Modelling the corrosion of radioactive waste containers during prolonged exposure to atmospheric conditions

Transcription

1 Modelling the corrosion of radioactive waste containers during prolonged exposure to atmospheric conditions Dr. Cristiano Padovani ICorr CED meeting, 24 April 2018, Warrington

2 Acknowledgements Technical contributions Nick Smart, Andy Rance, Paul Fennell, Rob Winsley, Cliff Harris, Kevin Christie (Wood) Angus Cook, Majid Ghahari, Alison Davenport (University of Birmingham) Octavio Albores-Silva, Alasdair Charles (University of Newcastle) Fraser King (Integrity Consulting) Sarah Watson, Peter Robison, Claire Watson, Jenny Burrows (Quintassa) Donal Krouse (Callaghan Innovation) and Nick Laycock Funding 2

3 Outline Context Key experimental observations Corrosion models Summary 3

4 Radioactive Waste Management in the UK Immobilise and containerise waste Interim storage Transport to an underground geological disposal facility (GDF) Utilise man-made and natural barriers to prevent/minimise release

5 Interim storage and operational phase of a geological disposal facility Example of ILW store at Trawsfynyyd Conceptualisation of GDF in higher-strength rock 5 Artist s impression of unshielded ILW containers stacked in GDF

6 6 Intermediate Level Waste (ILW) and its processing

7 Corrosion of stainless steel in atmosphere Corrosion induced by chloride containing aerosols: Generated by hygroscopic chloride salts deposited on surfaces Temperature (T) and relative humidity (RH) determine the characteristics of the electrolyte (e.g. concentration) and hence its corrosivity Particle Sat. Clˉ Solution Clˉ Solution Cl salt RH Cl salt RH Cl ˉ pitting 7 Stress corrosion cracking (SCC)

8 Experimental approach Environmental and corrosion monitoring Pit initiation studies SCC initiation studies Mechanistic studies 8

9 Summary of key experimental results to date: environmental conditions in ILW facilities Temperature and humidity fluctuate with daily and seasonal variations in surface facilities but are typically more stable underground Values of 0-30 C and 40-90% RH are broadly representative of expected envelope of conditions Deposition of aerosols indoors is much slower than outdoors and combines internal sources (cement dusts) with external sources (e.g. marine, industrial, urban and agricultural sources) Rate of deposition of chloride from aerosols expected to be of the order of 1 mg cm -2 year -1 (100 mg cm -2 after 100 years) Sulphate and sometimes nitrate (and carbonate?) found in similar 9 concentration to chloride

10 number of locations Examples of key experimental results to date: environmental conditions in ILW facilities number of locations Anions deposition on horizontal surfaces (various facilities) chloride sulphate nitrate Cumulative measurements for a number of facilities operated over periods of 0 to more than 20 years 5.0 chloride sulphate nitrate Cations deposition on horizontal surfaces (various facilitie < deposition density (ug cm-2) Chloride generally below 10 mg cm -2 Sulphate between mg cm -2 Very low Mg (Cl mostly from NaCl)? sodium calcium potassium magensium < deposition density (ug cm-2)

11 Summary of key experimental results to date: corrosion initiation on stainless steel MgCl 2 and CaCl 2 are inherently much more corrosive than NaCl (particularly with regard to SCC) Initiation requires minimum level of chloride deposited on the surfaces (at least 1-10 mg cm -2 ) Can be inhibited by nitrates if in high enough amounts (sulphates and probably carbonates are too insoluble) Depends on RH high enough RH can lead to dilution and prevent initiation (or lead to cessation) SCC initiation severely inhibited by surface treatments inducing compressive stresses 11

12 Examples of key experimental results to date: corrosion initiation NaCl, 80%RM, 40 C, Cl deposition = 40 mg cm -2 MgCl 2 + Mg(NO 3 ) 2, 36%RM, 30 C 60 TEMPERATURE [ C] NO ASCC ASCC MgCl 2, 33%RH, RT-50 C Cl deposition = 10-32,000 mg cm -2 CHLORIDE DEPOSITION LEVEL [ g cm -2 ]

13 Summary of key experimental results to date: corrosion propagation on stainless steel Mechanistic studies in bulk solution indicate that, under polarization, short-term pit propagation rates are very high Average pit depth stifles with time kinetic law of the type (D = A t n ) or maximum asymptotic value? SCC propagation rates very fast after apparent incubation period 13

14 Maximum pit depth (mm) Examples of key experimental results to date: corrosion propagation long-term lab tests (MgCl 2, 40%RM, 40 C) 70 short-term lab tests (NaCl, RT, polarized in bulk solution) year = 8760 hours Exposure time (hours) short term lab tests (r ~ mm s -1 ) long-term lab tests (r ~ 100 mm year -1 ) 14

15 15 Parametric model Atmospheric Corrosion of Stainless Steel in Stores (ACSIS)

16 Basic format of the ACSIS model Three basic components (modules) Environmental Is the surface wet? Is the surface wet? No Increment time Corrosion initiation Yes Does corrosion initiate? Corrosion propagation If so, how much damage results? Two forms of corrosion considered Pitting/crevice corrosion Stress corrosion cracking Can corrosion initiate? Yes How much damage occurs? No 16

17 Treatment of Wetting and Corrosion Initiation Criteria for wetting Threshold salt loading is exceeded RH > Deliquescence Relative Humidity (DRH) Criteria for the initiation of Localised Corrosion Temperature exceeds a critical value (CPT/CCT) Relative humidity below dilution threshold (URH) Cl deposit density higher than Cl/NO 3 inhibition ratio Additional criteria for the initiation of SCC For smooth surfaces, total stress must exceed a threshold value For pre-existing defects, no additional criteria 17

18 Treatment of Corrosion Propagation Any form of corrosion Corrosion assumed to stop immediately after container surface becomes dry Can either initiate at same site for successive wetting episodes (default) or re-start each time at a new location Localised corrosion User-specified choice of either constant or time-dependent rate of propagation (D = At n ) For time-dependent rate, rate reverts to initial rate for each wetting event (default) Stress corrosion cracking User-specified choice of either constant or stress(depth)-dependent crack growth rate (v = C (K I ) N ) Crack growth occurs for stress intensity factor (K I ) greater than K ISCC 18

19 Example simulation (pitting with NaCl) 10-yr storage period NaCl contamination Fluctuating RH and temperature Time-dependent pit propagation rate For each wetting event: Damage accumulates at same location Rate re-sets to initial rate 19

20 Simulation of wetting behavior (NaCl) NaCl deliquescence point short wet periods (~ 10 s of hours) long dry periods (up to 5-6 months) 20

21 Simulation of accumulated damage (NaCl) Pit depth ~ 50 mm in 5 years Prototype 4 m Box SS 304L Little signs of corrosion after about 10 years monitoring Some small pits (< 100 mm in size) in areas of higher contamination (e.g. ledges, Cl > 10 mg cm -2 ) No SCC Prototype 4 m Box SS 304L ledge Little signs of corrosion after about 10 years monitoring Some small pits (< 100 mm in size) in areas of higher contamination (e.g. ledges, Cl > 10 mg cm -2 ) No SCC 2 21 ledge

22 Mechanistic model Laycock-White Krouse (LWK) model 22

23 Simulation of pitting with LWK model 23

24 Summary The atmospheric corrosion behavior of stainless steels during prolonged periods of storage in atmospheric indoors conditions has been the subject of substantial R&D Parametric and mechanistic models have, or are being, developed to gain understanding and confidence in the expected behaviour So far results indicate that: Propagation of pitting corrosion may be stifled by a variety of factors; Initiation of SCC can only occur with relatively high rates of deposition of marine aerosols (MgCl 2 /CaCl 2 ) but, if it does, propagation may be fast 24

25 Key references 1. C. Padovani, O.E. Albores-Silva, E.A. Charles, Corrosion control of stainless steels in indoor atmospheres laboratory measurements in MgCl2 at constant RH (Part 1), Corrosion, Vol. 71, issue 3, March 2015, pages C. Padovani, R.J. Winsley, N.R. Smart, P.A.H. Fennell, C. Harris, K. Christie, Corrosion control of stainless steels in indoor atmospheres practical experience (Part 2), Corrosion, Vol.71, issue 5, May 2015, pages M. Ghahari, D. Krouse, N. Laycock, T. raiment, C. Padovani, M. Stampanoni, F. Marone, R. Mokso, A.J. Davenport, Synchrotron X-ray radiography studies of pitting corrosion if stainless steel: extraction of pit propagation parameters, Corrosion Science, 100(11), pp 23-35, A.J.M.C. Cook, C. Padovani, A.J. Davenport, Effect of nitrate and sulphate on atmospheric corrosion of 304L and 316L stainless steel, Journal of the Electrochemical Society, 164 (4), pp C1-C16, F. King, P. Robison, C. Watson, S. Watson, R. Metcalfe, J. Burrow, The Atmospheric Corrosion of Stainless Steel in Stores Model, AMEC-FW report 17391/TR/010, issue 2, available at 6. D. Krouse, N. Laycock, and C. Padovani, Modelling pitting corrosion of stainless steel in atmospheric exposures to chloride containing environments, Corrosion Engineering, Science and Technology volume 49, issue 6, August 2014, pp

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27 Crack length / mm Crack opening / mm Incubation time of SCC and rate of crack propagation Crack propagation measured with Digital Image Correlation (DIC) Crack length Crack opening 12 nominal stress 1.2σ y T = 40ºC, RH = 30%, μg cm -2 chloride Courtesy of Anthony Cook (Univerisity of Manchester) Time / h Incubation time for observable cracks of the order of 1000 hours (~40 days) 27 Growth rate is of the order of 1-10 mm year -1

28 28 Examples of key experimental results to date: corrosion propagation (II) Literature data on pit depth of SS in atmosphere 28 pit depth over 1-2 decades not much greater than after 1 year maximum depth found over 1-20 years is 120 mm

29 Example simulation (pitting with MgCl 2 ) Continuous wetness Dry periods ~ few hours MgCl 2 deliquescence point Pit depth ~ mm in 5 years 29