Neutron transmission experiments were performed on samples of an advanced nickel-chromium-molybdenum-gadolinium (Ni-Cr-Mo-Gd) neutron absorber alloy. The primary purpose of the experiments was to demonstrate the thermal neutron absorbing capability of the alloy at specific gadolinium dopant levels. The new alloy is to be deployed for criticality control of highly enriched DOE SNF. For the transmission experiments, alloy test samples were fabricated with 0.0, 1.58 and 2.1 wt% natural gadolinium dispersed in a Ni-Cr-Mo base alloy. The transmission experiments were successfully carried out at the Los Alamos Neutron Science Center (LANSCE). Measured data from the neutron transmission experiments were compared to calculated results derived from a simple exponential transmission formula using only radiative capture cross sections. Excellent agreement between the measured and calculated results demonstrated the expected strong thermal absorption capability of the gadolinium poison and in addition, verified the measured elemental composition of the alloy test samples. The good agreement also indirectly confirmed that the gadolinium was dispersed fairly uniformly in the alloy and the ENDF VII radiative capture cross section data were accurate

Wachs, Gregg W.

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This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint should not be cited or reproduced without permission of the author. This document 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, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use, or the results of such use, of any information, apparatus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. The views expressed in this paper are not necessarily those of the United States Government or the sponsoring agency. INL/CON-07-12838PREPRINTCharacterization of an Advanced Gadolinium Neutron Absorber Alloy by Means of Neutron TransmissionGlobal 2007 G. W. Wachs J. W. Sterbentz L. M. Montierth F. K. Tovesson T. S. Hill September 2007 CHARACTERIZATION OF AN ADVANCEDGADOLINIUM NEUTRON ABSORBER ALLOY BY MEANS OFNEUTRON TRANSMISSIONG. W. Wachs, J. W. Sterbentz, L. M.MontierthIdaho National LaboratoryIdaho Falls, ID 83415-3710F. K. Tovesson, T. S. HillLos Alamos National LaboratoryLos Alamos, NM 87545-H855ABSTRACTNeutron transmission experiments were performed onsamples of an advanced nickel-chromium-molybdenum-gadolinium (Ni-Cr-Mo-Gd) neutron absorber alloy. Theprimary purpose of the experiments was to demonstratethe thermal neutron absorbing capability of the alloy atspecific gadolinium dopant levels. The new alloy is to bedeployed for criticality control of highly enriched DOESNF. For the transmission experiments, alloy testsamples were fabricated with 0.0, 1.58 and 2.1 wt%natural gadolinium dispersed in a Ni-Cr-Mo base alloy.The transmission experiments were successfully carriedout at the Los Alamos Neutron Science Center (LANSCE).Measured data from the neutron transmissionexperiments were compared to calculated results derivedfrom a simple exponential transmission formula usingonly radiative capture cross sections. Excellentagreement between the measured and calculated resultsdemonstrated the expected strong thermal absorptioncapability of the gadolinium poison and in addition,verified the measured elemental composition of the alloytest samples. The good agreement also indirectlyconfirmed that the gadolinium was dispersed fairlyuniformly in the alloy and the ENDF VII radiativecapture cross section data were accurate.I. INTRODUCTIONThe National Spent Nuclear Fuel Program (NSNFP),located at the Idaho National Laboratory (INL),coordinates, and integrates national efforts in managingthe disposal of U.S. Department of Energy (DOE)-ownedspent nuclear fuel (SNF). These functions include thedevelopment of a DOE standardized canister forpackaging, storage, transport, and long-term disposal ofDOE SNF. DOE SNF will be packaged in standardizedcanisters with internal baskets. Since some types of DOESNF contain highly enriched uranium, an advancedcanister neutron absorber material may be required forcriticality control.By deploying a neutron absorber alloy as a basketmaterial, the number of DOE SNF packages can bereduced and handling of individual SNF elements will beeliminated at the repository. The internal basket materialis envisioned to be: 1) highly corrosion resistant, such thatunder the projected storage, transportation, and disposalconditions the neutron-absorbing element will not leachout, 2) acceptable mechanical properties based onAmerican Society of Mechanical Engineers (ASME) codeapproval, 3) material production using conventional ingotmetallurgy techniques, and 4) be weldable.Directed by the NSNFP, a research and developmentproject has been implemented to develop an advancednickel-chromium-molybdenum-gadolinium (Ni-Cr-Mo-Gd) neutron absorber alloy, or now more commonlyreferred to simply as the Advanced Neutron Absorber(ANA). The Idaho National Laboratory is leading allaspects of the development in conjunction with SandiaNational Laboratories supporting melt refractory andproduction studies, with additional support from LehighUniversity supporting mechanical propertymeasurements. Natural gadolinium (Gd) was chosen asthe neutron absorbing element based on its high thermalneutron absorption cross section and limited solubility inthe disposal environment. The base metal was chosen forits high corrosion resistance and consists of nickel-chromium-molybdenum.The alloy has been manufactured with conventionaltechniques involving a Vacuum Inductive Remelt process.The follow on conversion process from ingot form toplate involves standard hot working practices atcommercial rolling mills. Mechanical, physical, andchemical results were submitted to the American Societyfor Testing and Materials (ASTM). The ASTM acceptedthe results and is now designated as UNS N06464, ASTMSpecification B932-04 [1]. The alloy was also approvedfor use for ASME Boiler and Pressure Vessel Codeconstruction per Section III, Division 3 Code Case N-728in the non-welded condition [2].Additional studies beyond mechanical, physical, andchemical property measurements have been performedthat include neutron reactivity, corrosion and weldability.Reactivity experiments were carried out at the LosAlamos Critical Experiments Facility (LACEF) [3]. Theaverage weight percent of gadolinium in the experimentprototype Ni-Cr-Mo-Gd alloy was approximately 2.34wt%. The experiment had a measured keff of 1.002 versesa predicted value of 1.001, or a negative worth of the Gdalloy plates of about 8.8$ of reactivity. As a comparisonin the same configuration, the calculated negative worthof an equivalent volume of borated stainless steel plate(1.7% natural boron) was approximately 6.4$ ofreactivity. The results of these experiments aredocumented in the International Handbook of EvaluatedCriticality Safety Benchmark Experiments by the NuclearEnergy Agency Nuclear Science Committee [4].In addition, the INL has performed numerouscorrosion tests. The results have shown, the Gdintersecting the surface along the grain boundaries iseasily removed followed by repassivation of the Ni-Cr-Mo-Gd matrix, the corrosion rate will drop to anextremely low rate [5]. Welding studies continue to beperformed to generate the required data for the extensionof ASME Code Case N-728 to welded construction.Current results show that mechanical properties of weldjoints have reduced ductility relative to the base metal.However, the properties can be recovered with the use ofpost weld heat treatment [6].Future research and development is directed at alloyscale-up to 1000-3000 pound slabs and continued weldprocess development. One of the primary issues duringscale-up will be to focus on the loss of gadolinium whenintroduced during the melt process. Gadolinium has avery high attraction for oxygen. Techniques will need tobe developed to account for these gadolinium losses.Welding process development continues at the INLstudying process parameters and different weld wire.These efforts will result in future revision to the ASMECode Case to included welded conditions once theprocess is finalized.Preliminary results from neutron transmissionmeasurements on the ANA alloy are described herein.The neutron transmission measurements are intended toprovide data that can be compared to calculatedtransmission results using a simple exponentialtransmission formula with microscopic radiative capturecross sections for all ANA isotopic constituents.Agreement between measured and calculatedtransmission results provides verification of wet chemicalanalysis of the ANA materials isotopic constituents, butmore importantly, the verification of the thermal neutronabsorption capability of the natural gadolinium in theANA alloy.II. ANA SAMPLE CHARACTERIZATIONAlthough the final process for the fabrication of ANAmaterials has not been finalized, ANA samples wereselected that were available and provide a representationof the manufactured product currently envisioned. PlateD5-8302 and Plate E2-14827(C) were chosen since theseplates have been characterized by wet chemistry analysisat various locations across the finished plate. Theinvestigated samples were not randomly distributedsamples of the finished plate but in contrast were locatedadjacent to the chemical analysis samples.II.A Gadolinium SamplesSamples from two ANA plates manufactured duringFY-06 were chosen for these measurements. ANA platedesignation is D5-8302 with 1.58 wt% Gd and plate E2-14827(C) with 2.1 wt % Gd. To assess the isotopiccompositions of gadolinium added to the ANA plates,INL performed an isotopic analysis of the gadoliniumfeed stock samples. The gadolinium (Gd) neutron poisonin the ANA plates was natural Gd with the followingisotopic atom fractions: Gd-152 (0.20%), Gd-154(2.18%), Gd-155 (14.8%), Gd-156 (20.47%), Gd-157(15.65%), Gd-158 (24.84%), and Gd-160 (21.86%). Theresults clearly indicated that the gadolinium has not beenenriched from “naturally occurring” gadolinium [7].The largest thermal neutron absorber is the Gd-157with an estimated thermal neutron radiative capture crosssection of 255,000 barns [7]. All the plates measuredwere approximately 0.9525 cm (3/8-inch) thick and 5.08cm (2-inches) square. Chemical analysis of the two ANAplates, at varies locations, were taken from finishedplates. The chemical analysis results are provided inTable I.Table I. Chemical Analysis of Measured PlatesElement D5-8302, 02-S2 *(wt %)E2-14827(C), SC-1(wt %)Mo 14.50 14.63Cr 16.65 16.18Gd 1.58 2.1Mn <0.01 <0.001Mg <0.005 0.001Ni Balance (~67.24) Balance (~67.04)Fe 0.02 0.021Co <0.01 0.014C 0.010 0.0081Si <0.01 0.006S <0.001 <0.001N 0.000 <0.001*average of triplicate sample measurementsII.B Non-Gadolinium SamplesTo evaluate the gadolinium affects on neutrontransmission measurements samples with similarchemistries were measured that did not containgadolinium. Plate D5-7689 (M-319) was chosen due tosimilar chemistries as the ANA materials. The chemicalanalyses of the non-Gd plate are provided in Table II.Table II. Non-Gadolinium Chemical AnalysisElement D5-7689, M-319(wt %)Mo 15.3Cr 16.32Gd 0O 0.0069Mn 0.01Mg 0.0023Ni Balance (~68.34)Fe 0.01Co 0.01C 0.003Si 0.001S 0.0006N 0.0004III. EXPERIMENTAL AND FACILITYLANSCE requires experiment requesters to registertheir experiments. This registration process is the primarymechanism at LANSCE for counting users. An INLexperiment proposal was submitted (S083) and a proposalnumber (0062551) assigned. Neutron transmissionexperiments used standard LANSCE experimentaltechniques.Figure 1 depicts the layout of Lujan Centers’ largearray of capabilities. Each of the experimental ports islabeled with a flight path (FP) number. Short, intenseproton pulses are directed at a tungsten target. Thetungsten target is coupled to two different neutronmoderators for the production of cold (<0.025 eV),thermal, and epithermal neutrons. The neutrons arecollimated to form beams in each FP. FP-5 was chosenfor these experiments as shown in Figure 1. This flightpath is commonly used for fission cross sectionmeasurements, and it has a parallel-plate fissionionization chamber placed about 8 meters from thetungsten target. A 200 µg/cm2 thick U-235 fission foilwas loaded in the chamber, and neutron induced fissionfrom the foil was measured. The short flight path, thelarge fission cross section of U-235, as well as the highefficiency of the parallel-plate chamber all contribute tohigh count rates which allow reasonable sort irradiationtimes to be used to collect sufficient statistics. Figure 2shows the LANSCE measured detector counts as afunction of neutron energy at FP-5, without a samplepresent.Figure 1. Layout of the Lujan Center1101001000100001000001.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00Neutron Energy (MeV)EventsIncident neutronspectrumFigure 2. LANSCE Measured results without sampleIV. RESULTS TO DATEThree neutron transmission experiments on threedifferent material samples have been completed at theLANSCE facility. The three samples include the M-319(no Gd), 02-S2 (1.58 wt% Gd), and E2-14827(C) (2.1wt% Gd) samples. A fourth test measured the neutronspectrum and intensity without a sample present in thebeamline.IV.A. Calculated Macroscopic Cross SectionsEvaluated Nuclear Data File (ENDF) microscopiccross sections were provided for the four main elementsin the ANA material, mainly nickel, molybdenum,chrome, and gadolinium [8]. The total cross sections forwere imported into an Excel spreadsheet. The energyspectrum given for each microscopic cross section wascross referenced to the measured energy spectrumprovided by LANSCE for each sample measured. Thedata was further “normalized” based on a LANSCEcalculated charge (coulombs) delivered to the neutron-producing tungsten target for each of the sample runs.The number of events is proportional to the incidentnumber of neutrons on each transmission sample.Macroscopic cross sections as a function of energywere calculated for the three samples tested: (1) D5-8302(1.58 wt% Gd), (2) E2-14827 (2.1 wt% Gd), and (3) M-310 (0 wt% Gd). The calculated macroscopic crosssections are based solely on ENDF VII cross section dataand the measured isotopic concentrations of constituentsin each test sample. The macroscopic cross sections (were calculated using the following formula: ı*NWhere:ı= microscopic radiative capture cross section (cm2)N = atomic number density (atoms/cm3)andN = (ȡ * A * Wj * APi / Mi)ȡ= Alloy density (8.76g/cm3)A = Avogadro’s number (6.023 1023 atoms/mole)Wj = weight percent of alloy element j (based onchemical analysis)APi = atom percent of isotope i in naturaloccurring elementMi = atomic weight of isotope i (g/mole)Figure 3 shows the calculated total (N,TOT)microscopic cross section for sample 02-S2 (1.58 wt%Gd) by element.1.E-031.E-011.E+011.E+031.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01Neutron Energy (MeV)MacroscopicCrossSection(1/cm)680&5680*G6800R6801LFigure 3: Calculated total macroscopic cross sections forANA sample 02-S2 (1.58 wt% Gd) by element.The elemental macroscopic cross sections weretotaled and are shown in Figure 4.1.E-011.E+001.E+011.E+021.E+031.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01Neutron Energy (MeV)MacroscopicCrossSection(1/cm)680RIDOOFigure 4: Calculated total macroscopic cross section forSample D5-8302 (1.58 wt% Gd).IV.B. Results of Calculated Versus MeasuredThe transmission intensities (events) were calculatedbased on the given normalized events from the sample out(Io) measured values by using the following relationship:It = Io * e –[Where:It = number of events (calculated),Io = number of measured events (sample out),= macroscopic (N,G) or radiative cross sectionfor a given energy (cm2), andx = thickness of the neutron absorbing material(0.952 cmor 3/8-inch)The results are shown in Figures 5, 6 and 7 ascomparison plots between calculated and measuredevents.1.E+001.E+011.E+021.E+031.E+041.E+051.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00Neutron Energy (MeV)EventsCalculatedMeasuredFigure 5: Calculated versus measured events based ontotal macroscopic cross sections for ANA sample D5-8302 (1.58 wt% Gd).1.E+001.E+011.E+021.E+031.E+041.E+051.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00Neutron Energy (MeV)EventsCalculatedMeasuredFigure 6: Calculated versus measured events based ontotal macroscopic cross sections for ANA sample E2-14827 (2.1 wt% Gd).1.E+001.E+011.E+021.E+031.E+041.E+051.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00Neutron Energy (MeV)EventsCalculatedMeasuredFigure 7: Calculated versus measured events based ontotal e macroscopic cross sections for sample M-319 (0wt% Gd).V. DISCUSSION OF RESULTSThe calculated results are in very good agreementwith the measured results as shown in Figures 5, 6 and 7.The agreement is best over the thermal neutron energyrange, but good agreement extends up to approximately3.0E-3 MeV. The thermal range (0.01eV to 1eV) is therange where the total macroscopic cross section isdominated by the Gd-157 cross section.V.B. ConclusionsIt is clear that the addition of natural gadolinium inthe ANA samples absorbs thermal neutrons. The severelydepressed thermal flux (events) in Figures 5 and 6 is dueprimarily to the large thermal cross section of the Gd-157isotope.The very good agreement between the measured dataand the calculated results (Figures 5, 6 and 7) shows thatthe simple exponential transmission formula combinedwith ENDF energy-dependent total macroscopic crosssections for the ANA sample are sufficient to predict thephysical phenomena. The good agreement alsodemonstrates indirectly the integrity of the ENDF crosssection used in the exponential transmission formula andthe relative uniformity of the gadolinium distribution inthe ANA sample, since the formula implies ahomogenized or uniform gadolinium distribution in the3/8-inch thick samples.Additional transmission experiments with as-meltedANA material without rolling and heat treatment areplanned in the near future at LANSCE. Theseexperiments will be based on thinner plate.ACKNOWLEDGMENTSThis work was supported by the DOE, AssistantSecretary for Environmental Management, under DOEIdaho Operations Office; contract DE-AC07-99ID13727.This work was performed through support from theNational Spent Nuclear Fuel Program.REFERENCES[1]. “Standard Specification for Low-Carbon Nickel-Chromium-Molybdenum-Gadolinium Alloy Plate,Sheet, and Strip” ASTM B932-04, American Societyfor Testing and Materials (2004).[2] “Containment Systems and Transport Packagings forSpent Nuclear Fuel and High-Level RadioactiveWaste,” ASME Boiler and Pressure Vessel Code,Sec. III, Division 3, The American Society ofMechanical Engineers (2002); see also Case N-728,“Use of B-932-04 Plate Material for Non-PressureRetaining Spent-Fuel Containment Internals to 650F(343C)”, Section III, Division 3, (May 10, 2005).[3]. D. J. LOAIZA R. Sanchez, G. Wachs, W. Hurt, andR. E. Mizia, “Critical Experiment Analysis of aNeutron Absorbing Nickel-Chromium-Molybdenum-Gadolinium Alloy Being Considered for the Disposalof Spent Nuclear Fuel,” Journal of Nuclear MaterialsManagement, Volume 32 Number 2, Institute ofNuclear Materials Management.[4]. International Handbook of Evaluated CriticalitySafety Benchmark Experiments, “2 x 2 x11 Array ofHighly Enriched Uranium with Ni-Cr-Mo-Gd Alloy,Moderated and Reflected by Polyethylene,”NEA/NSC/DOC (95) 03, Organization for EconomicCooperation and Development, Nuclear EnergyAgency, (Sept 2004).[5]. R. E. Mizia, P. J. Pinhero, T. E. Lister, and T.L.Trowbridge, “Localized Corrosion of a NeutronAbsorbing Alloy Ni Cr Mo-Gd Alloy,” Paper No.600, Corrosion 2005, NACE International, Houston,TX.[6] C. V. Robino, J. R. Michael, J. J. Stephens, J. N.DuPont, D. B. Williams, and R. E. Mizia,“Solidification and Weldability of Gadolinium-Containing Nickel-Based Alloys, Trends in WeldingResearch, Proceedings of the 6th InternationalConference, Pine Mountain, GA, April 2002.[7]. Nuclides and Isotopes, Fourteenth Edition, Chart ofthe Nuclides, Revised 1999.[8]. Brookhaven National Laboratory (BNL), ENDF Filesfrom the National Nuclear Data Center, VersionENDF/B-VII.0 at 300oK, December 15, 2006, website (http://www.nndc.bnl.gov/exfor7/endf00.htm)

CHARACTERIZATION OF AN ADVANCED GADOLINIUM NEUTRON ABSORBER ALLOY BY MEANS OF NEUTRON TRANSMISSION

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CHARACTERIZATION OF AN ADVANCED GADOLINIUM NEUTRON ABSORBER ALLOY BY MEANS OF NEUTRON TRANSMISSION

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