Safety Investigation of in-box LOCA for DEMO Reactor: Experiments and Analyses

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UNIVERSITÀ DI PISA

DOTTORATO DI RICERCA IN INGEGNERIA INDUSTRIALE

Curriculum in Ingegneria Nucleare e Sicurezza Industriale

Ciclo XXIX

Safety Investigation of in-box LOCA for DEMO Reactor:

Experiments and Analyses

Supervisors

Author

Prof. Ing. Nicola FORGIONE

Ing. Marica EBOLI

Dr. Ing. Alessandro DEL NEVO

Dr. Werner MASCHEK

Coordinator of the Ph.D. Program

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ABSTRACT

The aim of this Ph.D. thesis work is the safety investigation of in-box Loss Of Coolant Accident for the Water Cooled Lithium Lead breeding blanket concept in DEMO reactor. The research work, conducted at ENEA C.R. Brasimone, is a crosscutting activity carried out under the international framework of the EUROfusion Consortium Breeding Blanket Design and Safety Projects.

The research activity starts with a comprehensive literature review. The interaction between heavy liquid metal and water is investigated with main focus on the phenomena relevant to the safety of the WCLL breeding blanket. Past experiments and numerical activities have been identified and reviewed. The study highlights the lack of qualified and reliable numerical codes able to predict the phenomena involved in such postulated accidental scenario. Starting from the outcomes of the literature review, main and innovative goal of the Ph.D. research activity is twofold: 1) setting-up a qualified computer code for deterministic safety analysis of the WCLL BB in-box LOCA, also in support to the design of the breeding blanket and its connected systems, 2) validating the code against experimental data available in literature or provided by the new LIFUS5/Mod3 campaign specifically designed for code validation purposes.

The first objective is fulfilled by the PbLi/water chemical reaction model implementation in SIMMER-III and SIMMER-IV codes and the successfully verification and validation processes. A methodology for code validation is established based on a three-step procedure: 1) the initial condition results, 2) the reference calculation results, and 3) the results from sensitivity analyses. The methodology is applied to all available LIFUS5 tests. The post-test analyses highlight open issues of test execution and of experimental data, as well as code limitations and capabilities. The qualitative accuracy evaluation is performed through a systematic comparison between experimental and calculated time trends based on the engineering analysis, the resulting sequence of main events, and the identification of phenomenological windows and of relevant thermo-hydraulic aspects. Finally, the accuracy of the code prediction is evaluated from quantitative point of view by means of selected, widely used, figures of merit.

Second key point of the research activity is the design and the follow up of a new experimental campaign in LIFUS5/Mod3. The experimental campaign is unique and innovative, focused on chemical reaction code model validation, thanks to the generation of meaningful, qualified and reliable data with well-known initial and boundary conditions. Supporting and pre-tests analyses by SIMMER-III and RELAP5/Mod3.3 codes are executed to provide useful data (e.g. injection pressure, water mass flow rate, volume of cover gas, temperature map, hydrogen measurement line) for the final design of the facility configuration and instrumentation choice.

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NOMENCLATURE

BB Breeding Blanket

BIC Boundary and Initial Condition BLAST BLAnket Safety Test

BSS Back Supporting Structure

BZ Breeding Zone

CAD Computer-Aided Design

CCI Coolant Coolant Interaction CDA Core Disruptive Accident

CEA French Atomic Energy Commission

CP Continuous Phase

CR Research Center

DAQ Data Acquisition System DCLL Dual Cooled Lithium Lead DEMO DEMOnstration Power Plant

DICI Department of Civil and Industrial Engineering DP Differential Pressure transducer

DSA Deterministic Safety Analysis

E Excellent

EC European Commission

EFDA European Fusion Development Agreement

ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development

EoPh End of Phase

EOS Equation of State

EoT End of Transient

EU European Union

FCI Fuel Coolant Interaction

FFTBM Fast Fourier Transform Based Method

FW First Wall

HCLL Helium Cooled Lithium Lead HCPB Helium Cooled Pebble Bed

HLM Heavy Liquid Metal

HTC Heat Transfer Coefficient

IAEA International Atomic Energy Agency I&B Initial and Boundary

IFA InterFacial Area

IPA Integral PArameter

ITER International Thermonuclear Experimental Reactor

JNC Japan Power Reactor and Nuclear Fuel Development Corporation JRC Joint Research Centre

LOCA Loss of Coolant Accident LMFR Liquid Metal Fast Reactor LWR Light Water Reactor

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PC Absolute Pressure Transducer P&ID Plan and Instrumentation Diagram PHTS Primary Heat Transfer System

PhW Phenomenological Window

PR Poloidal-Radial

PT Dynamic Pressure Transducer PWP Programmatic Work Package PWR Pressurized Water Reactor

R Reasonable

R&D Research and Development

RELAP REactor Loss of coolant Analysis Program RTA Relevant Thermal-hydraulic Aspect SAE Safety and Environmental

SC Superconducting Coils SET Separate Effect Test SoT Start of Transient

STH System Thermal Hydraulic SVP Single Valued Parameter

TC Thermocouple

TR Toroidal-Radial

TSE Time Sequence of Events

U Unqualified

UNIPI University of Pisa

US United States

VV Vacuum Vessel

V&V Verification and Validation WCLL Water Cooled Lithium Lead

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LIST OF FIGURES

Fig. 1.1 – Flow chart of the Ph.D. research activity. ... 4

Fig. 2.1 – Main DEMO tokamak systems (Ref [1]). ... 7

Fig. 2.2 – DEMO CAD model 2015: 3D isometric view (Ref. [3]) and WCLL BB DEMO sector piping system (Ref. [2])... 9

Fig. 2.3 – WCLL BB module internal structure (left) and track of baffle plates and TR stiffening plates (right) [2]. ... 10

Fig. 2.4 – Layout of BZ cooling tubes on an elementary cell (top) and section of the single module on toroidal-radial plane (bottom) [2]. ... 11

Fig. 2.5 – Contact modes between hot and cold fluid (liquid metal-coolant) ... 13

Fig. 2.6 – Lithium-lead alloy/water interaction (Refs. [7], [26]). ... 16

Fig. 2.7 – H2 production rate for H2O-Pb83Li17 reaction (Ref. [28]). ... 17

Fig. 2.8 – Pressure and temperature trends (left pinj=0.1 MPa, Tsub=75°C, right pinj=2.0 MPa, Tsub=75°C). Ref. [8] ... 17

Fig. 2.9 – Specific mechanical energy as a function of the subcooling (Ref. [8]). ... 18

Fig. 2.10 – BLAST Test#5: identification of the different phenomenological windows in the reaction vessel pressurization (Ref. [9]) ... 19

Fig. 2.11 – Reaction and expansion vessels pressure trends of BLAST experiments (Ref. [9]). ... 20

Fig. 2.12 – LIFUS5 experimental campaign: experimental trends (Refs. [18], [19]). ... 21

Fig. 2.13 – SIMMER-III code simulations of first peak of BLAST tests (Ref. [16]). ... 23

Fig. 2.14 – Cigalon model and its application to BLAST experiments (Ref. [16])... 23

Fig. 2.15 – SIMMER-III code results vs BLAST Test#5 experimental data: pressure trends in reaction and expansion vessels (Ref. [19]). ... 24

Fig. 2.16 – SIMMER-III code results vs LIFUS5 experimental data: pressure trends in reaction and expansion vessels (Ref. [18]). ... 25

Fig. 3.1 – Overall framework of SIMMER-III code ... 31

Fig. 3.2 – Pool flow regime map in SIMMER-III. ... 34

Fig. 3.3 – Channel flow regime map in SIMMER-III. ... 34

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Fig. 3.7 – SIMMER-III – Amount of reactants and products in case of Eq.(3.7). ... 39

Fig. 3.8– Evaluation of the expected and predicted mass of chemical products for Eq. (3.8). ... 40

Fig. 3.9 – SIMMER-III – Amount of reactants and products in case of Eq.(3.8). ... 40

Fig. 4.1 – Methodology for SIMMER code validation. ... 49

Fig. 4.2 – LIFUS5 facility: P&ID ... 52

Fig. 4.3 – LIFUS5 facility: drawing of S1 reaction vessel (part 1). ... 54

Fig. 4.4 – LIFUS5 facility: drawing of S1 reaction vessel (part 2). ... 55

Fig. 4.5 – LIFUS5 facility: drawing of the expansion tubes, the 3’’ connecting pipe and the expansion vessel S5 ... 55

Fig. 4.6 – LIFUS5 facility: cross section of the reaction vessel with the dimensions of the U-tube mock-up and the position of the TC (Tests#3-5). ... 56

Fig. 4.7 – LIFUS5 facility: cross section of the reaction vessel with the dimensions of the U-tube mock-up and the position of the TC (Tests#6-8). ... 56

Fig. 4.8 – SIMMER-III modeling of LIFUS5 facility and focus on reference mesh cells. ... 61

Fig. 4.9 – Macro-regions SIMMER-III BFCAL modeling of LIFUS5 facility. ... 62

Fig. 4.10 – LIFUS5 Test#8: V14 opening and closing signal. ... 70

Fig. 4.11 – LIFUS5 Test#8: pressure in reaction vessel S1, in expansion vessel S5, and in the injection line. ... 70

Fig. 4.12 – LIFUS5 Test#8: pressure in reaction vessel S1, in expansion vessel S5, and in the injection line. Focus on 0 - 2000 ms. ... 71

Fig. 4.13 – LIFUS5 Test#8: three characteristic zones of the interaction and position of thermocouples. ... 71

Fig. 4.14 – LIFUS5 Test#8: temperature measurements in different zones of reaction. ... 72

Fig. 4.15 – L5T8 RUN0: pressure in S1 at different level PK(I=6, J=4-7-10), in S5 PK(5,47), and imposed (Ref_BIC), and comparison with experimental data. ... 81

Fig. 4.16 – L5T8 RUN0: pressure in S1 at different level PK(I=6, J=4-7-10), in S5 PK(5,47), and imposed (Ref_BIC), and comparison with experimental data. Zoom on 0 - 3500 ms. .. 81

Fig. 4.17 – L5T8 RUN0: Pressure at the injector (PK) calculated by the code and imposed pressure as boundary condition (Ref_BIC). ... 82

Fig. 4.18 – L5T8 RUN0: initial TH condition of injected water (pressure PK and temperature TLK3). Zoom on 200 – 600 ms. ... 82

Fig. 4.19 – L5T8 RUN0: calculated volume fraction of vapor (ALPGK), and water (ALPLK3) at the injector. Zoom on 200 - 600 ms. ... 83

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Fig. 4.20 – L5T8 RUN0: pressure (PK) and water mass flow rate (TFLUXN) at the injector. Zoom on 0 – 12 s. ... 83 Fig. 4.21 – L5T8 RUN0: calculated water mass flow rate at the injector (TFLUXN) versus total hydrogen generation (MASFPG). Zoom on 0 – 12 s. ... 84 Fig. 4.22 – L5T8 RUN0: calculated hydrogen generation (MASFPG) in different macro-regions (S1, S5, expansion tubes and mock-up structures). ... 84 Fig. 4.23 – L5T8 RUN0: PbLi temperature trends (TLK1) in S1 and S5 at different rank and axial position (left column) compared with experimental data (right column). ... 85 Fig. 4.24 – L5T8 RUN0: mass flow rate of PbLi (BFLUXF) through expansion tubes and towards S5 expansion vessel. ... 86 Fig. 4.25 – L5T8 RUN0: mass flow rate of gas (BFLUXG) in the expansion tubes and towards S5 expansion vessel. ... 86 Fig. 4.26 – L5T8 RUN0: gas volume (GVOL) in different macro-regions (S1, S5, expansion tubes, mock-up structures). ... 87 Fig. 4.27 – L5T8 RUN0: total mass of PbLi (MASF3). ... 87 Fig. 4.28 – L5T8 RUN0: mass of water (MASN1) in different macro-regions (S1, S5, expansion tubes, mock-up structures). ... 88 Fig. 4.29 – L5T8 RUN0: mass of vapor (MASN2) in different macro-regions (S1, S5, expansion tubes, mock-up structures). ... 88 Fig. 4.30 – L5T8 RUN0: SIMMER-III material composition, screenshots at different times. ... 89 Fig. 4.31 – Sensitivity analyses: influence of temperature. Calculated pressure in S1 PK(6,7) and in S5 PK(5,47) compared with experimental data (zoom). ... 95 Fig. 4.32 – Sensitivity analyses: influence of temperature. Mass flow rate at the injector (TFLUXN) and total hydrogen production (MASFPG). ... 95 Fig. 4.33 – Sensitivity analyses: influence of chemical parameters. Calculated pressure in S1 PK(6,7) and in S5 PK(5,47) compared with experimental data (zoom). ... 96 Fig. 4.34 – Sensitivity analyses: influence of chemical parameters. Mass flow rate at the injector (TFLUXN) and total hydrogen production (MASFPG). ... 96 Fig. 4.35 – Sensitivity analyses: influence of gas volume. Calculated pressure in S1 PK(6,7) and in S5 PK(5,47) compared with experimental data (zoom). ... 97 Fig. 4.36 – Sensitivity analyses: influence of gas volume. Mass flow rate at the injector

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Fig. 4.38 – Sensitivity analyses: influence of nodalization. Mass flow rate at the injector (TFLUXN) and total hydrogen production (MASFPG). ... 98 Fig. 4.39 – L5T8 RUN0 “Reference”: FFTBM results for pressure in reaction (S1) and expansion (S5) vessels. ... 101 Fig. 4.40 – Values for the selected figures of merit: DEV2_SIGN for S1 and S5 pressures. 102 Fig. 4.41 – Values for the selected figures of merit: DEV2_ABS for S1 and S5 pressures. . 102 Fig. 4.42 – Values for the selected figures of merit: DEV2_RMS for S1 and S5 pressures. 103 Fig. 4.43 – Values of AA function of WF (FFTBM) evaluated at cut frequency of 100 Hz for S1 pressure. ... 103 Fig. 4.44 – Values of AA function of WF (FFTBM) evaluated at cut frequency of 100 Hz for S5 pressure. ... 104 Fig. 4.45 – LIFUS5 SIMMER-IV model. ... 105 Fig. 4.46 – SIMMER-IV L5T8 RUN0: calculated pressure (PK) in S1, in S5, and imposed (Ref_BIC), and comparison with experimental data. ... 107 Fig. 4.47 – SIMMER-IV L5T8: water mass flow rate at the injector (TFLUXN) and total hydrogen production (MASFPG). ... 107 Fig. 4.48 – SIMMER-IV L5T8 RUN0: PbLi mass flow rate (BFLUXF) through expansion tubes and towards S5. ... 107 Fig. 4.49 – SIMMER-IV L5T8 RUN0: gas mass flow rate (BFLUXG) through expansion tubes and towards S5. ... 108 Fig. 4.50 – SIMMER-IV L5T8 RUN0: water mass (MASN1) in different macro-regions. .... 108 Fig. 4.51 – SIMMER-IV L5T8 RUN0: vapor mass (MASN2) in different macro-regions. .... 108 Fig. 4.52 – SIMMER-IV L5T8 RUN0: gas volume (GVOL) in different macro-regions. ... 108 Fig. 4.53 – SIMMER-IV L5T8 RUN0: PbLi temperature trends (TLK1) in S1 and S5 at different rank and axial position (left column) compared with experimental data (right column). ... 109 Fig. 4.54 – L5T3 Run0: pressure in S1 at different level PK(I=6, J=4-8-11), in S5 PK(6,54), and comparison with experimental data. ... 117 Fig. 4.55 – L5T3 Run0: pressure in S1 at different level PK(I=6, J=4-8-11), in S5 PK(6,54), and comparison with experimental data (zoom). ... 117 Fig. 4.56 – L5T3 Run0: calculated water mass flow rate at the injector (TFLUXN) versus total H2 generation (MASFPG). ... 117

Fig. 4.57 – L5T4 Run0: pressure in S1 at different level PK(I=6, J=4-7-10), in S5 PK(5,47), and comparison with experimental data. ... 117

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Fig. 4.58– L5T4 Run0: pressure in S1 at different level PK(I=6, J=4-7-10), in S5 PK(5,47),

and comparison with experimental data (zoom). ... 117

Fig. 4.59 – L5T4 Run0: calculated water mass flow rate at the injector (TFLUXN) versus total H2 generation (MASFPG). ... 117

Fig. 4.60 – L5T5 Run0: pressure in S1 at different level PK(I=6, J=4-7-10), in S5 PK(5,47), and comparison with experimental data ... 118

Fig. 4.61 – L5T5 Run0: pressure in S1 at different level PK(I=6, J=4-7-10), in S5 PK(5,47), and comparison with experimental data (zoom). ... 118

Fig. 5.1 – S1_B reaction vessel and penetrations. ... 128

Fig. 5.2 – S4_B1 storage vessel and penetrations. ... 128

Fig. 5.3 – S4_B2 storage vessel and penetrations. ... 129

Fig. 5.4 – P&ID of LIFUS5/Mod3 facility ... 130

Fig. 5.5 – LIFUS5/Mod3 facility synoptic. ... 131

Fig. 5.6 – SIMMER-III modelling of S1_B reaction vessel. ... 132

Fig. 5.7 – Influence of gas volume - pressure trends (PK) in S1_B reaction vessel. ... 136

Fig. 5.8 – Influence of gas volume – water mass flow rate (TFLUXN) at the injector device. ... 136

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Fig. 5.11 – Influence of injection pressure - water mass flow rate (TFLUXN) at the injector

device. ... 136

Fig. 5.12 – Influence of injection pressure - calculated hydrogen generation(MASFPG) in S1_B reaction vessel... 136

Fig. 5.13 – Influence of TS configuration - pressure trends (PK) in S1_B reaction vessel. 137 Fig. 5.14 – Influence of TS configuration - water mass flow rate (TFLUXN) at the injector. ... 137

Fig. 5.15 – Influence of TS configuration - calculated hydrogen generation (MASFPG) in S1_B reaction vessel... 137

Fig. 5.16 – Influence of orifice diameter - pressure trends (PK) in S1_B reaction vessel. . 137

Fig. 5.17 – Influence of orifice diameter - water mass flow rate (TFLUXN) at the injector. ... 137

Fig. 5.18 – Influence of orifice diameter - calculated hydrogen generation (MASFPG) in S1_B reaction vessel... 137

Fig. 5.19 – Influence of temperature - pressure trends (PK) in S1_B reaction vessel. ... 138

Fig. 5.20 – Influence of temperature - water mass flow rate (TFLUXN) at the injector. .... 138

Fig. 5.21 – Influence of temperature – calculated hydrogen generation (MASFPG) in S1_B reaction vessel... 138

Fig. 5.22 – LIFUS5/Mod3 facility: S1_B water injection system. ... 141

Fig. 5.23 – Scheme of protective cap test section. ... 141

Fig. 5.24 – Pressure time trend for T#10. ... 142

Fig. 5.25 – SIMMER-III pre-tests: material composition and temperature maps during the transient. ... 144

Fig. 5.26 – LIFUS5/Mod3: design of the test section and arrangement of the TC in S1_B. 145 Fig. 5.27 – H2 system line numerical model by RELAP5/Mod3.3. ... 146

Fig. 5.28 – Relap5/Mod3.3 pre-test case#11: pressure trends. ... 148

Fig. 5.29 – Relap5/Mod3.3 pre-test case#11: volumetric flow rate. ... 148

Fig. 5.30 – Relap5/Mod3.3 pre-test case#11: volumetric flow rate (zoom). ... 148

Fig. 5.31 – Relap5/Mod3.3 pre-test case#11: temperature trends. ... 148

Fig. 5.32 – P&I of the hydrogen system measurement. ... 149

Fig. 6.1 – Comparison between CFD and SIMMER-IV model of WCLL-BB single breeding unit (water tubes in evidence). ... 152

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Fig. 6.3 – case a) Water pressure (PK) and temperature (TLK3) at the tube inlet. ... 156

Fig. 6.4 – case a) Water velocity at the tube inlet (WK2) and across the rupture (UK2). .. 156

Fig. 6.5 – case a) Pressure trends (PK) inside the module. ... 156

Fig. 6.6 – case a) Pressure trends (PK) inside the module (zoom). ... 156

Fig. 6.7 – case a) Water mass flow rate and integral value of injected water. ... 156

Fig. 6.8 – case a) H2 production (MASFPG) inside the module. ... 156

Fig. 6.9 – case a) PbLi temperatures (TLK1) around the rupture. ... 156

Fig. 6.10 – case a) Max H2O (TLK3) and PbLi (TLK1) temperatures inside the module. ... 156

Fig. 6.11 – case a) Material composition in different sections. ... 157

Fig. 6.12 – case b) Water pressure (PK) and temperature (TLK3) at the tube inlet. ... 158

Fig. 6.13 – case b) Water velocity at the tube inlet (WK2) and across the rupture (UK2). ... 158

Fig. 6.14 – case b) Pressure trends (PK) inside the module. ... 158

Fig. 6.15 – case b) Pressure trends (PK) inside the module (zoom). ... 158

Fig. 6.16 – case b) Water mass flow rate and integral value of injected water. ... 158

Fig. 6.17 – case b) H2 production (MASFPG) inside the module and in the H2O tube. ... 158

Fig. 6.18 – case b) PbLi temperatures (TLK1) around the rupture. ... 158

Fig. 6.19 – case b) Max H2O (TLK3) and PbLi (TLK1) temperatures inside the module. ... 158

Fig. 6.20 – case b) Material composition in different sections. ... 159

Fig. 6.21 – case c) Water pressure (PK) and temperature (TLK3) at the tube inlet. ... 160

Fig. 6.22 – case c) Water velocity at the tube inlet (WK2) and across the rupture (UK2). 160 Fig. 6.23 – case c) Pressure trends (PK) inside the module. ... 160

Fig. 6.24 – case c) Pressure trends (PK) inside the module (zoom)... 160

Fig. 6.25 – case c) PbLi temperature (TLK1) around the rupture. ... 160

Fig. 6.26 – case c) Max H2O (TLK3) and PbLi (TLK1) temperatures inside the module. ... 160

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LIST OF TABLES

Tab. 2.1 – Main differences between FCI and CCI... 14

Tab. 4.1 – LIFUS5 facility: reaction vessel main characteristics. ... 53

Tab. 4.2 – S2, S3 and S5 main characteristics. ... 53

Tab. 4.3 – Location of pressure transducers. ... 54

Tab. 4.4 – LIFUS5 reference model by SIMMER-III: correspondence of main dimensions. 63 Tab. 4.5 – LIFUS5 Test #: “Reference” input deck. ... 66

Tab. 4.6 – L5T8 RUN0: comparisons between measured and calculated initial condition results ... 78

Tab. 4.7 – L5T8 RUN0: Thermal-hydraulic condition of water at the injection time. ... 78

Tab. 4.8 – L5T8 RUN0: phenomenological analysis. ... 78

Tab. 4.9 – L5T8 RUN0: resulting sequence of main events. ... 78

Tab. 4.10 – L5T8 RUN0: parameters characterizing the RTA. ... 80

Tab. 4.11 – LIFUS5 Test#8: sensitivity calculation matrix ... 91

Tab. 4.12 – L5T8 RUN0: results of accuracy quantification for the selected parameters. 100 Tab. 4.13 – L5T8 RUN0: summary of results obtained by the application of FFT-BM. ... 100

Tab. 4.14 – Accuracy evaluation for sensitivity analyses. ... 101

Tab. 4.15 – LIFUS5 experimental Tests# relevant features (Refs. [1]-[13]) ... 110

Tab. 5.1 – Tanks S1_B, S4_B1, and S4_B2 geometrical features. ... 126

Tab. 5.2 – Penetrations on S1_B. ... 127

Tab. 5.3 – S4_B1 penetrations. ... 127

Tab. 5.4 – S4_B2 penetrations. ... 127

Tab. 5.5 – SIMMER-III S1_B model: correspondence of main dimensions. ... 133

Tab. 5.6 – Sensitivity pre-tests matrix and main parameters characterizing the calculations. ... 135

Tab. 5.7 – Test Matrix. ... 139

Tab. 5.8 – Data characterizing the protective cap tests. ... 142

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1 INTRODUCTION

The world demand for energy is set to increase significantly in the next decades, spurred by economic growth, especially in developing countries. Nevertheless, to prevent the most severe impacts of climate change, the international community has agreed to keep the global warming below 2°C compared to temperature in pre-industrial times [1].

In order to reach this goal, the European Council reconfirmed the European long-term policy of reducing greenhouse gas emission by 80-95% by 2050 compared to 1990 [2]. The EU Research and Innovation programme, Euratom HORIZON 2020, represents the first target of this roadmap (20% reduction greenhouse gas emission compared to 1990, 20% energy saving and 20% of renewable energies in the total energy mix [3]). In this international context, as an established source of low-carbon energy, nuclear power plays a key role in achieving the goal of reducing greenhouse gas emission. Fusion has advantages that ensure sustainability and security of supply: fuels are widely available and virtually unlimited, no production of greenhouse gases, intrinsically safe as no chain-reaction is possible, environmentally responsible with a proper choice of materials for the reaction chamber indeed radioactivity decays in a few tens of years and at around 100 years after the reactor shutdown all the materials can be recycled in a new reactor. At the beginning of 2012, the European Commission requested EFDA to prepare a technical roadmap to fusion electricity by 2050. In November, EFDA published “Fusion

Electricity – A roadmap to the realisation of fusion energy” [4] that set out a strategic vision

to demonstrate the generation of electrical power by a Demonstration Fusion Power Plant (DEMO) by 2050. 28 European countries signed the agreement to work on an energy source for the future. The roadmap has been developed within a goal-oriented approach articulated in eight different missions:

1) Demonstrate plasma regimes of operation (based on the tokamak configuration); 2) Demonstrate a heat exhaust system capable of withstanding the large load of DEMO; 3) Develop materials that withstand large 14 MeV neutron fluence without degrading

their physical properties;

4) Ensure tritium self-sufficiency through technological solution for the breeding blanket; 5) Implement the intrinsic safety features of fusion into the design of DEMO following the

experience gained with ITER;

6) Produce an integrated DEMO design supported by targeted R&D activities;

7) Ensure the economic potential of fusion by reducing the DEMO capital costs and developing long-term technologies;

8) Bring the stellarator line to maturity.

For each mission the critical aspects for reactor application, the risks and risk mitigation strategies, the level of readiness now and after ITER and the gaps in the programme have

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1.1 International framework

The cooperation between DICI (Department of Industrial and Civil Engineering) of University of Pisa and the Experimental Engineering Division (FSN-ING) of ENEA C.R. Brasimone is long lasting and EUROfusion has strengthened this cooperation. The FSN-ING division manages relevant and innovative experimental laboratories and facilities in support of nuclear R&D with several facilities related to fusion engineering development, such as LIFUS5, LIFUS6, IELLLO, HeFUS3, TRIEX, THALLIUM, HYDREX, PERI II.

The Ph.D. research activity is conducted at ENEA C.R. Brasimone in the international framework of the BB design Project, implemented under the EUROFusion Consortium. In particular, it is a cross cutting activity falling into specific tasks, i.e. “WPBB5.4.2 PbLi water reaction” and “WPBB3 WCLL Breeding Blanket design”, and it is connected with “WPSAE”.

1.2 Objectives of the research activity and expected technological

advancements

The renewed interest for the Water Cooled Lithium Lead breeding blanket concept has focused on R&D activity connected with the potential interaction between lithium-lead and water. In this framework, the research activity is devoted to perform deterministic safety analysis of the WCLL BB in-box LOCA. Currently, no code is able to deal contemporarily with phenomena connected to thermodynamic interaction and to chemical reaction. Such numerical code shall be multi-fluid and multi-phase, shall manage the thermodynamic interaction among the fluids, and shall include the exothermic chemical reaction between lithium-lead and water, generating oxides and hydrogen. To address this objective, the steps are:

 reviewing and summarizing studies and experiments carried out in the past to address the lithium-lead/water reaction;

 setting up a code for WCLL safety analysis investigation, implementing the empirical model to simulate the exothermic chemical reaction and the hydrogen production;  designing and implementing SET experiments specifically addressed for code

validation with suitable and reliable instrumentation. The experimental campaign will provide qualified data at well-known initial and boundary conditions;

 validating the simulation code/s (SIMMER-III/-IV, Refs. [9], [10]) for the PbLi/water chemical reaction by means of code assessment methodology constituted by three-step analysis; initial conditions, reference post-test analyses results, and sensitivity calculations results are compared with experimental data, available in literature or provided by new LIFUS5/Mod3 experimental campaign.

In Fig. 1.1, the flow chart of the performed Ph.D. research activity is reported. The safety issues connected with PbLi/water interaction can be prevented by adequate WCLL components design (e.g. the adoption of double wall tubes, detection systems, etc…) and mitigated by active or passive systems. However, it is fundamental to evaluate the possible consequences of a postulated PbLi/water interaction due to in-box LOCA

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accident, in order to assure the mechanical resistance of the component itself, or in the worst case, to avoid jeopardizing the entire system. The research activity focused on the latter aspect. Green background highlights the literature survey, interpretation of studies, and status of code applications connected with PbLi/water interaction. Red background highlights the numerical activities, focusing on the implementation and V&V of the chemical models in SIMMER code. The experimental activities, which provided qualified and reliable data for code validation, are identified by light blue boxes. Beyond the Ph.D. activity, but connected with it as future perspective of the research work, there are two boxes identified by number 2 and number 3. They are focused on the experimental and numerical activities of the deterministic safety analysis of the in-box LOCA in the WCLL. The first one will be addressed to perform an experimental campaign implementing a WCLL BB mock-up and considering scale factors of main thermo-hydraulic parameters. The latter aspect, which will involve also the qualified SIMMER code, will be focused on setting-up of procedure and chain of codes for the DSA of WCLL BB.

1.3 Structure of the thesis

This thesis is structured in seven sections. The first section is the introduction describing the background information, the framework of the research activity and its objectives, while § 7 presents conclusions and future perspectives.

The issues and phenomena connected with the in-box Loss of Coolant Accident in the WCLL BB are described and discussed in § 2, highlighting the current status of numerical code activities.

Then, the SIMMER code features and the implemented modifications for fusion application are described in § 3. The verification of the chemical reaction model between PbLi and water is presented in the same section.

The code assessment process and the methodology for SIMMER code validation is reported in § 4. This consists of performing post-test analyses against available experimental campaign on LIFUS5 facility, highlighting capabilities and deficiencies of the code, and open issues relative to the available experimental data.

The design of LIFUS5/Mod3 facility and the experimental campaign, suitable and specifically addressed to SIMMER code validation, is described in § 5.

Finally, safety investigation on postulated transient of WCLL in-box LOCA and preliminary results applying the validated SIMMER code are reported in § 6.

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1.4 References

[1] http://ec.europa.eu/clima/policies/strategies

[2] EC, A Roadmap for moving to a competitive low carbon economy in 2050,

http://ec.europa.eu/clima/policies/strategies/2050

[3] https://ec.europa.eu/programmes/horizon2020

[4] F. Romanelli et al., Fusion Electricity – A roadmap to the realisation of fusion energy, EFDA, November 2012, ISBN 978-3-00-040720-8.

[5] AA.VV., Work Plan for the Implementation of the Fusion Roadmap in 2014 – 2018, EFDA, October 2013.

[6] AA.VV., Annex 15 - Work Packages, EFDA, Issue 2, November 2012.

[7] L.V. Boccaccini, Breeding Blanket Project - Project Management Plan (PMP), February 2014, https://idm.euro-fusion.org/?uid=2MDAMJ

[8] N. Taylor, Safety and Environmental Project – Project Management Plan (PMP), March 2014, https://idm.euro-fusion.org/?uid=2L9WGS

[9] S. Kondo, W. Maschek, et al., Current status and validation of the SIMMER-III LMFR

safety analysis code, 7th International Conference on Nuclear Energy, ICONE-7249,

1999.

[10] AA.VV., SIMMER-III (Version 3.F) Input Manual, O-arai Engineering Center, Japan Nuclear Cycle Development Institute, May 2012.

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2 PHENOMENA CONNECTED WITH WCLL BB IN-BOX LOCA

2.1 Characteristics of the DEMO design

DEMO (DEMOnstration Power Plant) is a generic name for proposed magnetic confinement nuclear fusion power plants. The goal of DEMO will be to produce at least 2 GW of fusion power (Ref. [1]).

Fig. 2.1 – Main DEMO tokamak systems (Ref [1]). The tokamak structure is composed of three main systems:

1) The vacuum vessel (VV) is a torus-shaped double-walled pressure vessel. It provides the primary vacuum and shields the magnet system from neutrons. It supports the in-vessel components and other systems:

 Breeding blanket (BB) with first wall (FW). The actively cooled FW withstands the plasma heat radiation and additional heat loads due to hitting particles. The actively cooled breeder units contain lithium that is transmuted into tritium due to neutron radiation. The tritium is removed from the breeder unit in a closed loop and extracted from that loop by the tritium extraction system. The heat from both FW and breeder unit is exhausted by the primary coolant and transferred to the secondary cooling loop via the heat exchanger.

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 Auxiliary heating systems. These systems face the plasma and radiate electromagnetic waves into the plasma transferring energy to certain particles but also provide additional functions.

 Diagnostics. There is variety of different sensors that are installed. These measure plasma and magnetic field parameters mostly in the framework of the plasma control system.

 Fuelling system. It is located outside tokamak, forms pellets of frozen D, T, or D-T, accelerates these pellets, and guides them through a pipe up to the level of the FW. 2) The magnet system is an assembly of planar superconducting (SC) coils, which

provide the magnetic field required to breakdown and confine the plasma, to drive its current and to define its poloidal structure. It is actively cooled by liquid helium at ~4K.

3) The Cryostat is a large, single-walled, passively cooled vacuum vessel at room temperature. It provides the vacuum required to operate the magnet system in cryogenic condition and supports the two backbone structures of the tokamak: the vacuum vessel and the TF coil system.

2.2 WCLL BB system configuration

In DEMO, the BB System constitutes the primary interface to the plasma in the main chamber, and must provide a plasma-facing surface that is compatible with the plasma performance requirements. The choice of the Blanket System impact the overall DEMO plant design, availability, safety and environmental aspects and cost of electricity. The main functions of the Blanket System are:

 To interface the plasma and remove the heat generated by fusion reactors in the tokamak plasma and transfer it to the Primary Heat Transfer System (PHTS) insuring power conversion efficiency.

 To regenerate (breed) the tritium consumed in the fusion reactions in order to insure tritium self-sufficiency of the reactor.

 To shield, both for thermal and nuclear radiation, the Vacuum Vessel (VV) and external vessel components, and to contribute to shield the superconducting coils.

The WCLL Blanket System is based on the use of reduced activation ferritic-martensitic steel EUROFER as structural material, liquid Lithium-Lead (PbLi) enriched at 90% in 6_{Li as}

breeder, neutron multiplier and tritium carrier, and water at typical PWR conditions (pressure 155 bar, inlet temperature 285°C, and outlet temperature 325°C) as coolant. The WCLL BB design (Ref. [2]) is based on 2015 DEMO CAD model [3], which is characterized by 18 sectors. Each DEMO sector (20°) is composed by two inboard segments and three outboard segments (Fig. 2.2) and it has an upper port and a lower port to allow the feeding and the outlet of the water coolant and the PbLi pipelines (Fig. 2.2).

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The WCLL BB version 2016 [2] is designed with the single module segment approach with a basic breeding cell element repeated along the poloidal direction. The single BB segment is composed by the following components:

 First Wall (FW) and Side Walls (SW);  top and bottom caps;

 internal stiffening and baffle plates;  Back Supporting Structure (BSS);  Breeding Zone (BZ) cooling pipes;  Lithium Lead (PbLi) internal manifold;

 inlet and outlet breeding zone cooling water manifolds ;  inlet and outlet manifolds of first wall cooling system.

Fig. 2.2 – DEMO CAD model 2015: 3D isometric view (Ref. [3]) and WCLL BB DEMO sector piping system (Ref. [2]).

The FW of the segment (inboard and outboard) is single curved in poloidal direction, and its plasma-facing area is covered with a tungsten layer. The FW is cooled by water flowing in square channels in counter-current direction along a radial-toroidal path. The channels distance is calculated accounting for the pitch of two consecutive stiffening plates delimiting the basic breeding cell.

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The PbLi circulation is ensured by baffle plates of 2 mm thick, placed along the toroidal-radial direction. The distance between TR stiffening plates and baffle plates is 135 mm and is calculated on a curvilinear coordinate. The TR ribs and baffle plates are planar and locally normal the face plasma BB curve (Fig. 2.3). The FW channels are symmetrical with respect to the plane of TR and baffle plates. The TR stiffening plates define in each BB segment about 100 radial-toroidal elementary cells. The Breeding Zone (BZ) cooling tubes are placed (Fig. 2.4) in each cell. The BZ tubes are double walled and have external diameter of 13.5 mm, the internal one of 8 mm and the thickness is 1.25 mm. The BZ areas for the inboard and outboard module are respectively 450 mm and 800 mm thick (Fig. 2.4). The PbLi manifolds are placed inside the segment structures, delimited by walls of 30 mm thick (Fig. 2.4).

Fig. 2.3 – WCLL BB module internal structure (left) and track of baffle plates and TR stiffening plates (right) [2].

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Fig. 2.4 – Layout of BZ cooling tubes on an elementary cell (top) and section of the single module on toroidal-radial plane (bottom) [2].

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2.3 Key LB LOCA phenomena relevant for safety analysis

The use of double wall cooling tubes in WCLL breeding blanket is considered as reference design. This solution was proposed to decrease the probability of water leakage into the PbLi breeder. Nevertheless, taking into account the number of cooling tubes in the blanket, the probability of water leakage or pipe break is still not negligible. Consequently, the contact between water and lithium lead is a major safety concern in the design. Safety evaluations of the WCLL blanket design have to be carried out to ensure that the blanket module and its ancillary systems comply with the safety design limits and operating conditions. The main parameters affecting the safety in the case of a single tube rupture are:

 the pressure transient, governed by mixing and pressurization, which might exceed the design limits;

 the chemical reaction contributing to pressure and temperature increases, which might generate a more serious system condition;

 the H2 production, which might represent a potential source of energy;

 the release of radioactivity products.

Experiments with lithium-lead alloy breeder material were performed in US in the ’70 and ‘80, i.e. Westinghouse Hanford Company [4], [5], [6], to characterize the potential safety concerns, and University of Wisconsin [7], focused on the chemical kinetics. More recently separate effect experiments were carried out in Europe at JRC Ispra [8], [9] and ENEA CR Brasimone [10], [18]. The main numerical models that described these phenomena instead, were developed and applied in US at MIT [12] and at the University of Wisconsin [13], [14], [15], while first validation activities of numerical model and computer codes were carried out at CEA [16], [17], and UNIPI/ENEA [18]-[20].

According with Piet et al. [21]:

«the liquid metal/water reaction severity may be a strong function of several parameters, such as the contact mode, the location of the liquid metal and the water, the degree of separation between the two coolants, the phase of the water, and the order of the events in a transient».

Considering the injection contact mode of water (steam or liquid) into liquid metal, the vapor explosions and turbulent mixing may be the prevalent phenomena.

«This process falls into the generic category of Fuel-Coolant Interaction (FCI)… The FCI may involve also chemical reactions between the fuel and the coolant, which if exothermic, can dominate the consequences of the even».

2.3.1 Thermodynamic interaction between HLM and water

In the nuclear research scenario, the liquid metal-coolant interaction phenomenon constitutes one of the major crosscutting safety issues involving Light Water Reactor (LWR), Liquid Metal Fast Reactor (LMFR), and Water Cooled Lithium Lead Breeding Blanket. The interaction event assumes three main configurations, based on the contact

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mode between the fluids (Fig. 2.5) such as: a) dispersion of hot liquid fluid into cold liquid pool, b) dispersion of cold liquid droplets into a hot liquid pool, c) stratification between the two fluids. The likelihood of having a particular contact mode rather than another one depends on the application. The former, called “Fuel-Coolant Interaction” (FCI), represents the most common situation that might occur in LWR or LMFR following a Core Disruptive Accident (CDA) when the core melts and comes into contact with the coolant. The injection of cold fluid into a hot fluid pool is representative for the so-called “Coolant-Coolant Interactions” (CCI) which might occur in LMFR in case of the rupture of one or more tubes of the steam generator. The latter case is also the contact mode, which might occur in WCLL BB in-box LOCA. The stratification of the two fluids, instead, might occur in reactor if the hot dense phase reaches the bottom of the vessel and spreads out without freezing, creating two stratified liquid layers separated by a vapor film. Both in FCI and CCI, the energy transferred from the hot liquid to the colder more volatile one produces an amount of pressurized vapor that could be released in an extreme low time scale, presenting an explosion nature. This phenomenon is referred as thermal/vapor/steam explosion.

Many experimental and analytical studies (Ref. [22]) were conducted over the past fifty years aiming to clarify the physical nature of the vapor explosion. Qualitatively, the mechanisms at the basis of the sudden energy release are: premixing, triggering, propagation, and expansion.

Fig. 2.5 – Contact modes between hot and cold fluid (liquid metal-coolant)

A comparison of FCI and CCI scenarios was discussed by Dinh [23], summarized in Tab. 2.1. Another significant peculiarity of CCI is constituted by depressurization wave propagated in the tube upstream the rupture, causing the violent water flashing and consequent two-phase flow discharge in liquid metal. Beznosov et al. [24] experimentally investigated the two-phase flow distribution injected into molten lead, highlighting a disperse phase of small-diameter steam bubbles, which may contain fine evaporating droplets. The liquid drops were found to exist for long time, and eventually liquid fraction

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FCI CCI

Jet of molten metal injected into a pool of more

volatile coolant Coolant jet injected into a pool of less volatile molten metal Presence of high temperature liquid metal

drops, separated from coolant by a continuous vapor film

Presence of coolant droplets separated from the liquid metal by a vapor layer Available energy dependent on the amount of

heat given by molten liquid metal Available energy limited by the amount of coolant Main mechanism of heat exchange due to

radiation Main mechanism of heat exchange due to film boiling Tab. 2.1 – Main differences between FCI and CCI.

Concluding, the multiphase thermodynamic phenomena expected in case of a postulated water tube rupture in WCLL breeding blanket can be divided into four phases:

 1st_{Ph. starts from the tube break and the consequent injection of water in the BB}

module. The flow rate (choked) of water injected from the tube is relevant, and drives the depressurization of the water loop and the formation of pressure waves inside the module.

 2nd_{Ph. is characterized by the formation of a region of steam, in correspondence of the}

area in which the rupture is localized. The subsequent formation of steam and its expansion cause the mixing of the liquid metal. In particular, during the injection of the mixture of water and steam into the liquid metal, different types of hydrodynamic instabilities at the interface of the two fluids may occur. These instabilities cause further breakage of the jet of water and steam and consequently a formation of a flow regime characterized by water drops dispersed into the liquid metal and separated by a layer of steam Fig. 2.5b). The evaporation of the drops leads to the development and growth of a large vapor bubble in correspondence of the break.

 3rd _{Ph. involves the triggering of a CCI phenomenon, by means of contact between the}

two fluids, with the possible occurrence of steam explosion. Indeed, due to the expansion of the vapor bubble, and in correspondence of its critical dimension, the interface instability may cause its rupture with subsequent liquid-liquid contact. The consequent fragmentation into smaller droplets increases the surface area, therefore the heat exchange, and the rate of evaporation. This progression of the phenomenology is not frequent in case of water jet mode (the water tube rupture into a liquid metal pool). Indeed, the water/steam undergoes to an effective fragmentation because the different pressure between two systems. Bubbles are relatively stable, and the coalescence with the other causes the formation of a relatively large bubble. During the formation of these larger bubbles, the liquid drops evaporate almost completely. Therefore, when the critical size is reached and the liquid metal penetrates through the steam only a slight amount of water is available for the interaction.

 4th_{Ph. consists in the entrainment of the vapor bubbles finely dispersed into liquid}

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of steam explosion, the postulated water tube rupture leads to formation of pressure waves and system pressurization, which may affect the surrounding structures. Thus, a comprehensive study of the thermodynamic phenomena, besides the chemical reaction, is needed in order to evaluate the safety issues connected with the WCLL BB.

2.3.2 Chemical reaction between PbLi and water

Several experiments were performed in the past aimed at different purposes. Jeppson et al. [5] conducted a series of experiments with lithium alloy and different materials such as concrete, atmosphere, and secondary coolants to identify the safety concerns as consequences of postulated accidents. The performed tests evidenced that in case of eutectic lithium-lead alloy «…although the interaction may be benign in terms of energetics

or mobilized radioactivity, hydrogen could be produced from the metal-water interaction».

According with Jeppson and Muhlestein [6] and with Kuhlborsch [25], the chemical reactions that should be considered are:

2 2 2 2 2 1 204 2 2 186 Li H O LiOH H Li H O Li O H

kJ

mol Li

kJ

mol Li

        (2.1)

depending on the excess of reactant and on the temperature of the reaction zone. In fact if H2O excesses, a secondary reaction between Li2O and H2O occurs and therefore, the stable

product is the LiOH. At the same time, if the temperature of the reaction zone is higher than 450°C, the stable product is the Li2O.

The experiments by Herzog [7] and Lomperski [26] investigated the metal alloy/coolant interaction under stratified small-scale conditions. The intent was to measure the amount of hydrogen produced and metal reacted per unit area. For a given contact area, the rate of hydrogen production is dependent upon the chemical kinetics of the reaction. Therefore, measurements of the rate of hydrogen production of the lithium-lead/water reaction for a known contact area could be used in conjunction with suitable dynamic mixing models to estimate the extent of hydrogen production for postulated accident scenarios. The analysis of the data from the experiment showed that the extent of reaction was not a function of the water temperature, and varies over the range of initial liquid lithium-lead temperatures investigated (Fig. 2.6).

Large-scale lithium-lead experiments carried out at Hanford Engineering Development Laboratory by Jeppson and his staff [27] indicated that in the presence of excess lithium, the water and lithium would react as the second equation of (2.1).

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asymptotically to the values measured for lower metal temperatures. The author explained that the reason might be attributed to the mode of reaction product deposition, indeed above 462°C the reaction product is liquid and does not wet the metal so that no protective layer is built up impairing the reaction. The exponential decrease could then be a consequence of the lowering of the Li-inventory in the metal bulk during reaction. The reaction rate seems diffusion-controlled.

Fig. 2.6 – Lithium-lead alloy/water interaction (Refs. [7], [26]).

The more recently small-scale experiments performed by Kranert and Kottowski [8] were aimed at evaluating qualitatively the difference of the chemical and thermo-hydraulic behavior of the PbLi/water interaction.

The evolution of the reaction pressure and the temperature at different injection pressure is reported in Fig. 2.8. At low injection pressure, lithium-lead alloy shows a considerably lower pressure peak. This difference is due to the chemical reaction. If only a thermo-hydraulic alloy-coolant interaction occurs, heat transfer from the alloy to the coolant is reduced because of the vapour film at the interface between the alloy and the coolant. In contrast to this, lithium can chemically react either with coolant or with vapour. On the one hand, the produced hydrogen leads to a pressure increase, on the other hand, the hydrogen attenuates significantly the impact energy of the coolant. Fragmentation remains poor which results in a smaller heat and mass transfer contact surface. The pressure in the reaction tube remains low and no further pressure peak exist. The evolution of the reaction pressure and the temperature of the same alloys at higher injection pressure are shown in Fig. 2.8 on the right. Comparing the lithium-lead eutectic alloy with pure lead, the first pressure peak is lower, which is due to the cushioning effect of the hydrogen. This phenomenon leads to less fragmentation. As no further pressure peaks appear, the interpretation of the pressure trends suggests that the chemical reaction is of less significance for the interaction, which can also be proved by the temperature evolution that decreases below the initial temperature of the alloy.

The fragmentation has an important influence on the alloy-coolant interaction because it increases the contact surface between the alloy and the coolant. A meaningful parameter to evaluate the fragmentation is the release of mechanical energy due to the interaction

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(Fig. 2.9). For a low injection pressure, lead and Pb83Li17 show identical performances. In

contrast, the picture in the right side characterizes the behaviour for a high injection pressure. Due to improved fragmentation and mixing, the lithium alloys show higher values of the specific mechanical energy. Lead does not show such an increase because of the missing chemical reaction. At low values of subcooling, the specific mechanical energy of Pb83Li17 is higher than the other lithium alloy. Indeed, for the latter, the higher

hydrogen production shields the interface between the fluids, while for lithium-lead, the hydrogen is not enough, causing the opposite effect. The analyses of lithium-lead eutectic alloy show that the chemical reaction mainly attenuates a violent thermo-hydraulic reaction, only at high injection pressures and low values of coolant subcooling, the chemical reaction intensifies the interaction.

Fig. 2.7 – H2 production rate for H2O-Pb83Li17 reaction (Ref. [28]).

Fig. 2.8 – Pressure and temperature trends

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Fig. 2.9 – Specific mechanical energy as a function of the subcooling (Ref. [8]).

2.3.3 SET campaign: BLAST and LIFUS5

More recently, JRC Ispra [9] and ENEA CR Brasimone [10]-[11] performed a series of SET experimental campaigns aimed at evaluating the phenomena involved in large breaks of water tubes in the liquid metals. A summary of experiments are reported in the following. BLAST experimental campaign [9], carried out at JRC Ispra in ’80, consisted of nine tests. The description of the facility, of the tests procedure and of the test matrix is fully reported in reference. Notwithstanding the PbLi-water interaction depends by many factors, pressure evolutions of BLAST tests evidenced similar trends: geometry and initial conditions mainly determine the behavior of the system. The reaction vessel pressurization can be divided in three phenomenological windows as described hereafter (see Fig. 2.10):

 1st_{PhW [from 0 to few ms]: the water injection leads to a pressurization, which is}

mainly a function of the system compressibility and the geometry of the reaction vessel.

 2nd_{PhW [from few to 450 ms]: the water jet expansion causes pressure rising in the}

reaction vessel. This is function of the water evaporation, hydrogen production and the geometry of the reaction vessel and expansion tube.

 3rd_{PhW [> 450 ms]: the flow of gases into the expansion vessel causes the start of}

depressurization. It proceeds down to almost the saturation pressure value at the water injected temperature, equal to Psat (@225°C) = 25.4 bar.

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Fig. 2.10 – BLAST Test#5: identification of the different phenomenological windows in the reaction vessel pressurization (Ref. [9])

Summary of the pressure trends of reaction and expansion vessels of BLAST facility are reported in Fig. 2.11. The main outcomes of the experimental campaign are summarized hereafter.

 Mixing is the governing factor of the process,

 The effect of the tube bank shows a longer pressurization,

 There are indications that the chemical and physical interaction is self-limiting: i.e. due to the steam pressurization, hydrogen generation and production of solid oxide and hydroxide of Li, the melt is partially insulated against water, so the energetic vapor explosions appear unlikely,

 System compressibility and layout drive the trend of the first pressure peak. Therefore, the connection between the reaction and the expansion vessels determines the trend of the first peak,

 Higher temperature of the melt, as in test No. 8 causes a higher initial pressure peak, shorter in time. This is connected with the H2 production, indeed for T > 450°C the

hydrogen production is higher,

 Pressurization in reaction vessel did not exceed the water injection pressure with an expansion tube of 50 mm. Different results were observed with an expansion tube of 8 mm diameter, i.e. tests No. 7 and 9.

0 5 10 15 20 25 30 35 40 45 50 55 0 100 200 300 400 500 600 700 800 P re ss ur e [ ba r] Time [ms]

Rea ction Vessel

phase 2

phase 1

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(a) Test 5 (b) Test 7

(c) Test 8 (d) Test 9

Fig. 2.11 – Reaction and expansion vessels pressure trends of BLAST experiments (Ref. [9]). LIFUS5 experimental facility [10]-[11] was designed to investigate the consequence of LOCA accidents in liquid metals pools and to operate in a wide range of conditions. The experimental campaign was carried out in the ’00, and consisted in eight tests. The description of the facility, of the tests procedure and of the test matrix is fully reported in sect. 4, where a comprehensive and exhaustive analyses of the tests is conducted. Hereafter, a brief description of the results and of the phenomenological analysis is reported, based on Refs. [18], [19].

Tests #3-4-5 (Fig. 2.12a).

 The three characteristic phases of the interaction were identified, as in BLAST experiments. However, these are connected, with the geometrical features of the facility (compressibility and connections between reaction and expansion vessels) and to the boundary and initial conditions of the tests.

 The free volume in the expansion vessel is more relevant than the water enthalpy in determining the pressurization of the system. However, lower enthalpy means lower first pressure peak and slower pressure increase rate.

 Larger amount of injected water causes higher temperature in the melt due to chemical reaction. No data on the hydrogen is provided. The influence of the chemical reaction on the pressure trend is not quantified.

Tests #6-7-8.

 (Fig. 2.12b). The second series of experiments, carried out at higher gas free volume, evidenced a remarkably different behavior of the pressure trends, with respect to

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BLAST and LIFUS5 T#3-4-5: the pressure rises slower and the first pressure peak disappeared

 (Fig. 2.12c and d). The effect of the melt temperature on the pressure evolution appears not of primarily importance.

 (Fig. 2.12c and d). The content of Li in the PbLi alloy determines the effect exothermic reaction, thus the temperature of the melt. However, the influence of the chemical reaction, in this configuration (free level surface on the reaction vessel), is negligible notwithstanding the temperature trends differ of about 100°C in the expansion vessel.

(a) Tests #3-4-5: pressure trends in S1 (b) Tests #5-6: pressure trend in S1 and S5

(c) Tests #6-8: pressure trends in S1 (d) Tests #6-7-8: S5 temperature trends Fig. 2.12 – LIFUS5 experimental campaign: experimental trends (Refs. [18], [19]). 0 20 40 60 80 100 120 140 160 0 2000 4000 6000 8000 10000 12000 14000 Time [ms] P re s s u re [ b a r] n.5 n.4 n.3 Rupture disk failed 0 20 40 60 80 100 120 140 160 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Time [ms] P re s s u re [ b a r] PT1 (6) PT2 (6) PT1 (5) PT2 (5) 0 20 40 60 80 100 120 140 160 0 1000 2000 3000 4000 5000 6000 Time [ms] Pr essu re [b ar ] N.6 N.8 Test N.6 p = 160 bar T = 330 °C Test N.8 p = 160 bar T = 430 °C 300 350 400 450 500 550 600 0 5000 10000 15000 20000 25000 Time [ms] T e m p e ra tu re [° C ] N. 6 N. 7 N. 8

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2.4 Status of simulation numerical codes

The main numerical models that describe the PbLi/water chemical reaction and the first development of computer programs were carried out at MIT [12] and University of Wisconsin [13], [14], [15].

Yachimiak and Kazimi [12] developed the FULIB-2 program, while Corradini and Herzog [13] developed a lumped parametric model (MARSBURN). Both the models were slightly improved [14], following the experimental results of Jeppson [27]. Main assumptions of these models are listed hereafter:

 The reaction zone is assumed to be a spherically shaped region, containing a homogeneous mixture of reactants and products at thermal equilibrium;

 The flow rate of water into the reaction zone is modeled by the one-dimensional homogeneous equilibrium model (HEM) for critical flow;

 The reaction between the breeder and water is instantaneous, and is limited by the water injection rate;

 Pressurization of the system occurs only due to the generation of hydrogen gas, the behavior of which is ideal.

Herzog [7] developed two separate models, which predict the mass of hydrogen generated by the reaction as a function of time, implementing the kinetic of the reaction. One model is controlled by the kinetic rate of reaction at the interaction surface, while the second model is controlled by the rate of diffusion of reactants to the interaction surface and products in the liquid metal pool. Lomperski [15] extended these numerical models adding a set of experimental data to calculate the amount of hydrogen produced as a function of time at different initial liquid metal temperatures and liquid metal mass. Details on assumptions and correlations used to develop the numerical models are reported in Refs. [7], [12], [13], [14], [15], [29]. However, it is worth underling that all these numerical models were focused on the chemical reaction, neglecting the thermodynamic effects characterizing the short-term interaction.

The availability of references about code simulations of BLAST experiments is limited. P. Sardain et al. [16] reported the simulation of three BLAST experiments using SIMMER-III code. The simulations were focused on the first pressure peak of the tests (i.e. few tenths of seconds) because, during this phase, the effects of the chemical aspects, not modeled by the code, affected less the pressure trend. SIMMER-III results of Test#5 and Test#9 are reported in Fig. 2.13. The first pressure peak of Test#5 was governed by the thermal interaction and it was properly calculated. On the contrary, discrepancies were observed in the simulation of Test#9. The phenomenon inducing the fast high pressure peaks (i.e. phase 1) was not reproduced by the “standard” FCI model of SIMMER-III code. In this case, the experimental data trend seems driven by vapor explosion phenomenon. Indeed, the water jet hitting the tube bank is fragmented and instantaneously vaporizes. The resulting high pressure stops the injection. Then, the process repeats at high frequency (about 0.4 kHz) increasing the amount of vapor in the reaction vessel of BLAST facility (which is not reproduced by the simulation).

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(a) Simulation of T5 (b) Simulation of T9 Fig. 2.13 – SIMMER-III code simulations of first peak of BLAST tests (Ref. [16]).

Another approach [16] was used to simulate phase 1 of the experiment (without chemical reaction). This consisted in applying the Cigalon model, which is based on the theory and modeling developed for severe accident in light water reactors [22], [30].

(a) Conceptual sketch of small scale droplets PbLi/water interaction

(b) Cigalon model simulations of BLAST for different diameters of the expansion tube Fig. 2.14 – Cigalon model and its application to BLAST experiments (Ref. [16]).

In the framework of the WP13 T06-SYS [19], BLAST Test#5 was chosen to be post-analyzed by SIMMER code. Several issues were identified in the published description of the test execution, initial condition of the PbLi filling level and the geometrical configuration of the injection line. Moreover, no correlation is available in SIMMER-III to simulate the exothermic effect and hydrogen production due to chemical reaction between water and lithium. Therefore, an engineering approach was applied, based on 1)

0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06 1.0E+07 1.2E+07 1.4E+07 0.000 0.002 0.004 0.006 P re s s u re ( P a ) Tim e (s) 50 mm 20 mm 8 mm EXP Ppeak = 40bar EXP Ppeak = 170bar

Safety Investigation of in-box LOCA for DEMO Reactor: Experiments and Analyses

UNIVERSITÀ DI PISA

DOTTORATO DI RICERCA IN INGEGNERIA INDUSTRIALE

Curriculum in Ingegneria Nucleare e Sicurezza Industriale

Ciclo XXIX

Safety Investigation of in-box LOCA for DEMO Reactor:

Experiments and Analyses

Supervisors

Author

Prof. Ing. Nicola FORGIONE

Ing. Marica EBOLI

Dr. Ing. Alessandro DEL NEVO

Dr. Werner MASCHEK

Coordinator of the Ph.D. Program

ABSTRACT

TABLE OF CONTENTS

NOMENCLATURE

LIST OF FIGURES

LIST OF TABLES

1 INTRODUCTION

1.1 International framework

1.2 Objectives of the research activity and expected technological

advancements

1.3 Structure of the thesis

1.4 References

2 PHENOMENA CONNECTED WITH WCLL BB IN-BOX LOCA

2.1 Characteristics of the DEMO design

2.2 WCLL BB system configuration

2.3 Key LB LOCA phenomena relevant for safety analysis

kJ

mol Li

kJ

mol Li

2.4 Status of simulation numerical codes