AECL-5301

ATOMIC ENERGY ^ f f Q L'ENERGIE ATOMiQUEOF CANADA LIMITED • f f i j T DU CANADA LIMITEE

INTERACTION BETWEEN GRAPHITE AND UO2 IN

OPERATING NUCLEAR FUEL ELEMENTS

by

F.R. CAMPBELL

Chalk River Nuclear Laboratories

Chalk River, Ontario

August 1976

ATOMIC ENERGY OF CANADA LIMITED

INTERACTION BETWEEN GRAPHITE AND UO2 IN OPERATINGNUCLEAR FUEL ELEMENTS

by

F.R. Campbell

Fue± Materials BranchChalk River Nuclear Laboratories

Chalk River, Ontario-August 1976

AECL-5301

Interaction entre le graphite et l'Ut^ dansles éléments combustibles en service

par

F.R. Campbell

Résumé

Avant d'adopter le graphite comme lub r i f i an t entre le combustibleet sa gaine dans le concept CANLUB, i l a f a l l u s'assurer que touteinteraction chimique entre le graphite et l'UOo n'aurait pas d 'ef fetnuisible sur la performance du combustible. C est pourquoi, un morceaude graphite a été placé au centre de deux éléments combustibles d'U02hyperstoichiomëtrique, lesquels ont été i r rad iés. Les températures aucentre des éléments, estimées comme a l lan t jusqu'à 1900 K, ont permis desimuler l ' e f f e t d'une partie de la couche CANLUB de graphite tombant dansles régions chaudes du combustible. Les résultats s'appliquent égalementà l'emploi de disques de graphite entre les pasti l les de combustible oude dëlinâateurs d'espace clos constitués par du graphite.

La thermodynamique de la réaction C-UO2 à haute température indiqueque de très hautes pressions de gaz CO et CO2 peuvent être engendrées àpar t i r d'une te l l e réaction. L'expérience en réacteur a montré que lavitesse de la réaction dépend fortement de la température et qu'el le nedevient s ign i f ica t ive qu'aux températures supérieures à environ 1700 K.Cependant, une analyse théorique a suggéré et l'expérience a confirmél'existence d'un mécanisme qui maintient dans des l imi tes tolérables lavitesse et l ' i n tens i té de la pressurisation des gaz dans les élémentscombustibles. Le CO engendré par la réduction par t ie l le de 1'UO2 hyperstoi-chimëtrique (due au graphite) dans les régions chaudes du combustible setransforme en carbone et en CO2 à la surface du combustible et le carbonese dépose dans les parties moins chaudes de l'élément combustible-. L'accumula-t ion du CO2 est l imi tée et la séparation des deux réact i fs est réalisée parle mouvement concomitant de l'oxygène excédentaire, via la phase gazeuse,vers les régions très chaudes de l'élément combustible.

L'Energie Atomique du Canada, LimitéeLaboratoires Nucléaires de Chalk River

Chalk River, Ontario

Août 1976 AECL-5301

ATOMIC ENERGY OF CANADA LIMITED

INTERACTION BETWEEN GRAPHITE ANP U07 IN OPERATINGNUCLEAR FUEL ELEMENTS

by

F.R. Campbell

ABSTRACT

The adoption of graphite as a fuel/sheath lubricant in theCANLUB fuel design required assurance that any chemical interactionbetween graphite and UO2 had no detrimental effect on fuel perform-ance. Accordingly,a piece of graphite was placed at the centre ofeach of two hyperstoichiometric UO2 fuel elements and the elementsirradiated,with estimated centreline temperatures of up to 1900 K, tosimulate the effect of part of the graphite CANLUB layer falling intothe hot region of the fuel. The results are also pertinent to theuse of graphite inter-pellet discs or plenum delineators.

The thermodynamics of the C-UO2 reaction at high temperatureindicates that very high gas pressures of CO and CO2 can be generatedfrom such a reaction. The in-reactor experiment showed the reactionrate to be strongly temperature dependent, becoming significant onlyat temperatures above about 1700 K. However, a theoretical analysissuggested, and the experiment confirmed, that a mechanism existswhich limits the rate and degree of gas pressurization of a fuelelement to tolerable limits. The CO generated by the partial reduc-tion of the hyperstoichiometric UO2(by graphite) in the hot regions ofthe fuel decomposes to carbon (and CO2) at the fuel surface, thecarbon being deposited in the cooler regions of the fuel element.The accumulation of CO2 is limited, and the separation of the tworeactants is completed,by the concomitant movement of the excessoxygen, via the gas phase, towards the high temperature regions ofthe fuel element.

Fuel Materials BranchChalk River Nuclear Laboratories

Chalk River, OntarioAugust 1976

AECL-5301

INTERACTION BETWEEN GRAPHITE ANV UQ„ IN OPERATINGNUCLEAR FUEL ELEMENTS

by

F.R. Campbell

1. INTRODUCTION

The extersive use of graphite in UO2 fuel elementsis becoming a major feature of CANDU*fuel designs. Theuse of graphite in the fuel/sheath gap as a lubricant orchemical barrier (CANLUB) is now standard practice (1).The inclusion of graphite discs between pellets to act asan alternate heat path (2) to lower average UO2 temperaturesand the use of graphite as a plenum delineator are beinginvestigated. The proposed presence of graphite in operat-ing fuel elements raised the question of chemical compatibil-ity between UO2 and graphite. The temperature at whichinteraction between these species begins and the effectsof their interaction are of obvious concern to the fueldesigner. In particular, the production of CO and CO^from a C-UO2 reaction in amounts sufficient to cause fuelfailure due to overpressurization is a possibility whichhad to be closely examined. In addition, the possibilityof the formation of uranium carbide was investigated asuranium carbide may react explosively with the coolant ifthe sheath defects.

The interaction of graphite (C) and UO2 has been thesubject of considerable attention by the nuclear industry.Fuel fabricators use the carbothermic reduction of UO2 asa processing route to uranium carbide. Fuel designers havebeen concerned with the possibility of graphite and U0 2

reacting inside operating fuel elements. In graphite-coatedU02 fuel particles, the overpressurization of individualparticles by the formation of CO and COo gas as the resultof C-UO2 reaction (3) is an identified failure mechanism.Also, in these same coated particles, the migration ofcarbon from the hot to the colder side of the particlecauses the U0 2 kernels to migrate through the graphitecoatings (the so-called "amoeba" effect) (4). In pelletizedU02 fuel, the presence of CO and C02 (generated by C-UO2reaction) has been sited as the mechanism by which oxygenredistribution occurs (5), especially in the hyperstoinhio-metric range. Although there is considerable controversy overthis point (see, for example, references (6) and (7)) it is,nevertheless, the most widely accepted mechanism at thistime.

*CANada Deuterium Uranium

2.

Although none of these instances of chemical inter-action between UOj and graphite are all that similar tothe situation with which this report is concerned, thequalitative observations and the development of the thermo-dynamics required to interpret previous results have pro-vided a useful starting point for this investigation.

2. THEORETICAL ANALYSIS

The specific situation with which this report isconcerned is the presence of free carbon (as graphite) at,or near, the centreline of a UO2 fuel element during irrad-iation. In practice, this graphite may be in the form ofa cooling disc or may originate from material which hasbecome dislodged from the plenum or the fuel/sheath inter-face layer during handling.

It is assumed in this analysis that the UO2 pellet isoperating with a centreline temperature of 2300 K and asurface temperature of 750 K.

2.1 Assessment of Thermodynamic Equilibria

The relative importance of the various possible reactionswhich might occur between UO2* and graphite at elevated temp-eratures was assessed by examining their respective thermo-dynamic equilibria. The calculations, for the most likelyreactions, are outlined in Appendix 1. It is considered thatthe UO2 may be reduced by graphite to i) uranium metal,ii) uranium carbide or iii) a UO2 of lower O/U ratio.

The calculations show that the only reaction of the threethat is capable of producing high internal gas pressures inan operating fuel element is the last - the reduction of UO2

by graphite to form a UO2 of lower O/U ratio. At typicalfuel centreline temperatures, the equilibrium partial pressuresof CO and CO2 generated by the partial reduction of hyperstoichio-metric UO2 + to a UO2 of lower O/U ratio are very large (>1Q3MPa, see Table Al) certainly sufficient to overpressurize afuel element. Whether these equilibrium conditions will beapproached in practice is examined in subsequent sectionsof this report.

*In this report, UO2 will be used to refer to any uraniumdioxide near the stoichiometric composition, whereas U02+x

will be used to refer specifically to hyperstoichiometricuranium dioxide when the distinction is important.

tThe as-fabricated O/U ratios of fuel pellets may vary from2.000 to 2.015. In general the ratio is < 2.005.

3.

The formation of uranium carbide under these conditionsis unlikely (the formation of uranium metal even less so)since the gas phase in equilibrium with U02+x would have toohigh a partial pressure of CO to allow the reaction to proceed.The behaviour of a defected element may be different, however,since the CO could be vented, but even then it would not beexpected that the carbide forming reaction could compete withthe partial reduction reaction.

2.2 Limits on Approach to Equilibrium

The approach to the high equilibrium pressure calculatedfor the partial reduction of UO2+X m aY b e limited in thesystem under study, either by a shortage of reactants or bythe decomposition of the gaseous reaction products in coolerparts of the fuel element.

2.2.1 Availability of Reactants

Conceivably, in an operating fuel element, either insuf-ficient oxygen or insufficient graphite may be present toenable very high internal gas pressures to be achieved.

In a typical CANDU fuel element, there would seem tobe an abundant supply of oxygen, especially when it isconsidered that under strongly reducing conditions (e.g.excess graphite) the 0/U ratio of UO2 can be reduced to aslow as 1.85 at about 2300 K (8). Even though the CO andCO2 pressures in equilibrium with this O/U ratio would besomewhat less than those shown in Table Al, they would stillbe in excess of tolerable levels. Therefore, it must beconcluded that sufficient oxygen is. available in practice togenerate excessive CO and CO2 gas pressures.

In the case of graphite supply, where the source isgraphite discs placed between pellets, gas pressures willnot be limited to acceptable values by graphite availability.However, if the concern were the loss of part of the graphitecoating in the fuel/sheath gap or the relocation of a chipfrom a graphite plenum to hotter regions of the fuel, theremay be only a small amount of graphite involved. For example,if a portion of CANLUB layer became lodged near the fuelcenterline, say 10 mg, reaction could form about 2.0 x 104 mm-(at STP) of CO and CO2 gas. This would result in a pressurerise in a typical power reactor fuel element of about 7 MPa,not sufficient to induce fuel failure.

2.2.2 Decomposition of Product Gases

The equilibrium calculations in Appendix 1 did notconsider the effect of the conditions at the fuel surfaceon the centerline equilibrium nor the effect of the center-line conditions on the fuel surface equilibrium. In practice,

4.

these equilibria cannot coexist as written in Appendix 1.Through their interaction a mechanism exists whereby the gasgenerated by the C-U02+x reaction at the centerline may beremoved from the system by reverse reactions at the fuelsurface. As part of this process, excess oxygen is moved tothe hotter regions of the fuel and the graphite migrates tothe cooler regions.

The initial conditions in an operating fuel elementcontaining an amount of graphite at the fuel centerline areshown schematically in Figure la. The fuel has an initial0/U ratio of 2.015 and is operating with a central tempera-ture of 2300 K. It will be assumed that at this temperature,the graphite and DO2 will react rapidly to generate CO andCO2 via the reactions expressed by equations A3, A4 and A5(Appendix 1).

As shown in Table Al, the carbonaceous gases generatedat the centerline will be rich in CO. But then the CO/CO2ratio will be above the local equilibrium value at the fuelsurface. Therefore, there will be a tendency to move towardsequilibrium at the surface by removing excess CO and producingC02- This is obviously not a stable condition. The followingsection will discuss the changes that must occur in the fuelelement if stable conditions are to be achieved.

There are two routes by which the CO/CO2 ratio can bealtered. One can be described by the reaction of equationA3, i.e.

2(CO) t <C> + (CO2) (1)

and the other by the reaction

f <UO 2+ X> + (CO) t i- <UO2+x-y> + (CO2) (2)

which is the resultant of equations A4 and A5. Because atlow temperatures (fuel surface) these reactions are drivento the right and at high temperatures (fuel centerline) theyare driven to the left, they lead to the net migration ofthe main reactant species, oxygen and carbon. The migrationof these species will be dealt with separately, although theyare not, as will be seen, completely independent.

2.2.2. 1 Oxygen

The latter of the two reactions given in the previoussection (equation 2) provides a mechanism whereby oxygen isremoved from the fuel surface. As will become apparent inthe next section, this migration of oxygen fulfills a criticalrole in limiting the gas pressure and is a prerequisite tothe approach of a stable end point.

5.

At the fuel surface, equation 2 is driven to the rightand the excess CO picks up oxygen from the fuel (loweringthe 0/U ratio) forming C02« At the fuel centerline, sinceC02 is now in excess of the local equilibrium content, thereaction is driven to the left, the CO2 tending to oxidizethe UO2 to form CO. As a result, the excess oxygen is removedfrom the fuel surface (cooler region) and deposited at the fuelcenterline (hotter region). Oxygen migration continues untilthe 0/U varies across the fuel radius such that it will be inequilibrium with a constant CO/CO2 ratio. (Table Al showsthat the CO/CO2 ratio at the fuel surface and fuel centerlineapproach one another as the 0/U at the fuel surface drops andthat at the centerline rises).

That oxygen does migrate up the temperature gradient in hy-perstoichiometric UO2 fuel elements has been experimentallydemonstrated many times. Indeed, Perron (9) and later Jeffs(10) as well as many others (see, for example, references 5and 7) have shown that this gradient can be satisfactorilypredicted by considering the C-UO2+X equilibrium. (Theseworkers have assumed that the small amount of carbon requiredto establish a carbonaceous gas phase is present as an impurityin the fuel.) For typical CANDU fuel elements, Jeffs foundthat the oxygen redistribution was well advanced after 2 weeksof irradiation. Based on a moc.el developed by Rand andMarkin,(5), the 0/U profile for an element with an initialO/U ratio of 2.015 can be approximated by the curve sketchedin Figure lb.

2.2.2.2 Gnaphltz [Ccuibon]

As discussed, rapid reaction of C with UO2+X at thefuel center will result in a rapid increase in system pressure(Table Al). The only reaction capable of reducing or limitingthis pressure is that expressed by equation 1 and, as we haveseen, the formation of CO2 is favoured at the lower temperaturesat the fuel surface. But, Table Al also shows that, until thesurface 0/U ratio is lowered, no driving force exists toreduce the total (CO + CO2) pressure. Once redistribution ofthe surface oxygen has begun, equation 1 tends to the right atthe fuel surface. If this reaction does proceed, the systempressure is lowered and the graphite deposits in the coolerportions of the fuel element where, because of the low tempera-tures, it should not react further. The CO2 formed is thenconverted to more CO at the fuel centerline (via reactions iand 2) and the process may continue until all the graphite atthe center has reacted and the CO + CO2 pressures are reducedto the low values in equilibrium with stoichiometric UO2 atfuel surface temperatures.

The degree to which the gas pressures generated by theC-UO2 reaction at the fuel centerline can be limited by the

6.

decomposition reaction at the fuel surface depends predomi-nantly on the rate at which the surface reaction proceeds.

At low temperatures (around room temperature) thisdecomposition reaction would appear to take place only inthe presence of a catalyst. (Otherwise, CO would not beavailable in gas cylinders.) Nevertheless, it is expectedthat the reaction will proceed at or near the surface of anoperating fuel element since the temperatures are higher and/orbecause a suitable catalyst may be present in this chemicallycomplex system.

2.3 Experience with Graphite in UO2 Fuel Elements

No adverse effects have been attributed to graphite-U02chemical interaction during CRNL* irradiations of UO2 pelletfuels. No evidence of reaction has been observed, eitherfrom examination of the coating or presence of CO and CO2 inpost-irradiation gas analysis, in any test involving CANLUBfuel with a graphite interlayer. There has been some erosionof graphite disc material in the hottest areas of very highlyrated fuel; the eroded material seems to deposit near theperiphery of the graphite disc and on the inner surface ofthe sheath near the disc (11). However, no effect has beendetected on fuel performance. Because of the complex tempera-ture distribution in these "disc" elements and uncertaintiesin their operating history, it is difficult to assign atemperature at which interaction is noticeable, but points oferosion do appear to be reasonably coincident with the onsetof equiaxed grain growth (11).

At Idaho Nuclear, however, Lussie and Miller (12) haveconducted transient tests in SPERT in which carbon, presentas a fuel impurity {- 50 ppm), reacted to form appreciablequantities of CO and CO2. After a power burst of very shortduration (= 5 s) an element which exhibited 22% diametralstrain was found on puncture to contain over 80% CO in thereleased gas. Unfortunately, their analysis does not includean estimate of the C-UO2 reaction temperature.

2.4 Conclusions from Theoretical Analysis

From this theoretical treatment, one cannot assess withcertainty whether the presence of graphite at a UO2 fuel center-line presents a real hazard. A reaction has been identifiedwhich is capable of generating very high pressures of CO andCO2 as the result of the partial reduction of hyperstoichio-metric U02 at the fuel centerline. However, it has been shownthat where the graphite is supplied from a CANLUB layer, orfrom a small chip off a graphite plenum insert, the pressureswill be limited to fairly moderate levels (< 10 MPa) by the

*Chalk River Nuclear Laboratories

7.

small amount of graphite available. And, even when thegraphite supply is large, a realistic mechanism has beenidentified which may prevent a buildup of excessive pressureby decomposing the products of the high temperature reactionin cooler regions of the fuel element.

The formation of uranium carbide in this system wouldappear unlikely.

Because of uncertainties in the rate at which thedecomposition reaction will take place in an operating fuelelement, it was concluded that the effects of a graphite/UC>2reaction should be demonstrated in a controlled in-reactorexperiment. By monitoring the gas pressure within an operatingUO2 fuel element containing an amount of graphite at the center-line, one could observe whether high gas pressures are developedand how they vary with time. The rate of pressure rise, ifany, and the rate of any subsequent decline would be fairlyquantitative measures of reaction rates. Hopefully, theredistribution of graphite could also be demonstrated.

3. THE EXPERIMENT

Two fuel elements containing hyperstoichiometric UO2pellets and sheathed in type 304 stainless steel were assembledinto a rod with external dimensions similar to those of an NRXfuel rod. Each fuel element contained an amount of graphiteas a discrete "slug" placed snugly in a hole drilled at thecenterline of a UO2 pellet. These pellets were then positionedabout midway in their respective fuel stacks. Each elementwas fitted with a null balance gas pressure switch to monitorthe element's internal gas pressure and contained a plenum voidwhich was sufficiently remote from the fuel so as to remainat approximately the same temperature as the coolant. Thefuel elements were surrounded by a shroud tube (or flow restric-tor tube) on which were fixed continuous-reading neutron fluxmonitors.

The assembly was irradiated for two reactor cycles in theNRX reactor (about 75 EFPD*) . One element, identified as MTT,operated at an average linear power of 50 kW/m and the otherelement, MTW, operated at 60 kW/m. During operation, the inter-nal gas pressures and neutron fluxes of the two elements weremonitored and recorded.

3.1 Fabrication and Assembly

3.1.1 Preparation of Fuel Stack

The fuel pellets were prepared from enriched powdersupplied by Eldorado Nuclear and were pressed and sinteredby Canadian General Electric. The chemical compositions of

*Effective Full Power Days

8.

sample powder and pellets are given in Table 1. Two of thefuel pellets were ultasonically drilled to close tolerancesso that a graphite "slug" would fit tightly in a central hole.Following this, all the fuel pellets chosen for use (nominallystoichiometric) were equilibrated with known amounts of U3O8to raise their 0/U ratio to 2.015 (UO2.015)-

The graphite "slugs" were machined from a solid block ofPOCO-AXP-Q1 isotropic graphite and placed in the drilledpellets just prior to loading the elements.

3.1.2 Assembly Data

The fabrication parameters of the two elements aresummarized in Table 2. A sketch of an assembled element is shownin Figure 2. The relative positions of the two elements inthe irradiation site and the graphite "slug" positions areshown in Figure 3.

3.2 Irradiation Conditions

3.2.1 Irradiation History

The assembly was inserted into position S-13 (axiallyaveraged cell boundary flux of 3.3 x lO-*-' n m~^ s~l) of theNRX reactor for initial startup on March 10, 1972. Afterapproximately 75 EFPD, the assembly was removed on June 8,1972 and hung in the storage block in NRX for an appropriatecooling period.

3.2.2 Coolant Conditions

Coolant conditions for this experiment were similarto those for a standard NRX fuel rod. Thus the inlet headerpressure was 1.07 MPa and the inlet temperature ranged froma minimum of 276 K in March to a maximum of 285 K in June.The coolant flow was a minimum of 1 x 10~3 m-̂ /s correspondingto a minimum nucleate boiling power ratio of 1.5.

3.3 Post-Irradiation Examination

The two elements, after a suitable cooling period, weredismantled from the rest of the assembly in the UniversalCells. They were then transferred to the Metallurgical HotCells for examination. The elements were punctured forcontained gases and cut to obtain metallographic sections. Inaddition, an amount of fuel from each element was sent forburnup analysis.

4. EXPERIMENTAL RESULTS

4.1 Gas Pressure Readings

The results of the measurement of internal gas pressure

9.

for elements MTT and MTW are shown in Figures 4 and 5 (notethe difference in scale). These experimentally determinedpressures have been corrected for switch bias (the differentialpressure (internal minus external) required to close thepressure sensor diaphragm against the contact; this is takenas the average between pre- and post-irradiation determinations(see Table 2). Accompanying the plot is the power history ofthe corresponding element. Both graphs are plotted on thesame time-in-reactor base.

The internal gas pressure of element MTT (low power)rose only slightly over the first half of the test, exhibiteda relatively rapid rise as a result of a short term increasein power midway through the test, and generally declinedthereafter. The gas pressure in element MTW (high power) onthe other hand, rose sharply very soon after startup, reacheda maximum and thereafter declined steadily to about one tn.Ljidof the peak value.

4.2 Element Power Histories

The variation of element power with time shown in Figures4 and 5 was established from the output of the appropriateflux monitors, with corrections for both the decrease inmonitor output due to burnout of rhodium and for the burnupof the fissile isotope in the fuel. The power level wasthen normalized so that the integrated power/time curve wasconsistent with the chemical burnup data.

The powers of the two elements are shown to be essentiallysteady except for perturbations during and following reactorstartups.

4.3 Post-Irradiation Examination Results

4.3.1 Gas Release

The amount and composition of the gases collected duringthe puncture test are shown in Table 3.

4.3.2 Burnup Analysis

The results of the fuel chemical burnup analyses areshown in Table 4. Only one sample was analysed from elementMTW and two from MTT. The two isotopic ratios determined forMTT, although different, are within one standard deviation oftheir me an. It is their arithmetic mean value which is used toderive the element power history. Table 4 also shows the goodagreement between the element power ratio apparent from theburnup results and that from the flux monitor readings.

LO.

4.3.3 Metallography

Polished cross-sections of the two elements are shownin Figures 6 and 7. As Figure 6 shows, no significant graingrowth has occurred in element MTT whereas element MTW(Figure 7) exhibits a fractional radius of grain growth of0.28.

4.3.4 Graphite Redistribution

No significant deposits of graphite were noted on theinside sheath surface or in the gas plenums of these fuelelements during post-irradiation examination. This is perhapsnot too surprising if one assumes that the graphite may beuniformly distributed over all the available cold surface area.Also, since the CO decomposition reaction is thermodynamicallyfavourable at any temperature less than 1000 K, the graphitemay be deposited in fuel cracks near the fuel surface.

It was also intended to check if any graphite remained inits original position at the fuel centerline. Unfortunately,those portions of the fuel stacks originally containing th*igraphite "slugs" were inadvertently used as the samples forchemical burnup analysis. However, the chemists noted thata small lump of graphite was left behind after dissolution ofthe burnup sample from element MTT, but none from element MTW.

5. DISCUSSION

5.1 Interpretation of Behaviour

Qualitatively, the results of this experiment supportthe reaction mechanism suggested as the result of a theore-tical analysis in Section 2. The preponderance of CO and CO2in the gases recovered on puncture (Table 3) indicates thatthe observed increases in gas pressure were caused by theformation of these species as the result of a C-UO2 reaction.This is substantiated by the complete removal, in the morehighly powered element, of the graphite "slug" placed at thefuel centerline. Furthermore, the maximum CO + CO2 pressurereached (about 5.8 MPa in element MTW) is evidence that thedominant reaction is, as suggested, the partial reduction ofUO2+x to a lower 0/U ratio; the equilibria calculated for theformation of uranium metal and uranium carbide show thatneither of these reactions can generate CO + CO2 pressures ofthis magnitude. Finally, the postulated decomposition of COand CO2 in cooler regions of the fuel element is supportedby the observed decline in gas pressure after the initial peak.

In the theoretical analysis, it was assumed that thereaction rate at the fuel centerline would be rapid; the effect

11.

of a slow reaction rate was not discussed. However, sincethe surface temperatures of the two elements were quitesimilar (Table 5) , and, consequently, the decomposition rateof CO at the fuel surface for a given driving force should besimilar, the difference in the rate and degree of pressurerise between elements MTT and MTW must be due to their differentcenterline temperatures. The relatively small centerlinetemperature difference (= 300 K) indicates that the kineticsof the C-UO2 reaction are very strongly temperature dependent.The effect of temperature will be further discussed in Section5.3.

5.2 Rate and Extent of Reaction

The internal gas pressure and linear power of elementsMTT and MTW are shown in Figures 8 and 9, respectively, forthe first two days of the test. The time scale was chosento include the period during which the pressure in elementMTW was increasing. The reader's attention is drawn to thelarge difference in pressure scale between the two elements.

ELEMENT MTW

In element MTW (Figures 5 and 9), the rate of pressurerise reaches a maximum very quickly (at some critical powerlevel) and decreases continuously thereafter through a peakin gas pressure. From the slope of Figure 9 an initial max-imum rate of pressure rise of 0.15 kPa/s has been calculated.The gas storage capability of element MTW can be determinedfrom the end-of-life pressure and the amount of gas colle.after puncture; therefore this initial pressurization ratecan be converted to a net gas production rate of 2.5 mm-Vs.If the assumption is made that no significant reverse reactionha^ yet begun a<_ the fuel surface (since very little gas(CO + CO2) has as yet been generated), the rate of gas pressurerise can be considered as a direct measure of the reactionrate at the centerline. This has been calculated as equivalentto a graphite consumption rate of 1.3 x 10"^ g/s.

The observed decrease in the rate of pressure rise asthe test proceeds is partially explained by a slowdown inthe reaction rate at the fuel centerline (see below). However,it is also due to the increasing rate at which the productCO is being decomposed at the fuel surface as the CO partialpressure rises. If no significant surface reaction were takingplace, a maximum pressure of - 10 MPa (based on the amount ofgraphite initially present) would have been reached in elementMTW.

If the maximum reaction rate calculated above werecontinued, all the graphite would have been reacted after only6 x 104 seconds (about 18 hours). Instead, the gas pressure

12.

continues to rise for a full 48 hours, and even at thispoint it cannot be assumed that all the graphite has beenreacted. There are several possible reasons for thisapparent slowing of the reaction at the fuel centerline. Toa small extent, it will be due to the marginally lowereddriving force due to the presence of some product gas. Itmay also be due to a reduced surface area for reaction asthe graphite "slug" disappears. More probably, the slowdownis due to the reduced availability of oxygen. The reactionof graphite with UO2 would lower the 0/U ratio in the vicinityof the graphite "slug", reducing the driving force. Thereaction can then proceed only through the migration of oxygen,either carried from the surface as CO2 or diffused fromsome other part of the fuel element. Therefore, the ratedetermining step is judged to be, in the early stages, thechemical reaction itself but later to revert to the transportof oxygen to the reaction site as the local oxygen supply isdepleted.

By the same treatment used in calculating the forwardreaction rate, it is possible to calculate the rate of decom-position, at the surface, of gas produced at the centerline.At the end-of-life, all the graphite has been reacted (Section5.1). Therefore, the slope of the pressure curve is a directmeasure of the rate of decomposition. At the end-of-life, whenthe total pressure is 2.2 MPa, this slope is -0.4 Pa/s, equi-valent to an effective decomposition rate (or graphite recoveryrate) of 2.3 x 10-9 g (graphite)/s. The peak decompositionrate is 9 x 10~9 g (graphite)/s at 5.8 MPa. This approximatelylinear dependence of rate on pressure indicates that the rateof decomposition is first order in total (CO + CO2) pressure.This dependence is probably fortuitous (see section on elementMTT) since, with the driving force for reaction dependent onboth the CO/CO2 ratio and the total pressure, a more complexrate equation would be expected.

At end-of-life, the gas pressure within element MTW isstill decreasing, i.e. a stable equilibrium condition hasnot yet been reached. To what level it would, in time, decayis uncertain since the degree to which the local equilibriumcan be approached at these low temperatures is not known.Table Al indicates that at equilibrium the gas phase composi-tion will be rich in CO2 and the total CO + CO2 pressure willbe very low. The incompleteness of the approach to equilibriumis also evident from the still relatively high CO/CO2 ratio(Table 3).

ELEMENT MTT

Although the behaviour of elements MTW and MTT arequalitatively similar, the quantitative treatments appliedto element MTW are more difficult to apply to the results

13.

for element MTT. As Figures 4 and 8 show, the pressure inelement MTT rises only very slowly for the first twelvedays or so to a pressure peak of 0.55 MPa (an order of magnitudelower than that reached in element MTW). The pressure thendeclines even more slowly until a second rise in pressure isobserved. The rate of graphite consumption, calculated fromthe slope of Figure 8 is 3.8 x 10~9 g/s, a rate which, ifcontinued, would require about 260 days to consume all theavailable graphite. This is consistent with the presence ofsome graphite at the centerline when the test was complete.It is uncertain, however, if the slope of Figure 8 is dependentsolely on the rate of graphite - UO2 reaction. As the figureshows, the power of element MTT rose slowly over the firsttwo days as the reactor moderator level was being raised(element MTT was placed above the reactor centerline). Therising power would cause some increase in the internal gaspressure even if the amount of gas present was constant. Onthe other hand, with a slow reaction at the fuel centerline,the reverse reaction at the fuel surface may be having a signi-ficant effect on the observed rate of pressure rise.

Judging by the slow reaction rate, the temperature sensi-tivity and the presence of a second pressure peak, it wouldappear that the reaction rate remains chemically controlled inthis element, rather than changing over to transport controlas in the case of MTVJ.

The second pressure peak of about 0.8 MPa is associatedwith an increase in element power (see Figure 4). The resultantincrease in fuel centerline temperature has increased thereaction rate above any previous level. This increase in ratemay be enhanced by the increased oxygen level at the fuel center-line as a result of oxygen redistribution. The pressure risedoes not last long; the pressure quite quickly drops to nearits previous level and then drops more slowly to an end-of-lifepressure of 0.4 MPa. The rate of decline in the gas pressure,expressed as an equivalent graphite decomposition rate, variesfrom 4 x 10~9 g/s just after the second peak at about 0.7 MPadown to 3 x 10~ 1 0 g/s near end-of-life at 0.45 MPa. UnlikeMTW, the rate of decomposition does not appear linear (firstorder) in total (CO + CO2) pressure. As with element MTVJ, thestill decreasing gas pressure indicates that equilibrium isnot yet reached at end-of-life.

5.3 Temperature Dependence

5.3.1 Existence of "Threshold" or "Critical" Temperature

The marked effect of fuel centerline temperature onthe rate of C-UO2 reaction is evident in the large differencein the magnitude and rate of pressure rise between the twoelements. The effect of temperature is also evident in the

14.

early behaviour of element MTW and in the presence of asecond pressure peak in MTT. Figure 9 shows that in elementMTW no significant rise in pressure above that expected dueto the original filling gas is observed until a particularlevel of element power is reached. This is better demonstratedin Figure 10, in which the behaviour over only the first5 x 104 s (about 14 hour?) is showr. Note that until a powerof 54 kW/m is reached (at about 8 x 103 s) the rate of pressurerise is very small. At this power level, a steep but steadyrise in gas pressure begins, interrupted by a reactor shutdownat about 2.3 x 104 s. Then, on the subsequent approach topower, a very similar power level (56 kW/m at 4.3 x lC* s)initiates a further steady pressure rise. This behaviourwould suggest that a "critical" or "threshold" temperatureexists beyond which the reaction rate between graphite andUO2 is rapid. If so, the behaviour of element MTT would betypical of a fuel element whose centerline temperature wasbelow this "threshold" value and that of MTW typical of onewith a temperature somewhat greater than this value.

5.3.2 Derivation of "Threshold" Temperature

A calculation of fuel temperature corresponding to theonset of significant pressure rise is complicated by the lowfuel surface temperature, the high 0/U ratio and the factthat irradiation damage is being introduced in the low tempera-ture fuel during the early stages of operation.

It is known that the conductivity of UO2, below 723 K,is decreased considerably by irradiation damage and that thisdamage saturates fairly quickly. at a fluence of approximately5 x 1024 n/m3 (13). In power-reactor fuel design, thermalconductivity values corresponding to the fully damaged condi-tion are felt to be appropriate, (though the choice haslittle influence on the overall temperature distributionbecause little, if any, fuel will be at temperatures below723 K). In this test however, the fuel surface temperaturesare less than 460 K, so that the conductivity of a considerablefraction of the fuel will be affected by radiation damage.Consequently, the UO2 thermal conductivity was modified bycalculating values appropriate to the amounts of damage accruedin the burnup range of interest. From the data of Cloughand Sayers (13), it was determined that for the burnup inelement MTW at the time the first significant pressure risewas noted, the thermal conductivity was reduced by approxima-tely 10% below the unirradiated value. For the time at whichthe second increase in pressure (after the shutdown) was noted,the conductivity decrease was calculated as being 14%. Tosimplify the calculations an average value of 12% was used.

The amount of oxygen in excess of UO2.000 is also knownto have a marked effect on fuel thermal conductivity. Several

15.

sources of experimental information on this effect exist(14,15), but the largest single body of consistent datais found in (16) . For the 0/U ratios considered here acorrection to the stoichiometric conductivity based on thedata in (16) was used. It has been noted previously(Section 2) that oxygen migrates in an operating fuel ele-ment such that the fuel surface tends to the stoichiometriccomposition and the fuel centreline becomes oxygen enriched.However, because of the relatively short elapsed times thatare of interest here, it was assumed that no significantoxygen redistribution had taken place; the 0/U ratio isassumed uniform across the fuel at its initial level atthe times for which a temperature distribution is required.

Figure 11 shows how the fuel centreline temperaturevaries with fuel power for the element geometry and coolantconditions encountered in this test. These temperatureshave been determined by the fuel performance code, ELESIM II(17), with the modifications to the thermal conductivityequations described previously. Shown on the curve arethe two power levels at which significant reaction rates wereobserved in element MTW. The corresponding temperatures are1755 K and 1825 K respectively. These provide an upper limit,exclusive of experimental uncertainties, to the "threshold"temperature. A lower limit can be similarly calculated fromthe preceeding lower power levels at which no significantincreases in pressure were observed. These power levelsare (from Figure 10) both approximately 50 kW/m, equivalentto a central temperature of 1610 K.

A similar analysis was carried out on element MTT forthe power at which the second pressure rise was noted(52 kW/m). The fuel conductivity, for this case, was correctedassuming saturated radiation damage of the low temperaturefuel. However, because the degree to which the oxygen hasbeen redistributed at this point is uncertain, it was feltnecessary to determine the central temperatures for both theundistributed and fully redistributed cases. Again, theELESIM II code was used, amended to simulate oxygen redistri-bution in an operating fuel element using the model developedby Rand and Markin (5). A temperature range of 1550 K to1750 K was determined.

5.4 Application of Results to the Behaviour Expected in PowerReactor Fuel

In several respects, the experimental conditions foundin this test are not typical of those encountered in powerreactor fuel. Some of these differences tend to make thistest more severe than in practice, some less. In severalinstances, the effect of the difference is uncertain.

1 6 .

5.4.1 Amount of Graphite

The amount of graphite initially placed at the center-line of the fuel elements in this test is large comparedto the amount which might be expected to become dislodgedfrom a CANLUB coating. Since this source of graphite isthe major concern for present fuel designs, in this respectthe experiment can probably be regarded as a severe one.However, the presence of a larger amount of graphite maynot necessarily resuiL in higher pressures. As discussedin Section 5.2, the generation of carbonaceous gases isprogressively slowed by a limited local oxygen supply whilethe decomposition reaction becomes progressively more rapid.Thus, the gas pressure may well have stabilized before allthe graphite has been used up.

5.4.2 Stoichiometry

The high initial 0/U ratio of the fuel in theseelements has almost certainly increased the rate and degreeof interaction, and may also have lowered the "threshold"temperature. Thus a typical fuel element starting upwith graphite at its centreline would not be expected toexhibit as rapid a production of CO and C02 as has beenobserved in this test. However, if at the time an amountof graphite comes in contact with hot fuel the O/U ratiohas reached its equilibrium distribution, i.e. a high 0/Uratio at the centreline, the present results may not beoverly pessimistic.

5.4.3 Fuel Centreline Temperature

The centreline temperatures determined for the fuelelements in this test are somewhat lower than the peakcentreline temperatures encountered in power reactor fuels.As a result the rates of gas generation observed here maybe exceeded in practice. If, as suspected, the reaction iscontrolled at high temperature by oxygen transport, reactionrates at temperatures above those encountered here could bedetermined by using the activation energy for oxygen migration.

5.4.4 Fuel Surface Temperature

The fuel surface temperatures in this test are verymuch lower than those encountered in power reactor fuel ele-ments. Thus the decomposition reaction at the fuel surfacemay be slower than in practice. As a result it would beexpected that the gas pressure would not go as high and woulddecline more quickly in a fuel element with a higher surface

17.

temperature. However, if the decomposition reaction takesplace at an intermediate fuel radius where the temperaturewas higher, the surface temperature, as such, may beunimportant.

5.4.5 Internal Volume

The internal free volumes, at power, of the fuel ele-ments in this test are much larger (by as much as an orderof magnitude) than those of production fuel. Smaller volumeswill obviously result in higher pressures for the same amountof gas produced. However, higher gas pressures increase thedriving force for the surface reaction while decreasing itfor the centreline reaction. So, while the peak gas pressurewould probably increase, to a great extent this increase isself-limiting.

5.4.6 Sheath Composition

The use of stainless steel sheathing rather than Zircaloy(as used in CANDU reactors) has almost certainly affectedthe chemistry within the fuel elements tested. As Jeffsnoted, oxygen redistribution was essentially complete within"several hours" in stainless steel sheathed experiments doneby Christensen (18,19) whereas in Jeff's own Zircaloysheathed experiments, the process appeared incomplete after13 days (10) . The effect appears to be associated with thegettering of some of the excess oxygen by the Zircaloy. Thus,it might be argued that with less oxygen available, the rateof pressure rise, and consequently the peak pressure, wouldbe reduced in a Zircaloy sheathed element. On the otherhand, an additional, but probably less important, effectmight also be to lessen the decomposition rate at the fuelsurface. Therefore, it is felt that the stainless steels.heathed experiments have provided a more severe measure ofrate of reaction than would be experienced in production fuel(other things being equal).

6. CONCLUSIONS

The behaviour of a fuel element with an amount of graphiteat the fuel centreline has been elucidated by an examinationof the thermodynamics of the system and demonstrated by asubsequent experimental simulation. Although in some respectsthis test is not typical of power reactor conditions, quali-tatively the behaviour is expected to be similar.

Graphite and UO2, if placed in contact at elevated temp-eratures in a closed system, react chemically producing COand CO2 gas, the graphite acting to reduce the 0/U ratio of

18.

the fuel. The reaction rate exhibits a marked temperaturedependence, to the extent that a "threshold" or "critical"temperature can be identified above which the reaction proceedsrapidly. A "threshold" temperature of 1700 ± 150 K has beendetermined from this experiment. This should not be construedas indicating that graphite and UCU are necessarily compatibleat lower temperatures but rather that any reaction will benegligibly slow.

The high CO and COj partial pressures that can exist atequilibrium as the result of the reaction between graphite andhyperstoichiometric UO2 will not, fortunately, be reachedin practice. In the case of CANLUB fuel elements and elementswith graphite plenums, the pressure will be limited, usuallyto quite low values, by the small amount of graphite availableat the reaction site. Even when larger amounts of graphiteare available, excessive pressure will be prevented by thedecomposition, in the cooler regions of the fuel element, ofthe CO produced at the centreline. This decomposition is accom-panied by a progressive separation of the reactants, the excessoxygen moving to the hotter regions of the fuel element and thegraphite being deposited in the cooler regions.

A typical fuel element in which graphite and UO2 camein contact at temperatures in excess of 1700 K would exhibita fairly rapid rise in gas pressure up to something of theorder of coolant pressure (depending on the temperature andthe amount of graphite). This pressure would then declinemore slowly to much lower values.

REFERENCES

1) Robertson, J.A.L.Improved Performance for U0 2 Fuel,Engineering Journal (Eng. Inst. of Canada).Nov-Dec. 1972.

2) Lewis, W.B., Duret, M.F., Craig, D.S., Veeder, J.I.and Bain, A.S.Large Scale Nuclear Energy From the Thorium CycleAECL-3980 September 1971.

3) Nickel, H. and Balthesen, E.Status of HTP Fuel Element Development in the FRG andits Potential for Process Heat ReactorsKernteckn^K 17, Jahrgang (1975) No. 5.

19.

4) Lindemer, T.B. and deNordwall, H.J.Review of Thermodynamic Behaviour in the U-Pu-Th-O-CSystem and of Transport Behaviour in HTGR FuelsORNL-TM-4742, October 1974.

5) Rand, M.H. and Markin, T.L.IAEA Symposium on the Thermodynamics of Nuclear MaterialsVienna, 1968.

6) Leitnaker, J.M. and Spear, K.E.Elimination of C0 2 and H.,0 as Oxygen Transport Speciesin Mixed Oxide Fuel PinsORNL-TM-3849, June 1972.

7) Adamson, M.G. and Carney, R.F.A.Thermal Diffusion Phenomena in Non-Stoichiometric OxideFuels Part 1, Evidence of gas phase oxygen transport inU 0 2 + x from axial and radial temperature gradientexperiments.AERE-R-6830, 1972.

8) Edwards, R.K. and Martin, A.E.Thermodynamics 2IAEA, Vienna, 1966, p.423.

9) Perron, P.O.On the Chemical Stabil i ty of UO2- and PuO2~FuelledGraphiteAECL-2783, February 1968.

10) Jeffs, A.T.Oxygen:Metal Ratio Effects in (U,Pu)O2 FuelsAECL-3690, August 1970.

11) MacDonald, R.D., private communication.

12) Lussie, W.G. and Miller, R.W.Carbon Impurity in UO2 and its Effects on CladdingSwelling During Power BurstsIN-ITR-115, July 1970.

13) Clough, D.J. and Sayers, J.B.The Measurement of the Thermal Conductivity of U0 2 underIrradiation in the Temperature Range 150-1600°CAERE-R 4690, 1964.

14) Ross, A.M.The Dependence of the Thermal Conductivity of UraniumDioxide on Density, Microstructure, Stoichiometry andThermal-Neutron IrradiationAECL-1096, 1960.

20.

15) Thermal Conductivity of Uranium DioxideIAEA, Vienna, 1966, p.14.

16) Hobson, I.C., Taylor, R. and Ainscough, J.B.Effect of Porosity and Stoichiometry on the ThermalConductivity of Uranium DioxideJournal of Physics, Division of Applied PhysicsVol. 7, May 1974.

17) Notley, M.J.F.A Computer Program to Predict the Performance of UO2

Fuel Elements Irradiated at High Power Outputs to aBurnup of 10,000 MWd/MTU.Nuclear Applications and Technology, Vol. 9, Aug. 1970.

18) Christensen, J.A.Stoichiometry Effects in Oxide Nuclear Fuels1: Power Rating Required for Melting and Oxygen

Redistribution in Molten Center U0 2 + FuelsBNWL-536, December 1967.

19) Christensen, J.A.Research at Batelle-Northwest on Transport Processesin Oxide Nuclear Fuel,BNWL-1202, October 1969.

20) Kubachewski, O., Evans, E.L., and Alcock, C.B-Metallurgical ThermochemistryPergamon Press, Oxford, 4th Ed. 1967.

21) Perron, P.O.Thermodynamics of Non-Stoichiometric Uranium DioxideAECL-3072, May 1968.

22) Wheeler, V.J. and Jones, I.G.Thermodynamic and Composition Changes in UO2+X (x < 0.005)at 1950 K.Journal of Nuclear Materials, Vol. 42, 2_, 1972.

23) Markin, T.L. and Bones, R.J.The Determination of Some Thermodynamic Properties ofUranium Oxides with O/U Ratios Between 2.00 and 2.03 Usinga High Temperature Galvanic CellAERE-R 4178, November 1962.

21.

TABLE J

FUEL ANALYTICAL PATA

Voiodzfi

Batch Number

0/U Rat io

Ag (ppm)

Al (ppm)

B (ppm)

C (ppm)

Ca (ppm)

Cd (ppm)

Co (ppm)

Cr (ppm)

Cu (ppm)

F (ppm)

Fe (ppm)

Mg (ppm)

Mn (ppm)

Mo (ppm)

Ni (ppm)

Si (ppm)

130-7116-1819

2.100

-

10

0.25

130

25

<0.2

-

10

<1

20

75

3

1

9

15

40

632-1545-1-1828

2.000

<1

30

0.35

<10

15

0 . 3

<1

15

1

<5

165

5

3

9

20

30

22.

TABLE Z

ELEMEMT AND ASSEMBLE DATA

Fuel

Enrichment

0/U Ratio(as loaded)

Pellet Diameter

Land WidthDepth

Density

1.9 wt% U-235 in U

2.016 avg.

23.95 mm

2. 4 mm0.45 mm

10.66 Mg/m3

Shzath

Material

I.D.

O.D.

Wall Thickness

304 Stainless Steel (annealed)

24.03 mm

26.67 mm

1.32 mm

Material

Density

Diameter

Weight

POCO-AXF-Q1 (degassed for 16 hat 1300 K)

1.84 Mg/m3

3.1 mm

element MTT0.0849 g

MTW0.0847 g

23.

MTT MTW

152.32 mm

1.0 mm

8

0. 8 mm

2.27 m

1000 mm3

Argon

728.58 g

152.53 mm

1. 0 mm

8

0.8 mm

1.59 m

1000 mm3

Argon

728.36 g

Stack length

Axial clearance

Number of pellets

Diametral clearance

Element centrelineelevation (abovecalandria bottom)

Plenum void(includes void intransducer)

Filling gas

Total fuel weight

TnAtKumzntatlon

Null Balance Pressure Switches - supplied by Reuter StokesCanada Limited

Pre-irradiation Switch Bias 0.015 MPa 0.003 MPaPost-irradiation Switch Bias 0.006 MPa 0.004 MPa

Neutron Flux Monitors - continuous reading type with rhodium emitter- supplied by Reuter Stokes Canada Limted

24.

TABLE 3

AMALVS1S OF GAS RELEASED ON PUNCTURE

ConitA.tae.nt

Total Gas

CO

CO2

Kr

Xe

Ar

N2

O2

H2O

H2

He

CHi,

MTT

mm3 at STP

6.7

2.1

2.8

:0.1

<0.1

1.8

<0.1

<0.1

<0.1

<0.1

ND*

<0.1

%

100

31.3

41.8

—

—

26.9

—

—

—

—

—

—

MTW

m-,i3 at STP

34.3

10.5

21.7

<0.1

0.4

1.7

ND*

<0.1

<0.1

<0.1

<0.1

<0.1

%

100

30.6

63.2

—

1.2

5.0

—

—

—

—

—

—

*not detected

25.

TABLE 4

SUMMARY OF BURN-UP ANALYSES

Pre-irradiationisotopic ratio

Post-irradiationisotopic ratio

Repeat of post-irradiation ratio

Average change inisotopic ratio

Burnup from LATREP*

Power Ratio fromburnup

Power ratio fromflux monitors

MTT

(1.959 ±

(1.851 +

(1.838 ±

(.114 ±

22.8

/U 2 3 5\

\uUsJ0.008)10~2

0.006)10-2

0.009)10~2

o.oi6)icr2

MWhkgU

1

1

1UTW

(1.959

(1.821

(.138 ±

27.

(wiss\\U23S)

± 0.008)10"2

± 0.006)10"2

—

0.014)10"2

6 MWhkgU

1.21

1.19

*LATREP - a reactor physics code which predicts element power as afunction of isotopic composition, geometry and flux. Alsocalculates the integrated power (burnup) from a given changein the isotopic composition.

26.

TABLE 5

THERMAL STATE OF ELEMENTS AT NOMINALFULL POWER [FRESH FUEL)

Average Linear Power(kW/m)

Fuel Surface Temp.(K)

\ xdeTo (kW/m)

Fuel centrelinetemperature (K)

Surface heat flux(kW/m2)

MTT

50

428*

3.7

1500

660

MTW

60

455*

4.5

1850

800

*Based on:

Coolant temperature

Film coefficient

Sheath conductivity

Fuel/sheath conductance

= 290 K

= 20. kW/m2 K

= 0.019 kW/m K

= 10. kW/m2 K

FUEL TEMPERATURE

zo— rn

cc — oCO

CO^\\\\\\\\\\\\\\\\\\\\\\\\\\V 3II H d « H 9 \\VI\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\N

0)

O•H+J•H•dcou

id

c•H

U-l

oao

-P

(1)tn0)M

U•H

U33

•H

u u v a n/o

28.

\

2 05

2 000 -

^ UNIFORM P ( C 0 2 >

-2 2300K

FUEL RADIUS SURFACE

Figure lb Schematic representation of conditions after oxygenand graphite redistribution.

SENSORLEAD

PLENUM

/

/

PRESSURELINE PRESSURE

SENSOR

NEUTRONFLUX

DETECTOR

FUELSHEATH

DETECTORSUPPORT

BOTTOMPLUG

Figure 2 Fuel element assembly.

30.

MTT ELEVATION 227 cmABOVE CALANDRIABOTTOM

GRAPHITE"PLUGS"

- MTW - FLUX ELEVATION 159 cmABOVE CALANDRIABOTTOM

Figure 3 Schematic of graphite positioning and element elevations.

31.

I . 0

- 0.8 1—

n. 6 —

0.4 —

0. 2

(X

en

20 30 40 50 60 70

TIME (DAYS) SINCE START OF TEST

80 90

Figure 4 Power and gas pressure history for element MTT

32.

7. 0

6. 0

inGOLU

d.

5.

^.

0

0

I I I I I I I I I I I I I I I I I I

• +•

aa.

3. 0

2. 0

1 . 0

0

60

HO

20

0

J L(VI

10 20 30 HO 50 60 70 80

TIME (DAYS) FROM START OF TEST

90

Figure 5 Power and gas pressure history for element MTW

33.

7.5X

Figure 6 Polished section of element MTT.

34.

7.5X

Figure 7 Polished section of element MTW.

35.

I . 0

- 0.8 —

0.4 0.8 1.2

T I M E ( D A Y S ! F R O M S T A R T OF T E S T

2. 0

Figure 8 Record of linear power and gas pressure in element MTTfor first two days of operation

36.

7.0

6. 0

* 5.0

4. 0

3. 0

2.0 —

CDD-

\ . 0

0

60

40

20

0

J

/

0.4 0 8 1.2 1 6

TIME (DAYS) FROM START OF TEST

Figure 9 Record of linear power and gas pressure in elementMTW for first two days of operation

37.

7. 0

G. 0 —

Q.

- 5 . 0

(JO

a.

zor

4 0 ••

3. 0

2. 0

1 I 1 1 1 1 i 1 1

j—

• , - • • •

U — \ * * * \ i i i i i ii i i i

— \ ONSET OF_ REACTION

I 1 1 1 1

1 1 1 1

(^\ ONSET OFj REACTION

L 1 U 1 1 —

0

60

20 —J

TIME FROM START OF TEST ( X 10

Figure 10 Power and pressure record of first 5 x 10"* seconds forelement MTW

t-iCD

FUEL CENTRELINE TEMPERATURE IK)

CD—*t oCDCD

—*

Oo

enoo

—*COCDCD

N iOOO

l-oh oOCD

H- Cr t CD3* I--

LQ rtP> Wtn 3

13TD CDi~i i-iCD p)03 r ttn eC Hn CDCD

p. pi3 l-iO H-

CD r t

tn oCD 3W

1-"- H-3 rt

3"CDM t lCD O

3 SCD CD3 l-irt tnS 3"i-3 OS S• H-

3iQ

CD

CD

tn

LJ-Ctnrt

crCD

I—'

o

p)3Cb

OOH-nci<

Mip-3

mi—i • i

mi

j

f—H

~^-

m-^

-ao^̂m

—

3'—'

CD

T—

L n

L nCD

L nL n

cnCD

cnLn

"8C

39.

APPENDIX 1

THERMODYNAMIC EQUILIBRIA BETWEEN GRAPHITE AND UOg

Uranium dioxide (U02+x) may be reduced by graphite (C) to formuranium metal, uranium carbide or a uranium dioxide of lower 0/Uratio. The thermodynamic equilibria of each of these candidatereactions will be examined in this appendix.

A/ Uranium Metal Formation

The reduction of UO, by carbon at high temperatures to formuranium metal can be described by the two reactions*:

<uo2> *- {u} + (o2),

for which the standard free energy change is (20),

AG Q = 1.129 x 106 + 64.48 T log T - 406.1 T

joules per mole, and

<c> + M o ^ i (co),

for which (20) AG = -1.12 x 105 + 87.7 To

joules per mole.

These mciy be combined to form a single expression

<UO2> + 2 <C> 1 {U} + 2(CO) ...(Al)

fcr which

AG Q = 9.05 x 105 + 64.48 T log T - 581.5 T ...(A2)

joules per mole.

But AG = -RT £n Ko p

= -RT Sin p 2 (CO)

*The brackets denote the phase of the species considered, i.e.

< > - solid{ } - liquid( ) - gas

40.

where p (CO) denotes the partial pressure of CO and other constituentshave unit activity. Thus the partial pressure of carbon monoxide inequilibrium with graphite and UO2 at 2300 K would be given by

*np2(CO) = ^ 5 8 0 = _341f

or p(CO) = 18 kPa.

At 750 K, the uranium metal product is solid instead of liquid andequation Al becomes (20)

<U02> + 2 <C> * <U> + 2 (CO),

and & G Q = 8.565 x 105 - 342.8 T Joules/g mole.

Therefore p(CO) = 0.136 fPa (fPa = lo"15 Pa).

B/ Uranium Carbide Formation

The reduction of UO2 by carbon to form uranium carbide canbe described by the following three reactions

<uo2> t <u> + (o2)

Z <uc>,and <C> + J5(O2) J (CO)

at temperatures below the melting point of uranium. The overallreaction can be described by

where (20) AG = 7.660 x 105 - 349.1 T Joules/g mole.

<UO2> + 3<C> t- <UC> + 2 (CO)

(20) AG = 7.660 x 105 - 34=

Therefore p(CO) = 262 kPa at 2300 K

and p(CO) = 0.276 pPa at 750 K (pPa = 10~ Pa).

C/ Partial Reduction to Lower O/U Ratio

The as-fabricated O/U ratios ol fuel pellets may vary from2.000 to 2.015. In general, the ratio is less than 2.005. Thefundamental equations governing the equilibrium between hyperstoichio-metric UO2 and graphite (carbon) are

41.

2 (CO) t (CO2) + <C> ...(A3)

(CO) + h(O2) (from UO2+x) * (CO2) ...(A4)

I < U C W ^< U 02 +x-y> + T(02> "•<A5>

The equilibrium constants for each of these three equationscan be solved simultaneously to determine the equilibrium conditionswhich will pertain separately to a typical fuel centerline and to atypical fuel surface. The solution will be used to construct a tableof CO and C02 pressures in equilibrium with C and U02+x at selectedvalues of temperature and 0/U ratio.

The thermodynamics of equations A3 and A4 are well developedand can be found in many thermochemistry texts (20) . The thermo-dynamics of equation A5 are not so well characterized but have beeninvestigated by several workers. Perron (21) has correlated theirresults for the partial molar free energy of oxygen, G(02), as afunction of temperature and stoichiometry. For use in this study,his best fit data for G(O2) has had to be extrapolated to temperaturesoutside the range included in his work.

By analogy with the method described in part A of this appendix,the following expression is obtained from the free energy change forequation A3,

P(CO2) _ 1 ? 0 8 0 Q + 1 7 4 6 T _1X1 =

P2(co) RT

Similarly, the free energy change for equation A4 yields

P(CO2) 1 = -282 600 + 86.88 TP(CO) p5s(O2) RT

and equation A5 produces the expression-G(O-)

=

These equations have been solved simultaneously to obtain expressionsfor the equilibrium partial pressures of CO and C02 in terms of thepartial molar free energy of oxygen. Thus

22_ 13430In p(CO) = 4 - 1 6 T + ^^- + 10.56 ...(A6)

and J n P(CO2) = o ^ p + — ^ +0.12 ..

42.

Equations A6 and A7 have been used, along with Perron'sdata for G(C>2)» to calculate the partial pressures of CO and C0 2

in equilibrium with graphite and U0 2 for various O/U ratios atthe two temperatures of interest. The results of these calcula-tions are shown in Table Al.

These results show that at high temperatures, the equilibriumpartial pressures of CO and C02 produced as the result of the chemicalinteraction of graphite and UO2 are of the order of 10^ to 10^ MPaand that the gaseous product tends to be rich in CO. At low temper-atures and low O/U ratios these equilibrium pressures are very lowand the gas is rich in CO2.

It is worth noting that there are independent determinationsof the partial molar free energy of oxygen in non-stoichiometric UO2

that are at variance with Perron's values. This is particularly truein the slightly hyperstoichiometric range. Wheeler and Jones (22)have measured these values at 1950 K for a range of stoichiometries.Their work is in excellent agreement with the data of Markin and Bones(23). Their data for G(O2) at slightly hyperstoichiometric O/U ratios(between 2.0 00 and 2.005) would yield lower CO2/CO ratios and lowertotal pressures than the corresponding values of Perron (provided wecan assume that the differences in results of these workers can beextrapolated to 2300 K). In fact, the use of the data of Wheeler andJones would make the results in Table Al look somewhat more self-consistent in that at 2300 K the partial pressures of CO and CO2

would constantly increase as the O/U ratio was increased. However,these differences do not significantly affect the interpretation ofthe present experiment.

TABLE Al

Tempo la tu le(K)

2300

2300

M 2300L J 2300

H 11

M

n2300

750

L J 750H 750

U 750750

750

750

PARTIALWITH 110*4

Q/URatio

2

2

2

2

2

2

2

2

2

2.

2.

2.

2.

2.

.000

.001

.005

.010

015

050

100

000

001

005

010

015

050

100

PRESSURE OFly AMP CARSOWA.

IJ/

117

118

126

114

107

79

47

207

186

135

125

123

120

122

02)*mot)

030

305

015

305

645

132

286

596

049

870

920

001

707

202

CARBON MONOXIDEAT TVP1CAL FUEL

2.

6.

2.

4.

2.

1.

2.

p(CO)PTTOÏ)

5.9

6.3

9.1

5. 3

3.7

0.83

0.153

0 x 10~ 2

7 x 10~ 3

1 x 1 0 ~ 6

2 x 10~ 7

6 x 1 0 ~ 7

9 x 10~ 7

3 x 10~ 7

AHV CARBON DIOXÎVi INCENTRAL ANC SURFACE

2.

2.

1.

3.

4.

2.

1.

8.

2.

8.

4.

6.

9.

7.

Pt CO)(M Pa)

9

7

8

4

8

1

1

2

6

1

0

3

2

2

x

x

X

X

X

X

X

X

X

X

X

X

X

X

103

103

103

103

103

IO4

105

10" 4

io- 2

101

IO2

102

102

102

4

4

1

6

1

2

7

4

3

9

2

5

3

EÇU/UI8RIUMrf'HPERATURES

P(COZ)(MPa)

.9

.3

.9

.5

.3

.6

.2

.0

.9

.4

.4

.0

.1

x 10 2

x 10 2

x 10 2

x 10 2

x 10 3

x 10 4

x 10 5

x IO"3

1.0

x IO7

x IO8

x IO9

x IO9

x IO9

piCO) + piCOj)IMPa)

3.4 x IO3

3.1 x IO3

2.0 x IO 3

4.1 x IO3

6.1 x IO 3

4.7 x IO4

8.3 x IO5

4.8 x 10~ 3

4.0

3.9 x IO7

9.4 x IO8

2.4 x IO9

5.0 x IO9

3.1 x IO9

*As a result of the way equation A5 was expressed these values are - h the values forpublished by Perron (21) .

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