Effet des ultrasons sur l’interface liquide-solide

Sergueï Nikitenko

Cours de chimie séparative ICSM 2009-2010

Plan of presentation

Problems of spent nuclear fuel dissolution

General presentation of sonochemistry

Mechanical effect of ultrasound on solid surface

Chemical effects of ultrasound on solid surface

Problems of spent nuclear fuel (SNF) dissolution

In France: 59 nuclear reactors produce 79% of electricityNuclear plant La Hague: Reprocessing capacity is about 1700 tons of spent fuel per year

Dissolution is the first ″chemical″ step of SNF reprocessing (PUREX)

UOX Fuel – UO2;

Spent UO2 (Pu, Np, Am, Cm, Fission products)

Oxidative dissolution:

UO2(s) + 4/3(2+x)HNO3(aq) → UO2(NO3)2(aq) + 2xNO2(g) +4/3(1-x)NO + 2/3(2+x)H2O (0<x<1)

U(VI)/U(IV) E°= + 0.41 V vs NHE

NO3-/NO2 E°= + 0.81 V vs NHE

Dissolution of UO2 in 5-10 M HNO3 is rapid and complet (>95%)

Minor problem is a small amounts of insoluble residues

(noble metals etc.)

Problems of spent nuclear fuel (SNF) dissolution

MOX Fuel: (U,Pu)O2

Distribution of Pu is non-homogeneous for high content (>20%) of Pu

SEM of irradiated MOX

Dissolution is slow + significant amounts of insoluble residues (PuO2, ″molybdates″,

noble metals etc.)

″molybdate″: Mo2Zr(1-α)PuαO7(OH)22H2O

Problem: PuO2 is insoluble in HNO3

PuO2/Pu(VI) E°= + 1.22 V vs NHE

NO3-/NO2 E°= + 0.81 V vs NHE

Conventional solution:

heating at 90-95°C, ∼14 M HNO3, 0.05-0.1M HF

Problem: corrosion, reaction is slow

Problems of spent nuclear fuel (SNF) dissolution

Redox approach for PuO2 dissolution :

Rapid oxidative dissolution in HNO3 with Ag(II)

Ag(II)/Ag(I) E°= +1.98 V vs NHE

Problem: corrosion, low efficiency in the presence of organics

Rapid reductive dissolution with Cr(II), Ti(III) in H2SO4

PuO2/Pu(III) E°= + 0.67V vs NHECr(III)/Cr(II) E°= - 0.41V vs NHETi(OH)3+/Ti(III) E°= + 0.06V vs NHE

Problems of spent nuclear fuel (SNF) dissolution

UC and (U,Pu)C – potential fuel for Generation IV

gas-cooled fast nuclear reactors

5 mmDissolution of UC in HNO3 is rapid and congruent for (U,Pu)C

UC + 6 HNO3 ⇒ UO2(NO3)2 + CO2 + 3H2O + 3NO + NO2

PuC + 8 HNO3 ⇒ Pu(NO3)4 + CO2 + 4H2O + 2NO + 2NO2

Problem: large amounts of organic acids are formed (mellitic, benzoic, oxalic, formic, acetic acids etc.) which interfere in solvent extraction

General idea – control of actinide oxides and carbides dissolution with ultrasound

″Sonochemical Dissolution″

General presentation of sonochemistry

The origin of sonochemistry is acoustic cavitation:

nucleation, growth and implosion of microbubbles in liquides subjected to ultrasound

(f= 16 kHz – 1 MHz)

T = 5000 K≈ 1010 K/s

General presentation of sonochemistry

Cavitation bubble dynamics: Rayleigh-Plesset equation

3220

20

3 2 22

k

h v k a vRd R dRR P P P P P

dt dt R R Rθ θ⎡ ⎤ ⎛ ⎞ ⎛ ⎞⎛ ⎞ρ + = − + − − + −⎢ ⎥ ⎜ ⎟⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠⎢ ⎥ ⎝ ⎠⎣ ⎦

( )

ρ - density of the solvent, R – bubble radius Ph – hydrostatic pressure, Pa –acoustic pressure, Pv – vapor pressure, k= Cp/Cv – polytropic index, and θ

complex parameter taking into account the surface tension

Time of the collapse:

00.915ih a v

t RP P P

ρ≅

+ −

Bubble radius:

H2O:f, kHz R, µm18 1501000 3.3

is applied circular frequencyFaWa π2=

( )3/12/1

0 3(2

123

4⎥⎦

⎤⎢⎣

⎡ −+⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

h

hA

AhA P

PPP

PPWa

Rρ

f= 20 kHzI = 1 W/cm2

ti= 0.7-0.8 µsec

General presentation of sonochemistry

Geometry of the cavitation field

Observation by sonoluminescence

20 kHz

550 kHz

General presentation of sonochemistry

Asymmetric collapse at solide/liquide interface

Mechanical effects:

•acceleration of mass transfer

•surface erosion

Chemical effects:

•local heating

•radical reaction

Ultrasonic enhancement of the rate of dissolution

( )AAA CCkS

dtdC

−=− *

Increased by microstreaming Increased solubility due to the local heating, supersaturation

Increased by erosion

Redox reactions at solid/liquid interface (important for actinides):

•Local heating

•Radical reactions

Acceleration of the mass-transfer

Sonoelectrochemistry

Levich equation:

iL = (0.620) n F A D2/3 w1/2 v–1/6 C

w is the angular rotation rate of the electrode (radians/sec) v is the kinematic viscosity of the solution (cm2/sec). The kinematic viscosity is the ratio of the solution's viscosity to itsdensity.

C.Costa 2009

Cavitation erosion of glass surface

M. Virot 2009

Primary effects

Secondary effects

cracks

Mechanical and chemical effects

Sonochemistry of graphite in water

F. Guittonneau 2009

Chemical effects

Sonocatalytic dissolution of CeO2

0

1

2

3

4

5

6

7

8

9

% C

e pa

ssé

en s

olut

ion

Agitation : Eau Agitation : Acidenitrique 3M +

Hydrazine 0,2M

Ultrasons :Acide nitrique3M + Hydrazine

0,2M

Ultrasons :Acide formique

1M

Ultrasons :Acide formique1M + NPs de Pt

avant US après US

3h de traitement, T= 20-22C, I= 18 W/cm², Pac= 0,36W/mL, 2.5%NPsPt/CeO2 (mass.), V= 50 mL

Cavitating

bubble

Heat

CeO2/Pt

HCOOH

Products:

CO2 + Ce(III)T.Chave 2009

Chemical and mechanical effects

Dissolution of metallic Pu under ultrasound

Pu is passivated in HNO3

Pu dissolution is accelerated if HCOOH is added to HNO3

Problem: dissolution is slow and incomplete

0.5MHNO3+1MHCOOH 1MHNO3+1MHCOOH

S. Nikitenko, Ph. Moisy 2006