A modest proof-of-principle effort has been conducted to investigate melt casting as a process for compacting SrF/sub 2/ to near theoretical density. A nonradioactive SrF/sub 2/ mixture, similar in chemical composition and method of prepartion to SrF/sub 2/ encapsulated at the Hanford Waste Encapsulation and Storage Facility (WESF) was used for the test evaluations. Hard, dimensionally stable, monolithic ingots that are >98% of theoretical density have been produced. Significant chemical purification from Al, Fe, Cr, Ni, Na and Zr has been demonstrated

Montgomery, D. R.

Wheelwright, E. J.

UNT Digital Library

PNL-SA—12637 DE85 009429 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. FEASIBILITY OF MELTCASING STRONTIUM FLUORIDE TO PRODUCE HIGH-DENSITY HEAT SOURCES E. J . Wheelwright D. R. Montgomery PNL-SA~12637 DE85 009429 February 1985 To be presented at Waste Management '85 Tucson, Arizona March 24-28, 1985 Work Supported by the U.S. Department of Energy under Contract DE-AC06-76RL0 1830 Pacific Northwest Laboratory Richland, Washington 99352 DISTRIBUTION OF THIS DOCUMENT IS UMLIMITE.; DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER  Portions of this document may be illegible in electronic image products.  Images are produced from the best available original document.  FEASIBILITY OF MELTCASTING STRONTIUM FLUORIDE TO PRODUCE HIGH-DENSITY HEAT SOURCES E. J. Wheelwright and D. R. Montgomery Pacific Northwest Laboratory P. 0. Box 999 Richland. Washington 99352 I - ABSTRACT A modest p roo f -o f -p r inc ip le e f f o r t has been conducted to invest igate melt casting as a process for compacting SrF2 to near theoret ica l dens i ty . A nonradioactive SrFj mixture, s imi lar in chemical composition and method of preparation to SrFo encapsulated a t the Hanford Waste Encapsulation and Storage F a c i l i t y (WESF) was used for the t e s t evaluat ions. Hard, dimensionally s tab le , monol i thic ingots t ha t are >98 percent of theoret ica l density have been produced. S ign i f i can t chemical pu r i f i ca t i on from A l . Fe, Cr, Mi , Na and Zr has been demonstrated. INTRODUCTION DESCRIPTION OF EXPERIMENTS Fission product stront ium, a byproduct from the processing of i r rad ia ted f u e l , has been used as the energy source for over 60 radioisotope thermoelectr ic generators (RTGs) used in t e r r e s t r i a l app l i ca t ions . The only current source of t h i s material i s the encapsulated strontium f l uo r ide stored in the Hanford Waste Encapsulation and Storage F a c i l i t y (WESF) storage basins. Over-30 m in i on - cu r i es of stront ium-90 have been encapsulated In 427-capsules and placed i n storage. The primary mission of B-Plant and WESF has been the conversion of highly concentrated stront ium solut ions to dry s tab le-so l ids i n the form o f strontium f l u o r i d e , double encapsulated in metal containers. Manufacturing high-density strontium capsules for use in RTGs or other-'thermal converters was not a primary considerat ion. As a r e s u l t , highest achievable density and chemical pu r i t y were not obtained. The highest-wattage capsules current ly in the WESF storage basins have strontium f l uo r i de dens i t ies of 3.2 to 3.7 g/cnr (75 to 85 percent of theore t ica l densi ty) and thermal outputs of from 660 to 760 wat ts . For many heat sources or e l e c t r i c a l power generator app l i ca t ions , an optimized system requires encapsulated stront ium f luo r ide of higher power densi ty (W/cm-^) than current ly ava i l ab le . The use of stront ium f l uo r i de of lower power density increases the size and weight of the generator, increases the heat losses, and decreases the power conversion f i c i e n c y . Fission product strontium w i th a higher Sr/Sr i so top ic r a t i o cannot be obtained unless waste from future Purex operations i s processed in B-Plant and WESF. That leaves improvements in bulk density and chemical pur i ty as the only strontium f l uo r i de parameters a f fec t ing power density that can be modi f ied. Equipment The melt -cast ing tests were performed i n a c y l i n d r i c a l , water-cooled, s ta in less steel vacuum chamber 60 cm diameter by 90 cm high. The vacuum chamber was equipped wi th side and top viewing por ts . Melt ing and casting cruc ib les were heated by thermally insulated induction c o i l s ins ide the vacuum chamber. A few tests were performed using tantalum crucibles^ but most of the tes ts were performed in melt ing and cast ing crucib les fabr ica ted from graph i te . A mechanical roughing pump and a d i f f us i on pump were used to obtain the necessary-gas pressure ins ide the chamber. After c los ing the chamber and before high-temperature operat ions, a i r was removed from the chamber and replaced wi th argon to pro tec t the graphite or tantalum cruc ib les from ox ida t i on . Preparation o f Strontium Fluor ide Several 11.5 kg batches of nonradioact ive SrF2 feed mater ial were prepared and used. Weighed amounts of the n i t r a tes of Sr, Fe, Cr, Ba, Ca, Cd, A l , Hg, Mn, Nd, Ni and Pb were dissolved in water. Sol id sodium f l uo r i de was added to the so lu t ion to p rec ip i t a te the f l u o r i d e s . The s lur ry was digested one hour a t 80°C and then f i l t e r e d through #42 f i l t e r paper. The f i l t e r cake was dr ied a t 125°C in a dry ing oven. Weighed amounts o f KF, NaF and ZrF* were added to the dr ied p rec ip i ta te and thoroughly mixed wi th the p rec ip i t a te by b a l l m i l l i n g . The blended f l uo r i de mixture was then vacuum dr ied a t 275°C fo r 24 hours and then f i r e d a t 800*C. Af ter f i r i n g , the f l uo r i de mixture was again b a l l m i l l e d . The normal composition of the f l uo r i de mix ture , based on the composition of the feed so lu t ion i s given In Table I . The composition i s s im i la r to t h a t obtained in the WESF SrF2 operat ion. P roo f -o f -p r inc ip le experiments were conducted in 1983 to invest igate melt cast ing as a possible a l t e rna t i ve to hot pressing for densi fy ing SrF2 for heat or power source use. The primary requirements were that the cast cy l i nd r i ca l ingots be free of surface or I n t e r i o r vo ids, have near theore t i ca l dens i ty , have a predictable uniform diameter, and be strong enough to withstand handl ing. Chemical p u r i f i c a t i o n was not an i n i t i a l requirement but has proved to be an a t t r ac t i ve process fea tu re . TABLE I Nominal Composition'*^ of the SrF2 Salt Mixture De f i n i t i ve Melt Casting Tests Component Sr F Al Ba Ca Cd Cr Fe K Mg Mn (b) (c) Na Nd* NI ' ' " M I Zr(d) Weight 64.60 31.30 0.15 0.45 0.15 0.02 0.15 0.22 0.12 0.02 0.01 1.50 0.50 0.15 0.02 0.84 (a) (b) (c) (d) Based on composition of feed so lut ion Can vary depending of washing o f p rec ip i t a te Nd used as typ ica l of rare„earths Zr w i l l rare Increase in WESF ^"SrF, as ^": Sr decays I n i t i a l Mel t ing Tests Pure SrF2 melts a t 1473°C and the vapor pressure reaches one atmosphere a t 2489°C., During i n i t i a l mel t ing experiments performed-in-graphlte c ruc ib les , the impure feed powder was found to melt a t a temperature of about 1350 to 1400°C. Three phenomena were observed In the early t es t s . They w e r e : 1 . Impure feed powder cannot be melted in a high vacuum without excessive sp la t t e r i ng as the v o l a t i l e impur i t ies are released. However, the powder can be melted wi thout s i gn i f i can t sp la t t e r i ng under an argon pressure of 400 to 250 t o r r . 2. I r o n , chromium and nickel impur i t ies are reduced to metal by react ion with the graphi te c ruc ib le . When the melts were cooled and removed from the graphite c r u c i b l e , metal globules containing these elements were found imbedded In the outside surface of the ingots . 3. High density SrF2 ingots cannot be formed by mel t ing the powder under an argon pressure of 500 to 400 t o r r and then f reezing the molten s a l t in the melt ing c ruc ib le . Vo la t i l e Impur i t ies released during the process produce voids in the s o l i d i f i e d ingot . When the v o l a t i l e impur i t ies are vaporized from the molten sa l t by slowly reducing the pressure inside the vacuum chamber and then res tor ing the pressure to about 500 to r r before the sa l t i s f rozen, high density ingots are obtained. The observation tha t Fe, Cr and Ni are reduced to metal by react ion w i th the graphite melt ing c ruc ib le prompted the use of separate melt ing and cast ing c ruc ib les . A graphite melt ing cruc ib le was fabr i ca ted w i th a drain hole i n the bottom, s l i g h t l y elevated above the bottom of the c ruc ib le . A graphite stopper rod sealed the drain hole during powder load ing, melt ing and outgassing steps. When the stopper rod was ra i sed , the molten sa l t drained from the mel t ing cruc ib le in to a graphite cast ing c ruc ib le posi t ioned d i r e c t l y below the melt ing c ruc ib le . Fine me ta l l i c globules of Fe, Cr and Ni remained in the melt ing c ruc ib le attached to the inside surface of the graphi te . Some s a l t a lso remained attached to the stopper rod and i n the trap In the bottom of the c ruc ib le . Such sa l t can be recyc led. When molten SrFj i s cast in to a cold mold, f reezing s ta r t s a t the mold In ter face and progresses Inward. The r esu l t I s a pipe or cavi ty that extends from the top par t way down the center l i n e of the cast ing . Piping was minimized by cont ro l led d i rec t iona l cool ing of the mold. By ad just ing the spacing of the Induct ion c o i l s surrounding the graphite cast ing mold, i t was possible to cont ro l the cool ing and maintain a bottom-to-top temperature d i f f e r e n t i a l during s o l i d i f i c a t i o n . The ingot shown i n F i g . 1 was cast by t h i s technique. About 0.5 cm was cut o f f the top of the Ingot to remove a small cav i t y . The maximum and minimum diameters of the ingot were measured a t 1-Inch i n te r va l s down the height of the ingo t . The r e s u l t s given In Table I I show a range of 0.012 i n . between extreme diameter measurements. The density of t h i s Ingot was 4.23 q/cnr or 99.62 of the theore t i ca l density of pure SrFg. Several Ingots w i th bulk densi t ies above 4.15 g/cm-* (98Z of theore t i ca l density) have been prepared. Chemical p u r i f i c a t i o n can be achieved by care fu l control of the cruc ib le temperature and the pressure inside the vacuum chamber. Figure 2 shows the temperature and pressure measurements fo r the t e s t which y ie lded the ingot shown in F i g . 1 . The pressure Inside the chamber was held a t about 250 to 150 t o r r u n t i l the charge had melted. The pressure was then reduced to about 0.8 t o r r over a period of about 65 minutes. V o l a t i l e impur i t i es vaporized from the molten sa l t and condensed on cooler surfaces Inside the chamber. When the pressure reached 0.8 t o r r . the vacuum system was valved o f f . argon was admitted to the chamber u n t i l the pressure reached 500 t o r r , and then the stopper rod was raised to permit the molten sa l t to drain from the melt ing cruc ib le i n to the cast ing c ruc ib le . Power was reduced which allowed the temperature to f a l l as shown in F i g . 2 . A f te r the furnace had cooled and was opened, residues were recovered from the melt ing cruc ib le and from the Inside of the vacuum chamber. I t was obvious tha t there had been some sp la t t e r i ng o f mater ia l from the melt ing c ruc ib le . The ingot was recovered from the mold. A l l mater ia ls recovered were weighed, sampled, and the samples analyzed. The analyses and the mater ial balance recoveries are summarized In Table I I I . FIGURE 1 . Melt Cast Ingot of SrF» with a Density I of 4.23 a/cm3. Purity of the SrF2 was increased to 98.6Z but only 80Z of the SrF2 was recovered in the ingot. Twelve percent was recovered from the melting crucible and 7Z from the vacuum chamber.- The residue recovered from the melting crucible could be recycled. Refinements In equipment design and In the control of both temperature and pressure should significantly reduce strontium holdup in the melting crucible and splattering into the vaccum chamber. A f inal test was made to evaluate impurity decontamination using feed material containing much higher concentrations of NaF, SIF^ and ZrF4. Due to equipment problems, the vacuum chamber was pumped down to only about 50 torr before the molten salt was drained into the casting mold. As a result , the recovered ingot was not pur i f ied. The Ingot was remelted and the pressure reduced to 0.4 torr over a period of about 60 minutes before r e f i l l i n g the chamber with argon and casting the molten sa l t . The recovered ingot had uniform dimensions and a density of 4.23 g/cm'. The chemical analyses are given in Table IV. The good impurity decontamination achieved Is probably more a result of the lower pressure than the recasting. FIGURE 2. Temperature, Pressure and Time Relationships During Heltcastinq of Inqot in Fiq. 1 . TABLE I I Variation in Diameter of Cast Ingot Maximum Diameter ( i n . ) Minimum Diameter ( In . ) Top 1.451 1.448 1st Inch 1.451 1.446 2nd Inch 1.454 1.446 3rd Inch 1.449 1.444 4th Inch 1.448 1.446 Bottom 1.446 1.442 Range Between Extremes 1.442 to 1.454 = 0.012 i n . TABLE I I I Analysis of Material Balance Recovery From Melt Casting Residue Recovered . . Recovered Ingot From Melting Crucible"*' Residue Recovered From Vacuum Chaober Compound AIF3 BaFo CaF2 CrF^ FeFs NaF NdF, SrFo ZrFj Melting Charge wt=900g . Analysis^*' (wtZ) 0.33 0.65 0.23 0.15 0.19 2.06 0.38 95.2 — 0.77 Wt' >700g Z of charge-Analysis (wt2) NO 0.45 0.21 0.014 0.016 0.06 0.36 98.6 -0.24 =78 2 of Compound in Charge 54 71 7 6.6 2.3 71 80.6 24.2 1 2 of Analysi (WtZ) 0,77 2.2 0.23 0.55 0.24 5.04 0.35 88.1 1.86 i»t=116g Charge= s 2 of in =13 Compound Charge 30 44 13 47 16 32 12 12 31 wt 2 of ( Analysis (wt2) 3.34 0.87 0.25 0.33 0.25 11.8 0.31 76.5 1.94 =75q Charge 2 of In =8 Compound Charge 84 11 9 18 11 48 7 7 21 Material Balance Accountability (Z) 117 107 85 120 107 54 88 99.7 84 Ta] Analysis by Inductively-Coupled-plasraa/Optical Emission Spectroscopy (b) Does not include 1 gram of metal globules recovered separately TABLE IV Chemical Purification Achieved With Lower-Pressure Vaporization of Impurities Recast Ingot (wtZ) Impurity Material AIF3 BaFp CaFp CrF, FeF, NaF SIFd ZrFj Feed Material (WtZ) 0.28 0.65 0.26 0.26 0.34 5.04 9.34 9.34 F i rs t Cast Ingot (wtZ) 0.07 0.68 0.23 0.16 0.03 3.2 1.35 4.63 0.04 0.21 0.31 0.02 0.04 0.13 0.02 0.82 Impurlty Decontamination Factor 7 3 1 13 9 39 7100 11 Areas of Comparison Probab i l i t y of successful development f o r use in WESF Product Density Equipment of f -gas control Chemical p u r i f i c a t i o n Product ingot dimensional control E f fec t on WESF throughput Equipment ope rab i l i t y i n a hot c e l l Equipment ma in ta inab i l i t y i n a hot ce l l E f fec t on preparation of feed powder Capsule compat ib i l i t y TABLE V Comparison of Melt Casting and Hot Pressing Melt ing Casting High >9SZ TO Capsule power density Requires careful o f f -gas cleaning Has potent ia l for s i g n i f i c a n t p u r i f i c a t i o n Adequate High p robab i l i t y of adequate throughput Comparable to current WESF operations Comparable to current WESF operations Can probably el iminate current high temperature dry ing step Potent ia l fo r Improved capsule compat ib i l i t y due to reduced impur i t ies S i g n i f i c a n t l y increased power density because of p u r i f i c a t i o n for Use in WESF Hot Pressing High >95Z TD Less of f -gas cleaning requirement than melt cast ing Has potent ia l f o r s l i g h t Increase i n impur i ty contamination Very good Would reduce throughput More complicated than melt casting More complicated than melt casting Requires dry, uniform powder COMPARISON OF MELT CASTING WITH HOT PRESSING Table V shows a comparison--of-melt casting wi th hot pressing for densifying SrF2. Hot pressing i s superior to melt casting in three areas: i t i s a developed technology, i t has been used in hot c e l l s fo r many years, and i t requires a s i g n i f i c a n t l y lower temperature and less of f -gas c leaning. Although melt cast ing i s done at a somewhat higher temperature, operating a t atmospheric pressure or less leads to a s i g n i f i c a n t l y simpler process. No high-pressure equipment or hydraul ic f l u i d s are requ i red ; thus, the i n - c e l l operation and maintenance may be s i m p l i f i e d . The greatest advantage of melt cast ing over hot pressing i s the potent ia l fo r p u r i f i c a t i o n during the mel t -cast ing process. The prel iminary decontamination fac tors observed appear s u f f i c i e n t to remove most i n i t i a l impur i ty . Furtherpiore, melt cast ing appears capable of removing from 80 to 95Z of the decay-product zirconium present In stored strontium f l u o r i d e . Such p u r i f i c a t i o n may resu l t in an increased power density of up to 15Z in melt cast ing of SrF2 from capsules cur rent ly in the WESF storage basin. This p u r i f i c a t i o n i s in contrast to a probable small increase i n impur i ty content that i s expected to occur during mater ial handling pr ior to hot pressing. Melt cast ing I s believed to have a higher throughput rate and can d igest a lower qua l i ty feed material than hot pressing. SUMMARY Su f f i c i en t work has been done to provide a high degree of confidence that melt cast ing could be developed i n to a r e l i a b l e process for use in WESF. The work remaining to be done Is mostly equipment development. The work done to date has shown that SrF2 ingots wi th bulk dens i t ies that are greater than 98Z o f the theore t ica l densi ty of pure SrF2 can be cast to c losely cont ro l led s izes. The cast ingots have good physical strength and can be read i l y cut to any desired length wi th a ceramic saw. With the exceptions of CaFo and BaFj, good chemical p u r i f i c a t i o n i s obtained from impur i t ies in the WESF product. A decontamination fac tor o f 3 to 10 from the zirconium daughter i s a s i gn i f i can t advantage in improving the power densi ty o f aged ma te r i a l . ACKNOWLEDGMENT This work was supported by the U. S. Department of Energy under Contract DE-AC06-76RL0 1830. 

Feasibility of meltcasing strontium fluoride to produce high-density heat sources

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Feasibility of meltcasing strontium fluoride to produce high-density heat sources

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