NUREGER4740M LM-2597

1978 ANNUAL REPORT:AEROSOL CHARACTERIZATION FROM

A SIMULATED HCDA

W. A. ZanotelliG. D. Miller E. W. Johnson

Monsanto Research Corporation

1557 001

Prepared forU. S. Nuclear Regulatory Commission

2 011 1 sos 3 2

NOTICE

This report was prepared as an account of work sponsored byan agency of the United States Government. Neither theUnited States Government nor any agency thereof, or any oftheir employees, makes any warranty, expressed or implied, orassumes any legal liability or responsibility for any third party'suse, or the results of such use, of any information, apparatusproduct or process disclosed in this report, or represents thatits use by such third party would not infringe privately ownedrights.

1557 002

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3pringfield, Virginia 22161Price: Printed Copy $4.50 , Microfiche $3.00

The price of this document for requesters outsideof the North American Continent can be obtainedfrom the National Technical Information Service.

NUREGER4740MLM-2597R7

1978 ANNUAL REPORT:AEROSOL CHARACTERIZATION FROM

A SIMULATED HCDA ,

W. A. ZanotelliG. D. Miller E. W. Johnson

1557 003

Monsanto Research CorporationMound Facility

Miamisburg, OH 45342

Manuscript Completed: January 1979Date Published: March 1979

Prepared forDivision of Reactor Safety Research

Office of Nuclear Regulatory ResearchU. S. Nuclear Regulatory Commission

Washington, D.C. 20555NRC FIN No. A1171

,

Contents

Page

SUMMARY 3. . . . . . . . . . . . .. .... .. .....

1. INTRODUCTION. 3. . . . . . .. . .. . . . ......

2. THEORY. 4. . . . . . . . .. . . .... . ......

3. EXPERIMENTAL PROCEDURE

Laser Evaporation 5. . . . .. . .. .. . ......

4. THERM 0 CHEMICAL CONSIDERATIONS

Introduction. 17. . . . . .... .. . .. ......

Calcul.ation of Free Energies of Formation 22......

Calculation of Reactant Free Energies of Formation. 22.

Free Energies of Formation for the Products 25.....

Results 29. . . . . . . . . ... ... . . ......

5. FUTURE WORK . 31. . . . . .. .. . . .. . ......

6. ACKNOWLEDGEMENTS. 31. . . . .. . .. .. . ......

REFERENCES. 33. . . . . . . . . .. . .. ... ......

1557 vD4

-,

2

1978 AhlNUAL REPORT: AEROSOL

CHARACTERIZATION FROM A SIMULATED HCDA

(NRC PROGRAM A-1171-7)by

W. A. Zanotelli, G. D. Miller and E. W. Johnson

Summary

Simulated environmental conditions modelling theHCDA on a reduced scale have provided the followinginformation:

Aerosols resulting from the condensation ofgaseous constituents are generally spherical,small in diameter (0.01 to 0.25 pm), andbranched chain-like structures.

Electron diffraction analyses have identifiedactinide dioxides, the constituents of stain-less s, teel, and various sodium actinates(uranates and zirconates). Some superstoi-chiometric oxides (U02.2s and U 0 ) were noted3 8also.

X-ray analyses were not sensitive enough toverify presence of sodium uranates or plu-tonates. Electron diffraction of the pluton-ia-doped system should verify these componentsin this mixed-actinide system.

Reaction products are kinetically controlledduring cool-down rather than based uponequilibrium thermodynamics.

1. Introduction

In support of the Office of Nuclear Regulatory Research,Nuclear Regulatory Commission, MRC has initiated a programto produce and characterize the primary aerosols which couldresult from a hypothetical core-disruptive accident (HCDA)in a Liquid Metal Fast Breeder Reactor (LMFBR). These testsare being conducted under simulated conditions that can berelated to the actual events that would occur under such acritical excursion. The program includes the following:

1557 005 3

A feasibility study to determine the possibility of directacquisition of kinetic information regarding the chemicalreactions for the U-Pu-0-Na system in the simulated HCDAenvironment.

The acquisition of a thermodynamic data base of the pos-sible reactions and species formed from the i: ' actions

of the U-Pu-0-Na system.

The simulation of an HCDA event by the laser evaporationtechnique.

The investigation of the chemical reactions and compoundsformed from the short-lived vapor state existing in theHCDA environment.

The assessment of the role of reaction kinetics on boththe formation and post-HCDA environment effects on thematerials present throughout this accident sequence.

The resulting data are being compared with theoretical and ex-isting analytical data and coordinated with other groups thathave developed or are developing models and aerosol data.Large amounts of experimental data exist on the generationand behavior of single-component aerosols, and some limiteddata exist on aerosols containing both uranium and sodiumoxides; but little or no data exist on multi-component systemscontaining aerosols generated from (U,Pu)02, sodium, and stain-less steel produced under HCDA conditions.

MRC has designed and fabricated a system that will have thecapability to incorporate all of these materials in a labora-tory-scale simulation of an HCDA. The operation of a labora-tory-scale simulation will enable the variation of the operat-ing parameters and materials.

2. Theory

During the most severe hypothetical core-disruptive accidents(HCDA), caused for example by transient overpower (TOP) orloss of flow (LOF), substantial amounts of the reactor fuel,stainless steel cladding, and sodium coolant could be vaporized.The vapor species formed, including Pu0, U0, pug 2, U02, UO 3,

0, 02, Na, stainless steel, and fission products, are expectedto be contained initially in gas bubbles within the sodiumcoolant. A temperature gradient within the bubble will exist,initially, from probably greater than 2800 C in the gas phaseto about 900 C at the liquid sodium walls of the bubble. Asthe bubble moves away from the primary heat source, coolingwill probably result in formation of an aerosol by condensation

'. 1557 0064 : .

and reactions of various vapor species. In addition, condensa-tion and reactions will occur at the liquid sodium walls of thebubble.

The physical and chemical nature of the aerosol particles is ofimportance in the predictions of the eventual behavior and fateof the aerosol. The physical properties of the aerosol (par-ticle size, density, settling velocity, and coagulation tenden-cies) are presently being studied, chiefly, by vaporization ofUO2 The chemical nature of the aerosol particles, which isdetermined by the chemical species present in the various tem-perature regimes transited by the aerosol, must be consideredbecause it could affect the physical properties of the aerosoland would definitely influence the subsequent environmentala.id biological pathways of any aerosol which might escape fromthe reactor containment structure.

3. Experimental Procedure

LASER EVAPORATION

Materials

The first phase of this study involved the heating of nonrad-ioactive pellets of the following compositions (in volumepercent):

1) 75% UO2 - 25% 316 stainless steel

2) 75% UO 2 - 25% 316 stainless steel - Na(surface)

3) 75% U0 /Zr0 - 25% 316 stainless steel - Na(liquid)2 2

4) 75% UO2 - 25% 316 stainless steel - Na(liquid)

5) 75% UO2/Zr0 - 25% 316 stainless steel - Na(liquid)2

Most of the UO2 used in the study was depleted reagent-gradematerial; however, some was prepared by reducing U 0 8 in a hy-3

drogen atmosphere. The 316 stainless steel was Metco 41C pow-der (45 to ll2-pm size range). The sodium was reagent-grademetal which was stored under xylene. The Zr02 was 95.5%stabilized with 3.5% Ca0. It also contained a total of 1.0%of the following elements: Fe, Ni, Mg, A1, and Si.

The materials were mixed using a mortar and pestle, charged in-

to a 14-mm-diameter graphite-coated die, and hydraulicallypressed at 8.61 x 10 Pa in an argon atmosphere.

Sodium metal was rubbed on the surface of pellets No. 2 andNo. 3 and introduced into a depression prepared in the center

. 1557 0075

of pellets No. 4 and No. 5. The pellets were heated to meltthe sodium in the depressions.

The second phase of this study involved pellets of variouscompositions including plutonia:

1) 100% Pu0,

- 25% stainless steel2) 75% Pu02

3) 80% Pu02 - 20% U0 2

4) 75% UO2/Pu02 - 25% stainless steel

5) 75% U02/Pu02 - 25% stainless steel - Na(liquid)

The depleted UO2 was 99.5% pure. The 23'Pu02 had a plutoniumassay of 0.8767 g Pu/g Pu02 product. The isotopic analysiswas 93.7% plutonium-239, 5.9% plutonium-240, 0.50% plutonium-241, and less than 0.05% plutonium-238 and plutonium-242.

The stainless steel and the sodium metal were the same batchesas were used in the first phase of the study.

The materials were again pressed at 8.61 x 10e Pa in an argon-atmosphere glovebox. The moisture level was less than 1.0 ppmand the oxygen level was less than 10.0 ppm.

Method

The first phase of the study used a neodymium YAG laser. Laserheating was accomplished by first placing the pellet on a stand.The power was set at approximately 12 J with a laser pulse timeof approximately 1.5 msec. Spot diameters of 0.05 and 0.10 cmwere used. The power densities calculated for the two differ-ent spot sizes were 1.0 and 4.0 MW/cm2, respectively. At thesepower densities the laser can produce temperatures between5,000 and 10,000 K in the vapor plume (1). The laser beampassed through the quartz top of the static laser system, va-porizing a portion of the pellet which condensed on both trans-mission electron microscope (TEM) grids and scanning electronmicroscope (SEM) stubs. The laser heating was performed in anargon atmosphere. Figure 1 illustrates the experimental setupused in this portion of the study.

The energy inputs to the pellets were approximately 2600 and7000 J/g. The SEM samples were analyzed for particle size,distribution, and elemental constituents of the particles.The TEM samples were analyzed for particle size, shape, agglom-eration (chains), and compound formation (electron di f f raction).

1557 0086

Laser Beam

1/4 in. X 1/2-in. Slot Hole

Quartz WindowSEM Stub

G y Stub SupportPellet N

Chamber

\Heater ,

y odium LiquidS

i

Argon Flow Argon Flow

fin

;

HeaterLeads

Pellet Stand

Pellet StandAdjustment

1557 009O

FIGURE 1 - Revised laser system which permits inert-gas laserheating.

7

The laser used for the second phase of the study was a raillaser adapted to use a neodymium glass rod. The static pelletsystem was used here also (Figure 2).

The laser voltage was set at 2.70 kV (24-J output) with apulse time of 5 msec. The spot diameter was 0.10 cm. Thepower density calculated from these parameters was 4.0 MW/cm2,At this power density a temperature of 10,000 K can be expectedin the vapor plume (1). A portion of that material was con-densed on glass slides which were located under the quartz top.The laser heating was performed in an argon-atmosphere glove-box. The experimental setup is shown in Figure 2.

The condensate was removed from the slides and placed in glassspines which were mounted using Apiezon R grease. Thesesamples were analyzed for various compounds via Debye-Scherrerpowder x-ray diffractometry.

Results

Results for the first phase of the study were primarily assessedby examining the products via SEM and TEM analyses. The pelletcompositions, compounds identified, and aerosol size range datacollected from the TEM analyses are summarized in Table 1.

Pellets No. 3 and No. 5 are the closest simulations to the HCDAcompositions and conditions. The major products from the va-porization of UO2 were UO2, UO2.2s, and U30s. UO2 is known toform superstoichiometric oxides of UO2.2s, U O 2.s 2 , U03, and U 083

at high temperatures (2). Therefore U02, U0 .2 s , and U30s can2

be expected to form during an HCDA.

Zr02 was identified as a major compound. It was used as asimulant for Pu02 in that their chemical compositions andproperties are similar. Pu02 differs from Zr02 at high tem-peratures where Pu02 forms substoichiometric oxides of theform Pu0 -x above 1700 K (2). Thus Pu02 and some form of2

Pu02.-x can be expected to form in an HCDA event.

The stainless steel was observed to vaporize and condense intoits basic components of Fe, Cr, and Ni when analyzed on theSEM. The FeC and FeNi are combination condensation productsof the components of stainless steel. The Fe203 can form inthe HCDA bubble by the reaction

Fe2032Fe + 3/2 02 -*

The minor. compounds observed for the simulated HCDA event canbe used as an initial data base to predict sodium uranate andpossibly sodium plutonate compound formation. An x-ray line

1557 ofa8

To/from Gas, QuartzPurification

WindowsSystem

d k

Alpha -*- R ail' 'Glovebox /

90'

[ Directional'*"'

g,",';"''s= _J }m

Sample NeodymiumLaserPellet * *

,

Pt-R h"

Heater

* - TEM Grid and SEM Stub Locations

: AlignmentLaser

-.

FIGURE 2 - Purified atmosphere (argon) glovebox enabling experi-mentation using plutonium-239 and liquid sodium.

1557 0119

.

5Table 1 - NONRADI0 ACTIVE LASER HEATING SUMMARY

Compounds Identified by AerosolElectron Diffraction Analyses Size Range

Pellet System Composition Major Minor (um)

U 08, SS 0.05 to 0.2075% UO2 - 25% SS UO2, 3

U 08, SS, Na3 U0%, Na,U0s, 0.02 to 0.1075% UO2 - 25% SS - Na(surface) UO 32,

Fe203 Na2U2.s08.s,0 0sNa2 2

U 08, SS, N a ,. U 0 s , NaV0 0.01 to 0.2575% UO2/Zr02 - 25% SS UO2, 3 3,

- Na(surface) Fe2 0 Na2U20s,3

Y0NaZr0 3, 2 3

U 08, Na3 00% 0.02 to 0.1075% UO2 - 25% SS - Na(liquid) U O 2.2 s , 3

Fe20s, FeC,Nauu0s

U 0 .2 s , Na3 U0u, Nauu0s, 0.02 to 0.2575% UO2/Zr02 - 25% SS - Na(liquid) UO2, 2

Zr02 Na2U2.s08.s,Na20, Zr0_ FeNi

--

LnLn'M

CD--

scan of a TEM grid indicated that the minor compounds wereestimated to be approximately 3 to 4% of the total composition.

HCDA Aerosol Formation

Figures 3 and 4 are transmission electron photomicrographs ofthe UO -Zr02-SS-Na(surface) and the UO2-Zr02-SS-Na(liquid) sys-2

tem condensation aerosols. The particles are spherical in na-ture and the diameters ranged from 0.01 pm to 0.25 pm. Thephotomicrographs show branched chain-like structures which aretypical of vaporization condensation acrosols of most metal ox-ides (1). These branched chain-like s tures from the fieldsof Figures 3 and 4 are shown in Figures . and 6. The filmaround the particles in Figures 5 anJ 6 s attributed to ex-cess sodium condensed on some of the particles and the carbonfilm surrounding the particles. A secondary ion mass spec-trometric analysis of this film area was attempted to verifythis assumption, but the results were inconclusive. The irreg-ularities around some of the particles in Figure 4 are distor-tions of the carbon film caused by the laser heating effectduring collection of the particles. Electron diffraction pat-terns were photographed from the particles shown in the fieldsof Figures 3 through 6. The coupounds were discussed in theprevious section and are listed in Table 1. These compoundsand the spherical and branched chain-like particles are ex-pected to form as condensation products from an HCDA event.

Figures 7 and 8 are energy-dispersive x-ray analyses from par-ticles produced from the laser heating of the UO2-Zr02-SS-Na(surface) and the UO -Zr0 -SS-Na(liquid) systems. Figure 72 2

shows all the components were condensed and collected on thestub except sodium. Figure 8 indicates that all the componentsof the pellet were condensed on the sample stub. Both analy-ses indicate that stainless steel was vaporized and condensedas the components iron and chromium.

Plutonia Experiments

Six pellets of various compositions were prepared for thesecond phase of this study. These studies were conducted us-ing 23'Pu02 The pellets were pressed and laser heated in aninert argon-atmosphere glovebox. The aerosols were collectedon glass slides, transferred to glass spines, and analyzed forcompound formation by Debye-Scherrer powder x-ray di ff ractometry.

Table 2 summarizes the sample compositions and subsequent x-ray results. The results indicated that the compounds formedwere primary constituents of the starting materials. Thebasic elements of stainless steel such as Fe, Cr, and Ni werenot observed in three of the four pellet systems. This can berelated to the stainless steel aerosols which are smaller than

1557 01311

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FIGURE 3 - TEM photomicrograph of laser-produced particles froma 75% UO2/Zr02 - 25% SS - Na(surface) pellet.

*- 57 014

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FIGURE 4 - TEM photomicrograph of laser-produced particles froma 75% U02/Zr02 - 25% SS - t4a(liquid) pellet.

1557 015.

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1557 016

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FIGURE 7 - Energy-dispersive x-ray analysis of particles fromthe laser heating of a 75% UO /Zr02 - 25% SS - Na(surface) pellet.2

Q.!],'j ~ j]~'' j j f]AEoEX j {j!! !!|

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FIGURE 8 - Energy-dispersive x-ray analysis of particles from- 25% SS - Na(liquid) pellet.the laser heating of a 75% U0 /Zr02 2

,,

,

1557 018,,

Table 2 - RADI0 ACTIVE LASER HEATING SUMMARY

Compounds IdentifiedPellet Composition via X-Ray

(vol %) Di f f racti on

100% Pu02 Pu02

Pu0 , Ni, FeC75% Pu0 - 25% SS 22

2 - 25% SS U0275% UO

2 - 20% Pu0 50% U02 - 50% Pu0280% UO 2

solid solution

75% C02/Pu02 - 25% SS U02

75% U02/Pu02 - 25% SS - U02, Pu02, NaNa(liquid)

0.25 pm; these particles are too small for diffraction lines toappear. (Figures 9 and 10 are TEM photomicrographs which showtypical stainless steel aerosol chains.) Also, the x-ray analy-ses indicate that sodium uranates or zirconates are not formed.Previous experiments which had sodium uranates and zirconatesidentified by electron diffraction patterns also did not showany sodium uranates or zirconates by x-ray analysis. X-rayanalysis identified the compounds as UO2, U02.25, and Na. An-other reason for the absence of sodium uranates can be relatedto the sensitivity of x-ray analysis, which is 5%. Tne sodiumuranates and zirconates are estimated to be present at the 3-4%level. Therefore x-ray analysis would not be sensitive enocghto identify sodiun. uranates or zirconates which may be present.

4. Thermochemical Considerations

INTRODUCTION

As a first step toward identifying mechanisms through whichsodium uranates could be formed in a simulated HCDA, we haveconsidered the thermodynamics of chemical product formationf rom vapori zed U02<c) , Na(c), and stainless steel. In doingthis, we have attempted to answer the question, "What productswould be formed in an HCDA if thermodynamics controlled thecourse of the chemical reactions?" It is recognized thatkinetics, rather than thermoJynamics, may actually control thecourse of reactions, but because of the lack.of kinetic dataon this system a thermodynamic assessment of the HCDA is anessential first step to understanding product formation.

1557 019

17

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1557 020

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19

In performing the thermodynamic assessment it was desired tosimulate the experimental work described previously as closelyas possible. Figure 11 is a diagrammatic representation ofthe chemistry involved in the simulated HCDA experiment. On

the basis of the experimental observations reported in theprevicus section, the stainless steel can be left out of theanalysis since it does not react with either Na or UO . When2

the starting materials are vaporized by the laser pulse thetemperature of the vapor reaches 5,000 to 10,000 K. At thesehigh temperatures the system will consist of mainly Na, U, and0, essentially all as monatomic gases. Some UO is also ex-pected if the temperature is only as high as 5000 K, but ithas been neglected in the calculations because of a lack ofthermochemical data on it and because i ts use admits the pos-sibility of too many additional reactions. The high-tempera-ture mixture of monatomic Na, 0, and V is so randomized thatit has " forgotten" from what starting materials it originated.The starting point for a thermodynamic analysis, then, shouldbe a determination of the standard Gibbs free-energ changebetween the n possible final products (U02 and (n-1 sodiumuranates] anH the high-temperature monatomic gas mixture. Theproduct associated with the largest negative standard Gibbsfree-energy change at a given temperature will be the majorproduct if equilibration between the n products is allowed.If equilibration between the n products is not possible (be-cause of large kinetic barriers), then the interpretation ofthe thermochemical calculations becomes difficult and, in fact,cannot be used alone to explain the experimental results.

The standard Gibbs free-energy change is only a valid predic-tor of the most stable product under conditions of constanttemperature and pressure. Under constant temperature and vol-ume conditions it is the standard Helmholtz free energy whichdetermines stability. While the gas bubble rising through thesodium melt in an HCDA is not a constant-pressure system,neither is it a constant-volume system. The vapor cloud '

formed in the laser vaporization experiments described pre-viously is thought to be more closely described as constantpressure than constant volume. For this reason the standardGibbs free-energy change was chosen as the measure of stabil-ity.

The reactions for which we wish to calculate the standardGibbs free-energy change AG7 can be represented by

xNa(g) + yV(g) + z0(g) + Na U 0 (c) (I)xyZ

where x, y, and z are small integers that depend upon whichproduct species is being evaluated. For example, x = 0, y = 1,and z = 2 for the formation of U02(c). The standard free-energychange of the reactions can be expressed by

1557 02220

Na ,)+ U(,) . O(,)z

.

hi>

/\

uo (c) r Na(*)2

Product i

Product 2

Product n

FIGURE 11 - Representation of Physical process occurring in the-

simulated HCDA.

1557 023

21

'

r "m00 (2)AG *,m

m=1

where the vm terms are the stoichiometric coefficients of thechemical reaction of Equation 1, and the AG f m terms are thestandard Gibbs free energies of formation for the a productsand reactant species. The problem, then, is to calculate thefree' energies of formation of the products and reactants intemperature-dependent form so that AG can be calculated fromEquation 2 at several temperatures.

CALCULATION OF FREE ENERGIES OF FORMATION

From the thermodynamic relation dG = -SdT + VdP we know thatat constant pressure the temperature dependence of AG ris givenby (BAGr/3T)p = -ASr, which is integrated to yield:

AGf,m,T =AGf,m,T dT (3)ref ,m,T

Tref

where Trcr is a suitable reference temperature. For the sod-ium uranates , Tre f = 1000 K; for all other species Tref = 298 K.The integral on the right side of Equation 3 requires tempera-ture-dependent entropies of the reactants and the elements fromwhich they are formed. These entropies are, in general, calcu-lated for the ith species from the expression:

,298+fSy,T i (4)b*

298

CALCULATION OF REACTANT FREE ENERGIES OF FORMATION

We deal fi rs t wi th Na<g ), U< g; and Pu(g) which are all crystallinein their standard stetes at 298 K. Deri va ti on of AGr,m,7 isaccomplished most easily by consideration of the diagram ofFigure 12. Because the free energy of formation of an elemen-tal crystal at 298 K in its standard state is by definition

,zero, one has to consider only the sublimation process. Doingthat according to Figure 12 15: ads to:

T

AG|,m,T =AG[,m,298 -f (S m(c))dT (5)(g) - S298

1557 024

22

orT

AG ,m,T " ^ ,m,298 - 298AS[,m,298 ~[ (b (g) m(c))dT-S

298

(6)

From Equations 6 and 4 the standard free energy of formationof Pug) was determined to be:

2AG = 84,577 - 23.48~T + 1.80 x 10-3 T - 2.8 T in Tf,Pu(g ,T

(7)

D ? similar manner, the expression for Na(g) was found to be:

= 23,974 - 30.31 T + 1.59 T log Tnki.Na, g),T--

s

+ 1.63 x 10-3 T2 + 9.64 x 10-8 T (8)3

In the case vi 'a, the resultant expression was:

110 ?00 -47.2 T + 5.75 T log T (9)06f,U(g),T =

The final reactant to be (,v idered is 0(g). By inspection ofthe diagram in. Figure 13 o;.. 4crives the expression:

AGf,0(g),T " 06 ,0(g),298 g) 0 (g)/2)dT (10)- S

2

Then, using Equation 4, and the equipartif.'cn theorem, whichfor this case may be stated:

Cp0(g) v,tr + R = 3/2 R + R = 5/2 R (lla)C=

C =Cv.tr + Cv, rot + Cv,vib + R = 3/2 R + R + R + Rp0 (g)2

= 9/2 R . 15)

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23

AG suo,TT C g

T TU- [ S dT -[ S dT

g

298 298

298 K C - g

06 sub,298

FIGURE 12 - Free energy of formation of a gas for an elementhaving a crystalline standard state.

AGT

0 (g) 0(g)T 13 2

T T

k[S dT [S dT2

2(g) (9)298298

0 (g) 0(g)298 K k2 2

AG298

FIGURE 13 - Free energy of formation of 0(g).

1557 02624

as a good high-temperature heat-capacity approximation, oneobtains for 0(g):

aG = 57,032 - 11.65 T + 0.50 T ln T (12)f,g ,7

FREE ENERGIES OF FORMATION FOR THE PRODUCTS

For the products. U0 (c) and Pu02<c), sufficient thermochemical2

data exist in handbook tables to find the free energies offormation directly. The expressions found were for U02(c) andPu02(c) respectively:

AG,UO (c)'

' '

2

5 T-l (13)370 x 10-3 T2 + 2.13 x 10+

and

+M. T - 3.H T lg T (10AG =

f,Pu0 (c),T,

2

The only data on the sodium uranates were obtained by Battles,Shinn and Blackburn who give enthalpies and entropies of forma-tion at 1000 K (3). From these data, handbook third-law en-tropies, and equipartition theorem (for gases) or DJ1ong-Petit(for solids) heat capacities we find the temperature-dependentstandard free-energy change for sodium uranate (NaV0) formation

(15)aNa(s) + SU(s) + y0 (g) + Na,U 0g 2y2

to be expressed by:

aG + M A+WOBf,NaVO(c), f,NaVO(c),000

+ (B-A) T - B T in T (16)

A = aSf,NaV0(c) 'where: ' '

B = 4.97y (18)

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25

Table 3a summarizes the expressions for the temperature-de-pendent standard free-energies of formation for all reactantsand products. The sources of the thermochemical data used todetermine the coefficients in Table 3a are given in Table 3b.The standard free er.ergies of formation for each reactant andproduct species have been calculated at temperatures rangingfrom 1000 to 5000 K. These are given in Table 4

Table 3a - SUMMARY OF COEFFICIENTS FOR TEMPERATURE-DEPENDENT STANDARD FREE ENERGIES OF FORMATION EXPRESSEDIN THE FORM:a

AG = a + bT + cTinT + dT2 + eT3 + fT-IU

,i,T

3 8 10-5i a b c dx 10 ex 10 f x

Pu(g) 84,577 -23.48 -2.8 1.80 - -

U(g) 110,200 -47.2 2.50 - - -

Na(g) 23,974 -30.31 0.69 1.63 9.64 -

0(g) 57,032 -11.65 0.50 - - -

Pu0 (c) -246,450 52.48 -1.50 - - -

2

UO (c)-262,880 100.54 -8.65 3.70 - 2.13

2

Na U0 -489,605 127.29 -9.94 - - -

2 4NaV0 -363,461 131.01 -7.46 - - -

3Ma U 02 2 7 -764,796 292.0 -17.40 - - -

Na U0 -573,338 220.2 -12.43 - - -

4 5

Na UO -483,694 177.38 -9.94 - - -

3 4

U'All units are such that AG is in calories / mole.

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26

Table 3b - REFERENCES FOR U02, Pu02 AND S0DIUMURANATE FREE-ENERGY CALCULATIONS

Species Data Source

U0 , Pu0 Handbook of Chemistry and Physics, 57th Ed.,2 2

R. C. Weast (ed.), CRC Press, Cleveland, Ohio,1977, p. D-48.

Na Uranates J. E. Battles, W. A. Shinn and P. E. Blackburn," Thermodynamic Investigation of TrisodiumUranium (V) 0xide (Na300%), IV. Mass Spectro-metric Study of the Na + U + 0 System,"J. Chem. Thermodynamics, 4, 425-439 (1972).

Na(g) and Na(c) Handbook of Chemistry and Pnysics, pp. D-75and D-77.

0 (g), 0(g) Handbook of Chemistry and Physics, p. F-230;2 S. W. Benson, Foundations of Chemical Vinetics_,

McGraw-Hill Book Co., New York, N. Y., 1960,p. 662.

U(c), U(g) M. H. Rand and 0. Kubaschewski, The Thermo-chemical Properties of Uranium Compounds,Interscience Publishers, New York, N. Y.,1963, pp. 8-9.

Pu(c), Pu(g) Plutonium Handbook, Vol. I, 0. J. Wick (ed.),Gordon and Breach Science Publishers, NewYork, N. Y., 1967, p. 38.

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Table 4 - REACTANT AND PRODUCT STANDARD FREE ENERGIESOF FORMATION

Temperature (K)Species 1000 2000 3000 4000 5000

0.20 -18.9 -33.1 -42.1 -45.4Na(g)

U(g) 80.25 53.76 28.58 4.25 -19.45

Pu[g) 43.6 2.3 -36.9 -73.4 -107.0

0(g) 48.8 41.3 34.1 27.0 20.1

UO (s) -218.2 -178.4 -135.7 -88.5 -36.02

Pu0 (s) -204.3 -164.3 -125.0 -86.2 -47.92

NaV0 (s) -284 -215 -150 -87 -263

Na UO (s) -375 -280 -190 -104 -203 4

Na UO (,) -439 -322 -211 -105 -154 5

Na UO (s)-431 -386 -346 -310 -276

2 4

Na U 0 (s) -593 -445 -306 -174 -45227

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28

.

RESULTS

Using the free energies of formation in Table 3a and Equation2, the standard free-energy changes for the reactions ofEquation 1 were calculated. The results of the calculationsare shown in Figure 14. The thermochemical data upon whichthe calculations are based are in most cases considered accur-ate up to about 3000 K. Because of the near linearity of thecurves, it is reasonable to extend the calculations to about5000 K. Above this temperature the validity of further exten-sion is questionable because of the increasing importance ofelectronically excited states which, when accessible, addlarge amounts to the heat capacity of a species. Inspectionof Figure 14 shows that at temperatures above about 5000 KUO2(c) is more stable than are any of the sodium uranates ex-cept Na2 00%(c), while at temperatures below about 3000 K UO2(c)is less stable than are any of the sodium uranates. If equi-libration is allowed between all possible products, thenNa2U207(c) would be the expected product at low temperatures.This, of course, is not what is observed. The reason is thatequilibration between all the products is not possible becauseof prohibitively high kinetic barriers.

An alternate approach to predicting the major product from Fig-ure 14 is to consider a process beginning at a temperature inexcess of 5000 K and to simulate a cool-down process by follow-ing the temperature axis to the left. For the present, ignoreNa 2 U0ue ) . As one looks at a cool-down, it may be noted thatthe first product to become stable is UO (c). The next is NaV03cc).2

By this time, however, UO (c) formation is more stable by about2

50 kcal/ mole. Because of the essentially irreversible naturegenerally observed in solid formation it is felt that the first.product to begin to seed out (here U02(c)) will be the major pro-duct because its formation will use up all the reactants beforethe system cools enough to allow other possible products to be-come stable. Na2 U0%<c is an anomaly in this analysis; thermo-dynamically it should)be the first product to seed out. Evi-dently a kinetic barrier prevents this. This disagreement be-tween the experiment and the thermodynamic prediction illus-trates the need for kinetic studies of the HCDA.

There is experimental evidence that there can be equilibrationbetween some (but not all) of the possible products in an HCDA.Specifically Blackburn has studied the equilibrium (6)

Na UO (c) (21)U0 (c) + 3Na()) + 0 (g)2 2 3 4

at 900*C. His work shows that Na3U0gc) formation can befavored if excess oxygen is available either as dissolved gasor nonstoichiometric, oxygen-rich uranium oxide. Rough

1557 03129

200 -

-

'4UOS

Na3UO4

EUO2//|0 f f

NaUO3' 2 A

3T (10 ), g UOg

-

Naguyo,

~200 _

Naguo4$50 -

, ' ' .

S

~400 -

-

-600 -

-

- 800.

_

-1000

FIGURE 14 - Behavior of free-energy change as a function of tem-perature calculated for various potential products.

1557 03230

kinetic data from Blackburn's work show that essentially all

of the U02(c) formed in the simulated HCDA could be convertedto Na 3U0 <c) in 1 msec. We conclude that Na2 00u is the thermo-dynamically favored product at high temperatures. The reasonit is not observed experimentally is because its formation isprohibited by chemical kinetics. The next product to " seedout" according to the thermochemical analysis is UO2<c). Asthe cool-down continues to about 1200 K the reaction of Equa-tion 21 explains the experimental observation of Na3UO.(c) inaddi ti on to U02(c).

5. Future VVork

The future experiments planned are the static laser systemstudies of the U0 -Pu02-SS-Na(liquid) system using TEM and2

electron diffraction (ED) analyses for aerosol characteriza-tion and compound formations, and the dynamic laser systemstudies of the U02-Pu02-SS-Na(gas) system using x-ray, TEM,and ED analyses. Determination of the effects of fission-pro-duct additions on the chemical intcractions of sodium andmixed oxide fuel is also a short-term goal.

6. Acknowledgements

The authors would like to thank E. Stacy who operated thetransmission and scanning electron microscopes and who photo-graphed the aerosol particles and electron diffraction patternswhich were used in this study. We would also like to thankD. R. Kaser for performing the nonradioactive las r evaporationexperiments and for helping to design and set up the radioactivelaser facility, and D. B. Sullenger and D. R. Rohler for inter-preting the x-ray diffraction data.

1557 033

SYMBOLS

AG - the standard Gibbs free-energy change for1,m.n process 1, species m, at temperature n

UAH - the standard enthalpy change

l,m.nUAS - the standard entropy change1,m,n

S - the third-law entropy of the ith species at1,n

temperature n

C - the constant-pressure molar heat capacity ofpjthe ith species (in general, temperaturedependent)

C - the constant-volume molar heat capacity duev'k to energy mode k

R - the gas constant

Subscripts

1: f = formation; r = reaction; s = sublimation

k: tr = translation; rot = rotation; vib = vibration

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32

References

1. M. D. Allen and J. K. Briant, " Characterization of LMFBRFuel-Sodium Aerosols," Health Physics, 35, 237-254 (1978).

?. C. Keller, The Chemistry of the Transuranium Elements,Verlag Chemie, Weinheim/Bergstr., Germany, 1971.

3. J. E. Battles, W. A. Shinn and P. E. Blackburn, "Thermo-dynamic Investigation of Trisodium Uranium (V) 0xide(Na3U0s), IV. Mass Spectrometric Study of the Na + U + 0System," J. Chem. Thermodynamics, 4, 425-439 (1972).

4. M. G. Chasanov, Nucl. Sci. Engng., 30, 310 (1967).

5. P.A.G. O' Hare et al. , " Thermodynamic Investigation ofTrisodium Uranium (V) 0xide (Na300u), I. Preparation andEnthalpy of Formation," J. Chem. Thermodynamics, 4, 401-409 (1972).

-

6. P. E. Blackburn, " Reaction of Sodium with Uranium-Pluton-ium 0xide and Uranium 0xide Fuels," in Behaviour and Chem-ical State of Irradiated Ceramic Fuels, International AtomicEnergy Agency, Vienna, IAEA-PL-463/23, 1974, p. 393.

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33

7,[e Ru 335 "### 'v.s NUCLE AR REcVL ATORY COMMf SSION

BIBLIOGRAPHIC DATA SHEET M'HEG /CH-074 04 TITLE AND SUBTITLE (Add Volume No. of acprocreatel 2. (Leave blek)

1978 Annual Report: " Aerosol Characterization From Aa REclPiENT'S ACCESSION NO.Simulated HCDA"

7. AUTHOR tS) 5. DATE REPORT COMPLETED

W. A. Zanotelli, G. D. Miller, and E. W. Jonnson lb" ""Jan uuy

9 PE RF OAMING ORG ANIZATION N AME AND M AILING ADDRESS (/nclude I,p Code / DATE REPORT ISSUED

|"^"" "'"Monsanto Research CorporationM rch 1979Mound Facility

6''' " "''''Miamisburg, Ohio 453428. (Leave Nank)

12. SPONSORING ORGANIZ ATION N AME AND M AILING ADDRESS (/nclude lea Codel10. PRoJE CT/T ASK/ WORK UNIT NOOffice of Nuclear f.egulatory Research

Division of Reactor Safety Research M. CONTR ACT NO.Fast Reactor Safety Research Branch (MS-ll 30SS )Washington, DC 20555 A-l l 71 - 7

13 TY PE OF REPORT PE RIOD COV E RE D (/nclus,ve dams)

Annual Report (Technical Report) October 1,1977 - September 30, 1978

15. SUPPLEMENTARY NOTES , 14. (Leave Ne*/

16 ABSTH ACT Q00 *ords or lessiSimulated environmental conditions modelling the HCDA on a reduced scale have providedthe following information: Aerosols resulting from the condensation of gaseousconstituents are generally spherical, small in diameter (0.01 to 0.25 ), and branchedchain-like structures; electron di ffraction analyses have identi fied actinide dioxidesthe constituents of stainless steel, and various sodium actinates (uranates andzi rcona tes ) . Some superstoichiometric oxides (U0-2.25 and U 0 ) were noted also;38X-ray analyses were not sensitive enough to verify presence of sodium uranates orplutonates; Electron di ffraction of the plutonia-doped system should veri fy thesecomponents in this mixed-actinide system; Reaction products are kinetically controlledduring cool-down rather than based upon equilibrium thermodynamics.

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17. KE Y WORDS AND DOCUMENT AN ALYSIS 17a DESCRIPTORS

Chemical Characterization of LMFBR Aerosol LASER VaporizationSodium Uranates, Sodium Plutonates Mixed-0xide AerosolAerosols from LMFBR-HCDA's

17b IDENTIFIERS /OPEN-EN DE D TERMS

Aerosols from HCDA for LMFBRs18 AV AILABILITY STATEMENT 19. SE CURITY CLASS (This report / 21. NO. OF P AGES

Unlimi ted 2o. SECURITY CLASS (This papel 22. P RICE$

NRC FORM 335 (7 77)

UNITE D ST ATE 5NUCLE AH HEGULATORY COVMISSION

W ASHING T O N. O. C 20555, , ,, ,,,,,,

"' k"C''""""G"'"v "*OF F ICI A L BUSIN F SS' "'P t N A L T V F O R F H I V A Y L US E , $ 300 18 N M Alt

L -J

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