6\' CENTRAL Rl 3] & F - I T p AT TR ETI R TIM ™ 1 P T ‘ DO{"""-J "I""-.- J—J-I\‘. -‘l 1‘ v 4 J-' n.-‘ h AEC RESEARCH AND DEVELOPMENT REPORT 9%y 1434 Special Features of Aircraft Reactors MARTIN MARIETTA ENERGY SYSTEMS LIBRARIES IR AT 3 445k 0350527 7 0 SOME ASPECTS OF THE BEHAVIOR OF FISSION PRODUCTS IN MOLTEN FLUORIDE REACTOR FUELS M. T. Robinson W. A, Brooksbank, Jr. H. W. Wright T. H. Handley OAK RIDGE NATIONAL LABORATORY OPERATED BY UNION CARBIDE NUCLEAR COMPANY A Division of Union Carbide and Carbon Corporation UCC POST OFFICE BOX X * OAK RIDGE, TENNESSEE ERRATA for ORNL-2373 NOTICE The Molten Fluoride Reactor Experiment (MFRE) referred to in this report is officially designated as the Alircraft Reactor Experiment (ARE) ¥ & _ ORNL-237h4 J This document consists of 20 pages. Copy /23 of 251 copies. Series A. Contract No. W-T4O5-eng-26 SOLID STATE AND ANALYTICAL CHEMISTRY DIVISIONS SOME ASPECTS OF THE BEHAVIOR OF FISSION PRODUCTS IN MOLTEN FLUORIDE REACTOR FUELS M. T. Robinson W. A. Brooksbank, Jr. S. A. Reynolds H. W. Wright T. H. Handley DATE ISSUED AUG £ § 1957 OAK RIDGE NATIONAL LABORATORY Operated by UNION CARBIDE NUCLEAR COMPANY A Division of Union Carbide and Carbon Corporation Post Office Box X Oak Ridge, Tennessee 1ES MARTIN MARIETTA ENERGY SYSTEMS LIBRAR M u 3 445k 0350527 7 1. R. G. Affel 2. C. J. Barton 3. M. Bender , L. D. 8. Billington 5. F. F. Blankenshif 6. E. P. Blizard T. C. J. Borkowski 8. W. F. Boudreau 9. G. E. Boyd 10. M. A. Bredig 11. E. J. Breeding 12. W. A. Brooksbank, J7 13. W. E. Browning 14. F. R. Bruce 15. A. D. Callihan 16. D. W. Cardwell 17. C. E. Center (K-25) 18. R. A. Charpie 19. R. L. Clark 20. C. E. Clifford 2l. J. H. Coobs 22. W. B. Cottrell 23. 8. J. Cromer 24L. R. S. Crouse 25. F. L. Culler 26. D. R. Cuneo 27. J. H. DeVan 28. L. M. Doney 29. D. A. Douglas 30. E. R. Dytko 31. W. K. Eister 32. L. B. Emlet (K 33. D. E. Ferguson 34. A. P. Fraas 35. J. H. Frye, JiE 36. W. T. Furgergin 37. R. J. Gray 38. A. T. Greskj§ 39. W. R. 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Division of Regearch and Develop t, AEC, ORO . & ; - nsion, Oak Ridge Observations are reported on the behavior of several fission product elements in fused NaF-ZrFA- 4 fuels, irradiated in capsule experiments, forced-convection in-pile loop experiments, and in the Molten Fluoride Reactor Experiment (MFRE). .The rare pases have been ob- served to escape readily from the fuels in dynamic tests, although in static tests the rate of escape is very low. Ruthenium and niobium depogit on the Inconel walls of the fuel container, probably as metals. Other fission products studied (Sr, Zr, la, Ce) appear to remain in the %uel. The unsatisfactory nature of Cs137 as a fission .. monitor in such fuels is reported ang-the use of Zr95‘as a substitute is discussed. . The hypothesis 'is propesed that fission product deposition may serve to reduce corrosion of metals by molten fluoride fuels. e -D- The chemical behavior of the fission product elements is of great importance in any fluid-fueled nuclear reactor, as well as in the re- processineg of nuclear fuels of any sort. Observations are reported here on the tehavior of several important elements in fused fluoride fuels (1) of the type employed in the Molten Fluoride Reactor Experiment (MFRE)(2). Most of the fuels examined have been of the NaF-ZrFA—UF4 type, with various compositions. The samples examined were taken from three different types of experiment: l. Static fluoride irradiations: Observations are reported on samples of fuel from two in-pile static corrosion .tests (3). Two experiments are also reported on the removal of %6125 from static fluorides. 2. Dynamic fluoride irradiations: Observations are reported on fuel samples from two in-pile forced-convection loop tests and on metal samples from one of these (4, 5). 3. The MFRE: Observations are reported on a fuel sample and on a metal sample from the MFRE (6). Behavior of the Rare Gases The fission monitorine technique based on 09137, developed at the Argonne National Laboratory (Z), was applied to two samples of NaF~ZrE4- U23fiflflk(50n46n4 mole %, respectively) which had been irradiated in the MTR fdr 116 hrs and 325 hrs, respectively, at about 800°C, at a thermal neutron flux of (2.3640,16) x 1014 neutrons c@iRsec=d. The results are shown in Table I. It will be observed that although ascreement between the measured and calculated numbers of fissions occurring in the sample is good in the shorter irradiations, in the loneger omne it is very poor. -3 A portion of the capsule which was exposed to vapors from the molten salt was dissolved in each case and a Cs137 determination was performed on the resulting solution. The results (last column of Table I) show appreciéble amounts of Cs237 to have been present on these surfaces. These results are taken as evidence of the escape from the fuel of 3.9 minute Xel37, the parent of the cesium isotope. | An attempt was made to study directly the evolution of X3135 from irradiated fluorides. Two runs were made under identical conditioms, except that in one case the fuel was sparged by bubbling He throush it, while in the other case, the carrier gas merely swept over the surface of the melt. The helium, purified by passaee over hot copper turnines and marnesium perchlorate, was conducted to and from the capsule through 0.036 in. 0.d. stainless steel capillary tubing. The off=-ocas was passed throuch two Dry Ice acetone—cooled traps, the secéond filled with activated charcoal to hold the xenon. A helium eras flow rate of 15 ml/min was used in each experiment. The fuel sample in each case was 1 em of NaF—KF-UF4 (4605-26,0=27.5 mole #,meltine point 530°C), containing normal uranium. It was irradiated in the ORNL rraphite Reactor at 650 11 neutrons cm“z,Sec’l, for to 750°C, at a thermal neutron flux of 7 x 10 3] minutes. After waiting 4.5 hours for short~lived activities to decay, helium flow was started and continued for 6.5 hours. The capsules re- mained in the reactor durineg this periodo The thermal neutron dose was monitored with a ¢lip of Al-Mn~Co alloy, removed and counted immediately after the irradiation was completed. The amount of Xe135 was determined . Table I cs137 Analyses in MIR-Irradiated Static Fluorides Flux 2(2.36%0,16) x 104 neutrons cm=2ses~l Temp.= 800°C 18 03137 recovered from Time of cs137 (fissions/em x 10~18) Capsule tops 18 Irradiation (hrs) Observed (a) Calculated(b) (fission x 107 116 0,085 £ 0.005 0.11%0.01 0,001 325 0.091% 0,010 0,28+ 0,03 0,013 (a) Based on ANL calibration of Cs1?7 f£lux monitorine method (7). (b) Based on flux determined by Co activation; corrected for flux depression, _5- by transferring the contents of the charcoal trap to an appropriate vessel and counting in a 4= | peometry hich-pressure ionization chamber. The results are shown in Table ITin terms of the response of the instrument used. No absolute calibration was made. It may be said, however, that the amount of Xe13® recovered in the sparging expsriment was approximately that expected from the fission history of the sample. It is clear from the results of Table II that the rare cases do not aiffuse extremely readily from static fused fluorides umder the conditions of these experiments. Their removal is easily accomplished, however, by éfficient sparging of the fuel with helium, As one part of the operation of the MFRE (6), a so-called xenon experiment was performed. The control rods were calibrated during the period when the reactor was beine brought to criticality by measured additions of NazUF6 to the originally uranium-free salt. In the "xenon experiment", the rod positiqn was recorded as a function of time during a 20-hour run at a nominal power of 1.5 meeawatts. The rod position data were converted to A k/x values usine the previously estatlished calibration. When these results were corrected for Smi4? polsonine and for the decrease in reactivity due to <35 burnup; it was apparent that virtually all of the %6135 had been removed from the fuel. Whils no certain quantitative interpretation can be given of the poisoning remaining after correction for Sm14Y and burn-up effects, it appeared that no more than about 2% of the expected Xe135 remained in the reactor fuel during the period in question. During operatiqpfg{ the MFRE, an s¢cidential leak of gases occurred from the reactor .imto the pit im whigh. it was installed (6). This pas was dispersed by drawing it into an emercency off-gas line Table II Evolution of X9135 from Irradiated Static Fluorides 1 Flux =7 x 1011 neutrons em~?sec” Temp = 650 to 750°C Thermal Neutron Dose(b) Amount of Xel135 (t) ) Observed ~ Calcuiated ‘2 o ————— Fuel sparged with He 0.117 1.44 oo eam Fuel surface swept with He 0.097 0,032 1.22 (a) Based on results obtained in sparging experiment; corrected for slioht difference in uranium content of the two capsulss. " (b) Arbitrary units -7 inserted into the pit. A sample of the off-cas from this line, adsorbed on cooled charcoal, was examined by Bell, et al. (8), primarily by camma-ray scintillation spectrometry. They established the presence of bS8 (daushter of 2.8 hr. Krss), Xe135, and cs138 (daughter of 17 min.Xel38), but were unable to identify many of the observed peaks in the samma-ray spectrum. Determination of the amounts of cs?37 1n the fuel of the MFRE and of one of the in-pile loops indicated the escape of less than about 20% of the X9137 from these systems. The data obtained on both static and dynamic systems demonstrates that the rare eases are evolved readily from fused fluorides, although in static systems, the rate of evolution is very low. The fraction of any rare gas isotope which will be removed from a fluid fuel may be estimated using a theory developed for Xel35 poisoning kineties (9). This fraction depends on the geometry and flow conditions of the specific reactor, as well as on the radioactive half-life of the nuclide in question. Llonger-lived nuclides will be removed to a ecreater extent than shorter~ lived ones, very crudely in proportion to their half-lives. More detailed discussion of the matter is deferred here, since it is treated in another place (9). Bohavior of Ruthenium and Niobium Samples of fluoride fuel removed from two in=pile forced-convection loops and a sample from the MBRE were examined for the presence of Ru103 by radiochemical techniques. The results are shown in Table IlI. The very marked reduction below the expected levels of the R0 gontent of the fuel, especially in the LITR loop and in the FSRE; indicated the existence of an _8- Table IIY Rulo3 Analyses of Irradiated Fluoride Fuel from Dynamic Experiments LITR MFRE MTR loop —_— loop Fuel Composition (mole% NaF-ZrFA—U235F4) 62.5-12.5-25.0 53.5-40.0-6.5 53,5-40.0-6.5 Fissions/cm3 of fuella) x 10~16 12.9 8.7 . 655 Calculated Ru103 concn. in fusl (atoms/ecm3 x 10-15) 3.9 2.5 - 190 Observed Rul®3 conen. in fuel (atoms/em3 x 10-15) 0,001 0.00003 104, Ratio, surface/volume (em~—1) 3.5 1 5 Averace Ru103 surface concn. (atoms x 10“15) 1.1 205 17 (a) Estimated from hest generation for LITR loop and MFRE; estimated from Zr?> analyses for MIR loop. _9_ an efficlent means of ruthenium removal from molten fluorides. It was possitle to obtain salt-free sections of Inconel pipe from the LITR loop (4) and from the reactor. These sections were selected from parts of each system which were not exposed to high thermal neutron fluxes, thus avoiding activation of the cobalt content of the Inconel. One pipe section was selected from a reeion of the LITR loop upstream from the hish-flux region, another from a re-ion an equal distance down- stream. famma-ray spectrometry of these samples showed the presence of Ru103 activity and of Zr?5-Np?5 activity. The latter activity occurred to the same extent in each section, but the Ru103 éctivity in the downstream section was 40% greater than that in the upstream section. After a delay of 53 days, the two sections were re-examined. The Ru103 in both samples decayed with an apparent half-life of about 42 days, in eood acreement with published data (1Q). The Zr?9-Nb?2 activity, however, decayed with an apparent half-life of 40 to 43 days. This indicates that the active deposit must have contained ~ 95% Nb?> (35 days) and only ~ 5% Zr95 (65 days) at the time of reactor shutdown. The relative amount of Nb7° expected if no segrecation of the element occurred is about 5% of the total activity. The pipe section from the MFRE was a rineg cut out of the fuel inlst line to the reactor core. Three samples cut from this ring showed the presence of RulOB, Ru106, and Zr95—Nb95. Two of the samples were re- examined after a delay of 130 days. The apparent half-life of the Zr95-Nb95 activity was 50 days in each case, acain suggesting that the deposit was very largely Nb959 An autoradioeraph of the third pipe -10- sample showed the radiocactive deposit to be well localized at the fuel-metal interface, within the rather poor resolution obtainable with beta radiation. A pipe eltow, which served as the inlet end of the MFRE emergency off- eas line, was examined for radioactivity. A very small amount of Ru103 was detected, which was shown by chemical treatment to be entirely on the outside of the pipe. It appears likely that a small amount of RuFs or of RuOA (from reaction with air that may have been introduced into the reactor when the leak occurred) volatilized from the fuel. In view of the larpe amount of ruthenium found on fuel container surfaces, it is felt that volatilization of this element is of very little importance in its removal from the fuel. This view is supported ty experience to date with the fluoride volatility process (1l) for recovering uranium from spent fuel. It is evident from the results reported above that ruthenium is rapidly and efficiantly removed from fused fluoride fuels bty Inconel container surfaces. However, the data obtained for the MIR loop experiment : indicate that saturation of the walls with ruthenium was approached in that case. If this interpretation of the data is indeed correct, it seems reasonable to suecest that deposition of fission product metals may well interfere with the course of the ordinary corrosion process, (1, 12) and that long-term in-pile corrosion of metals by fluoride fuels may be signi~ flecantly less than predicted from comparable out-of-pile tests. Short-term in-pile corrosion tests to date are not in disacreement with this hypothesis (13). Niobium appears to deposit on Inconel alone with ruthenium. It appears likely that molybdenum also deposits, but there has not yet been an opportunity to examine samples soon enough after irradiation to observe 67 hroMb999 the -11- loneest-lived radiocactive isotope of this element which is known in fission. It is also possitle that zirconium may deposit from fuels not containing ZrF 4’ Behevior of Other Fission Products Samples of fuel drawn from the MFRE dump tank were examined by radio- but no experiments have yet been comducted on such materials. chemical methods. In order to eétimate the efficiency of retention of some typical fission products, these analyses were compared with similar results obtained on a sample of NaF—ZrF4=UF4 (50=46-4 mole 4) irradiated in the solid state in the ORNL fraphite Reactor. The irradiation time was matched approximately to the high ~power operating time of the MFRE. The comparative analyses of the MFRE fuel and the standard are shown in Table IV. It is - clear that, with the exception of RuloB, there 1s no gross loss of the fission product nuclides listed. The ratio obtained for SJ:°89 could be interpreted to show partial loss of its parent, 2.6 min. Kr89, but no explanation can be offered for the value obtained for zr??, It is likely that no loss occurred of any of these fission product elements from the fuel of the MFRE, and that the variation from 0.3 to 1.6 is a raflection of experimental errors, such as inhomogeneous samples, chemical difficulties in the complex fluoride system, etc. A determination was also made of the ratios of activities of 03136 and 09137_1n.the two samples. The result indicated the loss of less than 20% of the Xel37 parent of the latter nuclide. Analysis of the oross camma~-ray spectrum of a 7 mg. sample of fuel from the MFRE was continued throuch the period from 31 to 81 days after shutdown of the reactor. The total activity of the sample was determined in a high-pressure ionization chamber. These results were combined with gamma- ray spectral data to yield both total phOton emission rates and differential -12- decay data. The only activities detected were Ba140-La140, 091419 and Zr95~Nb95o Neither Ru103 nor 1131 were observed. The specific camma activity of the sample was.estimated as 16 mc/em 31 days after reactor shutdown and 3.5 mc/em 79 days after shutdown. The averace gamma-ray energies were 0,96 and 0.73 Msv, respectively. Bell and his coworkers (8) weré unable to establish the presence qf iodine and bromine in the sample of MFRE off-gas which they examined. Since 3 day 1431 could not be detected in the analysis of the eross ramma~-ray spectrum of the MERE fuel, the question of the fate of the halosen elements in molten fluoride fuels must be left open. Use of Zr?? as a Figsion Monitor Uncertainties as to the applicability of 05137 as a fission monitor in fluid reactor fuels, led to the adoption of Zr?? as a substitute,; at least for fuels containine macroscopic amounts of normal zirconium. In order. to calibrate the use of this nuclide, two samples of enriched uranium were irradiated as solutions for 2.375 days in the ORNL fraphite Reactor. One solution was prepared from U3089 the other from a typical NaF=ZrF4wUFA fuel, After 10 days decay, radiochemical determinations were made of~Gs137 and ZE95 o GComparison of these results, using the ANL 68137 standards,(7b), rave a fission yield for Zr’° of 0.0664 t 0,0013 atom/fission. Usine the inte~rated neutron dose, measured with a cobalt monitor, the yield is 0.0632 * 0.0021 atom/fission. The discrepancy between the two results is removed when the cobalt activation flux is corrected for the cadmium ratio prevailine in the irradiation facility (~ 30). The vyield recommended for use in fission monitorine with Zr95 is 0.0664 =+ 0.0013 atoms/Pission. -13- Table IV Fission Product Analyses in MFRE Fuel, Compared to a Standard Sample Nuclide sr89 2535 Ry103 1240 Colél Msan (Omitting Ru103) Activity Ratio, MFRE/Standard 0.6 0.3 1.6 x 102 1.5 1.6 1.0 £0.5 -~ -1k Conelugio While most fission product elements remain in solution in molten fluoride fuels, two important classes of elements have a sfirong tendency to escapes the rare cases by volatilization and the noble metals, Ru and Nb, by deposition on the walls of the container. Experiments are currently in proeress, which will reveal the behavior of other elements, particularly the helocens, Mo, and Zr, the latter in fuels not containing ZrF4 as a constituent, Additional information is required concerning the extent to which metallic deposition can occur on various metals and the desree of protection which the deposited coatineg affords acasinst corrosion of the container ty the fuel. It must be emplasized that in choosing suitable fission monitors for use in fluid fuels, dlose attention must be given to the chemistry of the fission product elements. While cs137 ig very satisfactory for this application in so0lid fuel elements, the escape of its parent, X5137, makes it unreliable in fluid fuels., Similarly, Zr?3 may be unsuitatle in fused fluorides which do not contain ZrFA as & major constituent. Acknoyledeements The data reported here could not have been obtained without the assistance of a oreat many members of the staff of the Oak Ridee National Laboratory. C. C. Webster, W. 5. Piper, and R. H. Shields otteined many of the samples. J. H. Oliver assisted in many of the experiments. E. I. Wyatt's oroup in the Analytical Chemistry Division performed many of the radiochemical ahalyses. We wish to thank these people for their assistance in the work reported. -15- References 1. W. R. frimes, et al., American Nuclear Society Meeting, Pittsburg, Pa., 2 3 4o 7 8. 9. 10, 11. 13, June 10-14, 1957. E. S. Bettis, et al., ibid. J. 0. Morgan, H. E. Robertson, A. E. Richt, and P. R. Klein, unpublished. 0. Sisman, W. W. Parkinson, and W. E. Brundase, ORNL=-1965, Jan. 15, 1957 (Secret). D. B. Trauger, et al. unpublished. W. B. Cottrell, et g8l., American Nuclear Society Meeting, Pittsbure, Pa., June 10-14, 1957. (a) W. A. Brooksbank, Jr., ORNL-1547, p. 7, June 24, 1953; (bt) D. Fredrickson,, et al., ANL-,922 (1952), P. R. Bell, R. C. Davis, W. K. Ereen, . . Kelley, and N. H. lazar, unpublished data. M. T. Robinson, American Nuclear Society Meeting, Pittsbure, Pa., June 10-14, 1957. E. ., J. 0. Blomeke, Nuclear Properties of U35 Fission Products, ORNL-1783, October 21, 1955. fn. I, Cathers, American Nuclear Society Meeting, Washington, D. C., December 10-12, 1956. W. D. Manly, et al., American Nuclear Sociely Meeting, Piitsburg, Pa., June 10-14, 1957 o W. Keilholtz; W. E. Browning, and J. G. Morgan, ibid.