IMMOBILIZATION OF HIGH LEVEL WASTE IN SYNROC

A. JOSTSONS and K. D. REEVE, Australian Atomic Energy Commission, Lucas Heights Research Laboratories, Locked Mail Bag No. 1 , Menai, N'SW 223-i Australia Telephone (02) 543-3111, Telex AA 24552

ABSTRACT occurring radioactive elements in a variety of geological cnvi ronmi.-nt s . Synroc. a polyphase titan, boina developed in Australia as a second generation The Synroc strategy for HLW immobiliza- waste form for the immobilization of high- tion was developed in 1978 by Ringwood and 3 11 level waste (HLW) from nuclear plants. The colleagues ' at the Australian National wasteform, produced by reactive hot -press i n _.-,,- University (ANU). The Australian Atomic has excellent resistance to leaching and there Energy Commission (AAEC) began studii-*. on is good evidence that the long-term release Synroc in collaboration with the ANU in 1979 rate of elements from Synroc will be of the and the program has expanded with strong order of 10~" - ] 0~s g/m2 per day at 90°C. support from the Australian Department of The durability of Synroc is not affected . Resources and Energy and the National Energy, significantly by impurities normally present Research, Development and Demonstration Pro- in liquid waste from reprocessing operations gram (NERDDP). This aupport has enabled the and is not sensitive to fluctuations in the construction at Lucas Heights of an inactive composition of the waste-stream. Reducing Synroc demonstration pilot plant with a capa- conditions prevalent during Synroc fabrication city of 10 kg/h of Synroc. Apart: from the ensure that the losses of volatiles such as Cs AAEC and ANU, other groups in Australia are and Ru are restricted to levels of around 0.17« engaged in Synroc research, the most signifi- or better. The important physical properties cant of these being Griffith University, of Synroc are also superior to those of cur- Queensland, which has attracted NERDD1' sup- rent generation wasteforms, allowing greater port . flexibility of options in managing storage and eventual disposal of high-level waste. The Australian Synroc. program benelitid s i gn i f i can t 1 v f*-om thf U.S. pr^grirr on alter- INTRODUCTION native waste forms in the early 1980s. Cur- rently, the major Synroc research outside For about three decades, research on Australia is proceeding under the cover of high—level waste (HLW) immobilization has been bilateral R & D agreements with Italy, Japan under way in many countries resulting in the and the U.K. choice of borosi1icate glass as a first generation waste encapsulation medium. The This paper summarizes our current under- first continuous commercial scale production standing of Synroc as a waste form, its chemi- of vitrified waste was achieved in 1978 in the cal durability and fabrication. AVM plant at Marcoule, France. This technology has been further scaled up in THE SYNKOC PHASE ASSEMBLAGE plants under construction at La Hague in France, and in the U.K. ' Addi- Synroc-C, a formulation chosen for the tional vitrification plants based on the Joule encapsulation of HLW from the reprocessing of melter have been constructed2 or are commercial light water reactor spent fuel, under construction. consists mainly of zirconolite CaZrTi2O7, barium hollandite Ba(Al,Ti)2 Ti6 0,6, perov- Synroc, a polyphase titanate ceramic, is skite CaTiO3 and rutile TiO,. A combination an advanced waste form which is receiving of the first three phases has the capacity to attention as a possible candidate for second accept most of the elements present in HLW. generation waste management. The major phases Under the redox conditions chosen for Synroc in Synroc are similar to naturally occurring fabrication a number of waste elements are minerals known to have retained naturally reduced to the. metallic state and form alloys

537 which are microencapsulatod within the tita- modification by cation ordering, crystallo- natc phases. The composition and mineralogy of graphic shear, or twinning on a unit cell scale Synroc-C is given in Table. 1. is an important mechanism for enhancing the capacity of Synroc phases to immobilize HLW and Table 1. Composition and Mineralogy of imparting flexibility to respond to inevitable Synroc-C variations in waste-stream composition.

Composition % Mineralogy (approx wt.%' The "forgiving" nature of Synroc is fur-

TiO2 57.=~1 hollandite 30 ther enhanced by the occurrence of spontaneous ZrO2 5.3 zirconolite 30 adjustments in the proportions of phases in the Al2O3 4.3 perovskite 20 Synroc assemblage in response to variations in BaO A.5 TiO2 + minor ph'ases 15 the waste-stream. One formulation of the Synroc CaO 8.8 allovs 5 constituents shown in Table 1 can immobilize a HLW 20 10.15 or 2O7= HLW loading without diminution in durabi lity.'3 Thus , a Synroc-C with a nominal 107„ HLW loading can accept fluctuations of An important aspect of Synroc solid state ±100% in the level of each constituent of the chemistry is the use of the Ti-TiO2 buffering waste-stream. system provided by the reaction between the excess TiO of the formulation and titanium 2 The Synroc microstructure is tine grained, powder added to tho calcined powder M v:m,with alloy phases with diameters in the before hot-pressing. The resultant non- range 0.01 - 0.1 pm encapsulated by the tita- stoichiometric rutile (actually Ti 0 nates. Transmission electron microscopy re- Magnel'i i• phasesi ;\ provide. • s a suppl, y on,2n-r i veals the presence of intergranular glassy Ti3 which stabilizes hollandite and increases films 1 - 3 nm thick.'" The glassy phase also the solubility limit of caesium therein ' . occurs at triple grain junctions. These glassy The buffer also controls the oxygen fugacity, films are not continuous, are rich in process ensuring that most noble metal species in HLW contaminants and usually caesium, and generally are reduced to the metallic state, and pre- account for less than XX of the total volume.' vents the formation of water soluble phases containing caesium. An excess of non- stoichiometric rutile provides Synroc with the CHEMICAL DURABILITY capability to react to unexpected fluctuations in the HLW stream composition. The partition- The most rigorous test of the chemical ing of HLW species in Synroc is summarized in durability of a waste form is provided by Table 2. leaching under conditions where the leach solu- tion remains undersaturated with respect to the wasteform. We use the MCC-l'5 test with regu- Table 2. Partitioning of HLW Species in lar replacement of leachant to measure the Synroc Host Phases""' intrinsic immobilization capacity of the waste form and to evaluate the effects of processing hollandite: Cs, Ba, Rb variables on the durability of Synroc. In all •zirconolite: U, Zr, tetravalent actinides cases, the leach rate of each element from the perovskite: Sr, Na, rare earths and tri- wastpform is calculated from the formula valent and tetravalent. actinides alloy: Mo, Ru, Rh, Pd, Te Differential leach rate (g/ni2 per d) = fraction of element * Lesser amounts of rare earths are also contained in leached x W/At zirconolite, whereas perovskito incorporates significant amounts of uranium. The TiO does not contain significant 2 where W is the initial mass of the test speci- - quantities of HLW species apart from Zr. men in grams, A is the geometric surface area Trace amounts of other phases are usually of the specimen in square metres, and t is the found but generally they do not contain signi- time in days. cant amounts of HLW species. Thus, in Synroc-C incorporating waste with high Na contents. Figure 1 shows the leach rates of the more White8 has observed freudenbergite, nomi- soluble elements in Synroc at 90 C. The rapid nally Na2Al2Ti6 016 which was previously obser- decrease in these leach rates with time is cha- ved in ceramic waste forms by Morgan et al.' racteristic of Synroc and makes the accurate Monazite (Ce,Nd)P0i, is invariably present if determination of long-term leach rates diffi- phosphorus is a contaminant of the HLW.8 cult because conventional methods, such as inductively coupled plasma and atomic absorp- The three major Synroc phases have a range tion spectroscopy, have insufficient sensiti- of cation acceptor sites and various substitu- vity. To overcome this difficulty, and to tion mechanisms which ensure that the solid generate results for the more leach-resistant solubility of HLW species in the host phases is fission products, a hot-cell line was construc- extensive.10 White et al.11, White12, and Kesson ted at Lucas Heights to incorporate into Synroc 99 m and White5'6 have demonstrated that structural a waste solution from the Mo/" Tc production

538 line. The resultan t leach data obtained bv constituents intersecting the specimen surfacesurf . Y-counting is shown in Figure 2. The so leach Elements such as caesium have been identifiied rates .ire based on total activity in the in the glassy regions. Actiniae elements, leaching vessel and include adsorbed and sus- usually simulated in inactive studies by ponded radionuclic s. The- results ag.iin show uranium, have not been detected in intergranu- the incongrucnt dis solution of Synroc over the lar films, which could explain the low initial period investigated and indicate that the total leach rates of this group of elements. leach rates of the rare earths, ruthenium and zirconium approach IXICT". g/m2 per d after three weeks. From 80-1007 of the ram earths and zirconium, and 1 roin 50-70», ol tlie ruthenium are removed by lilt ration through a <).hb yni Cs137 f iiter sug^e F> L i ng I hal most ol 1 h";H' c 1 oiwu ' Ba1«0 are uresent in l!ie le.ulute as 1 suspension. Zr-95 Ru-103 Ce-141 Pm-148

JO 30 TIME (days)

Fig. 2 Differential leach rate of fission pro- ducts from Synroc-C in ileionized water at 70 C. Leachant replaced after 1 and 7 davs and everv 7 days thereafter. 40 50 60 TIME (days)

Fig. 1 Differential leacii rates from Synroc-C i.md';r MCC-1 conditions in deioni-.ed wafer a I: 90°C.

To provide data on the leaching of acti- nide elements from Synroc, a small production line, consisting of four interconnected glove boxes, was constructed at the AAliC. The glove- bo/. procedures were similar to those used in the inactive laboratory for production of spe- cimens for waste form evaluation. Synroc spe- cimens hive been fabricated containing up to 0.03 wt" "'Am, 0.53 wt7„ 2 3 91'u, 0.002 wt" 2*"Cm 237 and 1.3 wt70 Mp. The resulcs of leach t?sts 20 30 4ti on Synroc specimens containing actinides are TIME (days) shown, in Figure 3. .The initial leach rates, based on measured leachate activity, are very fig. 3 Differential leach rate of actinide low and show only a very slight decrease with elements from Synroc-C at 70 C in 2 3 time. Apart from ' Np, the leacii rates are in deionized water. the vicinity of 1x10 5 g/m2 per d. These results, at temperatures below The relatively high initial leach rates of 100 C, suggest that the release rate of all some elements from Synroc a"-e inocl probably due elements will eventually converge to a constant to the preferential dissolution of the glassy value determined by :he titanate matrix. At intergranular regions and traces of minority present, this terminal value appears to be

539 below 1x10 " g/m' per d r. for i 7-e, . . V.'C-! a concentration vi 2 vfL above its normal level test at 90 C and could be less tli.ni wi, •.;.•"••' :n Synroo, had no effect on the lojch rate. The v.T dr.y as indicated by the leaching bel..wiour effect ot sili dl and phosphorus additions is the actinide elements. !! is noted thai the shown in Tiblr 3, and Table 4 shows the effect ab ve terminal leach rates '.orrespond to solu- ol varying sodium levels. These results indi— i i i t i c s (t o t a 1 dissolved s o 1 i d s ' o! the o r '. i •• '•-ue that the total SiCK content of 0.3 wt"(, cr :-10 ppb. Measurement and i nt t rpre; e • ion ol trebles the loaih rate for caesium but does not le-" -h rates of t h i s magnitude . •-". voi ••' •{}' ' ii \i 1 L ,'ilect ::ho leach rares of other waste elements. r a'.id c.-reful long-term leachiiij; t c .-.!•; e xt e ml i re The i usensi t i v i t v ot S'-'n or—C leach rates to beyond three years will bo required. levels oi sodium additions ns high as 2.57. has ilso been confirmed by independonr studies at The ol'ect ol temperature on t !v leach -.lie .I-ipan Atomic. Energy Research Institute 19 • •..•>• of S\:iroc-C his boon i nvc«! i r.i! •:•(! by K UAFRI). 7 et -1."' and Solomah." The overall loci; r.it <• ; i" c.iscs by a (actor of 25 <<•.>.•:• the topper .1- Table 4. Lffoct of Sodium on Synroc-v Leach fire raniv' 4'i-'!00 C and the utivat ion euer e,ies Rates (g/m'.d) tor elemental and mass loss oi Svnroc. range, (MCC-1 Test at 90°C tor 7 days) from 15-30 kJ/mol. This relatively small o '.'•.•et of temperature on the chemical durahil i ty 1 .07„ 2. 57. 4.07„ permits consideration of dispos.il of Svnroc i n Add i ti ve N.nno N,i?0 Na20 Na 20 deep drill-holes where the .mbient temporal are ?:.-..; s (1.021 0.011 0.007 0.023 will be higher than [r the •-l^ • 1 1 ov;-:-i: no rep o s i - n :'•.'. 0.031 U.0i7 0. 11 tori es favoured currentIv. .s 0.076 0.038 0.041 0.13 Mo 0.34 0.35 0.37 1.18 The incorporation into Synroc-C of some ri i 0.30 0.30 0.20 0.20 elements not norm.illy present in H! VJ r in nffpcf Sr 0.032 0.U19 0.017 0.013 the chemical durability. Impurities can be introduced through the precursor or rhrough the waste as a result of chemical additions during All I l,o abovo observations on the chemical reprocessing. Sulphate, silii.a and chloii.it: iliiraii: Lit;.1 üi Syr.roc refer to material that has may be present at low levels in sor.ie of the not been subject to radiation damage from alpha inert chemicals (precursors) i;sed to produce dec.i1. of acfinides. There is considerable evi- Synroc. Iron is often used i:o control pluto- dence ' " " ' r ha i. 7,irconoli te , the major host for nium partitioning during solvent extraction and actinides in Synroc, becomes metamict (amor- can be included as a result of plant corrosion: phous) al doses equivalent to that experienced phosphorus is present i .1 most wastes from the bv Synroc wi t h 107« HL,W in about 10s years. radiolytic decomposition of the Pl'RKX solvent: Measurements1 ° of the leachability of natural and sodium is introduced when the v.Mste solu- z.i rconol i tcs that have been subjected to vary- tion is neutralised with caustic soda. ing levels of radiation damage suggest10 that the increase in leach rates is only a factor of Table 3. Effect of Silica and Phosphorus f ive. Additions on Synroc-C I.each P.atos Siudies by Weber et al.22 on zirconolite (MCC-1 Tesr -:t ')0 C !'or 7 da V S ) radiation damaged to the amorphous state by Cm doping showed that Addi- o. 2;; o. 4"; 1 . 0". 1 .0% tive None Si02 Si02 Si02 P20, Cm leach rate was unchanged from that of: the crystalline form, Mass 0.021 0.041 Ü.02C 0.026 0.048 Pu leach rate increased 11 times, and 0.10 0.09 0.09 0.04 0.071 Ba mass loss was 2.5 times higher for the Cs 0.07& 0.22 0.32 0.52 0.072 amorphous form. Mo 0.34 0.41 0.26 0,42 0.25 Si 0.30 0.72. 0.48 0.71 0.30 There have been no studies of the leaching 0.032 0.032 0.018 0.032 0.060 Sr of Synroc, as distinct from zirconolite, doped with Cm to determine the effect of radiation * Ti leach rate was bellow detection limit. damage. Such measurements are expected to be- ** Contains 0.08% SiO2. come available in the near future from UKAV.A (Harwell) and JAER1. A systematic study ci these impurities on the leach rate oE Synroc with 10 wt7, U.S. simu- Dosch et al.21 have reported that Pb ion lated reference waste PW-4b has been reported simulation of damage equivalent to a-doses up by Levins et al.18 Chloride, fluoride and sul- to lxl025ct/m3 did not lead to significant phate introduced into simulated liquid waste in enhancement of matrix dissolution of titanate amounts corresponding to concentrations equiva- ceramic waste forms during leaching. Similar- lent to between 200 and 1000 ppm in Synroc had ly, Sethi and Bates21* found that Synroc con- no effect on the leach rate at 90 C. Iron, at taining 10 wt7. synthetic radwaste retained its

540 leach resistance alter irradiation wich 3.S MeV reasons, demonstration of remote operation has "He* ions and 250 keV "'"Kr* ions to doses been restricted to novel aspects of the plant equivalent to about 106 years ot" storage of not hitherto ieinonstrated in a radiochemical Synroc. Our results on fast neutron irradia- environment. The design philosophy has been tion o£ Synroc at about 70 C to doses equiva- described in some detail by Levins et al.25 lent to a-damage received over 4.5x10"" years Here only selected aspects of the process and show no systematic effect of radiation damage plant are discussed. on the leach rate. The Synroc Precursor PROCESSING The starting point for Synroc fabrication The Synroc fabricat ion process aims at a is a precursor containing the Synroc-forming fully dense ceramic- with a small (<1 um) uni- ox.des listed in Table 1. Ideally, the pre- form grain size with a minimum of continuous cursor should ainorphoiT. intergranu 1 ar phases. The small grain size is desirable to prevent microcrack- have a high specific surface area to permit ing from ,-inisot ropic stresses during thermal hot-pressing at the minimum temperature and contraction and during irradiation growth from pressure; a-recoil damage. be a chemically homogeneous blend of all its components to avoid segregation of some T.0- elements or formation of unwanted 7rfi. F>'^ ' eke :ter non-equilibrium phases; A|2Oj be of acceptable purity (e.g. concentrations CoO of silica above 0.17» are undesirable); BoO PRECURSOR MAKE-UP have good rheological properties when slurried (low viscosity and low shear stress Simulated HLW at relatively high pulp density); n 2MHNO3 after calcination, be free-flowing, WASTE MIXING To Stack non-dusty and have a high bulk density (to facilitate bellows filling and compaction); and Reducing Gas OFF-GAS TREATMENT be relatively cheap ($A30/kg is a reasonable

DRYINC/CA .CINAT ON target). (1-2 h at 750 °C No precursor has all these properties. Since Ti Powder — 1 TiO2 constitutes more than 70% of the mass of the precursor for Synroc-C, its role is most POWDER BLENDING important. Early work at the ANU and Lucas Heights indicated that the Sandia process developed by Dosen et al.26 yielded a precursor which incorporated radionuclides by ion- BELLOWS FILLING exchange/sorption reactions to yield a calcined powder which could be converted to a dense homogeneous ceramic by hot-pressing at moderate COLD PRESSING 110 MPci temperatures and pressures compared with those needed for densification of ground conventional ceramic oxide powders. More recent laboratory studies have shc.jn that precursors based on HOT PRESSING simpler alkoxide hydrolysis routes, hydrous titania pulp and spray-dried sols, have pro- • mise. CCOUNG/CANISTER LOADING The precursor in a hot plant can be slur- ried and tested outside the cell before being Fig. 4 1'rocess steps in the fabrication of pumped and mixed inside the cell with the Synroc. liquid 11LW before drying and calcination.

The overall flow diagram of the process is Dry ing/Calei nation shown in Figure 4. Based on this flow sheet, a demonstration plant with a nominal capacity of A number of equipment options were consi- 10 kg of Synroc per hour, at a loading of 10-20 dered for this process step rind a rotary calci- wt7D simulated waste, has been built at Luear ner was chosen to perform both drying and cal- Heights in collaboration with the ANU. At the cination because higher waste loading, the throughput is equiva- lent to the HLW generation rate from a 400 (a) the slurry atomizer required for either a r./year reprocessing plant. This plant will fl';idized bed or a spray drier was consi- never handle radioactive wastes. For economic dered a likely source o£ blockages;

541 (b) powder entrainmcnt is likely to be lower Extnnsive experience has been gained with in a rotary calciner; the hot-pressing stage. Examples of the bel- (c) the long-term practicality of operating a lows before and after hot-pressing are shown in rotary calciner in a hot cell has been Figure 5. After hot-pressing for 2 hours and demonstrated at the Marcoule plant; and 14-21 MPa, the Syrroc has consolidated to near (d) solids movement in a rotary calciner is theoretical density (4250-4350 kg/m3) and essentially by plug flow which ensures a bellows containers of 300 mm and 436 mm dia- minimum residence time. meter contain about 30 and 60 kg of Synroc respectively. Leach tests on specimens cut The calciner is designed for a maximum from cores trepanned from various regions of operating temperature of 900 C and a produc- the consolidated Synroc have confirmed that the tion rate of 10 kg solids/h. The tube is material produced on this scale is of compara- constructed of 310 stainless steel, 300 mm in ble quality to that produced in the extensive diameter and 7Q00 mm long. Heating is provided laboratory program. by an 80 kW resistance-heated refractory fur- nace comprised of eight independent heating zones which enable the temperature profile to be closely controlled.

Slurry is fed to the calciner at a rate of about 30 L/h. A reducing atmosphere is required during calcination to prevent the formation of leachable non-Synroc phases (particularly caesium molybdate). This is achieved by passing 40 Nm'/h of 3.5 vol7„ hydro- gen in nitrogen (below the explosive limit in air) counter-cjrrcnt to the slurry. The cal- ciner is designed for a solids residence time of two hours but this can be varied by adjust- ing the tilt and/or rotational speed. Calcina- tion is usually carried out at 750 C; the resulting free-flowing powder, completely denitrated, is grey to blue showing that the Ti02 has been reduced in the H2/N2 gas. Fig. 5 300 mm diameter bellows containers before and after hot-presing. (Holes The reducing gas effectively prevents the in the top of the compressed bellows formation of volatile oxides of Cs and Ru represent locations where cores were during calcination. Extensive laboratory tests removed for testing). v-'ith radioactive isotopes of Cs and Ru indicate losses less than 0.1% mostly because of The compressed bellows can be loaded in entrainment rather than volatilization. These stainless steel canisters, filled with an promising results have been obtained without appropriate heat transfer medium (e.g. lead). the use of organic materials for denitration. Thus the bellows container technique minimises the creation of secondary process wastes. Also, Dens i fication since the bellows do not constitute a waste disposal barrier, individual bellows can be A simple method of uniaxial hot-pressing cored and sampled lor compliance with regula- in stainless steel bellows has been developed27 tory requirements, on a statistical basis, to to produce dense monoliths of Synroc at modera- supplement quality assurance by careful control te pressures (14-21 MPa) and temperature and monitoring of the feed materials and pro- (1150 C). The powder from the cnlciner is cess parameters. blended with 2% of titanium metal powder (<300 mesh), transferred into the bellows container, POTENTIAL FOR FURTHER PROCESSING DEVELOPMENT welded, decontaminated and pre-prcssed cold to about two thirds of its original height before The demonstration plant design commenced transfer to a hot press. in 1983 and was frozen in July 1984. However further R & D on various precursors shows pro- The entire operation from bellows filling mise that a dry reactive granular precursor can to removal of the hot-pressed bellows after be developed which could be impregnated with appropriate cooling is performed automatically liquid HLW, calcined and hot-pressed to yield a with manipulators. Each bellows has a parti- homogeneous monolith of Synroc. This would late filter and off-gas tube in the base. No allow a considerable reduction in the size of loss of ruthenium has been detected during the calciner; in the present calciner 60% of hot-pressing and caesium losses of less than the heat load is devoted to evaporation of the 0.14% are condensed on a small filter located water associated with the slurry feed. Such a in the bellows off-gas line. solid precursor could be based on sol-gel raate-

542 rials of the type described by Gerontopoulos et assurance. al.:" or by Woodhead et al.29 Work is also pro- ceeding on the development of precursors which At this stage of the development of Synroc remain fluid at higher solids contents, and on processing, further process simplification is measures to increase the tap density of the confidently anticipated. calcined powder without sacrificing ease of sintering during hot-pressing. Some of the REFERENCES constraints on the precursor and calcined pow- der would be eased by development of materials 1. CG. SOMBRET, "The Vitrification of High of construction capable of permiting hot- Level Radioactive Wastes in Franco", Nucl. pressing at temperatures around 1250 C instead Energy, 24, 85 (1985). of the current range of 1150-1200 C imposed even by the most advanced alloys. Current 2. G. HOHLEIN, E. T1TTMANN, and H. WIESE, developments in the industry suggest "PAMELA: Adva.iced Technology for Waste that this requirement will be soon satisfied. Solidification", Nuclear Europe, 2, 16 (1985).

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