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The Effects of Gamma Radiation on the Corrosion of Candidate Materials for the Fabrication of Nuclear Waste Packages

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CA AECL The Effects of Gamma Radiation on the Corrosion of Candidate Materials for the Fabrication of Nuclear Waste Packages Les effets du rayonnement gamma sur la corrosion des materiaux
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CA AECL The Effects of Gamma Radiation on the Corrosion of Candidate Materials for the Fabrication of Nuclear Waste Packages Les effets du rayonnement gamma sur la corrosion des materiaux candidats pour la fabrication des colis de dechets nucleaires D.W. Shoesmith, F. King July 1999 juillet A AECL EACL THE EFFECTS OF GAMMA RADIATION ON THE CORROSION OF CANDIDATE MATERIALS FOR THE FABRICATION OF NUCLEAR WASTE PACKAGES by D.W. Shoesmith* and F. King REPORT PREPARED FOR THE USDOE/YMSCO UNDER THE TERMS OF CONTRACT #DE-AC08-95-NV11784 AECL TECHNOLOGIES INC. * Department of Chemistry University of Western Ontario London, Ontario N6A 5B7 Engineered Barriers and Analysis Branch Whiteshell Laboratories Pinawa, Manitoba ROE 1L July AECL-11999 A AECL EACL LES EFFETS DU RAYONNEMENT GAMMA SUR LA CORROSION DES MATERIAUX CANDIDATS POUR LA FABRICATION DES COLIS DE DECHETS NUCLEAIRES par D.W. Shoesmith* et F. King RESUME L'influence du rayonnement gamma sur la corrosion des materiaux candidats pour la fabrication des colis de dechets nucleaires a ete examinee en profondeur. La comparaison de la corrosion des divers materiaux a ete effectuee dans trois milieux distincts : 1) Milieu A; saumures enrichies en Mg 2+ dans lesquelles l'hydrolyse du cation produit des milieux acides et ou le Mg 2+ perturbe la formation des pellicules protectrices; 2) Milieu B; milieux salins a faible teneur en Mg 2+ qui demeurent neutres; 3) Milieu C; etats humides aeres. Parmi ces milieux, seul B, dans des conditions d'immersion complete en presence d'egouttements, et C, en l'absence d'egouttements, constituent des milieux faisables a Yucca Mountain. Dans le milieu A, les vitesses de corrosion sont elevees pour les aciers au carbone et les alliages de nickel, C-4. L'influence du rayonnement sur les vitesses de corrosion de l'acier au carbone est de les restreindre jusqu'a des debits de dose de 10 4 R/h, mais de les accelerer aux debits de dose superieurs. Le phenomene de restriction semble etre du a la formation induite par le rayonnement de depots de magnetite partiellement protecteurs. Le C-4 subit une corrosion par piqures et une corrosion caverneuse induites par le rayonnement a des debits de dose 10 3 R/h. On peut attribuer cela au retrecissement de la region de potentiel passif sur cet alliage dans la saumure acidifiee. Les alliages de titane (plus precisement le Ti-7) ne subissent qu'une acceleration tres minime de la croissance de la pellicule passive a des debits de dose 10 3 R/h. Dans le milieu B, l'acier au carbone subit une corrosion generale et une piquration superficielle amplifiees par le rayonnement a des debits de dose 300 R/h dans les solutions salines (eau de mer). Les alliages de cuivre et les cupronickels ne subissent qu'une acceleration minime de la corrosion generale a des debits de dose 10 4 R/h. Aux debits de doses inferieurs, la croissance de la pellicule favorisee par le rayonnement restreint la vitesse de corrosion du cuivre. La suite.. passivité générale de l'acier inoxydable 304L n'est pas touchée par le rayonnement jusqu'à des débits de dose élevés ( 10 5 R/h), mais certains signes indiquent que la propagation de la corrosion caverneuse peut être accélérée à des débits de dose d'au plus 10 3 R/h. Aucune donnée n'a été relevée pour les alliages de nickel, mais on pourrait s'attendre à ce qu'ils soient bien moins sensibles que le 304L à la corrosion localisée influencée par le rayonnement. Le titane et les alliages de titane ne sont pas sensibles aux effets du rayonnement au-dessous de 10 4 R/h. Aux débits de doses plus élevés, on note une accélération minime de la croissance de la pellicule dans le cas du Ti-2, mais pas dans celui du Ti-12. Au contraire, on constate avec le Ti-12 une absorption directe de l'hydrogène produit radiolytiquement à un débit de dose 5 x 10 4 R/h, mais pas dans le cas du Ti-2. Cette différence peut être attribuable à la présence de fenêtres de Ti2Ni dans la pellicule passive par ailleurs imperméable. Dans le milieu C, un débit de dose 10 3 R/h est nécessaire avant que l'on note une accélération minime de la corrosion de l'acier au carbone. Dans le cas du cuivre, on constate un effet du rayonnement au faible débit de dose de ~ 10 2 R/h dans la vapeur aérée à 150 C. Dans l'air humide à T 90 C, un débit de dose 10 4 R/h est nécessaire avant de noter quelque influence du rayonnement. Un fort débit de dose semblable est nécessaire avant de constater quelque influence du rayonnement sur les alliages Ni-Cu ayant fait l'objet d'essais (90/10 Cu/Ni). En supposant une période d'assèchement d'au moins 100 ans à la surface du colis de déchets, ni le modèle de colis de déchets de référence (acier au carbone supérieur à C-22) ni le modèle de rechange (C-22 sur Ti ou vice versa) ne subiraient de corrosion influencée par le rayonnement. Dans le cas du concept de référence, même un mouillage immédiat du colis de déchets (c'est-à-dire à la mise en place) n'entraînerait pas des effets de rayonnement. Dans le cas du concept de rechange, si le Ti se trouvait à l'extérieur, un mouillage immédiat n'aurait aucun effet néfaste. Si le C-22 était à l'extérieur, le mouillage immédiat pourrait entraîner des dommages minimes de corrosion. Cette conclusion pessimiste est fondée sur l'hypothèse très prudente que le C-22 ne serait pas plus résistant à la corrosion localisée que l'acier inoxydable 304L. En outre, puisque le colis en C-22-Ti possède des parois plus minces, il se refroidira plus lentement que le colis de référence et cela prendrait beaucoup de temps pour réaliser les conditions humides qui pourraient entretenir un processus de corrosion influencé par le rayonnement. D'ici-là, le débit de dose à la surface du colis de déchets de rechange se sera désintégré à des niveaux inoffensifs. * Département de chimie University of Western Ontario London (Ontario) N6A 5B7 Barrières ouvragées et Analyse Laboratoires de Whiteshell Pinawa (Manitoba) ROE 1L0 Juillet 1999 AECL-11999 AECL EACL THE EFFECTS OF GAMMA RADIATION ON THE CORROSION OF CANDIDATE MATERIALS FOR THE FABRICATION OF NUCLEAR WASTE PACKAGES by D.W. Shoesmith* and F. King ABSTRACT The influence of gamma radiation on the corrosion of candidate materials for the fabrication of nuclear waste packages has been comprehensively reviewed. The comparison of corrosion of the various materials was compared in three distinct environments: (1) Environment A; Mg 2+ -enriched brines in which hydrolysis of the cation produces acidic environments and the Mg 2+ interferes with the formation of protective films; (2) Environment B; saline environments with a low Mg 2+ content which remain neutral; (3) Environment C; moist aerated conditions. Of these environments, only B, under fully immersed conditions in the presence of drips, and C, in the absence of drips, are feasible environments at Yucca Mountain. In Environment A, corrosion rates are high for carbon steels and for the nickel alloy, C-4. The influence of radiation on the carbon steel rates is to suppress them up to dose rates of 10 4 R/h, but to accelerate them at higher dose rates. The suppression appears to be due to the radiationinduced formation of partially protective magnetite deposits. The C-4 suffers radiation-induced pitting and crevice corrosion for dose rates 10 3 R/h. This can be attributed to the narrowing of the passive potential region on this alloy in the acidified brine. Titanium alloys (specifically the Ti-7) suffer only very minor acceleration of passive film growth for dose rates 10 3 R/h. In Environment B carbon steel suffers radiation-enhanced general corrosion and shallow pitting for dose rates 300 R/h in saline (seawater) solutions. The copper and copper/nickel alloys suffer only minor acceleration of general corrosion for dose rates 10 4 R/h. At lower dose rates radiation-enhanced film growth suppresses the corrosion rate of copper. The general passivity of 304L stainless steel is not affected by radiation until high dose rates ( 10 5 R/h) but there is evidence that the propagation of crevice corrosion can be accelerated for dose rates as low as 10 3 R/h. No data has been recorded for nickel alloys, but it would be expected to be much less sensitive to radiation-influenced localized corrosion than the 304L. Titanium and titanium alloys continued... show no sensitivity to radiation effects below 10 4 R/h. At higher dose rates minor acceleration of film growth is observed for Ti-2 but not for Ti-12. In contrast, a direct absorption of radiolytically-produced hydrogen is observed with Ti-12 for a dose rate 5 x 10 4 R/h, but not for Ti-2. This difference may be attributable to the presence of Ti2Ni windows in the otherwise impermeable passive film. In Environment C, a dose rate 10 3 R/h is required before a minor acceleration of carbon steel corrosion is observed. For copper a radiation effect is observed at the lower dose rate of ~ 10 2 R/h in aerated steam at 150 C. In moist air at T 90 C a dose rate 10 4 R/h is required before any influence of radiation is noticed. A similar high dose rate is required before any influence of radiation is observed on the Cu/Ni alloys tested ( 90/10 Cu/Ni). Assuming a minimum dry out period at the surface of the waste package of at least 100 years, neither the reference waste package design (carbon steel over C-22) or the alternative design (C-22 over Ti or vice versa) would be subject to radiation-influenced corrosion. For the reference design, even immediate wetting of the waste package (i.e., on emplacement) would not lead to radiation effects. For the alternative design, if Ti were on the outside immediate wetting would have no adverse effects. If C-22 were on the outside, a minor amount of corrosion damage could be sustained if immediate wetting occurred. This pessimistic conclusion is based on the very conservative assumption that C-22 would be no more resistant to localized corrosion than 304L stainless steel. Also, since the C-22/Ti package is thinner walled it will cool more slowly than the reference package and a substantial time would be required to establish the wet conditions that could sustain a radiation-influenced corrosion process. By this time the dose rate on the surface of the alternative waste package will have decayed to innocuous levels. * Department of Chemistry University of Western Ontario London, Ontario N6A 5B7 Engineered Barriers and Analysis Branch Whiteshell Laboratories Pinawa, Manitoba ROE 1L July AECL-11999 CONTENTS 1. INTRODUCTION 1 2. POSSIBLE EFFECTS OF RADIATION 1 3. CORROSION ALLOWANCE MATERIALS IRONS AND CARBON STEELS COPPER AND COPPER ALLOYS CORROSION RESISTANT MATERIALS STAINLESS STEELS NICKEL ALLOYS TITANIUM AND TITANIUM ALLOYS COMPARISON OF THE CORROSION PERFORMANCE OF VARIOUS MATERIALS RADIOLYTICALLY-INFLUENCED CORROSION UNDER YUCCA MOUNTAIN CONDITIONS SUMMARY 23 REFERENCES 25 TABLES 32 FIGURES 37 Page LIST OF TABLES 1 Groundwater Compositions in Which the Influence of Radiation Has Been Investigated 32 2 Summary of Radiation Effects on the Corrosion Rate of Irons, Carbon Steels, and Low Alloy Steels 33 3 Summary of Radiation Effects on the Corrosion of Copper and Copper Alloys 34 4 Summary of the Radiation Effects on the Corrosion Rates of Passive Materials 35 5 Crevice Corrosion Data for Grade-2 Titanium at 100 ± 5 C Using Artificially Creviced Coupons 36 Page LIST OF FIGURES 1 General Corrosion Rates of Forged 0.2% Steel in Deaerated Synthetic Seawater at 90 C With and Without Radiation 37 2 Average General Corrosion Rates of Unalloyed Steels in Q-Brine at 90 C With and Without y-radiation Effect of a y-radiation Field on E CORR for Copper in Deaerated SCSSS at 150 C (lgy=10 2 R) 39 4 Anodic Polarization Curves for 316L Stainless Steel in mol/l NaCl (= 100 x J-13 Well Water Concentration) With (o) and Without (D) Radiation (3.3 x 10 6 R/h); E CORR and Ep are the Corrosion Potential and Pitting Potential, Respectively (* With Irradiation) 40 5 Variation of the Crevice Current and Crevice Potential With Time for 304L Austenitic Stainless Steel in 10 u.g/g Cl Solution at 30 C 41 6 General Corrosion Rates for Hastelloy C-4 and Ti 99.8 Pd in Q-Brines at 90 C With and Without Radiation (1 Gy s 10 2 R) 42 7 Change in Oxide Film Thickness on Ti-12 With Time in Brine A at 108 C With (1.5 x 10 5 R7h) and Without Radiation 43 8 Effect of Radiation (~ 10 4 R/h) on the Crevice Corrosion Behaviour of Ti-2 (Containing a Low Concentration of Fe and Ni Impurities) in 0.27 mol/l NaCl at 150 C; (a) Variation of the Crevice Current; (b) Variation of the Crevice Potential 44 9 Crevice Current and Crevice Potential for Ti-2 (Containing Fe and Ni Impurities) as a Function of Time in 0.27 mol/l NaCl at 150 C With (~ 10 4 R/h) and Without Radiation Amount of Hydrogen Absorbed by Ti-12 as a Function of Time of Exposure to Brine A at Two Temperatures With (1.5 x 10 5 R/h) and Without Radiation Amount of Hydrogen Absorbed by Ti-12 at 250 C in a Basaltic Environment as a Function of Radiation Dose Rate 47 Page continued... LIST OF FIGURES (concluded) 12 The Influence of Radiation Dose Rate on the Corrosion of Various Candidate Materials for the Fabrication of High Level Nuclear Waste Packages in Environment A-Brines Containing High Mg 2+ Concentrations (Q-Brine, Brine A in Table 1), (1 Gy/h = 10 2 R/h) The Influence of Radiation Dose Rate on the Corrosion of Various Candidate Materials for the Fabrication of High Level Nuclear Waste Packages in Environment B-Groundwaters With Low Mg 2+ Concentrations (SCSSS, Seawater, Basalt, Tuff in Table 1), (1 Gy/h = 10 2 R/h) The Influence of Radiation Dose Rate on the Corrosion of Various Candidate Materials for the Fabrication of High Level Nuclear Waste Packages in Environment C - Moist Air and Aerated Steam (lgy/h=10 2 R/h) Radiation Rates (R/h) Calculated for the Surface of the Reference Waste Package (C22/A516) and the Alternative Design of Waste Container (Ti/C22) as a Function of Time of Emplacement of the Package in the Yucca Mountain Repository 51 Page 1. INTRODUCTION Although alternatives are under consideration, the present reference design of nuclear waste package for emplacement in the proposed waste repository in Yucca Mountain, Nevada, employs a dual wall arrangement, in which a 2 cm-thick nickel alloy inner barrier is encapsulated within a 10 cm-thick mild steel outer barrier. It is felt that this arrangement will give considerable containment lifetimes, since no common mode failure exists for the two barriers. The corrosion performance of this waste package will be determined by the exposure environment established within the emplacement drifts. Key features of the Yucca Mountain repository in controlling waste package degradation are expected to be the permanent availability of oxygen and the limited presence of water. When water contacts the surface of the waste package, its gamma radiolysis could produce an additional supply of corrosive agents. The gamma field will be produced by the radioactive decay of radionuclides within the waste form, and its magnitude will depend on the nature and age of the waste form as well as the material and wall thickness of the waste package. The effects of radiation on the corrosion of various metals and alloys have been reviewed (Byalobzheskii 1970, Stobbs and Swallow 1962, Glass 1981). Also, a large body of information exists on the study of radiation effects on in-reactor corrosion processes (Andresen 1992). The present review will be confined to a discussion of the effects of radiation on various candidate container materials. The significance of these findings with respect to the corrosion of packages emplaced at Yucca Mountain will then be discussed. These studies have been performed in a range of groundwaters and solution compositions, some of which are for potential disposal sites no longer under consideration. To facilitate the following discussion, a summary of these compositions is given in Table 1. The boldly typed rows emphasize the constituents which seem most important in determining the corrosivity of the groundwater. Qualitatively, the corrosivity decreases from left to right across this table. Of these environments, the first two are particularly aggressive due to their high Mg 2+ -content. Based on recent measurements and calculations by Nagies and Heusler (1998), the ph of Q-Brine and Brine A will be suppressed to 3 and 4, respectively, for temperatures of 100 C or greater. 2. POSSIBLE EFFECTS OF RADIATION Only gamma radiation need be considered, since induced mechanical damage of the metal or any protective passive oxide on its surface, by alpha particle or neutron bombardment, will be negligible. Gamma irradiation of the package surface and the surrounding environment may have a number of effects on the corrosion behaviour of the package: (1) Radiolysis of the vapour and aqueous phases to produce oxidizing and reducing radicals and molecular products; (2) Interaction with semiconducting passive oxide films; (3) Reduction in the number of viable microbes at, or near, the waste package surface. -2- The nature of the radiolytic species produced, and their concentration, will depend very much on the environment undergoing radiolysis. For relatively dilute groundwaters, like J-13 well water, the predominant oxidants would be OH', O 2, H 2 O 2, and O 2 while the predominant reductants would be H\ e^ and H 2 In the open system existing within a repository drift, the hydrogen would be expected to escape rapidly, and oxidizing conditions would prevail. In aerated solutions the production of radical oxidants, especially O 2, would be increased. In the concentrated saline solutions that could be produced by a sequence of wetting/drying cycles on the package surface, additional species such as CIO , C1O 2, ClOj, CIO4, CI2, and Cl 2 could be formed (Jenks 1972). While definitive experiments to determine the individual reactivities of these species have not been performed, and, indeed, may prove effectively impossible, there is good evidence to suggest that both the radical oxidants OH* and O 2, as well as the molecular oxidant H 2 O 2, are important in corrosion processes in dilute groundwaters (Marsh et al. 1986), while electrochemical experiments have identified CIO' as an additional oxidant in saline solutions (Farvaque-Bera and Smailos 1994). In moist air environments, radiolytic processes leading to the fixation of nitrogen as NO, NO 2, and especially HNO 3, are also important (Reed and Van Konynenburg 1988). This production of HNO 3, coupled with the large metal surface area to solution volume ratio expected in moist vapour systems, could produce aggressive local acidic conditions on the waste package surface. For passive package materials, it is possible to induce photo effects by absorption of radiation energy in the semiconducting passive oxide. This would lead to the creation of electron/hole (e/h) pairs in the conduction (e) and valence (h) bands. Providing redox conditions are sufficiently oxidizing for band bending at the surface of the oxide, separation of these charge carriers will be achieved and their recombination prevented. The migration of holes to the oxide/solution interface could then lead to one of a number of possible reactions; (i) (ii) (iii) oxygen formation, 4h + + 2H 2 O - O 2 + 4H + (1) photo corrosion, 4h + + 2MO - 2M 2+ + O 2 (2) enhanced film growth M + H 2 O - MO + 2H + + 2e (3) One would expect reaction (2) to lead to a combination of enhanced general corrosion, and possibly the initiation of localized corrosion sites (pits/crevices), whereas reaction (3) would be expected to re-enforce passivity, thereby protecting the metal from localized corrosion. This photo effect will be an inefficient process, since the oxides are thin ( nm) and will absorb only small amounts of energy, the majority of the high energy gamma photons (MeV cf. to oxide band gap energies in the range 1 to 3 ev) pa
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