forge report, single column layout · seals, in particular gas flow along contacts; role ... mr p....

FORGE Report: D2‐3 – Ver.0

Fate of repository gases (FORGE)

The multiple barrier concept is the cornerstone of all proposed schemes for underground disposal of radioactive wastes. The concept invokes a series of barriers, both engineered and natural, between the waste and the surface. Achieving this concept is the primary objective of all disposal programmes, from site appraisal and characterisation to repository design and construction. However, the performance of the repository as a whole (waste, buffer, engineering disturbed zone, host rock), and in particular its gas transport properties, are still poorly understood. Issues still to be adequately examined that relate to understanding basic processes include: dilational versus visco‐capillary flow mechanisms; long‐term integrity of seals, in particular gas flow along contacts; role of the EDZ as a conduit for preferential flow; laboratory to field up‐scaling. Understanding gas generation and migration is thus vital in the quantitative assessment of repositories and is the focus of the research in this integrated, multi‐disciplinary project. The FORGE project is a pan‐European project with links to international radioactive waste management organisations, regulators and academia, specifically designed to tackle the key research issues associated with the generation and movement of repository gasses. Of particular importance are the long‐term performance of bentonite buffers, plastic clays, indurated mudrocks and crystalline formations. Further experimental data are required to reduce uncertainty relating to the quantitative treatment of gas in performance assessment. FORGE will address these issues through a series of laboratory and field‐scale experiments, including the development of new methods for up‐scaling allowing the optimisation of concepts through detailed scenario analysis. The FORGE partners are committed to training and CPD through a broad portfolio of training opportunities and initiatives which form a significant part of the project.

Further details on the FORGE project and its outcomes can be accessed at www.FORGEproject.org.

Contact details:

EC FORGE Work Package 2 Leader:

Dr Delphine PELLEGRINI

Organisation: IRSN

Tel: 33 (1) 58 35 74 53

Fax 33 (1) 58 35 79 76 email: [email protected]

web address: www.irsn.fr

Address: 31 avenue de la Division Leclerc

92260 Fontenay aux Roses Cedex

FRANCE

..

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http://www.forgeproject.org/

FORGE Report: D2‐3– Ver.0

Acknowledgements

Of the many individuals who have contributed to the project, the authors would particularly like to thank the following:

Mr P. POLIVKA from NRI (Czech Republic)

Mr D. DOBREV from NRI (Czech Republic)

Dr L. BERGER from IRSN/PSN‐RES/SCA/BIREN (France)

Mr. M. RABOIN from IRSN/PSN‐RES/SCA/BIREN (France)

Mr. B. MARCILLAUD from IRSN/PSN‐RES/SCA/LPMA (France)

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Contents

Acknowledgements .............................................................................................................ii

Contents .............................................................................................................................iii

Summary ............................................................................................................................vi

1 Introduction..................................................................................................................7

2 NRI Experiments ...........................................................................................................8 2.1 Experimental ........................................................................................................... 8 2.2 Results and discussion........................................................................................... 12 2.2.1 Carbon steel........................................................................................................... 12 2.2.2 Iron powder........................................................................................................... 19 2.3 Conclusion ............................................................................................................. 22

3 IRSN Experiments........................................................................................................23 3.1 Materials and methods ......................................................................................... 23 3.2 Results and discussion........................................................................................... 25 3.2.1 Irradiation of pure water....................................................................................... 25 3.2.2 Iron corrosion without irradiation ........................................................................ 26 3.2.3 Iron corrosion under irradiation............................................................................ 28 3.3 Conclusion and further experiments..................................................................... 30

References.........................................................................................................................32

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List of Figures

Fig. 1: The device for hydrogen evolution measurements. 1 – Heater, 2 – Constant pressure chamber, 3 – Sample, 4 – Gas sampling valve, 5 – Temperature regulator, 6 – Data acquisition equipment, 7 – Volume change detector, 8 – Source of constant pressure, 9 – Temperature sensor .............................................. 10

Fig. 2: The corrosion chamber for measuring hydrogen evolution under radiation before modification. ................................................................................................... 11

Fig. 3: Hydrogen evolution rates from experiments conducted in anaerobic box .................. 13

Fig. 4: Hydrogen accumulation curves from experiments with 1 sample of carbon steel in 2 l of bentonite water in anaerobic box.................................................................. 14

Fig. 5: Hydrogen accumulation curves from experiments with 10 sample of carbon steel in 2 l of bentonite water in anaerobic box.................................................................. 14

Fig. 6: The change of Eh in corrosion of 10 samples of carbon steel in bentonite water........ 15

Fig. 7: Change of pH during corrosion of 10 samples in 2 l of bentonite water ...................... 15

Fig. 8: Change of Eh during corrosion of one sample in 2 l of bentonite water ...................... 16

Fig. 9: Change of pH during corrosion of one sample of carbon steel in 2 l of bentonite water ........................................................................................................................... 16

Fig. 10: Raman spectrum from different carbon steel plate placed at 60 °C. ......................... 17

Fig. 11: Raman spectrum from different carbon steel plate placed at 70 °C. ......................... 17

Fig. 12: Hydrogen generation rate from carbon steel cylinder with surface removed by lathe before the experiment....................................................................................... 19

Fig. 13: Hydrogen and Eh evolution with corrosion of iron powder in an anaerobic box in solution of bentonite water at 70 °C. ..................................................................... 20

Fig. 14: The change of pH of bentonite water during corrosion of iron powder at 70 °C ....... 20

Fig. 15: The hydrogen evolution rate from iron powder in solution with solid bentonite...... 21

Fig. 16: Hydrogen evolution rate from iron powder at 80 °C under laboratory conditions with bentonite bubbled with nitrogen ..................................................... 22

Fig. 17: Experimental cell in IRMA facility, before irradiation (left) and after irradiation (12 hours; dose rate = 1kGy/h) ................................................................................... 24

Fig. 18: Experimental device .................................................................................................... 24

Fig. 19: Evolution of hydrogen concentration versus time under irradiation (1 kGy/h) ......... 25

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Fig. 20: Variations with time of oxygen and nitrogen concentration during an irradiation period (1 kGy/h) ........................................................................................ 26

Fig. 21: Evolution of hydrogen concentration with time (m Fe(o) = 9.226 g ; V H2O ‐ 85ml H2O) ............................................................................................................................. 26

Fig. 22: Variations of oxygen concentration with time (m Fe(o) = 9.226 g ; V H2O ‐ 85ml H2O) ............................................................................................................................. 27

Fig. 23: Variations of nitrogen concentration with time (m Fe(o) = 9.226 g ; V H2O ‐ 85ml H2O) ............................................................................................................................. 27

Fig. 24: Evolution of H2 concentration versus time (mFe(o)=10.394 g; VH2O=84.64 ml; 1kGy/h)........................................................................................................................ 28

Fig. 25: Evolution of oxygen content with time (mFe(o)=10.394 g; VH2O=84.64 ml; 1kGy/h)........................................................................................................................ 29

Fig. 26: Evolution of nitrogen content with time (mFe(o)=10.394 g; VH2O=84.64 ml; 1kGy/h)........................................................................................................................ 30

List of Tables

Table 1: Composition of carbon steel ........................................................................................ 8

Table 2: Compositions and chemicals for preparation of synthetic bentonite water............... 9

Table 3: Corrosion layer composition measured at different temperatures. Number 0 means a sample without a corrosion layer................................................................. 18

Table 4: Corrosion layer composition after evaporating (etching) a thin surface layer by an electron beam. 0 is a sample without a corrosion layer....................................... 18

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vi

Summary

This report presents the experimental progress made after the first 2‐years period of the European project FORGE by the two partners involved in Work Package 2 on hydrogen generation from corrosion of carbon steel and iron.

The experiments carried out by NRI consist in measuring continuously the volume of hydrogen generated form iron powder or carbon steel samples of various reactive surfaces in synthetic bentonite water, within a closed system at various temperatures (40 to 70°C) and anerobic conditons, during about 30 days. The presence of oxygen in low concentration in the solution seems to determine the initial rate of hydrogen generation (up to 10 mol H2.m

‐2.yr‐1 for the investigated conditions) and the nature of the corrosion products formed on the metallic surfaces. The stability and thickness of the protective corrosion layers, which strongly slows down corrosion and consequently reduces hydrogen generation to much lower rates, appear to depend strongly on the initial conditions and on the evolution of these conditions as a result of corrosion processes. This could lead to a rise in hydrogen generation rates due to reactivation of corrosion processes after the dissolution of corrosion layers. Besides, differences were observed in the pH evolution between the “iron” and the “carbon steel” corrosion systems, which could be explained by impurities contained in carbon steel. At last, one experiment was carried out to investigate the effect of bentonite in the solid form and showed a slight increase in the hydrogen generation rate. More generally, NRI investigations show that carbon steel corrosion under anaerobic conditions is not yet fully explored and understood. Additional experiments are being carried out, including on longer durations.

IRSN experimental program aims at assessing the impact of radiation on hydrogen generation. A specific set up have had to be developed in order to match with the objective of online hydrogen measurements under irradiation conditions, in an “open” (flushed) system with low hydrogen levels. Preliminary tests carried out with the corrosion chamber developed by NRI for measurements under irradiation highlighted the need for modifications of the chamber design so as to be operational for IRSN set up and investigation plans. A glass vessel was temporary used to carry on the validation of the set up and to identify lines for optimisation of the experiment specifications, while a new cell was manufactured. Results in terms of hydrogen concentrations generated under irradiation of pure water, by corrosion of iron powder and by corrosion under irradiation (1Gy/h) are presented together with orientations for revised specifications of future experiments (flushing gas flow rate, solid mass/ solution volume, dose rate, iron sample post‐mortem analysis…).


1 Introduction It is well known that gases generated in radioactive waste repositories could affect adversely the functional properties of engineered or natural barriers if the pressure was allowed to increase beoynd some limits. The problem can be solved either by preventing fast generation of gases in a repository or by using barriers permeable enough not to allow accumulation of gas leading to the increase of pressure above some barriers strength limits. The largest source of gas in repositories comes primarily from the corrosion of carbon steel based canisters. Despite a huge number of publications devoted to the study of carbon steel or iron corrosion with hydrogen generation (Jelinek, Neufeld, 1982, Simpson, 1984, Simpson et al., 1985, Anantamula et al. 1987, Marsh, Taylor, 1988, Schenk, 1988, Grauer et al., 1991, Hara et al., 1992, Honda et al., 1991, Kozaki et al., 1994, Fujisawa et al.1997, Fujiwara et al. 2000, Xia et al., 2005, Smart et al., 2001, 2008), there is still a lot of uncertainties concerning hydrogen generation rate in a repository conditions. The problem is that hydrogen evolution rate is affected by a large variety of factors such as, pH, temperature, and composition of water, radiation or the amount of oxygen in water. Reported values of hydrogen evolution rate therefore range from less than one tenth of moles per square meter and year to values exceeding several tens of moles of generated hydrogen per square meter and year.

Hydrogen generation rate from iron or carbon steel corrosion strongly decreases with pH (Schenk, 1988, Simpson, 1984, Simpson et al., 1985, Grauer, 1991, Matsuda 1995, Fujiwara, 2001) from values of ten of moles of hydrogen per year at low values of pH until tenth of moles per year at high pH. This is related to much easier formation of passive layers on iron or carbon steel under alkaline conditions.

Much more complicated seems to be the effect of oxygen on hydrogen generation rate. It is well known that hydrogen is not practically formed under fully aerobic conditions under which the iron reacts with oxygen and water using the following reaction without hydrogen generation:

4 Fe + 2 H2O + 3 O2 → 4 FeO(OH) → 2Fe2O3 + 2H2O (1)

But both Hara et al., 1992 and later Fujiwara et al, 2001 found that hydrogen generation increases significantly with the amount of oxygen in water. Hara et al, 1992 conducted the experiments in synthetic sea‐water and Fujiwara et al., 2001 at cement equilibrium water. Hara et al., 1992 have found that in the case of high amount of oxygen (1, 3 or 10 ppm) dissolved in water, the hydrogen evolution rate is ranging between 20 to 30 mol H2 m

‐2 yr‐1, but low values (0.5 mol H2 m

‐2 yr‐1) were reported for very low amount of dissolved oxygen (0.01 ppm). No pH values were reported in their report. A systematic study of the effect of pH of the range from 11.5 to 14 and the amount of oxygen was conducted in the work of Fujiwara et al., 2001. The amount of oxygen (1 x 10‐3 to 1 x 105 ppm) in solution was controlled by the amount of oxygen in nitrogen gas flowing through solution. They also found that hydrogen generation increases with the content of oxygen in water. Experiments were however conducted at very high pH values, so that passivating iron oxide layers were formed on the surface of carbon steel and consequently hydrogen generation rates were order of magnitude lower than values measured

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by Hara et al, 1992. They found that at very low values of oxygen, generation of hydrogen tends to be higher than at medium concentrations of oxygen. This was explained by the fact that under very low concentration of oxygen passive films on the surface of carbon steels cannot be reproduced sufficiently and therefore the corrosion rate becomes higher, but this does not explain, why the hydrogen evolution rate is greater at greater content of oxygen in water if it is supposed that under the presence of oxygen in water, corrosion follows equation (1).

It seems that no systematic study of the effect of temperature on hydrogen generation rate from iron metal was conducted. Only some indications were observed that the maximal values of hydrogen generation rates are at temperatures between 50 and 80 °C (Schenk,1988, Simpson, 1984, Simpson et al., 1985).

Several studies have been devoted to the effect of radiation on carbon steel corrosion (Daub et al, 2010, Marsh and Taylor…). Many of these studies, however, were devoted to the conditions to which are exposed materials in nuclear reactors. For example systematic experiments conducted by Daub et al, 2010 were carried out at pH 10.6 and in borate buffer. Marsh and Taylor, 1988 in granitic water, found that corrosion rate of carbon steel increases in the presence of radiation. Smart et al, 2008 conducted experiments in bentonite or granitic water and found a slight increase of hydrogen evolution in the presence of radiation also, but he found that it depends on dose rate and on the time of irradiation. They found that hydrogen generation rate strongly decreases over time. In these papers no systematic effort was devoted to determination of the change of redox potential during experiments.

A number of literature reviews was also compiled on this issue (Wiborgh et al, 1986, Platts et al, 1994, Johnson and Smith, 2000, King, 2007), but no clear conclusions were derived concerning the values of hydrogen generation rates in a repository.

The main aim of this work conducted in the framework of 7th FM EC project FORGE was primarily to decrease the uncertainties concerning the possible values of hydrogen generated from corrosion of carbon steel and iron under repository conditions. A first part carried in NRI was devoted to conducting experiments under strictly anaerobic conditions in an anaerobic box with simultaneous measuring of Eh and pH (chapter 2). The second part carried out by IRSN was devoted to the investigation of radiation on hydrogen generation from carbon steel (chapter 2).

2 NRI Experiments

2.1 EXPERIMENTAL

The corrosion system consisted of 1 or 10 carbon steel plates in 2 L of synthetic bentonite water or 30 g of the iron powder in the same amount of water or carbon steel cylinder in 0.3 L of synthetic bentonite water. The carbon steel correspond to the Czech standard ČSN ISO 11321 (Table 1) (dimension of the plate is 70x70x1 mm with circle cut out in center of diameter 8 mm). The surface of carbon steel plates was 99.7 cm2/plate and the carbon steel cylinder 141 cm2.

Table 1: Composition of carbon steel

Element C P Mn S

% 0.1 0.035 0.45 0.035

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The iron powder was Alfa Aesar A Johnson Matthey Company (< 10 micron, 99.9+% (metal basis), surface 0.205 m2/g). Before the experiment carbon steel plates were polished by sandpaper of grid 600 and washed by ethanol and carbon steel cylinder using a lathe to remove surface oxidized layers.

The composition of synthetic bentonite water and quantity of chemicals used for it is given in the Table 2. The pH of this water was 7.5, approximately, conductivity 2100 mS.m−1, and ionic strength 0.29 mol.l−1.

Table 2: Compositions and chemicals for preparation of synthetic bentonite water

Salt m [g.l–1] ion m [mol.l−1]

NaCl 0 Na+ 0.2

KCl 0.2013 K+ 0.0036

MgCl2.6H2O 1.5491 Mg2+ 0.0190

MgSO4.2H2O 0.5438 Ca2+ 0.0087

CaCl2.2H2O 0 Cl− 0.065

CaSO4.2H2O 1.5842 SO42− 0.1

SrCl2.6H2O 0.0216 F− 0.0003

Na2SO4 12.843 HCO3− 0.0008

NaHCO3 0.0748

NaF 0.0092

The bentonite water should correspond to the composition of sodium bentonite Volclay KWK 20‐80 of a density of 1600 kg.m‐3.

The hydrogen generation rate was determined by measuring hydrogen evolved from corroding iron or carbon steel using equipment developed in NRI (Fig. 1).

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Fig. 1: The device for hydrogen evolution measurements. 1 – Heater, 2 – Constant pressure chamber, 3 – Sample, 4 – Gas sampling valve, 5 – Temperature regulator, 6 – Data acquisition equipment, 7 – Volume change detector, 8 – Source of constant pressure, 9 – Temperature sensor

This device enables to measure hydrogen evolution continuously at different temperatures and at a constant pressure in water. The source of pressure is heavy piston, moving in mutually interconnected pressurized cylindrical vessels with a rolling membrane (Brůha and Pelech, 1989). The measurement of volume is based on the detection of the piston position, which depends on the medium volume.

Most of the experiments were conducted in anaerobic box filled with argon with concentration of oxygen below 0.1 ppm. For comparison sake some of the experiments were carried out in closed vessels under normal laboratory conditions.

In the framework of this project the device, which scheme is shown in Fig. 2 was developed in NRI to enable to conduct experiments under irradiation. It uses the same principle of measuring hydrogen generation rate from the volume change with LVDT Displacement Tranducer as described above. However, difficulties were encountered by IRSN to operate this set up in its laboratory conditions (see below in chapter 3), which led to the development of a new device.

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Fig. 2: The corrosion chamber for measuring hydrogen evolution under radiation before modification.

1. Chamber case 2. Chamber body 3. Nuts 4. Cover 5. Sample in the form of a tube 6. Filtration textile 7. Watering tube 8. Temperature sensor 9. Heater 10. Sealing O circles 11. Solution, bentonite or other materiál

Two independent electrodes, platinum wire electrode of diameter 1 mm, length of 20 mm (Beckman Instruments, USA) and gold wire electrode of diameter 1 mm, length 50 mm (Safina, Czech Republic), were used to measure redox potential continuously in the corrosion system with carbon steel and bentonite water. The redox potential was measured between work electrode (platinum, gold) and reference Ag/AgCl electrodes. These reference electrodes were prepared by coating Ag wire by silver chloride in our laboratory. During the experiments working and reference electrodes were cleaned from precipitated corrosion products by hydrochloric acid. Combined platinum electrode ORC 103–BAZ (Theta ‘90, Czech Republic, platinum disc of diameter 4 mm) was also used to measure Eh in system with steel plates (measuring period 1 day). The same platinum electrode and additional XCL 101 XB2 platinum electrode (Gryf HB, Czech Republic, platinum disc of diameter 2 mm) were used for measuring the redox potential in the corrosion system with iron powder. For measuring pH HC 113 electrode (Theta ‘90, Czech Republic) was used and in the system with the iron powder also pH PCL 321 XB2 electrode (Gryf HB, Czech Republic). The electrodes from Gryf HB company were used also for measuring in system with carbon steel wire.

The experiments were carried out at various temperatures controlled by a TDC 2 Temperature Controller (Gamry Instruments Inc, USA) with a Series 988 PID control unit (Watlow Controls, USA). The concentration of Fe3+ and S2‐ ions in the solution after experiments was measured at UV/Vis spectrometer Specord 205 – 222A358 (Analytic Jena Co., Germany), Fe3+ by modified

11


ferozzine method (Viollier et al., 2000) and for sulphide detection Spectroquant test kit (No. 167, Merck) was used. Total iron concentration in solution was measured at atomic absorption spectroscope Varian SpectrAA 200 (Varian, U.S.A).

Corrosion products on the carbon steel plates after the experiments were measured by Raman spectrometer LabRam (Horiba ‐ Jobin Yvon, France) with two lasers (He – Ne and Ar), by X‐ray diffraction Philips‐Xpert PRO (PANalytical, Netherlands) and by ESCA Probe P (Omikron Nanotechnology, Germany) in CAE mode (Constant Analyser Energy).

2.2 RESULTS AND DISCUSSION

2.2.1 Carbon steel The results of hydrogen generation rates form the experiments conducted in an anaerobic box with 10 or 1 carbon steel samples in 2 L of bentonite water are shown in Fig. 3. The curves were obtained by derivation of hydrogen accumulation curves shown in Figs 4 and 5. The pictures show a considerable dispersion of values measured at 10‐minute intervals, so the results of the hydrogen release rate values obtained by derivation of these curves can not be overestimated. It can be seen, however, that in the case of the experiments with 10 samples of carbon steel in 2 L of solution the corrosion rates decrease and in longer times approach 2 µm/yr, but in some of the experiments some indication of growth can be seen in the end of experiments. The indication of growths after decrease in the beginning of corrosion is clearly visible in the system with one sample in 2 L of solution.

These results can be explained by the formation and dissolution of protective corrosion product layers on the surface of carbon steel samples. The stability of corrosion layers depends on the evolution of the conditions as a result of corrosion. Both pH and Eh are affected by corrosion process itself (Figs. 7 ‐ 10). Eh very quickly decreases to low values immediately after immersion of samples in the solution in the system with 10 samples (Fig. 6). The initial Eh of the solution in the anaerobic box after 14 day equilibration was slightly above zero mV. This indicates that remained oxygen quickly reacted with carbon steel surface under the formation of corrosion products, such as ferrous hydroxide or magnetite. Then Eh increases due to oxidation and the formation of corrosion products with ferric ions, such as maghemite or hematite. pH decreases for all the time of corrosion (Fig. 7). This might be due to impurities contained in carbon steel. The Eh of the corrosion system with 1 carbon steel sample in 2 l of solution also decreases for all the time of experiments, but a fall of Eh in the beginning is much smaller (Fig. 9). The least stable corrosion product layers were formed under corrosion at 40 °C and 50 °C as will be seen later, at which Eh decreased at least. This suggests that the reaction of iron with oxygen was not sufficient to form a stable protective corrosion product layer.

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Fig. 3: Hydrogen evolution rates from experiments conducted in anaerobic box

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Fig. 4: Hydrogen accumulation curves from experiments with 1 sample of carbon steel in 2 l of bentonite water in anaerobic box.

Fig. 5: Hydrogen accumulation curves from experiments with 10 sample of carbon steel in 2 l of bentonite water in anaerobic box.

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Fig. 6: The change of Eh in corrosion of 10 samples of carbon steel in bentonite water

Fig. 7: Change of pH during corrosion of 10 samples in 2 l of bentonite water

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Fig. 8: Change of Eh during corrosion of one sample in 2 l of bentonite water

Fig. 9: Change of pH during corrosion of one sample of carbon steel in 2 l of bentonite water

The nature of corrosion products layers was analysed using X‐ray diffraction and Raman spectroscopy. Corrosion product layers formed on the surfaces of the samples from experiments with one carbon steel plate conducted at 40 °C and 50 °C were under the detection limit of Raman spectroscopy and X‐ray diffraction. This confirms that the thickness of these layers was very thin and can explain why the hydrogen evolution rates of some samples started to increase after some time (Fig. 2).

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The corrosion product layer of the sample from the experiment carried out at 60 °C and 70 °C was also under the detection limit of X‐ray diffraction but by Raman spectroscopy magnetite/maghemite (band 676 cm–1), maghemite (1430 cm–1) and hematite (band 220 cm–1) were detected (Fig. 10 and Fig. 11).

Fig. 10: Raman spectrum from different carbon steel plate placed at 60 °C.

Fig. 11: Raman spectrum from different carbon steel plate placed at 70 °C.

Interesting results that emerged from our experiments were achieved by analyzing the elementary composition of the corrosion layers by ESCA. A relatively large number of elements

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were detected, but with very low percentages, so that only the 3 most important elements: C, O, and Fe are shown in Tables 2 and 3. Table 3 shows the results after etching (evaporation) a very thin surface layer (thickness nm) by an electron beam. It can be seen that after removing the first layer affected by atmospheric corrosion, the percentage of iron on the surface of a non‐corroded sample is much higher than on the surfaces of corroded samples. The oxygen content in surface layers increases with the corrosion temperature. Therefore we can say that this method provided evidence that an adhering corrosion layer of non‐negligible thickness was formed on the surface of carbon steel during anaerobic corrosion at 60 and 70 °C, protecting carbon steel against further fast corrosion. The oxygen content in the surface layers of samples corroded at 60 °C and 70 °C, is, however, higher than the stoichimetric ratio of Fe/O in magnetite or maghemite so that the results suggest that the composition of passive corrosion products layers is not only formed by an inner layer of magnetite and outer layer of maghemite as proposed in previous papers (Kruger, 1989, Lu and Macdonald, 2008).

Table 3: Corrosion layer composition measured at different temperatures. Number 0 means a sample without a corrosion layer

0m sample/temperature, °C

At% 0 40 50 60 70

C 1s 49.37 64.33 37.4 29.2 50.27

O 1s 34.72 23.07 44.77 47.7 35.92

Fe 2p 9.89 1.47 4.21 5.33 2.75

Table 4: Corrosion layer composition after evaporating (etching) a thin surface layer by an electron beam. 0 is a sample without a corrosion layer.

etching sample/temperature, °C

At% 0 40 50 60 70

C 1s 7.44 53.2 19.74 4.89 12.95

O 1s 6.41 19.47 33.56 52.67 50.10

Fe 2p 86.15 9.77 26.04 21.14 14.57

The results confirm that protective corrosion product layers are formed in bentonite pore water solutions on the surface of carbon steel samples, but their stability depends strongly on the conditions of corrosion, primarily temperature, Eh and presumably pH. It also explains the wide range of corrosion and hydrogen generation rates mentioned above.

A number of experiments under various conditions have been carried out using device shown in Fig. 2. Very low values of hydrogen generation rates were obtained if the temperature of corrosion was below 50 °C (Fig. 12).

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0

5

10

15

20

25

0 5 10 15 20 25 30 35

Time (days)

V (m

l)

50 °C60 °C70 °C80 °C

Fig. 12: Hydrogen generation rate from carbon steel cylinder with surface removed by lathe before the experiment

A long‐term experiment conducted at 80 °C in an anaerobic box is now under way. The preliminary results suggest that hydrogen generation rate is by order of magnitude higher than in previous experiments shown in Fig.13. This surprising result cannot be completely explained now, but suggests that the carbon steel corrosion under anaerobic conditions is not yet fully explored and understood.

2.2.2 Iron powder In many research works focusing mainly on modelling of the effect of canister material corrosion on the evolution of the conditions in a repository, iron powder is used instead of carbon steel ((Lantenois, et al., 2005, Perronet et al., 2007, de Combarieu et al., 2007, Osacky et al., 2010). For comparison sake and also to accelerate corrosion reactions, particularly under low temperatures, iron powder with large surface was used in some of the experiments instead of carbon steel samples.

The experiment with iron powder was also conducted in the anaerobic box. Hydrogen accumulation curve for iron powder corroded at 70 °C in synthetic bentonite pore water is shown in Fig. 14. It can be seen that contrary to the results with carbon steel samples hydrogen accumulation curve started to increase only after several days and the rate of this increase was linear and corresponded to the corrosion rate of 0.8 molH2 m‐2 yr‐1 (corrosion rate of 4.3 µm.yr‐1). It could suggest that an initial protective iron oxide layer was dissolved during the first 10 days under these conditions and only after this time active corrosion of iron started or corrosion took place in the presence of oxygen without hydrogen evolution according to the equation (1).

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Fig. 13: Hydrogen and Eh evolution with corrosion of iron powder in an anaerobic box in solution of bentonite water at 70 °C.

In this experiment, very low values of Eh (Fig 13) were achieved rather quickly after beginning of the corrosion process. The pH in 30 days of the experiment contrary to carbon steel samples did not change (Fig. 14).

Fig. 14: The change of pH of bentonite water during corrosion of iron powder at 70 °C

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To determine the effect of bentonite in the solid form on the rate of corrosion, the same experiment was repeated with only one exception that a solid bentonite in a permeable bag was added to the solution. It can be seen (Fig. 15) that the character of the hydrogen accumulation curve is practically the same as in previous one. The rate of hydrogen evolution is little higher (1.2 to 0.8 molm‐2yr‐1), but it is difficult to conclude on the basis of this comparison that the presence of solid bentonite will accelerate hydrogen generation rate.

0

2000

4000

6000

8000

10000

12000

0 5 10 15 20 25 30 35t [den]

V [m

l]

-700

-600

-500

-400

-300

-200

-100

0

100

Eh [m

V]

objem H2 Redox externi Pt elektroda Theta Redox externi Grif

Fig. 15: The hydrogen evolution rate from iron powder in solution with solid bentonite.

Some of our experiments with iron powder were conducted also under normal laboratory conditions without anaerobic box in solution, which was bubbled for 20 minutes with nitrogen. The results of hydrogen generation are shown in Fig. 16. High hydrogen generation rates, approaching 10 mol H2/m2 yr, were observed at the beginning of corrosion, but after a rather short time the hydrogen generation rates decrease to low values well below 1 mol H2/m2 yr. Similar results were obtained under these conditions also with carbon steel samples (Fig. 3) and also in a number of experiments carried out in other laboratories (e.g. Smart et al.2001). The explanation is usually seen in the formation of protective iron oxide layers on the surface of iron but this can explain a decrease of hydrogen evolution rates, but not a high hydrogen generation rates in the beginning of corrosion. It seems that other plausible explanation can be seen in effect of oxygen on the formation of hydrogen. Commonly reported reactions of iron with water such as:

Fe + 2H2O → Fe(OH)2 + H2 (3)

3Fe(OH)2 ↔Fe3O4 + 4H2 + 2H2O (4)

under the formation of hydrogen are probably to much simplified and may not reflect the real mechanism of hydrogen generation. This also strongly suggests that estimation of corrosion

21


rate of carbon steel under not fully specified anaerobic conditions from the hydrogen generation rate can be wrong.

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8t [days]

H2

gene

ratio

n [m

olm

2yr-

1]

Fig. 16: Hydrogen evolution rate from iron powder at 80 °C under laboratory conditions with bentonite bubbled with nitrogen

2.3 CONCLUSION

Experimental investigation of hydrogen generation conducted in this work suggests that hydrogen generation rates cannot be predicted with any certainty if exact corrosion conditions are not known. It seems that the content of oxygen of the solution determines the rate of hydrogen generation, particularly in the beginning of corrosion, and the nature of corrosion products formed on the surface of carbon steel or iron. It was clearly shown that hydrogen generation rate in the presence of oxygen can reach values approaching 10 mol H2 m

‐2 yr‐1, but it always lead to quick depletion of oxygen from the solution and the formation of more or less stable corrosion product layers, which strongly slows down corrosion and consequently hydrogen generation rates to much lower. Our findings support the results of Honda et al., 1991 and Fujiwara et al., 2001 suggesting that the presence of oxygen accelerates the hydrogen generation rates. On the other hand it was shown that the presence of oxygen leads to the formation of protective corrosion layers, which stability and thickness depends strongly on the conditions of corrosion (notably T, Eh and presumably pH). This could also mean that hydrogen generation rate could be increased after dissolution of protecting corrosion layers.

It was observed that the behaviour of iron and carbon steel under anaerobic conditions is not the same. A decrease of pH was observed only in the experiments with carbon steel contrary to iron powder. This could be caused by the effect of impurities contained in carbon steel.

The problem of the investigation described in this work is that only short‐term, usually 30‐day experiments were conducted. The results suggest that carbon steel corrosion under anaerobic conditions is not yet fully explored and understood.

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For deeper understanding a larger set of samples should be tested in long‐term experiments, exceeding usual time of common projects.

3 IRSN Experiments

This first 2‐years period of the project was devoted to the definition of the experimental setup accounting for strong constraints, which are irradiation condition and hydrogen online measurement at low levels, as well as to first tests aiming at identifying the optimized conditions to acquire representative results. Thus, the following presents a summary of this specification work together with first results obtained related to corrosion under irradiation.

3.1 MATERIALS AND METHODS

The objective of WP2 experiments being to assess the possible influence of irradiation on hydrogen production by corrosion, it was proposed that the two partners of the project (NRI and IRSN) use the same device and the same conditions (solution composition, carbon steel). so that a comparison of hydrogen produced by corrosion alone and under irradiation would be facilitated. However, preliminary experiments conducted with NRI device (see Fig 2 in chapter 2.1) were unsuccessful because:

• in spite of rings, gas and solutions leakages were observed,

• corrosion of stainless steel (massive piece) is too low to be easily detected,

• presence of organic rings which would be damaged under irradiation,

• uneasy manipulation of the cell (2 persons are necessary).

Therefore, a new device was defined and manufactured. Because of the delay to obtain the new cell, first tests were conducted with glass vessels (Fig. 17).

Irradiation experiments were performed in IRMA (IRradiation MAtériaux) facility which is devoted to the study of the interactions radiation‐matter and more specifically to the behavior of materials and devices under γ radiation. Because of its large volume (24 m3), this facility allows a wide range of measures considering different geometries.

Four cylindrical sealed sources (60Co) with different activities are available for irradiation (see Deliverable D2.1). Their diameters are 10 mm and their lengths vary between 229 and 452 mm. The dose rate available is comprised between 5 μGy/h and 15 kGy/h. For the first set of experiments presented in this report, a single cylindrical source was used and was disposed in front of the cell at a distance of 15 cm. In these conditions the dose rate was 103 Gy/h.

A first set of tests has allowed defining the experimental setup (Fig. 18). The cell alone was introduced in IRMA irradiator and submitted to irradiation. An inert gas (helium) is introduced at the bottom of the cell in order to degas the solution (limit oxygen content in solution) and to carry out hydrogen at a constant gas flow rate at the exit of the cell. The gas mixture (O2, H2, N2 in helium) was regularly (10 min) sampled (1 ml) and analyzed by Gas Chromatography (Varian GC 450) equipped with a pulsed discharge helium ionization detector (PDHID).

A mass close to 10 g of iron powder (d<10µm) provided by alpha Caesar is added to a given volume (~100 ml) of water (Milli‐Q quality). The degassing of the solution is ensured by Helium 4.5 at a constant flowrate (approximatively 5ml/min). The gas content (O2 and N2) decrease was

23


followed by Gas Chromatography (GC). Gas chromatography was not yet calibrated; therefore the amounts of gas are expressed in arbitrary units.

Temperature is also monitored both in IRMA (“Temperature IRMA”) close to the vessel and in the laboratory (“Temperature hall”) close to gas chromatography (GC). At the end of the experiment, pH, redox potential and conductivity of the solution were measured.

Fig. 17: Experimental cell in IRMA facility, before irradiation (left) and after irradiation (12 hours; dose rate = 1kGy/h)

Gas chromatography (GC)

Fig. 18: Experimental device

solution degassing

Helium 4.5

Control of gas flow rate

He 6.0

Mixture of O2/H2 for GC standardization

10 m

He 6.0

6 Bars

Carrier gas

Sampling loop (1 ml)

Pressure measurement

outlet

Pneumatic valve monitoring

PC

Synthetic air

IRRADIATION

Iron powder

Solution

24


3.2 RESULTS AND DISCUSSION

Three experiments were conducted in order to test the device and to compare the gas concentration under irradiation alone, by corrosion and by corrosion under irradiation.

3.2.1 Irradiation of pure water The evolution of hydrogen amount (expressed in arbitrary unit (a.u.)) released by pure water irradiated during 4 hours at a constant dose rate 1 kGy/h is plotted on Fig. 19. As soon as irradiation begins, hydrogen concentration increases sharply during the first hour of irradiation. Between 1 and 4 hours of irradiation, the increase is going on but more smoothly. As soon as irradiation is stopped the hydrogen concentration decreases. No influence of temperature on hydrogen content is observed.

0

10

20

30

40

50

60

0 60 120 180 240 300 360 420

Time (min)

[H2]

(a.

u)

18

19

20

21

22

23

24

25

26

27

28

tem

pera

ture

(°C

)

irradiation

irradiation stopped

temperature IRMA

temperature hall

Fig. 19: Evolution of hydrogen concentration versus time under irradiation (1 kGy/h)

The evolution of oxygen and nitrogen concentrations versus time is plotted on Fig. 20. No significant variation was noticed.

25


0

50

100

150

200

250

300

0 60 120 180 240 300 360

Time (min)

[O2]

a.u

.

18

19

20

21

22

23

24

25

26

27

28

O2

N2

temperature hall

temperature IRMA

Fig. 20: Variations with time of oxygen and nitrogen concentration during an irradiation period (1 kGy/h)

3.2.2 Iron corrosion without irradiation The evolution of hydrogen produced by iron corrosion versus time is plotted on Fig. 21.

0

10

20

30

40

50

60

70

80

0 740 1480 2220 2960 3700 4440 5180 5920 6660 7400

time (min)

[H2]

a.u

.

20

21

22

23

24

25

26

27

28te

mpe

ratu

re (

°C)

H2

temperature hall

Fig. 21: Evolution of hydrogen concentration with time (m Fe(o) = 9.226 g ; V H2O ‐ 85ml H2O)

26


The concentration of hydrogen increases sharply during the first hours (~30 hours) until to reach a short plateau around 45 a.u.. Then, hydrogen concentration increases again until a pseudo‐plateau corresponding to a mean value of 65 a.u. But the concentrations vary around this value between 55 and 75 a.u.. These variations seem partly linked to temperature variations. Similar variations of both oxygen and nitrogen content with temperature were observed (Fig. 22 and 23).

0

20

40

60

80

100

0 740 1480 2220 2960 3700 4440 5180 5920 6660 7400

time (min)

O2

(a.u

.)

20

21

22

23

24

25

26

27

28

29

O2

temperature

Fig. 22: Variations of oxygen concentration with time (m Fe(o) = 9.226 g ; V H2O ‐ 85ml H2O)

40

50

60

70

80

90

100

0 740 1480 2220 2960 3700 4440 5180 5920 6660 7400

time (min)

N2

(a.u

.)

20

21

22

23

24

25

26

27

28

29

N2

temperature

Fig. 23: Variations of nitrogen concentration with time (m Fe(o) = 9.226 g ; V H2O ‐ 85ml H2O)

27


Solubility of gases in solution depends on temperature. When temperature increases, solubility of gases decreases and their concentrations in gaseous phase increase.

But some variations observed in the case of hydrogen can’t be explained by temperature variations. The experiment in glass vessel gives the opportunity to observe formations of gases bubbles at the surface of iron particles. When these bubbles reach a sufficient size they are released in gaseous phase inducing a flush of gas and a sudden increase of hydrogen concentration in this phase.

At the end of the experiment, the glass vessel has been dismantled and the physico‐chemical parameters of the solution were measured. The pH was 9.15 and the redox potential has decreased to ‐285 mV (ENH). The conductivity of the final solution was 3 times higher than those of pure water, traducing the presence in solution of iron species. After few hours, the colour of the solution has changed and became brunish, linked to the oxidation of Fe2+ to Fe 3+.

3.2.3 Iron corrosion under irradiation This experiment was conducted as followed:

Phase 1: Iron in contact with deaerated water during 22hours.

Phase 2: First irradiation during 4 hours (1kGy/h).

Phase 3: Irradiation is stopped during 15 hours

Phase 4: Second irradiation during 8 hours (1kGy/h)

Phase 5: Irradiation is stopped and the online hydrogen measurement goes on during 96 hours

The gases (hydrogen, oxygen and nitrogen) concentrations were monitored during all the experiment. The figure 24 presents the variation of hydrogen concentration as a function of time.

0

50

100

150

200

250

0 1440 2880 4320 5760 7200 8640

time (min) BEF 27-04-11

[H2]

(a.

u.)

18

19

20

21

22

23

24

25

26

27

tem

pera

ture

(°C

)corrosion

irradiation (4h)

irradiation stopped

irradiation (8h)

temperature hall

temperature IRMA

Fig. 24: Evolution of H2 concentration versus time (mFe(o)=10.394 g; VH2O=84.64 ml; 1kGy/h)

28


The first part of the curve is quite similar (same hydrogen concentrations for same contact durations) to those obtained in the experiment without irradiation (Fig. 21). As soon irradiation begins the concentration of hydrogen in gaseous phase increases and this lasts during all the irradiation phase. When irradiation is stopped, hydrogen concentration decreases sharply until to reach the level of concentration observed in the previous experiment for same contact iron‐water duration (Fig. 21).

The same sharp increase in hydrogen content is observed for the second irradiation phase (8 hours). Again, hydrogen content decreases when irradiation is stopped. Then, hydrogen content during about 4days (end of the experiment) varies between 70 and 110 a.u., i.e. slightly higher than in previous experiment.

Hydrogen content variations seem linked to temperature variations in the laboratory (green curve). Excepted during the irradiation phases for which temperature in the irradiation cell increase, the variations of the temperature in IRMA are small (2 °C) and seem to have minor influence on the gas content value.

The same influence of the temperature of the laboratory is observed on the variations of oxygen and nitrogen content. At 20 °C, the mean values are respectively 85 a.u. for oxygen (Fig. 25) and 160 a.u. for nitrogen (Fig. 26).

Physico‐chemical parameters of the solution were not measured at the end of the experiment, due to various hindrances; nevertheless the same change in the solution colour (as that observed in the previous experiment) due to contact with atmosphere (FeII oxidation) was observed.

40

60

80

100

120

140

360 1080 1800 2520 3240 3960 4680 5400 6120 6840 7560 8280 9000

time (min)

O2

(a.u

.)

18

19

20

21

22

23

24

25

26

27

28

tem

pera

ture

(°C

)

corrosion

irradiation (4h)

irradiation stopped

irradiation (8h)

temperature IRMA

temperature hall

Fig. 25: Evolution of oxygen content with time (mFe(o)=10.394 g; VH2O=84.64 ml; 1kGy/h)

29


120

130

140

150

160

170

180

190

200

210

220

360 1080 1800 2520 3240 3960 4680 5400 6120 6840 7560 8280 9000

time (min)

N2

(a.u

.)

18

20

22

24

26

28

30

tem

pera

ture

(°C

)

corrosion

irradiation (4h)

irradiation stopped

irradiation (8h)

temperature IRMA

température hall

Fig. 26: Evolution of nitrogen content with time (mFe(o)=10.394 g; VH2O=84.64 ml; 1kGy/h)

3.3 CONCLUSION AND FURTHER EXPERIMENTS

This first set of experiments has allowed testing the feasibility of continuous gas measurements by gas chromatography (GC) over a period of a week. In the conditions of these experiments (high dose rate, iron (o)), hydrogen was easily detected and as soon as GC will be calibrated a quantification of hydrogen concentration in gaseous phase will be possible. The results presented in this report also show the importance of temperature especially in the laboratory at the level of GC.

Further experiments will be conducted with the new cell, which equipment allows the control of pressure and He 4.5 flowrate. Thus, after the optimization of the conditions of the experience (flushing gas flow rate, ratio Fe(o) mass / solution volume) the quantification of hydrogen production will be possible.

This first set of experiments was conducted with a higher dose rate (1 kGy/h) than those expected outside HLW overpacks in a geological disposal. Future experiments will be realized with lower dose rates and if possible (capability of IRMA irradiation facility), additional experiments will be carried out to study the relation [H2] ‐ dose rate.

To complete this study on the influence of dose rate on hydrogen generation, few vials containing iron and water or carbonate solution will be placed in IRMA facility for several weeks at such a distance of sources that dose rate will be low.

Then the influence of solution composition on hydrogen production will be studied. Instead of using a solution which composition exactly mimics those of bentonite, we will use a Na2CO3 solution for simplification purpose, carbonate ion being sensitive to irradiation.

30


31

Some experiments will be conduct using iron powder (d<210 µm) and iron pieces which offer lower specific areas to corrosion processes than powder used in the first set of experiments. but allows solid observation (SEM…) and characterisation (RAMAN…) at the end of the experiment (contrary to iron particles which diameter is lower than 10 µm). A comparison of corrosion layers obtained with and without irradiation would complete data obtained on gas production.

Some analytical developments are still necessary to better characterize the solution after each experiment. First, the determination of hydrogen peroxide according to Ghormley method (Joseph et al., 2008) needs to be validated and to be tested in presence of iron species in solution. Secondly, a method of determination of iron speciation in solution (Fe2+/Fe3+) will developed (Viollier et al., 2000).

At last, the modelling of these experiments regarding solution radiolysis (Chemsimul calculations) and chemistry mass balance (using the code Chess) planned for the second 2‐years period of this project will help the interpretation of the experiments.


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forge report, single column layout · seals, in particular gas flow along contacts; role ... mr p....

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