V,

A Mixed-Potential Model for the Prediction of theEffects of Alpha-Radiolysis, Precipitation and

Redox Processes on the Dissolution of Used Nuclear Fuel

by

Fraser King, David W. Shoesmith and Miroslav Kolar

AECL, Whiteshell LaboratoriesPinawa, Manitoba Canada ROE iLO

This work is funded by Ontario Hydro, Nuclear Waste Management Division(J.E. Villagran, Project Engineer).

1998 Spent Fuel Workshop, Las Vegas, 18-20 May 1998.

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What are we trying to model?

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Summary of existing data on U02 electrochemistry

Outline of mixed-potential model (MPM)* reaction scheme* reaction-diffusion equations* model geometry* boundary conditions* assumptions

"Model predictions"* validation/verification of MPM* effects of a-radiolysis, precipitation and redox processes

Summary of UOo+ Electrochemist (1)

* Behaves like a p-type semiconductor* Rates of reaction dependent on potential (E)

rest6Aj ,Ox.aiz;,qj ,

led o6c", M�-V

Summary of UO2 Electrochemistry (2)

* The degree of oxidation of the surface is E dependent* Above a certain E, dissolution becomes oxidative in

nature (i.e., the anodic dissolution to U(VI) iselectrochemically coupled to the cathodic reduction of asuitable oxidant).

g74480.,1

U0 2 ..300 _ - -_ - - - - - - - - - - -

UOa(COs32)

100 - ______-_-1

> !- w/ U032H20

E -100 --------- __ __

-300 -4I -- _____ ---

UOz -X

3.500

Summarv of UO9+x Electrochemistry (3)

In non-complexing environments, corrosion productfilms build up on the U02 surface, leading to a decreasein the dissolution rate.

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102

4.

Cb.A.

0C

c

ina

cl

10

1

10-1

10.2

10-3 L..0.1

-�fb0.1 0.3 0.5

Potential (V vs. SCE)

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Summary of U0 2+x-

* The addition of complexa&rate of dissolution.

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102

OC,- ~~~~CO x 1

E 1 XX X

'0 /1 ~00 X*X~

A/

~10.1 Xx10 0 1 S-f A*AC14 4 (jli~ ~ )

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Electrochemistry (4)

ats for U(VI) increases the

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Potential (V. vs SCE)

Summary of U02+x Electrochemistry ()

* The cathodic reduction of 02 on U02 has beenextensively studied.

* Less is known about the cathodic reduction of H 2 0 2 , orthe surface-mediated decomposition of H 2 0 2.

Va 103 , uo? elechw& .

C

°d~~~~~~~~~~~~ 1eawo 0 ' {

lo?- ~ ~ ~ ~ ~ ~ ~ 40

0~~~~~~~~~~~~~

10~~~~~~~

-0.2 -0.4 -0.6 -0.8

Potential (V. vs SCE)

&)Summary of Electrochemical Data (6)

Potential-Dependent Dissolution Rate of Feud4

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Outline of UO0 Mixed-Potential Model (1)

The Reaction Scheme

(UO2+)ads

U02+

UN(s)

Fel(aq)

FeM(s)

U.2-

up, -

., o

UO?2

uk,

U0 3 .2H20 (s)

k2U[ (LC03

I UO, (Co,)2-

Juoi

_ ~~~~~.

JUC)2(%)2

I

Uls

C032- k6I Fel(aq)

D2 (C0 3 )2

[Ic Flk erns)2.

lU0 2 + )ads

A/

Fe(s)

Fe1(aq) q Fe1(aq)

J02

H2 02H2 02 _

20H' OH'

10202 E(s)

7i [k7

Fen(s)02

98-009.13

Outline of U02 Mixed-Potential Model (2)

The Reaction-Diffusion Equations

The various mass-transport and chemical processes involving theten species (UO2+, U0 2 (CO3 ) 2 -, U0 3 2H2 0(s), C0 3 , 2,

H 20 2, Fe(II)AQ, Fe(II)ppT, U(IV)pr and U02+ (ads)) are describedby reaction-diffusion equations of the general form

-c = a 9i)+sR for c=l,...,1O0

at DX ( ax j=1

e.g., for 02 (species #7)

e ac 7 ax TD7 ax + G 7 R(x, t)-ek3 C 7 C 9

and for U03 * 2H2 0(s) (species #3)

£ aC3= ek1 max(O, (c, - cdat ) + k2 max(O, (c 2 - cat) -k _ 2c 3cP

Since Di and kj are temperature dependent, we also solve a heat-conduction equation

aT a ( aT)PC -xi K-at -ax -axi

to predict T(x,t) and, hence, Di(x,t) and kj(x,t) (depending uponmodel geometry, may assume T(t) only).

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Outline of UO, MPM (3)

The Spatial Grid

* one-dimensional representation of the container andvault geometries

* two possible model geometries are:

C 1) 174 %OR e.^ #4 !P

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.*�.�

* - S -* **t **

�' r� *� MO%* ** .� ..� �

9 � 4.�e boc,,i%. .- � U*. *E * �.

* 4 0 * 0� S�*�*.* C. .

- .�.D 0- S

*Oi* -0'

*5

I�.

4 ' -

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LOGd40 0%) 0

C7 a# 000d

v &a 0a a

S.oe es0 fr0d 0:

Fio, V,~

fluxi .Afi t

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Ile

000'.0.10"

meCIk

(2t) .! A Ido5s sb SS

Iroveldwatse-

14.a

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Outline of UO2 MPM (4)

A Closer Look at the Porous U(VI) Layer

The U(VI) precipitate layer has several effects:* blocks the surface electrochemical reactions* inhibits mass transport to and from the U02 surface

* affects the yield and location of radiolysis products

Aqueous Solution

t. aI 09Outline of UO2,MPM (5)

Boundary Conditions

Uo2-

kA P u ol!"

UO- (

2*or kFlj

U02

-I

t1 1ckAh c

*k N

1'1~c zT2 -Z.C-*e

I

`111�o x.

<: '-F,4O0 4 krFc~ e4

IA + IB + IC = -(ID + IE)

I = ±nFPTD ac(ot)Ox

I = ±feAkc(O, t) eXP(V ± (E - ED))

Derived parameters:

IJ = -(IF + IG +IH + IJ)

I = ±n~rc'D (1c(Xrght , t)

8xI = ±F'Akc(xfi&, t) ea( F (E-EO))

Derived parameters:

,CORR = IA + IB

ECO =EA +- RT I ICORROAF F2FcAkA J

6 ra4eOutline of U0 2 MM (4

Assumptions

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a k

* the reaction scheme and 1-D model geometryadequately describe the oxidative dissolution of U0 2 ' i

* uniform dissolution of the fuel surface (no localizedeffects, such as gb etching)

* mass transport by diffusion only* system is completely saturated and the supply of H 20 is

not limiting* no effect of Zircaloy fuel cladding* effects of a-radiolysis can be simulated by 02 and H 20 2 .

only* U(VI) and Fe3O 4 films can be treated as equivalent

porous media with spatially and temporally constantporosity and tortuosity

* U(VI) film is assumed to be electrically insulating withelectrochemical reactions restricted to base of pores

* U(VI) film attenuates ct-dose rate at the fuel surface andcontains co-precipitated ct-emitters

* film growth occurs at the film/solution interface with noprecipitation within the pores

* Fe30 4 is the stable C-steel corrosion product* constant pH throughout the system (pH 7)* various assumptions involved in derivation of input data

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eAV f,$

a, so'

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at -I

CY I d;

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4Fi-

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"Model Predictions" (1)

Validation/Verification of the Model

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(1b)

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"Model Predictions" (2)

The most basic prediction is of IcORR and EcORR of theU02 surface

* ICORR is equivalent to the dissolution rate* EcORR indicates the period of oxidative dissolution

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B #&xf.o/uho\ff432 cV.saofe E ,0

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'Model Predictions" (3)

* To what extentdissolution?

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does film growth inhibit U0 2

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"Model Predictions" (4)

* The ability of the C-steel to act as a redox barrier

(b)

AIP=R�rvLw-

(C)

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Cator0

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Future Developments (1)

(1) Inclusion of multiple growing layers..

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rx. °

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U 1".01"', ,

(2) More detailed radiolysis modelling

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Future Developments (2)

Inclusion of H2 produced by radiolysis and corrosion(3)

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Hz

(4) Prediction of localized acidification and chemistrywithin deposits

lUOz4 .n x,

*\,* ~- a4~ -,

ms@w7S5 C.. fpAo&'r4l'

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(5) Improvements to treatment of FeAffie interface

Conclusions

* An electrochemically based model has beendeveloped for predicting the effects of a-radiolysis,precipitation and redox reactions on the dissolutionbehaviour of used fuel.

* The reaction-diffulsion of ten chemical species isincluded in the model (UO2+, 2-(U 2 , U0 2(C0 3)27U0 3 2H20(s),C0 3 , 02, H 20 2 , Fe(II)AQ, Fe(II)PPT,

U(IV)ppT and UO2+ (ads)).

* The container/vault system is described by a 1 -Dsystem of porous mass-transport barriers representingcorrosion product films, the container and buffer andbackfill materials.

* The use of electrochemical boundary conditionspermits the prediction of the corrosion potential(EcORR) and dissolution rate (Icopa) of both the UO2and C-steel surfaces.

* As yet, no predictions have been made!