New reprocessing method overcoming environmental problems

Progress in NuclearEnergy, Vol. 29 (Supplement), pp. 227-234, 1995 1995 Elsevier Science Ltd Printed in Great Britain 0149-1970/95 $29.00

Pergamon

0149-1970(95)00047-X

NEW REPROCESSING METHOD OVERCOMING ENVIRONMENTAL PROBLEMS

H. Tomiyasu and Y. Asano Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan

ABSTRACT

A new method for the nuclear fuel reprocessing has been proposed. This method consists of several processes, all of which are carried out in low acidic solutions at room temperature (we call such conditions as mild conditions hereafter) and the separations are made based on precipitation methods without using organic solvents. Therefore, any potential danger of explosion can be avoided. The dissolution of uranium dioxide UO2 fuel were carried out in dilute HC1 and HNO3 solutions in the presence of strong oxidants such as chlorine dioxide C102 or ozone.

In the latter case Ce3+ was used as a catalyzer.

Uranium and

transuranium elements are separated by a precipitation method as follows. Since penta or hexa valent ions of these actinides form extremely stable complexes with carbonate ion and the complexes formed are soluble in aqueous solutions at pH 7-9, these actinide ions are separated from precipitated fission products. Under such basic solutions most of transition elements, lanthanides, zirconium and platinum group elements as well as alkaline earth metals are precipitated. The platinum group elements, Ru, Rh and Pd are recovered by using the complex formations of these elements with trichlorostannate(II) [SnC13]-, which precipitates by the addition of some cations, such as Cs+ or quaternary ammonium ion. It was found that [SnC13]- acts an effective precipitant of Cs+.

KEYWORDS

A new method; nuclear fuel reprocessing; mild conditions; dissolution; separations

227

228

H. Tomiyasuand Y. Asano INTRODUCTION

Purex is the only nuclear fuel reprocessing method, which has been applied to commercial plants. Even though, this method involves various problems as follows: dissolutions of UO2 fuel are made under severe conditions, i.e.in HNO3 at nearly boiling temperatures; organic solvents used are of potential danger of explosion; hazardous minor actinide elements, Np, Am and Cm and strong radioactive elements, such Sr and Cs are all mixed into high-level waste solutions; the process is accompanied by a large quantity of wastes, particularly nitrate and phosphorus compounds.The objective of the present study is to develop a new method which consists of the following major processes; 1. dissolution of UO2 in mild acidic solutions(Asano et al., 1994a); 2. separations of U, Pu, Np and Am from fission products by a simple precipitation method; 3. separations of Cs and Sr from high-level waste solutions; 4. recovery of Noble metals, Ru, Rh and Pd(Asano et al., 1994b). All processes are carried out under mild conditions without using any flammable organic solvent and hence the possibility of danger due to the explosion of organic compounds should be ultimately zero. From an environmental point of view, the reduction of radioactive waste is most important. Since transuranium elements can be used as nuclear fuels, strong radioactive elements such as Cs and Sr be used as radiation sources, if separated independently, and the industrial use of noble metals recovered might be possible, these elements are no longer wastes. Therefore, the effective separation of the above elements results in the reduction of high-level wastes. The aim of the present study is to establish a new reprocessing method which minimizes the high level-wastes.

RESULTS AND DISCUSSION

As described above on the Purex method, UO2 is dissolved in nitric acid at nearly boiling temperature. The dissolution yields nitrous acid, which is a reduction product of nitric acid and is in equilibrium with NOx. Under such conditions uranium exists as UO22+, plutonium mainly as pu4+ and neptunium as NpO2 + and partly as NpO22 +. Then, the extraction with tributyl phosphate (TBP) results in the separation of UO22 +, pu4+ and NpO22+ (a part of neptunium) from fission products. A problem still remains, however, in the Purex method as follows: strongest radioactive elements, Cs and Sr, valuable platinum group elements, Ru, Rh and Pd, and the most hazardous minor actinides, Np, Am and Cm, together with lanthanide elements, are all mixed into high-level waste solutions. Because of the oxidative nature of nitric acid and nitrous acid, the purex solutions are basically oxidative in nature and the valence adjustment toward a reductive direction is difficult unless reduction potentials of adjusting elements are larger than the redox potentials of nitric and nitrous acids. The existence of nitrous acid makes the valence adjustment even more difficult, since nitrous acid is also oxidized by strong oxidant such as Pu(VI).

New reprocessingmethod overcomingenvironmentalproblems

229

In Fig. 1 and 2, new reprocessing systems are illustrated. Both systems are basically the same consisting of the following main processes: dissolution of UO2 under mild conditions(Asano et al., 1994a); separation of U and Pu as well as minor actinides, Np and Am; separation of strong radioactive elements, Cs, Sr and Sn; recovery of platinum group elements, Ru, Rh and Pd(Fig. 3) (Asano et al., 1994b); separation of lanthanides and Cm. In contrast with Purex method, the formation of nitrous acid is not allowed in these systems, even though the dissolution is made in nitric acid solutions, since nitrous acid formed is oxidized quickly by ozone. Therefore, the valence adjustments towards higher oxidation states, i.e. the oxidation of Am3+ to Am(V) or Am(VI), is relatively simple. In hydrochloric acid solutions, after the removal of chlorine dioxide no oxidant exists and hence the valence adjustment towards

lower oxidation states, i.e. the

reduction of NpO2 + to Np4+ or Np3+, becomes possible. The system 1 (Fig. 1) is in favor for the reprocessing of UO2 fuels, while the system 2 (Fig.2) for the reprocessing of metal fuels. As a matter of fact, the inside of block by dotted line in the system 2 reveals the reprocessing of metal fuels, because uranium metal is dissolved in hydrochloric acid without any oxidant giving uranium(IV).

In an earlier study on the dissolution of UO2 fuels(Ikeda et al., 1993a). it was found by means of 170 Nuclear Magnetic Resonance that oxygen atoms labeled in

UO2 remained in

UO22+ during

the

dissolution. This indicates that one or two electrons transferred from UO2 oxidant resulting in the formation of UO2+ or UO22+. Based on the above mechanism, the dissolution of UO2 is expected to be made under mild conditions if a strong oxidant exists in solutions. Experiments were carried out in dilute nitric acid solutions in the presence of 0.1 M (M = tool dm-3) Ce3+ ion under bubbling of ozone. Figure 4-1 shows dissolution ratios against time.The dissolution was observed in 0.5 M HNO3 solution and the dissolution progressed more rapidly at higher acidic solutions, but was nearly constant at

HNO3 above 3 M. In

hydrochloric acid solutions, the dissolution behavior was similar to that in nitric acid. The dissolution was markedly observed in the presence of NaC103 (Fig. 4-2) and again the dissolution rate was almost constant in the region HC1 above 3 M. As seen in Fig. 4-3, the dissolution was even more remarkable, if solutions contained small amount of chlorine dioxide C102. It is interesting to see in Fig.4-4, that the dissolution rate was independent of acid species, HNO3, HC1 and HC10~ in solutions containing 0.17 M C102.

It is most characteristic that U, Pu, Np and Am form stable carbonate complexes in their penta- or hexavalent ions, i.e. for hexa-valent uranium (VI) ion the stability constant of [UO2(CO3)3] 4- is the order of 1023. These carbonate complexes are soluble in basic aqueous solutions at around pH 9, under which conditions most of metal ions are precipitated with only few exceptions such as zinc and alkalin ions. By filtrating precipitate, U, Pu, Np and Am can be separated from precipitated fission products (Solution 1).

230

H. Tomiyasuand Y. Asano

Spent Nuclear Fuel Dissolution in dilute

I ~.- Off Gas (12, Ru04) l~Addition of (NH4)2CO3 or Na2CO3 /to adjust pH = 8~9 filtration I I Precipitate 2 Solution 1 Rare earth, Cm, U,Pu,Np,Am Ru,Rh,Pd,Sr,Nb, and Zr,Mo,Sn,Ba,etc. Cs,Na I Addition of dilute Separation Separation HCI to adjust of Sr pH -- s of U and TRU r-filtration- ! r-Cation Exchange Resin--]

HCI with CI02, or HNO3

[Precipitate

3I

Solution Sr,Ba,Sn

k

Addition of zM HCl and 0.1M SnCl2

Recovery of Noble Metals [

filtrationq

I Precipitate Ru,Rh,Pd

4

Cm,Zr,Mo,Nb,etc. Raer Solution earth, 5

I

Complex

4llCo,umnI Cs,Na

l Addition of ~2M HCI land l S n C l z , °r [NaB(C6H5)4 I- f i l t r a t i ° n

I IPreci CspitaltJe formation

with macrocyclic I filtration --1 Precipitate 5 Rare earth [ Zr, Mo,Nb,etc. and Cm

ligands

I.Solution6]

Separation I Photochemical of Cm

reaction

I--- filtration --] Precipitate

Cm

o,u,,oo

7

Rare earth

Fig 1. Schematic Diagram of New Reprocessing System

I s°'uti°n zl U,Pu,Np,Am

Separation of Cs I Solution Na 3

Fig, 2

I I U, andPu. AmNp, 1

filtrate

I

Separation of U and TRU by using anion exchange resin

fillration

I

IU andTRU Fission Products)

filtrate

Addition of CsCI or Et3NHC1

Schematic Diagram of New Reprocessing System

I Lanthanides Cm,Zr,Sr,Ba, etc.

Neutralization by Carbonate

Oxidation of U and TRU

Main Process

Ru, Rh, and I'd 1

I

Recovery of Noble Metals iprecipitate ( filtration

I

Complex formation with SnC13- I-~

i

( reflux tbr 1~2 hours)

Addition of SnC12

~Removal of Cs by precipitaion

t with SnCI2

Reducing reagent

in dilute HC1 or HNO3 Reduction

Dissolution of U O 2

Spent Nuclear Fuel )

Idition

dryingin desiccator

= 1.0mol/dm s

Fig 3. Recoveryof Rutheniumby Using the Complex Formation of [RuCI(SnCis)s]4-

gravimetric analysis)

Determination of [Ru]

filtrate

~;t3NHCI]

[ CsC1 ] = 1.0tool/din s

I

Assignment (uv .......... t)

)

ddition o f anti oxidantS/<

I

reflux for 70rain

p r e c i pi~at~

(

Recovery of Ru[

Separation

Precipitation Reactions

Complex formation of [RuCI(SnCI3)5]4-

[ HCI ] = 2,3 tool/din 3 [ RuCI3 I = 4.0 × t 0 3 tool/din 3 [ SnC12 ] = 1.0 N 10 1 tool/din 3

Sample Solution 5 0 × 1 0 "3 dan 3

@

o

<

8

O e~ O <

0~

o

z

232

H. Tomiyasu and Y. Asano

10 0

..~'. ?i.

....... o--" ...It'"

~;,,,=,

-~.

•~

80

80

• ......

.'

o

,?.'

2o ~" .."

;=,

. ,,,I .... I .... I .... I .... I .... 0 I~ 50 100 150 200 250 300

20 0

50

100

150

200

250

300

350 Time / min

Time / rain ---.e---- Initial

[HN03]

= 0.5M

----*----Initial

[HN03]

= 5M

---.w---- Initial

[HN03]

= 3M

F i g . 4 - 1 D i s s o l u t i o n of U O 2 ( 1 0 0 , ~ 3 0 0 ~ m) in HNO

s o l u t i o n s in the p r e s e n c e of 0,1M Ce(IV) " 3 at 30~C under c o n d i t i o n s of bubbling ozone.

100

....

80

:

i/ :.-

°O~ O

40

-///

""4t"-lnitial

[HCI] = 3M

[HCI]

.... v----Initial

[HCI]

Fig. 4 - 2

Dissolution o f U O 2 ( 1 0 0 - - 3 0 0 u m )

100

,.,, ¢

. . . . .

O

6O

.o ..b _=

,..

":7

o 40

I ....

I ....

50

100

[CI02] = O . 0 6 M

I ....

150

.g ,,I,,,I,.,I,,,I,,,I,,,

I ....

200

250

0

20

in 3M

---'='---Initial

HCI

Dissolution

[C102] = 0 . 1 2 M

solutions

---.u----Initial ""'*"--Initial

ofUO

2 (100--300

containing

40

60

80

100

120

Time / min

[C102] = 0 . 1 7 M

4-3

I - : . - { t ~ . . . . . " r ........... ~. . . . .

°~

Time / min

Fig.

to

/' ,,',' ,/ ,~" ,, /,,

e~

f/

20

---'e----Initial

3

,,# ,$;'Y

"~- 8 0

i/' /

---+---Initial

at 2 0 %

p

20 ....

= 7M

HCI s o l u t i o n s at v a r i o u s c o n c e n t r a t i o n s .

. . . . .

:i ?

0

= 5M

in the s o l u t i o n s p r e p a r e d by adding 3M NaCIO

w'"'""

,'

"i

60

J..-'*'"'-'"l

[HC]] = 1M

"-'-*--'-Initial

f

/'

/ ,,"

'..

:" ' = '

f

/ /

.O ~

--

L-":"z"z"- 1 '

//' ,,," F

" ' " ~ ---[n i t i a l

CIO

2

~ m) at 20°C.

[HCI04] = 3 M [HCl]

-"-*---- Initial

[HN03]

= 3M

= 3M

Fig, 4 - 4 D i s s o l u t i o n o f U O

(100--300

~ m)

in v a r i o u s acid s o l u t i o n s bubbling 0 . 1 7 M C I O at 2 0 ~C.

2

New reprocessingmethodovercomingenvironmentalproblems

233

However, Cm is not oxidized to higher oxidation states in aqueous solutions and mixed into tri-valent ianthanide elements. In order to separate Cm from lanthanides, we are investigating on complex formations of lanthanides with macrocyclic ligands and on photochemical behavior of these complexes. Separation of Cs is very important, because Cs is the strongest "r emitter of fission products and the mixing of Cs into high-level waste should be avoided. Furthermore, Cs can be used as a radiation source, if separated purely. It was found that the addition of SnC12 yielded the effective precipitation of cesium compound. The separation ratio by this method is over 99% from aqueous solutions.

In Fig. 1, precipitate2 contains the following elements: Sr and rare earth involving Cm; platinum group elements, Ru, Rh and Pd; Zr, Mo, Tc, Sn and Nb. It is of particular importance to separate Sr from other fission products because of its strong radiative(/3 emitter) nature. Unlike other heavy metal ions, Sr is precipitated as carbonate salt and is easily dissolved when solution is slightly acidified (Solution 4). The recovery of platinum group elements were carried out by using the complex formation of these dements with SnCI3- as seen in Fig.3. In case of Ru, the complex is identified as [RuCI(SnCI3)5]4-. These complexes are easily precipitated when mixed with cations, such as Cs+ and triethylammonium ion (Precipitate 4). The recovery ratios were extremely good in 2M HCI solutions for Ru, Rh and Pd to be nearly 100% as Cs+ was used as a precipitant. In solutions containing HNO3, the recovery ratios were slightly lower than in HCI solutions.

The separation of Cm from Solution 5 is somewhat complicated. Recent studies including our works(Ikeda et

al., 1993b,1994)

on this

subject using carbamoylmethylene-phosphoric acid (CMP)

and

carbamoylmethylphosphine oxide (CMPO)seems to be successful for extracting Cm together with all rare earth elements from HNO3 solutions. However, non of the results shows the specific separation of Cm from high-level waste solutions. In order to separate Cm specifically, a study based on the complex formations with macro-cyclic ligands and their photochemical reactions is in progress.

It should be noted finally that the Purex is the well established reprocessing method and should be operative safely under severe controls. However, the severe controls are unfavorable as far as the operative cost is concerned. In view of the operative conditions, the present system would be more favorable economically.

234

H. Tomiyasuand Y. Asano SUMMARY

The new reprocessing proposed in the present paper enables us to separate U, Pu, Np and Am from fission products; to separate Cs and Sr independently; to recover platinum group elements. All processes are carried out under mild conditions without using any flammable organic solvent and hence any potential danger of explosion can be avoided.

ACKNOWLEDGMENT The present study has been carded out in cooperation with Dr. Yasuhisa Ikeda of Institute of Research innovation, Mr. Yukio Wada of PNC, and Prof. Kunihiko Mizumachi of Rikkyo University. The authors wish to acknowledge their cooperations.

This paper was partly supported by a grant-in-Aid for

Developmental Scientific Research No. 0655006 for Ministry of Education, Science and Culture.

REFERENCE~

Asano, Y., M. Kataoka,Y. Ikeda, Tomiyasu and S. Hasegawa(1994a). New method for dissolving UO2 using ozone. GENES~TIT "94, Abstracts, 33 Asano, Y., T. Yamamura, H. Tomiyasu, K.Mizumachi, Y. Ikeda, and Y. Wada(1994b). Recovery of noble metals from high-level liquid waste by precipitation method. Spectrum "94, 2_836-841 Ikeda, Y., Y. Yasuike, Y. Takashima, Y.Y. Park, Y. Asano and H. Tomiyasu(1993a). 170 NMR study on dissolution reaction of UO2 in nitric acid. J. Nucl. Sci. Technol., 30, 962-964. Ikeda, Y., A. Miyata, Y.Y. Park, K. Hatakeyama and H. Tomiyasu(1993b). Nuclear resonance study of ligand exchange reaction in lanthanum nitrate complex with n-Octyl(phenyl)-N,NDiisobutylcarbamoylmetylphosphine Oxide.J. Nucl. Sci. Technol., 30, 720-723 Ikeda, Y., K. Hatakeyama, Y.Y. Park and H. Tomiyasu(1994). A nuclear magnetic resonance study of ligand exchange reactions in La(III),Nd(III),U(IV) nitrate complexes with n-Octyl(phenyl)-N,NDiisobutylcarbamoylmetylphosphine Oxide. RECORD-94, 3.