Effects of process-generated hydrogen on RPV walls

Potential effects of process-generated hydrogen on RPV walls during thermal gradients

Kristof Dockx

Thesis submitted for the degree of Master of Science in Nuclear Engineering Thesis supervisor: Prof. dr. ir. W. Bogaerts Assessor: Prof. dr. ir. J. Lecomte-Beckers

Prof. dr. D.D. Macdonald Prof. dr. ir. P. Van Iseghem

Academic year 2016 – 2017

c Copyright KU Leuven

Without written permission of the thesis supervisor and the author it is forbidden to reproduce or adapt in any form or by any means any part of this publication. Requests for obtaining the right to reproduce or utilize parts of this publication should be addressed to Faculteit Ingenieurswetenschappen, Kasteelpark Arenberg 1 bus 2200, B-3001 Heverlee, +32-16-321350. A written permission of the thesis supervisor is also required to use the methods, products, schematics and programs described in this work for industrial or commercial use, and for submitting this publication in scientific contests.

Master of Science in Nuclear Engineering

Thesis Summary Page

Name of the student:

Kristof Dockx

Title:

Potential effects of process-generated hydrogen on RPV walls during thermal gradients.

Abstract:

The reactor pressure vessel (RPV) is an important component in the safety system of a pressurized water reactor. In 2012, several flaws have been detected in the RPVs of Doel 3 and Tihange 2 in Belgium. These flaws were identified as hydrogen flakes and are concentrated in the upper and lower core shells of the RPV. As these flaws originate from the presence of hydrogen, this work focusses on the evolution of hydrogen gas and atoms in the RPV both during normal operation as for accidental conditions. Three different sources of hydrogen are identified, i.e. hydrogen dissolved in the primary water, corrosion generated hydrogen and radiolysis of the primary water. These hydrogen sources are quantified using literature and a chemical modelling code. After quantification, one can calculate the corresponding hydrogen pressure in the RPV wall for different operating conditions. During hot in-service conditions, according to the model, the radiolysis generated hydrogen corresponds to a mechanical hydrogen pressure of 1.5 10 5 atm, while the effect of corrosion might be limited to 3.62 Pa. However, d uring a cold shutdown, the mechanical hydrogen pressure increases up to 1.7 10 5 atm and between 166 - 1.3 10 5 Pa for radiolysis and corrosion, respectively. Finally a pressurized thermal shock is considered as an accidental operating condition. During a PTS, the hydrogen pressure is found to be 1.6 10 5 atm for radiolysis and up to 4.7 10 4 Pa due to corrosion. It has been found that these mechanical pressures are in the same order of magnitude as the required pressure for cracks to grow in a steel material. Therefore, these simulations and models can not assure the stability of the hydrogen flakes in the RPV.

Promotor:

Prof. dr. ir. Walter Bogaerts

Approval for submission by promotor:

Assessors:

Prof. dr. ir. Jacqueline Lecomte-Beckers Prof. dr. Digby D. Macdonald Prof. dr. ir. Pierre Van Iseghem

Academic Year 2016-2017

Belgian Nuclear higher Education Network, c/o SCK  CEN, Boeretang 200, BE-2400 Mol, Belgium

i

Preface

A little more then one year ago, I started this study to satisfy my interest in the field of nuclear science and engineering. However, as time passed and I became more involved in this topic, it became so much more than just an interest. The complete project allowed me to use all of my previous acquired knowledge in a new and hot topic. However, this did not make it any easier to progress. The project made me step in the middle of a very instable land were various opinions were thrown at me. Therefore, it was very important during the complete project to act independent and unbiased, which was one of many great lessons to learn for me. Everything I achieved in this thesis, would not have been possible without the help and involvement of my promotor prof. dr. ir. Walter Bogaerts and prof. Digby D. Macdonald. As mentioned, prof. Bogaerts gave me the opportunity to work on this very interesting and hot topic. His guidance and knowledge on various disciplines in this work, e.g. corrosion and material degradation, helped me to complete the project as it is today. Therefore, I want to express my sincere gratitude to him. Furthermore, I want to thank prof. Digby D. Macdonald. Especially for all the opportunities he gave me during the project. Furthermore, he was a big help for the radiolysis calculations and much more. He definitely was an indispensable factor in the complete project. Furthermore, I would like to thank dr. George Engelhardt. Together with prof. Macdonald, he provided me with the basic radiolysis program used during this work. Last but not least, I want to express some words of gratitude for everyone else involved in this project. Without all of them, this work would not have been possible.

Thanks to all of you.

Kristof Dockx

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Samenvatting Het reactordrukvat is een belangrijk onderdeel in het veiligheidssysteem van een drukwaterrector. In 2012 werden er verschillende foutindicaties ontdekt in de reactor- drukvaten van Doel 3 en Tihange 2 in België. Deze indicaties werden geïdentificeerd als waterstofvlokken, ook wel waterstofscheurtjes genoemd. De fouten zijn vooral geconcentreerd in de twee ringen van het drukvat die het dichtst bij de kern van de reactor gelegen zijn. Aangezien deze fouten het gevolg zijn van de aanwezigheid van waterstof, legt dit werk zijn focus op de evolutie van waterstofgas en waterstof atomen in het reactordrukvat. Deze evolutie wordt onderzocht tijdens zowel normale operatieve condities als tijdens mogelijke ongevallen en incidenten. Drie verschillende bronnen van waterstof werden geïdentificeerd, i.e. opgelost waterstof in het primaire water van de reactor, waterstof gevormd door corrosie van het reactordrukvat en radiolyse van het primaire water. Deze verschillende bronnen van waterstof wer- den gequantificeerd door middel van chemische modellen en eerder gerapporteerd onderzoek. Wanneer de grootte van de verschillende bronnen gekend is, wordt het mogelijk om de overeenkomstige waterstofdruk in the wand van het reactordrukvat te bepalen voor de verschillende condities die worden beschouwd. Ten eerste is er de warme operatieve conditie. Hier draait de reactor aan 100% van zijn vermogen in de normale omstandigheden. Berekeningen volgens het model hebben gewezen op een waterstof productie als gevolg van radiolyse die resulteren in een mechanische waterstofdruk van 1.5 10 5 atm. Corrosie resulteert slechts in een overeenkomstige waterstofdruk van 3.62 Pa voor de meest pessimistische en conservatieve corrosie condities. Vervolgens is er ook de koude uitschakeling van de reactor. Hierbij wordt de reactor over een periode van ongeveer 25 uur uitgeschakeld en afgekoeld tot kamertemperatuur. In deze omstandigheden gaan de mechanische waterstofdrukken als gevolg van radiolyse en corrosie in the reactorwand toenemen tot respectievelijk, 1.7 10 5 atm en 166–1.3 10 5 Pa. Tenslotte, wordt ook een ongeval beschouwd, i.e. een thermische schok onder druk. De reactor wordt dan heel snel afgekoeld met extern koelwater. De grote temperatuursverschillen in de wand van de reactor samen met de hoge druk die mogelijks wordt opgebouwd, zorgen dan voor extra spanningen en krachten in het materiaal. Hierbij komt nog de waterstofdruk als gevolg van de gevormde waterstof door radiolyse en corrosie. Deze druk werd berekend en is gelijk aan 1.6 10 5 atm als gevolg van radiolyse en tot 4.7 10 4 door corrosie. Eerder onderzoek heeft uitgewezen dat een druk met een grootteorde van enkele GPa kan zorgen voor een verdere groei van scheurtjes in staal. Dit komt exact overeen met de gevonden mechanische drukken in de wand van een reactorvat. Hierdoor, kan dit werk geen uitsluitsel bieden over de stabiliteit van waterstofvlokken in de stalen wand van een reactordrukvat. iii

List of Figures

1.1 Schematic representation of a PWR. . . . . . . . . . . . . . . . . . . . . 5 1.2 Schematic representation of a RPV. . . . . . . . . . . . . . . . . . . . . 7 1.3 Schematic representation of a RPV. . . . . . . . . . . . . . . . . . . . . 9 2.1 Examinated zones in 10-yearly quality control of the RPV. . . . . . . . 12 2.2 Schematic representation of an underclad defect. . . . . . . . . . . . . . 12 2.3 Stresses in a RPV wall. . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.4 Machine used for UT inspection. . . . . . . . . . . . . . . . . . . . . . . 14 2.5 Distribution of the indications as function of depth in Doel 3 (2012). . . 14 2.6 Distribution of the indications as function of depth and height in Doel 3 (2012). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.7 Schematic representation of macro-segregation in a large ingot. . . . . . 16 2.8 Solubility of hydrogen in iron as a function of temperature. . . . . . . . 17 2.9 Micrograph of hydrogen flake with MnS precipitates. . . . . . . . . . . . 17 2.10 Micrograph of a hydrogen flake fracture surface. . . . . . . . . . . . . . 18 2.11 Material susceptibility to hydrogen flaking as function of hydrogen and sulphur concentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.1 Reaction mechanism of molecular hydrogen absorption in metallic material. 22 3.2 Octahedral and tetrahedral interstitial lattice sites in a bcc lattice. . . . 23 3.3 Temperature and pressure dependent solubility of hydrogen in iron. . . 24 3.4 Types of microstructural lattice defects acting as hydrogen traps present in a steel component. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.5 Experimental values for the solubility constant and the diffusion coefficient of hydrogen in steel as a function of temperature. . . . . . . . 28 4.1 Typical primary water chemistry conditions in a PWR during operation. 31 4.2 Reactions due to ionizing radition in water. . . . . . . . . . . . . . . . . 34 5.1 Henry constant for hydrogen gas in light water at high temperatures in GPa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.2 Density of water as a function of temperature at a pressure of 150 bar. . 41 5.3 Henry constant for hydrogen gas in light water at high temperatures in mol/m 3 Pa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 iv

List of Figures

5.4 Equilibrium pressure of H 2 for a concentration between 25 and 50 STP cm 3 /kg. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.5 Pourbaix diagram for Fe-Cr-H 2 O and Fe-H 2 O system at a temperature of 320 ◦ C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.6 Density of radiolytic species for high and low LET radiations. . . . . . . 47 5.7 Stability test of the model for extreme conditions. . . . . . . . . . . . . 54 5.8 Variation of H concentration during the fuel cycle. . . . . . . . . . . . . 56 6.1 Equilibrium pressure of H 2 for a concentration of 35 STP cm 3 /kg. . . . 61 6.2 Experimental values for Henry coefficient of noble gases as function of temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.3 Relation between the maximum Henry coefficient and the Van der Waals radii of the noble gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.4 Relation between the temperature, where the maximum Henry coefficient is reached, and the Van der Waals radii of the noble gases. . . . . . . . . 65 6.5 Henry coefficient in GPa for atomic hydrogen as a function of temperature. 66 6.6 Henry coefficient in mol/m 3 Pa for atomic hydrogen as a function of temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 6.7 Temperature dependence of the H 2 dissociation constant. . . . . . . . . 68 6.8 Concentration of species at begin of cycle in absence of radiation. . . . . 70 6.9 Sieverts’ constant for hydrogen in ferritic steel. . . . . . . . . . . . . . . 73 6.10 Schematic representation of the RPV wall for the diffusion model and its solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 7.1 Typical cooling path for the reactor coolant in a pressurized water reactor during a cold shutdown. . . . . . . . . . . . . . . . . . . . . . . . 79 7.2 Temperature profile in the RPV wall as a result of the typical cooling path during a cold shutdown. . . . . . . . . . . . . . . . . . . . . . . . . 81 7.3 Concentration profile in the RPV wall as a result of the typical cooling path during a cold shutdown considering only corrosion generated hydrogen with a 10% absorption coefficient and a hydrogen generation rate of 50 mol H/yr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 7.4 The hydrogen fugacity in the RPV wall as function of time due to corrosion. 84 7.5 Concentration profile in the RPV wall as a result of the typical cooling path during a cold shutdown considering only corrosion generated hydrogen with a 10% absorption coefficient and a hydrogen generation rate of 150 mol H/yr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 7.6 Concentration profile in the RPV wall as a result of the typical cooling path during a cold shutdown considering only corrosion generated hydrogen with a 90% absorption coefficient and a hydrogen generation rate of 50 mol H/yr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 7.7 Concentration profile in the RPV wall as a result of the typical cooling path during a cold shutdown considering only corrosion generated hydrogen with a 90% absorption coefficient and a hydrogen generation rate of 150 mol H/yr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 v

List of Figures

7.8 Concentration profile in the RPV wall as a result of the typical cooling path during a cold shutdown considering only radiolysis generated hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 7.9 Concentration profile in the RPV wall as a result of the typical cooling path during a cold shutdown considering only radiolysis generated hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 7.10 Temperature profile in the RPV wall as a result of a PTS. . . . . . . . . 89 7.11 Concentration profile of hydrogen in the RPV wall as a result of a PTS considering only radiolysis generated hydrogen. . . . . . . . . . . . . . . 90 7.12 Hydrogen concentration profile in the RPV wall as a result of a PTS considering only corrosion generated hydrogen with a 90% absorption coefficient and a hydrogen generation rate of 150 mol H/yr. . . . . . . . 91

vi

List of Tables

1.1 Material composition of the different steels used in a RPV . . . . . . . . 8

2.1 Number and size of indications measured in the core shells of the Doel 3 and Tihange 2 RPV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.1 G-values for the different species in the primary water. . . . . . . . . . . 48 5.2 Reaction rate constants for the chemical reactions. . . . . . . . . . . . . 50 5.2 Reaction rate constants for the chemical reactions. . . . . . . . . . . . . 51 5.3 Conditions for stability test of model. . . . . . . . . . . . . . . . . . . . 53 6.1 H concentration in the RPV components after production. . . . . . . . . 60 6.2 Maximum Henry coefficient and corresponding temperature for the noble gases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.3 Conditions for begin of cycle without radiation. . . . . . . . . . . . . . . 69 6.4 Hydrogen charging rates of H in the RPV due to corrosion. . . . . . . . 74 6.5 Hydrogen charging ratesin the RPV due to corrosion. . . . . . . . . . . 75 7.1 Data used for the calculation of the temperature profile in the RPV wall during a cold shutdown. . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 7.2 Data used for the calculation of the time dependent concentration profile in the RPV wall during a cold shutdown. . . . . . . . . . . . . . . . . . 82 7.3 Maximum hydrogen fugacity in the base material of the RPV due to corrosion after cold shutdown. . . . . . . . . . . . . . . . . . . . . . . . . 86 7.4 Maximum hydrogen fugacity in the base material of the RPV due to corrosion after a PTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

vii

Symbols

Chemical activity of specie i Concentration of specie i

a i c i

Heat capacity

C p D i E a

Diffusion coefficient of specie i

Activation energy for the Arrhenius law

Faraday constant

F ˜ F f i

Conversion factor from rad/s to eV/gs

Fugacity of specie i

G-value for the production of specie i due to γ -radiation G-value for the production of specie i due to α -radiation G-value for the production of specie i due to neutron-radiation

G γ i G α i G n i H i

Henry constant for specie i

Current density

I

Flux of specie i due to corrosion

J c i

Thermal conductivity

k

Equilibrium constant for reaction i

K i k j k S

Reaction rate constant for chemical reaction j

Sieverts’ solubility constant Ionic product of water

K W

Molar mass

M ˙ n i

Production rate of specie i

Avogadro constant Pressure of specie i

N v

P i

Chemical production rate of specie i

R c i R y i

Production rate of specie i formed by radiolysis

Reaction rate for reaction i

RR i

Temperature Specific volume

T

v

Volume

V

Molar volume

V m

Molfraction of specie i

x i

viii

List of Tables

Number of exchanged electrons in the reaction

z i

Activity coefficient

γ

Γ γ i Γ α i Γ n i

γ -radiation energy dose rate α -radiation energy dose rate neutron-radiation energy dose rate

Fugacity constant Density of material i

ν

ρ i

ix

List of Abbreviations

BWR Boiling Water Reactor ECP Electrochemical Potential HAZ Heat Affected Zone LET Linear Energy Transfer NIST National Institute for Standards and Technology NPP Nuclear Power Plant PTS Pressurized Thermal Shock PWR Pressurized Water Reactor RDM Rotterdamsche Droogdok Maatschappij

RPV Reactor Pressure Vessel SCC Stress Corrosion Cracking STP Standard Temperature and Pressure UT Ultrasonic Testing VT Visual Testing

x

Contents

Preface

ii

List of Figures List of Tables Introduction

iv

vii

1

I Literature Review 1 Reactor pressure vessel

3

5 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Function of a reactor pressure vessel . . . . . . . . . . . . . . . . . . 6 1.3 Design of a reactor pressure vessel . . . . . . . . . . . . . . . . . . . 7 1.4 Production of a reactor pressure vessel . . . . . . . . . . . . . . . . . 9 2 Findings in Doel 3 and Tihange 2 11 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2 Flaw indications in Doel 3 and Tihange 2 . . . . . . . . . . . . . . . 13 2.3 Origin of the indications . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3 Hydrogen in steel 21 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2 Solubility of hydrogen in steel . . . . . . . . . . . . . . . . . . . . . . 21 3.3 Hydrogen trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4 Sources of hydrogen 29 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.2 Dissolved H 2 in primary water . . . . . . . . . . . . . . . . . . . . . 29 4.3 Corrosion of the RPV wall . . . . . . . . . . . . . . . . . . . . . . . . 30 4.4 Radiolysis of the primary water . . . . . . . . . . . . . . . . . . . . . 33

II Modeling

37

5 Hydrogen production in PWR 39 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 xi

Contents

5.2 Dissolved H 2 in primary water . . . . . . . . . . . . . . . . . . . . . 39 5.3 Corrosion of the RPV wall . . . . . . . . . . . . . . . . . . . . . . . . 43 5.4 Radiolysis in the primary water . . . . . . . . . . . . . . . . . . . . . 46 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 6 Hydrogen concentration in PWR wall 59 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 6.2 Hydrogen from RPV production . . . . . . . . . . . . . . . . . . . . 59 6.3 In-service hydrogen generation . . . . . . . . . . . . . . . . . . . . . 60 6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 7 Hydrogen pressure in PWR 77 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 7.2 Pressure during normal operating conditions . . . . . . . . . . . . . . 77 7.3 Pressure during incidental or accidental conditions . . . . . . . . . . 88 7.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 8 Conclusion 95 A Radiolysis model code 101 Bibliography 121

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Introduction

Relevance and problem statement This master thesis aims at acquiring a fundamental understanding of the so called “hydrogen cracks” or “hydrogen flakes” formed in nuclear reactor pressure vessel steel. Over the past few years, a lot of attention in Belgium and worldwide has been spent on the investigation of these flaws found in two of the Belgian nuclear reactor pressure vessels. The reactor pressure vessel is the heart of a nuclear reactor containing the nuclear fuel. It is subject to high temperatures and high pressures. For a safe operation of the reactor, the structural integrity of the reactor pressure vessel must be assured at every moment in time. Due to the number and density of these hydrogen induced flaws, some people have expressed their concerns about the safety of these nuclear units. A lot of research has already been done to assure the current integrity of both reactor pressure vessels. The conclusion of these investigations was evaluated positive, resulting in the continued operation of the reactors. As these reactors are currently planned to remain in operation for the next 6 and 7 years, it is important to investigate the possible evolution of these flaws in time. The above mentioned investigations did not focus into detail on the further development of the flaws. Therefore, the objective of this master thesis is to investigate the stability of these hydrogen induced flaws found in the reactor pressure vessels. The question whether it is possible for the cracks to grow under normal operating or accidental conditions will be handled. This problem will be treated from a theoretical point of view and by modeling. Thesis organization To cover the complete investigation on the possible evolution of the hydrogen induced flaws in two of the Belgian reactor pressure vessels, the thesis will be divided in several parts. First there is an explanation about the function and production of reactor pressure vessels. This is continued by a discussion about the discovery of flaws in the reactor pressure vessels of Doel 3 and Tihange 2 in Belgium. Furthermore, the effects of hydrogen in steel will be handled, followed by the possible sources of hydrogen during operation of the reactor and the uptake of this hydrogen by the 1

Contents

reactor pressure vessel steel. This hydrogen uptake by the steel will result in elevated pressures in the cracks, discussed in the next chapter. Finally there is a conclusion for final words about the safety and possible concerns for the reactor pressure vessels.

2

Part I Literature Review

3

Chapter 1 Reactor pressure vessel

1.1 Introduction A nuclear power plant (NPP) is a very complex high technology system. Several types of NPP’s are used for the production of electricity all over the world. The most popular types are the pressurized water reactor (PWR) and the boiling water reactor (BWR). The further discussion will be focussed on the PWR, since this is the type of nuclear reactor in Doel and Tihange, where the hydrogen flaws were found. A schematic representation of this type of reactor is given in Figure 1.1. [1]

Figure 1.1: Schematic representation of a PWR similar to Doel 3 and Tihange 2. [1] One of the most important parts in a nuclear power plant is the reactor pressure vessel (RPV). This vessel contains the core of the reactor, i.e. the nuclear fuel and the coolant. It is subjected to high pressures and temperature during operation. 5

1. Reactor pressure vessel

Therefore, it is a very important component for the safety system of a nuclear reactor. Furthermore, the RPV is the major part that limits the lifetime of a nuclear power plant. This is because the RPV is considered to be an irreplaceable component. The replacement of the RPV can not be economically justified by the owner of the plant due to the very high cost to replace it. 1.2 Function of a reactor pressure vessel The main function of a RPV is to assure the containment of the nuclear fuel and the coolant, both during normal operation and possible accidental conditions. These accidental conditions might be initiated by external causes such as seismic activity or internal causes leading to for example pressurized thermal shocks (PTS). The RPV is one of the three safety barriers for the containment of radioactive materials in a nuclear reactor unit, according to the so called “defense in depth” principle. These three barriers consequently are: • Fuel cladding: The cladding is a metallic sheet around the fuel and forms a barrier between the nuclear fuel and the coolant. It is supposed to confine the nuclear fuel and avoid loss of fuel into the coolant and cooling system. [2] • RPV: The RPV contains the fuel and coolant. Its integrity is not only impor- tant when the fuel cladding has failed. It contains the coolant of the reactor. Therefore, a failure of the RPV can result in a loss of coolant which can lead to overheating and possible damage to the fuel. [2] • Containment: This is a concrete wall encapsulating the RPV and most often also the primary cooling circuit. In case of a failure of the RPV or pipes, it should contain all the radioactive material and prevent it from escaping towards the environment. Some reactors have a double containment wall, e.g. Doel 3 and Tihange 2, as shown in Figure 1.1. [2] Besides the forces resulting from the high temperatures and pressures of the coolant, also external forces can act on the RPV, e.g. resulting from earthquakes. These kind of accidents have to be considered to a certain extend when designing a RPV. For a frequency larger than 10 − 4 per reactor year such initiating events have to be taken into account for the safety measures of the NPP. [3] This means that these events must not have any radiological impact at all. Lower frequency events are still limited in the amount of radioactivity that can be released. As the RPV is the most important of the three barriers, these demands impose design requirements and extra safety margins for the mechanical properties of the RPV. 6 This is a clear application of the defense in depth principle, as each barrier encapsulates the other. This means that a failure of one single barrier can not lead to the release of radioactivity to the environment.

1.3. Design of a reactor pressure vessel

1.3 Design of a reactor pressure vessel These mechanical requirements lead to a typical 20 cm thick steel wall for the RPV of a PWR. The reactor units of Doel 3 and Tihange 2 are build to the design by Framatome, now AREVA, in accordance with the ASME Code, Section III for Class 1 components. [1] A schematic representation of this type of RPV is shown in Figure 1.2. It can be seen that the total vessel is made of multiple rings which are welded together.

Figure 1.2: Schematic representation of a RPV like Doel 3 and Tihange 2 taken from FANC. [4] Each of these componenents is welded together to form the complete RPV. For Doel 3 and Tihange 2, the RPV is composed of 6 forged rings of which one has 6 nozzles, one inlet and one outlet nozzle for each of the three primary loops. All of these components are made in a low-alloy steel type SA-508 Class 3, except for the bottom plate and RPV head, which are made in SA-533 Grade B Class 1. [1] Both materials are low-alloy Mn-Mo-Ni steels. The inside of the RPV is cladded with a 7 mm thick stainless steel Type 308L/309L lining. The composition of each of these materials is given in Table 1.1. The total height of the RPV including the RPV head and the bottom plate is 13.2 m and has an inner diameter of 4.0 m. [1]

7

1. Reactor pressure vessel

Table 1.1: Material composition in wt% of the different steels used for a RPV [2]. Specification (wt%) SA-508 Class 3 [2] SA-533 Grade B Class 1 [2] SS 308L [5] SS 309L [5] C ≤ 0 . 25 ≤ 0 . 25 ≤ 0 . 03 ≤ 0 . 03 Si ≤ 0 . 40 0 . 15 – 0 . 40 ≤ 1 . 00 ≤ 1 . 00 Mn 1 . 20 – 1 . 50 1 . 15 – 1 . 50 ≤ 2 . 00 ≤ 2 . 00 P ≤ 0 . 025 ≤ 0 . 035 ≤ 0 . 045 ≤ 0 . 045 S ≤ 0 . 025 ≤ 0 . 040 ≤ 0 . 030 ≤ 0 . 030 Ni 0 . 40 – 1 . 00 0 . 40 – 0 . 70 10 . 0 – 12 . 0 12 . 0 – 15 . 0 Cr ≤ 0 . 25 – 18 . 0 – 21 . 0 22 . 0 – 24 . 0 Cu ≤ 0 . 10 – – – Mo 0 . 45 – 0 . 60 0 . 45 – 0 . 60 – – V ≤ 0 . 05 – – –

8

1.4. Production of a reactor pressure vessel

1.4 Production of a reactor pressure vessel The production of different rings of the RPV is done in multiple successive steps. Due to the limited size of the RPV for a PWR (4 m), the rings can be produced as a whole, in contrast to RPVs for BWRs, which are larger. The starting point for the production of the RPV rings is a solidified ingot of the base material, SA-508 Class 3. From this ingot, a central “carrot” is drilled out. The diameter of this carrot depends on the amount of material necessary to produce the final product, in this case a ring. The remaining donut shaped material will be forged to obtain the proper dimensions of the ring. This is done with high power hydraulic presses, which can produce a force up to 15 MN. This process can be seen in Figure 1.3. [3] After forging, the rings are given a heat treatment. Next, there are some machining steps to obtain the correct final dimensions for the rings. Finally, different types of quality examinations are performed on the rings. These tests include non-destructive tests, e.g. ultrasonic testing and magnetic testing, and destructive tests on samples from the waste material, e.g. micrographic, mechanical and chemical analysis, conform to the ASME Code Section III regulations. [6] To complete the RPV, a cladding is weld-deposited to the different rings and the rings are welded together. Further quality examinations are performed before accepting the RPV.

Figure 1.3: Schematic representation of the forging process. The donut shaped ingot is shaped to the RPV ring dimensions.

The production of the RPV for Doel 3 and Tihange 2 was performed by multiple companies. Marrel Frères provided the plate material for the bottom cap and the vessel head. The base material for the rings was produced by Fiedrich KRUPP Hüttenwerke. KRUPP also pre-forged all the different parts of the RPV, except for the transition ring. The pre-forged parts, called blooms, were delivered to Rotterdamsche Droogdok Maatschappij (RDM) together with the ingot for the transition ring. At RDM, all of these parts were further forged to the required dimensions. RDM also 9

1. Reactor pressure vessel

performed the heat treatments and quality examinations. [6] The forged parts for the bottom side of the RPV, i.e. both core shells, the transition ring and the bottom plate, are delivered to Cockerill, a Belgian steel company. Also the vessel head and the transition flange were delivered to Cockerill. Cockerill performed the cladding and assembling of these parts. The remaining parts of the RPV, i.e. the vessel transition flange, the nozzle shell and the inlet and outlet nozzles, are delivered to Framatome, which was responsible for the cladding and assembly of these parts. Furthermore, Framatome performed the final assembly of both top and bottom of the RPV. [7]

10

Chapter 2 Findings in Doel 3 and Tihange 2

2.1 Introduction

According to the ASME Code Section XI, a 10-yearly examination of the RPV must be done. The ASME Code requires a surface examination by Visual Testing (VT) of the complete inner side of the RPV and a volumetric examination by Ultrasonic Testing (UT) of the weld lines between different rings of the RPV and the Heat Affected Zones (HAZ) around the welds. [8] The areas to be examined are indicated in Figure 2.1. Back in February 1999, an extra volumetric examination by UT was performed on the whole core shell, also called the beltline, of the RVP of the PWR of Tricastin 1 in France. This inspection resulted in the detection of some underclad defects. [10] These type of defects are formed during the cladding process. As the cladding is weld-deposited on the base material, the cooling after deposition will result in thermal stresses. These stresses can overcome the yield stress of the base material and result in cracks. The formed cracks are oriented perpendicular to the cladding- base material interface (Figure 2.2). [11] Due to their orientation, they will be subjected to both hoop and axial stresses (Figure 2.3). Temperature and pressure gradients over the RPV wall can result in significant stresses in the hoop and axial directions. [9] Therefore, the existence of underclad defects are a particular concern for the mechanical integrity of the RPV. As a result of the discovered underclad defects in Tricastin 1 and its possible detrimental consequences for the integrity of the RPV, Electrabel, the operator of the Belgian NPPs, performed UT inspections to find such underclad defects in the beltline of the Doel 3 RPV during the planned outage in June 2012. These UT inspections examined the first 30 mm from the inner pressure vessel surface, additional to the UT inspections according to the ASME Code mentioned above. [1] 11

2. Findings in Doel 3 and Tihange 2

Figure 2.1: Examinated zones in 10-yearly quality control of the RPV, according to the ASME Code Section XI. [8] The blue zones are examined by Visual Testing, while the red zones are examined by Ultrasonic Testing. [9]

Figure 2.2: Schematic representation of an underclad defect in the RPV wall. [12]

Figure 2.3: Stresses in a RPV wall. Hoop and axial stresses can be signifi- cant as a result of thermal and pressure gradients over the RPV wall. [9]

12

2.2. Flaw indications in Doel 3 and Tihange 2

2.2 Flaw indications in Doel 3 and Tihange 2 The UT inspections on the Doel 3 RPV in June 2012 were performed over a height of 4080 mm, starting from 3320 mm under the transition flange RPV up to 7400 mm, and cover the complete circumference of the RPV. The complete height of the core was covered by this examination. This inspection showed no presence of underclad defects over the complete investigated area. However, 158 indications of a different type of defect were found. These indications were found to be quasi-parallel to the RPV wall surface, were “almost circular” in shape, had a mean diameter of 15 mm with a maximum up to 30 mm and a flaw density up to 40 indications per dm 3 . However, as this inspection technique is only qualified for the detection of underclad defects, it is not reliable for the measurement of indications, which are different in shape, orientation and depth compared to underclad defects. [7] To overcome the limitations of the inspection method used before, a new UT inspection over the complete height of the RPV is performed in July 2012. This UT inspection is the same as used for the inspection of the welds and HAZ of the RPV performed on a 10-yearly basis and investigates the full thickness of the RPV wall. The so called “MIS-B”, Machine d’inspection en Service Belge , used is shown in Figure 2.4. [1] The inspection confirms the presence of a large amount of quasi-parallel indications, especially in the core shells of the RPV. The highest number of these indications were found in the lower core shell, where 7205 indications were detected. In the upper core shell 857 indications were detected. All indications are found in the base material, outside the weld regions, of which the major part is situated at a depth between 10 and 100 mm from the inner surface. Their distribution as a function of depth can be seen in Figure 2.5. [1] Similarly, the distribution as a function of depth and height is shown in Figure 2.6 [1]. The figure shows the detected indications over a cumulative angle of 20 ◦ , where the orange indications are the larger ones compared to the pink colored indications. One can see that the indications are found deeper in the RPV wall with increasing height, along a diagonal. The indications are interpreted to be parallel to the wall surface and slightly smaller than estimated from the first UT inspection, with a mean diameter between 8 and 10 mm, while some indications are measured to be up to 70 mm. [13, 14] A quantitative control of the UT inspection on independent samples in 2014 revealed that the detection parameters of the machine and the interpretation method of the signals might result in an underestimation of the real number of indications and their size. Therefore, the detection parameters were adapted in order to have a conservative measurement of the indications, i.e. to guaranty that the measured size of the indication is equal to or larger than its real size and that every indication is detected. A result of this adaptation is that closely neighboring indications might be detected as one big indication. [15] During the outage of Doel 3 in 2014, this improved UT inspection was performed. An approximate increase of 60% more indications were detected compared to the inspection in 2012, 1440 indications in the upper core shell and 11607 indications in the lower core shell. Together with this increase in number of indications, also the dimension of the indications was found to be larger. The average size of the indications was found to be 12 to 16 mm, while 13

2. Findings in Doel 3 and Tihange 2

Figure 2.4: Representation of the MIS-B machine used to perform the UT inspection on the RPV wall of Doel 3. [1]

Figure 2.5: Distribution of the detected indica- tions in the RPV wall of Doel 3 during the UT inspection performed in 2012. [1]

Figure 2.6: Distribution of the detected indications in the RPV wall of Doel 3, over a cumulated angle of 20 ◦ , as a function of depth and height during the UT inspection performed in 2012. The orange indications are measured to be larger compared to the pink indiations. The axes give the depth in the material in mm (x-axis) and the inverse height in the RPV in mm (y-axis). The indications are found deeper in the RPV wall for increasing height. [1]

14

2.3. Origin of the indications

the largest indications was measured being 179 mm large. In the mean time, these results are converted to the detection parameters used during the inspection in 2012, in order to be able to compare them and see any possible evolution of the indications over this period. It was concluded by Electrabel that the indications were stable and did not grow between both inspections. [14] For the examination of the Tihange 2 RPV, a similar timeline can be constructed. First UT inspection of the RPV was performed in September 2012 during a planned outage of the reactor. The inspection resulted in a detection of 1931 indications in the upper core shell and 80 indications in the lower core shell. Similar to the procedure in Doel 3, a new improved UT inspection was performed on the Tihange 2 RPV in 2014, resulting in 1901 indications in the upper core shell and 76 indications in the lower core shell. The decrease of the number of indications between both inspections is attributed to the merged detection of some closely neighbouring indications for the 2014 UT inspection. As for Doel 3, an increase in the mean indication size is observed going from 8 – 10 mm in 2012 to 14 – 16 mm in 2014 with maximum dimensions up to 155 mm. [16] Table 2.1 gives an overview of the number and dimension of the indications measured in the core shells of the Doel 3 and Tihange 2 RPV. Table 2.1: Number and size of indications measured in the core shells of the Doel 3 and Tihange 2 RPV by the UT inspection methods used in 2012 and 2014. The size of the indications is given in mm and expressed as dimension in X and Y direction. [7, 13, 14, 16]

Reactor Unit Core shell

number/size

2012 inspection 2014 inspection

number

857

1440

upper

average size [mm] (X–Y) maximum size [mm] (X–Y)

8.8 – 7.6

13.7 – 12.3 56.4 – 45.3

31.0 – 26.4

Doel 3 [7, 14]

number

7205

11607

lower

average size [mm] (X–Y) maximum size [mm] (X–Y)

9.6 – 7.6

16.0 – 12.7 179.0 – 72.3

67.9 – 38.4

number

1931

1901

upper

average size [mm] (X–Y) maximum size [mm] (X–Y)

9.8 – 7.9

14.8 – 13.8 154.5 – 70.9

38.0 – 25.4

Tihange 2 [13, 16]

number

80

76

lower

average size [mm] (X–Y) maximum size [mm] (X–Y)

10.2 – 9.3 27.4 – 19.1

15.5 – 15.4 33.1 – 27.6

2.3 Origin of the indications Lots of investigations have been carried out to find the origin of these indications. [1, 14, 16–22] These investigations pointed so called “hydrogen flakes” to be the root 15

2. Findings in Doel 3 and Tihange 2

cause of the defects.

2.3.1 Formation mechanism of hydrogen flakes The first cases of defects associated with hydrogen flakes date back to the beginning of the 20th century. After World War II, a lot of research was done related to the problem of hydrogen flaking. [23] It was found that the occurrence of hydrogen flakes is closely related to segregation in the ingot. It is generally known that solidification of a large ingot does not occur homogenously. Due to a difference in solubility of alloying elements and impurities in the liquid and solid phase, segregation will occur. These segregated areas are the latest to solidify and are enriched in these alloying elements, like carbon, and impurities, like hydrogen and sulphur. Furthermore, these areas contain the most inhomogeneities in the ingot. [24] A schematic representation of the macrosegregation is shown in Figure 2.7. [25]

Figure 2.7: Schematic representation of macro-segregation in a large ingot. The “+” indicate positive segregated area’s, whereas “-” indicate negative segregated area’s. [25] The exact mechanism for the formation of hydrogen flakes is not yet fully understood. However, most likely, the formation of hydrogen flakes is related to the difference in solubility for hydrogen between the solidified and non-solidified material. As one can see in Figure 2.8, the solubility of hydrogen varies strongly between the different phases. As the material used for the reactor pressure vessel walls is a low alloy ferritic steel, the material has to make all of the microstructural transitions shown in Figure 2.8. The transitions from the liquid to the ferritic δ -phase and 16

2.3. Origin of the indications

from the austenitic γ -phase to the ferritic α -phase are the most critical, as these transitions are accompanied by a large decrease in hydrogen solubility. [24] As the ingot solidifies, the hydrogen, together with the other impurities and alloying elements, will be enriched in the liquid phase. In a later stage, the γ - to α -phase transition will have a similar effect, increasing the concentration of impurities in the γ -phase. When this phase transition occurs for the last γ -phase areas, the newly formed α -phase can become supersaturated in hydrogen. As a result, the hydrogen atoms in the supersaturated regions will recombine at so called “trapping positions” to form molecular hydrogen gas, H 2 . The increasing internal pressure from the hydrogen gas together with transformation or residual stresses can result in the formation of brittle cracks, called “hydrogen cracks” or “hydrogen flakes”. [24] [23] Figure 2.9 shows a cross section of a hydrogen flake. The crack is thought to be initiated on the MnS precipitates, shown as the 3 dark grey areas.

Figure 2.8: Solubility of hydrogen in iron as a function of temperature. [24]

Figure 2.9: Light optical micrograph of a hydrogen flake found in a steam generator. The crack is found in the close neighborhood of MnS inclusions. These precipitates act as hydrogen traps and play a crucial role in the initiation of the hydrogen cracks. [24]

As mentioned above, it is the recombination of supersaturated hydrogen that results in the formation of hydrogen flakes. The complete flake formation occurs as a 17

2. Findings in Doel 3 and Tihange 2

stepwise process. After a first crack is formed by the recombined hydrogen gas, the pressure and stress close to the crack are relieved. Diffusion of hydrogen towards the free surface of the crack will lead to a new increase in the local stresses and pressure inside the crack and result in further growth of the crack. Again, more hydrogen will diffuse towards the crack and the process starts again. This continues until the material has reached an equilibrium between the dissolved hydrogen and the pressure inside cracks. The high concentration of hydrogen around the crack results in an embrittlement of the this material. [23] Therefore, the fracture surface of such a hydrogen flake typically has brittle fracture characteristics, e.g. a quasi-cleavage fracture. [26] Figure 2.10 shows a typical fracture surface of a hydrogen flake. Besides the brittle appearance of the fracture, one can also see the diagonal lines, running from the right top to the left bottom, indicating the stepwise growth of the crack. [27]

Figure 2.10: Micrograph of the typical appearance of the fracture surface of a hydrogen flakes. One can recognize a quasi-cleavage fracture structure together with diagonal lines indicating the stepwise growth of the crack. [27]

2.3.2 Parameters influencing the formation of hydrogen flakes It is clear from the above formation mechanism, that the hydrogen content is one of the major parameters influencing the formation of hydrogen flakes. Furthermore, (residual) stresses and the sensitivity of the microstructure also affect the probability for hydrogen flake formation. [24] The goal of this section is not to give an exhaustive list of all possible parameters that can influence the formation of hydrogen flakes, as every parameter which influences the local hydrogen concentration, the stresses or the microstructure will also affect the susceptibility to hydrogen flaking. However, the most important influencing factors are given here. As can be expected, the more brittle the microstructure, the more susceptible it is to hydrogen flaking. As a result, martensitic and bainitic phases must be 18

2.4. Conclusion

avoided in order not have a sensitive microstructure. The formation of these phases is mainly determined by the cooling rate from the austenitization temperature. [24] Similarly, the higher the toughness of the material, the less susceptible it is to hydrogen flaking. Higher inclusion volume fractions generally result in a decreased toughness and therefore one would expect it to result in an increased susceptibility to hydrogen flaking. However, as these inclusions, mainly MnS precipitates, also act as hydrogen traps, they also influence the local hydrogen concentration. There are numerous reports referring to these hydrogen traps as the initiation point for hydrogen flakes. [23, 24, 26, 28, 29] However, simply reducing the number of these strong traps is not a solution as in that case a larger amount of dissolved hydrogen will be accumulated over a lesser volume of hydrogen traps, resulting in higher local hydrogen recombination. Therefore, there is a more complex relation between the inclusion volume fraction and the susceptibility of the material. [23] Also the grain size can affect the probability of hydrogen flaking. As larger grains result in a lower material toughness, this will again increase the susceptibility. On top of that, larger grains mean that the total grain boundary surface area will decrease and therefore result in an increase of the hydrogen concentration in the grain boundary, since these grain boundaries can also act as hydrogen traps as explained later. [23] Of course the hydrogen content in the material is one of the most influencing parameters for hydrogen flaking. In literature, there is consensus on the existence of a threshold hydrogen concentration for hydrogen flaking. This threshold is determined by the materials microstructural susceptibility and the presence of hydrogen traps in the material. In the 1970’s and 1980’s, this threshold was thought to be 1.5 to 2 ppm, however in this period hydrogen flakes were found in ultra clean steels, containing less then 20 ppm sulfur and 20 ppm oxygen and lower hydrogen concentrations. [23, 24] Furthermore, it is difficult to determine such a threshold as it is the local hydrogen concentration which determines the formation of hydrogen flakes. Therefore, a different degree of segregation can result in a different threshold for the global hydrogen concentration in the material. Fruehan [23] tried to visualize the threshold hydrogen concentration in steel as a function of the sulphur content (Figure 2.11). Furthermore, there is the existence of local stresses which is an important pa- rameter. In the absence of stresses, there will be no cracking. These stresses can have a different origin, e.g. residual stresses, transformation stresses, the pressure by buildup of hydrogen gas. These transformation stresses can become very high due to the γ to α transformation in the material. When the total force is high enough for the initiation or propagation, a hydrogen flake will form. [23, 24] 2.4 Conclusion Multiple arguments have lead to pointing to hydrogen flakes as the origin for the indications found. One of the main reasons is the clustering of the indications in the macrosegregated areas. One can see that the A-segregations in Figure 2.7 correspond to the position of the found indications (Figure 2.6). This is completely as expected 19