3 research outputs found
Intermetallic Density Based Group Separation of Actinides and Lanthanides Using Liquid Bi to Decontaminate High Level Wastes from Pyroprocessing
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ผ๋ฌธ (๋ฐ์ฌ)-- ์์ธ๋ํ๊ต ๋ํ์ : ๊ณต๊ณผ๋ํ ์๋์ง์์คํ
๊ณตํ๋ถ, 2018. 8. ํฉ์ผ์.The pyrochemical processing technology for recycling spent nuclear fuel (SNF) has been developed to recover useful resources and to reduce the toxicity and longevity of final radioactive wastes with the enhanced proliferation-resistance and accident-tolerance over aqueous reprocessing technology. However, the final vitrified waste stream from the used molten salt released from current pyroprocessing technology is still classified into high level waste (HLW) due to non-negligible actinide concentration and heat density. SNF and high level waste (HLW) from reprocessing should be eliminated to avoid long-term natural hazards in normal scenarios and man-made risk of abnormal scenarios. If the final high level wastes can be further decontaminated down to intermediate level wastes, their institutional control period can be drastically shortened and all the long-term risk can be ameliorated. In this thesis, an innovative waste decontamination process has been developed to adequately extract actinides collectively out of the final molten salt waste stream discharged from conventional pyroprocessing, based on intermetallic density differences utilizing liquid bismuth cathode. By electrolytic reduction of both actinides and lanthanide groups that are remaining in the molten salt waste from the conventional pyroprocessing, bismuth intermetallic particles are produced and separated into two groups by their density differences within the liquid bismuth cathode.
The innovative density-based separation has been developed based on the fact that bismuth intermetallic of actinides (AnxBiy) have higher density than liquid bismuth that however has higher density than its intermetallic of Ln (LnxBiy). It has been shown both theoretically and experimentally that actinides and lanthanides can be deposited onto a liquid Bi cathode to readily form intermetallics due to their low solubility limits at 500 oC. In addition, the density differences is shown to provide adequate separation forces, overcoming surface tension effect, by externally applying acceleration.
In order to investigate thermodynamic and kinetic characteristics of intermetallics, a series of cyclic voltammetry experiment have been carried out for the cations of U, Ce, Hf and Lu on a set of key cathode materials (solid W, Bi film on W and liquid Bi pool in eutectic LiCl-KCl salt at 500 oC). Ce is chosen as a representative material for lanthanides whereas Lu and Hf are employed as surrogates for actinides due to their high Bi-intermetallic densities relative to pure liquid Bi. Galvanostatic electrolyses have been conducted to reduce Ce, Lu and Hf ions in the eutectic salt so as to form their Bi-intermetallics in liquid Bi. Upon the completion of electrolysis, the vertical cross sections of solidified Bi cathode were analyzed to identify intermetallics phase and their spatial distribution in Bi to demonstrate density-based group separation behaviors of actinides and lanthanides, respectively.
Both thermodynamic and kinetic properties have been derived from the experimental results that are verified and complemented with available literature data for the both electrochemical deposition monitoring and 1-D computational electrolysis modeling. The flowsheet of purification process for waste LiCl-KCl salts from the conventional pyroprocessing has been established by using experimental results and computational modeling on density separation of AnxBiy and LnxBiy.
Computational modeling has been developed to design a pilot-scale purification process system for the innovative decontamination of high level waste salts from the conventional pyroprocessing into intermediate level waste (ILW) that can meet the waste acceptance criteria of U.S. Waste Isolation Pilot Plant (WIPP). Benchmarking of modeling results against the experiments in this thesis and literature is made to verify a validity of the models. The pilot-scale unit process system, designated as PyroRedSox, is integrated into the integral innovative process designated as PyroGreen that can eliminate HLW and associated risks of abnormal scenarios.Chapter 1 Introduction 1
1.1 Background 1
1.1.1 Long term uncertainties with disposal of spent nuclear fuel 1
1.1.2 Separation goal for decontaminating SNF into ILW 3
1.1.3 SNF partitioning technology overview 7
1.1.4 Actinides group separation in conventional pyroprocess 10
1.2 Motivation 13
1.3 Objective 17
Chapter 2 Literature Review 18
2.1 Survey of for liquid metal process for pyrochemical group separation 18
2.1.1 Electrolysis on liquid Cd cathode 18
2.1.2 Electrolysis on liquid Bi cathode 22
2.2 Intermetallic formation in Cd and Bi 24
2.2.1 Cd intermetallic formation in LiCl-KCl and Cd system 24
2.2.2 Bi intermetallic formation in LiCl-KCl and Bi system 29
2.2.3 Uranium dissolution into liquid Bi 35
2.3 Intermetallic density based separation process 37
Chapter 3 Rational and Approach 39
3.1 Rationale 39
3.2 Research Questions 41
3.3 Research approaches 42
3.3.1 Surrogate Elements determination 45
3.3.2 Cyclic voltammetry and electrochemical properties 48
3.3.3 REFIN model description 55
Chapter 4 Electrochemical Studies for U, Ce, Hf and Lu in LiCl-KCl using inert electrode 57
4.1 Test matrix 57
4.2 Experimental setup 60
4.2.1 Apparatus setup 60
4.2.2 Electrode preparation 62
4.2.3 Reagents preparation 62
4.3 Cyclic voltammetry results and discussions 66
4.3.1 LiCl-KCl 66
4.3.2 LiCl-KCl-UCl3 68
4.3.3 LiCl-KCl-CeCl3 72
4.3.4 LiCl-KCl-HfCl4 75
4.3.5 LiCl-KCl-LuCl3 79
4.4 Data reproducibility 82
Chapter 5 Electrochemical Studies for U, Ce, Hf and Lu in LiCl-KCl using Bi electrode 84
5.1 Test matrix 84
5.2 Experimental setup 86
5.3 Cyclic voltammetry results and discussions 90
5.3.1 LiCl-KCl 90
5.3.2 LiCl-KCl-UCl3 94
5.3.3 LiCl-KCl-CeCl3 97
5.3.4 LiCl-KCl-HfCl4 101
5.3.5 LiCl-KCl-LuCl3 103
5.4 Data reproducibility 106
Chapter 6 Intermetallic Formation and Vertical Distribution in Liquid Bi Cathode 108
6.1 Test matrix and the results summary 108
6.2 Metal dissolution into liquid Bi 111
6.2.1 Experimental setup and procedure 111
6.2.2 Bi alloy ingot analysis procedure 115
6.2.3 Bi-Ce results 117
6.2.4 Bi-Hf results 126
6.2.5 Bi-Lu results 131
6.3 Electrolysis of Ce, Hf and Lu on liquid Bi pool electrode 135
6.3.1 Experimental setup 135
6.3.2 CeCl3/Bi results 137
6.3.3 HfCl4/Bi results 143
6.3.4 LuCl3/Bi results 149
6.4 Discussions 156
6.4.1 Temperature history during solidification by quenching and annealing 156
6.4.2 Mixing effect on the liquid Bi phase 159
6.4.3 The evidence for metal powder dissolution in liquid Bi 161
6.4.4 Settling time of intermetallic particles 161
6.4.5 Formation of ternary intermetallics Bi, actinides and lanthanides 165
Chapter 7 PyroRedSox Process Design for Spent Nuclear Fuel Pyroprocessing 167
7.1 Molten salt-liquid bismuth system benchmark 167
7.1.1 Reductive extraction 168
7.1.2 Electrolysis of Pu on Bi 172
7.2 PyroRedSox process mass balance simulation and the results for spent nuclear fuel pyroprocessing 174
7.2.1 Electrolysis modeling results 174
7.2.2 Discussions 177
7.3 Integral flowsheet of PyroRedSox 180
Chapter 8 Conclusions and Future Work 183
8.1 Conclusions 183
8.2 Future Work 186
Bibliography 187
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