Some properties of heptavalent plutonium in molten KOH-NaOH eutectic

INORG. NUCL. CHEM. LETTERS

Vol. 8,

pp. 861-867, 1972. Pergamon Press. Printed

in

Great

Britain.

SOME PROPERTIES OF HEPTAVALENT PLUTONIUM IN MOLTEN KOH-NaOH EUTECTIC L. Martinot* and G. Duyckaerts Laboratory of Nuclear Chemistry University of Liege rue A. St@vart 2 B - 4000 LIEGE (Belgium) (Received 12 May 1972)

ABSTRACT We have investigated

the electrochemical

reduction of Pu

in

molten KOH-NaOH at 240°C. This reduction proceeds in two steps whose last one is rate determining PuO~- + e-

, >

PuO~ + + 2e-

PuO~ + + 302>

PuO 2

-'7INTRODUCTION In study

a previous

of

the

paper

reduction

(1), of

we h a v e

heptavalent

reported

a chronopotentiometric

Neptunium

in

molten

KOH-NaOH

eutectic. A similar

work has

now been

carried

out

with

Plutonium.

of Pu(VII) has allowed only to approach the qualitative

The short

aspect of the

question.

* Chercheur

i

l'Institut

Interuniversitaire

861

des

Sciences

life

Nucl6aires.

:

862

H E P T A V A L E N T PLUTONIUM

Vol. 8, No. 10

EXPERIMENTAL The melt preparation,

the chronopotentiometric

electrodes have been described earlier

circuit and the

(1). The indicator electrode

was a graphite single crystal with cleavage plane exposed to the melt

(S = O.16 cm2). All attempts to oxidize PuO 2 dissolved in the bath by continuous

flowing of 02 were unsuccessful The Pu(VII)

: only Pu(VI) was obtained.

solutions were easily prepared by dissolving weighted

amounts of Na3PuO 5 in the melt whose color turned immediately

to dark

green. This compound was obtained according to the method proposed by Keller and Seifert

(2). A typical sample was obtaimed by mixing

together and heating 54.2 mg of PuO 2 and 46.6 mg of Na202 for 26 hours, at 420°C, in a platinum erucible and in a pure oxygen flow. To check the efficiency of the reaction,

a ~ 10 mg sample is dissolved in

3M NaOH and the absorption spectrum is taken : Pu(VII) peaks appear at 400 - 410 n.m. and at 620 - 630 n.m. peaks at 820 - 830 n.m. and at 1010 - 1020 n.m.

characteristic

(2,3). Pu(VI)

(3) never had appeared.

The composition of the melt under investigation was also checked by comparing the visible spectrum of a "frozen" part of the melt, dissolved in water, with the literature data (2,3)

RESULTS

Preliminary

experiments have shown that when Na3PuO 5 is introduced

into the melt at 240°C, Pu(VI) the Pu(VII)

appeared within 10 - 15 minutes. At 290°C,

life time was even shorter and no experiment was ~udertaken.

Vol. 8, No. 10

H E P T A V A L E N T PLUTONIUM

A typical is

shown in

Fig.

steps reduction

1.

chronopotentiogram The s h a p e o f

but no overshoot

the

of

the

863

reduction

w a v e seems t o

fit

of

Pu(~I)

with

a two

for the first transition

appears

clearly. We use the classical Sand's io~ I

I/2

formula

- where i o is the current

time - for the first reaction. calculations

and further

(I) to calculate

the product

density and ~I the first transition

Large discrepancies

estimations

arise in these

of the diffusion

coefficient

would be meaningless.

FIG.

1

A typical chronopotentiogram for the reduction

of PuO53-

~Pu053-~ = 1.285.10-3 i~ = 0.55 mA

m.f.

864

HEPTAVALENT

Pu(VII~

x 10 3

PLUTONIUM

Vol.

i ° I 11/2(m A sec 1/2)

Number of

at 240 ° C

determinations

0.635

O. 18 + 0.09

5

1.285

0.39 + 0.25

8

1.490

0.43 + 0.2

6

1.510

0.38 + 0.2

4

m.f.

8,

The ratio between the second transition time 1 2 and the first one, is about 11 but this value increases markedly with the elapsed time. c) Tafel lines Tafel lines were drawn for the last two solutions according to the galvanostatic method as already reported by us (2,4). The experimemtal slopes ~ E / ~log i

o

- E beimg the potential off,he

indicator electrode - for these experiments equal - 0.048 and - 0.055

(Fi~. 2) E (volt -0.20

o

Oo

O

-0.2@

® L

i

,

0.1

FIG. 2

•

,

,

1 l

,

,

Tafel lines for the reduction of PuO 3-

[ uo 1 o

•

O.5 L0 ImAl.--~

No.

I0

Vol. 80No. 10

HEPTAVALENT PLUTONIUM

865

DISCUSSION As a preliminary hypothesis, and Pu(VII)

we shall consider that both Np(VII)

exist in the melt as anions

: PuO~-, NpO~-. The same situ-

ation holds true in concentrated KOH or NaOH aqueous solutions. Furthermore,

the chronopotentiometric

reduction proceeds in two steps,

the last one involving PuO 2 precipitation

on the indicator electrode

as it is the rule in uranyl and neptunyl ions reduction According to Delahay's theory

(1).

(5) on chronopotentiometry,

two consecutive reactions occur, the transition time ~ 2

when

of the second

one is given by :

~nl/

in1 / J

n I : number of electrons of the first reaction n 2 : number of electrons for the second reaction For

these

two consecutive

Pu(VII) Pu(VI)

reactions

:

+ e-

>

Pu(VI)

nI = 1

+ 2e-

>

Pu(IV)

n2 = 2

the

ratio

%2/~1

the

best

one to

will

be 8.

account

This

for

As the experimental ratio of of Pu(VI) in the melt

the

combination high

~2/'~1

of

partial

experimental

value

reductions obtained

is :

increases with spontaneous formation

- checked by visible spectra - we may explai~ that

the theoretical value ~2/~[I = 8 is not observed, ning of each experiment,

even at the very begin-

because a small amount of Pu(VI) is immediately

formed when Na3PuO 5 is poured into the bath.

866

H E P T A V A L E N T PLUTONIUM

Vol. 8, No. 10

The preparation of a pure Pu(Vll) solution is therefore impossible and this would lead to a greater value of the ratio T2/~I. A similar phenomenon is also observed for the reduction UO2(VI) + e-

~

UO2(V) | UO2(V) + e-

--~

UO 2 in LiC1-KC1 (6)

where the theoretical ratio ~ 2/~q = 3 is only observed with pure UO2(VI) solutions. If UO2(V) appears in the melt, the ratio 1 2 / ~ 1 increases rapidly. Thus the most probable reaction scheme for the electrochemical reduction of Pu(VII) will be : PuO~- + e-

~

PuO~ + + 2e-

PuO~+ + 302~> PuO 2

When Na3PuO 5 is added to the bath at 240°C, a chemical reduction by the OH a)

ions or an internal oxido-reduction occurs :

2PuO

+ 2OH-

~

2PuO + + 602- + 2H + + 02

which is followed by 2H + + 2OHb)

2PuO~-

~

~

2H20 ; 2H20 + 202-

~

40H-

22PuO~ + + 402- + 02

This electrochemical two steps mechanism is well supported by the analysis of the slope~ E / ~ l o g

i ° of the Tafel lines.

When an electrochemical reaction proceeds in several steps whose jt~ is the slow determining step, Mauser (7) uses the formula E/~log

i° =

F2.3Aj RT

for the calculation of the Tafel slope values.

Aj is a complex term depending on the number of electrons nj transferred in the slow step, the electrochemical transfer coefficient a and the number of electrons _ ~ n i in all the steps preceding the jth one.

AJ = m nj + ~ j-lq

ni

Vol. 8, No. I 0

HEPTAVALENT PLUTONIUM

From the above quoted values f o r ~ E / ~ l o g

867

io, it comes that A J C 2

with 2"3F RT = 0.102 at 240°C. If we assume that PuO~ + + 2eis the rate determining step, nj = 2 and

~ 1 j-1

>

PuO 2

ni

Aj = 2 fits well the experimental data for = = 0.5. This value of the electrochemical transfer coefficient has already been pointed out for the reduction of NpO~" (I) in molten KOH - NaOH

CONCLUSION As it is observed in aqueous alkaline solutions, the stability of heptavalent Plutonium is inferior to that of heptavalent Neptunium in a KOH-NaOH melt at 240°0. Spontaneous reduction of Pu(VII) gives only Pu(VI) as a reaction product. The electrochemical reduction proceeds in two steps : PuO;- + ePuO~ + + 2e-

>

PuO~+ + 302>

PuO 2 (rate determining step)

ACKNOWLEDGMENTS We thank the I.I.S.N. for the financial and general support of this work.

REFERENCES

(1) L. MARTINOT and (].. D~CKAERTS, I . N . C . L . , (2) e . KELLER and H. SEIFERT, I . N . 0 . L . ,

6 , 546 (1970)

~ , 51 (1969)

(3) V.I. SPITSYN et al., J.I.N.C., 31, 2733 (1969) E. HENRICH, Thesis from Ruprecht - Karl - UniversitMt, Heidelberg

1971 (4) L. MARTINOT and G. DUYCKAERTS, Anal. Letters, 4, 1 (1971)

(5) P. DELAHAY, New I n s t r u m e n t a l

Methods i n E l e c t r o c h e m i s t r y ,

Interscienoe Publishers, New-York 1964, P. 179 (6) F. CALIGARA, L. MARTINOT and G. DUYCKAERTS, I.N.C.L., 4, 169 (1960) (7) F. MAUSF~, Zeit. Elektrochemie, 62, 419 (195S).