RWMC Engineering Report
RWMC-TRE-03003

Evaluation of Gas Generation Rateby Metal Corrosion in the Reducing
Environment

Masaaki Kaneko, Norihiko Miura, Ai Fujiwara, Masafumi Yamamoto

March 2004
The Radioactive Waste Management Funding
and Research Center

This technical report summarizes the research results of the “Evaluation of effects of gas
generation” carried out from 1999 to 2002 in the research project of “Verification test on
advanced radioactive waste disposal systems” by the Radioactive Waste Management
Funding and Research Center (RWMC) under the contract with the Ministry of Economy,
Trade and Industry (METI).

EVALUATION OF GAS GENERATION RATE ARISING FROM METAL CORROSION
IN THE REDUCING ENVIRONMENTABSTRACTThis technical report summarizes the research results of the “Evaluation of effects ofgas generation” carried out from 1999 to 2002 in the research project of “Verification
test on advanced radioactive waste disposal systems” by the Radioactive Waste
Management Funding and Research Center (RWMC).

Background and Objectives
Low-level radioactive wastes with comparatively high radiation levels generated as aresult of the operation of nuclear power plants are planned to be disposed of below
ground, for example, at a depth of 50-100 m. The environment of this kind of repository
can be assumed to be extremely low-oxygen condition. And in this environment,
hydrogen gas is expected to be generated by metal corrosion of metal wastes, metallic
waste containers and steel rebars in concrete structures of repositories.
Depending on the generation and migration rates, these gases accumulate inrepositories, and might increase the pressure inside repositories, and possibly exert
mechanical effect on the barriers. The increased gas pressure might also push out thewater and air inside the repositories polluted with radioactive substances to thesurrounding hostrock. This phenomena can cause influences including migration ofradioactive substances to the outside of repositories. In order to evaluate theinfluences of the gas generation from the viewpoint of realistic safety assessment, it isimportant to evaluate the long-term generation rate of hydrogen gas at high accuracy.
In our previous studies, hydrogen gas was quantitatively analyzed using HID gas
chromatography, but since the measurement limit of this method was about 1 ppm,
direct on-line measurement of the gas generation rate was impossible. Consequently,
this method was insufficient to evaluate the gas generation rate at high accuracy.
In the present study, a new test system was fabricated and used to measurecontinuously the gas concentration from anaerobic corrosion of carbon steel. With thistest system, we could determine the gas generation rate at high accuracy and examine
the factors that affect the gas generation rate.

Experimental System for High-Accuracy Evaluation of Gas Generation (ESHG)
In fabricating the ESHG , requirements to evaluate the gas generation rate at highaccuracy were examined, and it was decided to evaluate the gas generation rate at an
accuracy of 0.001 µm/y level (up to about 1.4 ppb in terms of hydrogen gas
concentration) of an equivalent corrosion rate.
It was confirmed that the target could be achieved by using an atmospheric pressure
ionization mass spectrometer (APIMS) for an apparatus to analyze the gas generation
rate and by eliminating background inside the test system. In addition, by documenting
the reproduction and operating methods as operating instructions, a high-accuracy gas
generation rate evaluation system that generates highly reliable data was constructed.
(i)The new test system enables a more precise and reliable measurement of corrosion
rate over time than the corrosion weight loss measuring method. Thus, the mechanismof long-term corrosion can be reliably estimated with this system.

Outline of the Results
Using the ESHG, the gas generation rate from carbon steel over the long term underconditions that simulated the low-oxygen and anaerobic disposal environment wasevaluated and at the same time, effects of changes in environmental conditions were
examined. The conclusions obtained in the present study are as follows.
When the gas generation rate was studied by the hydrogen gas generation rate fromcarbon steel, the equivalent corrosion rate of carbon steel in the low-oxygen calciumequilibrium water rapidly increased at first and indicated the maximum value; then, it
gradually decreased. Except for the initial period, the value was kept to 0.05 µm/y orlower, and after 800 days or more passed, it evaluated 0.02 µm/y or lower.
It was confirmed that blast-furnace carbon steel and electric-furnace carbon steel
exhibited equal equivalent corrosion rates. Based on this, it was assessed that under
the recent test conditions, different carbon steel grades do not affect the hydrogen gas
generation.
Regarding the effect of carbon steel covered with cementitious material on the gas
generation, the test results for about 500 days indicated a release of traces ofhydrogen gas, but it delayed by about 100-300 days and the generation rate wasextremely small.
With respect to chloride ion in the test solution on gas generation, it was confirmed that
there is scarcely any difference in the long-term equivalent corrosion rates at thechloride ion concentration of 5000 ppm or lower. However, at the chloride ion
concentration of 20,000 ppm, a remarkable increase of the corrosion rate wasobserved.
For the effect of the difference in pH of the test solution of gas generation, it was
confirmed that in the range from pH 10.5 to pH 13, the effect of pH is extremely small
and was 0.05 µm/y in terms of the equivalent corrosion rate.
With respect to the solution temperature on the gas generation, effects of solution
temperature on the hydrogen gas generation rate were preliminarily tested in thetemperature range from 20 to 45°C, and the large effects of environmental temperature
were confirmed.
As long-term behavior of gas generation, no increase in the hydrogen gas generation
rate from carbon steel that was immersed for more than 100 days was observed.

( ii )

Summary
This report summarizes the basic data (main test conditions: 35°C for temperature and1 ppb or lower for oxygen concentration) concerning the evaluation of hydrogen gas
generation.
The results of the present research are applicable in the evaluation of mechanicaleffects of gas generation inside repositories or migration of radio nuclides, or
performance evaluation such as pressure evaluation by gas generation inside disposal
containers. When the environmental conditions of disposal are specified, detailed
confirmation testing is required with specific environmental conditions taken into
account.
It is necessary to carry forward the evaluation of long-term corrosion rate in order toexamine long-term barrier performance of disposal containers, leaching of nuclides
from wastes, etc. as well as the examination with variations of corrosion rate due tounstable elements, which will be required for evaluation of the disposal system.

( iii )

Contents

INTRODUCTION ････････････････････････････････････････････････････････ 1

CHAPTER 1 BACKGROUND AND PURPOSE OF THE PRESENT RESEARCH
1.1 Background of the Research ･･･････････････････････････････････････････ 2
1.1.1 Outline of Gas Generating Mechanism ････････････････････････････････ 2
1.1.2 Factors of Gas Generation (Case Studies) ･････････････････････････････ 5
1.2 Purpose of the Research ･･････････････････････････････････････････････ 6

CHAPTER2 INVESTIGATION OF TEST POLICY
2.1 Factors Contributing to Gas Generation ･････････････････････････････････ 8
2.1.1 Corrosion Mechanism in Weak Alkaline Solution (pH: about 8-10) ･････････ 8
2.1.2 Corrosion Mechanism in Strong Alkaline Solution (pH: about 10-14) ･･･････ 9
2.1.3 Factors Contributing to Gas Generation ･･･････････････････････････････ 12
2.2 Relation of Hydrogen Gas Generation Rate to Equivalent Corrosion Rate ････ 12
2.3 Problems of Assessment of Gas Generation Rate ････････････････････････ 13
2.4 Master Plan ･････････････････････････････････････････････････････････ 16

CHAPTER3 FABRICATION OF EXPERIMENTAL SYSTEM FOR HIGH-ACCURACYEVALUATION OF GAS GENERATION (ESHG)
3.1 Concept of ESHG ････････････････････････････････････････････････････ 18
3.1.1 Corrosion Rate and Measurement Accuracy････････････････････････････ 18
3.1.2 Measurement Apparatus and Analytical Instrument ･･････････････････････ 19
3.1.3 System Configuration ･･･････････････････････････････････････････････ 21
3.2 Approaches to Improving Data Reliability ････････････････････････････････ 21
3.2.1 Fabrication of ESHG ････････････････････････････････････････････････ 21
3.2.2 Performance Verification of the ESHG ･････････････････････････････････ 28
3.2.3 Setting Test Conditions ･･････････････････････････････････････････････ 29
3.3 Analysis Performance of ESHG ････････････････････････････････････････ 31i3.3.1 Calibration of ESHG ････････････････････････････････････････････････ 31
3.3.2 Data Reproducibility ････････････････････････････････････････････････ 33
3.3.3 Operation Method ･･････････････････････････････････････････････････ 34

CHAPTER4 RESULTS OF GAS GENERATION RATE EVALUATION TEST BY
CORROSION OF CARBON STEEL
4.1 Effect Assessment of Chloride Ion Concentration on the Gas Generation
Rates ･･･････････････････････････････････････････････････････････････ 35
4.1.1 Objectives ･････････････････････････････････････････････････････････ 35
4.1.2 Test Conditions and Test Method ･････････････････････････････････････ 35
4.1.3 Test Results ･･･････････････････････････････････････････････････････ 38
4.1.4 Discussion ･････････････････････････････････････････････････････････ 49
4.2 Effects Assessment of pH of Liquid Phase on the Gas Generation Rates ････ 50
4.2.1 Objectives ･････････････････････････････････････････････････････････ 50
4.2.2 Investigation of pH Range for Testing･･････････････････････････････････ 51
4.2.3 Test Conditions and Test Method ･････････････････････････････････････ 51
4.2.4 Test Results ･･･････････････････････････････････････････････････････ 53
4.2.5 Discussion ･････････････････････････････････････････････････････････ 66
4.3 Effect Assessment of Carbon Steel Grade on the Gas Generation Rate ･･････ 67
4.3.1 Objectives ･････････････････････････････････････････････････････････ 67
4.3.2 Investigation on Test Material (Rebar Material) ･････････････････････････ 67
4.3.3 Test Conditions and Test Method ･････････････････････････････････････ 68
4.3.4 Test Results ･･･････････････････････････････････････････････････････ 69
4.3.5 Discussion ･････････････････････････････････････････････････････････ 77
4.4 Effect Assessment of Cementitious Covering Materialon the Gas Generation Rates ･････････････････････････････････････････ 78
4.4.1 Objectives ･････････････････････････････････････････････････････････ 78
4.4.2 Test Conditions and Test Method ･････････････････････････････････････ 78
4.4.3 Test Results ･･･････････････････････････････････････････････････････ 83
4.4.4 Discussion ･････････････････････････････････････････････････････････ 86ii

4.5 Evaluation of Difference between Corrosion Weight Lossand Equivalent Corrosion Rate ･･･････････････････････････････････････ 88
4.5.1 Objectives ･････････････････････････････････････････････････････････ 88
4.5.2 Test Conditions and Test Method ･････････････････････････････････････ 88
4.5.3 Test Results ･･･････････････････････････････････････････････････････ 90
4.5.4 Discussion ･････････････････････････････････････････････････････････ 98
4.6 Evaluation of Long-Term Behavior of Gas Generated from Carbon Steel ･････ 100
4.6.1 Objectives ･････････････････････････････････････････････････････････ 100
4.6.2 Test Conditions and Test Method ･････････････････････････････････････ 100
4.6.3 Test Results ･･･････････････････････････････････････････････････････ 101
4.6.4 Discussion ･････････････････････････････････････････････････････････ 108

CHAPTER5 SUMMARY OF GAS GENERATION RATE EVALUATION
5.1 Metallic Material (Specimens) ･･････････････････････････････････････････ 110
5.1.1 Hydrogen Gas Generation from Carbon Steel ･･････････････････････････ 110
5.1.2 Effects of Steel Grades of Carbon Steel ････････････････････････････････ 111
5.1.3 Effects of Gap ･･････････････････････････････････････････････････････ 112
5.1.4 Effects of Corrosion Product ･････････････････････････････････････････ 113
5.1.5 Effects of Cementitious Covering Material ･･････････････････････････････ 113
5.2 Groundwater Environment (Test Solution) ･･･････････････････････････････ 115
5.2.1 Effects of Chloride Ion Concentration ･･････････････････････････････････ 115
5.2.2 Effects of pH ･･･････････････････････････････････････････････････････ 115
5.2.3 Effects of Oxygen ･･･････････････････････････････････････････････････ 116
5.2.4 Effects of Types of Solution ･･････････････････････････････････････････ 117
5.2.5 Effects of Solution Temperature ･･････････････････････････････････････ 117
5.3 Other Effects ････････････････････････････････････････････････････････ 119
5.3.1 Long-Term Behavior of the Hydrogen Gas Generation ･･･････････････････ 119
5.3.2 Potential of Test Environment ････････････････････････････････････････ 119
5.3.3 Difference Between Equivalent Corrosion Rate and Corrosion Rate ･･･････ 120
5.3.4 Gas Generation of Other Metals ･･････････････････････････････････････ 120iii

5.4 Prediction of the Gas Generation Rate by Metal Corrosion under
the Repository Environment ･･･････････････････････････････････････････ 121
5.5 Subjects in Future Research ･･･････････････････････････････････････････ 123

REFERENCES ･･････････････････････････････････････････････････････････ 125
ACKNOWLEDGEMENTS ････････････････････････････････････････････････ 127iv

Contents of Figures

Fig. 1.1-1 Accumulated gas generation volume by generation factors ･･･････････ 5
Fig. 1.1-2 Evaluation of hydrogen concentration at canister surface asa function of time for different corrosion rates ･･････････････････････ 6
Fig.2.1-1 Electrochemical schematic diagram pertaining to Fe corrosion
reactions in weak alkaline solution environment ･･････････････････････ 9
Fig.2.1-2 Electrochemical schematic diagram pertaining to Fe corrosion
reactions in a strong alkaline solution environment ･･･････････････････ 11
Fig.2.3-1 Relationship between immersion time and corrosion rate ･････････････ 14
Fig.2.3-2 Estimated corrosion rate over a super long period of time･････････････ 15
Fig. 3.1-1 Gas flow type measurement system and gas accumulation typemeasurement system ･････････････････････････････････････････ 20
Fig. 3.2-1 System diagram of ESHG ･･･････････････････････････････････････ 22
Fig.3.2-2 Working principle of APIMS ･･･････････････････････････････････････ 24
Fig.3.2-3 Measurement section of APIMS ･･･････････････････････････････････ 24
Fig. 3.2-4 Full view of ESHG ･･････････････････････････････････････････････ 25
Fig. 3.2-5 Gas purifier (Ultra purity Purifier) ･････････････････････････････････ 25
Fig.3.2-6 Pretreatment equipment and APIMS ･･･････････････････････････････ 26
Fig.3.2-7 Schematic representation of test vessel and photoshowing the Interior ･････････････････････････････････････････････ 27
Fig.3.2-8 Changes of gas concentration with external test vessel･･･････････････ 28
Fig.3.2-9 Change in hydrogen gas concentration after system is switched ･･･････ 30
Fig.3.3-1Calibration conditions of ESHG ････････････････････････････････････ 31
Fig.3.3-2 Calibration measurement results of ESHG
(Calibration results from point B using standard hydrogen gas) ･･････ 32
Fig. 3.3-3 Measurement reproducibility in AEA cement equilibrium water(Test vessel-6 is a background while carbon steel is mounted
to test vessels 7 through 9.) ････････････････････････････････････ 33
Fig.3.3-4 Changes in background hydrogen gas concentration ･････････････････ 34v

Fig. 4.1-1 Test flow pertaining to the effects evaluation of chloride ion
concentration ･･････････････････････････････････････････････････ 37
Fig.4.1-2 Changes with time of the generated hydrogen gas concentration(effects of chloride ion concentration) ･･････････････････････････････ 40
Fig.4.1-3 Changes with time of the accumulated hydrogen gas generation
rate per unit area (effects of chloride ion concentration) ･･････････････ 40
Fig.4.1-4 Changes with time of the equivalent corrosion rate
(effects of chloride ion concentration) ･･･････････････････････････････ 41
Fig.4.1-5 Changes with time of the accumulated equivalentcorrosion thickness (effects of chloride ion concentration) ･･･････････ 41
Fig.4.1-6 Chloride ion concentration and mean equivalent corrosion rate ････････ 42
Fig. 4.1-7 Visual observation results of specimens when chloride ion
concentration is 5,000 ppm ･････････････････････････････････････ 44
Fig. 4.1-8 Visual observation results of specimens when chloride ion
concentration is 20,000 ppm ････････････････････････････････････ 45
Fig.4.1-9 Composition distribution to the depth direction of thespecimen when chloride ion concentration is 5,000 ppm ･････････････ 46
Fig.4.1-10 Surface film analysis results of specimens when chloride ion
concentration is 5,000 ppm(broad photoelectron spectrum after 0.3 µm sputtering) ･･････････････ 46
Fig.4.1-11 Composition distribution to the depth direction of thespecimen when chloride ion concentration is 20,000 ppm ･･･････････ 47
Fig.4.1-12 Surface film analysis results of specimens when chloride ion
concentration is 2,000 ppm(broad photoelectron spectrum after 0.3 µm sputtering) ･･････････････ 47
Fig.4.2-1 Example of measurement results of effects of pH on the hydrogengas generation rate ･･････････････････････････････････････････････ 50
Fig. 4.2-2 pH change of leachate of cementitious material ･････････････････････ 51
Fig.4.2-3 Changes with time of the generated hydrogen gas concentration(effects of pH) ･････････････････････････････････････････････････ 55

vi

Fig.4.2-4 Changes with time of the accumulated hydrogen gas generation rate
per unit area (effects of pH) ･･････････････････････････････････････ 55
Fig.4.2-5 Changes with time of the equivalent corrosion rate
(effects of pH) ･･････････････････････････････････････････････････ 56
Fig.4.2-6 Changes with time of the accumulated equivalent corrosion thickness(effects of pH) ･･････････････････････････････････････････････････ 56
Fig. 4.2-7 Mean equivalent corrosion rate with respect to liquid-phase pH ･･･････ 57
Fig.4.2-8 Appearance observation results of pH14 specimens ･････････････････ 59
Fig.4.2-9 Appearance observation results of pH13.5 specimens ････････････････ 60
Fig.4.2-10 Appearance observation of pH13 specimens ･･･････････････････････ 61
Fig.4.2-11 Composition distribution in the depth direction of pH14 specimens ････ 62
Fig.4.2-12 Surface film analysis results of pH14 specimens(broad photoelectron spectrum after 0.3 µm sputtering) ･････････････ 62
Fig.4.2-13 Composition distribution in the depth direction of pH13.5 specimens ･･ 63
Fig.4.2-14 Surface film analysis results of pH13.5 specimens(broad photoelectron spectrum after 0.3 µm sputtering) ･････････････ 63
Fig.4.2-15 Composition distribution in the depth direction of pH13 specimens ････ 64
Fig.4.2-16 Surface film analysis results of pH13 specimens(broad photoelectron spectrum after 0.3 µm sputtering) ･････････････ 64
Fig.4.3-1 Changes with time of the hydrogen gas concentration generated(effects of carbon steel grades) ･･････････････････････････････････ 70
Fig.4.3-2 Changes with time of the accumulated hydrogen gas generation
rate per unit area (effects of carbon steel grades) ･･･････････････････ 70
Fig.4.3-3 Changes with time of the equivalent corrosion rate
(effects of carbon steel grades) ･････････････････････････････････ 71
Fig.4.3-4 Changes with time of the accumulated equivalent corrosion
thickness (effects of carbon steel grades) ･･･････････････････････････ 71
Fig.4.3-5 Appearance observation results of blast furnace steel specimens ･･････ 73
Fig.4.3-6 Appearance observation results of electric furnace steel specimens ････ 74
Fig.4.3-7 Composition distribution in the depth direction ofvii

blast furnace steel specimens ････････････････････････････････････ 75
Fig.4.3-8 Surface film analysis results of blast furnace steel specimens(broad photoelectron spectrum after 0.3 µm sputtering) ･･････････････ 75
Fig.4.3-9 Composition distribution in the depth direction ofelectric furnace steel specimens ･･････････････････････････････････ 76
Fig.4.3-10 Surface film analysis results of electric furnace steel specimens(broad photoelectron spectrum after 0.3 µm sputtering) ･･････････････ 76
Fig. 4.4-1 Shape and dimensions of the specimen ･･･････････････････････････ 80
Fig.4.4-2 Cross-sectional view of the mortar covered specimen fabricatedin the preliminary test ･･･････････････････････････････････････････ 80
Fig.4.4-3 Preparation condition of the mortar covered specimen ･･･････････････ 81
Fig. 4.4-4 Test flow pertaining to the effect evaluation of cementitious
covering material ･･････････････････････････････････････････････ 82
Fig.4.4-5 Change with time of the gas concentration of hydrogen generated(effects of mortar covered specimen) ･･････････････････････････････ 84
Fig.4.4-6 Change with time of the accumulated hydrogen gas generation
rate per specimen (effects of mortar covered specimen) ････････････ 84
Fig.4.4-7 Change with time of the equivalent corrosion rate
(effects of mortar covered specimen) ･････････････････････････････ 85
Fig.4.4-8 Change with time of the accumulated hydrogen gas generation
rate per specimen (effects of mortar covered specimen) ･････････････ 85
Fig.4.4-9 Hydrogen migration behavior in the mortar covered specimen ･････････ 86
Fig.4.5-1 Measurement period and corrosion weight loss by various tests ･･･････ 89
Fig.4.5-2 Changes with time of the generated hydrogen gas concentration ･･････ 91
Fig.4.5-3 Changes with time of the accumulated generation rate of hydrogengas per unit area ････････････････････････････････････････････････ 91
Fig.4.5-4 Changes with time of the equivalent corrosion rate ･･････････････････ 92
Fig.4.5-5 Changes with time of the accumulated equivalent corrosion thickness ･･ 92
Fig. 4.5-6 Visual observation results of specimens immersed for 5 days ･････････ 94
Fig. 4.5-7 Visual observation results of specimens immersed for 35 days････････ 95

viii

Fig.4.5-8 Composition distribution to the depth direction of the specimen
immersed for 5 days ･････････････････････････････････････････････ 96
Fig.4.5-9 Surface film analysis results of specimens immersed for 5 days
(broad photoelectron spectrum after 0.3 µm sputtering) ･･･････････････ 96
Fig.4.5-10 Composition distribution to the depth direction of the specimen
immersed for 35 days.･･････････････････････････････････････････ 97
Fig.4.5-11 Surface film analysis results of specimens immersed for 35 days
(broad photoelectron spectrum after 0.3 µm sputtering) ･･････････････ 97
Fig.4.6-1 Long-term test evaluation: Changes with time of the generatedhydrogen gas concentration ･････････････････････････････････････ 102
Fig.4.6-2 Long-term test evaluation: Changes with time of the accumulatedhydrogen gas generation rate per unit area ････････････････････････ 102
Fig.4.6-3 Long-term test evaluation: Changes with time of the equivalentcorrosion rate ･･････････････････････････････････････････････････ 103
Fig.4.6-4 Long-term test evaluation: Changes with time of the accumulated
equivalent corrosion thickness･･･････････････････････････････････ 103
Fig. 4.6-5 Mean equivalent corrosion rate by periods ･････････････････････････ 104
Fig. 4.6-6 Visual observation results of SPHC specimens (No. 5) ･･････････････ 106
Fig.4.6-7 Composition distribution to the depth direction of the SPHC specimen(No. 5) ･････････････････････････････････････････････････････････ 107
Fig.4.6-8 Photoelectron spectra of iron of the SPHC specimen (No. 5) ･･････････ 107
Fig.5.1-1 Equivalent corrosion rate of carbon steel in calcium equilibrium water ･･ 110
Fig. 5.1.2 Effects of difference in steel grades of carbon steel on
the accumulated equivalent corrosion thickness･････････････････････ 112
Fig.5.1-3 Equivalent corrosion rate of carbon steel covered withcementitious material ･･･････････････････････････････････････････ 114
Fig. 5.2-1 Effects of chloride ion concentration on the mean equivalentcorrosion rate ･････････････････････････････････････････････････ 115
Fig. 5.2-2 Effects of pH on the mean equivalent corrosion rate ･････････････････ 116
Fig. 5.2-3 Relation between oxygen concentration and the hydrogen gasixgeneration rate ････････････････････････････････････････････････ 116
Fig. 5.2-4 Effects of solution temperature on the equivalent corrosion rate ･･･････ 117
Fig. 5.2-5 Measurement results on solution temperature and the equivalentcorrosion rate in Japan and abroad ･･･････････････････････････････ 118
Fig.5.3-1 Changes of natural potential with time ･････････････････････････････ 119
Fig.5.3-2 Equivalent corrosion rate and changes with time of stainlesssteel and Zircaloy ･･･････････････････････････････････････････････ 121xContents of Tables

Table 2.4-1 Overall schedule for assessment on effects of gas generation ･･･････ 17
Table 3.1-1 Example of test conditions for past studies ･･･････････････････････ 19
Table 3.1-2 Relation between equivalent corrosion rate and hydrogengas concentration in carrier gas ････････････････････････････････ 19
Table 3.1-3 Comparison evaluation of measurement systems ･･････････････････ 20
Table 3.2-1 Specifications of gas purifier ････････････････････････････････････ 22
Table 3.2-2 Specifications of pretreatment equipment ･････････････････････････ 23
Table 3.2-3 Specifications of atmospheric pressure ionization mass
spectrometer (APIMS) used for testing ･････････････････････････ 23
Table 3.2-4 Conditions to be verified by test runs ････････････････････････････ 28
Table 3.2-5 Background measurement results ･･･････････････････････････････ 29
Table 3.2-6 Gas system switching conditions ････････････････････････････････ 30
Table 3.3-1 Calibration conditions of ESHG ･････････････････････････････････ 32
Table 4.1-1 Test conditions of effect evaluation of chloride ion concentration ･････ 35
Table 4.1-2 Test conditions of X-ray photoelectron spectroscopy ･･･････････････ 36
Table4.1-3 Comparison between corrosion thickness obtained fromcorrosion weight loss and equivalent corrosion thickness ･･････････ 48
Table 4.1-4 Effects assessment of chloride ion concentration andchanges of solution composition ････････････････････････････････ 49
Table 4.2-1 Test conditions for effect assessment of pH ･･･････････････････････ 52
Table 4.2-2 Comparison of corrosion thickness obtained from corrosion
weight loss and equivalent corrosion thickness ･･･････････････････ 65
Table 4.2-3 Effect assessment of pH and changes in the solution composition ･･･ 66
Table 4.3-1 Effect evaluation and test conditions of carbon steel grades･････････ 68
Table 4.3.2 Composition analysis results of carbon steels used for testing ･･･････ 68
Table 4.3-3 Comparison of corrosion thickness obtained from corrosion
weight loss and equivalent corrosion thickness ･･･････････････････ 77
Table 4.3-4 Effects assessment of carbon steel grades and changes inxisolution composition ･･････････････････････････････････････ 77
Table 4.4-1 Effect evaluation of cementitious covering material and test
conditions ････････････････････････････････････････････････････ 79
Table 4.4-2 Specimen specifications ･･･････････････････････････････････････ 79
Table 4.5-1 Evaluation and test conditions of difference between corrosion
weight loss and equivalent corrosion rate ････････････････････････ 89
Table 4.5-2 Corrosion thickness computed from corrosion weight loss ･･････････ 98
Table 4.5-3 Comparison between corrosion thickness and equivalentcorrosion thickness obtained from corrosion weight loss ･･･････････ 99
Table 4.6-1 Evaluation of Long-term gas generation behavior and test conditions 100
Table 4.6-2 Equivalent corrosion rate by periods ･････････････････････････････ 104
Table 4.6-3 Comparison between corrosion thickness obtained fromcorrosion weight loss and equivalent corrosion thickness ･････････ 108
Table 5-1 Factors that would affect hydrogen gas generation ･･････････････････ 109
Table 5.1-1 Analysis results of the chemical composition of carbon steel
used for tests ････････････････････････････････････････････････ 111
Table 5.1-2 Effects evaluation of steel grades of carbon steel: Equivalent corrosion rate for each immersion period ･･････････････ 112
Table 5.4-1 Summary of the present study ･･････････････････････････････････ 122

xii

INTRODUCTION

Low-level radioactive wastes with comparatively high radiation levels generated
as a result of the operation of nuclear power plants are planned to be disposed of
below ground, for example, at a depth of 50-100 m. In the disposal facilities, barriersystems combining artificial barriers and natural barriers are under investigation so thatthe general public will be free of exposure exceeding the radiation effect level even if
groundwater that passes the facilities moves to the biosphere.
Examples of disposed waste include metal waste such as carbon steel andstainless steel, and organic matter such as plastic. From this waste, hydrogen gas
associated with the corrosion of metals and methane and carbon dioxide gas resulting
from microbial degradation are assumed to be generated. In addition, it is assumed
that gas is generated similarly from metal waste containers and rebars used for
concrete structures of repositories, too.
Depending on the generation and migration rate, these gases accumulate within
the repository, increase pressure inside the repository, and possibly exert dynamic
effects on the barriers. In addition, increased gas pressure forces out water and air
inside the repository, which are contaminated with radioactive material, possibly
affecting areas outside of the repository. In order to evaluate the effects of thesegenerated gases, it is necessary, in particular, to evaluate the generation rate ofhydrogen gas from metal material over a long period of time to a high degree of
accuracy.
The Radioactive Waste Management Funding and Research Center (hereinafter
called “RWMC”) has carried out “Verification test on advanced radioactive waste
disposal systems” (hereinafter called the “verification test”) in order to establish a safeand rational disposal system. As a part of this, tests and investigations were carriedout to evaluate the hydrogen gas generation rate from 1999 to 2002. In the present
engineering report, research results are discussed on the evaluation of the hydrogengas generation rate involved in the corrosion of carbon steel under a low-oxygen
environment.

1

CHAPTER 1 BACKGROUND AND PURPOSE OF THE PRESENT RESEARCH

1.1

Background of the Research

1.1.1

Outline of Gas Generating Mechanism
The following can be assumed as factors for the generation of gas in theunderground repository environment:[1]

Metal corrosion;[2]

Microbial degradation;[3]

Radiolysis; and[4]Radioactive decay.
The gas generation mechanism by these factors is briefly described as follows.

(1)

Metal corrosion
Because of the existence of groundwater in the geological disposal environment,

metal corrosion occurs due to both aerobic conditions and anaerobic conditions, andhydrogen gas may be generated due to corrosion reactions.

Structural members(rebars, etc.), metal waste (core internals, etc.), waste containers, etc. are subject to thismetal corrosion.
Principal metals to be used and buried in repositories include the following:• Carbon steel (rebars, waste containers, metal waste);• Stainless steel (metal waste);• Zircalloy (metal waste);• Aluminum (metal waste);
Examples of corrosion reactions of the above metals are shown as follows:• Aerobic corrosion of iron
4Fe + 3O2 → 2Fe2O3↓
4Fe + 6H2O + 3O2 → 4Fe(OH)3↓• Reduction of iron oxide film
4Fe2O3 + Fe → 3Fe3O4↓2

4Fe(OH)3 + Fe + 2H2O → 3Fe3O4↓ + 5H2↑
• Anaerobic corrosion
3Fe + 4H2O → Fe3O4↓ + 4H2↑
Fe + 2H2O → Fe(OH)2↓ + H2↑
3Fe(OH)2 → Fe3O4↓+ 2H2O + H2↑• Zircalloy
Zr + 2H2O → ZrO2 + 2H2↑
• Aluminum
Al + 3H2O → Al(OH)3 + 3/2H2↑(2)

Microbial degradation
Gas generation resulting from the decomposition of organic materials by microbial

degradation means that organic materials contained in the backfilling material andgroundwater are decomposed by microorganisms and generate gas.

In addition, gas

is also generated by the decomposition of inorganic salts such as sulfate, nitrate, andothers.

Microorganisms possibly contributing to gas generation under the repository

environment and their generated gas include the following:• Fermentation microorganism - CO2, H2• Methane-generating bacteria - CH4• Denitrification bacteria - N2
• Sulfate-reducing bacteria - H2S
In addition, gas is generated from organic materials in radioactive waste, which issupplied and decomposed as nutrient sources for microorganisms.

Examples oforganic materials decomposed by microorganisms include the following:• Cellulose (paper, wood, cloth)• Plastic (polyethylene, vinyl chloride)• Rubber• Organic complex (mixture of the above)• Asphalt
Among these materials, cellulose is considered to be main source of gas
generation and gases generated by its decomposition are CO2, CH4, and H2.
3

However, microorganisms under anaerobic conditions provide lower activity thanthat under aerobic conditions and gas generation by microbial degradation underanaerobic conditions tends to be small.

(3)

Radiolysis
The mechanism of gas generation by radiolysis can be explained by radiation(γ-ray, α-ray, etc.)from radioactive waste that decomposes water as well as organic

materials around waste to generate gas.

As the concept of repository will not bedesigned to exit a large volume of organic materials around waste,

only waterradiolysis is discussed in this part of the section.
Water radiolysis occurs in the following mechanism.
Water absorbs the radiation energy and H radical (H・) and OH radical (OH・) are
generated.

Furthermore, when the radiation energy is high, water is ionized togenerate H2O+ and hydrated electron (eaq-).

The H・ and eaq- generated by these water

decompositions have high activity and it is assumed that the hydrogen generation
reactions of the following equation occur1).
H・ + H・ → H2↑
eaq- + H・ → H2↑ + OHeaq- + eaq → H2↑ + 2OH・

(4)

Radioactive decay
The alpha-decay of actinide nuclides such as U, Th, etc. contained incauses the generation of He gas.

wastesIn addition to the He gas, Rn, Kr, I2, etc. are

generated but they are only traces.
The He gas volume V(He) generated by radioactive decay can be found from thefollowing equation:
V(He) =∫tＡ×3.5136×107×ＮA×dt

where

V(He): He gas generation volume (mol)A: alpha radiation at time t
NA: Avogadro’s constant4

t: time (years)

1.1.2

Factors of Gas Generation (Case Studies)
Many reports released in various countries are stating that; the hydrogen gas

generated from repositories after closures is dominant;the greatest volume ofhydrogen gas is generated from metal corrosion, and carbon steel accounts for thegreatest percentage.
Fig. 1.1-1 shows an example reported by ENRESA2) in which the gas generation
rate is trially computed with time.

Gas generation by carbon steel corrosion is thegreatest from the initial stages of disposal, which is followed by radiolysis, andradioactive decay in that order.

Since gas generation by microbial degradation

requires time for forming a site where microorganisms proliferate, the timing of initiation

Accumulated gas generation volume (mol)for gas generation lags behind the others.

6

10

Corrosion of carbon
steel

5

10

Microbial
degradation

4

10

3

Radiolysis

10

2

10

101

Radioactive decay
放射線崩壊

0

10

10-1
-2

10

1

10

100

1000

104

105

106

Fig. 1.1-1 Accumulated gas generation volume by generation factors

It has been pointed out that the hydrogen gas generated destroys the engineered
barrier functions of the repository and possibly accelerates the transition rate ofradioactive materials.

In addition, depending on the circumstances, the possibility oflowering the soundness of hostrock, etc. around disposal tunnels has been recognized,
too.

5In ANDRA3), France, 0.1 to 0.3 µm/y (value of the hydrogen gas generation rate
converted to the corrosion rate of carbon steel) is presented as one good-rule-of-thumbwith respect to high-level radioactive waste (see Fig. 1.1-2).
It is assumed that the gas generation rate that would affect the repository would
vary in accordance with the composition, etc. of the repository.

Fig. 1.1-2 Evaluation of hydrogen concentration at canister surface as a function oftime for different corrosion rates 3)

1.2

Purpose of the Research
The hydrogen gas generated by carbon steel corrosion may increase pressure

inside the repository and exert mechanical effects on engineered barriers due to thepressure, depending on the generation rate and migration rate.

In addition, the gaspressure would push out water and air inside the repository contaminated withradioactive materials into the surrounding host rock and give rise to effects such as
promoting the migration of radioactive materials to outside of the repository.

Based onthis, it is necessary to further assess even more accurately the gas generation rate andtake appropriate measures as required.
Experimental examination of the hydrogen gas generation rate by the metal
corrosion requires an instrument that can maintain high accuracy over a long period ofexperiment term and measure continuously because only traces of hydrogen gas are
generated, and the corrosion rate is great in the initial stages of immersion and isstabilized with time.

6

In the previous researches4), HID gas chromatography was used for thequantitative analyses and measurement of hydrogen gas, but since the measurement
limit of the hydrogen gas concentration was around 1 ppm, large errors resulted when
the gas generation rate was small, and it was impossible to assess the gas generation
rate with high accuracy.
In the present research, to solve the problems above, an Experimental system
for high-accuracy evaluation of gas generation (ESHG) was fabricated. This evaluation
system was designed to measure continuously the gas volume generated from thelong-term corrosion of carbon steel to further assess even more accurately the behaviorof the gas generation rate and various factors assumed to exert effects on the gas
generation.

7

CHAPTER 2 INVESTIGATION OF TEST POLICY

2.1

Factors Contributing to Gas Generation

This chapter summarizes the electrochemical generation mechanism of hydrogengas generated from metal corrosion and identifies factors affecting gas generation.
2.1.1

Corrosion Mechanism in Weak Alkaline Solution (pH: about 8-10)

Figure 2.1-1 is an electrochemical schematic representation of corrosion
reactions of Fe in weak alkaline solution (pH: about 8-10) such as artificial seawater or
bentonite equilibrium water based on the potential-pH equilibrium diagram and themeasured polarization curve, etc. with respect to Fe, which is the main component ofcarbon steel.
The inner anodic polarization curve rapidly rises from the Fe equilibrium potential
and indicates the corrosion form of the active state region. The anode reaction is Fe →Fe2++2e-, and it is assumed that dissolved Fe2+ immediately precipitates asthermodynamically stable Fe(OH)2 or Fe3O4.
• Fe2++2OH- →Fe(OH)2• Fe2++4H2O →Fe3O4+8H++2eThe inner cathodic polarization curve falls into the following two broad generalcategories in accord with the presence of oxygen in the solution.
Cathodic reactions under high oxygen conditions:
O2+2H2O+4e- →4OH- primarily occurs but if the O2 concentration is low,
2H2O+2e- →H2↑+2OH- coexists, too.
Cathodic reactions under low oxygen conditions:
When O2 scarcely exists, 2H2O+2e- →H2↑+2OH- occurs.
The corrosion rate of Fe corresponds to the current density (i corr.) at theintersection between inner anodic polarization curve and inner cathodic polarization
curve and indicates corrosion behavior of the active state region.
The hydrogen gas generation rate arising from Fe corrosion corresponds to i corr.
when O2 is not present, but when O2 exists, hydrogen gas of a value lower than i corr. is
generated in accord with its amount. The existence of O2 encourages localized
corrosion to occur, and hydrogen gas is generated because of changes in solution
composition at the localized corrosion section.

8

Ｅ0O2

High
oxygen concentration
内部カソード分極曲線
O2+2H2O+4e-→4OH-

電 位

Low
oxygen低酸素濃度

Potential

concentration

Ｅcorr．

Ｅ0Fe

cathodic
高酸Inner
素濃度
polarization curve

Inner cathodic polarization curve
内部カソード分極曲線
2H2O+2e-→H2+2OH-

ｉcorr．

anodic polarization curve
内Inner部アノード分極曲線（Fe→Fe2++2e-）

Log (current
density)Log（電流密度）Fig. 2.1-1

2.1.2

Electrochemical schematic diagram pertaining to Fe corrosion reactions
in weak alkaline solution environment

Corrosion Mechanism in Strong Alkaline Solution (pH: about 10-14)

Figure 2.1-2 is an electrochemical schematic representation of Fe corrosion
reactions in strong alkaline solution (pH: about 10-14) such as cement equilibrium
water based on the potential-pH equilibrium diagram and measured polarization curve,
etc. for Fe, the main component of carbon steel.
The inner anodic polarization curve is passivated from the Fe equilibrium
potential via the active state region. When the potential is sufficiently noble, thepotential reaches the level of generating pitting corrosion in accord with the amount of
Cl- ion in the solution and localized corrosion results. The anode reaction is Fe →9

Fe2++2e-, and it is assumed that dissolved Fe2+ immediately precipitates asthermodynamically stable Fe(OH)2 or Fe3O4.
• Fe2++2OH- →Fe(OH)2• Fe2++4H2O →Fe3O4+8H++2e• In the passive state region, γ-Fe2O3 is generated as the surface film.

The inner cathodic polarization curve falls into the following two broad generalcategories in accord with the presence of oxygen in the solution.
Cathodic reactions under high oxygen conditions:
O2+2H2O+4e- →4OH- primarily occurs but if the O2 concentration is low,
2H2O+2e- →2OH-+H2↑coexists, too.
Cathodic reactions under low oxygen conditions:
When O2 scarcely exists, 2H2O+2e- →2OH-+H2↑occurs.
The corrosion rate of Fe corresponds to the current density (i corr.) at theintersection between the inner anodic polarization curve and the inner cathodic
polarization curve and indicates corrosion behavior of the active state region, but whena sufficient amount of O2 exists in the solution, Fe is passivated (since potentials of EoFe
and EoH2O are close, the issue arises over whether passivation of Fe occurs because of
the oxidizing power of H2O).
The hydrogen gas generation rate arising from Fe corrosion corresponds to i corr.
when O2 is not present, but when O2 exists, hydrogen gas of a value lower than i corr. is
generated in accord with its amount. The existence of O2 encourages localized
corrosion to occur in accord with the Cl- concentration, and hydrogen gas is generated
because of changes in the solution composition at the localized corrosion section.

10

Cl-

OH-

Localized
局部腐食域
corrosion
regionＥ0O2

Potential

電 位

Low oxygen
低酸素濃度
concentration

Inner cathodic polarization curve
内部カソード分極曲線（O2+2H2O+4e-→4OH-）
Passivestate region
不働態域
High
oxygen高酸素濃度
concentration

Inner
cathodic polarization curve
内部アノード分極曲線（Fe→Fe2++2e-）

Ｅcorr．
Ｅ0Fe

ｉcorr．
Activestate
活性態域
regionLog Log(current
density)（電流密度）
Fig. 2.1-2

Electrochemical schematic diagram pertaining to Fe corrosion reactions
in a strong alkaline solution environment11

2.1.3

Factors Contributing to Gas Generation

As clear from the above discussion on the corrosion principle, the oxygen
concentration in the solution and pH of the solution are the basic factors that contributeto gas generation. In addition, it is assumed that factors such as solution composition(chloride ion concentration), composition of carbon steel, etc. may exert effects on thegas generating behavior.
Because the repository environment that the present study investigates isassumed to have an extremely low oxygen concentration, it is necessary to conduct the
tests under low-oxygen concentration conditions.
The plan is to use cementitious materials from disposal facilities and, because it
is likely that the pH under the repository environment will be maintained to highalkalinity over a long time, investigation will be conducted with primary emphasis
placed on pH 12.5 of the calcium oxide saturated solution.
The effects of the chloride ion concentration must be investigated with theseawater system conditions set to the maximum (20,000 ppm).

2.2

Relation of Hydrogen Gas Generation Rate to Equivalent Corrosion
Rate

The following two chemical formulae can be considered for hydrogen gas
generation by corrosion of carbon steel.

Fe + 2H2O → Fe(OH)2↓ + H2↑

Eq. (1)

3Fe + 4H2O → Fe3O4↓ + 4H2↑

Eq. (2)

When the hydrogen gas generation rate is assessed from the corrosion rate ofcarbon steel, Eq. (2) is used for a conservative assessment, and therefore, in thepresent research, using Eq. (2), the corrosion rate equivalent to the hydrogen gas
generation rate (equivalent corrosion rate) was investigated.
An example method for converting the hydrogen gas generation rate per unit
area to the equivalent corrosion rate is shown as follows.
From Eq. (2), 4 moles (=89.6 L) of hydrogen gas are generated by corrosion of 3moles (=167 g) of Fe.
Now, Let S (m2) denote the surface area of the specimen, X (m/y) the corrosion
rate, and ρ (=7.8x106 g/m3); then, the corrosion weight W (g/y) per year is given by thefollowing equation:
W = S x X x ρ.

12

In such event, the hydrogen gas generation ratio Q (L/y) per year is found by thefollowing equation:
167 (g) : 89.6 L = W (g/y) : Q (L/y).
Based on this, the hydrogen gas generation ratio Q/S (L/m2/y) per unit area peryear is expressed as follows:
Q/S = 4.18 x 106 x X.
Consequently, the corrosion rate of 1 µm/y (X = 1.0 x 10-6 m/y) corresponds to4.18 L/m2/y per unit area.
For example, if the surface area of the specimen is 0.106 m2, the corrosion rateof 1 µm/y is 443 mL/y as follows:
4.18 (L/m2/y) x 0.106 (m2) = 0.443 (L/y).

2.3

Problems of Assessment of Gas Generation Rate

Outlines of assessment of corrosion rates and hydrogen gas generation rates ofcarbon steel with the underground repository environment assumed in previous
researches implemented from 1993 to 1998 are summarized below. The carbon steel
corrosion rates were evaluated under the following conditions.
• The oxygen concentration in injected gas is 1 ppm or lower.
• The test period is 30 days or more.
• The solution to be immersed is blast-furnace cement equilibrium water (pH: about 12)[Note: AEA selected groundwater composition].
• Carbon steel free of crevice corrosion between metals.
• Test specimens free of surface treatment (to be free of mill scale).

Figure 2.3-1 shows the results of corrosion rates calculated from corrosion weightloss averaged by measurement periods as well as the results of corrosion rates
calculated from the hydrogen gas generation rates. In addition, Fig. 2.3-2 shows theresults of long-term forecast corrosion rates. Integrating all these results produces thefollowing:• The corrosion rate calculated from the corrosion weight loss is more than 10 times the
corrosion rate (equivalent corrosion rate) calculated from the hydrogen gas generation
rate (see Fig. 2.3-1).
• The corrosion rates after about 2 years passed are given as follows:
0.1-0.4µm/y.....
Calculation
resultsfromcorrosionweightloss.
0.02-0.08 µm/y ..... Calculation results from hydrogen gas generation rate.

13• There are several factors that exert effects on the gas generation rate, but thefollowing three factors exert the greatest effect: kind of solutions (blast furnace cement
equilibrium water, bentonite equilibrium water, seawater, etc.), pH of solution, anddissolved oxygen concentration in the solution.
• Directly after carbon steel was immersed in the solution (some scores of days), thegas generation rate gives rise to violent changes but stabilizes over time, and after
about 500 days passed under some environmental conditions (blast furnace cement
equilibrium water; oxygen concentration in injected gas: 100 ppb), the gas generation
rate becomes about 0.005 µm/y in terms of the equivalent corrosion rate (see Fig. 2.32).
• Based on several test results1), the gas generation rate resulting from corrosion ofcarbon steel under repository environment is assumed to be 0.2 µm/y or less in termsof equivalent corrosion rate (value obtained by converting the hydrogen gas
generation rate to iron corrosion rate).

1

10
Corrosion rate (µm/y)

Corrosion rate from0

corrosion weight loss

10
-1

10
Corrosion rate fromhydrogen gas
-2

10
-3

10

0

100

200

300

400

500

600

700

800

Immersion time (days)

Fig. 2.3-1

Relationship between immersion time and corrosion rate1)

14

Corrosion rate (µm/y)

10 1
10 0
10 -1
10

-2

10 -3
10 -4
10

-5

0

Fig. 2.3-2

2000

4000
6000
8000
Immersion time (days)

1X104

1.2X10 4

Estimated corrosion rate over a super long period of time1)

The following points require further research.
Because the hydrogen gas concentration is measured by the use of HID gas
chromatography, from the viewpoint of measurement limit, test containers were closed
at the time of sampling and hydrogen gas generated for about 1 day was measured ina batch method. Consequently, the measurement frequency was limited andcontinuous measurement of hydrogen gas generation rate was unable to be carried out.
In addition, because of the design of the testing apparatus, the measurement values
over a long term exceeding 2 years are not warranted.
For this subject, in the present research, tests will be carried out from thefollowing viewpoints.
Using the equipment and system which can measure the hydrogen gas
concentration to the level of about 0.1 ppb, the generated hydrogen gas will be
continuously measured. If this becomes possible, the hydrogen gas generation rate
will be able to be assessed to about 0.001 µm/y level in terms of the equivalentcorrosion rate.
Tests will be carried out on many test specimens, and errors will be able to beproperly evaluated systematically.
Tests will be carried out in compliance with an appropriate quality control system,
and by this, data with high reliability will be generated, which can be applied toperformance evaluation.

15

2.4

Master Plan
Table 2.4-1 shows the master plan of the present study.

The object of the master plan is to accurately measure and evaluate for long
periods the hydrogen gas generation rate caused by corrosion under the repository
environment of carbon steel (low-oxygen atmosphere, high alkali conditions).
Consequently, in 1999, test facilities (Experimental system for high-accuracy
evaluation of gas generation) that enable long-term high-accuracy evaluation of gas
generation rate were fabricated and the operation performance was confirmed.
For about 3 years from 2000 to 2002, a long-term test was performed forevaluating the gas volume generated from corrosion of carbon steel under repository
environment (low-oxygen atmosphere, pH 12.5).
During this period, as complementary tests, investigations were carried forward
on the following items, which are assumed to exert effects on the assessment of thegas generation rate from corrosion of carbon steel.
• Assessment on the effect of chloride ion concentration on the gas generation rate;• Assessment on the effect of the pH of a liquid phase on the gas generation rate;• Assessment on the effect of steel grade of carbon steel on the gas generation rate;• Assessment on the effect of cementitious material shielding on the gas generation
rate; and• Assessment on the difference between corrosion weight loss and equivalent corrosion
rate.

The tests are discussed in detail in “Chapter 4 Results of Gas Generation Rate
Assessment Tests by Carbon Steel Corrosion.”

16

Table 2.4-1

Overall schedule for assessment on effects of gas generation

Year

1999

2000

2001

2002

Investigation of test
policySelection of test items anddecision of test methods

Survey andplan
Design andfabrication oftest facilities

Commissioning

Long-term test

Preliminary test
(initial immersion
assessment test)

Long-term test
(Long-term test on gas generation rate by metal corrosion)

Complementary test

Complementary
test

Assessment(Complementary test on gas generation rate by metal corrosion)

Assessment oncommissioning of test facilities

Assessment ofcomplementary tests

17

Primary assessment of long-term
test and complementary test

Comprehensive
assessment

CHAPTER 3 FABRICATION OF EXPERIMENTAL SYSTEM FOR
ACCURACY EVALUATION OF GAS GENERATION (ESHG)

3.1

HIGH-

Concept of ESHG

Requirements for ESHG must be defined in fabricating a gas generation rateevaluation system. Specifically, the evaluation was conducted based on the following:• Corrosion rate and measurement accuracy;• Measurement equipment and analysis apparatus• System configuration.

3.1.1

Corrosion Rate and Measurement Accuracy

In this section, corrosion rates requiring evaluation and measurement accuracy
required for the evaluation will be discussed.
In the current confirmation testing, an investigation was conducted to evaluate
the effects of hydrogen gas generation on repository systems. As a result, it was
revealed that the effect of hydrogen gas generation varies according to the repository
system, thus it is difficult to quantitatively specify required measurement accuracy
under the current situation in which the configuration of the repository system has not
yet been defined.
Yet, in discussing disposal environment conditions studied with current testing
techniques, the following results were obtained. Note that the equivalent corrosion rate
shown in the current study is the gas generation rate that is replaced by the corrosion
rate when the corrosion of carbon steel is assumed to be a magnetite generation
reaction.
• At an equivalent corrosion rate of 0.01 µm/y or lower, hydrogen gas diffuses andmigrates in a dissolved state where no gas phase is generated. The repository
system is only slightly affected.
• At an equivalent corrosion rate of approx. 0.1 µm/y, the effect of hydrogen gas
accumulation on the repository system must be studied.

Thus the current apparatus must be able to evaluate to the level where it can be
determined whether the equivalent corrosion rate is greater or lower than 0.01 µm/y.
As a result, a 0.001 µm/y level, one digit smaller than required, was evaluated formeasurement accuracy.
The equivalent to the corrosion rate of 0.001 µm/y is equal to 4.17 mL/m2・y visà-vis the hydrogen gas generation rate. Yet, examples of the surface area ofspecimens and carrier gas flow rate used in past studies are shown in Table 3.1-1.

18

Based on this, converting the equivalent corrosion rate of 0.001 µm/y to the hydrogenconcentration in carrier gas produces the results shown in Table 3.1-2.

Table 3.1-1

Table 3.1-2

Example of test conditions for past studies
Item

Conditions

Surface Area of Specimen

0.035 m2

Carrier Gas Flow Rate

1.05 × 108 ml/y(200 ml/min)

Relation between equivalent corrosion rate and hydrogen gas
concentration in carrier gas
Equivalent Corrosion Rate

Hydrogen Gas Concentration

0.001 µm/y

1.4 ppb

Thus in order to evaluate an equivalent corrosion rate of 0.001 µm/y, we mustuse an apparatus which can measure hydrogen gas concentration to about 1.4 ppb.
The atmospheric pressure ionization mass spectrometer (APIMS), an analytical
instrument, provided a 1000-mL/min standard flow rate at measurement, thus if carrier
gas is allowed to flow at a conventional flow rate of 200 mL/min in test vessels, thegenerated hydrogen gas is diluted 5 times. The required measurement limit in thisevent is about 0.3 ppb.
However, it is possible to increase the specimen surface area 5-fold, thus in orderto evaluate 0.001 µm/y, apparatus that can measure the hydrogen gas concentration toabout 1.5 ppb would suffice. Thus, while there are various uncertainties, it was judged
as necessary to design and fabricate a gas generation rate evaluation system that canmeasure to approx. 1.5 ppb.

3.1.2

Measurement Apparatus and Analytical Instrument

The gas flow type measurement system and gas accumulation typemeasurement system are proven methods for measuring the generated hydrogen gas.
Fig. 3.1-1 describes this concept.
The gas flow type measurement system detects hydrogen gas generated while
gas is constantly permitted to flow in the measurement system and has a provenrecord in current verification and other tests. The gas accumulation type measurement
system hermetically seals the measurement system and samples, and detects
generated gas as required.

19

Table 3.1-3 shows comparison results of these two measurement systems. As
shown in the table, while there is a large economic burden, the gas flow typemeasurement system would be suitable for current study according to measurement
accuracy and testing environment monitoring. Thus, gas flow type measurement
systems were studied for potential apparatus in this section.

Table 3.1-3Comparison evaluation of measurement systems

Item

Gas Flow Type Measurement
SystemGas Accumulation Type
Measurement System

Continuous Measurementof Hydrogen Gas{(Continuous hydrogen gas
measurement is enabled.)∆(Continuous measurement isunavailable.)

Test Environment
Monitoring{(Enabled with continuously
monitoring oxygen gas, etc.)∆(Difficult to continuously
monitor temporal response.)

Test Apparatus Control∆(Continuous control isrequired.){(Extremely simple control)

Entry side of coolingwater for cooler
Exit side ofcooling water
Purging gas inside desiccatorreleased to atmosphere

Purified argon gas
for maintaining low
oxygen in test vessel
Purified argon gas
for purging internal
desiccator

CoolerTo gas analyzer(APIMS)
Desiccator

Hydrogengas

Pressuredifference

Test vessel

Test item

Constant-temperature bathFig. 3.1-1

Mercury

Gas flow type measurement system and gas accumulation typemeasurement system

The following must be studied as apparatus when the gas flow typemeasurement system is selected.
• Carrier gas purifier• Analytical instrument

20• Gas transfer system (piping, valves, etc.)

The requirements for carrier gas purifier include hydrogen gas concentration
sufficiently smaller than the hydrogen gas concentration to be generated and small
oxygen concentration in order to reduce the effects of oxygen-induced corrosion.
The analytical instrument must have the ability to analyze at least approx. 1.5 ppbhydrogen concentration as described above.
The gas transfer system must satisfy requirements for minute gas leak-in frompiping, for piping that resists residual gases from remaining and others. In addition,
valves which have minute leak-in when switching and few dead spaces are required.

3.1.3

System Configuration

The system configuration includes a gas supply system, gas generating system,
analysis system and gas transfer system.
The gas supply system must purify and supply carrier gas, and consists of liquid
gas cylinders, a liquid gas vaporizer and purifier.
The gas generating system comprises test pieces (specimens), test solution, andtest vessels.
The analysis system must have a function to continuously analyze transferred
gas at high accuracy, and consists of analysis apparatus and pre-treatment equipment.
The gas transfer system is intended to transfer carrier gas and analysis gas andcomprises piping, valves, etc.

3.2

Approaches to Improving Data Reliability

3.2.1

Fabrication of ESHG

Fig. 3.2-1 illustrates an ESHG diagram. The current system mainly consists of thefollowing:• Gas purifier;• Pretreatment equipment• Analysis apparatus.

The gas purifier is a device to purify carrier gas. Liquid argon gas contains a large
amount of impurities and transfers liquid argon gas as is to the testing system, thus
impurities (oxygen, etc.) may exert effects on metal corrosion and they must be
removed. Table 3.2-1 shows specifications on the gas purifier used in the ESHG. The
21

concentrations of hydrogen gas and oxygen gas in purified carrier gas are kept to 1
ppb or lower thanks to the gas purifier.
Table 3.2-1

Specifications of gas purifier

Item

Specifications

Manufacturer

SAES GETTERS JAPAN CO. LTD.

Equipment Name

Phase II 1500

Rated Flow Rate

15 L/minMax. Rated Flow Rate

30 L/minOperating Temperature

400°C

Rated Pressure

9.9 kg/cm2

Power Consumption
Regular/Maximum

175/600 W

Max. Pressure Loss

0.7 kg/ cm2

Guaranteed Purity(Ar Gas)H2: 1 ppb or lower
O2: 1 ppb or lower
H2O: 1 ppb or lower

Oxygen analyzer

酸素濃度分Mass flow controller

ﾏｽﾌﾛｺﾝﾄﾛ
FIC

Release to atmosphere

FIC

大
気
放
出

Vaporizer

気 化

Test vessel-1

試験容器

Constanttemperature
恒 温
bath

Release to
大気放出
atmosphere

大気放

Dehumidifier

除湿装Switching
切換バル
valve

Gas purifier

精製装Release to atmosphere

大
気
放
出

Liquid argon

Test vessel-30

液 アルゴ

試験容器

Fig. 3.2-1

System diagram of ESHG

22

Gas analyzer
ガス分析(APIMS)

Pretreatment equipment is intended to maintain the performance of analysis
apparatus, and is installed particularly to remove moisture in the measuring gas. Table
3.2-2 shows the specifications of this pretreatment equipment.

Table 3.2-2

Specifications of pretreatment equipment

Item

Specifications

Manufacturer

Kobe Steel, Ltd./Hitachi Tokyo Electronics

Gas Used (Carrier Gas)

High-purity argon gas

2

Number of Gas Lines
Number of Moisture
Adsorption Columns

2 (2 columns are used alternately. While one column isin service, the other is being recreated.)

1000 cm3/minStandard Gas Flow Rate
Control System

Sequencer control

Power Supply

AC100V 15A

An atmospheric pressure ionization mass spectrometer (APIMS) 1), 2), 3) was used
for the analysis apparatus to measure hydrogen gas to 0.1 at 1 ppb levels. The working
principle of this apparatus is shown in Fig. 3.2-2 and the measurement section of theapparatus in Fig. 3.2-3. In addition, Table 3.2-3 shows specifications for the apparatus.

Table 3.2-3Specifications of atmospheric pressure ionization mass spectrometer(APIMS) used for testing
Item

Specifications

Manufacturer/Model

Hitachi Tokyo Electronics/UG-400-P

Applicable Gas

N2,Ar,He,H2

Analyzed Ion

Positive ion

Mass Range

m/Z＝3 – 360

Sensitivity

S/N ≥ 1000 (O2 peak in N2)

ResolutionM/⊿M=2M

Ionization Method

Atmospheric ionization method

Analysis Section

Quadruple mass spectrometer

Analysis Scanning Time

0.06-8 sec/mass

Ion Monitoring

Simultaneous monitoring of 16 separate peaks

23

Primary ionization

Secondary ionization

C+S+
S:Impurities

Ion
Corona dischargeS+

Ion-particle reactions

C++S→C+S+C+SSNeutral particle

C:Main component
CC

Fig. 3.2-2

Working principle of APIMS

Ionization section
Operating exhaust section
Sample introducing section
Analysis section

105Pa 105Pa 50Pa

4× 10-4Pa
Quadruple mass filter

Electron multiplier tube

Lens

Needle electrode

AmplifierSampleComputer
Lens

ExhaustExhaustFig. 3.2-3

Measurement section of APIMS

24

Printer
PlotterFig. 3.2-4 shows a full view of the ESHG. In addition, Fig. 3.2-5 shows the gas
purifier, while Fig. 3.2-6 shows the atmospheric pressure ionization mass spectrometer(APIMS) and pretreatment equipment.
Fig. 3.2-7 is a schematic representation of the test vessel and a photo showing
the interior.

Fig. 3.2-4

Fig. 3.2-5

Full view of ESHG

Gas purifier (Ultra purity Purifier)25

Atmospheric pressure ionization mass
spectrometer (APIMS)

Fig. 3.2-6

Pretreatment equipment

Pretreatment equipment and APIMS

26

Entry side of cooling waterfor cooler
Exit side ofcooling water

Purging gas insidedesiccator released toatmosphere

Purified argon gas formaintaining low oxygen in
test vessel

To gas analyzer(APIMS)

Purified argon gas forpurging internal desiccator

CoolerDesiccator
Hydrogen gas

Test vessel

Test item

Constant-temperatureb thFig. 3.2-7

Schematic representation of test vessel and photo showing the interior27

3.2.2

Performance Verification of the ESHGThe ESHG was continuously operated for about 30 days in order to verify thefunctions shown in Table 3.2-4, and verification results were also indicated.
As the table shows, verification results indicate that the ESHG possesses
functions to evaluate gas generation rates with a high level of accuracy.
Table 3.2-4
Item Checked
Air-tightness ofFacilities
Controllability
Facilities
Carrier Gas
PurityAir-tightness of
Test System

Conditions to be verified by test runs

Check Method

Results

Piping and other facilities must be
free of oxygen inflow from theatmosphere.
The facilities are able toaccommodate 30 test systems, andthese systems must be controlledsmoothly.
Carrier gas used for tests mustachieve required purity.

It was confirmed that the facilities
were free of any oxygen inflow fromthe atmosphere.
Switching was performed on 10
systems connected, and it was
confirmed that all systems could beswitched smoothly.
It was confirmed that oxygen
concentration of carrier gas was 1
ppb or lower.
Whether or not the external test
vessel (desiccator) was present, it
was confirmed that the hydrogengas concentration was constant,and there were no leaks confirmed(See Fig. 3.2-8).

Hydrogen gas concentration ismeasured with and without external
test vessel (desiccator), and noleakage of hydrogen gas must be
detected.

Gas concentration
(ppb)
1000

100

O2

10

H2

1

0.1

0.01

Fig. 3.2-8

Top lid of desiccator
removed020

Top lid of desiccator
attached

40
60
Elapsed time (min.)

80

100

120

Changes of gas concentration with external test vessel.

28

Note)The test vessel is exposed to atmosphere (oxygen concentration: 20%; hydrogenconcentration: 50 ppb) when the desiccators is kept open. The oxygen concentration

inside the test vessel increases approx. 100 times, but very little change is observed in
the hydrogen concentration. Thus it is assumed that as long as the gas concentrationdifference is approx. 500 times, leak-out and leak-in would hardly occur at all.

3.2.3

Setting Test Conditions
The following requirements must be defined in order to carry out the test.
• Background stability;• Switching time of measuring systems• Test vessel adequacy

The background stability is an important requirement for evaluating hydrogen gas
generation rates. The results of measurements carried out for about one month are
shown in Table 3.2-5.
The measurement results indicate that while there are large standard deviationsof oxygen and moisture, the mean values are small, thus there are no measurement
problems. The results indicate that the standard deviation is small and stable vis-à-vis
hydrogen gas. As current testing is performed in order to measure hydrogen gas
concentrations with a high degree of accuracy, the measurement results indicate thathydrogen gas concentrations can be measured in an adequately stable condition.

Table 3.2-5

Background measurement results

Gas

Mean concentration (ppb)Standard deviation (ppb)

Hydrogen

1.52

7.24×10-2

Oxygen1.26×10-1

4.74×10-2

Moisture

2.66E×10-1

2.48×10-2

With respect to the analysis measurement system, measurements will be madeby switching 30 systems in the future, thus the appropriate switching time must be
defined. The measurement results of a certain system are provided in Fig. 3.2-9. As
shown in the figure, the gas concentration after 50 minutes indicates almost no change,
thus the switching time is set as shown in Table 3.2-6.

29

Table 3.2-6

Gas system switching conditions

Item

Condition

Switching Time

1 hour

Evaluation Data

Data for 50 minutes after switching

Hydrogen gas concentration
(ppb)
1000

100

10
System switched
System switched

1

Fig. 3.2-9

-10

0

10
20
30
40
50
Elapsed time after system is switched (min)60

70

Change in hydrogen gas concentration after system is switched

The adequacy of the test vessels were confirmed by tests carried out before 1998
and their air-tightness was verified. This study, the following points were improved tofurther improve air-tightness.
• This study, gas sampling was no longer required. All cocks were removed and test
vessels were integrated into a glass test vessel.
• Test vessels were integrated into a glass test vessel except joints for taking in test
pieces in order to increase air-tightness.

30

3.3

Analysis Performance of ESHG

3.3.1

Calibration of ESHGThe ESHG is calibrated by using standard hydrogen gas. Fig. 3.3-1 illustrates
how calibration is performed, while Fig. 3.3-2 and Table 3.3-1 present test results.
Based on Fig. 3.3-1, standard hydrogen gas was aerated from point A andcalibration of the ESHG was performed. In addition, standard hydrogen gas wasaerated from point B and the ESHG was checked for any sign of leakage, etc.
Fig. 3.3-2 summarizes measurement values after the lapse of each measurement
time when standard hydrogen gas was aerated from point B.
In Table 3.3-1, increased errors as the gas concentration drops are attributed tothe measurement interrupted in a comparatively short time, under which the hydrogengas concentration was not fully stabilized. Based on these results, it is assumed thatthe measurement error for hydrogen gas generation rates in the ESHG is within 10%.

Oxygen analyzer
Mass flow controller
Release to atmosphere

FIC

Vaporizer

Test vessel-1

Constant-temp.
bath

Release toatmosphere
Dehumidifier

Switching valve

Ｂ
Test vessel-30

Liquid argon

Fig. 3.3-1

Release to atmosphere

Gas purifier
Gas analyzer

Ａ

Calibration conditions of ESHG

31

Hydrogen gas concentration

500
228ppb 115ppb

450

59ppb

25ppb

14ppb

400
350
300
250
200
150
100
50
0
0

20

40

60

80

100

120

140

160

180

200

Elapsed time (min)Fig. 3.3-2
Calibration measurement results of ESHG
(Calibration results from point B using standard hydrogen gas)

Table 3.3-1

Calibration conditions of ESHG

Gas flow rate (mL/min)
Case

Calculatedhydrogen gas
concentration(ppb)

Measuredhydrogen gas
concentration(ppb)

Measuredconcentration/
calculated
concentration

Argon
gas

Standardhydrogen
gas

1

200

0

−

2.6

−

2

0

200

228.1

231.3

1.01

3

100

100

115.3

121.1

1.05

4

150

50

59.0

62.4

1.06

5

180

20

25.1

26.3

1.05

6

190

10

13.9

15.4

1.11

32

3.3.2

Data Reproducibility

The hydrogen gas concentration measurement error in the ESHG is assumed tobe approx.10% as described above, yet errors caused by the test specimen fabrication
method, errors caused by the solution preparation method, errors caused by the test
specimen mounting method and other errors must be taken into account. Of theseerrors, the solution is analyzed before testing thus the error is assumed to be small. In
addition, with respect to test specimen mounting method errors, the test specimen is
mounted in compliance with the Quality Assurance Protocol, and it is assumed that theerror would be small. Thus, in this section, errors caused by the test specimen
preparation method are studied.

Equivalent corrosion rate (µｍ／ｙ)

The test specimen is machined after material is obtained (with a mill sheet or
chemical composition analysis as required), then shot-blasted and used for the
immersion test. When the test specimen is shot-blasted, shot-blasting material, shotblast pressure, shot-blast distance, etc. are set, yet subtle differences may be createdbecause all operations are carried out manually. These subtle differences can beevaluated by the difference in hydrogen gas generation rates using the same specimen.
Fig. 3.3-3 shows such measurement examples. The figure indicates several times
differences in hydrogen gas generation rates in the initial stages of immersion.
However, the differences become small after approx. 200 to 300 days elapse, while thestandard deviation is 25% or lower of the mean value. Yet, in these results, the effect ofsolution fluidity on the test specimen surface is also included, and it is assumed that a
far smaller effect is exerted by the test specimen preparation method.

Test Vessel-6

Test Vessel-7

Test Vessel-8

Test Vessel-9

50.00

100.00150.00200.005.00E-01
4.00E-01
3.00E-01
2.00E-01
1.00E-01
0.00E+00
0.00

250.00Elapsed time (day)Fig. 3.3-3

Measurement reproducibility in AEA cement equilibrium water(Test vessel-6 is a background while carbon steel is mounted to test
vessels 7 through 9.)33

3.3.3Operation Method

The current test was carried out in compliance with ISO, and the operation
method, test specimen preparation method and other methods for the ESHG conformto “Test and Operation Procedures for Gas Generation Rate Evaluation by Metal
Corrosion” (prepared on June 28, 2000 and revised June 19, 2001) which was createdspecifically for the current test.
Note that there are portions which cannot be controlled due to external factors(atmosphere, etc.). For example, background hydrogen gas concentration changes
with the seasons as shown in Fig. 3.3-4. This change in background hydrogen gas
concentration is attributed to a release of internal hydrogen as a result of temperaturechanges in the stainless steel piping in the ESHG. In the ESHG, the temperature
control is carried out, yet its control is insufficient and thus changes the results.

Hydrogen gas concentration (ppb)

However, 1 ppb of hydrogen gas concentration corresponds to approx. 0.001µm/y to 0.003 µm/y in long-term tests, which is a small value in comparison to 0.01
µm/y under evaluation. In addition, the background hydrogen gas is assumed to begenerated from carbon steel, thus it may be a safer evaluation method.

1999/12/6

2000/4/6

2000/8/6

2000/12/6

2001/4/7

2001/8/7

2001/12/7

2002/4/8

2002/8/8

2002/12/8

2003/4/9

Date of measurement

Fig. 3.3-4

Changes in background hydrogen gas concentration

34

CHAPTER 4 RESULTS OF GAS GENERATION RATE EVALUATION TEST BY
CORROSION OF CARBON STEEL

4.1

Effect Assessment of Chloride Ion Concentration on the Gas
Generation Rates

4.1.1

Objectives

The chloride ion concentration in a solution within an oxidizing atmosphere exerts
great effects on the carbon steel corrosion rates. On the other hand, under thegeological repository conditions, chloride ions may be contained in groundwater.
However, to date, there has been no sufficient quantitative evaluation of the effects ofchloride ion concentration in the solution on the corrosion rates of carbon steel within
the anaerobic condition.
This chapter purports to measure the rates of hydrogen gas generated by varying
the chloride ion concentration in calcium hydroxide (Ca(OH)2) saturated solution andevaluates the effects of chloride ion concentration on the rates of hydrogen gas
generated from carbon steel.

4.1.2

Test Conditions and Test Method
Table 4.1-1 depicts the test conditions.

The specimens used were plate-type specimens with carbon steel SPHC (Steel
Plate-Hot Commercial) surface-treated by shot-blasting. For the test liquid, in order tomaintain calcium hydroxide equilibrium water (pH 12.5), a solution with calcium
hydroxide added in super saturation was used. The chloride ion concentration wasinvestigated in 20,000-ppm seawater conditions set to the maximum. The test period
was set at 100 days or more in order to evaluate the effects on gas generation.

Table 4.1-1

Test conditions of effect evaluation of chloride ion concentration
Test pieceNo.
Material

Shape (mm)Test solution
Surfacetreatment

Type

1
2
3
4

Carbonsteel
SPHC

80×120×3
×5 pcs.
＋60×30×3
×1 pcs.

Calciumhydroxide
Shotequilibrium water
blasting
10 g/L(supersaturated)

535

Chloride ion
concentration
(ppm)

Test
period(days)

5 or lower

192.8

500

191.8

1,500

191.9

5,000

537.8

20,000378.8

Figure 4.1-1 illustrates the test flow pertaining to the effect evaluation of chloride
ion concentration.
The test solution with chloride ion concentration adjusted is prepared by adding
sodium chloride (NaCl) to the calcium hydroxide equilibrium water. The initial
composition of this test solution is analyzed separately.
This test solution is placed in a test vessel inside a low-oxygen atmosphere globe
box, and after deaerating, pretreated carbon steel test pieces are mounted on the test
vessel.
The inside of the test vessel is purged with high-purity argon (Ar) gas and is heldin the low-oxygen atmosphere. The hydrogen gas generated as a result of carbon
steel corrosion is automatically measured by a gas analyzer (APIMS: AtmosphericPressure Ionization Mass Spectrometer) and the data on changes in the hydrogen gas
generation rates with time can be accumulated.
After the completion of the test, the test pieces and test solution are recoveredfrom the test vessel and analysis is conducted.
The test pieces are photographed and their weight is measured after removal,
and after washing by acetone, etc. (descaling), the test pieces are again photographed
and their weight measured. The corrosion thickness is obtained from the corrosion
weight loss. In addition, the element distribution in the direction from the test piecesurface to depth is measured by X-ray photoelectron spectroscopy (XPS). Table 4.1-2
shows the measurement conditions. The test solution undergoes component analyses(Ca, Na, Si, Cl, Fe, etc.) and effects associated with changes before and after testing
are investigated.
These test results are comprehensively evaluated and the effects of chloride ion
concentration on the gas generation rate are evaluated.

Table 4.1-2

Test conditions of X-ray photoelectron spectroscopy

Item

Conditions

Remarks

Analyzer

X-ray photoelectron
spectroscopic device

PHI5400MC available from PerkinElmer

X-ray source

MgKαAnalysis area

1.1 mmφ

Angle

45°

Ar+ sputter rate

5 nm/minAngle between detector and specimen
in terms of SiO2

36

Preparation of calcium equilibrium water(Calcium hydroxide concentration: 10 g/L)

Preparation of frame for test vessel

Pretreatment of test piece(Machining and shot blasting)

Chloride ion concentration adjustment using NaCl

(Chloride ion concentration: 0-20,000 ppm)

Measurement and storage of test pieces(Weight measurement, appearance
observation)

Analysis of test solution(Sampling, analysis)

Preparation of specimens inside low-oxygen atmosphere globe box(Deaeration of solution, loading of specimens, assembly of test vessel, etc.)

Mounting of specimens in the gas generation rate evaluation system
and measurement of amount of gas generated

Detaching and disassembling of test vessel from the gas generation
rate evaluation system

Removal and surface analysisof specimens for surface
analysis

Removal and drying by dryer ofweight measurement specimens

Sampling of solution (about 100 cc)
and component analysis (Ca, Na, Si,Cl)• Photographing(immediately after removal)• Weight measurement(immediately after removal)

Component analysis of remaining
solution (Fe)• Washing with tap water• Cathode electrolyzation• Pure water ultrasonic washing• Acetone washing• Photographing (after

descaling)• Weight measurement (after
descaling)

Fig. 4.1-1

Test flow pertaining to the effects evaluation of chloride ion
concentration
37

4.1.3

Test Results(1)

Hydrogen Gas Generation Rate

Figure 4.1-2 illustrates changes with time of the hydrogen gas concentration ineach chloride ion concentration test.
The figure confirms that from the initial stage up to about 50 days after, thehydrogen gas concentration rose under all conditions, and thereafter, lowered and wasstabilized. At the 20,000 ppm chloride ion concentration, the maximum hydrogen gas
concentration was about 550 ppb, and it lowered to 100 ppb or less when 300 days
had elapsed. In addition, at the chloride ion concentration lower than 5,000 ppm,
nearly similar trends were exhibited under all conditions, confirming that on and after
200 days, it lowered to about 50 ppb with about 150 ppb attained as the maximum
value. Thus, it was confirmed that on and after 200 days, the hydrogen gas
concentration lowered to about 500 ppb.
Figure 4.1-3 illustrates changes with time of the accumulated hydrogen gas
generation rate per unit area.
From the figure, it was assumed that 1000 mL/m2 of hydrogen gas was
generated per unit area after the elapse of 400 days when the chloride ion
concentration was 20,000 ppm. In addition, under the conditions of chloride ion
concentration of 5,000 ppm or lower, the hydrogen gas generation rate was assumed
to be about 300 mL/m2 after the elapse of 400 days.
Assuming that the measured hydrogen gas concentration is generated only frommagnetite generation reactions (3Fe+4H2O→Fe3O4+4H2), Fig. 4.1-4 shows changes
with time of the value converted into the iron corrosion rate (hereinafter called the“equivalent corrosion rate”*).
The figure confirms that in the initial stages of immersion at the chloride ion
concentration of 20,000 ppm, the equivalent corrosion rate became extremely large ascompared to other concentrations and the equivalent corrosion rate lowered to about
0.1 µm/y after the elapse of about 400 days. In addition, under the conditions of thechloride ion concentration of 5,000 ppm or lower, the equivalent corrosion rate lowered
to about 0.05 µm/y.
Figure 4.1-5 shows changes with time of the accumulated equivalent corrosion
thickness obtained from the equivalent corrosion rate.
From the figure, it was assumed that the accumulated equivalent corrosion
thickness was 0.2 µm after the elapse of 400 days at the chloride ion concentration of
20,000 ppm. In addition, it was assumed that the accumulated equivalent corrosion
thickness was 0.1 µm after the elapse of 400 days at the chloride ion concentration of
5,000 ppm or lower.
Figure 4.1-6 illustrates the mean equivalent corrosion rates per elapsed timeagainst chloride ion concentration.

38

The figure confirms that as the elapsed time increased, the equivalent corrosion
rate lowered. In addition, it was confirmed that when the chloride ion concentration
was 5,000 ppm or lower, no effect was exerted on the equivalent corrosion rate, that is,
the hydrogen gas generation rate.

39

Background

Chloride ion concentration: 5 ppm

Chloride ion concentration: 500 ppm

Chloride ion concentration: 1500 ppm

Chloride ion concentration: 5000 ppm

Chloride ion concentration: 20000 ppm

Hydrogen concentration(ppb)

600
500
400
300
200
100
0
0

100

200

300

400

500

600

Elapsed time (days)

Fig. 4.1-2

Changes with time of the generated hydrogen gas concentration (effects
of chloride ion concentration)
Chloride ion concentration: 5 ppm

Chloride ion concentration: 500 ppm

Chloride ion concentration: 1500 ppm

Chloride ion concentration: 5000 ppm

Accumulated hydrogen gas generation rate per unit area (ml/m2)

Chloride ion concentration: 20000 ppm

Fig. 4.1-3

1000
800
600
400
200
0
0

100

200

300

400

500

600

Elapsed time (days)

Changes with time of the accumulated hydrogen gas generation rate perunit area (effects of chloride ion concentration)
40

Chloride ion concentration: 5 ppm

Chloride ion concentration: 500 ppm

Chloride ion concentration: 5000 ppm

Chloride ion concentration: 20000 ppm

Chloride ion concentration: 1500 ppm

Equivalent corrosion rate (µm/ｙ)

1.E+00

1.E-01

1.E-02
0

100

200

300

400

500

600

Elapsed time (days)

Fig. 4.1-4

Changes with time of the equivalent corrosion rate (effects of chloride
ion concentration)

Chloride ion concentration: 5 ppm

Chloride ion concentration: 500 ppm

Chloride ion concentration: 1500 ppm

Chloride ion concentration: 5000 ppm

Accumulated equivalent corrosion thickness (µm)

Chloride ion concentration: 20000 ppm

1.E+00

1.E-01

1.E-02

1.E-03Fig. 4.1-5

0

100

200
300
400
Elapsed time (days)

500

600

Changes with time of the accumulated equivalent corrosion thickness(effects of chloride ion concentration)
41

value calculated from gas generation rates for 100 days
value calculated from gas generation rates for 200 days
value calculated from gas generation rates for 300 days
value calculated from gas generation rates for about 400 days

Mean equivalent corrosion rate (µm/y)

0.50

0.40

0.30

0.20

0.10

0.00
1

10

100

1000

10000

100000Chloride ion concentration (ppm)Fig. 4.1-6(2)

Chloride ion concentration and mean equivalent corrosion rate

Appearance and Surface Changes of Specimen and Weight Changes

Figure 4.1-7 shows the appearance changes of specimens (carbon steel) before
the start of testing and after the end of testing using the photos of specimens at 5,000
ppm chloride ion concentration. In addition, Fig. 4.1-8 presents photos of specimens at
20,000 ppm chloride ion concentration.
Figure 4.1-7 and Fig. 4.1-8 show that immediately after specimens were takenout after testing, white deposits arising from calcium hydroxide or sodium chloride wereobserved. However, the specimen surface after descaling (rust removal) lost the initial
metal luster but no localized corrosion was observed for both specimens.
Figure 4.1-9 shows the composition distribution in the depth direction as a resultof XPS analysis of the specimen surface at 5,000 ppm chloride ion concentration.
Figure 4.1-10 is a broad photoelectron spectrum after 0.3 µm sputtering.
Figure 4.1-9 shows that calcium (Ca), sodium (Na), and silica (Si) are distributed
from the surface to the depth direction. Based on Fig. 4.1-10, at 0.3µm, Ca, Na, and Si
were observed in addition to Fe. Si is assumed to elute from the glass of the test
vessel.

42

Figure 4.1-11 shows the composition distribution in the depth direction as a resultof XPS analysis of the specimen surface at 20,000 ppm chloride concentration. In
addition, Fig. 4.1-12 shows a broad photoelectron spectrum after 0.3 µm sputtering.
Figure 4.1-11 shows that calcium (Ca), sodium (Na), and silica (Si) were
distributed from the surface to the depth direction. In addition, based on Fig. 4.1-10, at
0.3 µm, Ca, and Na were observed in addition to Fe. No clear difference was found in
the specimen surface when chloride ion concentrations were 20,000 ppm and 5,000
ppm.

43

Before test

Immediately after removal

After descaling

Fig. 4.1-7

Visual observation results of specimens when chloride ion concentrationis 5,000 ppm.

44

Before test

Immediately after removal

After descaling

Fig. 4.1-8

Visual observation results of specimens when chloride ion concentrationis 20,000 ppm.
45

Fig. 4.1-9

Composition distribution to the depth direction of the specimen whenchloride ion concentration is 5,000 ppm.

Fig. 4.1-10

Surface film analysis results of specimens when chloride ion
concentration is 5,000 ppm (broad photoelectron spectrum after 0.3µm
sputtering).
46

Fig. 4.1-11

Composition distribution to the depth direction of the specimen whenchloride ion concentration is 20,000 ppm.

Fig. 4.1-12

Surface film analysis results of specimens when chloride ion
concentration is 2,000 ppm (broad photoelectron spectrum after 0.3µm
sputtering).
47

The corrosion thickness was computed from the weight changes in specimens
before and after testing. Incidentally, in computation, the corrosion thickness was
assumed to be uniform.
Table 4.1-3 indicates the results of the comparison of the corrosion thickness(corrosion weight reduction) obtained from the changes in specimen weights before
and after the test with the equivalent corrosion thickness obtained from theaccumulated hydrogen gas generation rates. Incidentally, in the same table, smallequivalent corrosion thickness is obtained at 5,000 ppm chloride ion concentration in
the chloride ion concentration range from 5 to 5,000 ppm at which specimens provide
similar hydrogen gas generation behavior because the test period is long and theperiod when the hydrogen gas generation rate became small is considered.
The table indicates that the corrosion weight loss at 5,000 ppm chloride ion
concentration is smaller than that of the specimens at 5 to 1,500 ppm chloride ion
concentration, which indicate similar hydrogen gas generation behavior, though the test
period is longer. This may be attributed to the descaling method (cathode electrolysis),storage method after specimens were taken out, etc.

Table 4.1-3

Comparison between corrosion thickness obtained from corrosion
weight loss and equivalent corrosion thickness
Equivalent corrosion
thickness[2]
YearlyEquivalentYearlyequivalentcorrosion corrosion
corrosion
thickness thickness
thicknessduring testtwtg(µm/y)
period(µm/y)(µm)

Corrosion thickness

No.

(3)

Chloride ion
Testconcentration period(days)（ppm）[1] Test
periodcorrosion
thickness(µm)[1]-[2](µm)

1

5 or lower

192.8

0.543

1.03

0.05020.095

0.4928

2

500

191.8

0.733

1.39

0.04130.079

0.6917

3

1500

191.9

0.545

1.03

0.04170.079

0.5033

4

5,000

537.8

0.249

0.169

0.081

0.055

0.168

5

20,000

378.8

0.459

0.442

0.228

0.220

0.231

Composition Change of Test Solution
Table 4.1-4 shows the results of analyzing the composition of the test solution.

The table confirms that the pH is constant. In addition, the Fe dissolving rate ishigher (25 mg) at a chloride ion concentration of 20,000 ppm as compared to otherconditions. This is assumed to cause the possible difference in the hydrogen gas
generation rates.
48

Table 4.1-4 Effects assessment of chloride ion concentration and changes of solution
composition.

No.

Chloride ion
concentration
setting (ppm)

1

5 or lower

2

500

3

1500

4

5,000

5

20,000Measurement timepH

Fe(total)mg

Before testing

12.5

–

After completion of testing

12.4

0.05

Before testing

12.5

–

After completion of testing

12.4

0.10

Before testing

12.5

–

After completion of testing

12.4

0.15

Before testing

12.5

–

After completion of testing

12.5

9.2

Before testing

12.4

–

After completion of testing

12.4

25

Note 1)

pH is a measurement value at 35°C.

Note 2)Fe (total) denotes the iron amount (mg) in the total solution.

4.1.4

Discussion

Based on the foregoing test results, the effects of chloride ion concentration on thehydrogen gas generation rates were assessed.
• Because there is no significant difference in the hydrogen gas generation rates atthe chloride ion concentration of 5,000 ppm or lower, it is assumed that there is
scarcely any effect of chloride ion concentration on the hydrogen gas generation
rates of carbon steel.
• For the chloride ion concentration of 5,000 ppm or lower, it is assumed that after theelapse of one year, the equivalent corrosion rate becomes 0.05 µm/y or lower.
• It was confirmed that at the chloride ion concentration of 20,000 ppm, theaccumulated hydrogen gas generation rates during this test period become about
two to three times as much as those at the chloride ion concentration of 5,000 ppm
or lower.

49

4.2

Effects Assessment of pH of Liquid Phase on the Gas Generation
Rates

4.2.1

Objectives

Carbon steel used in repositories assumed in this testing is presumed to be
exposed to the high alkaline environment because of the components eluted from
cementitious material of engineered barriers. In addition, it is assumed that thedeterioration of cementitious material would give rise to changes in the pH, too. In paststudies1), short-term test results indicate that the hydrogen gas generation rates ofcarbon steel are greatly subject to the pH (Fig. 4.2-1).
This part of the section evaluates the effects of the pH of the liquid phase(alkaline atmosphere) on the hydrogen gas generation rates.

1.E+00Equivalent corrosion rate （µm/y)

pH14.0

1.E-01

pH13.5

1.E-02

pH13.0pH11.8pH12.5

1.E-03
0

10

20

30

40

50

60

Elapsed time (day)Fig. 4.2-1

Example of measurement results of effects of pH on the hydrogen gas
generation rate50

4.2.2

Investigation of pH Range for Testing

Under the repository environment, pH changes in groundwater in contact withcementitious material are reported by Atkinson et al.2) as indicated in Fig. 4.2-2.
Incidentally, the solution is assumed to be concrete pore water.
The figure shows the initial pH changes to about 13 to 12.5 because sodiumhydroxide (NaOH) and potassium hydroxide (KOH) are predominant. When NaOH and
KOH finish leaching, the pH becomes about 12.5 by portlandite (Ca(OH)2). Thereafter,
the pH is governed by the calcium-silicate hydrate (C-S-H) of the cement hydrate andis shifted to about 12.5 to 10.
Based on this, it is assumed that the pH of ground water under the repository
environment in contact with cementitious material is about 10 to 13. Considering thetolerances, it was decided that investigation would be conducted up to pH14.0 at the
maximum for the range of effects to be tested.

Fig. 4.2-2

4.2.3

pH change of leachate of cementitious material2)

Test Conditions and Test Method
Table 4.2-1 depicts the test conditions.

The specimen used was plate-form carbon steel SPHC that was surface-treatedby shot-blasting. For the test solution, the calcium hydroxide solution (Ca(OH)2; 2 g/L)
whose chloride ion concentration was adjusted to 5,000 ppm was used. For pH
adjustment, NaOH was used to increase the pH and HCl was used to lower the pH.

51

The test period was set to 200 days or more in order to evaluate the effects on
gas generation.

Table 4.2-1

Test conditions for effect assessment of pH

Specimen
No.
Material

Shape
(mm)

Test solution
Surface Type and chloride
treatment ion concentration

pH

Test period
(day)

1-1

10.5

453.0

1-2

10.5

453.0

11.5

441.0

12.5

537.8

13.0

403.0

5

13.5

403.0

6

14.0

432.2

2
3
4

Carbonsteel
SPHC

80×120×3
× 5 pcs.
+
60×30×3
×1 pc.

Ca(OH)2 solution:
2 g/L
Shotblasting

Chloride ion
concentration :5,000 ppm

The test flow pertaining to the liquid-phase pH on the gas generation rates andeffect assessment is the same as that of “Section 4.1 Effect Assessment of Chloride
Ion Concentration on the Gas Generation Rates” mentioned above.
To the Ca(OH)2 solution, sodium chloride (NaCl) is added and the test solution
with pH adjusted is prepared. The initial composition of this test solution is thenanalyzed separately.
This test solution is placed in a test vessel inside the low-oxygen atmosphereglobe box, and after deaerating, pretreated carbon steel specimens are mounted.
The inside of the test vessel is filled with high-purity argon (Ar) gas and the lowoxygen atmosphere is maintained. The generated hydrogen gas involved in the
corrosion of carbon steel is automatically measured by the gas analyzer (APIMS) andthe data of changes in the hydrogen gas generation rates accompanied by the elapsed
time can be accumulated.
After completion of testing, the specimens and test solution are recovered fromthe test vessel and analyzed.
The specimens have their photos taken and their weight measured after removal,
and after washing (descaling) by acetone, etc., the specimens have their photos taken
and their weight measured again. From this corrosion weight loss, the corrosion
thickness can be obtained. In addition, by X-ray photoelectron spectroscopy (XPS),
molecular distribution from the surface to the depth direction of the specimens ismeasured. The test solution is measured for Fe increase after testing.

52

These test results are comprehensively evaluated, and the effects of liquid-phase
PH on the gas generation rates are assessed.

4.2.4

Test Results(1)

Hydrogen Gas Generation Rate

Figure 4.2-3 depicts changes with time of the hydrogen gas concentration in the
pH tests of each liquid phase.
The figure confirms that at pH14, the hydrogen gas concentration is as high as
1200 ppb or more from the initial stages, and changes to the order of 500 ppb evenafter 200 days. At pH13.5, after the elapse of 100 days, the hydrogen gas
concentration achieved 500 ppb or higher, but lowered thereafter, and when 200 days
had elapsed, it changed to the order of 50 ppb.
In addition, we confirmed that at pH13 to pH10.5, there was hardly any differencein changes of the hydrogen gas generation concentration, and after the elapse of 100
days, the hydrogen gas concentration changed to the order of 50 ppb.
Figure 4.2-4 depicts changes with time of the accumulated hydrogen gas
generation rates per unit area.
The figure confirms that at pH14, the hydrogen gas generation rates linearly
increased and exceeded 2500 mL/m2 per unit area after 400 days had elapsed. It was
confirmed that at pH13.5, up to 100 days, trends the same as those of pH14 were
exhibited but thereafter, the generation rates decreased, and even after the elapse of
400 days, the hydrogen gas generation rates changed to the order of 1500 mL/m2. In
addition, under the conditions of pH13 to 10.5, there was little difference, and thehydrogen gas generation rate was 500 mL/m2 or lower after the elapse of 400 days.
Figure 4.2-5 shows changes with time of the equivalent corrosion rates of ironwith the measured hydrogen gas concentration assumed to be generated frommagnetite generation reactions of iron (3Fe+4H2O→Fe3O4+4H2) only.
The figure confirms that at pH14, the equivalent corrosion rate changed at about
0.5 µm/y even after 400 days had elapsed. At pH13.5, up to 200 days, the equivalentcorrosion rate was 0.1 µm/y or more, but on and after 200 days, it lowered up to about
0.05 µm/y. It was confirmed that the equivalent corrosion rate was 0.05 µm/y or lower
at pH13 to 10.5. It was assumed that in this range, as the pH lowers, the equivalentcorrosion rate tends to slightly decrease.
Figure 4.2-6 shows changes with time of the accumulated equivalent corrosion
thickness obtained from the equivalent corrosion rate.
Based on the figure, it was assumed that at pH14, corrosion took place to about
0.6 µm after the elapse of 400 days. At pH13.5, it was assumed that the corrosion
thickness was slightly small and the specimen corroded up to about 0.4 µm.

53

Furthermore, under the conditions of pH13 to pH10.5, it was assumed that thecorrosion thickness was all 0.1 µm or lower even after the elapse of 400 days.
Figure 4.2-7 shows the average equivalent corrosion rates by elapsed periods
and by periods with respect to liquid-phase pH.
The figure confirms that as the elapsed time increased, the equivalent corrosion
rate lowered. In addition, under the conditions of pH13-10.5, the equivalent corrosion
rate was scarcely affected.

54

2000
pH:11.5
pH:12.5
pH:13.0
pH:13.5■pH:12.5◆pH:13.0 ▲pH:13.5
pH:14.0
ｐＨ10.5（29）ｐＨ10.5（30）Hydrogen concentration (ppb)●pH:10.5 ▲pH:10.5 ●pH:11.5

1500■pH:14.0

1000

500

0
0

100

200

300

400

500

600

Elapsed time (day)Accumulated hydrogen gas generation rate per unit area (ml/m2)

Fig. 4.2-3

Changes with time of the generated hydrogen gas concentration(effects of pH)

3000
pH:11.5
pH:12.5●pH:10.5▲pH:10.5 ●pH:11.5
pH:13.0
pH:14.0■pH:14.0
ｐＨ10.5（30）2500

pH:13.5
ｐＨ10.5（29）■pH:12.5 ◆pH:13.0 ▲pH:13.5

2000

1500
1000

Fig. 4.2-4

500
0
0

100

200

300

400

500

600

Elapsed time (day)

Changes with time of the accumulated hydrogen gas generation rate perunit area (effects of pH)
55

1.E+01●pH:10.5▲pH:10.5
pH:11.5
pH:12.5●pH:11.5
pH:13.0

pH:13.5

pH:14.0
ｐＨ:10.5（30）1.E-03 ▲pH:13.5■pH:12.5◆pH:13.0■pH:14.0
Equivalent corrosion rate (µｍ/ｙ)

ｐＨ:10.5(29)1.E+00

1.E-01

1.E-02

1.E-03
0

100

200

300

400

500

600

Elapsed time (day)Fig. 4.2-5

Changes with time of the equivalent corrosion rate (effects of pH)

Accumulated equivalent corrosion thickness(µｍ)

1.E+01

pH:11.5 ▲pH:10.5
pH:12.5●pH:10.5●pH:11.5
pH:14.0

pH:10.5（29）

pH:13.0
pH:10.5（30）

pH:13.5

400

500■pH:12.5 ◆pH:13.0 ▲pH:13.5 ■pH:14.0
1.E+00

1.E-01

1.E-02600
1.E-02

1.E-02600
1.E-03
ｐＨ:10.5(29)
0

100

200

300

600

Elapsed time (day)Fig. 4.2-6

Changes with time of the accumulated equivalent corrosion thickness(effects of pH)

56

Mean equivalent corrosion rate（µm/y）

1.0
0.9
0.8
value calculated from gas generation rates for 100 days

0.7

value calculated from gas generation rates for 200 days

0.6

value calculated from gas generation rates for 300 days
value calculated from gas generation rates for about 400 days

0.5
0.4
0.3
0.2
0.1
0.0
10.0

11.0

12.0

13.0

14.0

ｐＨ
Immersion period and mean equivalent corrosion rate

Mean equivalent corrosion rate（µm/y）

0.55
0.50
0.45
0.40

0th to 100th day
10th to 200th day
200th to 300th day

0.35

300th to 400th day

0.30
0.25
0.20
0.15
0.10
0.05
0.00
10.0

11.0

12.0

pH

13.0

14.0

Mean equivalent corrosion rate according to period
Fig. 4.2-7

Mean equivalent corrosion rate with respect to liquid-phase pH

57

(2)

Appearance and Surface Changes as well as Weight Changes ofSpecimens

Specimens of pH14, pH13.5, and pH13, which exert large effects on thehydrogen gas generation rate, were analyzed.
Figure 4.2-8 includes photos showing the appearance of the pH14 specimens(carbon steel) before the test was started and after the test was completed. In addition,
photos of pH13.5 specimens are given in Fig. 4.2-9 and those of pH13 specimens in
Fig. 4.2-10.
Figure 4.2-8, Fig. 4.2-9 and Fig. 4.2-10 show that the surfaces of specimens after
descaling (rust removal) did not provide initial metal luster but were all free of localized
corrosion.
Figure 4.2-11 shows the composition distribution in the depth direction when the
pH14 specimen surfaces were analyzed by XPS. In addition, broad photoelectron
spectra after 0.3 µm sputtering are shown in Fig. 4.2-12.
Figure 4.2-11 shows that the ingredients contained from the surface to the depth
direction, sodium (Na), silica (Si), iron (Fe), and calcium (Ca) are distributed in thatorder. In addition, based on Fig. 4.2-12, even at 0.3 µm, Na, Ca, and Si were observedin addition to Fe. Si was assumed to elute from the glass of test vessels in trace
amounts.
Figure 4.2-13 shows the composition distribution in the depth direction when thepH13.5 specimen surfaces were analyzed by XPS. In addition, broad photoelectron
spectra after 0.3 µm sputtering are shown in Fig. 4.2-14.
Figure 4.2-13 indicates that from the surface to the depth direction with sputtering
for up to 5 minutes, Na is distributed more than Fe, but in the area deeper than that, Fe,
Na, Ca, and Si are distributed in that order. Furthermore, from Fig. 4.2-14, Ca, Na, andSi were observed in addition to Fe even at 0.3 µm.
Figure 4.2-15 shows the composition distribution with respect to the depth
direction when pH13 specimen surfaces were analyzed by XPS. In addition, Fig. 4.216 shows broad photoelectron spectra after 0.3 µm sputtering.
Figure 4.2-15 indicates that very near the surface, Na is slightly more distributed
than Fe but as the depth increases, Fe, Na, Ca, and Si are distributed in that order.
Furthermore, from Fig. 4.2-16, Ca and Si are observed in addition to Fe even at 0.3 µm,
and Na is scarcely observed.
These results suggest that at pH14, which exerts large effects on the hydrogengas generation rates, Na is likely to infiltrate into specimens (carbon steel). Based on
this, it is assumed that Na may possibly exert effects on the hydrogen gas generation
rates.

58

Before test

Immediately after removal

After descaling

Fig. 4.2-8

Appearance observation results of pH14 specimens
59

Before test

Immediately after removal

After descaling

Fig. 4.2-9

Appearance observation results of pH13.5 specimens
60

Before test

Immediately after removal

脱After descaling

Fig. 4.2-10

Appearance observation of pH13 specimens

61

Fig. 4.2-11

Fig. 4.2-12

Composition distribution in the depth direction of pH14 specimens

Surface film analysis results of pH14 specimens (broad photoelectron
spectrum after 0.3 µm sputtering)62

Fig. 4.2-13

Fig. 4.2-14

Composition distribution in the depth direction of pH13.5 specimens

Surface film analysis results of pH13.5 specimens (broad photoelectron
spectrum after 0.3 µm sputtering)63

Fig. 4.2-15

Fig. 4.2-16

Composition distribution in the depth direction of pH13 specimens

Surface film analysis results of pH13 specimens (broad photoelectron
spectrum after 0.3 µm sputtering)64Based on the weight changes of specimens before and after testing, mean
corrosion thickness was computed.
Table 4.2-2 compares the corrosion thickness (corrosion weight loss) obtained
from the weight changes of specimens before and after testing with the equivalentcorrosion thickness obtained from the accumulated hydrogen gas generation rates.
These test periods ranged from 400 to 550 days, nearly the same, and because
as the pH increases, the hydrogen gas generation rate increases, too, in the range
from pH 10.5 to 14.0, the corrosion thickness increases as the pH increases. In
addition, when the corrosion thickness and equivalent corrosion thickness arecompared, a difference of about 0.2 µm is generated in the corrosion thickness.
However, with respect to pH14 specimens, the difference is too large, suggesting that
corrosion products other than Fe3O4 might be included in the scale composition.

Table 4.2-2

Comparison of corrosion thickness obtained from corrosion weight loss
and equivalent corrosion thickness
Corrosion thickness

(3)

No.

pH
setting

Test
period(days)

1-1

10.5

1-2

Equivalent corrosion thickness[1]Test period
corrosion
thickness(µm)

Yearlycorrosion
thicknesstw (µm/y)[2] Test period
equivalentcorrosion
thickness (µm)

Yearlyequivalentcorrosion
thickness
Tg (µm/y)[1]-[2]
(µm)

453.0

0.160

0.129

0.038

0.031

0.122

10.5

453.0

0.262

0.211

0.043

0.034

0.219

2

11.5

441.0

0.271

0.224

0.060

0.050

0.211

3

12.5

537.8

0.249

0.169

0.081

0.055

0.168

4

13.0

403.0

0.379

0.343

0.101

0.092

0.278

5

13.5

403.0

0.737

0.668

0.363

0.329

0.374

6

14.0

432.2

1.630

1.377

0.656

0.554

0.974

Composition Change of Test Solution
Table 4.2-3 shows the analysis results of test solution composition.

The table confirms that the pH lowered after immersion for pH10.5 and pH11.5 ofthe liquid phase.
Also, under the pH14 conditions, the Fe dissolving rate is about one digit greater(286 mg) than that under other conditions. This Fe dissolution is assumed to exert
effects on the hydrogen gas generation rate.
65

Table 4.2-3

Effect assessment of pH and changes in the solution composition

Measurement No.

pH setting

1-1

10.5

1-2

2

3

4

5

6

Measurement timepH

Fe(total)mg

Before testing

10.6

-

After completion of testing

9.4

2.8

Before testing

10.6

-

After completion of testing

9.4

4.3

Before testing

11.5

-

After completion of testing

10.0

0.62

Before testing

12.5

-

After completion of testing

12.5

9.2

Before testing

13.1

-

After completion of testing

13.0

17

Before testing

13.5

-

After completion of testing

13.4

5.6

Before testing

13.9

-

After completion of testing

13.8

286

10.5

11.5

12.5

13.0

13.5

14.0

Note 1)

pH is a measurement value at 35°C.

Note 2)

Of ion concentrations, Fe (total) denotes the iron amount (mg) in the total solution.

4.2.5

Discussion

The findings obtained by testing with respect to the effects of liquid-phase pH on
the hydrogen gas generation rates can be summarized as follows:•

In the range from pH10.5 to 13.0, as the pH rises, the hydrogen gas generation
rates tend to slightly increase, but they are nearly the same.

•

At pH14.0, when compared to pH10.5 to 13.0, a large volume of hydrogen gas is
generated, and even after 400 days have elapsed, the hydrogen gas generation
rate is unlikely to drop. After about 400 days have elapsed, specimens generate
more than one digit greater hydrogen gas than those at pH13.0 or lower.
This may be explained by the stability of the passivation film being impaired byexcess hydroxide ion (OH-) or sodium ion (Na+) at pH14.0.

•

Because in the range from pH10.5 to 13.0, the mean equivalent corrosion rate in
the period from the 300th day to 400th day is about 0.05 µm/y and tends to lower
further, it would be sufficient to assume the equivalent corrosion rate under therepository environment as 0.05 µm/y at the maximum over the long term.

66

4.3

Effect Assessment of Carbon Steel Grade on the Gas Generation Rate

4.3.1

Objectives

In disposal facilities below the generally used depth, a large amount of metalmaterial (carbon steel, etc.) is embedded. Examples of the metal material to beembedded include carbon steel as rebars used as structural material, carbon steel
used for repository containers, and carbon steel, etc. which are embedded as waste.
With respect to the long-term effects of gas generation, investigation is conducted by
the use of hot-rolled steel SPHC, which is typical material used for repository
containers.
In the conventional findings, the difference in carbon steel grades does not exert
effects on the hydrogen gas generation rates but these are the test results under the
aerobic environment exceeding 100 ppb.
This part of the section explains the effects of the difference in carbon steel
manufacturing methods (blast furnace steel and electric furnace steel) on the hydrogengas generation rates in the super-low oxygen environment of 1 ppb or lower.

4.3.2

Investigation on Test Material (Rebar Material)

The type of rebar material can be broadly classified into blast furnace steel andelectric furnace steel.
For the blast furnace steel, pig iron is produced by first dumping iron ore into asmelting furnace (blast furnace) together with coke and limestone, injecting hot blastfrom the bottom, and allowing reduction reactions to slowly take place. Because thepig iron contains a large amount of carbon and impurities, molten pig iron is further
poured into a converter, and oxygen is blown in to encourage oxidation reactions to
produce steel. And the produced steel is worked by rolling, etc.
On the other hand, electric furnace steel is produced by directly melting scrap
steel or scrap substitutes in an electric arc furnace and working by rolling, etc.
The chemical composition of these two types of rebar material and theirproperties are discussed as follows:•

Blast furnace steel contains high percentages of Si and Mn and has chemicalcomposition with primary emphasis placed on the quality such as ductility,
toughness, etc. In addition, application of Al-Si-killed steel is observed as part ofthe blast furnace steel.

•Electric furnace steel contains residual components of Cu, Ni, Cr, Sn, etc. and has
a high percentage of N, which exerts effects on strain aging (type of embrittlement
phenomena). In addition, since it contains high percentages of Cu, Ni, Cr, and Sn,
product strength can be easily obtained, and the carbon equivalent is lower thanthat of blast furnace steel.

674.3.3

Test Conditions and Test Method

Test conditions were established in order to observe the effects of the difference
in rebar material SD345, namely, blast furnace steel and electric furnace steel, on thehydrogen gas generation rates. Table 4.3-1 shows the test conditions.
The specimens used were sheet-form SD345 made from blast furnace steel andthat made from electric furnace steel with the surfaces treated by shot blasting. For thetest solution, the supersaturated calcium hydroxide solution (Ca(OH)2; 10 g/L) withchloride ion concentration adjusted to 5,000 ppm was used. The test period was set to
200 days or more to evaluate the effects on gas generation. Table 4.3-2 summarizes
the results of composition analyses of SD345 used for testing.
Now, investigation is being made on the effects of the difference in Cr, Al, Mo, Ni
contents on the hydrogen gas generation rates, which are assumed to provide anticorrosion effects. The test flow pertaining to affect the evaluation of carbon steel
grades on the gas generation rates is the same as that described in “Section 4.1 EffectEvaluation of Chloride Ion Concentration on the Gas Generation Rates.”

Table 4.3-1

Effect evaluation and test conditions of carbon steel grades
SpecimenNo.

1
2

Test solution

Material
SD345

Shape
(mm)

Surfacetreatment

Blast furnace
steel

25×120×3
×5 pieces
60×25×3×
1 pieceShotblasting

Electric
furnace steel

Table 4.3.2

Type and chloride
ion concentration
Ca(OH)2 solution,
10 g/L
Chloride ion
concentration:
5,000 ppm

pH

Test period(days)
360.2

12.5
361.0

Composition analysis results of carbon steels used for testing

No.

1

2

(Reference)

SD345 classification

Blast furnace steel

Electric furnace steel

JIS G 3112

C

0.22

0.24

0.27 or less

Si

0.35

1.10

0.55 or less

Mn

1.46

0.028

1.60 or less

P

0.012

0.034

0.040 or less

S

0.011

0.031

0.040 or less

Cu

0.01

0.31

Ni

0.02

0.10

Cr

0.04

0.29

Mo

-

0.01

Al

0.016

0.005

V

0.021

0.008

N

0.005

Chemicalcomposition(%)

684.3.4

Test Results(1)

Hydrogen Gas Generation Rate

Figure 4.3-1 illustrates changes with time of the hydrogen gas concentration
using blast furnace steel and electric furnace steel.
The figure indicates that when compared to blast furnace steel, electric furnacesteel provides slightly higher hydrogen gas concentration in the initial stages, but after
the elapse of 200 days, there is scarcely any difference between the two steels, andthe hydrogen gas concentration changes at around 50 ppb.
Figure 4.3-2 shows changes with time of the accumulated hydrogen gas
generation rate per unit area.
The figure indicates that the accumulated hydrogen gas generation rate per area
is about 300 mL/m2 after the elapse of 300 days for both blast furnace and electric
furnace steels.
Assuming that the measured hydrogen gas concentration is generated only frommagnetite generation reactions of iron (3Fe+4H2O→Fe3O4+4H2), changes with time ofthe equivalent corrosion rate of iron are shown in Fig. 4.3-3.
The figure indicates that the equivalent corrosion rate after 300 days changes ataround 0.5 µm/y for both blast furnace and electric furnace steels.
Figure 4.3-4 shows changes with time of the accumulated equivalent corrosion
thickness obtained from the equivalent corrosion rates.
The figure indicates that the equivalent corrosion thickness per unit area after theelapse of 300 days is around 0.04 µm for both blast furnace and electric furnace steels.
It was confirmed that the foregoing results provide no difference from the values
obtained when carbon steel SPHC is used (see data of “Section 4.1 Effect Evaluationof Chloride Ion Concentration on the Gas Generation Rates” and “Section 4.2 EffectEvaluation of liquid-phase pH on the Gas Generation Rates”). Based on these, it isassumed that there would be no effect of the difference in carbon steel grades on thehydrogen gas generation rates.

69

SD345/blast furnace steel

SD345/electric furnace steel

Hydrogen concentration (ppb)

250

200

150

100

50

0
0

100

200

300

400

Elapsed time (days)

Accumulated hydrogen gas generation rate per unit area(‘ml/m2)

Fig. 4.3-1

Changes with time of the hydrogen gas concentration generated (effectsof carbon steel grades)

SD345/blast furnace steel

SD345/electric furnace steel

500

400

300

200

100

0
0

100

200

300

400

Elapsed time (days)

Fig. 4.3-2

Changes with time of the accumulated hydrogen gas generation rate perunit area (effects of carbon steel grades)70SD345/electric furnace steel

SD345/blast furnace steel

Equivalent corrosion rate (µm/ｙ)

1.E+00

1.E-01

1.E-02

1.E-03
100

0

200

300

400

Elapsed time (days)

Changes with time of the equivalent corrosion rate (effects of carbon
steel grades)
Accumulated equivalent corrosion thickness(µm)

Fig. 4.3-3

SD345/blast furnace steel

SD345/electric furnace steel

1.E-01

1.E-02

1.E-03

1.E-040

Fig. 4.3-4

100

200
Elapsed time (days)

300

400

Changes with time of the accumulated equivalent corrosion thickness(effects of carbon steel grades)

71

(2)

Appearance and Surface Changes of Specimen and Weight Changes

Figure 4.3-5 shows the appearance changes of specimens (carbon steel) before
the start of testing and after the end of testing using the photos of specimens of blast
furnace steel. In addition, Fig. 4.3-6 presents photos of specimens of electric furnacesteel.
Figure 4.3-5 and Fig. 4.3-6 show that the surfaces of specimens of both blast
furnace steel and electric furnace steel after descaling (rust removal) did not provide
initial metal luster but were both free of localized corrosion.
Figure 4.3-7 shows the composition distribution in the depth direction when theblast furnace steel specimen surfaces were analyzed by XPS. In addition, broad
photoelectron spectra after 0.3 µm sputtering are shown in Fig. 4.3-8.
Figure 4.3-7 indicates that as compositions contained from the surface to thedepth direction, calcium (Ca), silica (Si), sodium (Na), and iron(Fe) are distributed inthat order. From Fig. 4.3-8, Na, Ca, and Si were observed in addition to Fe even at 0.3µm. Si is assumed to elute from the glass of the test vessel.
Figure 4.3-9 shows the composition distribution with respect to the depth
direction when electric furnace steel specimen surfaces were analyzed by XPS. In
addition, Fig. 4.3-10 shows broad photoelectron spectra after 0.3 µm sputtering.
Figure 4.3-9 indicates that as the composition contained from the surface to thedepth direction, calcium (Ca), silica (Si), sodium (Na), and iron (Fe) are distributed inthat order. Furthermore, from Fig. 4.3-10, Na, Ca and Si are observed in addition to Feeven at 0.3 µm.
The foregoing suggests that there would be no effect of carbon steel grades,
which is also confirmed by the surface analysis results.
The corrosion thickness was calculated from weight changes in specimens
before and after testing. Table 4.3-3 compares the corrosion thickness (corrosion
weight loss) obtained from the weight changes of specimens before and after thetesting with the equivalent corrosion thickness obtained from the accumulatedhydrogen gas generation rates.
From the figure, the results of comparison between the corrosion thickness andthe equivalent corrosion thickness of blast furnace steel and electric furnace steel
indicate a difference of about 0.4 µm for blast furnace steel and that of about 0.2 µm forelectric furnace steel. This cannot be attributable to the difference in material because
the difference of about 0.2 to 0.4 µm was also obtained in the tests conductedpreviously.

72

Before test

Immediately after removal

After descaling

Fig. 4.3-5

Appearance observation results of blast furnace steel specimens

73

Before test

Immediately after removal

After descaling

Fig. 4.3-6

Appearance observation results of electric furnace steel specimens

74Fig. 4.3-7

Composition distribution in the depth direction of blast furnace steel
specimens

Fig. 4.3-8

Surface film analysis results of blast furnace steel specimens (broad
photoelectron spectrum after 0.3 µm sputtering)75

Fig. 4.3-9

Composition distribution in the depth direction of electric furnace steel
specimens

Fig. 4.3-10

Surface film analysis results of electric furnace steel specimens (broad
photoelectron spectrum after 0.3 µm sputtering)76

Table 4.3-3

Comparison of corrosion thickness obtained from corrosion weight loss
and equivalent corrosion thickness
Corrosion thickness

Equivalent corrosion thickness

No.

Steel grade

Test
period(days)

1

SD345 blast
furnace steel

360.2

0.420

0.426

0.03950.040

0.38052SD345electric
furnace steel

361.0

0.271

0.274

0.04350.044

0.2275(4)[1]Testperiodcorrosion
thickness(µm)

Yearlycorrosion
thicknessTw(µm/y)[2] Test period
equivalentcorrosion
thickness(µm)

Yearlyequivalentcorrosion
thickness tg(µm/y)[1]-[2](µm)

Composition Change of Test Solution
Table 4.3-4 shows the results of analyzing the composition of the test solution.

Based on the table, there is no difference in the liquid-phase ingredientcomposition of blast furnace steel and electric furnace steel before and after testing.

Table 4.3-4

Effects assessment of carbon steel grades and changes in solution
composition.

No.

Steel grade

1

SD345 blast
furnace steel

2

SD345 electric
furnace steel

Measurement timepH

Fe(total)mg

Before testing

12.4

-

After completion of
testing

12.3

11Before testing

12.4

-

After completion of
testing

12.2

6.7

Note 1)

pH is a measurement value at 35°C.

Note 2)Fe (total) denotes the iron amount (mg) in the total solution.

4.3.5

Discussion

To clarify the effects of the difference in carbon steel grades on the hydrogen gas
generation rate, using two types of specimens, testing was conducted for about 360
days.
The findings obtained by testing with respect to the effects of carbon steel gradeson the hydrogen gas generation rates can be summarized as follows:•

There is scarcely any effect of the difference in SD345 blast furnace steel/electric
furnace steel on the hydrogen gas generation rates. Based on this, it is assumed
that there is scarcely any effect of the difference in ingredients of blast furnace steel
and electric furnace steel on the hydrogen gas generation rates.
77

4.4

Effect Assessment of Cementitious Covering Material on the Gas
Generation Rates

4.4.1

Objectives

The corrosion behavior of carbon steel (waste containers, rebars in concrete
structures, etc.) covered over with cementitious material in depositories differs from thecorrosion behavior of carbon steels directly immersed in the solution, which has been
implemented before. That is, the carbon steel covered over with cementitious material
comes in contact with pore water from the cementitious material in the short term, but it
is assumed to come in contact with seepage groundwater entering cementitious
material over the long term. In addition, hydrogen gas generated in the cement system
accumulates in the cementitious material and discharge suppression effects of thecementitious material can be envisioned.
In this part of the section, with a view to the differences of these corrosion
environments, immersion corrosion tests are conducted on specimens prepared bycovering carbon steel with mortar (3 specimens; hereinafter called the “mortar covered
specimens”), and the behavior of hydrogen gas discharged outside the cementitious
material is examined.

4.4.2

Test Conditions and Test Method
Table 4.4-1 shows the test conditions.

The specimens used were blank test mortar specimens (containing glass flat
plate) and mortar-covered specimens (containing SPHC flat plate). For the test
solution, the supersaturated calcium hydroxide solution (Ca(OH)2; 10 g/L) with chloride
ion concentration adjusted to 5,000 ppm was used. The test period was set to 400
days or more to evaluate the effects on the gas generation.
Table 4.4-2 summarizes the detailed specifications of the specimens.
The glass flat plate was mounted in place of carbon steel as a plank test. For
carbon steel, SPHC flat plates (50 mm × 100 mm × 3 mm × 6 pieces) were used. The
total surface area of the flat plates used was determined as a size enabling themeasurement of hydrogen gas concentration even at the equivalent corrosion rate of
0.001 µm/y.
For the cement, regular Portland cement was used and quadratic prisms
measuring 70 mm each in width and in length and 120 mm in height were fabricated.
Figure 4.4-1 shows the fabricated shape of the specimen. In addition, a crosssectional photograph of the specimen preliminarily fabricated is shown in Fig. 4.4-2.
Figure 4.4-2 confirms that the test specimen, which was nearly equivalent to thatscheduled to be fabricated, was fabricated. In addition, Fig. 4.4-3 shows the fabrication
condition.
Figure 4.4-3 shows the test flow.

78

The mortar covered specimens were fabricated and cured in water for one week,
and were mounted on the test vessel, which contained the test solution. However, thetest solution was not aerated. Inside the test container, high-purity argon (Ar) gas waspurged and the low oxygen atmosphere was maintained. The hydrogen gas generated
as a result of the corrosion of carbon steel was automatically measured by gas
analyzer (APIMS).
Based on this test result, the effects of cementitious covering material on the gas
generation rates are evaluated.

Table 4.4-1

Effect evaluation of cementitious covering material and test conditions

Specimen
nameShape (mm)Mortarspecimen

70

×120H

70

×120H

Mortarcoveredspecimen 1
Mortarcoveredspecimen 2
Mortarcoveredspecimen 3

70

×120H

70

×120H

Test solution

Loaded test
piece (mm)Type and chloride
ion concentration

Glass:
50×100×3
×6 pcs.

400 ormore
Ca(OH)2 solution:
10 g/L
Chloride ion
concentration:
5,000 ppm

Carbon steel:
50×100×3
×6 pcs.

Table 4.4-2

Mortar12.5

400 ormore
400 ormore
400 ormore

Specimen specifications

Item
Embeddedmaterial

pH

Test period(days)

Test conditions

Glass

Glass flat plate (50x100x3: 6 pcs.)

Carbon steel

SPHC flat plate (50x100x3: 6 pcs.)

CementRegular Portland cement (472 kg/m3)

Water/cement ratio W/C(%)

65

Sand/cement ratio S/C

2.85 (mass ratio)

Fine aggregate

Pit sand from Shizuoka Prefecture (1345 kg/m3)

Coarse aggregate

None

Water

Tap water (307 kg/m3)

Shape (mm)70Placement atmosphere

Atmosphere

Curing method

Water curing (20°C)

Curing period

Wet dressing for 1 day + water curing for 6 days

Compression strength of 7-dayold mortar

21.3 (N/mm )

×120H

2

79

Glass or carbon steel

70mm

70mm

70mm

70mm
120mm

50mm
10mm

Full view

10mm

Cross-sectional viewFig. 4.4-1

Shape and dimensions of the specimen

From left edge

From right edge

10 mm
10 mm
10 mm
10

9 mm (From top surface)

8 mm (From top surface)

10 mm
10 mm

5 mm

3 mm

mm

10
mm
10 mm

6 mm
7 mm

mm
mm

12 mm
11.5 mm

10 mm
10 mm
10 mm

《Allowance》
10.5
10.5

10 mm

11 mm
11 mm

9 mm

6.5 mm

6 mm
6 mm

7 mm

8 mm

10 mm

10.5 mm
11 mm

10 mm

11 mm (From bottom surface)

9 mm (From bottom surface)mm

Fig. 4.4-2

Cross-sectional view of the mortar covered specimen fabricated in the
preliminary test80

Kneading scene

Scene of casing the first layer

Scene of charging iron plate

Scene after completion ofplacement

Fig. 4.4-3

Preparation condition of the mortar covered specimen81Procurement of carbon steel
test material

Determination of mortar
specimen shape

Machining of carbon steel

Procurement of cement andfine aggregate

Shot-blasting of carbon steel

Fabrication of mold

Visual inspection and weightmeasurement of specimen

Blending (W/C, etc.)
Preliminary fabrication test

・ Vacuum treatment of specimen
・ Dispatching of specimen(1 piece/pack)Fabrication of mortar covered specimen(atmospheric fabrication: 4 specimens)
Curing of mortar covered specimen (1 week)
Fabrication of calcium
equilibrium water

Dispatch of mortar covered specimen(wet-dressing curing)

Adjustment of chloride
ion concentration

Obtaining mortar covered specimen

Fabrication of stand for test piece

Analysis of test solution

Assembly of specimen (Work in atmosphere, 4 specimens, solution
Mounting of specimen on gas generation rate evaluation facilities
Measurement of hydrogen gas concentration (gas generation rate

Fig. 4.4-4

Test flow pertaining to the effect evaluation of cementitious covering
material82

4.4.3

Test Results

In this part of the section, measurement results of the hydrogen gas generation
rate are described.
Figure 4.4-5 shows changes with time of the hydrogen gas concentration withmortar covered specimens.
The figure shows that the hydrogen gas concentration is below the backgroundfor all specimens until 100 days elapse. After the elapse of 400 days, slight differences
are found between mortar covered specimens, but it is clear that 10-5 ppb lowconcentration hydrogen gas is generated. In addition, mortar specimens of the blanktest generate a small amount of hydrogen gas after 400 days.
Figure 4.4-6 shows changes with time of the accumulated hydrogen gas
generation rate per specimen.
The figure indicates that though there are slight differences for each mortar
covered specimen, the accumulated hydrogen gas generation rate is 3.5 mL or lower
even after the elapse of 400 days.
Assuming that the measured hydrogen gas concentration is generated only frommagnetite generation reactions (3Fe+4H2O→Fe3O4+4H2), Fig. 4.4-7 shows changes
with time of the equivalent corrosion rate of iron.
The figure shows that the equivalent corrosion rate after the elapse of 400 days
changes at around 0.02 µm/y for all mortar covered specimens.
Figure 4.4-8 shows changes with time of the accumulated equivalent corrosion
thickness obtained from the equivalent corrosion rate. In the figure, the test results ofcarbon steel alone are given, too.
The figure indicates that the equivalent corrosion thickness is smaller than that of
the carbon steel alone up to 100 days. In addition, even after 400 days, the equivalentcorrosion thickness is about 0.01 µm.

8310Mortar specimen

9Mortar covered specimen 1

Hydrogen concentration (ppb)8

Mortar covered specimen 2

7

Mortar covered specimen 3

6

Background

5
4
3
2
1
0
0

Fig. 4.4-5

100

200

300

Elapsed time (days)

400

500

Change with time of the gas concentration of hydrogen generated(effects of mortar covered specimen)

Accumulated gas generation rate(ml)4.0Mortarspecimen 累積水素ガス発生量の経時変化
図 4.4-6 試
験体当たりのﾓﾙﾀﾙ試験体（ｶﾞﾗｽ）（モルタル被覆試験体の影響）
Mortarcovered specimen 1
ﾓﾙﾀﾙ試験体-1（炭素鋼）

3.5
3.0

ﾓﾙﾀﾙ試験体-2（炭素鋼）
Mortarcovered specimen 2
ﾓﾙﾀﾙ試験体-3（炭素鋼）

2.5

Mortar covered specimen 3

2.0
1.5
1.0
0.5
0.0
0

Fig. 4.4-6

10
0

200

300

Elapsed time(days)

40
0500

Change with time of the accumulated hydrogen gas generation rate per
specimen (effects of mortar covered specimen)
84

Equivalent corrosion rate (µm/y)

1.E-01
Mortar specimen

Mortar covered specimen 1

Mortar covered specimen 2

Mortar covered specimen 3

1.E-02

図 4.4-7 等価腐食速度の経時変化（モルタル被覆試験体の影響）
1.E-03
0

100

200

300

400

500

Elapsed time (days)

Accumulated equivalent corrosion thickness (µm)

Fig. 4.4-7

Change with time of the equivalent corrosion rate (effects of mortar
covered specimen)

1.0E-01

1.0E-02

1.0E-03

Mortar specimen
Mortar covered specimen 1
Mortar covered specimen 2
Mortar covered specimen 3
Carbon steel alone

1.0E-04

1.0E-05
0

100

200

300

400

500

Elapsed time (days)

Fig. 4.4-8

Change with time of the accumulated hydrogen gas generation rate per
specimen (effects of mortar covered specimen)85

4.4.4

Discussion

When four mortar covered specimens are immersed in the saturated Ca(OH)2
solution, the release of hydrogen gas to the outside of the cementitious material was
hardly observed for 100 days after immersion. Thereafter, around the 100th day after
immersion, release of hydrogen gas from mortar covered specimen 3, release ofhydrogen gas from mortar covered specimen 2 on around 200th day after immersion
and release of hydrogen gas from mortar covered specimen 1 on around 350th day
after immersion. In addition, the release of hydrogen gas was detected from the mortar
specimen with no steel material from around the 450th day.
The reasons why the release of hydrogen gas from the mortar covered specimen
is smaller and there is delay from the carbon steel alone are explained by the following(see Fig. 4.4-9).
[1]

Atmospheric corrosion of specimen: While the specimen is being
prepared in the atmosphere and while the specimen is being watercured (for 6 days), passive film is formed on the carbon steel surface
and suppresses the hydrogen gas generation.

[2]

Delay of water penetration: It takes time for the test solution to reachthe carbon steel specimen and the release of hydrogen gas isdelayed.

[3]

Delay of penetration of hydrogen gas in air: It takes time for thegenerated hydrogen gas to move in the mortar (penetrate into air).

[4]

Adsorption of hydrogen gas: The generated hydrogen gas isadsorbed by the cementitious material and the release of hydrogengas is suppressed.

Mortar[2]

Carbon
steel

[1]
[3]
[4]

Test solution

Fig. 4.4-9

Hydrogen migration behavior in the mortar covered specimen

86With respect to Reason [1], it is common for film similar to gaseous phase oxide
film or passive film to be formed, and hydrogen gas generation is possibly suppressedby this film.
With respect to Reason [2], it is the case in which it is assumed that carbon steel
does not react with free water in mortar but reacts with the test solution (saturated
calcium solution) only in which the carbon steel is immersed. It is difficult to judge
because there are no sufficient data on water permeability in the present composition.
However, when the diffusion coefficient is assumed to be 1.40x10-9 m2/s (effective
diffusion coefficient of concrete structures of “JNC TY1400 2000-002, Progress Reporton Disposal Concept for TRU Waste in Japan, March 2000”) 3), the water diffusion
distance for 100 days is about 100 mm; therefore, it is assum ed that the test solution
has sufficiently reached carbon steel of the mortar covered specimen.
Consequently, there is little possibility of cause [2].
With respect to Reason [3], since hydrogen gas provides high permeability andcementitious material provides low physical adsorption functions, there would be little
delaying effect on hydrogen gas release.
With respect to Reason [4], the hydrogen gas permeability of mortar is low andthis might be one of the causes of the delayed hydrogen gas release. However, since
the value of the hydrogen gas permeability coefficient greatly varies from measurement
to measurement, satisfactory evaluation cannot be made.
For the reason why hydrogen gas was generated from the mortar specimen of
the blank test after 450 days, it is reported that traces of metallic iron are contained in
the cementitious material 1). It is assumed that hydrogen gas was generated by thismetallic iron.
With the foregoing discussion, a significant amount of hydrogen gas was
detected with mortar covered specimens after several hundred days had elapsed. It
could be assumed that gas permeability of the passive film formed at the time of
preparing the specimens and mortar would be one of the causes.
In addition, it was confirmed that when specimens were covered withcementitious material, an even smaller gas generation rate (equivalent corrosion rate:
about 0.02 µm/y) was obtained than for carbon steel alone. However, it could beassumed that if the test period was further extended, the gas generation rate would
become similar to that of carbon steel alone.

87

4.5

Evaluation of Difference between Corrosion Weight Loss andEquivalent Corrosion Rate

4.5.1

Objectives

In the tests implemented, 10-fold differences occur between the corrosion weight
reduction and the equivalent corrosion rate. This may be attributed to short-period
oxygen consumption type corrosion, etc. by residual oxygen during descaling or in theinitial stages of immersion because of the small corrosion thickness, which is 1 µm orless.
In this part of the section, carbon steel is immersed in a solution for a short period,
and this corrosion weight loss is measured to contribute to the evaluation of thedifference between the corrosion weight loss and equivalent corrosion rate.

4.5.2

Test Conditions and Test Method

In corrosion tests conducted under low-oxygen atmosphere, inconsistency isfound in the corrosion rate and equivalent corrosion rate computed from the corrosion
weight loss. Figure 4.5-1 summarizes the corrosion thickness computed from theverification test on advanced radioactive waste disposal systems (1997 to 2001). The
figure indicates that the corrosion weight loss occurs in a large quantity in the initialstages of immersion and scarcely increases even when the test period is increased.
Based on this, it is assumed that oxygen consumption type corrosion possibly occurs
due to air remaining in the test system (in the present test, piping, etc. of the ESHG) in
the initial stages of immersion or contact with the atmosphere after specimens aretaken out of the test vessels. In addition, loss, etc. of weight count due to dropout
during descaling of the polishing agent (alumina) remaining on the specimen surface atthe time of shot blasting could be assumed, but exact causes cannot be identified.
To confirm these possibilities, short-time tests were conducted and by measuring
the corrosion weight loss, the corrosion rate in the initial stages of immersion wasmeasured. For the test periods, one day and five days were selected in order toobserve the condition immediately after immersion.
Furthermore, because in the present evaluation facilities, deaeration water is
supplied to test vessels periodically (about once every month), air (oxygen) may havemixed in the test vessels in such an event and promoted the corrosion weight loss.
Consequently, specimens were taken out before and after deaeration water was
supplied to the test vessels (test period: 33 days and 35 days), the corrosion weightloss was measured, and the effects were evaluated.
Table 4.5-1 shows the test conditions.
The specimens used were sheet-form SPHCs with their surfaces treated by shot
blasting. For the test solution, the supersaturated calcium hydroxide solution (Ca(OH)2;
10 g/L) with chloride ion concentration adjusted to 5,000 ppm was used. The test
period was set to 1, 5, 33, and 35 days to evaluate the difference between the
corrosion weight loss and equivalent corrosion rate.
88

The test flow pertaining to effect evaluation of the difference between the
corrosion weight loss and equivalent corrosion rate is the same as that described in“Section 4.1 Effect Evaluation of Chloride Ion Concentration on the Gas Generation
Rates.”

Flat plate, batch sampling

Round bar, with millscale

Round bar, withno mill scale

Flat plate, gas generation rateevaluation facilities

Corrosion thickness (µm）

5.0

4.0

3.0

2.0

1.0

0.0
100

0

200

300

400

500

600

700

800

Measurement period (days)

Fig. 4.5-1

Measurement period and corrosion weight loss by various tests(Note: Round bar with mill scale is a specimen with adhering scale before the test begins.)

Table 4.5-1

Evaluation and test conditions of difference between corrosion weightloss and equivalent corrosion rate
Test conditions
Test material

No.
Material

Shape
(mm)

Test solution
Surfacetreatment

Type and chloride
ion concentration

pH

1
2
3

1

Carbonsteel
SPHC

80×120×3
×5 pcs.
+60×30×3
×1 pc.

Shot
blasting

Ca(OH)2 solution:
10 g/L
Chloride ion
concentration:
5,000 ppm

5
12.5
33
35

4
Note 1)

Test
period(days)

Water is supplied to the test vessels on the 34th day.

89

4.5.3

Test Results(1)

Hydrogen Gas Generation Rate
Figure 4.5-2 shows changes with time of the hydrogen gas concentration.

The figure indicates that there is no difference in the hydrogen gas concentration
generated during periods from 5 days to 35 days and it changes at around 130 ppb.
However, in one-day immersion, the generation rate was too small to measure.
Figure 4.5-3 shows changes with time of the accumulated hydrogen gas
generation rates per unit area.
The figure indicates that the accumulated hydrogen gas generation rates per unit
area linearly increase as the immersion period increases.
Assuming that the measured hydrogen gas concentration is generated only frommagnetite generation reactions (3Fe+4H2O→Fe3O4+4H2), Fig. 4.5-4 shows changes
with time of the equivalent corrosion rate of iron.
The figure confirms that the equivalent corrosion rate from 5 days to 35 days
changes at about 0.1 µm/y.
Figure 4.5-5 shows changes with time of the accumulated equivalent corrosion
thickness obtained from the equivalent corrosion rate.
The figure confirms that the accumulated equivalent corrosion thickness
increases as the immersion period increases. Based on this, it was assumed that there
is a difference between corrosion thicknesses, as the corrosion thickness in 5 days of
immersion is about 0.002 µm but it becomes about 0.01 µm in 35 days.

90

200

Hydrogen concentration (ppb)

1-day immersion

5-day immersion

33-day immersion

35-day immersion

150

100

50

0
0

10

20

30

40

Elapsed time (days)

Fig. 4.5-2

Changes with time of the generated hydrogen gas concentration

Accumulated hydrogen gas generation rate per unit area (ml/m2)

1-day immersion

Fig. 4.5-3

5-day immersion

33-day immersion

35-day immersion

100

80

60

40

20

0
0

10

20

30

40

Elapsed time (days)

Changes with time of the accumulated generation rate of hydrogen gas
per unit area91

1-day immersion

5-day immersion

33-day immersion

35-day immersion

Equivalent corrosion rate (µｍ/ｙ)

1.E+00

1.E-01

1.E-02

1.E-03
0

10

20

30

40

Elapsed time (days)

Fig. 4.5-4

Changes with time of the equivalent corrosion rate

Accumulated equivalent corrosion thickness (µm)

1-day immersion

5-day immersion

33-day immersion

35-day immersion

1.0E+01.0E-01

1.0E-02

1.0E-03
0

10

20

30

40

Elapsed time (days)

Fig. 4.5-5

Changes with time of the accumulated equivalent corrosion thickness

92

(2)

Appearance and Surface Changes of Specimen and Weight Changes

Figure 4.5-6 shows the appearance changes in specimens (carbon steel) before
the start of testing and after the end of testing using the photos of specimens immersed
for 5 days. In addition, Figure 4.5-7 presents photos of specimens immersed for 35
days.
From Figure 4.5-6 and Figure 4.5-7, the specimen surface after descaling (rustremoval) of both specimens immersed for 5 days and 35 days lost the initial metal
luster but no localized corrosion was observed, and there was no difference.
Figure 4.5-8 shows the composition distribution in the depth direction as a resultof XPS analysis of the surface of the specimen immersed for 5 days. Figure 4.5-9 is abroad photoelectron spectrum after 0.3 µm sputtering.
Figure 4.5-8 shows that a higher composition ratio of oxygen (O) than iron (Fe)was observed up to 5 minutes of sputtering, but thereafter, the composition ratio of Fe
became higher. In addition, based on Figure 4.5-9, the Fe peak could also be clearly
identified at 0.3 µm.
Figure 4.5-10 shows the composition distribution in the depth direction as a resultof XPS analysis of the surface of the specimen immersed for 35 days. In addition,
Figure 4.5-11 shows a broad photoelectron spectrum after 0.3 µm sputtering.
Figure 4.5-10 shows that the oxygen concentration ratio was higher than Fe up to
25 minutes of sputtering. It is assumed that oxygen penetrated deeper than thespecimen immersed for 5 days. In addition, at the 0.3 µm of Figure 4.5-11, a similar
peak to that of the specimen immersed for 5 days could be confirmed.

93

Before test

Immediately after removal

After descaling

Fig. 4.5-6

Visual observation results of specimens immersed for 5 days.

94

Before test

Immediately after removal

After descaling

Fig. 4.5-7

Visual observation results of specimens immersed for 35 days.

95Fig. 4.5-8

Composition distribution to the depth direction of the specimen
immersed for 5 days.

Fig. 4.5-9

Surface film analysis results of specimens immersed for 5 days (broad
photoelectron spectrum after 0.3 µm sputtering)96

Fig. 4.5-10

Composition distribution to the depth direction of the specimen
immersed for 35 days.

Fig. 4.5-11

Surface film analysis results of specimens immersed for 35 days (broad
photoelectron spectrum after 0.3 µm sputtering)
97

(3)

Corrosion Weight Loss

Table 4.5-2 shows the results of corrosion thickness computed from weightchanges of specimens before and after testing. Incidentally, the corrosion thickness
was computed on the assumption that corrosion takes place uniformly.
The table confirms that the corrosion thickness computed from the corrosion
weight loss was 0.18 to 0.19 µm for 1-day-immersed specimens, 5-day-immersed
specimens, and 35-day-immersed specimens. In addition, the corrosion thickness of
33-day-immersed specimens was slightly small but 0.14 µm.
Even in one-day immersion in which corrosion was assumed not to take place,the corrosion thickness of about 0.18 µm was computed from the corrosion weight loss.
This may be attributed to effects that do not arise from the corrosion phenomena.

Table 4.5-2
No.

Test
period(days)

1
2
3
4

1
5
33
35

4.5.4

Corrosion thickness computed from corrosion weight loss
Weightevaluatedtotal surfacearea（m2）
0.103
0.103
0.103
0.103

Total weight
beforetesting (g)Total weightafter descaling(g)

Corrosionweight(g)

Corrosion
thickness(µm)

1288.6873
1288.9205
1287.9239
1287.7199

1288.5330
1288.7681
1287.8049
1287.5596

0.154
0.152
0.119
0.160

0.183
0.181
0.141
0.191

Discussion

To clarify the difference between the corrosion weight loss and equivalentcorrosion rate, four pieces of the same specimens were fabricated and tests withimmersion periods from 1 day to 35 days were conducted. As a result, even during theperiod when hydrogen gas was scarcely generated (1 to 5 days), the corrosion
thickness obtained from the corrosion weight loss was about 0.2 µm. Examples ofcauses of corrosion equivalent to this 0.2 µm possibly include the following:•

Corrosion due to residual oxygen in the solution when specimens were
mounted;•Corrosion due to oxygen remaining in the test system during operation;•

Corrosion due to contact of the specimens with the atmosphere when
the specimens were removed (specimens were in the wet condition);and•Detachment from specimens at the time of descaling (cathodeelectrolyzation).

When the data of immersion periods for 5 days and 35 days (corrosion thicknessof 5-day immersion period is 0.18 µm and that of 35-day immersion period is 0.19 µm)
are compared, the corrosion weight loss thickness during the immersion period from 5days to 35 days is 0.01 µm. On the other hand, the equivalent corrosion thickness dueto the hydrogen gas generation during the immersion period from 5 days to 35 days is98about 0.01 µm (equivalent corrosion thickness for the immersion period of 5 days is0.002 µm and that for the immersion period of 35 days is 0.013 µm). That is, in theimmersion period from 5 days to 35 days, the corrosion thickness obtained fromcorrosion weight loss well coincides with the equivalent corrosion thickness obtained
from the hydrogen gas generation rate.
Based on this, the cause of corrosion in the initial stages of immersion has not
yet successfully been identified, but it was assumed that the hydrogen gas generation
rate is not seriously subject to the calcium hydroxide adhering condition to thespecimen surface, and the corrosion weight loss is greatly dependent on the surface
condition of the specimen. Furthermore, if the surface condition is the same and thelow oxygen condition can be maintained, the corrosion thickness and the equivalentcorrosion thickness obtained from the corrosion weight loss satisfactorily coincide. In
addition, it is assumed that effects at the time of water supply would be negligibly small
as compared to the initial corrosion weight loss.
Based on the results of the present investigation, in the calcium equivalent water(chloride ion concentration: 5,000 ppm; pH: 12.5), assuming that specimens corrode by
about 0.18 µm in the initial stages of immersion, in the testing with specimens
immersed for 537 days under the same test conditions, corrosion thickness as shownin Table 4.5-3 is obtained, indicating good coincidence between the corrosion thickness
obtained from the corrosion weight loss and the equivalent corrosion thickness
obtained from the hydrogen gas generation rate.

Table 4.5-3Comparison between corrosion thickness and equivalent corrosion
thickness obtained from corrosion weight loss
Corrosion thickness (µm)

Chloride ion
concentration

Test
period(days)

5,000

537.8

Test period
corrosion
thickness

Corrosion
thickness from
5 to 537.8
(tw)

0.249

0.069

Equivalentcorrosion
thickness from
5 to 537.8 days
(tg)
(µm)

tw/tg

0.080

0.86

Note 1)

Corrosion for 5-day immersion was evaluated as 0.18 µm.

Note 2)Equivalent corrosion thickness for 5 days was 0.001 µm and that for 537.8 days was 0.081 µm.

99

4.6

Evaluation of Long-Term Behavior of Gas Generated from Carbon
Steel

4.6.1

Objectives

In this part of the section, carbon steel planned to be used in the greatest quantity
as metal is studied, and the hydrogen gas generation behavior by the reactions
between this carbon steel and the solution is experimentally evaluated over as long a
period as possible.

4.6.2

Test Conditions and Test Method

Using carbon steel SPHC and SPCC with different processing methods, longterm behavior was evaluated with respect to the hydrogen gas generation rate fromcarbon steel. Table 4.6-1 shows the test conditions.
The specimens used were sheet-form SPHC (Steel Plate-Hot Commercial) and
SPCC (Steel Plate-Cold Commercial) with their surfaces treated by shot blasting. For
the test solution, the supersaturated calcium hydroxide solution (Ca(OH)2; 10 g/L) withchloride ion concentration adjusted to 5,000 ppm was used. The test period was set tobe as long as possible and in the present study, the test was conducted for 892 days at
the maximum.
The test flow pertaining to the difference between corrosion weight loss and theequivalent corrosion rate is the same as that of “Section 4.1 Effect Assessment of
Chloride Ion Concentration on the Gas Generation Rates” mentioned above.

Table 4.6-1

Evaluation of Long-term gas generation behavior and test conditions
Specimen

No.
Material
1

SPCC

2

SPCC

3

SPHC

4

SPHC

5

SPHC

Shape
(mm)

Test solution
Surfacetreatment

Type andchloride ion
concentration

pH

Test period(days)

613.0
80×120×3
×5 pcs.
+60×30×3
×1 pc.

Ca(OH)2
Shot
blasting

solution: 10 g/L
Chloride ion
concentration:
5,000 ppm

718.0
12.5

719.0
892.1
892.1

100

4.6.3

Test Results(1)

Hydrogen Gas Generation Rate
Figure 4.6-1 illustrates changes with time of the hydrogen gas concentration.

From the figure, it was confirmed that there was no difference in hydrogen gas
generation behavior due to the difference in the processing method of carbon steel.
When about 50 days had elapsed, the hydrogen gas concentration rose up to
about 200 ppb at the maximum but thereafter up until the elapse of100 days, thehydrogen gas concentration rapidly decreased and thereafter, it changed gently. In
addition, it was confirmed that the hydrogen concentration was about 20 ppb after theelapse of 892 days.
Figure 4.6-2 illustrates changes with time of the accumulated hydrogen gas
generation rate per unit area.
From the figure, it was confirmed that the accumulated hydrogen gas generation
rate per unit area increased as the immersion period increased. It could be assumed
that the accumulated hydrogen gas generation rate is 600 mL/m2 or lower after about
the elapse of 900 days, though there are slight variations in accordance with thespecimens.
Assuming that the measured hydrogen gas concentration is generated only frommagnetite generation reactions (3Fe+4H2O→Fe3O4+4H2), Fig. 4.6-3 shows changes
with time of the equivalent corrosion rate of iron.
From the figure, it was confirmed that the equivalent corrosion rate gently lowers
as the immersion period increases. In addition, the equivalent corrosion rate after theelapse of 892 days is about 0.02 µm/y.
Figure 4.6-4 shows changes with time of the accumulated equivalent corrosion
thickness obtained from the equivalent corrosion rate.
From the figure, it was confirmed that the equivalent corrosion thickness per unit
area increases as the immersion period increases. However, it can be assumed thateven when 900 days elapse, the equivalent corrosion rate is about 0.1 µm.
Table 4.6-2 summarizes the data of the mean equivalent corrosion rate for every
100 days after immersion. Figure 4.6.5 is the bar graph of the table. The figure showsthat the equivalent corrosion rate decreases with time.
Based on these findings, it was confirmed that the equivalent corrosion ratemonotonously decreases in time intervals of 100 days, though there are increases anddecreases in the time intervals of about 10 days.

101

200

Hydrogen concentration (ppb)180

計測番号 1

計測番号 4

計測番号 3

計測番号 21

■No.1

160

▲No.2

◆No.4

●No.3

●No.5

計測番号 22

140
120
100
80
60
40
20
0
0

100

200

300

400

500

600

700

800

900

1000

Elapsed time (days)

Note) Measurement No. 3, 4, 5: SPHC; Measurement No. 1, 2: SPCC

Long-term test evaluation: Changes with time of the generated hydrogengas concentration
Accumulated hydrogen gas generation rate per unit area(ml/m2)

Fig. 4.6-1

600

500

400

300

200

▲No.2
計測番号 1 ■No.1
計測番号
4
◆No.4

100

●No.3
計測番号 3●No.5

計測番号 21

計測番号 22

400

600

0
0

100

200

300

500

700

800

900

1000

Elapsed time (days)

Note) Measurement No. 3, 4, 5: SPHC; Measurement No. 1, 2: SPCC

Fig. 4.6-2

Long-term test evaluation: Changes with time of the accumulatedhydrogen gas generation rate per unit area102

Equivalent corrosion rate (µm/y)

1.E+00
SPHC(計測番号1）■No.1

SPCC（計測番号▲No.2●No.3 4）SPCC(計測番号 3）
SPHC(計測番号 21）◆No.4 ●No.5
SPHC(計測番号 22）

1.E-01

1.E-02

0

100

200

300

400

500

600

700

800

900

1000

Elapsed time (days)

Note) Measurement No. 3, 4, 5: SPHC; Measurement No. 1, 2: SPCC

Accumulated equivalent corrosion thickness(µm)

Fig. 4.6-3

Long-term test evaluation: Changes with time of the equivalent corrosion
rate

1.E+00

1.E-01

計測番号 1
計測番号 4
計測番号 3■No.1 ▲No.2 ●No.3
計測番号 21
計測番号 22◆No.4 ●No.5

1.E-02

1.E-03
0

100

200

300

400

500

600

700

800

900

1000

Elapsed time (days)

Note) Measurement No. 3, 4, 5: SPHC; Measurement No. 1, 2: SPCC

Fig. 4.6-4Long-term test evaluation: Changes with time of the accumulated
equivalent corrosion thickness
103

Table 4.6-2

Equivalent corrosion rate by periods
Mean equivalent corrosion rate (µm/y)

No.

200 –
300 day300 –400 day

400 –500 day500 –600 day600 –700 day700 –800 day800 –900 day1

0.07900.05760.04990.05280.04730.0301−−

2

0.05730.05250.04540.03830.03130.02490.0164−3

0.08650.04900.03720.03370.03350.02900.0204

40.04950.0456

0.03620.02830.02550.02400.02340.021150.05530.05760.05090.03650.02880.02480.02230.0199Meanvalue

0.06550.05250.04390.03790.03330.02650.02060.0205

Mean equivalent corrosion rate (µm/y)

100 –200 day

0.07
0.06
0.05
0.04
0.03
0.02
0.01
0

100–200day 200–300day 300–400day 400–500day 500–600day 600–700day 700–800day 800–900day

Immersion period
図 4.6-5 期間別の平均等価腐食速度

Fig. 4.6-5

Mean equivalent corrosion rate by periods(For example, 400 to 500 days are the mean values of the equivalent corrosion rate on
the 400th day to 500th day after immersion.)

104

(2)

Appearance and Surface Changes of Specimen and Weight Changes

Figure 4.6-6 shows the appearance changes in specimens (carbon steel) before
the start of testing and after the end of testing using the photos of SPHC specimens
immersed for 892 days.
In the figure, a large spot appears on the specimen sides, which is assumed toresult from the adhesion of calcium hydroxide powders. Though the cause of this has
not yet been identified, no localized corrosion occurs at this spot position, and it isassumed that it causes no problem in evaluating the gas generation.
Figure 4.6-7 shows the composition distribution in the depth direction as a resultof XPS analysis of the specimen surface of the SPHC specimens immersed for 892
days. Figure 4.6-8 shows electron spectral changes for every depth direction in theenergy range related to iron (Fe).
Figure 4.6-7 shows that the oxygen (O) composition ratio is higher than iron (Fe)in the sputtering range up to 200 nm. In addition, based on Figure. 4.6-8, even at thedepth of 0.2 µm (200 nm), metallic iron was scarcely identified.

105

Before test

Immediately after removal

After descaling

Fig. 4.6-6

Visual observation results of SPHC specimens (No. 5)106Fig. 4.6-7

Composition distribution to the depth direction of the SPHC specimen(No. 5)

Fig. 4.6-8

Photoelectron spectra of iron of the SPHC specimen (No. 5)
107

Table 4.6-3 shows the comparison results between the corrosion thickness andequivalent corrosion thickness. The corrosion thickness and the equivalent corrosion
thickness obtained from the corrosion weight loss indicate different values.
Specimens No. 1, 2, and 3 indicated good coincidence in corrosion thickness andequivalent corrosion thickness obtained from corrosion weight loss when about 0.2 µm
corrosion occurs in descaling. With respect to Specimens No. 4 and No. 5, thedifference between corrosion thickness and equivalent corrosion thickness was
reduced to about 0.4 µm during the test period. The cause of this is unclear but since alarge volume of calcium powders adhered to the surfaces of these specimens after
removal, it took time to dry the specimens and it is assumed that during this period,
specimens may have generated atmospheric corrosion.
Table 4.6-3

Comparison between corrosion thickness obtained from corrosion
weight loss and equivalent corrosion thickness
Corrosion thickness

Equivalent corrosion thickness

No.

Test
period(days)[1] Test
periodcorrosion
thickness(µm)

Yearlycorrosion
thicknesstw(µm/y)[2] Equivalentcorrosion
thickness during
test period (µm)

Yearlyequivalentcorrosion
thickness
tg(µm/y)

1

613.0

0.364

0.217

0.114

0.068

0.250

2

718.0

0.233

0.118

0.0925

0.047

0.145

3

719.0

0.303

0.154

0.118

0.060

0.185

4

892.1

0.555

0.227

0.0963

0.039

0.459

5

892.1

0.510

0.209

0.110

0.045

0.400

[1]-[2]Note) Measurement No. 3, 4, 5: SPHC; Measurement No. 1, 2: SPCC

4.6.4

Discussion

To clarify long-term gas generation behavior, testing was conducted in a period
close to 1000 days and the hydrogen gas generation rate was measured.
From the testing of long-term gas generation behavior, the following was clarified:•

The hydrogen gas generation rate simply decreases with time inincrements of 100 days, though there are some variations.

•

In the initial stages of immersion, the hydrogen gas generation rate of
0.1 µm/y or higher in terms of an equivalent corrosion rate is observed,
but it becomes 0.1 µm/y or lower when one year or more elapses, andafter 800 days elapse, it becomes about 0.02 µm/y. It is assumed thatthe hydrogen gas generation rate under the calcium water environmentof the carbon steel under the repository environment is 0.02 µm/y orlower in the long term.
108

CHAPTER 5 SUMMARY OF GAS GENERATION RATE EVALUATION
The present study was conducted to improve the reliability of data that werecharacterized for the hydrogen gas generation rate by corrosion under carbon steel
reducing conditions, and to evaluate conditions that would affect the hydrogen gas
generation under the carbon steel reducing conditions based on the measurement
method and their effects.
This chapter summarizes the whole aspect of the tests conducted, and organizes
the results and future deployment.
Table 5-1 shows factors that would affect the hydrogen gas generation when thedisposal of radioactive waste is considered. In the same table, expertise andproblems1),2) obtained before the start of the present study are described, too.

Table 5-1
Classification
Metallic
material(specimen)

Factors that would affect hydrogen gas generation

Item
Generation ofhydrogen gas fromcarbon steel.
Effects of steel gradeof carbon steels
Effects of gap
Effects of corrosion
products

Groundwater
environment(test solution)

Effects of
cementitious covering
material
Effects of chloride ion

Effects of pH
Effects of oxygen

Effects of type ofsolution

Miscellaneous

Effects of solution
temperature
Long-term behavior ofgas generation
Potential of test
environment
Difference incorrosion thickness
Gas generation ofother metals

Expertise and problems before study
Corrosion rate of carbon steel is 0.1-100 µm/y and theequivalent corrosion rate is 0.003 to 10 µmy, indicating a large
evaluation width.
Difference in steel grades is assumed to be negligible and is
not investigated.
They are not verified in tests under a low-oxygen environment
but crevice corrosion is suggested to occur.
It is reported that the corrosion rate increases many times dueto the accumulation of corrosion products, but there also exist
negative opinions.
There is no sufficient knowledge.

In atmosphere, as the chloride ion concentration increases,
localized corrosion is more likely to occur, and the corrosion
rate increases.
There is no clear report on the effects of pH. However, it has
been reported that it has large effects.
Effects on low oxygen have been reported but evaluation is
not possible because the oxygen concentration is notmeasured.
Tests have been conducted with various solutions but
measurements have not yet been made on the composition of
the repository environment.
Many reported that they can be displayed on the Arrhenius
plot. However, gradients differ.
Though it may arise from the test method, it has been reported
that the hydrogen gas generation rate increases after a
specified period.
Test environment is not thoroughly monitored.
Many tests including electrochemical tests have been
conducted, but there are large differences according to thetest methods.
It has been reported that stainless steel provides a sufficiently
small equivalent corrosion rate as compared to carbon steel.

109

In this chapter, these tests are investigated based on the findings which havebeen obtained to date. Incidentally, the tests conducted in the present study are tests
using the high-accuracy gas generation rate evaluation system, and basically, the tests
were conducted with calcium equilibrium water used as the test solution and at asolution temperature of 35°C and at an atmospheric oxygen concentration of 1 ppb or
lower.

5.1

Metallic Material (Specimens)

5.1.1

Hydrogen Gas Generation from Carbon SteelThe hydrogen gas generation from carbon steel is the main focus of the present
study and the hydrogen gas generation rate was evaluated as the behavior of anequivalent corrosion rate by long-term tests (about 900 days at the maximum).

Equivalent corrosion rate (µm/y)

The test solutions were calcium equilibrium water only, but it was confirmed thatthe equilibrium corrosion rate lowers with time and further lowers even at the time when
900 days have elapsed. Under the test conditions of calcium equilibrium water, asolution temperature of 35°C and an atmospheric oxygen concentration of 1 ppb or
lower, it was confirmed that the equivalent corrosion rate was about 0.02 µm/y whenabout 800 days had elapsed (see Fig. 5.1-1).

2.0E-01
1.8E-01
Long-term test data

1.6E-01
1.4E-01
1.2E-01
1.0E-01
8.0E-02
6.0E-02
4.0E-022.0E-02

0.0E+00
0

100

200

300

400

500

600

700

800

900

1000

Elapsed time (days)

Fig. 5.1-1

Equivalent corrosion rate of carbon steel in calcium equilibrium water1105.1.2

Effects of Steel Grades of Carbon SteelIt is assumed that carbon steel is used for the vessel material (for example,
SPHC), rebar material (for example, SD345), etc. in radioactive waste repositories. If
the hydrogen gas generation rate varies in accordance with the type of carbon steels
used, there might be cases where the imposing of steel grades of carbon steel,manufacturing method, etc. is necessary. Consequently, in the present study, effectsof the difference in steel grades of carbon steel on the hydrogen gas generation rate
were studied using blast-furnace steels and electric-furnace steels with comparatively
different chemical compositions.
Table 5.1-1 summarizes the elemental compositions of steel grades of carbon
steel used for comparison.
Table 5.1-2 summarizes the equivalent corrosion rate for each immersion period.
In addition, Figure. 5.1.2 depicts the equivalent corrosion thickness for the immersion
period.
Based on these results, there was scarcely any difference in the hydrogen gas
generation rate caused by the difference in steel grades of carbon steel.

Table 5.1-1

Analysis results of the chemical composition of carbon steel used for
tests
No.

1

2

Classification

SD345 blastfurnace steel

SD345 electricfurnace steel

C

0.22

0.24

0.27 or less

Si

0.35

1.10

0.55 or less

Mn

1.46

0.028

1.60 or less

P

0.012

0.034

0.040 or less

S

0.011

0.031

0.040 or less

Cu

0.01

0.31

Ni

0.02

0.10

Cr

0.04

0.29

Mo

-

0.01

Al

0.016

0.005

V

0.021

0.008

N

0.005

Chemicalcomposition
(%)

111

(Reference)
JIS G 3112

Table 5.1-2

Effects evaluation of steel grades of carbon steel: equivalent corrosion
rate for each immersion period.
Mean equivalent corrosion rate (µm/y)

Mean equivalent corrosion rateby periods (µm/y)

100
days

200
days

300
days

360
days

100-200days

200300 days

300360 days

4.9E-02

4.3E-02

4.1E-02

4.0E-02

3.8E-02

3.6E-02

3.5E-028.0E-02

5.8E-02

4.9E-02

4.4E-02

3.5E-02

3.0E-02

2.1E-02

Steel grade

Accumulated equivalent corrosion thickness (µm)

SD345 blastfurnace steel
SD345electricfurnace steel

SD345/blast-furnace steel

SD345/electric-furnace steel

1.E-01

1.E-02

1.E-03

1.E-040

Fig. 5.1.2

5.1.3

100
200
Elapsed time (days)

300

400

Effects of difference in steel grades of carbon steel on the accumulated
equivalent corrosion thickness

Effects of Gap

A thickness difference occurs in oxygen concentration and crevice corrosion
occurs if there is any gap in metallic material in the atmospheric environment3). The
high-accuracy gas generation rate evaluation system is constructed to have a gap
between the specimen and the support, but during the recent test (accumulated
operating hours: about 50 years), such type of corrosion was not observed.
We do not mean to conclude that crevice corrosion would not occur, but since thedissolved oxygen concentration is originally extremely low, the generation of crevice
corrosion was assumed to be extremely difficult.
112

In the future, in order to identify the effects of gap, one method would be toinvestigate the occurrence probability of crevice corrosion with the oxygen
concentration used as a parameter.
5.1.4

Effects of Corrosion ProductKojima et al.4),5) reported that large corrosion rates were obtained when
specimens with Fe3O4 reagent fines (compressed condition) that simulated corrosion
products provided to the circumference of carbon steel and covered with compressed
bentonite were fabricated with respect to the corrosion of overpack material for highlevel radioactive waste. In addition, by a series of these tests, they reported that “thecorrosion rate of carbon steel is accelerated by the accumulation of corrosion products.
In addition, under the bentonite environment, the corrosion rate is comparatively small
in the initial stages and accelerated by the accumulation of corrosion products.” With
respect to this, Fukaya et al.6) suggest that on the magnetite of the simulated bentonite
equilibrium water environment, hydrogen generation cathodic reactions increase andhydrogen generation reactions are accelerated. However, Watanabe et al.7) reported
that the corrosion of carbon steel was accelerated by the coupling of magnetite andcarbon steel but this acceleration was attributed to the reduction of magnetite and noacceleration of hydrogen generation reaction was confirmed (against this, Akashi etal.8) refutes by stating that magnetite self-reduction reactions scarcely occur in the
bentonite environment.) The report by Fushiwaki et al.9) concludes that magnetite
hardly exerts effects of accelerating hydrogen generation, too.
Because effects of the corrosion products on the hydrogen gas generation rate
may exert large effects on the repository environment, thoroughgoing investigation isrequired.
In the tests that were conducted on the high-accuracy gas generation rateevaluation system for about 1,000 days, the results of reduction in the equivalentcorrosion rate with time were obtained. Though corrosion products of magnetite,hydroxide, etc. could not be identified, acceleration effects of gas generation by theaccumulation of corrosion products have not been recognized.

5.1.5

Effects of Cementitious Covering Material

In the repository environment, carbon steel such as rebars, etc. are covered withcementitious material. The hydrogen gas generation rate associated with the corrosionof carbon steel in such an event was measured for about 500 days. As a result, thetime in which hydrogen gas was released from the cementitious material was delayed
for about 100 to 400 days (covering thickness: about 10 mm) and it was confirmed thata low equivalent corrosion rate is obtained in the initial stages of immersion (see Fig.
5.1-3).

113

Equivalent corrosion rate (µm/y)

1.0E+00
Mortar specimen
Mortar covered specimen 1
Mortar covered specimen 2
Mortar covered specimen 3
Carbon steel

1.0E-01

1.0E-02

1.0E-03
0

100

200

300

400

500

Elapsed time (days)

Fig. 5.1-3

Equivalent corrosion rate of carbon steel covered with cementitious
material

114

5.2

Groundwater Environment (Test Solution)

5.2.1

Effects of Chloride Ion Concentration

With respect to the effects of chloride ion concentration, tests were conducted ina chloride ion concentration range from 5 to 20,000 ppm in the calcium equilibrium
water. As a result, it was confirmed that in the range of chloride ion concentration from
5 to 5,000 ppm, which is assumed in the repository environment, the chloride ion
concentration scarcely exerts effects on the mean equivalent corrosion rate (see Fig.
5.2-1).

Mean equivalent corrosion rate (µm/y)

0.5

0.4

0th-100th day

100th-200th day

200th-300th day

300th-400th day

0.3

0.2

0.1

0.0
1

10

100

1000

10000

100000Chloride ion concentration (ppm)Fig. 5.2-1

5.2.2

Effects of chloride ion concentration on the mean equivalent corrosion
rate

Effects of pH

Effects of pH were investigated from pH 10.5 to 14.0 in calcium water. It was
confirmed that in the range of pH10.5 to 13.0, the hydrogen generation rate gradually
increases as the pH increases but the trend is extremely small. In addition, in therange from pH10.5 to 13.0, which is assumed in the repository environment using
cementitious material, it was confirmed necessary to study the 0.05 µm/y equivalentcorrosion rate for a long time (see Fig. 5.2-2).

115

Mean equivalent corrosion rate (µm/y)

0.55
0.50
0th-100th day

0.45
0.40

100th-200th day
200th-300th day

0.35

300th-400th day

0.30
0.25
0.20
0.15
0.10
0.05
0.00
10.0

Fig. 5.2-2
5.2.3

11.0

12.0
pH

13.0

14.0

Effects of pH on the mean equibalent corrosion rate

Effects of Oxygen

In the low-oxygen environment, the oxygen concentration (dissolved oxygen
concentration in the solution) has large effects on the hydrogen gas generation rate but
this is not measured by the high-accuracy gas generation rate evaluation system.
Figure 5.2-3 is a report10) which investigated the relation between the oxygen
concentration in the injected gas and hydrogen gas generation, indicating the oxygen
concentration dependency of the hydrogen gas generation rate. However, since thetest period is short, the reliability of the data is questionable. In the same figure, thereason why the hydrogen gas generation increases at 1 ppb oxygen concentration is
attributed to the fast consumption of air formed film in the low oxygen atmosphere andthe increased hydrogen gas generation rate at 10 ppm or higher is attributed to thegeneration of local corrosion.

Hydrogen gas generation rate (ml/m2・y)

1.E+05
Cement equilibrium water (pH11.8)
1.E+04

1.E+03

1.E+02
1 day
5 days

1.E+0115 days
30 days
1.E+0045 days
60 days
1.E-01
0.001

0.01

0.1

1

10

100

1000

10000 100000 100000
0Oxygen concentration (ppm)Fig. 5.2-3

Relation between oxygen concentration and the hydrogen gas
generation rate10)116

5.2.4

Effects of Types of Solution

In the high-accuracy gas generation rate evaluation system, tests wereconducted using calcium equilibrium water and AEA cement equilibrium water as test
solutions.
In the future, when the data at the original location of the repository is obtained, it
will be desirable to conduct tests using the solution. However, since it is assumed thatthe groundwater (solution) composition might change in the long run, it isrecommended that the most severe conditions assumed from the viewpoint of gas
generation should be selected and the gas generation rate should be evaluated bytests. Specifically, tests should be conducted using several solutions for about 200
days, and solutions with which the equivalent corrosion rate after 200 days becomes
lower than the setting (for example, equivalent corrosion rate of 0.02 µm/y) should be
regarded as no problem, and it is realistic to conduct long-term tests for the solutions
that exceed the setting only.

5.2.5

Effects of Solution Temperature

In the high-accuracy gas generation rate evaluation system, with the maximum
temperature at 100 m deep below ground assumed, tests were conducted at a solution
temperature of 35°C. In the initial stages of immersion, the possibility of achieving hightemperature over 35°C cannot be denied due to the decay heat, etc. of radioactive
waste. Consequently, the effects of the solution temperature on the equivalentcorrosion rate were investigated preliminarily. Figure 5.2-4 depicts this result.

Equivalent corrosion rate (µm/y)

Figure 5.2-4 confirms that the solution temperature exerts extremely large effects.
Figure 5.2-511),12),13) organizes data in Japan and abroad, but possibly because theoverseas data has comparatively high oxygen concentration, the temperaturedependency is small. On the other hand, the Japanese data provides higher sensitivity
to temperature possibly because tests are conducted at comparatively low oxygen
concentration. At any rate, since the effects of the solution temperature on theequivalent corrosion rate cannot be ignored, detailed investigation is required in the
future.
1.E+00No.1
計測番号No.2
1
計測番号 4

No.3計測番号
No.43
計測番号 20

1.E-01

1.E-02
3.1E-03

3.2E-03

3.2E-03

3.3E-03

3.3E-03

3.4E-03

3.4E-03

1/T (1/K)

Fig. 5.2-4

Effects of solution temperature on the equivalent corrosion rate117

Knecht
Hironaga

PlattsKursttenSchon

Fujisawa

No.2

No.3

No.4

2.8E-033.0E-03

3.2E-03

3.4E-03

40

21

No.1

Equivalent corrosion rate (µm/y)

1.E+02

1.E+01

1.E+00

1.E-01

1.E-02

1.E-032.2E-03

2.4E-03

2.6E-03

1/T (1/K)

181

Fig. 5.2-5

143

112

84
60
Temperature (°C)

Measurement results on solution temperature and the equivalentcorrosion rate in Japan and abroad11),12),13)

118

5.3

Other Effects

5.3.1

Long-Term Behavior of the Hydrogen Gas Generation

With respect to the hydrogen gas generation accompanied by carbon steel
corrosion, long-term behavior for about 1,000 days was investigated. As a result,neither an increase in hydrogen gas generation after a specified period which wasobserved by NARGA nor acceleration of hydrogen gas generation due to magnetite
accumulation reported by Kojima et al.4),5) was observed.
The results obtained in the present study are conclusions that can betheoretically satisfied, too, and the monotonous decrease of the equivalent corrosion
rate with time in increments of 100 days means that when the equivalent corrosion rateis measured for 100 days and if this is below the setting, it poses no problem withrespect to hydrogen gas generation. This is a useful finding for conducting tests andevaluations in the future.

5.3.2

Potential of Test Environment

Changes of natural potential with time in the present test environment wereconfirmed using the ESHG 1). This result was reported as shown in Fig. 5.3-1 and it
was confirmed that the natural potential was stable at -700 to -600 mV. Incidentally, inthe same figure, there are places where the potential rises instantaneously, and sincethese are immediately after deaeration water is injected into the test vessel, it isassumed that the natural potential instantaneously rises because the dissolved oxygenin the deaeration water is high.

酸化還元電位（mV
vs vs
SHE）
Redox
potential (mV
SHE)

0.0

・No.1計測番号18
・No.2

-0.1

計測番号20

-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
0

50

100

150

浸漬時間（日）Immersion
time (days)

Fig. 5.3-1

Changes of natural potential with time

119

200

5.3.3Difference Between Equivalent Corrosion Rate and Corrosion Rate

There are severalfold differences between the equivalent corrosion rate obtainedby assuming the hydrogen gas generation rate from carbon steel as magnetite
generation reactions and converting it into the corrosion rate and the corrosion rate
obtained from the corrosion weight loss, and for large differences, there are cases ofmore than 10-fold differences between the corrosion and equivalent corrosion rate.
This is attributed to descaling. Furthermore, Nishimura et al. made exhaustive
studies on this and concluded as follows:•

Various test results have clarified that the difference in corrosion thickness(=[corrosion thickness obtained from corrosion weight loss] - [corrosion thickness
obtained from hydrogen gas generation rate]) is about 0.2 µm.

•

Thoroughgoing investigation of the test process indicated that at least about 0.17µm oxygen consumption type corrosion resulted. This oxygen consumption typecorrosion is primarily attributed to the descaling process including the drying ofspecimens.

•

The equivalent corrosion rate may be measured when the hydrogen gas
concentration is unstable, and may be underestimated by about 10%.

•When the corrosion products are assumed to be iron hydroxide, the difference
between the corrosion thickness and equivalent corrosion thickness coincides in
the range of ±0.1 µm.

5.3.4

Gas Generation of Other Metals

Metals other than carbon steel were confirmed by Nishimura et al.1),2) by similar
tests and reported as follows (see Fig. 5.3-2), though they were not tested in the scopeof the present study:•

The equivalent corrosion rate of stainless steel (SUS304, SUS316) is nearly thesame as the equivalent corrosion rate of carbon steel when sufficient time elapses.
The reason why stainless steel provides corrosion resistance in the atmosphere isthat primarily Cr in stainless steel combines with oxygen to form chromium oxide
which covers the surface. In the low oxygen environment, as in the case of thepresent test environment (oxygen concentration: 1 ppb or lower), since oxygen islacking, It is difficult for Cr to form chromium oxide and stainless steel does notprovide corrosion prevention functions.

•Zircaloy provides an extremely small equivalent corrosion rate.

120Equivalent corrosion rate (µm/y)

0.14
0.12

SUS304

SUS316

Zircaloy

0.10
0.08
0.06
0.04
0.02
0.00
0

200

400

600

800

Elapsed time (days)

Fig. 5.3-2

5.4

Equivalent corrosion rate and changes with time of stainless steel andZircaloy 1), 2)

Prediction of the Gas Generation Rate by Metal Corrosion under theRepository Environment
Table 5.4-1 summarizes the conclusions obtained by the present study.

Based on these, it is assumed that the basic data pertaining to the hydrogen gas
generation evaluation (temperature: 35°C; oxygen concentration: 1 ppb or lower as
main test conditions) have been summarized.
The results of the present study could be applied to the evaluation of dynamic
effects and transition of radionuclide by gas generation in the repository facilities,
pressure evaluation by gas generation in repository vessels, and other performance
evaluations.
However, when repository environment conditions are concretized, confirmatory
tests with conditions specific to the site taken into account become necessary.

121

Table 5.4-1
Classification
Metallic
material(specimen)

Item
Generation ofhydrogen gas
from carbon
steel.
Effects of steel
grade ofcarbon steels
Effects of gap

Effects ofcorrosion
products
Effects of
cementitious
covering
material
Test solution

Effects ofchloride ion
Effects of pH
Effects ofoxygenEffects of typeof solution

Effects ofsolution
temperature
Miscellaneous

Long-term
behavior ofgas generation
Potential of
test
environment
Difference ofcorrosion
thickness
Gas
generation ofother metals

Summary of the present study

Conclusions of the present study
Equivalent corrosion rate of carbon steel in low-oxygen
calcium equilibrium water is 0.05 µm/y or lower, and it
becomes 0.02 µm/y after 800 days or more.
It has been confirmed that carbon steels (SD345) made fromblast-furnace and electric-furnace steel provide a similarequivalent corrosion rate. Based on this, it is presumed that
there is no effect of the difference in steel grades of carbon
steel on hydrogen gas generation.
It is not verified, but no effects of gap were observed during
the 50-year accumulated operating time in the high-accuracy
gas generation rate evaluation system. It is presumed thatno crevice corrosion would occur under the low-oxygen
condition.
It is reported that the corrosion rate increases many times
due to the accumulation of corrosion products, but negative
opinions also exist.
By testing for about 500 days, hydrogen gas release wasobserved, but it was confirmed that the release delays until
about 100 to 300 days and the generation rate is small, too.
However, even after 500 days, the hydrogen gas release
tends to increase.
It has been confirmed by tests that there is no effect when
the chloride ion concentration is 5000 ppm or lower.
However, the hydrogen generation rate increases at 20,000
ppm chloride ion concentration.
It has been confirmed that in the range from pH10.5 to 13,
effects of pH are extremely small and are 0.05 µm/y or lower
in terms of the equivalent corrosion rate.
In the high-accuracy gas generation rate evaluation system,
tests were conducted with oxygen concentration controlled
to about 1 ppb or lower. However, the effects of oxygen
have not yet been verified.
Investigation was made with two types of test solutions of
AEA cement equilibrium water and calcium equilibrium
water. However, effects of the type of solution have not yet
been verified with groundwater composition of the assumed
repository.
Preliminarily, in the temperature range from 20 to 45°C,
effects of the solution temperature on the hydrogen gas
generation rate were tested and it was confirmed that theeffects of temperature are great.
After immersion for 100 days or more, phenomena ofincreasing the hydrogen gas generation rate from carbon
steel and increasing the hydrogen gas generation rateaccompanied by magnetite accumulation were not observedin the scope of the present study.
In the present study, the test was not conducted but in a
separate report, it was confirmed that the natural potential isstable at -700 to -600 mV.
There is about 0.2 µm difference in the corrosion rate andequivalent corrosion rate obtained from corrosion weightloss. However, it was confirmed that this is caused byoxygen corrosion at the time of descaling.
Equivalent corrosion rate of stainless steel is equivalent tothat of carbon steel. Hydrogen gas generation rate isextremely small from Zircaloy.

122

5.5

Subjects in Future Research

In the years to come, investigation must be carried forward with the evaluation of
long-term corrosion rates, variations in corrosion rates due to unstable elements, etc.
taken into account in order to determine the long-term barrier performance of repository
containers, leachate of nuclides from waste, etc., all of which will be required whenrepository systems are evaluated.
To evaluate these corrosion rates, the following points must be investigated:•

Evaluation of effects of oxygen concentration;•

Evaluation of effects of type of solutions;•

Evaluation of effects of solution temperature;•

Evaluation of gas generation rates of other metals;•

Evaluation of effects of gaps;•

Evaluation of effects of corrosion products; and•Evaluation of effects of cementitious covering material.

With respect to the evaluation of effects of oxygen concentration, it was
confirmed by tests that the oxygen concentration exerts great effects on the hydrogengas generation rate. In the future, it is necessary to confirm by tests the changes in thehydrogen gas generation rate with time by simulating the changes in oxygen
concentrations in repositories.
With respect to the evaluation of effects of types of solutions, in the present study,
it was confirmed that there is no significant difference by the evaluation using AEA
cement equilibrium water and calcium equilibrium water. In the future, changes withtime of the hydrogen gas generation rate must be confirmed by tests using solution
compositions assumed in repositories.
With respect to the evaluation of effects of solution temperature, the preliminary
test results suggested that the solution temperature would exert great effects on thehydrogen gas generation rate. In addition, recently, it has been reported that thetemperature rises at a rate of 10°C/100 m under the ground, and the effects of solution
temperature on the hydrogen gas generation rate must be confirmed by tests.
In addition, though the degree of importance is low, investigation should be
conducted on the following items, too.
In the evaluation of the gas generation rates of other metals, the evaluation ofembedded metals other than carbon steel is required.
In the evaluation of the effects of gaps, in the long-term test results, no crevice
corrosion is observed in the low-oxygen atmosphere, but this must be confirmed byelectrochemical tests, etc.

123

In the evaluation of effects of corrosion products, acceleration of the equivalentcorrosion rate by the accumulation of corrosion products is assumed to be attributed totest methods (impurities in magnetite, etc.), and the confirmation by tests are essential.
With respect to the evaluation of effects of cementitious covering material,
behavior for about 500 days only has been confirmed so far, and long-term tests must
be conducted. At the same time, tests to elucidate water penetration and gas transition
behavior are required.

124

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CHAPTER 1
1)“Radwaste Radiolytic Gas Generation Literature Review,” EPRI NP-5977, pp32

2)

M. R. Minguez, PEGASE PROJECT REPORT, ENRESA, 1995

3)

“Gas Generation and Migration in Radioactive Waste Disposal Safety-relevant
Issues,” OECD, p153-p155, 2000

4)

“Verification test on advanced radioactive waste disposal systems, 1998 Report”

CHAPTER 2
1)“Verification test on advanced radioactive waste disposal systems, 1998 Report”

CHAPTER 3
1)

Katsumi Kuriyama: “Analysis of Impurities in Gas by APIMS,” Surface Control &Cleaning Design, Springtime Number (separate volume), 1990

2)

Hideki Kambara: “Atmospheric Pressure Ionization Mass Spectrometry,” Bunseki,Vol. 12, 1979

3)

Katsumi Kuriyama et al.: “Atmospheric Pressure Heating Desorption Gas
Analyzer,” Clean Energy, Vol. 10, 1997

CHAPTER 4
1)“Verification test on advanced radioactive waste disposal systems, 1998 Report”

2)

A. Atkinson: “The Time Dependence of pH within a Repository for Radioactive
Waste Disposal,” AERE R 11777

CHAPTER 5
1)

Tsutomu Nishimura, Ryutaro Wada, and Kazuyuki Fukutome: “Monitoring ofHydrogen Gas generated as a result of Metal Corrosion under Low-Oxygen
Environment,” Kobe Steel Engineering Reports, 2003

2)

Morihiro Mihara, Tsutomu Nishimura, Ryutaro Wada, and Akira Honda:“Estimation on Gas Generation and Corrosion Rates of Carbon Steel, StainlessSteel, and Zircalloy in Alkaline Solution under Low Oxygen Condition,” Japan
Nuclear Cycle Development Institute Engineering Reports, No. 15, 2002

125

3)

Osamu Yamazaki and Toshio Shibata: “Measurement of Critical Pitting
Temperature and Pitting Potential of Stainless Steel by Crevice Corrosion
Prevention Electrode,” Zairyou-to-Kankyou, No. 51, 2002

4)

Yoichi Kojima and Shigeo Tsujikawa: “Acceleration by Accumulated Corrosion
Products Layer of Carbon Steel Corrosion Rate in Compressed Bentonite
Environment,” Proceedings of the 44th Roundtable Discussion on Material andEnvironment, p. 421, 19975)

Yoichi Kojima, Toru Yabuuchi, and Shigeo Tsujikawa: “Acceleration by
Accumulated Corrosion Products Layer of Carbon Steel Corrosion Rate inCompressed Bentonite Environment,” Proceedings of the ‘98 Zairyo-to-Kankyo, p.
233, 1998

6)

Yuichi Fukaya and Masatsune Akashi: “Investigation on Corrosion Rate
Acceleration Mechanism of Carbon Steel by Magnetite Accumulation,”
Proceedings of the 48th Roundtable Discussion on Material and Environment, p.
329, 2001

7)

Hirokazu Watanabe, Atsushi Nishikata, and Toru Tsuru: “Acceleration ofCorrosion Carbon Steel by Magnetite and its Reaction Mechanism,” Proceedingsof the 46th Roundtable Discussion on Material and Environment, p. 261, 1999

8)

Masatsune Akashi, Yuichi Fukaya, Eiji Sasaki, and Shigeo Tsujikawa:“Electrochemical Discussion concerning “Magnetite Acceleration” in Corrosion ofHigh-level Radioactive Waste Repository Containers,” Zairyo-to-Kankyo 2002D, p.
331, 2002

9)

Yusuke Fushiwaki, Takumi Haruna, and Toshio Shibata: “Effects of Fe3O4 on H2
Generation Reactions associated with Corrosion of Carbon Steel,” Proceedingsof the 47th Roundtable Discussion on Material and Environment, p. 391, 2001

10)

“Verification test on advanced radioactive waste disposal systems, 1998 Report”

11)

W. R. Rodwell, A. W. Harris, S. T. Horsemann, P. Laleieus, W. Muller, L. Ortix
Amaya, and K. Pruess, “Gas Migration and Two-Phase Flow through Engineered
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Michihiko Hironaga and Yasuhiko Umahara: “Hydrogen Gas Generation Behavior
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13)

Ryutaro Fujisawa, Tetsunari Kurashige, Yusuke Inagaki, and Muneaki Seno:“Gas Generation Behavior of Transuranic Waste under Waste Disposal
Conditions,” Proceedings of the 45th Roundtable Discussion on Material andEnvironment, p. 39, 1998

126

ACKNOWLEDGEMENTS

In carrying on the present study, special mention should be given to Professor
Atsuyuki Suzuki, the University of Tokyo, who acted as a chairperson of the “Gas
Generation Investigation Committee” until 2000, and gave us excellent and extensive
advice and help in carrying forward the studies pertaining to the disposal ofradioactive waste.

Professor Toshio Shibata, Osaka University, participated in the present study as
a committee member from the very beginning of the project, and since 2001, has acted
as a chairperson of the “Gas Generation Investigation Committee.” He was the mosthelpful in providing us with intellectual support and practical assistance in the conceptof evaluation techniques, significance of measured data, organizing policies, etc. fromthe standpoint of a metal corrosion specialist.

In the “Gas Generation Investigation Committee”, enthusiastic discussions took
place at each meeting with respect to the measurement data evaluation methods, etc.
of traces of gas generation rates, which no one had experienced before, anddirectionality of the studies and organizing data were actively taken into deliberation.
As members of the present committee, specialists from various research fields got
together and cutting-edge opinions and advice were exchanged and given from eachresearch field, thereby obtaining never-before-available useful research results.

Tsutomu Nishimura, Kobe Steel, Ltd. and Kazuyuki Fukutome, Kobelco ResearchInstitute, Inc. took charge of tests and measurements and highly sophisticatedanalyses for the present studies, and gave valuable opinions as persons in charge ofbusiness practice during discussions on organizing the report.

In organizing the present technical report, Professor Emeritus Toshio Shibata
again provided us with excellent and extensive advice despite his busy work schedule.

We would appreciate the interest and assistance of the above-mentioned people
concerned and owe each one of them special thanks.

127

Radioactive Waste Management Funding and Research Center(RWMC)
No.15. Mori Bldg, 2-8-10, Toranomon, Minato-ku, Tokyo, 105-0001, Japan
TEL +81-3-3504-1081 FAX +81-3-3504-1297 http://www.rwmc.or.jp