DEVELOPMENT AND TESTING OF AN ADVANCED NEUTRON-ABSORBING GADOLINIUM ALLOY FOR SPENT NUCLEAR FUEL STORAGE

Transcription

1 DEVELOPMENT AND TESTING OF AN ADVANCED NEUTRON-ABSORBING GADOLINIUM ALLOY FOR SPENT NUCLEAR FUEL STORAGE CRITICALITY OF NUCLEAR MATERIALS KEYWORDS: neutron absorbing, gadolinium, spent fuel RONALD E. MIZIA,* TEDD E. LISTER, PATRICK J. PINHERO, TAMMY L. TROWBRIDGE, and WILLIAM L. HURT Idaho National Laboratory, P.O. Box 1625, Idaho Falls, Idaho CHARLES V. ROBINO and JOHN J. STEPHENS, Jr. Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico JOHN N. DUPONT Lehigh University, 5 East Packer Avenue Bethlehem, Pennsylvania Received November 18, 2005 Accepted for Publication December 22, 2005 The U.S. Department of Energy requires nuclear criticality control measures for storage of its highly enriched spent nuclear fuel. A new alloy based on the Ni-Cr-Mo alloy system with a gadolinium addition has been developed. Gadolinium has been chosen as the neutron absorption alloying element because of its high thermal neutron absorption cross section. The metallurgical development, mechanical and physical properties, thermal neutron absorption properties, and accelerated corrosiontesting performance of this Ni-Cr-Mo-Gd alloy is described. A brief comparison is also included of the corrosion performance of this alloy as compared to borated stainless steel, which is commonly used as a neutronabsorbing, structural alloy. I. INTRODUCTION The National Spent Nuclear Fuel Program, located at the Idaho National Laboratory, coordinates and integrates national efforts in management and disposal of U.S. Department of Energy ~DOE! owned spent nuclear fuel ~SNF!. These management functions include using the DOE standardized canister for packaging, storage, treatment, transport, and long-term disposal in the Yucca * Ronald.Mizia@inl.gov Mountain Repository. Nuclear criticality must be prevented in the postulated event in which a waste package is breached and water ~neutron moderator! is introduced into the waste package. These measures will be implemented by using a corrosion-resistant, neutron-absorbing material to fabricate the structural inserts ~fuel baskets! that will be placed in the standardized canister. This design is illustrated in Fig. 1. This alloy must be corrosion resistant under the projected storage conditions so as not to leach out the neutron-absorbing element ~neutron poison!. The objective of this program is to develop an alloy with appropriate mechanical and physical properties, thermal neutron absorption capability, and corrosion resistance to meet the SNF disposal requirements. In addition, Federal guidelines encourage the use of peer-reviewed national consensus standards. The program has sought and received industry standard approvals for both material specifications and design use. II. BACKGROUND II.A. Metallurgical Development The steps used in the development of this alloy are shown in Fig. 2. Alloy compositions were chosen, and ingots were cast by vacuum induction melting ~VIM!, which was followed by a vacuum arc remelting ~VAR! secondary refining step for most of the alloys that were produced. These ingots were then hot rolled into a plate, NUCLEAR TECHNOLOGY VOL. 155 AUG

2 Fig. 1. DOE standardized canister with neutron-absorbing structural insert. and samples were removed for metallography, mechanical and physical property testing, and corrosion testing. The initial alloy development work focused on a Type 316L stainless steel composition alloyed with gadolinium. Gadolinium was chosen as the neutron absorption alloying element because of its high thermal neutron absorption cross section. Type 316L stainless steel was chosen as the base alloy because of localized corrosion performance superior to Type 304L stainless steel. Borated stainless steel was not chosen for this application because of boron s lower thermal neutron absorption properties. The microstructure, mechanical properties, and corrosion resistance of these stainless steel alloys have been described elsewhere. 1 3 These studies showed that gadolinium has no solubility in the austenitic matrix of the Type 316L stainless steel and solidifies as a second phase. The study of Type 316L based alloys was discontinued because of hot workability problems ~low hot ductility! associated with the low liquation temperature and extended melting temperature range of this ~Fe,Ni,Cr! 3 Gd secondary, intermetallic phase. With these alloys, the hot workability range was restricted to a narrow temperature range around 9508C. Nickel-based alloys were then evaluated because of the high solid solubility of many alloying elements in its austenite matrix. The Ni-Cr-Mo alloy system was chosen because of proven corrosion performance in a wide variety of industrial applications and environments. The initial ingot chemistry target compositions were based on commercial alloy compositions. The target level for the gadolinium additions was 2.0 wt%, which was based on the minimum required level for neutron absorption capabilities. Additional information on the development of these alloys is available elsewhere. 4,5 The final alloy chemistry is shown in Table I. TABLE I Chemical Composition Limits for the Ni-Cr-Mo-Gd Alloy as Defined in ASTM B Composition Limits ~wt%! Element Alloy N06464 Molybdenum 13.1 to 16.0 Chromium 14.5 to 17.1 Iron 1.0 maximum Cobalt, maximum 2.0 Carbon, maximum Silicon, maximum 0.08 Manganese, maximum 0.5 Phosphorus, maximum Sulfur, maximum Nickel Remainder a Oxygen Nitrogen, maximum Gadolinium 1.9 to 2.1 Fig. 2. Project work flow. a Shall be determined arithmetically by difference. 134 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

3 II.B. Microstructure A typical microstructure for the Ni-Cr-Mo-Gd alloy ~Heat M340; see Table II! is shown in Fig. 3. The microstructure consists of a Ni-Cr-Mo austenitic matrix with a composition similar to UNS N06455 ~darker structure! and a dispersed secondary phase ~lighter structure! with a crystal structure based on the Ni 5 Gd gadolinide. This secondary phase is generally found at the austenite grain boundaries. The Ni 5 Gd phase also contains small amounts of dissolved chromium and molybdenum ~on the order of 1 wt%! ~Ref. 4!. The size, shape, and distribution of the secondary phase evolves from its initial solidification morphology in the interdendritic regions of the as-cast ingot through hot rolling and heat treatment to the wrought structure illustrated here. Gadolinium has extremely limited solubility in the nickel austenite matrix and has not been detected there. In general, the alloy can be described as one containing a hard dispersed secondary phase within a softer, ductile austenitic matrix. II.C. Standards This new material is now covered under American Society for Testing and Materials ~ASTM! B ~UNS N06464! with the chemistry requirements shown in Table I ~Ref. 6!. An American Society of Mechanical Engineers Code Case ~Section III, Division 3! has been approved. 7 III. EXPERIMENTAL PROCEDURE III.A. Ingot/Plate Preparation Mizia et al. The compositions of the Ni-Cr-Mo-Gd alloy heats used to generate the mechanical property data are shown TABLE II Alloy Compositions* Element M337 M338 M339 M340 Molybdenum Chromium Gadolinium Oxygen Manganese,0.001,0.001,0.001,0.001 Magnesium Nickel Balance Balance Balance Balance Iron Cobalt,0.001, Carbon Silicon , Sulfur,0.001,0.001,0.001,0.001 Nitrogen 0.012,0.001, Phosphorus,0.005,0.005,0.005,0.005 *In weight percent. in Table II and conform to ASTM B932. The alloys were produced by VIM and VAR. The alloys were hot rolled to a 17.7-mm ~0.5-in.!-thick plate and solution using straightand cross-rolling procedures. The plate was annealed by heating for 4hat12008C ~22008F! followed by water quenching. The heat chemistries for Ni-Cr-Mo-Gd alloys used for the physical properties, corrosion measurements, and neutron absorption properties are shown in Table III. Heat M322 was produced by VIM with a 300-lb initial melt that was vacuum cast into two 10-cm ~3.9-in.!-diam TABLE III Chemical Analysis of Heats M322, M326, and M327 Element M322 ~VIM0VAR! M326 ~VIM0VAR! M327 ~VIM0VAR! Molybdenum Chromium Gadolinium Oxygen Manganese,0.001,0.001,0.01 Magnesium, Nickel Balance Balance Balance Iron Cobalt, Carbon, ,0.001 Silicon, Sulfur,0.0002, Titanium,0.005 NR a NR Aluminum NR NR Fig. 3. Light optical micrograph of Heat M340. a NR means not reported. NUCLEAR TECHNOLOGY VOL. 155 AUG

4 equal-length round bars. These bars were vacuum arc remelted under ;130 Pa ~1 torr! of helium pressure into 14.3-cm ~5.63-in.!-diam ingots that weighed ;64 kg ~140 lb! each. M322 has a higher gadolinium level than allowed by ASTM B932 because it was prepared early in the development program where a target level of 2.25 to 2.5 wt% gadolinium was being studied for better neutronic performance. M326 and M327 were smaller VIM0 VAR ingots with a finished size before rolling of 12.7-cm ~5-in.! diameter cm ~8.5-in.! length. Heat M326 meets the chemistry requirements of ASTM B932. M327 was formulated to study the effect of a higher chromium level on performance. All ingots were hot rolled to a plate with the approximate dimensions of 1.5-cm ~0.6- in.! thickness 15-cm ~6-in.! width. The ingots were homogenized at 11608C ~21258F!6148C ~258F! for 16 h prior to rolling and then reheated between rolling passes as necessary. Following rolling, the plate was solution annealed at 11608C ~21258F!6148C ~258F! for 4 h and water quenched. Corrosion performance comparisons were also made with UNS N06022 and a borated stainless steel, whose chemistries are shown in Table IV. III.B. Mechanical Properties Tensile and Charpy impact tests were conducted following ASTM E8 ~Ref. 8! and ASTM E23 ~Ref. 9!, respectively, on the cross- and straight-rolled plate from Heats M337 through M340. Tensile tests were conducted over the temperature range of 23 to 4008C ~75 to 7528F!, and Charpy impact tests were conducted over the range of 40 to 4008C ~ 40 to 7528F!. Element TABLE IV Chemical Analysis for UNS N06022 and Borated Stainless Steel UNS N06022 Heat Type 304B6, Grade A Heat Carbon Cobalt 0.74 NR a Chromium Iron 3.54 Balance Boron 1.70 Manganese Molybdenum NR Nitrogen NR Nickel Balance Phosphorus Sulfur Silicon Tungsten 2.83 NR a NR means not reported. III.C. Low Cycle Fatigue Test Experimental Methods The low cycle fatigue ~LCF! testing procedures and fixtures were based on the practices shown in ASTM E606 ~Ref. 10!. Button-ended LCF samples, with a 6.35-mm ~ 1 4 _ -in.!-gauge diameter and a mm ~ 3 4 _ -in.!- gauge length, were machined from transverse plate samples of Heat M339. The full grip diameter of the samples was mm ~0.625 in.!. These samples were run at room temperature as fully reversed LCF tests, using a 30 kip MTS servohydraulic frame, using an MTS632.13B- 20, 615%, 12.7-mm ~ 1 2 _ -in.! extensometer, with the signal conditioner set to the 5% strain range. For total strain ranges of 1% and higher, tests were run at a strain rate of s ~0.1%0s!. This included tests run at 2% ~61%!, 1.6% ~60.8%!, 1.3% ~60.65%!, and 1.0% ~60.5%!. For total strain ranges,1%, a somewhat faster strain rate of s ~0.15%0s! was used. The latter types of tests were run at 0.6% ~60.3%! and 0.5% ~60.25%!. The faster strain rates are used at slower strain amplitudes because of the significantly longer test durations involved. III.D. Coefficient of Thermal Expansion The thermal expansion behavior of Heat M326 was determined by using a calibrated Netzsch DIL 402 ED dilatometer. The heating and cooling rates for all experiments were 58C0min ~98F0min! and 28C0min ~3.68F0 min!, respectively. The samples were 6.35-mm ~ in.!-diam cylinders with a nominal length of 25.4 mm ~1 in.!, with the long dimension of the cylinders oriented parallel to the plate rolling direction. Samples were tested in either the solution-annealed and water-quenched condition, or following a stabilization thermal cycle used to reduce the effects of residual stresses in the quenched plate. The stabilization thermal cycle consisted of heating to and cooling from 8008C ~14708F! using the rates given above. All tests were conducted in a flowing helium protective atmosphere at 1 psig. III.E. Thermophysical Properties Samples from Heat M326 were tested for thermophysical properties in the temperature range of 40 to 4008C ~ 40 to 7528F!. These properties were then used to calculate the thermal conductivity ~l! using the relationship l ac P d. The thermal diffusivity ~a! was measured using the laser flash technique per the requirements of ASTM E ~Ref. 11!. The specific heat ~C p! was measured per the requirements of ASTM E ~Ref. 12!. The bulk density ~d! values ~ g0cm 3! were calculated from the sample s geometry and mass. III.F. Elastic Properties The elastic properties of Heat M326 were measured by using acoustic methods. For an isotropic solid, the second-order elastic constants C 11 and C 44 are related to 136 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

5 the longitudinal and shear wave acoustic velocities V L and V S by the following relationships: 2 C 11 rv L and C 44 rv S 2, Mizia et al. where r is the density. Young s modulus E, shear modulus G, bulk modulus K, and Poisson s ratio n are then derived from these constants by using standard relationships. The room temperature longitudinal and shear acoustic velocities in samples of M326 were measured by pulse echo and through-transmission techniques. Samples were evaluated in both the longitudinal and transverse directions relative to the rolling direction. III.G. Neutron Absorption Measurement A critical experiment was carried out at the Los Alamos Critical Experiments Facility ~LACEF! to benchmark the Ni-Cr-Mo-Gd alloy ~Heat M322!. The experiment was performed using the Planet critical assembly at LACEF. The Planet critical assembly is a vertical lift machine residing in Critical Assembly and Storage Area I. The Planet critical assembly consists of a movable platform powered by a hydraulic lift and jackscrews. The vertical displacement of the movable platform can be measured and controlled within cm ~ _ in.!. The maximum speed of the movable platform is adjustable for each experiment to limit the insertion rate of reactivity to,0.05 $0s. To disassemble the configuration, the movable platform is dropped to its initial position. There are no other control or safety rods inside the assembly. 13 Figure 4 shows the experimental configuration. Fig. 4. Experimental setup for the criticality experiments. III.H. Electrochemical Corrosion Testing The corrosion properties of the Ni-Cr-Mo-Gd alloy heats were evaluated using two electrochemical methods: cyclic potentiodynamic polarization ~CPP! and potentiostatic ~PS! polarization. The following electrolytes were used: ~a! 0.1 M HCl, 308C and ~b! M NaCl, 308C. The acidic chloride solution was chosen for known localized corrosion initiation and is a bounding case for early waste package failure. Cylindrical specimens ~6.3 mm 42 mm long! were machined from a plate of ;15-mm ~0.59-in.! thickness. The specimens were degreased by sonication in acetone followed by ethanol. An initial mass was measured. The clean, mounted specimen was placed into a freshly purged corrosion cell, as specified by ASTM G5 ~Ref. 14!, fitted with two graphite rod auxiliary electrodes, a saturated calomel electrode ~SCE! reference connected through a luggin capillary, and a gas purge. The cell contained 1 of test solution. The entire assembly was thermostatically maintained in a water bath. The CPP test followed the requirements of ASTM G61 ~Ref. 15!. The corrosion potential was first measured for 50 min prior to the CPP scan. CPP scans were then acquired from the equilibrated corrosion potential to 1.0 V versus SCE. The potential sweep rate was 0.6 V0h. PS testing of a duplicate specimen, which is prepared in the same manner as the CPP tests, was performed at 0.20 V versus SCE in all solutions up to 50 h. The value of 0.20 V was selected because this potential corresponds to the dissolution of the surface-exposed gadolinide phase 4,5 and would be considered a bounding corrosion potential value. III.I. Long-Term Immersion Testing In addition to electrochemical testing, two long-term exposure tests have been performed in simulated Yucca Mountain Project solutions J-13 and J-13 50X ~Refs. 16 and 17! as shown in Table V. The J-13 solution is considered to be representative of the in-drift seepage water chemistry. The 50X J-13 solution multiplies the ionic content of J-13 fifty times. The testing procedure, which is based on ASTM G31 ~Ref. 18!, involves exposing triplicate M322 specimens ~ cm! to the solutions at 308C ~868F! with gravimetric analysis performed at selected intervals. The specimen weight loss and the calculated surface area were used to calculate the general corrosion rate.adescaling procedure fromastm G1 ~Ref. 19! was used to clean the specimens exposed to both the J-13 and J-13 50X solutions. III.J. Scanning Electron Microscopy Following corrosion testing, the microstructural changes were documented with scanning electron microscopy ~SEM!. Both secondary electron and backscattered electron imaging provide information about the NUCLEAR TECHNOLOGY VOL. 155 AUG

6 TABLE V Composition of Yucca Mountain Project Corrosion Solutions Ion Concentration ~mg0! J-13 Well Water 50X J-13 Potassium Sodium Magnesium Calcium Fluorine Chlorine NO SO HCO Si ~aq! ph 7.14 topography and secondary phases, respectively. In addition to imaging, energy dispersive spectra were also acquired to provide information about the chemistry of each phase. Dual-beam focused ion beam ~FIB! SEM, courtesy of Sandia National Laboratories, was also used to cross section through secondary phases and corroded areas. SEM of FIB cross-sectioned specimens was completed at the Idaho National Laboratory. IV. RESULTS AND DISCUSSION IV.A. Mechanical Properties The mechanical test results for the cross-rolled plate are summarized in Figs. 5 through 8. In general, the strength levels in both the longitudinal and transverse orientations are ;10% higher than of the base ~gadolinium-free! Ni-Cr-Mo alloy.as might be expected, Fig. 5. Transverse tensile data: cross-rolled heats. 138 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

7 Fig. 6. Transverse Charpy and lateral expansion data: cross-rolled heats. Fig. 7. Longitudinal tensile data: cross-rolled heats. NUCLEAR TECHNOLOGY VOL. 155 AUG

8 Fig. 9. Strain-life diagram for Ni-Gd alloy and various commercial Ni-based alloys. 20 Fig. 8. Longitudinal impact energy and lateral expansion: crossrolled heats. however, the gadolinide particles, Fig. 3, have a significant impact on the overall tensile ductility in both plate orientations. The typical transverse elongation for the cross-rolled plate at room temperature is on the order of 20 to 30%, compared with typical values of 50 to 60% for N06455 in the annealed condition. The gadolinide particles have a similar effect on the Charpy impact energy and lateral expansion, reducing the transverse impact energy from.240 ft{lb for the gadolinium-free alloy to ;15 ft{lb. The mechanical property test results from the straight-rolled plate are not shown here but were similar to those for the cross-rolled plate ~Figs. 5 through 8! except that the transverse ductility is generally somewhat lower and the longitudinal ductility is higher than that of the cross-rolled material. IV.B. LCF Test Results The number of cycles to failure ~N f!, when plotted versus total strain range and compared to a number of commercial nickel-based alloys ~Fig. 9!~Ref. 20!, shows some interesting trends. The major trend is that the Ni-Gd alloy ~Heat M339!, while having lower LCF endurance than the commercial nickel alloys, shows clear signs of approaching the trend curve at lower values of strain range. This trend is apparent in the data shown in Table VI, where the average value of N f for the Ni-Gd alloy has been compared on a percentage basis to the commercial nickel-based alloy trend line. The data for Ni-Gd are very similar to the trends observed when comparing the LCF results for borated stainless steels to the results for stainless grades without boron. At small values of strain range, the data for borated stainless also approached that of regular annealed Type 304 stainless steel. 21 The trends discussed above are also consistent with the observations made by Minicozzi, 22 who examined the effect of increasing tensile strain on metallographic sections of both borated stainless steel and the Ni-Gd Total Strain Range ~%! TABLE VI Fatigue Data Comparison N f Trend Line for Commercial Alloys ~N fcomm! Average N f for Ni-Gd ~N f Ni-Gd! Percentage Ratio of N f Ni-Gd 0N fcomm NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

9 alloy. Two percent strain is sufficient to show evidence of second phase cracking in both alloys. Clearly, lower total strain amplitudes will avoid strain excursions that promote the cracking tendency of the brittle second phase particles distributed at random throughout the wrought alloy. IV.C. Coefficient of Thermal Expansion For determination of the coefficient of thermal expansion ~CTE!, three conditions were compared, and these are shown in Fig. 10. The conditions evaluated were cooling from 6008C ~11108F! during the stabilization cycle, heating of a stabilized sample to 6008C ~11108F!, and cooling from 6008C ~11108F! following the stabilization cycle. For reference, the published expansion of gadolinium-free N06455 Ni-Cr-Mo alloy is also shown in Fig. 10. As shown, the expansion results for the three conditions for the gadolinium-containing alloy are closely matched, and the expansion of the gadolinium-containing alloy is only slightly greater than that for the gadoliniumfree alloy. For each of the three gadolinium-containing alloy conditions, the CTE was determined for room temperature to the specified temperature in 288C ~508F! intervals. These three CTE determinations for each temperature range were then averaged to provide the CTE values presented in Table VII. IV.D. Thermophysical Property Results Mizia et al. The thermal diffusivity values are given in Table VIII. The values are plotted in Fig. 11 and compared to those for UNS N The specific heat results are given in TABLE VII Coefficient of Thermal Expansion, Heat M326 Temperature Range ~8C! CTE ~ C 1! 23 to to to to to to to to to to to to to to to to to to to to to TABLE VIII Thermal Diffusivity, Heat M326 Temperature ~8C! Diffusivity ~cm 2 {s 1! Fig. 10. Thermal expansion data used for CTE determinations. For reference, the expansion of UNS N06455 is also shown. Table IX and are plotted in Fig. 12 along with those for UNS N The results for thermal conductivity calculations are given in Table X. The values are plotted in Fig. 13 and compared to UNS N NUCLEAR TECHNOLOGY VOL. 155 AUG

10 TABLE IX Specific Heat, Heat M326 Temperature ~8C! Specific Heat ~W{s0g{K! Fig. 11. Thermal diffusivity of Ni-Cr-Mo-Gd alloy Heat M326 and UNS N Fig. 12. Specific heat of Ni-Cr-Mo-Gd alloy Heat M326 and UNS N IV.E. Elastic Properties The elastic constants for Heat M326 are shown in Table XI and, as might be expected, are similar to the N06455 Ni-Cr-Mo alloy. IV.F. Criticality Measurement Results The number of units necessary to reach criticality ~no gap between bottom and top stack! was 11 plates of Ni-Cr-Mo-Gd alloy and _ units of highly enriched uranium ~HEU! foils. A complete unit consisted of one plate of polyethylene, four HEU foils, and one plate of the Ni-Cr-Mo-Gd alloy. Each HEU foil has a nominal weight of 65 g and is the only component of the system that can be a fraction of a unit. The total critical experiment mass consisted of 3207 g of HEU and g of the Ni-Cr- Mo-Gd alloy. This translates to ;239 g of gadolinium based on the average weight percent of gadolinium in the Ni-Cr-Mo-Gd alloy plates of ;2.34%. The Ni-Cr-Mo-Gd alloy-heu experiment had a measured k eff of versus a predicted value of These initial measurements and calculations suggest that the negative worth of the gadolinium alloy plates was ;8.8 $ of reactivity. In the same configuration, the calculated negative worth of an equivalent volume of borated stainless steel plate ~1.7% boron! is ;6.4 $ of reactivity. 23 The results of the experiment were of high quality and were documented according to International Criticality Safety Benchmark Evaluation Project guidelines. 24 Similar experiments have been performed at LACEF with various waste matrix materials. 25,26 The data have also been peer reviewed and incorporated into an international standards forum. 27 Additional neutron attenuation measurements to evaluate the effects of nonhomogeneity of the gadolinium are being considered. IV.G. Electrochemical Corrosion Test Results IV.G.1. Ni-Cr-Mo-Gd Alloy Corrosion Performance Figure 14 shows CPP scans for specimens of M327 and UNS N06022 in 0.1M HCl at 308C ~868F!. The 142 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

11 TABLE X Thermal Conductivity, Heat M326 Temperature ~8C! Density ~g{cm 3! Specific Heat ~W{s{g 1 {K 1! Diffusivity ~cm 2 {s 1! Conductivity a Temperature ~W{cm 1 {K 1! Conductivity a ~8F! a BTU{in.{h 1 {ft 2 {8F 1. M327 specimen displays a large rise in current starting from the corrosion potential, which has been attributed to the dissolution of the secondary gadolinide phase. 4,5 Following a peak at 0.2 V, the current declines until the transpassive region ~;0.8 V!. The reverse scan has a lower current over most of the passive region. Although not shown here, a second CPP scan of the same specimen results in a much lower current due to the removal of most of the exposed gadolinide particles from the surface from the first scan. SEM analysis following CPP testing has shown that the gadolinide particles have been dissolved to the point where none were found by SEM backscatter imaging. The surfaces were roughened by the 10- to 20-mm-diam pits where the particles resided. 4,5 The UNS N06022 specimen does not show any indication of localized attack because of the passive nature of the material and lack of a secondary phase. Figure 15 compares the performance of alloys M326 and M327 in M NaCl at 308C ~868F!. The M327 alloy has a higher chromium level as compared to M326 ~Table III!. The additional chromium was added to study its effect on passivity. Figure 15a shows that in the CPP test the current density seems to be controlled by dissolution of the gadolinide phase. The extra chromium content appears to have a measurable effect on the passive current as shown in the PS test ~Fig. 15b!, where at 0.2 V, the current density of the M327 specimen is much lower than the M326 specimen. This shows the beneficial effect of the additional chromium in M327 where it appears to enhance the passivation of the base metal. TABLE XI Elastic Constants, Heat M326 Sample Orientation Poisson Ratio, n Young s Modulus, E ~10 5 MPa! Shear Modulus, G ~10 5 MPa! Bulk Modulus, K ~10 5 MPa! Longitudinal Transverse Fig. 13. Thermal conductivity for Ni-Cr-Mo-Gd alloy Heat M326 and UNS N NUCLEAR TECHNOLOGY VOL. 155 AUG

12 Fig. 14. CPP scans for M327 and UNS N06022 specimens obtained in 0.1 M HCl at 308C. IV.G.2. Ni-Cr-Mo-Gd Versus Type 304B6 Stainless Steel Figure 16 shows CPP scans for M327 and 304B6 specimens in M NaCl as a comparison of the corrosion properties of the two materials. The M327 specimen again shows a large rise in current starting from the corrosion potential and peaking at 0.2 V in a manner much like that described above for tests in 0.1 M HCl. Hysteresis is not observed in this case, suggesting that the base material is stable in the passive region. The 304B6 specimen does not show significant current in the forward scan until ;0.25 V, where the initiation of breakdown occurs. There is a significant hysteresis current flow on the reverse scan with the sample not reaching a repassivation state at the original corrosion potential value ~ V! where the scan ends. This suggests that the material may not be stable to localized corrosion in this solution. Recently published results on the corrosion performance of borated stainless steel suggest that these alloys are susceptible to localized corrosion at or near the secondary boride phase when exposed to an acidic, chloride-containing solution. 28,29 Damage to the M327 specimen observed following the CPP is much like that observed in 0.1 M HCl as documented in previous work 4,5 and as discussed above. The 304B6 suffered far worse damage from the test. SEM images of the specimen examined in Fig. 16 are shown in Fig. 17. The type of pitting shown in Fig. 17 was observed throughout the specimen. The damage appears to be much more extensive as the bottom of the pits could not be observed with SEM or light optical microscopy ~LOM!. FIB milling revealed that the pits have extensive undercutting as shown in Fig. 18. Following removal of an area around a small pit, the entire exposed area was a Fig. 15. Comparison of ~a! CPP and ~b! PS performance of alloys M326 and M327, M NaCl, 308C. skin of material with an open region below where a large cavity apparently has been formed. This type of phenomenon of extensive damage was also observed for the 304B6 specimen in the tests below. Figure 19 shows PS curves obtained by poising M327 and 304B6 specimens at 0.2 V at 608C ~1408F! in 0.1 M HCl. The 304B6 specimen shows almost four orders of magnitude higher current during the test and resulted in extensive damage to the surface as shown in Fig. 20. The 304B6 specimen lost 32% of its mass and displayed extensive sugaring ~similar to severe intergranular corrosion!. The potentiostat could not deliver enough current to maintain the potential at 0.2 V for the 304B6 specimen, possibly the reason that a relatively flat response curve was observed. The M327 specimen shows a high current initially, then quickly drops to below 1 ma0cm 2, where a corrosion rate of 4200 nm0yr was calculated at 144 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

13 Fig. 16. CPP scans of 304B6 and M327 specimens in M NaCl at 308C. Note that the reversal potential was 0.8 V for 304B6. Fig. 17. SEM micrographs of 304B6 following two CPP scans, magnification 500, same specimen as shown in Fig. 5. the final current point using ASTM G102 procedures. 30 The photograph of the M327 specimen in Fig. 20 reveals very little change in the appearance. LOM imaging ~not shown! indicates that the gadolinide phase was removed from the specimen during the test. Fig. 18. SEM images ~a! before and ~b! after FIB sectioning of a 304B6 pit formed by potentiodynamic testing. IV.G.3. Long-Term Immersion Test Results The Ni-Cr-Mo-Gd alloys were immersed in J-13 and J-13 50X solutions at 308C ~868F! for long periods with intermittent examination. Figure 21 shows a plot of the corrosion rate derived from the weight loss during the tests using the ASTM G31 ~Ref. 18! immersion method. The corrosion rates for specimens exposed to J-13 decrease to values below 100 nm0yr with the final value of 8.3 nm0yr obtained at just under 2 yr of exposure. The J-13 50X specimen exhibited higher corrosion rates, as would be expected, because of the higher ionic concentration. Note that early tests used a cleaning method that did not adequately remove scale for J-13 50X specimens, and thus early corrosion rates for those specimens are not shown. The immersion tests were terminated at the last data point in Fig. 10, and further microstructural analysis was performed as described below. Images taken of the immersion specimens after the total exposure time are shown in Figs. 22 and 23. The NUCLEAR TECHNOLOGY VOL. 155 AUG

14 Fig. 19. Potentiostatic curves for M327 and 304B6 specimens held at 0.20 V at 608C in0.1m HCl. Fig. 20. Photographs of M327 and 304B6 specimens following PS test shown in Fig. 19. Fig. 21. The average corrosion rate for three M322 samples immersed in J-13 and J-13 50X solutions at 308C. Fig. 22. LOM images taken at 500 magnification of M322 samples exposed to ~a! J-13 and ~b! J-13 50X analyzed after the final data point taken in Fig. 21. LOM images of the J-13 exposed sample ~Fig. 22! indicate staining and light pitting isolated at the gadolinide particles while the high-resolution SEM image ~Fig. 23! shows the initiation of microscopic pitting on the gadolinide particles. The J-13 50X sample shows more extensive loss of the gadolinide particles in both the LOM and SEM images. The SEM image of the J-13 50X sample ~Fig. 23! shows a partially dissolved gadolinide particle ~see arrow designating particle! where microscopic pitting is evident. Most of the gadolinide particles were totally dissolved by the nearly 2-yr exposure to the J-13 50X solution. No evidence of attack of the base material was observed in the analysis, suggesting that the base material is resistant to localized attack under these conditions and may protect the remainder of the surface from further degradation. This also points to the use of the corrosion rate in work, as this is a general corrosion description, and the attack here is localized in nature. It does, however, provide a useful number to describe the amount of damage. 146 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006

15 which will determine the amount of the gadolinide that will be present in the alloy matrix. The gadolinide that intersects the surface exposed to acidic aqueous solutions and other solutions simulating the Yucca Mountain environment might be preferentially attacked and removed. However, the underlying Ni-Cr-Mo matrix will then repassivate, and the corrosion rate will drop off to an extremely low rate. The borated Type 304B6 stainless steel was found to be susceptible to localized corrosion in electrochemical tests. ACKNOWLEDGMENTS This work was supported by the DOE, Assistant Secretary for Environmental Management, under DOE Idaho Operations Office, contract DE-AC07-99ID This work was performed at the Idaho National Laboratory, formerly the Idaho National Engineering and Environmental Laboratory through support from the National Spent Nuclear Fuel Program. REFERENCES 1. Development of Gadolinium-Containing Stainless Steels, DOE0SNF0REP-066, Idaho National Engineering and Environmental Laboratory ~Jan. 2001!. 2. R. E. MIZIA, P. J. PINHERO, T. E. LISTER J. N. Du- PONT, and C. V. ROBINO, Corrosion Performance of a Gadolinium-Containing Stainless Steel, Proc. Conf. Corrosion 2001, Houston, Texas, 2001, NACE International. Fig. 23. SEM images taken at 5000 magnification of M322 samples exposed to ~a! J-13 and ~b! J-13 50X analyzed after the final data point taken in Fig. 11. V. CONCLUSIONS AND SUMMARY The strength levels in both the longitudinal and transverse orientations are ;10% higher than of the base ~gadolinium-free! Ni-Cr-Mo alloy. The second phase gadolinide particles were found to reduce the overall tensile ductility in both plate orientations. The physical properties were reasonably close to that of alloy UNS N The material has been approved as an ASTM standard under Specification B and has ASME Boiler and Pressure Vessel Code approval under Code Case N-728. These approvals validate the mechanical properties, physical properties, and design requirements measured and developed for this alloy. The neutronabsorbing properties have been measured and reported in an international, peer-reviewed forum. The corrosion resistance of these Ni-Cr-Mo-Gd alloys is dependent on the amount of gadolinium addition, 3. J. N. DuPONT, C. V. ROBINO, and R. E. MIZIA, Development of Gadolinium Containing Stainless Steels for Nuclear Criticality Control, presented at TMS 2000 Fall Mtg., The Minerals, Metals, and Materials Society, St. Louis, Missouri, October 8 12, R. E. MIZIA, T. E. LISTER, P. J. PINHERO, C. V. ROBINO, and J. N. DuPONT, Microstructure and Corrosion Performance of a Neutron Absorbing Ni-Cr-Mo-Gd Alloy, Proc. Conf. Corrosion 2003, Houston, Texas, 2003, NACE International. 5. Interim Report on the Corrosion Performance of a Neutron Absorbing Ni-Cr-Mo-Gd Alloy, DOE0SNF0REP-086, Idaho National Engineering and Environmental Laboratory ~Mar. 2004!. 6. Standard Specification for Low-Carbon Nickel-Chromium- Molybdenum-Gadolinium Alloy Plate, Sheet, and Strip, ASTM B932-04, American Society for Testing and Materials ~2004!. 7. Containment Systems and Transport Packagings for Spent Nuclear Fuel and High-Level Radioactive Waste, ASME Boiler and Pressure Vessel Code, Sec. III, Division 3, The American Society of Mechanical Engineers ~2002!; see also Case N-728, Use of B Plate Material for Non-Pressure Retaining Spent-Fuel Containment Internals to 650F ~343C!, Section III, Division 3 ~May 10, 2005!. NUCLEAR TECHNOLOGY VOL. 155 AUG

16 8. Standard Methods for Tension Testing of Metallic Materials, ASTM E8-01, American Society for Testing and Materials ~2002!. 9. Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, ASTM E23-02,American Society for Testing and Materials ~2002!. 10. Standard Practice for Strain-Controlled Fatigue Testing, ASTM E606-04, American Society for Testing and Materials ~2004!. 11. Standard Test Method for Thermal Diffusivity by the Flash Method, ASTM E , American Society for Testing and Materials ~2001!. 12. Standard Test Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry, ASTM E , American Society for Testing and Materials ~2001!. 13. LACEF, Los Alamos Critical Experiments Facility Safety Analysis Report, LA-CP , Los Alamos National Laboratory ~1992!. 14. Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements, ASTM G5-94, American Society for Testing and Materials ~2002!. 15. Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, ASTM G61-86, American Society for Testing and Materials ~2002!. 16. J. HARRAR, J. CARLEY, W. ISHERWOOD, and E. RABER, Report of the Committee to Review the Use of J-13 Well Water in Nevada Nuclear Waste Storage Investigation, UCID-21867, Lawrence Livermore National Laboratory ~Jan. 1990!. 17. TRW, In-Package Chemistry for Waste Forms, ANL- EBS-MD , Rev. 00, U.S. DOE Office of Civilian Radioactive Waste Management ~Mar. 15, 2001!. 18. Standard Practice for Laboratory Immersion Corrosion Testing of Metals, ASTM G31-72 ~Reapproved 1999!, American Society for Testing and Materials ~1972!. 19. Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens, ASTM G1-03, American Society for Testing and Materials ~2003!. 20. Fatigue Information of Several Nickel Based Alloys, Haynes International Corporation ~1986!. 21. W. J. O DONNELL, Letter to J. J. STEPHENS, Summary of LCF Results on Borated Stainless Steel: Fatigue Design Curve Calculations, SG on Fatigue, ~SC-Design!, Sandia National Laboratories ~May 11, 1992!. 22. M. J. MINICOZZI, The Investigation of the Toughness of a Ni-Based Alloy with Gd Enrichment for Spent Nuclear Waste Containment Systems, MS Thesis, Lehigh University ~July 13, 2005!. 23. D. J. LOAIZA, R. SANCHEZ, G. WACHS, W. L. HURT, and R. E. MIZIA, Critical Experiment Analysis of a Neutron Absorbing Nickel-Chromium-Molybdenum-GadoliniumAlloy Being Considered for the Disposal of Spent Nuclear Fuel, J. Nucl. Mater. Manage., 32, 2 ~Winter 2004!. 24. International Criticality Safety Benchmark Evaluation Project Guide to the Expression of Uncertainties, in International Handbook of Evaluated Criticality Safety Benchmark Experiments, NEA0NSC0DOC ~95! 03, Nuclear Energy Agency, Organization for Economic Cooperation and Development ~Sep. 2001!. 25. D. J. LOAIZA and R. SANCHEZ, Sensitivity Analyses for Polyethylene-Moderated and Polyethylene-Reflected Highly Enriched Uranium Experiments Mixed with Waste Matrix Experiments, J. Nucl. Sci. Eng., 143, 2, 132 ~Feb. 2003!. 26. D. J. LOAIZA and R. SANCHEZ, Examination of the 2 2 SiO 2, Al and Fe Experiments with Highly Enriched Uranium on the Thermal Energy Region, J. Nucl. Sci. Eng., 145, 2, 256 ~Oct. 2003! Array of Highly Enriched Uranium with Ni- Cr-Mo-Gd Alloy, Moderated and Reflected by Polyethylene, in International Handbook of Evaluated Criticality Safety Benchmark Experiments, NEA0NSC0DOC ~95! 03, Organization for Economic Cooperation and Development, Nuclear Energy Agency, ~Sep. 2004!. 28. D. A. MORENO, B. MOLINA, C. RANNIGER, F. MONTERO, and J. IZQUIERDO, Microstructural Characterization and Pitting Corrosion Behavior of UNS S30466 Borated Stainless Steel, Proc. Conf. Corrosion 2004, Houston, Texas, 2004, Vol. 60, No. 6, NACE International ~2004!. 29. D. V. FIX, J. C. ESTILL, L. A. WONG, and R. B. REBAK, General and Localized Corrosion of Austenitic and Borated Stainless Steels in Simulated Concentrated Ground Waters, PVP , Proc. American Society of Mechanical Engineers0Japan Society of Mechanical Engineers Pressure Vessels and Piping Conf. (PVP 2004), San Diego, California, July 25 29, 2004, American Society of Mechanical Engineers ~2004!. 30. Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, ASTM G102-89, American Society for Testing and Materials ~reapproved 1999!. 148 NUCLEAR TECHNOLOGY VOL. 155 AUG. 2006