermodynamic oxygen potential at AGo*- -417 M mol-’ Oa. Ah three alloys were severely corroded and the extent was in the order M316 > PE16 = FV448. Oxygen diminished the extent of corrosion but did not change the order. All three alloys carried two layers of corrosion products on top of a damaged metal surface. The outer layer contained Fe,,sNi,,,Te, for PE16 but Fe 2.25Te, for M316 and FV448. The inner layer contained Cr,Te, for all three alloys. The damaged metal surface was largely CrrTe,. The substrate alloys (PE16 and FV448) were penetrated intergranularly by Te, but M316 exhibited an even band of G-depleted steel. The results are explained by the diffusion of Te into the alloy and the diffusion of the alloy components in the opposite direction. The corrosion is exacerbated by dissolution of alloy into the liquid Te from which the metal tellurides subsequently precipitate. 1. Introduction
- 500 to - 400 W mol-’ 0, for bum-up from 3.8 to 11.2 at% f5]. The Cs : Te ratio, derived from the fission
The economics of the Fast Breeder Nuclear Reactor fuel cycle might be improved by increasing the burn-up of the uranium-plutonium oxide fuel. T&s demands fuel cladding alloys that possess a lower neutron-induced swelling and a greater creep resistance than the M316 steel which was the original reference cladding. Resistance to internal clad corrosion is also important, however, and this paper describes the corrosion of PE16 and FV448 advanced clad alloys at 948 K by elemental tellurium under known constant thermodynamic oxygen potentials. The results are compared with those for M316 steel. Future papers will deal with Cs/Te mixtures. Post-irradiation examination of 316 clad fuel pins has indicated that the fission products Te and Cs accumulate in the gap between the fuel and the clad [I]. These elements, together with oxygen, have been designated as the prime corrodants of the internal steel surface [2]. The consensus of opinion is that the oxygen potential in the gap increases with bum-up [3], and measurements of oxygen potential on simulated high bum-up fuel have supported this 141. Similarly for irradiated fuel (U,,sPu,,O,~,) it has been shown that AGo, increases with burn-up. For example, at 1300 K, the measured value for the fuel increases from about
product yields, is relatively constant (ea. 7 : 1) throughout radiation except during the first few days when Te is in large excess over Cs 161. It was this background that shaped the method and the conditions for the present work which attempts to find the effect on the corrosion of each of the variables alloy type, corrodant composition and oxygen potential.
2. Experimental
procedure
The alloys were supplied by UK Atomic Energy Authority, Windscale, and had the compositions given in table 1. The main features are that Nimonic PE16 contains much more Ni and less Fe than does Austenitic M316, but the Cr content is comparable. The Ferritic FV448 contains less Cr than the other two alloys and possesses very Little Ni. The corrosion experiments were carried out in sealed Ni or 316 steel capsules (40 mm tall, 20 mm wide, 1.5 mm wall thickness) which contained an Also, crucible (20 mm tall, 14 mm wide, 3 mm wall thickness) to hold the Te plus the alloy. The capsules were initially degreased with l,l,l-t~c~or~~~e, washed with water and then degassed at 1123 K under 10T2 N m-a for 12
0022.3115/90/%03.50 6 1990 - Elsevier Science Publishers B.V. (North-Holland)
R.J. Pulham, M. W. Richards / Chemiral reactions of Cs, Te and oxygen. I
320
Table 1 Composition (wt%) of test alloys
PE16 M316 FV448
Cr
Ni
Fe
MO
Mn
Al
Ti
Nb
V
si
c
P
16.5 17.0 10.5
43.5 13.5 0.65
33.7 64.7 86.1
3.3 2.4 0.65
0.1 1.75 0.95
1.3 -
1.3 0.005 -
0.3
0.17
0.2 Q 0.6 G 0.55
0.08 d 0.006 GO.12
d 0.045 -
hours. The alloys were in the form of foils (8 X 24 x 0.38-0.295 mm) which had been cut from clad tubing (FV448) or from sheets (PE16, M316). They were degreased, washed with water followed by boiling acetone, and finally dried at 373 K under vacuum. The capsules were loaded under Ar in a glove-box with the A1,Os crucibles which contained the required weight (0.33 g) of solid Te (99.9% Aldrich) packed round the foil (fig. 1). The Al,O, crucible sat on top of the oxygen buffering mixture which was an equimolar mixture (1.4 g) of MO + MOO* and was prepared by heating Moos with Mostar grade, Hopkin and Wil~ams) in a 1: 2 molar ratio at 973 K for 140 h under Ar. The X-ray diffraction pattern of this mixture showed the lines of MO and MoOz only. This MO/MOO, couple was chosen to give an oxygen potential (AGo>) of - 417 f 25 kJ 1.1 x lo-l8 N m-‘1 at 948 mol-’ Oz [pressure (9)s K [7], The threshold oxygen potential for oxidation of Cr in M316, PE16 and FV448 alloys is estimated from activity data [6] to be - 574, - 573 and - 568 kJ mol-’ 4, respectively, at 948 K and hence all three alloys were expected to be oxidised by the MO/MOO, buffer. The choice of Ni for these capsules was deliberate because NiO is the~od~~~ly unstable with respect to MO and hence the capsule would not remove 0, from the system. In some experiments, the oxygen potential was intentionally reduced below that which was capable of oxidising the alloy foils by using a 316 stainless steel capsule instead of Ni, or by using a getter of Ti sponge (1 g) inside a 316 capsule. The equilibrium 0, pressure of the Cr(in 316)/Cr,O, couple is 2.1 X lo-” N mm2 (Acool= -575 kJ mol-’ 4) at 94% K, and Ti establishes a much lower equilibrium Oz pressure. It was unlikely, however, that there was enough oxygen available to establish the equilibrium, so that the prevailing 0, potential was considerably less than that required to oxidize any of the alloys. Each Ni (or 316) capsule was sealed by welding on a lid in an Ar-filled glove-box. The capsules were heated at 948 K for 168 hours in a furnace which was filled with Ar to prevent external oxidation of the capsules. This is probably typical of the clad temperature in an operating fuel pin [6]. After heating, each capsule was
cooled and was opened with a circumferential cutter in a glove-box filled with Ar. The foils were extracted from the Al24 crucible and it was seen that all of the liquid Te (melting point 723 K) had been drawn up and over the alloy foils. The surfaces were subjected to X-ray diffraction analysis (XRD), and the foils were then mounted in bakelite. They were metallurgically polished and their sections were examined by optical microscopy and by scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). In all cases, the buffer remained uncontaminated (as determined by Xray diffraction powder analysis) by Te at the end of the experiments.
3. Results of corrosion
Seven corrosion experiments were carried out. The nature of the corrosion is shown in figs. 2, 3 and 4 and analytical data are presented in table 2. The corrosion of all three alloys occurred with the formation of three zones. There were two distinct corrosion scales (labelled outer and inner layers, respectively, figs. 2, 3 and 4) on top of a damaged metal surface. The outer layer was rich in Fe + Ni + Te for PE16 and M316 (figs. 2 and 3) and rich in Fe i- Te for FV448
1-t 1
NICKEL ----+--ARGON
JM
I
J
Fig. 1. Corrosion capsule.
R.J. Pulham, M. W. Richards / Chemical reaciions of Cs, Te and oxygen. I
Fig. 2. SEM photomicrograph
of the corrosion of PE16 by liquid Te at 948 K after 168 h. A = outer layer, B = inner layer, C = damaged alloy surface, and D = alloy subsurface.
{fig. 4). This is shown by the numerical data (table 2). Tellurium was present as shown by the bigb values of the ratios Te : Fe and Te : Ni (1.2 and 0.8, respectively,
Fig. 3. SEM photo~cro~aph
321
for PEX), but ratios involving Cr were zero (PE16). With M316, the pattern was similar except that the outer layer contained also a small proportion of Cr.
of the corrosion of M316 by liquid Te at 948 K after 168 h. A = outer layer, B = inner layer, C = damaged alloy surface, and D = alloy subsurface.
322
R. J. Pufham, M. W. Richards
Fig. 4. SEM photomicrograph
Table 2 Analysis by SEM/EDAX
Te:Cr
PEI6,/Te Surface Inner layer Outer layer M316
g a) gbb’ gb
g gb
Subsurface Surface Inner layer Outer layer Islands FV448 FV448/Te Surface Inner layer Outer layer a)
of the corrosion of FV448 by liquid Te at 948 K after 168 h. A = outer layer. B = inner la)ier, C = damaged alloy surface, and D = alloy subsurface.
of surfaces of alloys corroded by tellurium
Atom ratios
Alloy
PE16
/ Chemical reacrions of Cs. Te and oxygen. I
g gb gb
_ 0.18 1.92 1.59 _ 1.35 1.54 1.73 35.9 1.43 1.70 1.91 _
Te:Fe
Te:Ni
Cr:Fe
Ni:Fe
Cr:Ni
0.07 1.51 0.81
0.54 0.55 0.37 1.26 13.1 0
1.23 1.24 1.03 1.60 0 I.47
0.44 0.44 0.36 0.79 _ 0
0.32 0.33 0.05 0.98 6.29 0.02 0
0.19 0.14 0.02 0.04 _ 0.42 0.05
1.70 2.33 2.76 -
0.12 0.13 0.41 0.24 2.55 0
-
-
_ 0.07 1.39 20.8 1.2 _ 0.07 1.51 11.0 0.85 0 0.59 0.40 4.88 0.62
g = grain; b, gb = gram boundary;
3.73 27.0 2.0
-
‘) = from XRD.
_
0 -
Comments
Assignment ‘)
Te penetration, Cr/Ni depletion Fe depletion Cr/Te rich Te/Ni/Fe rich
CraTe3 Fer.sNit.sTe2
Te penetration, Cr/Ni depletion Te/Cr rich Cr/Te rich Te/Ni/Fe rich Pure Fe
Te/Cr Te/Cr Cr/Te Te/Fe
rich very rich very rich very rich
CrsTe, Fe,.,sTez
Cr,Te, %..asTea
R.J. Pulham, M. W. Richards / Chemical reactions of Cs, Te and oxygen. I
ratio remained high at 1.91. These data all strongly indicated that the inner layer was predominantly CrzTe,. The damaged alloy surfaces were also rich in Cr and Te; the Cr : Te ratio was about the same as in the inner scale layer (table 2). The damaged surface contained a high proportion of Cr,Te, but the texture, as shown by optical and scanning electron miscroscopy, was completely different (figs. 2, 3 and 4). The analytical data (table 2) showed that Fe had diffused out of the surface of PE16 (cf. Cr : Fe of 1.26 with 0.54, and Ni : Fe of 1.60 with 1.23) to leave an Fe-depleted surface. Similarly for M316 except that the damaged steel surface had been supplemented by the diffusion of Cr from the subsurface (cf. Cr : Fe of 0.98 with 0.05 and 0.23). The analytical data can be used in a similar fashion to show that the substrate metal (PE16 and FV448) was penetrated intergranularly by Te. With PE16, there was depletion in the grain boundaries of Cr and Ni but with FV448 there was Cr-enrichment. The substrate of M316 steel consisted of an even band into which Te had diffused but which was depleted in Cr and Ni.
With FV448, the Cr : Fe ratio was zero so that there was no Cr in the outer layer; this consisted entirely of Fe + Te (ratio Te : Fe = 0.62). The XRD powder pattern of the outer layer of scale showed the characteristic lines of Fe,.,Ni,.,Te, for PE16 but those of Fe,z,Te, [9] for M316 and FV448. This reflected the high concentration of Ni in PE16 and the lack of it in FV448. An obvious feature of the corrosion of M316 was the occurrence of islands of nearly pure Fe (table 2) embedded in the outer layer but adjacent to the inner layer. This might reflect competition by Cr for Te since, in general, the thermodynamic stability of the tellurides, as indicated by the values of their standard Gibbs free energy of formation relative to one mole of Te, increases in the order Fe < Ni < Cr [6,8,10-131. Both layers of scale readily separated from the alloy and from each other. The XRD powder pattern of the inner layer of scale showed good agreement with that of Cr,Te, and ‘CrTe’ [9]. These compositions he in the solid solution region of the phase diagram and both have the NiAs type of structure, but with varying degrees of occupancy by Cr of the octahedral holes which leads to similarity in XRD powder patterns [14]. The analytical data (table 2) showed, however, that this inner layer was Te-rich so that the XRD powder patterns were deemed to indicate the presence of Cr2Te, rather than ‘CrTe’. For PE16 (M316), the high ratios of 20.8 (11.0) and 1.59 (1.73) for Te: Cr and Te: Fe, respectively, showed the wealth of Te but lack of Fe, and the values of 13.1 (6.29) and zero for Cr : Te and Ni : Fe, respectively, showed the wealth of Cr but lack of Ni. Similarly for FV448; the high ratio of 4.88 for Te : Fe showed the high concentration of Te, and the high Cr : Fe ratio of 2.25 indicated a preponderance of Cr. Even though the Cr content was so high, the Te : Cr
3.2. The thickness of the corrosion zones The thickness of each corrosion zone was measured by optical microscopy on sectioned and polished alloy foils mounted in bakelite. The thicknesses are recorded in table 3. The overall depth (surface and subsurface) of damaged metal was less for PE16 and FV448 (58-59 pm) than it was for M316 (73 pm). The probable order is PE16 = FV448 < M316. The thinnest inner layer (CrzTes) occurred with the alloy (FV448) of lowest Cr content. The thinnest outer layer (Fe,,,TQ) occurred with the alloy (FV448) which contained no Ni, and the thickest outer layer (Fe,.SNiI,,Te,) occurred with the
Table 3 The thickness of the corrosion zones Thickness (pm)
MOY
Scale identification
Scale
Alloy Subsurface
Surface
Inner
Outer
a)
20 20
38 38
38 32
230 250
Cr,Te, + Fe,,,Ni,,,Te,
M316 M316/M
=)
35 33
38 40
80 40
200 150
Cr,Te, + Fe,.,,Te,
M316/Ti
a)
43
40
80
180
CrzTe, + Fe,.,STe,
25
34
20
115
CrzTe, + Fe2.25Te, Cr2Te3 + F%.zT~,
PE16 PE16/M
FV448 Fv448/M
a)
*) M denotes Mo/Mo02
buffer; Ti denotes Ti getter.
323
324
R.J. F&am,
M. W. Richards
/ Chemicul reuctions of Cs, Te and oxygen. I
Table 4 Corrosion depths (am) Alloy
Maximum
Minimum
Typical
PE16 PE16/M
a)
145 90
60 33
90 40
M316 M316/M M316/Ti
a> a)
168 142 122
39 21 33
115 95 115
FV448 FWW/M
a)
143 100
40 30
80 60
‘) M denotes MO/MOO,
buffer; Ti denotes Ti getter.
alloy (PE16) which contained the most Ni. With M316 steel, an increase in the oxygen potential (provided by the MO/MOO, buffer) to above that needed to oxidise Cr in the steel caused a decrease in the thickness of the inner and outer layers, but these were unchanged on PE16. Gettering of oxygen by Ti was slightly detrimental overall. 3.3. The extent of corrosion of the steels The approximate depths of corrosion of the alloy were derived from measurements of the thickness of the alloy before and after corrosion. Optical measurements on the corroded alloy were the least accurate due to problems in mounting and polishing the specimens, and in assessing exactly the position of the boundary between undamaged and damaged metal. The results are collected in table 4. The maximum and the typical depths show that the extent of corrosion decreased in the order M316 > PE16 = FV448. This order was unchanged by an increase in the oxygen potential. The effect of oxygen was to decrease the extent of corrosion whether this was expressed as maximum, typical or minimum depth.
4. Discussion The outstanding feature is the broad similarity in the nature of the corrosion of the three alloys. Each alloy carried two layers external to the damaged metal surface. These layers had a very different morphology from the metal surface. The combination of SEM and XRD analysis showed that the outer layer was iron or ironnickel tellurides whereas the inner layer was mainly chromium telluride. These layers had a very different morphology from that of the alloy surface so that the
formation of two scales is attributable to a dissolution mechanism whereby the metals of the alloy react to form tellurides which dissolve in the liquid Te. The binary phase diagrams [l&16,17] show that the solubility of Fe, Ni and Cr are 26, 10 and 6 mol%, respectively, at 948 K so that extensive and probably rapid dissolution of the alloy on an even front is expected initially. Chromium is least soluble so that the damaged metal surface is rich in Cr which reacts with inward diffusing Te and this diminishes the amount of free Te. Chromium tell&de, being the least soluble, is the first to pr~ipitate and does so on the damaged alloy surface which itself is mainly chromium telluride. The phase diagram indicates that the telluride which is formed at 948 K is Cr,Tes but that this transforms to Cr,Te, on cooling. Precipitated iron or iron-nickel tellurides comprise the outer scale. This is confirmed by the SEM/EDAX analysis (table 2) which shows that the inner and outer scales are ea. 60 and ca. 30 at% Te, respectively, and both of these compositions are solid at 948 K. As liquid Te is consumed, the diffusion through solid matrices becomes clearer. Tellurium diffused into all three alloy subsurfaces (this was p~rn~ly via grain boundary penetration with PE16 and FV448) and Cr diffused from grains into grain boundaries to combine preferentially with Te as expected from thermodynamics. Chromium (all three alloys) and Ni (PE16 and M316) diffused outwards. With PE16, the diffusion was via grain boundaries which eventually gave Cr and Ni depletion between the grains, With FV448 there was less movement through grain boundaries and no pronounced Cr-depletion between grains. With M316, the outward diffusion of Cr through the lattice left an Fe-rich subsurface which took the form of an even, broad ferritic band. There was some Cr-depletion also of the subsurface of FV448 ahoy. An increase in the oxygen potential inhibited the extent of corrosion as shown by the reduction in the depth of alloy which was corroded and by a corresponding diminution in the thickness of the corrosion scales. The beneficial effect of oxygen is presumably due to the preferential formation of Cr,O, (instead of Cr,Te,) which hinders diffusion of the relevant elements. The present results are broadly similar to the findings of other workers. Lobb and Robins [18J found that with a 20Cr, 25Ni, Nb-stabilized alloy in contact with Te vapour at 723 and 1123 K, there was the initial formation of an external layer which was rich in Fe, Ni and Te. but subsequently a layer of CrTe formed below this. This duplex layer was eventually converted to a single tayer of CrTe; and Fe or Ni were confined to the
R.J. Pulham, M. W. Richards / Chemical reactions of Cs, Te and oxygen. I
outermost edge. The intergranular penetration by Te increased with increasing temperature and time. Anand and Pruthi f19] conducted a similar study using 316 steel and again short term anneals produced a single layer rich in Fe, Ni and Te but a duplex layer was later formed with the inner zone being rich in Cr and Te. With Te vapour at 773 to 1073 K, Sallach et al. [20] found F~z.~~T~ on 316 steel and Ni,s,Te, on Inconel 600 together with small amounts of CrTe in both cases. The corrosion was inhibited by pre-oxidizing the alloys. With SUS-316 steel, Saito et al. [21) found that Te vapour formed FeTe,,s and NiTe,., outside of an ,inner layer of iron, nickel and chromium tellurides at 973K. The use of an Mo/MoOz buffer reduced the extent of corrosion and the inner layer now contained Cr,Os and MnCr,O,. Liquid Te studies by Batey and Bagley 1221 at 823 K showed the formation of FeTea, on alloys, and the times (h) taken to reach a depth of corrosion of 150 pm were ca. 1000 (PE16), 2000 (FV448) and 4000 (M316). Giitzmann et al. [23] stated that at low temperatures Fe, Cr and Ni diffuse into the Te giving a homogeneous attack on a plane front, but at high temperatures the corrosion occurs preferentially along the grain boundaries of austenitic steels. At high concentrations of Te, nickel tellurides are favoured at the reaction front whereas low concentrations encourage the formation of chro~um tellurides. In the presence of Cr and Ni, no iron telluride is formed. Oxygen again inhibits corrosion. GeneraBy, these data show that the nature of the corrosion depends upon the availability of Te! the annealing time at 723-1073 K and that the inmrgranular penetration by Te increases with increasing time and temperature.
5. Conclusions All three alloys were severely attacked by liquid Te. The depth of overall damage decreased in the order M316) PE16 = FV448. The effect of oxygen (MO/ MOO, buffer) was to miminise the extent of corrosion of each alloy but not change the order. The nature of the corrosion was broadly the same for each of the alloys; there were two layers (an outer and an inner) of corrosion products (transition metal tellurides) on top of a damaged metal surface, and Te penetrated the substrate alloy (PE16 and FV448) intergranularly. The corrosion of M316 differed in that the intergranuhu penetration was replaced by a Cr-depleted even band beneath the damaged surface. For PE16, the outer layer consisted mainly of Fe,,sNi,.sT+. The damaged alloy surface was largely Cr,Te, or CrTe, and the grain
325
boundaries of the substrate metal were enriched in Te + Cr. For M316, the outer layer was mainly F+.,,TQ, and the inner layer was mainly Cr;Te,. The damaged steel surface was largely Cr,Te, and the substrate steel was evenly denuded of Cr. With FV448, the outer layer was nearly pure Fe,,25Tez, and the inner layer contained mainly Cr,Te,. The substrate metal was penetrated intergranularIy by Te which was associated with Cr, and there was some Cr depletion at the corroding front. Overall there was Te diffusion into the alloy and diffusion of Fe, Cr and Ni towards the Te. In the early stages of corrosion this diffusion appeared to have taken the form of dissolution of the alloy into the liquid Te. The results show many facets of previously reported work.
Acknowledgement This work was sponsored by UKAEA under agreement number IIZ55615.
Windscale
References R.F. Hifbert, K.J. Perry, WK. Appleby, WE. Baily and C.N. Craig, General ELecttic Report, GEAP-13538 (April 1973). 121J.E. Anti11 and J.B. Warburton, J. Nucl. Mater. 71 (1977) 134. 131H. Kleykamp, J. Nucl. Mater. 131 (1985) 221. [41 R.E. Woodley, J. Nucl. Mater. 74 (1978) 290. (51 H. Matzke, J. Ottaviani, D. Peflottiero and J. Roualt, J Nucl. Mater. 160 (1988) 142. 161 M.G. Adamson, E.A. Aitken and T.B. Lindemer, J. Nucl. Mater. 130 (1985) 375. 171 0. Kubaschewski and C.B. Alccck, Metalhrrgical Thermochemistry (Pergamon, Oxford, 1979). (81 K.C. Mills, Thermodynamic Data for Inorganic Sulphides, Seienides and Telhnides (Butte~o~hs, London, 19745. 191ASTM Powder Diffraction File: 24-794 for FeI,Ni,,,Ter; 29-729 for Fq,,sTe; 37-134 and 28-458 for Cr,Te,; 2-722 for ‘CrTe’. WI B. Saha, R. Viswanathan, M. Sai Baba, and C.K. Mathews, High Temp.-High Press. 20 (1988) 47. u11 M. Sai Baba, R. Viswanathan, R. Balasubramanian, D. Darwin Albert Raj, B. Saha and C.K. Mathews, J. Chem. Thermodyn. 20 (1988) 1157. [12] R. Viswanathan, M. Sai Baba, D. Darwin Albert Raj, R. Balasubramanian, B. Saha and C.K. Mathews, J. Nucl. Mater. 149 (1987) 302; and 167 (1989) 94. [13] R. Prasad, V.S. Iyer, Z. Singh, V. Venugopal, S. Mohapatra and D.D. Sood, J. Chem. Thermodyn. 20 (1988) 319.
326
R.J. Pulham, M. W. Richardr / Chemical reactions of Cs, Te and oxygen. I
[14] H. HaraIdsen and A. Neuber, Z. Anorg. AIlgem. Chem.
[15] [16] [17] [18] [19]
234 (1937) 363. H. Ipser, K.L. Komarek and H. MikIer, Monatsh. Chemie 105 (1974) 1322. K.O. Klepp and K.L. Komarek, Monatsh. Chemie 103 (1972) 934. H. Ipser, K.L. Komarek and K.O. Klepp, J. Less-Comm. Met. 92 (1983) 265. R.C. Lobb and I.H. Robins, J. Nucl. Mater. 62 (1976) 50. H.S. Anand and D.D. Pruthi, Government of India AEC,
Bhaba Atomic Research Centre, Bombay, BARC-1009 (1979). [20] R.A. SaIlach, C.J. Greenholt and A.R. Taig, USDOE Report, NUREG/CR-2921 (1984). [21] M. Saito, H. Furuya and M. Sugisaki, Mater. Sci. and Eng. 87 (1987) 211. [22] W. Batey and K.Q. Bagley, J. Brit. Nucl. Energy Sot. 13 (1974) 49. 12310. Gtitzmann, P. Hoffman and F. Thiimmler, J. Nucl. Mater. 52 (1974) 33.