Uranium Contamination of Stainless Steel in Nuclear Processing

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Uranium Contamination of Stainless Steel in Nuclear Processing Plants Timothy Kerry, Anthony W. Banford, William Bower, Olivia R Thompson, Thomas Carey, J. Frederick Willem Mosselmans, Konstantin Ignatyev, and Clint Alan Sharrad Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05139 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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Industrial & Engineering Chemistry Research

Uranium contamination of stainless steel in nuclear processing plants Timothy Kerry,*† Anthony W. Banford,†,‡William Bower,§ Olivia R. Thompson,‡ Thomas Carey,‡ J. Frederick W. Mosselmans,║ Konstantin Ignatyev║ and Clint A. Sharrad*† †

School of Chemical Engineering and Analytical Science, The University of Manchester,

Oxford Road, Manchester, M13 9PL, UK ‡

National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park,

Warrington, WA3 6AE, UK §

School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL,

UK ║

Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire,

OX11 0DE, UK Abstract Stainless steel coupons have been exposed to uranium-containing nitric acid solutions, in conditions similar to those found in various uranium handling nuclear facilities across the nuclear fuel cycle. Solid state analysis of the stainless steel samples and solution composition analysis were undertaken to gain a better understanding of the contamination process mechanisms. Stainless steel coupons were immersed in 12 M HNO3 containing uranium (1 g/

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

L), in the form of uranyl, for periods of up to 255 days. Uranium contamination was observed across all time lengths of exposure. Solution analysis indicated that the levels of contamination reached an equilibrium state after ~14 days. Investigations using Raman microscopy, synchrotron microfocus X-ray fluorescence and X-ray absorption spectroscopy showed inhomogeneous localization of uranyl species associated with the oxide layer of the stainless steel surface. Over longer time lengths of exposure these contaminant species were predominantly found to locate within intergranular regions of the stainless steel. This finding should be taken into consideration when developing decontamination protocols for corroded stainless steel that has been exposed to uranium, to facilitate metal reuse/recycle and minimize hazardous waste volumes. Introduction In the United Kingdom the primary component of nuclear fuel is uranium. Historically the Magnox reactor fleet utilised metallic uranium, but later reactor designs have utilised uranium in its oxide form. The manufacture of the fuel and the handling of used fuel post power-generation, requires facilities that are able to manipulate uranium in various chemical and physical states. The most common medium for the dissolution of uranium is relatively concentrated nitric acid. In the UK, this is the case in fuel manufacture and in processing of used nuclear fuel. The dissolution of used nuclear fuel to extract uranium and plutonium requires copious quantities of concentrated nitric acid. Nitric acid is a corrosive, oxidising mineral acid and as such requires suitable construction materials to maintain containment. Stainless steel is widely utilised for this purpose due to its inherently high corrosion resistance.1 It has previously been shown that radioactive material can be deposited on the surfaces of nuclear facility infrastructure leading to regions of contamination.2-4 The presence of


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radioactive contamination on steel surfaces has numerous repercussions for the nuclear industry as a significant amount of stainless steel is used across the nuclear estate. In the UK alone, there is approximately 32,000 tonnes of contaminated stainless steel that is classified as Intermediate Level Waste (ILW).5 Disposal of contaminated stainless steel is expensive and has significant environmental consequences.6 Decontamination is a possible solution to this issue. Material that can be reclassified to a lower level waste stream (potentially even allowing the material to be reused or recycled) is highly desirable. As such, research has taken place to understand the binding mechanism/s of uranium species to stainless steel surfaces and ultimately define a decontamination methodology that most effectively removes the contaminant of concern. Previously reported studies on managing stainless steel from nuclear plants have predominantly focussed on the latter subject with only limited work available on understanding fission product, uranium and transuranic binding mechanisms.4-7 Moreover, previous studies that have investigated solution based contamination processes with austenitic stainless steels have mostly taken place over relatively short time lengths (i.e. hours), in weakly acidic media.3,8 Most nuclear processing plants that handle uranium in relatively concentrated nitric acid operate for more than ten years. Furthermore there will be limited, if any, opportunities to replace parts and components. It has been shown that there are multiple possible contamination mechanisms, leading to contaminants that can be generally described as either ‘loose’ or ‘fixed’.9 The former are easily removed using non-aggressive approaches whilst the latter are more tenacious and potentially require a more complex decontamination strategy. Grade 304L stainless steel is suitable for applications in HNO3 media to a maximum concentration of 8 M and at uppermost temperatures of 80 °C.10 Under these conditions only low uniform dissolution of the surface should be seen due to the formation of a passive oxide



film.11 Outside of idealised operations, variables such as an increase in acidity, temperature or the presence of oxidising species can lead to an increased corrosion rate and/or cause localised corrosion to occur.10 This study aims to expose 304L stainless steel to conditions beyond its usual operating standard in order to study the interaction between contaminant and corroded metal. This work investigates the extent and nature of uranium contamination onto 304L stainless steel exposed to 12 M HNO3 at 50 °C for up to 255 days, in order to understand the impact of long term corrosion on uranium contamination processes on stainless steel surfaces. These studies provide an improved understanding of contamination behaviour in plants that handle acidic solutions of uranium over extended periods. The outcome of which will allow improved decontamination strategies to be developed for post-operational clean out and decommissioning of such facilities. Experimental Materials and Sample Preparation Type 304L (1.4307) steel was received from Aalco (elemental composition in Table 1, production has been detailed elsewhere).4 The material was ground with SiC paper (P120 to P2500) and polished (6 µm followed by 1 µm diamond paste) prior to investigation in this study. Uranyl nitrate was obtained from the Centre for Radiochemistry Research (University of Manchester) isotope stocks. Accurately weighed stainless steel coupons (2.0 × 2.0 × 0.4 cm) were placed in vials with the side to be exposed to the contaminant facing upwards. Contaminant solutions of 12 M HNO3 (250 mL) were prepared containing uranyl nitrate hexahydrate (0.59 g, 1.17 mM) and poured over the stainless steel coupons. The glass reactant jars were sealed and placed in an oven at 50 °C for a time length from 1 to 155 days. Control samples were prepared using uranyl containing 12 M HNO3 solutions without any ACS Paragon Plus Environment

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stainless steel coupon, and with solutions containing just 12 M HNO3 with stainless steel coupons. All experiments were performed in triplicate. Aliquots (0.25 mL) were removed for solution analysis at various time lengths with the total amount of solution removed corresponding to < 1% of the initial solution volume. At the conclusion of the allocated time period for each sample, the coupon was removed from the contaminant solution and prepared for surface analysis and characterisation. Coupons were not returned to solution after completion of the analyses. Upon removal of the steel from solution, deionised water was used to rinse the surface. This was performed in order to remove any standing acidic solution from the coupon and avoid post-immersion contamination. Any contaminant removed during the wash was assumed to have been so weakly bound that it was not relevant to this study. The samples were then left to air dry at room temperature (~30 minutes) before weighing and analyses of the coupons were undertaken. A second group of stainless steel samples were prepared for X-ray spectroscopic studies. These coupons were exposed to the same experimental conditions but over an extended time period (255 days). The grade of this steel remained the same but the elemental composition of the material (Table 1) and volume of coupon (1.0 × 1.0 × 0.09 cm) differed. Furthermore, starting concentration of uranium was higher at 1.23 mM in 250 mL of 12 M HNO3. Table 1 Comparison of the elemental composition of steels tested in 1st and 2nd run of experiments Element

C

st

1 run wt%

0.023

0.32

1.55

0.32 0.001 18.31

8.06 0.064 Balance

2nd run wt%

0.018

0.35

1.15 0.033 0.001 18.24

8.07 0.059 Balance

Si

Mn

P

S

Cr


Ni

N

Fe


Microscopy Optical microscopy was undertaken using a Carl Zeiss Axio Lab A1 microscope fitted with a Zeiss Axio CAmERc5s camera. Images were manipulated in Zeiss Axio Vision Rel. 4.8 software. Micro-Raman Spectroscopy An XploRA plus Raman microscope system (Horiba scientific) was used to analyse coupon surfaces. A 532 nm diode laser (15 mW laser power), set at either 1% or 10% power was used in order to avoid sample transformation. This was utilised along with either a ×50 or ×100 objective lens. The Raman signal was acquired using a 1200 gr/mm grating centred between 50 and 1200 cm-1. Data was analysed using LabSpec 6. Microfocus X-ray Spectroscopic Analysis Microfocus X-ray absorption spectroscopy and fluorescence mapping of the stainless steel coupons was undertaken at beamline I18 of the Diamond Light Source. I18 has a double crystal monochromator with a working X-ray energy range of 2 – 20 keV, and with a beam size of ~ 2 µm × 2 µm.12 Samples were primarily investigated at a grazing exit angle (80° to the incident beam and 10° to the line of the detector) in order to reduce the Fe signal and increase U sensitivity. Analysis was undertaken using PyMca X-ray Fluorescence Toolkit and Demeter software.13,14 Uranium LIII-edge spectra were collected, calibrated, the background subtracted and normalised to a standard position of E0. Solution Analysis Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the concentration of uranium in the acidic solutions. Analysis was conducted using a Thermo iCap 6300 ICP-OES by the Microanalysis Laboratory at The University of Manchester. ACS Paragon Plus Environment

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Aliquots of 0.010 mL were removed from solution after various time periods from triplicate solutions. Results and Discussion Corrosion behaviour During the first experimental run, stainless steel samples were exposed to uranium containing solutions of 12 M HNO3 for various time periods of up to 155 days. This was undertaken in order to determine the effect corrosion has on contaminant uptake over long time periods. Weight loss of the coupons was assessed after removal from solution (Figure 1). A linear relationship between the total loss of material from the coupons (relative to surface area) and the time of exposure was observed.

2.5

Weight Loss (mg/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

1.5

1.0

0.5

0.0 0

20

40

60

80

100

120

140

160

Exposure Length (days)

Figure 1 Corrosion rate across the 155 days exposure of polished 304L stainless steel coupons (2.0 × 2.0 × 0.4 cm) in contact with 12 M nitric acid (250 mL) containing uranyl nitrate (1.17 mM) at 50 °C.{Fitted line: y = 0.017x, r2 = 0.989}



Figure 2 shows optical micrographs of the stainless steel coupons post-immersion in 12 M HNO3 (without uranium present). No evidence of corrosion was observed until 14 days of exposure under these conditions. Within the first month of exposure the grain structure of the stainless steel coupons was revealed through general corrosion of the surface. In the

14 days

31 days

93 days

Figure 2 Optical micrographs of 304L stainless steel removed from solution after exposure to 12 M HNO3 at 50 °C for 14, 31 and 93 days, from left to right, respectively. A grain vacancy is highlighted with the red circle. following months grain dropping was observed (as highlighted by the red circle in Figure 2). Uranium contamination The average change in the solution concentration of uranium during contaminant exposure of stainless steel coupons over a five month period is shown in Figure 3. It can be seen that after two weeks an interim-equilibrium state was reached which was maintained until measurements were taken again at 155 days. At this point in the first run of experiments the uranium solution concentration reduced further. This experiment was subsequently repeated with another source of 304L stainless steel (2nd run), where coupons were of smaller dimensions compared to the 1st run (see experimental section). The same interim equilibrium was observed over a similar timeframe but the subsequent reduction in uranium concentration was not observed until seven months of exposure. The apparent reduction in U solution concentration appeared to coincide with the


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Figure 3 ICP-OES measurements of uranium solution concentration against time length of exposure of 304L stainless steel coupons to solutions of uranyl nitrate in 12 M HNO3 (250 mL) at 50 °C. The 1st run (coupon dimensions: 2.0 × 2.0 × 0.4 cm) was conducted in triplicate whilst the 2nd run (coupon dimensions: 1.0 × 1.0 × 0.09 cm) was an individual sample. appearance of particulates in the solution. Such particulates were also observed in the control solutions over a similar timeframe. These particulates were isolated from the uranium containing solutions and control solutions on SEM specimen stubs. They were then investigated using optical microscopy and Raman microscopy (Figure 4). Contaminant speciation and location Raman spectroscopy is generally unsuitable for ordered metal systems but is suitable for imperfect crystals with strong covalent bonds such as those produced by metal corrosion. Furthermore, uranium, in the form of uranyl, has a Raman active symmetric stretch. The frequency of this stretch is dependent on the uranyl coordination environment.15,16 A typical spectrum taken from a dropped grain in the uranium containing solution is compared to a spectrum from a dropped grain control in Figure 4. The broad peak centred at ~710 cm-1 and associated shoulder at lower wavenumber can be assigned to the oxide film of the stainless steel.


1043.4

838.3

476.2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

713.8


Figure 4 Raman spectrum of uranium contamination on an isolated dropped grain of stainless steel (black) obtained from the exposure of 304L stainless steel to a solution of uranyl nitrate in 12 M HNO3 at 50 °C. Also shown is signal from a dropped grain recovered from the control solution (red). Inset shows micrograph of a typical steel grain. It is clear that no single species can account for the observed spectra. Previous work has described a layered structure of oxide films grown on austenitic stainless steel, a chromium oxide rich inner layer and iron oxide rich outer layer.17,

18

γ-Fe2O3 shows intense peaks at

~710 cm-1 and between 645 and 680 cm-1, while Fe3O4 also gives intense peaks in this latter region.19, 20 Previous work has also shown the presence of mixed oxide spinel phases such as NiFe2O4 and FeCr2O4 in 304L steel oxide layers display intense peaks in the 400 – 710 cm-1 region of the Raman spectrum.21 It can be concluded that the Raman spectra of the oxide layer of the stainless steel exposed to the solution conditions explored in these studies display a substantial proportion of γ-Fe2O3 or spinel phases. This indicates that it is primarily the outer layer of the oxide film that is observed in these Raman spectra. The presence of FeCr2O4 and Cr2O3 cannot be ruled out however due to overlapping bands with the aforementioned species.


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The shoulder observed at ~840 cm-1 for the uranium contaminated grain sample (Figure 4) along with the free nitrate band at 1043 cm-1 was indicative of the presence of the uranyl cation and nitrate anion on the grain surface.22 This suggested that the presence of dropped grains in solution may have contributed to the reduction in uranium concentration in solution. A larger surface area for uranyl binding has been produced by the loss of grains into the nitric media. Spectra collected from the uranyl nitrate liquor deposited and dried in air on a polished steel coupon provided comparable spectra. Crystalline uranyl nitrate shows a Raman shift of ~860 cm-1. The red shift of the uranyl stretch has been previously observed as a result of changes in bonding in the equatorial plane. Changes in the number and type of the ligands in this plane can result in the strengthening or weakening of the axial U=O bonds causing a shift of wavenumber for the O=U=O bond stretching frequencies. The observed reduction in wavenumber, hence weakening the axial bonds, may occur due to interaction with metal oxides within the oxide layer. Uranyl sorption to iron oxides has been reported previously,23, 24

including upon corroded stainless steel surfaces.25 Dodge et al. observed that uranyl

coordination with iron oxide resulted in shifts of uranyl assymetric stretch in the IR spectrum to lower energies.25 The group observed both sorption of uranyl to the surface of various iron oxides and co-precipitation of uranyl with iron. Further study is required in order to definitively understand the coordination environment of uranyl observed on the stainless steel grains studied. Micro-Raman spectroscopy was also used to investigate the stainless steel coupons themselves in an attempt to gain a better understanding of the contaminant speciation. A representative Raman spectrum taken from a stainless steel coupon immersed in 12 M HNO3 for three months is shown in Figure 5.



Figure 5 Raman spectrum of a uranyl contamination spot seen on 304L stainless steel after three months exposure to a solution of 12 M HNO3 containing uranyl nitrate at 50 °C. Inset shows optical micrograph of spot where the spectrum was recorded (red circle). This indicates the presence of uranyl nitrate on the stainless steel coupons. This is also evident in the optical micrograph showing the typical yellow material for uranyl nitrate. The Raman spectrum shows a characteristic unbound nitrate band appearing at 1045 cm-1 and a relatively weak uranyl-bound nitrate band at 744 cm-1.22, 26 The uranyl symmetric stretch is also observed at ~860 cm-1. As this stretch has not shifted from the uranyl nitrate standard it can be assumed that minimal interaction between uranyl and metal oxides within the oxide layer is observed. Contaminant localisation could not be linked to specific surface features over short contaminant exposure time lengths. However, at longer time periods it could be seen that uranium contaminants preferably aggregate within certain sites. The contaminant appears to be localised within the intergranular region of the stainless steel, an observation also seen across the surface of multiple samples held in solution for long time periods. The interaction of the contaminant with grain boundaries has been seen before by Woodhouse et al. who speculate that this is an energetically favourable point of binding.27


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Microfocus X-ray analysis was undertaken at the Diamond Light Source on the I18 beamline in order to investigate the localisation of the uranium contaminant. Surface mapping using Xray fluorescence and X-ray absorption near edge structure (XANES) analysis of the U LIII edge were conducted.

Mapping studies found localised uranium contamination of the

stainless steel surface with particular ‘hot-spots’ of the uranium. Figure 6 illustrates the inhomogeneous nature of the contaminant with uranium (indicated by yellow/green)

Figure 6 µXRF map (pixel size 2 µm x 2 µm) of a 304L stainless steel coupon held in 12 M HNO3 containing uranyl nitrate (initial conc.: 1.23 mM) for three months at 50 °C. Cr signal is shown in red and U in yellow/green associated with the darker regions displayed (depleted in chromium), indicating grain vacancies. A representative normalised absorption spectrum for a polished steel sample held in uranium solution with nitric acid for 93 days is shown in Figure 7. The shape and the position of the spectrum showed that uranium was predominantly in the U(VI) state. The presence of two multiple scattering resonances beyond the uranium edge are diagnostic for the presence of uranyl. The observation of localised contamination in the U(VI) oxidation state suggests agreement with the results from micro-Raman spectroscopy. Furthermore, Dombovári et al. investigated the kinetics and binding mechanism of uranium on stainless steel tubing through a pilot plant model system.3 They concluded that the contaminant was in the U(VI) oxidation state and at least partially present in the UO2(NO3)2·6H2O form. This observation corroborates the aforementioned Raman and U LIII-edge X-ray absorption spectra.



Figure 7 U LIII-edge X-ray absorption spectrum of a stainless steel surface following uranium (as uranyl nitrate; initial conc. 1.23 mM) contamination in 12 M HNO3 over a one month period at 50 °C Contaminants can become trapped within the surface oxides formed on stainless steel.3, 28, 29 It is thought that these layers can overlay contaminants thus lodging them in the materialoxide interface.28 Another possibility is contaminant diffusion through the surface oxides promoted by the low pH of the acidic solution in contact with the material.29, 30 This oxidelocalised contamination is known to play a significant role both in reactor cooling loop systems and in reprocessing plants.31, 32 Depth profiling will be the subject of future study in order to ascertain whether long term corrosion allows diffusion of contaminants in to the bulk of the material. Conclusions The impact of long term corrosion on the levels of uranium uptake onto stainless steel has been studied. The 304L stainless steel coupons were corroded over a period of 255 days in 12 M HNO3. Despite the corrosive attack on grain boundaries and the observation of grains dropping from the surface, it was observed that across the first 124 days in solution uptake remained in an equilibrium state that was formed after two weeks. However, measurement after 155 days showed a drop in solution concentration. This has been ascribed to the


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presence of a considerable quantity of dropped grains emanating from the steel coupon into the solution mixture. Raman spectroscopy showed the presence of uranium on the grains that were collected from the solution indicating uptake had taken place. In terms of postoperational clean out strategy this should be taken in to account upon decontamination of stainless steels that have been operational in radioactive, concentrated acid media. The presence of dropped grains in solution should be considered a source of mobile activity. Despite the level of contamination remaining unaffected by the corrosion levels of the material, the potential for association with grain boundaries or grain vacancies may lead to more aggressively bound contaminants. These results suggest that optimization of decontamination methodologies for stainless steel exposed to highly acidic conditions must take in to account surface state. Chemical decontamination techniques that aim to remove the oxide layer but leave bulk metal may be insufficient for removing the more tenacious contamination associated with grain boundaries.

AUTHOR INFORMATION Corresponding Author Email: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.



ACKNOWLEDGMENT This work was jointly supported financially by the EPRSC, from an Industrial CASE (voucher number: 11440232) award, and the National Nuclear Laboratory, through the Waste Management and Decommissioning Strategic Research Programme. We thank Diamond Light Source for access to Beamline I18 (SP12505) which contributed to the results presented here. REFERENCES 1. Shaw, R. Corrosion prevention and control at Sellafield nuclear fuel reprocessing plant. Brit. Corros. J. 1990, 25, 97-107. 2. Bower, W. R.; Morris, K.; Mosselmans, J. F. W.; Thompson, O. R.; Banford, A. W.; Law, K.; Pattrick, R. A. D. Characterising legacy spent nuclear fuel pond materials using microfocus X-ray absorption spectroscopy. J. Hazard. Mater. 2016, 317, 97-107. 3. Dombovári, P.; Kádár, P.; Kovács, T.; Somlai, J.; Radó, K.; Varga, I.; Buják, R.; Varga, K.; Halmos, P.; Borszéki, J.; Kónya, J.; Nagy, N. M.; Kövér, L.; Varga, D.; Cserny, I.; Tóth, J.; Fodor, L.; Horváth, A.; Pintér, T.; Schunk, J. Accumulation of uranium on austenitic stainless steel surfaces. Electrochim. Acta 2007, 52, 2542-2551. 4. Kerry, T.; Banford, A. W.; Thompson, O. R.; Carey, T.; Schild, D.; Geist, A.; Sharrad, C. A. Transuranic contamination of stainless steel in nitric acid. J. Nucl. Mater. 2017, 493, 436-441. 5. NDA, 2013 UK Radioactive Waste Inventory: Radioactive Waste Composition. Moor Row, Cumbria, 2014. 6. Wallbridge, S.; Banford, A.; Azapagic, A. Life cycle environmental impacts of decommissioning Magnox nuclear power plants in the UK. Int. J. Life. Cycle. Ass. 2013, 18, 990-1008.


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24. Duff, M. C.; Coughlin, J. U.; Hunter, D. B. Uranium co-precipitation with iron oxide minerals. Geochim. Cosmochim. Act. 2002, 66, 3533-3547. 25. Dodge, C. J.; Francis, A. J.; Gillow, J. B.; Halada, G. P.; Eng, C.; Clayton, C. R. Association of uranium with iron oxides typically formed on corroding steel surfaces. Environ. Sci. Technol. 2002, 36, 3504-3511. 26. Bryan, S. A.; Levitskaia, T. G.; Johnsen, A. M.; Orton, C. R.; Peterson, J. M. Spectroscopic monitoring of spent nuclear fuel reprocessing streams: an evaluation of spent fuel solutions via Raman, visible, and near-infrared spectroscopy. Radiochimica. Acta. 2011, 99, 563-571. 27. Woodhouse, G. J. Contamination and decontamination of pond furniture used in the nuclear power industry. PhD Thesis, University of Manchester, Manchester, 2008. 28. Hirschberg, G.; Baradlai, P.; Varga, K.; Myburg, G.; Schunk, J.; Tilky, P.; Stoddart, P. Accumulation of radioactive corrosion products on steel surfaces of VVER type nuclear reactors. I. 110mAg. J. Nucl. Mater. 1999, 265, 273-284. 29. Chen, L.; Chamberlain, D. B.; Conner, C.; Vandegrift, G. F. A survey of decontamination processes applicable to DOE nuclear facilities. ANL-97/19, Argonne National Laboratory, 1997. 30. Jawarani, D.; Stark, J. P.; Nichols, S. P. Critical discussion of relevant physical issues surrounding the weeping of nuclear-waste casks. J.Nucl. Mater. 1993, 206, 57-67. 31. Demmer, R.; Snyder, E.; Drake, J.; James, R. Understanding Contamination; Twenty Years of Simulating Radiological Contamination-12430. WM2012 Conference, Phoenix, AZ, USA, 2012. 32. Ocken, H.; Wood, C. J. Radiation-field control manual: 1991 Revision. EPRI Report TR100265, Palo Alto, 1992.



Figure 1. Corrosion rate across the 155 days exposure of polished 304L stainless steel coupons (2.0 × 2.0 × 0.4 cm) in contact with 12 M HNO3 (250 mL) containing UO2(NO3)2 (1.17 mM) at 50 °C.{Fitted line: y = 0.017x, r2 = 0.989} 49x40mm (300 x 300 DPI)


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Figure 2. Optical micrographs of 304L stainless steel removed from solution after exposure to 12 M HNO3 at 50 °C for 14, 31 and 93 days, from left to right, respectively. A grain vacancy is highlighted with the red circle. 90x22mm (300 x 300 DPI)



Figure 3. ICP-OES measurements of uranium solution concentration against time length of exposure of 304L stainless steel coupons to solutions of UO2(NO3)2 in 12 M HNO3 (250 mL) at 50 °C. The 1st run (coupon dimensions: 2.0 × 2.0 × 0.4 cm) was conducted in triplicate whilst the 2nd run (coupon dimensions: 1.0 × 1.0 × 0.09 cm) was an individual sample. 53x38mm (300 x 300 DPI)


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Figure 4. Raman spectrum of uranium contamination on an isolated dropped grain of stainless steel (black) obtained from the exposure of 304L stainless steel to a solution of UO2(NO3)2 in 12 M HNO3 at 50 °C. Also shown is signal from a dropped grain recovered from the control solution (red). Inset shows micrograph of a typical steel grain. 48x42mm (300 x 300 DPI)



Figure 5. Raman spectrum of a uranyl contamination spot seen on 304L stainless steel after three months exposure to a solution of 12 M HNO3 containing UO2(NO3)2 at 50 °C. Inset shows optical micrograph of spot where the spectrum was recorded (red circle). 71x42mm (300 x 300 DPI)


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Figure 6. µXRF map (pixel size 2 µm × 2 µm) of a 304L stainless steel coupon held in 12 M HNO3 containing UO2(NO3)2 (initial conc.: 1.23 mM) for three months at 50 °C. Cr signal is shown in red and U in yellow/green. 98x23mm (300 x 300 DPI)



Figure 7. U LIII edge X-ray absorption spectrum of a stainless steel surface following uranium (as uranyl nitrate; initial conc. 1.23 mM) contamination in 12 M HNO3 over a one month period at 50 °C. 60x36mm (300 x 300 DPI)


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Table of contents graphic 126x96mm (96 x 96 DPI)


Uranium Contamination of Stainless Steel in Nuclear Processing

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