Neutron cross sections have been measured near 14 MeV for reactions leading to long-lived radioisotopes in fusion materials. Results are summarized for various reactions leading to /sup 26/Al (720,000 y), /sup 55/Fe (2.73 y), /sup 63/Ni (100 y), /sup 59/Ni (76,000 y), and /sup 94/Nb (20,300 y). New data are presented for the production of /sup 91/Nb (680 y) from /sup 92/Mo, and from /sup 93/Mo (3500 y) and /sup 93m/Nb (13.6 y) for /sup 94/Mo. Our measurements suggest that the half-life of /sup 91/Nb may be shorter (350 y) than previous estimates. Measured cross sections are used to predict the production of these isotopes for various fusion reactor designs. 20 refs., 4 tabs

Bowers, D.L.

Greenwood, L.R.

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Production of Long-Lived Activities in Fusion MaterialsCONF-871036--15DE88 002963L. R. Greenwood and D. L. Bowers"Chemical Technology Division"Analytical Chemistry LaboratoryArgonne National LaboratoryArgonne, IL 60439 USADISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, 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, orprocess 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 Unwed States Governmen'. or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency (hereof.Proceedings of Third Internationa] Conference on Fusion Reactor MaterialsOctober 4-8, 1987, Karlsruhe, F. R. GermanyDISTRIBUTION OF THIS DOC'McKT iS !l\" T'iTEDProduction of Long-Lived Activities in Fusion MaterialsL. R. Greenwood and D. L. BowersArgonne National Laboratory,Argonne, II USAAbstractNeutron cross sections have been measured near 14 MeV for reactions leadingto long-lived radioisotopes in fusion materials. Results are summarized for variousreactions leading to 26A1 (720,000 y), 55Fe (2.73 y), 63Ni (100 y), 59Ni (76,000 y),and 94Nb (20,300 y). New data is presented for the production of 91Nb (680 y) from"Mo, and for 93Mo (3500 y) and 93mNb (13.6 y) from MMo. Our measurementssuggest that the half-life of 9INb may be shorter (350 y) than previous estimates.Measured cross sections are used to predict the production of these isotopes forvarious fusion reactor designs.IntroductionPreviously we have reported measurements of various neutron reaction crosssections leading to long-lived radioisotopes in fusion reactor materials.1"8 Such dataare needed to assess possible restrictions on the waste disposal of materials. Thecross sections are also used for neutron dosimetry, plasma diagnostics, and reactormaintenance applications. In particular we have focussed on the most importantlong-lived radioisotopes formed by (n,2n) reactions near 14 MeV. Since these reac-tions generally have thresholds above 10 MeV, the 14 MeV cross section is usuallysufficient to determine production rates in fusion materials. A wide range of ma-terials have been irradiated at the Rotating Target Neutron Source (RTNS) II atLawrence Livermore National Laboratory to neutron fluences on the order of 1022n/m2. A variety of techniques have been used to measure the activities followingradiochemical separations.Experimental ProceduresSamples were prepared from the pure metals of Al, Fe, Mo, Cu, Zr, Ni, andNb and the separated isotopes of 56Fe, 60Ni, MNi, 94Zr, and 94Mo. The separated94 Mo isotope consisted of a metallic powder pressed into thin discs measuring 3.0mm o.d. by 1 mm long. The 94Mo enrichment was 92.03% with 5.2% 95Mo and lessthan 1% of the other Mo isotopes. All of the samples were irradiated in thin alu-minum tubing along with dosimetry samples. Several irradiations were conductedat the RTNS II using both short irradiations close to the source and longer, pig-gyback run6 farther away. Details concerning the neutron energy spectra at RTNSII have been published previously.1*3 In all cases, the neutron fluences were deter-mined from radiometric dosimetry data based on either the 93Nb(n,2n) MmNb (10day) and S4Fe(n,p)S4Mn (312 day) reactions. Measurements are thus referenced tostandard cross sections of 463 mb (±4%) for 93Nb9 or to values between 331 mb(14.5 MeV) and 303 mb (14.8 MeV)(±7%) for "Fe1 . Many dosimeters were used tomap tiie neutron fluence over the sample assemblies. Radiometric measurements ofthe samples themselves were used to further refine the neutron ftuences relying onpreviously reported data near 14 MeV.J The precise geometry was determined foreach sample from the overall map of the neutron fluence. The average angle thendetermines the average neutron energy for each sample based on the calculatedneutron spectra.1 The neutron energies have a typical spread (fwhm) of about 0.4MeV.In most cases, radiochemical separations were used to isolate the desired activ-ity from interfering species. Details of these separations were published previously.7Gamma and x-ray spectroscopy were performed on most of the samples using high-purity, thin-window germanium detectors. This technique provided a good monitorfor the presence of undesirable activities. Direct gamma measurements were usedto detect 94Nb, 9 1 mNb, and selected samples of J6A1. X-ray measurements wereperformed for 59Ni, 91»Nb, "Mo, and 9 3 mNb. Liquid scintillation spectrometry wasused to detect S5Fe which decays by electron capture and the low-energy beta de-cay of 63Ni. In these cases, radiochemical separations and analyses of the energyspectra were critically important. The energy spectra of the measured activitieswere matched to standard spectra from similarly prepared samples (55Fe, 63Ni)to insure proper identification. The relatively new technique of accelerator massspectrometry was used to detect 2SA1.Summary of Previous MeasurementsAll of the reactions which we have studied are listed in Table I and referencesare given for each measurement. The cross sections for these reactions are summa-rized in Table IT and compared to previous data. Activation cross sections for 22reactions were measured at five energies between 14.5-14.9 MeV at the RTNSII,1and these data were very useful in refining neutron fluence maps for each irradiation.The 27Al(n,2n) data include measurements to the ground state (720,009 y) and iso-meric state (6.34s).4 The 54Fe(n,2n) measurements5 to 61Fe have been shown to bequite useful for plasma diagnostics at fusion reactors.6 Furthermore, the reactionis of interest since 53Fe decays to 83Mn (3.7xlO8 y). Due to the very steep energydependence of these two reactions near 14 MeV, data is not included in Table IIor in later estimates of fusion reactor activities. Additional measurements are inprogress or are planned, as detailed in Table I.New Measurements for Mo ReactionsWe have published results for the production of 9IfnNb and 94Nb from naturaland enriched 94Mo targets.3 New measurements have been completed for 91*Nb,9 3 mNb, and 93Mo from these same targets. Samples were dissolved in a mixture ofHCI and HNO3. Small aliquots were then flame dried on tantalum discs and cov-ered with a thin layer of tape for x-ray counting. The deposits generally measuredabout 1 mg of Mo spread over an area of about 0.5 cm3. In this way self-absorptioncorrections were less than 1% for x-rays in the 15-19 keV region. Absolute calibra-tion of the detector was determined with a 93mNb x ray standard (NBS-SRM4267).93rnNb has a half-life of 13.6±0.3 years and the K x-ray (16.5-19.1 keV) intensity is10.57±0.54 %. The half-life of 93Mo is 3500±700 years and it has the same x-raysas 93tnNb except that the intensity is 73.0±2.7 %. 91*Nb decays with a lifetime of680±130 years and the K x-rays (15.7-18.1 keV) have an intensity of 63.6±2.2 %.All of the above decay data is taken from reference 10.Since we irradiated both natural Mo and 9<Mo-cnriched samples, measurementsof the K x-ray activity can be used to uniquely determine the fraction of the activityin each sample which is due to either 92Mo or 94Mo. At the long decay timesof our measurements (1090 days), there are no other known Gources of Nb or ZrK x-rays from the other Mo isotopes, except for very long-lived isotopes such as92Nb (3.7x107 y) and 93Zr (1.5xlO6 y). Four samples were counted (2 natural, 2enriched) and a simultaneous fit to the data yields the cross section values listed inTable III. In the case of 92Mo, the only known activity with x-rays in this energyregion is 91ffNb. The 62 day isomer of 91mNb has already decayed away in thesesamples. 91ffNb can be made by several reactions, primarily 93Mo(n,2n)9IMo whichrapidly decays to 91Nb. Although there is an isomer in 91Mo and in 9JNb, allof these reaction products eventually decay to 91*Nb. 91Nb can also be made bythe 92Mo(n,d+np+pn) reactions. Hence, our cross section value represents the netproduction of 91srNb from 92Mo.In the case of 94Mo, we can make 93Mo from the (n,2n) reaction and MmNbfrom the (n,d+np+pn) reactions. 93mNb can also be made from the (n,t+nd-f-nnp)reactions on 95Mo; however 95Mo is only 5.18% of the enriched sample and thus canonly make a minor contribution (x 1%). The situation is further complicated bythe fact that 93Mo decays to 93TONb. We plan to perform a radiochemical separationof Nb and Mo in order to uniquely determine the cross sections for both species.However, at present we can only place limits on the cross sections for both reactions.The decay of 93Mo to 93mNb is rather siow due to the long half-life of 3500 yearand we calculate that at least 2% of the present activity must be due to 93mNb.Hence, we can place upper limits on both cross sections, as is done in Table III.It is, however, most probable that we produced more 93Mo than 93mNb during theirradiation, as discussed below.Comparison With Previous DataThere are no known previous measurements of the 94Mo(n,2n)93yMo cross sec-tion. The cross section to the 6.9 hr isoroeric state of MMo is reported to be 2.4-6.4mb11*12'13; however, the reaction proceeds mainly to the ground state of 93Rlo andthe total cross section should be much larger.Haight,et al.,M have reported a value of 9±3 mb for the 94Mo(n,d) reaction crosssection. This would be quite consistent with our results; however,our measurementsalso include other possible reaction channels such as 94Mo(n,np-f pn).We have used the THRESH215 semi-empirical computer code to estimate thestrength of various reaction cross sections in order to gain some insight concerningthe relative importance of unobserved reaction's. These calculations predict a rela-tively flat energy dependence near 14 MeV for the 94Mo(n,2n) reaction with a crosssection of about i barn. This calculation is close to our maximum value of 810 mb.THRESH also predicts that the 94Mo production cross section for 93mNb is 51 mband that the (n,np-f-pn) cross sections are about equal to the (n,d) value. However,these reaction cross sections are steeply rising near 14 MeV. If we assume that theratio of the production cross sections from THRESH2 for 93Mo / """Nb is roughlycorrect, then we would estimate that the 9*Mo(n,2n)93Mo cross section is about 280mb and that for 94Mo(n,x)93mNb is about 17 mb. This latter value would agreewith the (n,d) measurement14 assuming equal strength for the (n,np+pn) reactions.There have been several measurements16"18 of the 93Mo(n,2n)91Mo reactionto both the ground and isomeric states. Unlike the 94Mo(n,2n) reaction, the92Mo(n,2n) cross section is steeply rising with energy near 14 MeV. Hence, somecare must be taken in comparisons with our data since our energy resolution isabout 0.4 MeV. Previous measurements give a value of about 200-250 mb in ourenergy region between 14.5-14.9 MeV. However,our measurements also include theproduction of 9 l9Nb by the 92Mo(n,d+np+pn) reactions. Haight,et ai.,14 measureda value of 22±7 mb for the (n,d) reaction at 14.8 MeV. Using the THRESH2 com-puter code, we estimate that the (n,pn+np) reaction cross sections sum to about70 mb. Thus the total production cross section of 91*Nb should be about 300 mb,much less than our measurement of 603 mb.There are several possible explanations for the discrepancy between our mea-surements and previous data for the production of 91flNb from 93Mo. Since ourmeasurement is based on the strength of the Zr K x-rays, it is possible that thereare other contributing isotopes. However, due to the long decay time before anal-ysis (1090 days), we are unable to identify any known interferences from otherMo isotopes or impurities in the material. It may also be that the unobserved92Mo(n,np+pn) reactions have a much larger cross section than predicted by THRESH2;however, this seems highly unlikely 6ince we expect these reactions to be muchweaker than the (n,2n) reaction. A more likely explanation is that our measure-ments could be reconciled with previous data if the half-life of 91Nb were reduced toabout 350 years. The present value of 680±130 years has a rather large uncertainty(19%). Further measurements are needed to resolve this issue.Discussion and ConclusionsFurther measurements are in progress to improve our Mo results. We planto chemically separate Mo and Nb, thereby resolving the cross sections for theproduction of 93Mo and 93mNb from 94Mo. Higher-resolution x-ray measurementsmay also help to resolve the Nb and Zr x-rays and should separate any possibleinterferences with the 9INb reaction.The measured neutron cross sections can be used to calculate the productionof long-lived isotopes in fusion reactor materials, as shown in Table IV. These cal-culations are for the STARFIRE19 fusion reactor design for a six year operatingtime (21.6 MW-y/rn2); however, effects due to thermal neutrons have been ne-glected due to uncertainties in shielding calculations. In most cases, the activationis mainly due to the interaction with 14 MeV neutrons since there is no signifi-cant neutron flux above the typical threshold energies of 7-13 MeV. However, inthe case of the 63Cu(n,p) reaction, lower energy neutrons increase the productionof 63Ni significantly. Since we do not know the energy-dependent cross section forthis reaction, we assumed that the relative contribution from lower energy neu-trons would be about equal to that from the 14 MeV neutron flux, as reportedpreviously.17 Calculations were not included for the 27A1 and 54Fe(n,2n) reactionssince the activities are crucially dependent on the plasma temperature and neutralbeam heating parameters.6 In the case of MMo and MntNb, we can only place limitson the activities since we cannot separate them in our data. For 91Nb, we note thatthe cross sections are steeply rising near 14 MeV and we have thus used a lowervalue in our fusion calculations based on the energy dependence in previous (n,2n)reaction data.The fusion reactor activities reported in Table IV can be compared to previ-ous estimates.20 This comparison is not straightforward since we have neglectedtransmutation effects due to thermal neutrons. However, it is not certain that therelatively high thermal fluxes given for the STARFIRE first-wall spectrum should beused for bulk materials due to neutron shielding effects. If we compare our measure-ments with the 14 MeV cross sections used in that study, we find that the S3Cu(n,p)values are 80% higher, the 60Ni(n,2n) values are 3.0 times higher, the MMo(n,p)values are about 50% lower, values for 9aMo(n,2n)91Nb are 40% higher, and thosefor 93Mo and 93'nNb from 94Mo, as well as other reactions not mentioned, are quitesimilar. Hence, we would expect that the activity levels would 6cale accordingly ifwe included secondary effects due to thermal neutrons and side reactions as done inreference 20. Of course, since we cannot separate the 93Mo and 93mNb reactions, iiis likely that the activity levels will be substantially reduced from those in previouscalculations20. For example, if the cross section for the 93Mo is only about 280mb, as discussed earlier, then the fusion reactor activity will be reduced to only14 mCi/cc. In any case, it is probable that there will still be stringent limits onthe presence of Mo in fusion reactors if we are to meet the NRC requirements forclass C waste disposal. The implications of t.hese activities for fusion reactor designdepend crucially on the details of the design as well as our assumptions regardingfuture regulations for the disposal of waste materials.AcknowledgementsThis work was supported by developmental funding from the Analytical Chem-istry Laboratory at Argonne and by the Office of Fusion Energy, U. S. Departmentof Energy. We would also like to thank the staff of the RTNS II facility for assistingwith the neutron irradiations.References1. L. R. Greenwood, Recent Research in Neutron Dosimetry and Damage Analysisfor Materials Irradiations, Symp. on Effects of Rad. on Mater., Seattle, June 1986,ASTM-STP956, in press.2. L. R. Greenwood, D. G. Doran, and H. L. Heinisch, Phys. Rev. 34C,76-80(1987).3. D. W. Kneff, B. M. Oliver, H. Farrar IV, and L. R. Greenwood, Nucl. Sci. Eng.92,491-524(1986).4. R. K. Smither and L. R. Greenwood, J. Nucl. Mater. 122, 1071-1077(1984).5. R. K. Smither and L. R. Greenwood, Damage Analysis and Fundamental StudiesProg. Rep. DOE/ER-0046/17,11-13 (1984).6. R. K. Smither, L. R. Greenwood, and H. Hendel, Rev. Sci. lustrum. 56,1078-1080(1985).7. D. L. Bowers and L. R. Greenwood, Analysis of Long-Lived Isotopes by LiquidScintillation Spectrometry, Proc. Int. Conf. on Meth. and Appl. of RadioanalyticalChem., Kona, Hawaii, April 1987, to be published.8. L. R. Greenwood and D. L. Bowers, Measurement of Long-Lived Isotopes in FusionMaterials, Proc. Sixth ASTM-EURATOM Symp. on Reactor Dosimetry, Jackson,VVY, June 1-5, 1987, to be published.9. D. R. Nethaway,J. Inorg. Nucl. Chem. 40,1285(1978).10. E. Browne and R. B. Firestone, Table of Radioactive Isotopes, V. S. Shirley,Ed.,John Wiley, New York (1986).11. Fujino, NEANDC(J)-5lU,p.60(1977).12. S. Amemiya, Nucl. Sci. Tech. 19,p. 781(1982).13. Katoh, NEANDC(J)-116,p.75(1985).14. R. C. Haight, S. M. Grimes, R. G. Johnson, and H. H. Barschall, Phys. Rev.C23,700(1981).15. S. Pearlstein, Nucl. Cross Sections for Tech., NBS-SP425,332(1975).16. M. Bormann, H. K. Feddersen, H. H. Holscher, W. Scobel, and H. Wagener, Z.Phys. A277,203(1976).17. Y. Kanda, Nucl. Phys. A185,177(1972).18. W. Lu, N. Kumar, R. Fink, Phys. Rev. Cl,350(1970).19. STARFIRE- A Commercial Tokamak Fusion Power Plant Study, ANL/FPP-80-1,September 1980.20. H. L. Heinisch, F. M. Mann, and D. G. Doran, Fusion Technol. 8,2704-2707(1985).Table I: Long-lived Activities in Fusion MaterialsIsotope Half-life,y Reactions Status5730 14N(n,p) in progress26 Al53Mn55F e63Ni59N i93Zr92Nb91Nb9 4Nb93mNb93Mo7.2xlO53.7xlO62.73100.7.6xlO41.5xlO63.2xlO7680.2.0xlO413.63500.27Al(n,2n)54Fe(n,2n)/354Fe(n,d+np)56Fe(n,2n)63Cu(n,p)64Ni(n,2n)60Ni(n,2n)94Zr(n,2n)93Nb(n,p)93Nb(n,2n)92Nb(n,2n)94Nb(n,p)94Mo(n,x)94Mo(n,2n)Ref 4Ref 5PlannedRef 8Ref 8Ref 8Ref 8In progressIn progressIn progressThis workRef. 2This workThis workTable IIFe Isotopic Mass Spectrometry Measurements"(Values are atomic percents)(Calculations based on ENDF/B-V9)Fluence(1026n/rn2)54 FeExp. Calc. Exp. Calc.57Fe 58FeExp. Calc. Exp. Calc.6.37 5.02 5.01 78.36 77.70 14.84 15.00 1.28 1.683.48 5.33 5.35 83.54 83.73 10.12 9.73 0.70 0.803.21 5.41 5.39 84.83 84.83 8.86 9.21 0.59 0.74Mean(C/E) 0.999 0.996 1.003 1.23"Uncertainties: 54Fe 0.4%, 56Fe 0.07%, 57Fe 0.3%, 58Fe 1.2%Table III: Mo Cross Section MeasurementsReaction92Mo(n,2n)91eNb94Mo(n,x)s3Mo94Mo(n,x)93mNbEnergy,MeVa14.714.714.7cr,mb6O3.±1196-<810c-< 26daMean energy; width w0.5 MeVHJncert.: Stat. 1%, std. 2.4%, T 1 / 2 19%, fluence 5%, net 20%cUncert.: Stat. 1%, std. 2.4%, T 1 / 2 20%, fluence 5%, net 21%dUncert.: Stat. 1%, std. 2.4%, T 1 / 2 2.2%, fluence 5%, net 6%Table IV: Fusion Reactor Activation Calculations(STARFIRE design, 21.6 MW-y/m2, 3000 day cooling)Material Isotope Activity,mCi/ccfron ^Fe 25,000.Copper 63Ni 1795.Nickel 63Ni 227.59Ni 0.99Molybdenum 91Nb 243.94Nb 0.7793Mo -< 41.-< 140.

Production of long-lived activities in fusion materials

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Production of long-lived activities in fusion materials

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