Determination of the oxygen-to-uranium ratio in hypostoichiometric uranium dioxide

Awlylicn Chimica Acla Elscvicr Publishing Company, Amsterdam Printed in The Ne~hcrlands

437

DETERhLINATION OF THE OSYGEN-TO-URANIUM HYPOSTOICHIOMETIiIC URANIUM DIOSIDE”

RATIO

IN

In general, the reactor fuel uranium diosidc is l~ylxrstoicllionwtric. Howcwr, if a specimen of this material is l~eatecl at temperatures near or above 2000° in citlicr inert or reducing atmospheres hylxxtoichiomctric UOC..~ is formed*. As far as is lanown, the UOZ.-~, so obtained is composed upon cooling of uranium metal in a mntris of uo 2.000 -3. ITor the dctcrmination of the ovcmll os3’~eu-to-uranium ratio in UO:!.-,,, tile following methods have txen mentioned in tlic literaturc”~“. (a) The snmplc is osiclizccl in air:

and the O/U ratio is calculatccl

from the incrcasc in weight. in nitric acid can lx followed

Altcrnativel~,

dissolution

3 U&_,,

3 l_JOz”+ -+ U&H

fr!“!?:

and the O/U ratio is calculntccl from the increase in weight. (0) Partial oxidation of the sample in air is followed UO&r/

UOz.,.z “L

-2

by

COll\‘CfSiOIl

to

U:IOH:

by reduction in hyclrogen :

UOz.aoo + .V Hz0

The O/U ratio is calculated from tile increase in wei&it for the overall reaction : --+ LJOz.ooo uoa-,, (c) Hydride formation is usccl, the free uranium metal in UOO-,, Ixing converted first to uranium hydricle, after which the hydride is dccomposecl : 2 uoz-,,

-L

(2 -3~)

UOa.oon + _J’UH:j --f

2

UOz-,,

+ L&J, H:! T

In this case the O/U ratio is calculated from the amount of released hydrogen. The methods described under (a) are simple and reliable. However, adsorbed moisture in the samples may cause erroneous resultkP, and on account of the small increases in weight, large samples are needed. Method (0) does not appear to have any aclvantage over methods (a), and it has the serious drawback that the UO 2.000finally formed shows considerable reactivity (with oxygen and water)3. Method (c) is more attractive, but has the disadvantages2 that uranium metal * This paper WE prcscntccl :Lt the ‘IYhirtccnth Confcrcncc Tcchnolo~y, Gntlinbur~, Tenncsscc, U.S.A., October, rgbg.

on .4nnlytid

Chctnistry in Nuclear

occ:luclecl within the UOs-q-ains may not form the hydride and thus will escape subIAank is obtained, which makes t11e use of large sequent detection, * and ZL consideral>lc samples incvitalAe. I’(Jr the dcterminatic~n of the O/U ratio in small samples of hypostoichiornetric uranium diosicle, none of tlie met:hods mentioned above is attractive. ITor tllis reason the incrits of CL new mctliod wherein the sample is dissolved in concentrated pl~ospl~oric acid llavc lxcn studied. During dissolution the valency of uranium in UO~.ooo will uranium present is convertecl to uranium( IV) remain uncl~aiigcd~~, while the metallic kuncl an erluivalcnt amount of hyctrogcn is rcleasccl. Mcncc the overall reaction sl~-~.~lcl lx : UO~-,,

.-5

U‘1.k -j- (2 -,I))

Ha0

+ y 1-h t

‘lThf2 0/U ratio is calculatccl froin tlic amount of l~ydt-ogcn rclcascct. I f uraniuin( 1 I 1) is to uraniurn(lV) during dissolution and the prcsciit in the U03-I,, it: is a,lso convcrtccl saiiic ova-all reaction Jvill occur. I!or the measurement of the amount of llyclrogen, the method used involves removal of the hyclrogcn from the phosphoric acid solution by a helium stream. ‘IYE hydrogen is stripped from the helium by Molecular Sieve 5A powder at liquid nitrogen tcmpcrnturc. Afterwards the hydrogen is clcsot-lxx1 from the nwlccular sieve at room tempxakurc nnd clcterrninecl by gns cllrc.)matojiraphy”. This method WZLS tested. successfully on rnctallic uranium samples of :&out I mg. With s:~mples ol UO2._,, weighing =g--~~pmg, tlu3O/Uratios measured corresponclcd cluitc acccptnlAy lvith tlie values ol~tainccl by gravimctric method (a) on ~ooo-nq samples.

Hcli~rf~/lr.‘I’lrc cylinclor gas usctl CO~ltilillWZl tlic following masimum conccntr:Ltions of impurities: 0.5 11.pm. 11yclrogcn, S p.pm. nitrogen, 1 p.p.ni. osygen, 14 p.p.ni. argc-)n by weight. I~W~C nfhhdn~ s~~~~c+,I . This Lv;Ls Used iLS 30-410 mcsll powclcr obtainccl front I / 1 O-in pclkts by grinding mcl sieving. l’liis powder W;LS heutxd at 300~ for 4 11in ;L l~clium stream before USC.

The helium cylinder gas was first p.ssccl through a rccluction valve and then through a tulx filled xvith Molecular Sieve held at liquicl nitrogen temperature in order to reduce the concentrations of tlic impurities to a level whet-c they dicl not cause ;L measurable blank. Then the gas W;LS introduced into the apparatus. The apparatus used was a slightly moclifiecl version of the instrument for the determination of hydrogen, nitrogen and osygen in water 5. The general layout is shown in l:ig. I. Calibration was pc~rformecl with hydrogen generated coulometri.cally in a hydrazine sulphatc solution”. ‘4 ILlll.

Chir,r

.

.‘Icltc,

‘\cJ

(1970)

‘t_~7--LJ.~”

.

Dissolution tube A was made of quartz; the other parts of A, electrolysis cell IS, water trap C, four-way valves VI, VZ and their connecting leads were made of pyres. All the other tubing, absorption column I> and chromatoe;raphic column E, were nlade of stainless steel. Columns D and I2 had an internal diameter of 4 mm and a total length of 75 andzoo cm, respectively; they were filled with Molecular Sieve powcler. Eightway valve V:3 was made of stainless steel and Teflon. A catharometcr cletector 1; was used.

^Rl, Yy

ITig. I. I>)iagraln of cquipn~cnt usctl. (:\) I)issolution system With tlissolution ..\I, (13) sq’stcin for the coulonictric gcncrxtioii of hytlrogcii with clcctrdysis aq~~cous hy~lr;~~inc sul~)llCatc solution xntl cquiplxxl with pli~tillU~Tl clcctrotlcs ~JCttOlll

pl:LtC

pcra,tllrr?,

tluring pCr;~t.Urc.

fOr

(ID)

diSpXSiUl1

I-Iz-al~sorptioli

Of

thC

tlil~c

StrippiiiK fillctl

with

Kits,

stripping. (I<) cllro~ni~togr~rp1lic coluiiin (Vi) lmtliaroinctcr tlctcctor, (ICI, I<-)

1Vil.y V:ilVCS.

(\‘:I)

cigtit-wry

((::) water

molccrllar

fillctl

trap

to

1x2

sicw

irlltlItcpt

with

1nolecul:tr

flow rcfplatcei,

(ml,

kept

at liquid

siovo

1112) flO\v

purlfled

_

tulx :\ x11(1 \\‘iLtCrtl-al) cell 131 fillctl with Xl1 iIIlC1 il porous sititcrccl at

liquid IlitrrJgCll

and

Itcpt

IllCtCrS,

nitrcqpl

teni.

tCmpCrxtiirC

at (\‘I,

ro0111 \'z)

tctnfOUr-

ValVCS.

The helium carricx gas stream was adjusted to a speed of IOO ml/min (at atmospheric pressure) with regulator 1x1; the stripping gas stream was adjusted at a speed of LO ml/min with 1x2. Dissolution tube A was hcatccl with a small electric tube furnace.

About 0.3-1.2 mg of metallic uranium mechanically cleaned and stored in a dr)? atmosphere, or about 3o-140 mg of a crushed sample of hypostoichiometric uranium dioxide was placed in dissolution tube A; valves V1_3 were placed in the position shown in l?g. T and trap C and column D were immersed in liquid nitrogen. After the addition of about 5 ml of phospl~oric acid to the dissolution tube A, this tube was connected to the rest of system A and valve VI was turned. go”. After a

A. TOLK,

44”

W. A. LINGERAK,

I). L3tiRG13R

few seconds valve V:j was turned 45”, so that the stripping gas was passed through absorption column D, while the carrier gas was led directly to the chromatograplzic column E. Then the temperature of the phosphoric acid in tube A was increased gt-adually to 350” and it was kept at this value until ca. IO min after tile generation of hydrogen in the tube had ceased (total duration of the process about 20-&o min). After this period the furnace was I emoved and valve Va was turnecl 45”, so that the carrier gas was passed through the ahsorption column before going to the cln-omatographic column. After the chromato~raphic recorder had returned to its original lxxition the Dcwar flaslc with liquid nitrogen was removed from absorption tube D :~cl tllc tube was a!!owcd to warm up in air to room temperature. During the warming up, hyclrogen and the other gases present were desorlxcl from the Molecular Sieve and in clutcd through the cliromato~rnpl~ic column. The lieight of the peak for hydrogen tlic cliromntogram was used for the dctcrmination of its quantity.

For calibration purposes valves VI and V:, were !>laced in the positions of l;ig. 1 wllile V3 was turned go”. Coulometrically a known amount of hydrogen in the range of Z~--SOO ~1 (corrcctcd to o” and 76 cm pressure) was generated. The lx-occdurc for hydrogen stripping and measurement was the same as that mentioned above.

The results of the determinations are collected in Tables I and I I. Tl-lc values in Tahlc I show that the prolx~sccl method can be used for the analysis of nlctrillic uranimn. Tlic mean value of the rneasurwneiits is about I(‘)$ low, lmt tliis

A1\1011NT - -- -...-...

OF .._

IIS131~0G15N -._ .._ _ ._.....

1<151.12,\S721~

l)lJI
Thcorctical v;tluc of lX!lci.lS~cl hytlrogcii Mw.n of mcasurccl vales ((x1) ‘I~strcllles Stalltlarcl

_

I,lSSOI_UTION _ . .

‘I’llI: .

o/IT

.__..,. _.. ~.

3.3 7 0.32-I .2 I JO-236

_ .~ .._._..___..__ .._

l<,\TIOS

. _ -.

C;raviinctric values of C)/ LJ

IN

_, ShhlI’l.l~S

_

A4

OP

__

. _.

I *929/ I .92,4

iVI~~U~S

R~C:LIIvnlucs of 0/I Jratios accorcling to the I-L:lI’O.I-n~cthotl

.Estrcnics St~tllCliLrCl

clcvintions

Nurllbcr of clctcrminations

Smnplc aliquots (rng) Volumes of ITI3(pl)

.----- --.-.--- .____..___” . .._ _..

ACIIYJ

98.8

..___,.

1.927 1.947/1.91~ 0.009 IG 31s l-53.8

r41-34 .._ ,._.

URhNlUhl

DIOSll~J~

__. . _.______-..- __... -- --__.-_..- --..-. C

13 ._..---.-

1.934 ca. 1000

1.9541’ .955 I.955 ca. 1000

x.994/1.992 I.993

1.932

1.9Go

1.9‘\0/1.92.~

1.970/‘*95’ 0.005 =5 43.3-77.4. 155-240

13 _...

__

1.93311.935

0.005 18 35.3-75.7 196.455

._ _

_

llYPOS’I’C~IClIIOMTSTI~IC

. .._.. - .__..._ _.__ - __..._..__--.___ -__-_._

..

I’lIOS1’1IOl
‘)5*rj/lO.~.O

.St/Jl/pll!S -.-------_.-

IN

LOO.0

clcviation (‘X) Number of dctcrtniiiatiolls Sainplc aliquots (inff) Voluiiics of rclwrsccl I-12(161)

OP ..--

UI~ANlUhl

_

(‘x,)

(“6)

lXIE’I‘~~l7hlINA’IlON --..-_-.-----....._

OI1 hlISTAI.LIC _

.-._.

CCL.

-._. --

1000

.9YSG 1.9925il.9857 0.0018 19 36.6-137.0 35.0--IGz I

ANALYSIS

OF

HYPOSTOICHIO~IETl~IC

URASIUhI

DIOSII)E

441

deviation is considerably smaller than the value of the standard deviation. A standard deviation of about 3”/” is quite a normal value for the procedure of gas collection and measurements. For the figures mentioned in Table II small sintered pellets were used, showing a radial increase in O/U ratio: in sample A, O/Uratiosof 1.0~53 and~.gar weremeasured for the inner core of the pellet while values of 1.947 and 1.943 were measured for its outermost layer. The other sample sliquots used in Table II were chosen at random from the crushed pellets. On account of the small samples (30-140 nig) used in the proposed method, the inhomogeneity of the samples resulted in rather high values of thestandard deviations. In tllc case of gravimetric analysis much more sample material (ca. 1000 mg) was used, so that tile problem of inhomogeneity became less important liere. The mean values of tlic O/U ratios obtained l)y the phosphoric acid method correspond quite rcasonably with the values obtained by grnvimetry if an accuracy within 40.004 O/U units of the samis claimed for the grnvimetric method 3. On account of the inhomogeneity ples, it is not possible to give esperimentally verified figures about the accuracy and precision of the method. It is assumed that the accuracy and precision of the collecting and measuring procedure will be the limiting factor. In the present measurements no hydrogen blanks were detected. In. connection with the long dissolution time and the restricted absorbing capncity for hydrcJ#_!n of the Molecular Sieve, the speed of the stripping gas flow must be low. All impurities in the samples that generate hydrogen during dissolution in phosphoric acid will interfere in the cleterminations. During the dissolution process, the temperature of the phosphoric acid hns to considerable dehydration occurs, resulting be kept ‘below 400’. At higher temperatures in a consideral)le increase in the viscosity of the solution. For the determination of the O/U ratio, samples weighing 30-140 mg wore used; these generated hydrogen volumes of 35-455 /A. This range can be extended to If the limited sensitivity of the catharonleter the lower side LAthout xny ol>jection. becomes the restricting factor, an ionisa tion detector with a radioactive source can be used5. It stands to reason that any other sensitive and accurate method for the determination of the total amount of hydrogen in the stripping gas can be used for the measurements. The authors ing the samples.

wisll to thank G. VERSTEI~G nncl A. .J. G. ESGEL

of RCN

for supply-

For the determination of the osygen-to-uranium ratio in llypostoiclliometric uranium dio_xide, a method was developed in which the siI.mI>le is dissolved in pllosphoric acid. During dissolution, all uranium present is converted to uranium(I\r) while an amount of hydrogen equivalent to the degree of hypostoichiometry is released, The O/U ratio is determined from the amount of released hydrogen. Satisfactory results, in accordance with those obtained gravimetrically on r-g samples, can be obtained on 30-140 mg samples. .47t&. Ckirn. Acta, 49 (1970)

437-442

442

On CLcl~veloj~p~ une mdthodc de determination du rapport oxy&.ne/uranium dans un dioxyde d’uranium hypostoechion-&rique, avec dissolution de 1’Cchantillon dans l’ncicle phosphorique. Au tours de la dissolution tout l’uranium est converti en uranium( IV) tandis cfu’une certaine cluantitd d’hydrogene est lib&Cc, dquivalente au dcgre d’lly~~ostoecl~iomdtrie. Le rapport O/U est d6termine A partir de la quantitd I)es rfsultats satisfaisants correspondant a ceux trouves par cl’l~ydrog&ne lilS&e. qavimctrie avec clcs &A~antillons de I g, ont Ct.6obtcnus avec des priscs de 30-1~0 mg .

Ftir die I+&timmung cles Verlr%ltnisscs Sauerstoff zu Uran in unterstijchioInctrischem Urandioxid wurdc cinc Mcthode entwickelt, bei cler die Probe in Phospllors?lure gelijst wircl. Wlihrend des Aufldsevorganges wird das gesamte vorhanclenc Uran in Uran(IV) tiberfiiln t, wobei eine Wasserstoff-Menge freigesctzt wird, die dem Grad cler Untcrstiichiometrie Squivalent ist. Das VerlCltnis O/U wird aus der entwickelten Wasserstoff-Menge ermittelt. 13efriedigende Ergebnisse, die im Einklang mit jcnen stehen, die gravimetriscll bei Proben von I g zu erllalten sind, ktinnen bei Prohen von 30-140 mg erhalten werden.