Fabrication of uranium-plutonium oxide microspheres by the hydrolysis process

Journal

of Nuclear

0 North-Holland

Materials Publishing

FABRICATION PROCESS R. FORTHMANN

64 (1977)

OF URANIUM-PLUTONIUM

OXIDE MICROSPHERES

BY THE HYDROLYSIS

and G. BLASS

Institut ftir Reaktorwerkstoffe,

Received

275-280

Company

Kernforschungsanlage

Jiilich, Federal Republic of Germany

27 July 1976

The application of the Hydrolysis Process to the fabrication of (U, Pu)Oz microspheres is described. A uranium feed solution of high uranium concentration is prepared from concentrated uranyl nitrate-urea solution by adding solid hexamethylentetramine. The uranium feed solution is mixed with a concentrated acid-free plutonium nitrate solution and processed into high dense (U, Pu)O2 microspheres with Pu/(U + Pu) atomic ratios between 0.1 and 0.2. The Hydrolysis Process is applied also for pure substoichiometric plutonium oxide microspheres. L’application du processus d’hydrolyse a la fabrication de microspheres de (U, Pu)O2 est decrite. Une solution d’alimentation en uranium de concentration &levee en Uranium est p&par&e a partir d’une solution concentree de nitrate-uree d’uranyle en ajoutant de l’hexamethylene tetramine solide. La solution d’alimentation en uranium est melangee avec une solution concentree de nitrate de plutonium exempte d’acide et transform&e en microspheres de (U, Pu)Oz de haute densite ayant des rapports atomiques U/(Pu + U) compris entre 0,l et 0,2. Le pro&de d’hydrolyse est applique aussi a la preparation de microspheres d’oxyde de plutonium pur sous-stoechiometrique. Die Anwendung des Hydrolyse-Verfahrens auf die Herstellung von spharischen (U, Pu)OZ-Teilchen wird beschrieben. Eine Urangiessliisung hoher Urankonzentration wird aus konzentrierter Uranylnitrat-Harnstoff-Losung durch Zugabe von festem Hexamethylentetramin hergestellt. Die Urangiesslosung wird mit einer konzentrierten saurefreien Plutoniumnitratliisung gemischt und zu hochdichten spharischen (U, Pu)Oz-Teilchen verarbeitet, deren Pu/(U + Pu) -Atomverhaltnisse zwischen 0,l und 0,2 betragen. Das Hydrolyse-Verfahren wird such auFreine unterstijchiometrische Plutoniumoxidteilchen angewendet.

1. Introduction

powders, - (U, Pu)Oz microspheres, prepared directly from aqueous plutonium nitrate solutions from the reprocessing plant. Fuel pins containing vibro-compacted microsphere fuel (sphere-pat) are suitable for LWRs as well as for FBRs. Several irradiation tests on these sphere-pat fuel elements have shown a good performance [3-S]. For HTGR application, fuel is also needed in the form of microspheres for coated fuel particles. Preliminary irradiation experience on coated particles with plutonium containing kernels is available [6] .

The development of plutonium containing fuel elements for use in different types of nuclear reactors will be an important technological problem in the future. Presently, plutonium is produced by neutron capture of U-238 in Light Water-cooled Reactors (LWR) in increasing amounts, which, in Germany, will reach about 30 t Pu in the year 1985 [ 1,2] . As this production of plutonium will exceed its need for Fast Breeder Reactors (FBR), the use of plutonium in Light Water Reactors (LWR) or in High Temperature Gas-cooled Reactors (HTGR) has to be considered. For the application of plutonium oxide containing fuel to LWR fuel elements two different fuel forms are being considered : - (U, Pu)02 pellets, prepared from mixed oxide

For the production of plutonium containing microspheres several processes have been reported [6-l 1 ] . The Hydrolysis Process (“H-process”), which was developed for Urania microspheres having sizes in the range of 100-1200 pm [12,13], is another very prom275

king process. Because of its simpie ~ra~t~~abi~~~y~this new process is alsc considered for the ~efabri~at~on of U-233 ~o~ta~~~ng microspheres in the E-i-U fret cycle. Supplementary to the experience of Lahr 1141 1who tised the H-process previously for the preparation of (U, I%)02 sphere-pat LWR experimental fuel starting from PuO~ sol and s~bsto~~h~om~tr~~ uranyl nitrate ~olutions~ we demonstrated the a~~~~~ab~~~tyof the Kprocess to @Ju,pil)Oz and PuOz microspheres by usmg nearly stoicbiometric solutions, rather than ~01s. The results of these investigations are described in this pa. per.

2. Chemistry sf the hy~r~~ys~~ process 2.1. Uru~iumoxide microspheres A detailed ~esc~~t~on of the H-process for pure hlC12micros~heres was given in a previous paper [I 31. ‘fhe process is based on a rapid solidification of dropiets from a rather concentrated uranyl nitrate solution (about 1.4 M) containing urea and hexamethylenetetramine (““hexa”) at tern~~r~t~~res of about 90°C. This “~~aniui~ feed solution” is prepared by dissolving urea in a uranyl nitrate solution of sufficiently high uranium concentration (about 500 g U/l) and addition of solid hexa at temperatures below tlO”C. At these temperatures the solution is metastable: no increase in its viscosity occurs within 24 h. From ~~~otornetr~~ meas~iremEnts in the region of the optical absorption maximum at wavelengths between 400 and 450 nm, it was concluded that the solution contains ionic species each consisting of three uranium atoms connected by OH-bridges. These cations are additionally corn” plexed by two urea molecules per ~ran~u~?~atom. At 9C”C a very rapid ~olym~r~~a~~on of this cationic species occ‘rlrs:

e resulting product is a positively charged BkQz gel. Viscosity and optical absorptjon at U wavelengths between 400 and 4.50 nm increases during this chemical reaction, but the initial pW value of 5 .O-5.5 remains constant. No precipitation is observed: the

a diluted Pu(N03)4 solution in nitric acid (NO$/I’u Z 8) using the cation ion exchange resin DOWEX3, followed by vacuum evaporation of water at 38°C (40 ton), until a concentration of 1.2-I .3 kg l’u/l is reached. During this evaporation step the Pu(IV) polymers are depolymerized. The final dark brown solution is free of polymers and can be stored for an unlimited period of time without any tendency for polymerization. The chemical composition of black crystals obtained from such solutions was found to be Pu(OH)(NO~)~ ..5Ha0 [ZO] . If the concentrated Pu(OH)(NOs)s solution is diluted by water, very rapid hydrolysis occurs forming a green colloidal solution of Pu(IV) polymers. From theoretical considerations, the compatibility of the concentrated acid-free plutonium nitrate solution with the uranium feed solution of the H-process is expected to be good. Both solutions have low acid content and comparable high heavy metal concentrations. Consequently, the thermodynamic activity coefficients of UOZ’ and Pu4+ are both sufficiently small for a retarded hydrolysis at room temperature even at rather high pH values. For these reasons both solutions show very rapid hydrolysis if they are diluted by water.

3. Expe~mental

procedure and rest&s

Since no plutonium laboratories were available at KFA, the experimental work was performed at the Eidgeniissisches Institut fur Reaktorforschung (EIR), Wiirenlingen, Switzerland. From the fabrication of (U, Pu)C sphere-pat fuel for use in FBR fuel test pins by applying a modified KEMA gel precipitation process [2 1] , complete sphere forming equipment was available in the plutonium facility of the “Hot Laboratory”, which was very useful for our experimental work. The feed solution was forced under pressure from a cooled steel container through a vibrated nozzle forming single droplets which were solidified in a glass column with flowing hot silicon oil. After solidification of the droplets the microspheres were collected on a sieve. The apparatus is described in detail in EIR reports [ 18,191.

The uranium feed solution was prepared completely outside the plutonium facility, having the following

composition: 41.2 10.9 18.2 29.7

wt.% wt.% wt.% wt.%

UOz(N0s)a*6H20 (NHz)zCO (urea) Hz0 (CH2)eN4 (“hexa”).

The hexa was added as a.powder to the uranylnitrateurea solution. The complete uranium feed solution was then chanelled into the plutonium glovebox and mixed with the calculated amounts of acidfree plutonium nitrate solution having a plutonium concentration of 298 mg Pu,‘g and a nitrate content of NO~/Pu = 3.77 (table 1). Table 1 Composition of uranjum-pl~~tonium feed solutions -g Pu solution per 100 g U-feed ___..-...

wt.%Pu

wt.%U

Pu --._ u + Pu

NO; -u + Pu

1.32 16.44

2.03 4.21

18.21 16.79

0.1 0.2

2.18 2.35

As expected, the two solutions were fully compatible: no precipitation occurred after mixing of the solutions or during the whole sphere forming procedure. The uranium-plutonium feed solutions were dispersed in droplets, which were solidified in flowing silicon oil at 80-9O’C. The microspheres were dark brown and transparent. Due to the very fast solidification no cluster formation on the column was observed. From a more detailed study of the solidification conditions, an influence of the aging time in the hot oil after the solidi~~at~on step was found. If the aging time was about two min, several cracked microspheres were observed during the following hot water washing step. After aging for 30 min in the hot oil, the colour of the microspheres changed from dark brown to greenish brown and they became turbid. These microspheres survived the washing procedure without any crack. Probably the heating at SO-90°C leads to the formation of green plutonium(IV) polymers which are nuclei for PuOZ crystallite growth similar to the behaviour of ThOz in the H-process flowsheet [ 131. During the hot water washing step the U-Pu oxide microspheres became greenish yellow due to further crystallite growth. After drying in air at 70-80°C they were calcined in flowing argon using the following time/ temperature program:

to 11O”C, 60 min lo 45o”c,

120 min te zwc, 60 min at 450°C.

40 min

Reduction phere

and

5 h at i46O”C. were

black

Micrographs show

sintering

by heating

was

at a rate The

and

in general

sintered

glossy

of the

in Ar/4% and

(U, I”u)O,

wIthout

ncn-etched

a single-phase

done

of 210”C/h

any

Hi atmossintering

defects

(fig.

ceramographic structure

for

microspheres i;R~ sections

without

poro-

density near the theoretical density. in some microspheres, ‘however, so.me single large spherical pores were observed, whici? may have been caused either by incomplete degassing of the feed solutions or by radiolytic gas bubble formatian in the wet gel microspheres. sity

(fig.

2). This

Fig, 2. Micrograph

indicates

a high

of (U, Pu)Oz kernels (as polished).

unbroken fraction consisted of substoich~omet~~ PKIO~_~microspheres with a heterogeneous structure: the photomi~rographs show two different oxide phases (fig. 3). A smaller amount of white h~perstoi~hio~~~tri~ Puz 03 phase appears in the gray substoichiometric FuQz phase. The presence of two oxide phases can be explained from thermodynamic ~ons~derat~o~sof the sistering conditions and the plutonium-oxygen phase diagram. Assuming a water contamination of about 10 vpm Hz0 in the Ar-4% I-&Jatmosphere, the oxygen potential

spheres became dark green. At the same wasb~~~conditions as used for (U, Pu)O, many micraspheres cracked during c&in&ion between 200 and 45O”C, Pure plutonia microspheres need longer washing times, similar to pure thoria microspheres prepared by the Hprocess [ 131 . As the fresh microspheres consist of colloidal Pu02 PII&O (PuOz gel), this effect can be explained by the well known difficulties of removing NO, ions from PuOz sols 131, The remaining nitrate (NH4N03) is decomposed violently during the ~a~~jna~oustep causing cracked microspheres. An optimization of the washing conditions of pure plutonia microspheres was not achieved within the scope of this work. After sintering at 1460°C in Ar/& atmospheres the

calculated from the ratio of partial pressures P~QQJ = 2.5 X 204 would be about ??Q = -130 kcdJ PH2 mol at 146O’C. This gives an oxygen partial pressure of p(-& = 5 x 10+-17bar. From the phase diagr~l of Riley [22,16] (fig. 4) it can be concluded, that at 1460°C the single phase microspheres have a composition of approximately Pu01.a5. During cooling down disproportio~t~un in two oxide phases occurs when the rn~s~~b~l~t~ gap between Fu01.52 and PIIO~,~~is reached at temperatures below 800°C. For preparing stoichiometric PuOz microspheres sintering must be done in air.

4, Summary and conclusions It was demonstrated that (U, Pu>Oz mi~rospheres with high density can be produced by the Hydrolysis Process without any difficulties. A concentrated acidfree p~uto~um nitrate solution prepared after a flowsheet of the EiR [ 17- $91 was fully compatible with the u~nium feed solution of the H-Process. The manium feed solution can be made completely outside of the piutou~~ facility and mixed with the plutonium

Table 2

Experimentalconditionsfor the fabrication of (U, pU)Oz microspheres u&g the f&process --ll~-.

0.0

2.00

2.33

0.1

2.00

2.32

20.2

0.2

2.00

2.06

21.0

1.0

2.Oa -

1.83

20.3

19.5

.-

Hza(95"C) NH3 cx”C)

Ar(4S0°C)

ArlH2 fl460") Uq

1)

I',

31

t~,~)u~

W

15

3,

mwoz

8,

,s

,,

solution directly before the sphere forming step. The main expe~me~tal conditions for the fabr~~at~~r~ of (U, F’u)O2 ~nicrospher~s with different ~~uto~~~~~lcontents ranging between 0 < Pu/(U + wt) < 1 are givw in table 2. Any marked influence of the alpha radiation of plutonium on the chemistry of the process could not be observed.

Acknowledgement The authors wish to thank the EIR W~renl~~ge~l Switzerland for making their hot laboratory available for the experimental investigations, and parti~~llar~y Dr. K. Bischoff and his coworkers V. Birchmeier and M. Gehringer for their helpful assistance and cosperaCon.

[I ] W.J. Schmidt-Kiister, Atomwirtschaft 21 (1975) 282. [Z] W. Stall, Atomwirtschaft ‘1 (1975) 419. j3] CM. Cox, A.R. D&en, R.B. F&s, AL. Loits. Symposium on Sol-gel processes and reactor fuel cycles, Gat~~~bur~~ Tenn. CONF-700502, May (1970) p. 359. [4] A. Cervellati, G. Cogliati, G. Falcinelli, P. Guermani; C. Lepsckp, 6. Testa, ibid., p. 374. [5] C.J. Baroch, E.N. Harbinson, ibid., p.414.

cesscs and reactor fuel cycles, Gat!inbu*g/Tenn. f06P700502 (May 1970) p. 253. 1221 B. RileJi, Sci. teiam. 5 (1970) 8.3,