Electrochemical behaviors of U, Th in fluoride molten salts

Dewu Long, Wei Huang, Qingnuan Li

Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences(CAS)

INTERNATIONAL PYROPROCESSING RESEARCH CONFERENCE 2014 IDAHO FALLS, IDAHO, OCT. 19-23, 2014

TMSR Project at SINAP (CHINA)

Fluorides coolant

Homogenous fuel

Thorium breeding

On-line fuel treatment

Background

Features

3

Conceptual flowsheet of TMSR Fuel Processing

Fluorination Reactor Th,U,Pa,FPs

LiF-BeF2

UF6

Th,Pa,FPs LiF,BeF2

Storage 233Pa 233U

FPs Distillation

Fuel reconstitution

LiF BeF2

U，Th LiF-BeF2-UF4-ThF4

Waste proper to later disposal

Hydrofluoric acid dissolution LiF BeF2

Pyrohydrolysis

Modified Thorex

Th,Pa,FPs LiF,BeF2

Fluorides Oxides

Electrochemical separation

Electrochemical separation

2014-10-20 reprocessing and recycling Off-line reprocessing

U in FLiBe or in FLiNaK

Th and U in FLiNaK or in LiCl-KCl

Background

4

Background

Fluoride-based molten salt medium. One or more steps of the

electrochemical separation in TMSR fuel processing have to

be conducted in fluoride electrolyte, e.g. LiF-BeF2.

Fundamental database unavailable. The lack of reliable and

systematic database of Ans and Lns in fluoride molten salt.

The feasibility for recovering of U or Th and the efficiency of

the separation process. The applicability of electrochemical

separation method in TMSR fuel processing and the recovery

efficient should be evaluated.

Challenges for the implementation of electrochemical separation technology in TMSR

Due to the toxicity of LiF-BeF2, LiF-NaF-KF (46.5- 11.5- 42.0 mol%, FLiNaK, m. p. 454℃) is initially adopted as the medium.

FLiBe (7LiF-BeF2

66-34 mol%, m. p. 460 ℃)

Molten salts used in TMSR

The R&D of electrochemical technology in TMSR

Fundamental data acquisition Electrochemical measurement method Electrochemical deposition

Thermal stability Low vapor pressure Manageable melting point Low neutron cross section Radiation resistance Dissolvable media for Ans and Lns Toxicity and poison High cost for Li-7

Background

Ni wire

BN tube

Inner salt

Ion Channal

Potential shifting (< 5 mV)

Performance of as-prepared RE Over 50 hrs stability，shifting less

than 10mV Excellent repeatability, potential

deviation less than 5mV Suitable for experimental

requirement

Fabrication and Performance of Reference Electrode

performance of reference electrode is essential .

Non-commercial electrode available.

Illustration of reference electrode

Progress

7

On mechanism understanding the performance of an reference Electrode

Performance of an RE exhibits strong Ni2+ speciation dependence of the inner electrode couple.

20 wt%

40 wt%

70 wt%

Progress

K2NiF4

KNiF3 Li2NiF4

KNiF3 Li2NiF4

Ni2++2e-→Ni0

NiF2

NiF2

NiF2

~

-2.0 -1.5 -1.0 -0.5-0.02

0.00

0.02

Curre

nt /A

Potential/V

U3+→U0

U0→U3+

U3+→U4+

U4+→U3+

FLiNaK

Reaction Redox potential（V）vs. NiF2/Ni D/cm2 s-1 Separation

Gd3++3e→Gd0 -2.02/-1.68 3.2×10-4 Y Y3++3e→Y0 -1.96/-1.91 5.4×10-6 Y

Sm3++e→Sm2+ -1.65/-1.53 7.4×10-6 N Eu3++e→Eu2+ -1.07/-0.89 1.3×10-5 N Nd3++3e→Nd0 -1.95/-1.88 1.1×10-5 Y Zr4++4e→Zr0 -1.86/-1.50 2.5×10-6 Y Th4++4e→Th0 -1.91/-1.66 8.4×10-7 Y

U4++e→U3+ U3++3e→U0

-1.42/-1.24 -1.83/-1.69 1.3×10-6 Y

Determination the parameters of key elements

FLiBe LiCl-KCl

U4+→U3+

U3+→U0

U3+→U0 U4+→U3+

Progress

9

CV of U 4+ and U3+ in FLiNaK

Progress

FLiNaK, 823K D(U4+) = (4.7±0.9)× 10-6 cm2 s-1 D(U3+) =(2.6±0.2) × 10-6 cm2 s-1

ip=0.44（nF)3/2AC0U4+D1/2v1/2(RT)-1/2

Reduction sequence and electro-deposition of U

U deposition

Deposition product

Product after removal of salt Ni Electrode

U %～ 30%

U %～ 3% C0

U in MS

CU in product

20 40 60 80 100

Inte

nsity

/Cou

nts

2θ/°

♦--UO2

∇--KF• --LiF♠--Martix

♦

♦♦ ♦

♦♦♦ ♦ ♦

∇

∇

∇ ∇∇ ∇•

•

•

♠

♠

Progress

11

0 500 1000 1500

-1.85

-1.80

-1.75

-1.70

-1.65

-1.60

-10 0 10 20 30 40 50 60 70

-200

-150

-100

-50

0

50

i/mA

time/s

-200mA

-40mA

30mA0mA

1s

60s

3s

3s

Pote

ntia

l/V

time/s

Advanced method for electro-deposition of U

Current pulse electrolysis

Pre-concentration

deposition

dissolving

Post-stabilization

Progress

12

Electro-deposition of Th in LiCl-KCl

Progress

Deposited product

In molten salt

ICP-AES

XRD

Product

13

High performance reference electrode has been fabricated and the influence

of Ni2+ speciation on the performance of the electrode is evaluated.

Electrochemical behaviors of U and Th in various molten salts have been

investigated and preliminary deposition of U and Th was conducted. The

feasibility of recovery of U and Th was confirmed.

Future work would be focused on

Improve the reliability of the measurement of fundamental data in fluoride

salt.

Develop in-situ electrochemical method for process monitoring and TMSR

salt chemistry controlling (U4+/U3+ ratio represent the redox chemistry).

R&D of electro-chemical separation technique for TMSR fuel cycle as well

as designing of large-scale apparatus.

Summary & future work

2014/10/23 14

Thank you for attention