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
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