Nuclear Technology
ISSN: 0029-5450 (Print) 1943-7471 (Online) Journal homepage: www.tandfonline.com/journals/unct20
Behavior of a UF6 Container during a Fire&I:
Analysis and Phenomenological Interpretation of
Tenerife Experimental Results
Eric Pinton, Bernard Duret & Georges Berthoud
To cite this article: Eric Pinton, Bernard Duret & Georges Berthoud (1999) Behavior of a
UF6 Container during a Fire&I: Analysis and Phenomenological Interpretation of Tenerife
Experimental Results, Nuclear Technology, 127:3, 332-351, DOI: 10.13182/NT99-A3005
To link to this article: https://doi.org/10.13182/NT99-A3005
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BEHAVIOR OF A UF6 CONTAINER
DURING A FIRE—I: ANALYSIS
AND PHENOMENOLOGICAL
INTERPRETATION OF TENERIFE
EXPERIMENTAL RESULTS
THERMAL HYDRAULICS
KEYWORDS: safety of nuclear
fuel transport, nuclear safety, thermal hydraulics
ERIC PINTON,* BERNARD DURET, and GEORGES BERTHOUD†
Commissariat à l’Energie Atomique, Direction des Réacteurs Nucléaires
Grenoble, France
Received December 30, 1997
Accepted for Publication March 3, 1999
I. PRESENTATION OF THE PROBLEM
To improve the knowledge of the behavior of a UF6
container during a fire, an experimental project called
Tenerife was conducted by the Commissariat à l’Energie
Atomique. Three tests with UF6 with different kinds of
heating and temperature furnaces were carried out. The
main information obtained from monitoring temperature
and pressure during the heating tests is as follows:
1. The presence of a strong thermal contact resistance at the solid UF6-steel interface.
2. The rupture of the solid crust at the top of the container, a crust formed during container cooling after filling, for a pressure reaching 1.5 bars (triple point). This
leads to the beginning of boiling heat transfer and notably film boiling, followed by transition boiling and nucleate boiling.
3. The appearance of the liquid stratification with the
beginning of nucleate boiling. It can accelerate the rise in
pressure because of the reduction of mass transfer by condensation to the liquid-gas interface. This stratification is
preserved with the natural convection regime that replaces
the nucleate boiling after the end of heating.
4. After rupture of the upper UF6 crust, the pressure
increase may be delayed by different wetting of the UF6
on the steel wall.
Also, these tests were allowed to build and validate a scenario that has been reproduced in a numerical model.
*Current address: Robatel, rue de genève, BP 203, 69741 Genas Cedex, France.
†E-mail: georges.berthoud@cea.fr
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I.A. Introduction
Uranium hexafluoride is the raw material from which
the fuel for nuclear power plants is obtained. It is stored
and transported in the solid state in steel industrial containers similar to the one shown in Fig. 1.
The International Atomic Energy Agency ~IAEA! envisages a revision of current regulations and suggests that
a container should withstand a specific fire test 1 ~engulfing fuel fire of 8008C for 0.5 h, for a steel emissivity of
0.8 and flame emissivity of 0.9!. To study the safety of
the containers under these conditions a numerical model
was created by the Commissariat à l’Energie Atomique
~CEA!. A two-dimensional model that uses the finite element computation code ANSYS was developed. It takes
into account thermal and mechanical phenomena as well
as mass transfer.2 Modeling and validation are presented
in Part II of this paper.3 This model must be validated
with reliable and representative experimental results.
Over the past 30 yr, a number of experiments on UF6
containers under fire conditions have been performed.
The best instrumented and the most interesting was conducted by Suzuki et al.4 However, they do not in any way
reflect realistic conditions. The dimensions of the test
containers were much smaller than those of a 48Y container ~107 kg instead of 12 500!, and the simulated fire
temperatures were too low ~4008C instead of 800!.
Although it is possible to estimate the thermohydraulic behavior of the UF6 from these tests, many uncertainties remain ~rupture of the UF6 top crust and its influence
on transfer, liquid emergence, boiling type at the steelliquid UF6 interface, value of the thermal contact resistance steel-UF6 solid, pressure evolution, mass transfer,
etc.!. A numerical model can only be partially validated
with these results. Therefore, it was necessary for us to
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 1. Schema and description of 48Y container.
conduct tests where the experimental container shape and
the fire simulation are close to the real conditions.
Consequently, an experimental project called Tenerife 5 was defined and conducted in the scope of a joint
research program between France ~CEA! and Japan ~Central Research Institute of the Electric Power Industry!.
The experimental side of this project is carried out by the
Institute for Protection and Nuclear Safety in the facility
for research on fires at the CEA Cadarache ~France! Research Centre.
The fire is simulated with an electric furnace consisting of radiant INCONEL a panels capable of reaching
a temperature up to 10008C in 4 min. The Tenerife container is identical to a 48Y container except that its length
is reduced by one third to limit the amount of UF6
~4 tonnes instead of 12.5! and also to limit the overall
dimensions of the furnace ~see Fig. 2!. This paper presents
the phenomenological interpretation and the lessons
a
INCONEL is a trademark of the Inco family of companies.
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learned from these tests carried out with different furnace temperatures and heating time.5
I.B. Different Rupture Modes and Description of the UF6
Uranium hexafluoride, the only material inside the
container, is a solid at ambient temperature in equilibrium. When heated, the vapor sublimates without melting, as shown in the phase diagram ~Fig. 3!, which also
shows that
1. The solid-liquid phase change appears at the triple point ~648C, 1.52 3 10 5 Pa!.
2. The UF6 melts at a constant temperature of 648C.
3. For a pressure above that of the triple point, the
three phases coexist.
4. The vapor pressure rises steeply with the liquid
temperature to reach the critical point at a value of
46 bars with a corresponding temperature of 2308C.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 2. Schema and description of Tenerife container with the furnace.
Increase of the internal pressure with the rise of UF6 temperature coupled with the decrease of the steel mechanical properties at high temperature @see Fig. 16 of Part II
~Ref. 3!# can lead to rupture of the metal casing.
The container-filling regulation involves a gaseous
cavity at the top of the container of about one-third of
the inner volume. Thus, during the heating, melting of
UF6 entails a significant decrease in density Dr0rS 5 25%
~Fig. 4!, and thus, the liquid level will increase progressively with time until it eventually occupies all of the
inner volume. There is then a risk that the container will
tear open under the force of hydraulic pressure. The overall physical properties can be found in the compilations
of De Witt 6 and Anderson et al.,7 which assemble and
examine practically all the literature published on the
properties of UF6 .
I.C. Initial State of the UF6 and Cooling
of the Container After Filling
Fig. 3. Phase diagram for UF6 .
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The initial structure of the solid UF6 within the container is still not well known, since it is very difficult to
see inside the 48Y. However, it can be estimated by analyzing the cooling process of UF6 after filling by taking
internal temperature measurements during cooling of a
Tenerife container. The quantity of UF6 input in the liquid
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 5. Cooling of UF6 after filling.
Fig. 4. Density of UF6 .
state at 808C complies with the regulation that only 95%
of the volume must be occupied by the UF6 at 1208C.
The measurements indicate that the temperature of
the liquid UF6 becomes rapidly uniform at 648C, and then
the gas occupies ;15% of the inner volume as shown on
Fig. 5. Subsequently, the UF6 solidifies on the cold wall
of the steel cylinder. As the density of the liquid is less
than the solid, the liquid level is reduced. Crystallization
of UF6 gas at the top of the container forms a solid crust.
This one has been observed during an endoscopic visualization attempt.
In the region not close to the wall, which we call the
“core,” the crystallization seems different. Indeed, measurements reveal a thermal gradient in the solid in the
vicinity of the steel, while in the core, temperatures remain homogeneous at 648C until all the liquid disappears. Then, temperatures fall together.
This type of solidification is comparable to solidification of metal castings: Externally there is nucleation at
the wall; from these points a dendrite grows, following
the thermal gradient; and bulk freezing of the liquid occurs in an equiaxed configuration. These two types of
crystallization entail equally a difference of porosity. The
UF6 depositing gradually along the wall is very compact
and has a normal density of ;5000 kg0m 3. On the other
hand, in the solid core, inner voids appear. These are estimated at ;25%, which corresponds to the difference
between the liquid and solid densities.
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The thickness of the solid crust at the top of the
container and that of the solid next to the wall depend
on the external cooling conditions. Moreover, conditions of storage ~sunniness, external temperature, etc.,
leading to inner mass transfer! can also influence the
initial configuration of the UF6 . Therefore, the initial
UF6 configuration will not be identical for all containers and will depend on their history.
II. INTERPRETATION OF THE TEN2 TEST
II.A. Introduction
Two tests were carried out on the container called
TEN2, enclosed in the furnace at 8008C: the first with a
10-min heating ~June 29, 1995! and the second with an
18-min heating ~July 6, 1995!. Since the 18-min test provides most of the information, the analysis of the 10-min
test will not be presented in this paper.
The steel temperature evolution, then that of UF6 ,
and finally the pressure evolution will be discussed. On
the figures presented in this paper, time t 5 0 corresponds to the heating initiation. The instrumentation consists of the following:
1. 2 pressure sensors P1 and P2
2. 16 thermocouples ~T1 to T16! in three tubes inside the container
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Fig. 6. Instrumentation of the TEN2 container.
3. 8 thermocouples ~TP1 to TP8! on two combs near
the inner wall
4. 15 thermocouples ~A1 to A15! on the inner surface of the container
5. 10 thermocouples ~TJ1 to TJ10! and 10 strain
gauges ~J1 to J10! on the outer surface of the container.
Details on the positions of these measurement points can
be found in Fig. 6. On some of the figures, the saturation
temperature is shown, calculated from the pressure measured by sensor P1 ~range 0 to 7 bars! or P2 ~range 0 to
20 bars!.
Fig. 7. Definition of the median plane and the steel internal
thermocouples in this plane.
II.B. Interpretation of the Steel Temperatures
II.B.1. The Steel Inner Temperatures
in the Median Plane
The median plane is the plane running through the
middle of the container as shown in Fig. 7. The temperatures in the median plane are discussed as follows:
1. 0 , t , 250 s: The steel receives the radiant energy from the furnace, and its temperature rises progressively. At time 165 s, all the steel temperatures reach a
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plateau that lasts for ;20 s, and then they continue to
rise until 230 s, where the same phenomena seem to occur. We explain these two phenomena by increased heat
transfer between steel and UF6 . Indeed, these two phenomena are accompanied by a significant increase in pressure @see Tsat ~P1! on Fig. 8#, so it seems that a crack occurs
in the crust, leading to an infiltration of the gas contained between the steel and the solid UF6 @which is at a
higher pressure than the gas blanket Psat ~TUF6 ! . P1#
toward the gas volume, as shown in Fig. 9. This
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Fig. 8. Evolution of the steel inner temperatures in the median plane from 0 to 250 s.
Fig. 9. Interpretation of the temperature plateau.
contributed to increasing the pressure and to improving
exchanges at the steel inner surface.
As soon as the crack is sealed by condensation of
the vapor, the steel resumes its normal temperature evolution. The thermocouples measuring the inner temperatures of the metal wall are simply kept in contact
~Fig. 10! by a spot-welded steel sheet. The strange temperature decrease, corresponding to the second phenomenon, surely results from significant arrival of UF6 cold
gas on the thermocouples.
2. 350 , t , 540 s: The temperature continues to
rise, as shown in Fig. 11. At time t 5 420 s, all the tem-
Fig. 10. Location of the internal steel thermocouples.
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peratures are .2008C, even while the inside UF6 is still
in solid state. Now, in the case of perfect contact, the
metal temperature should not exceed the UF6 melting temperature because the UF6 is still solid ~if it was liquid,
wetting of the wall would have entailed a change of slope!.
This confirms the existence of a significant contact resistance also present at the level of the crust on the top of
the container.
At time 440 s, it seems that liquid appears almost
simultaneously at the levels of A14, A13, and A15. There
is in fact a slight change of slope in the steel temperature curve, indicating improved heat exchange. Even in
the presence of the liquid, exchange is limited by the
depth and conductivity of the liquid UF6 . Therefore, the
steel temperature continues to increase steadily. However, this temperature is higher than the critical temperature ~2308C!, indicating the presence of a supercritical
fluid next to the metal wall. Nevertheless, it must stay
liquid in the vicinity of the solid UF6 interface, which
is at 648C. Actually, the UF6 fluid ~whose state is not
well known! is confined between the wall and the solid
UF6 .
Referring to Fig. 11, the contact resistance would
seem to be greater in the lower part of the container
than in the upper part since A14, A13, and A15 are
greater than A9, A10, A11, and A12. If this were the
case, the UF6 in the upper part would receive more energy, and the liquid should therefore appear first in this
zone. However, it seems first to appear at the bottom,
which is an apparent contradiction. In our opinion, it is
rather that the lower part of the steel container receives
more energy than the part near the top. This is confirmed by the fact that temperatures of the INCONEL
furnace for positions facing the steel are found to be
higher in the bottom zone.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 11. Evolution of the steel inner temperatures in the median plane from 350 to 540 s.
3. 540 , t , 800 s: At 540 s the upper crust collapses as will be demonstrated later ~in Sec. II.C.1 from
540 to 650 s and in Sec. II.C.2 at 540 s!. Thus, the pressurized interstitial liquid is back at the pressure of the
cavity and starts to boil. Single-phase exchange is replaced by film boiling because of important superheat
between wall and saturation temperature Tp 2 Tsat .
2008C. Heat transfer at the wall improves and leads to a
change of the slope of the measurements at A13, A14,
and A15, as shown on Fig. 12. The liquid level gradually
rises and seems to reach A11 at 600 s and then A9 at
720 s ~change in slope!.
There is then an acceleration in the fall of the steel
temperatures. This is characteristic of the switch from
film boiling to nucleate boiling that goes through a transition boiling phase where the flux exchanged at the wall
qBoil increases with the decrease in wall temperature Tw ,
as shown on Fig. 13.
In nucleate boiling, heat exchange is excellent so that
the flux exchanged on the boiling surface qBoil rapidly
balances the external flux qext , which in our case is the
radiant flux from the furnace. As this flux is practically
constant over the whole surface, the temperatures of the
steel in contact with the liquid do not change anymore,
as shown on Fig. 12 ~at t 5 800 s!.
4. 800 , t , 1080 s: Over this period, the steel temperature is practically Tsat and follows the evolution of
Fig. 12. Evolution of steel inner temperatures in the median plane from 500 to 1500 s.
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Fig. 13. General characteristic curve of boiling heat exchanges.
Tsat . The reason needs explanation. During boiling, the
flux is governed essentially by the superheat Tw 2 Tsat .
The external flux is nearly constant; thus to conserve flux
balance qBoil 5 qext , the superheat must be kept constant.
The wall temperature Tw therefore follows the evolution
of saturation temperature Tsat . In nucleate boiling, the exchange coefficient is also very high, so a small superheat
is sufficient to transmit a high flux. This explains why
Tw ' Tsat .
However, in the bottom zone, the wall temperature
is below saturation temperature—a phenomenon that
could be attributed to the presence of natural convection
with no boiling. In fact, the internal thermocouples are
laid on the metal surface so that they are in contact with
both the steel and the UF6 ~liquid or solid!, as shown on
Fig. 10. Thus, we believe that the value measured at the
end of the thermocouples is some mean value of the steel
and UF6 temperatures. Depending on the exchange between the thermocouple sheath and the UF6 , the measured value may underestimate the true inner temperature
of the steel.
5. Particular Case of A9 and A10 (Near the Liquid
Level): During transition boiling, the major part of the
flux is used to produce vapor. When nucleate boiling is
established, the vapor output decreases rapidly. There is
a reduction in the volume occupied by the bubbles causing a drop in the liquid level, as shown on Fig. 14. We
consider that at time t 5 780 s, A9 is no longer in contact with the liquid, and since there is less exchange
with the gas, the temperature of A9 increases slightly
~until 850 s!. However, since A9 is close to the interface, it falls within the lateral thermal gradient, as illustrated on Fig. 15. As heat exchanges continue, the UF6
level rises with solid melting. The steel thermal gradient moves upward, leading to a decrease in the temperature of A9 ~from 850 to 1080 s!.
6. t . 1080 s: The radiant energy coming from the
INCONEL and received by the steel gradually decreases,
leading to a drop in the steel temperature. The boiling
regime then switches from nucleate boiling to natural convection. The decrease in temperature begins first at the
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Fig. 14. Variation in liquid level versus type of boiling.
Fig. 15. Tangential thermal gradient in the steel near 800 s.
bottom of the container and then propagates upward, indicating that natural convection starts at the bottom because the proximity of the solid improves the exchange
and develops gradually upward to the top of the cylinder.
When heating is stopped, the temperature values of
A9 and A10 follow that of Tsat , indicating that the regime here is nucleate boiling. Lower down, at the level
of thermocouples A13, A14, A15, A11, and A12, the regime is natural convection. Boiling near the liquid level
is due to the thermal energy stored in the upper zone of
the container that continues to stream down to the lower
zone by conduction ~see Fig. 15!. This conductive flux
~along with exchanges with the furnace due to its thermal inertia! is therefore sufficient to maintain nucleate
boiling close to the liquid-gas interface.
Figure 16 shows that the temperature of A8 ~top of
the container! increases almost linearly from 540 to
1080 s, i.e., from the time the crust collapses until heating is stopped. The radiant flux received on the outer surface of the steel is therefore still higher than that of the
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 16. Temperature evolution of A8 and of the INCONEL opposite A8.
radiant and convective flux exchanged on the inner
surface.
When the heating is stopped, the temperature of A8
gradually decreases, and eventually it becomes higher than
that of the INCONEL. From then on ~1500 s!, the steel is
no longer receiving energy from the furnace but, on the
contrary, transfers energy to the INCONEL.
II.B.2. The Steel Temperatures in the Horizontal
Axial Plane
The horizontal axial plane is the plane through the
middle of its axis, as illustrated in Fig. 17. The evolution
of A5, A6 and A11, A12 is practically identical ~see
Fig. 18!. On the whole, edge effects are practically nonexistent on the cylindrical part.
The temperature rise of TJ1 falls off at ;180 s, which
suggests that contact is improved. The liquid appears at
;500 s ~change of the slope!. Thus, TJ2 remains low be-
cause it receives less energy due to its location ~see
Fig. 6!.
II.C. Interpretation of the UF6 Temperatures
In what follows, we will begin by interpreting the
evolution of UF6 temperatures in the upper part of the
median plane ~T2, T3, T4, T5, T6, and T7!, then in
the lower part ~T8, T9, T10, T12, T13, T15, and T16!,
and conclude by the interpretation of the temperatures
corresponding to comb 2 ~TP5, TP6, TP7, and TP8!. Thermocouple locations are given in Fig. 6.
II.C.1. The UF6 Temperatures on the Upper Part
of the Median Plane
The UF6 temperatures on the upper part of the median plane are as follows ~see Fig. 19!:
Fig. 17. Definition of the horizontal axial plane and the thermocouples positioned in this plane.
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Fig. 18. Evolution of steel temperatures in the horizontal axial plane.
Fig. 19. Temperature evolution in the upper part of the median plane from 0 to 800 s.
1. t 5 0 s: The saturation temperature Tsat should
be that of the initial temperature of the system Tini , i.e.,
;368C in this test. However, Tsat ~P1! , Tini , which means
that the value measured by sensor P1 slightly underestimates the pressure, possibly because of calibration errors.
4. 180 , t , 240 s: T2, T3, T4, and T44 increase
with Tsat ~P !, whereas the other thermocouple values remain unchanged, except for T7, which rises very slightly.
2. t , 180 s: T3 and T2 evolve with Tsat ; this means
that T2 and perhaps T3 are in the gas phase. The other
temperatures do not change.
6. 240 , t , 540 s: All the thermocouples in the
upper part follow Tsat ~P !.
3. t 5 180 s: There is a sudden rise in temperatures
of T2, T3, T4, and T44, which increase with Tsat ~P!.
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5. t 5 240 s: The readings of T5, T6, and T7 rise
suddenly to the temperature of Tsat ~P !.
7. t 5 540 s: All the UF6 in the upper part reaches
the triple point temperature, i.e., 648C, where the liquid
phase can begin to exist.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
12. t 5 780 s: The UF6 at the levels of T5 and T6
becomes liquid.
Fig. 20. Evolution of the Tsat ~P ! front in the solid UF6 .
8. Analysis: Thermocouples T3 and T2 seem to be
in the UF6 gas and the other thermocouples in solid UF6 .
There seems to be a gradual diffusion of heat from the
solid-gas horizontal interface toward the bottom of the
solid and a Tsat ~P ! front that propagates as shown on
Fig. 20. In the inner part of the front where the temperatures are at Tsat ~P !, the heat exchanges are excellent since
the temperatures evolve in a uniform manner. The heat
input would be due in particular to the condensation of
the vapor ~cold surface! that sublimated at the solid-gas
interface of the crust ~hot surface!.
9. 540 , t , 650 s: At ;540 s, T2 temperature
increases suddenly and becomes significantly higher than
Tsat ~P !. This seems to be the moment the top crust collapses allowing the gas to come into contact with the “superheated” steel and to rise in temperature. At 590 s, liquid
UF6 running along the thermocouple encounters the weld
and causes a drop in temperature. Thermocouple T3 increases with Tsat ~P !; it must be near the liquid-gas interface. The other temperatures remain unchanged ~648C!
at the solid-liquid equilibrium temperature.
10. t 5 650 s: There is a change of phase at T4 and
T7 from the solid to the liquid phase because their temperatures, constant up to now, begin to rise.
11. 650 , t , 780 s: From 700 s, T2 temperature
decreases due to the switch from film boiling to nucleate
boiling. In fact, the flux exchanged on the boiling surface and the generation of vapor increase considerably
during the transition. In addition, with the reduction in
wall temperature and the change in the type of transfer
between liquid and steel, the vapor is less and less superheated and reaches saturation under nucleate boiling
conditions. A large quantity of cold vapor arrives and then
mixes with the gas blanket and causes its temperature to
drop.
Thermocouples T4, T7, and T3 are approximately at
the saturation temperature of Tsat ~P !. We can consider
that the motion in the liquid at the top, caused by the
generation of a large amount of vapor ~under film boiling and then transition boiling conditions!, tends to equalize the temperature of the liquid.
342
13. 780 , t , 870 s: Thermocouple T3 still evolves
with Tsat ~P !. The liquid level seems to remain constant.
Temperatures of T5 and T6 rise rapidly to that of T7. The
motion in the upper liquid remains important although
its equalizing effect is not so obvious.
On approaching equilibrium between the external flux
and the nucleate boiling flux, the generation of vapor is
considerably reduced. The energy absorbed by the gas,
by absorption in infrared radiation and by convection with
the steel, becomes higher than that added by the cold vapor released by boiling. The temperature of vapor located in the gas pocket rises as indicated by T2.
14. 870 , t , 1080 s: At 880 s, the motion in the
liquid appears to cease and give way to a thermal stratification. In fact, the temperature of the liquid now decreases regularly from the liquid-gas horizontal interface
at Tsat to the solid-liquid interface at 648C ~Fig. 21!.
We see that the liquid temperatures in the same horizontal plane ~T5, T6, and T7! are identical. This phenomenon is explained in Sec. II.C.3. At 1020 s, T3 ,
Tsat ~P !, the free surface of the liquid pool becomes above
the position of T3 @if Tinterface 5 Tsat ~P1!#.
15. t . 1080 s: There is a gradual change from nucleate boiling to natural convection along the steel wall.
Gradually, the free surface of UF6 decreases, which is
shown on the thermocouples that reach the saturation temperature as seen on Fig. 22 ~temperature of the liquidgas interface!. However, that evolution has a time delay,
as is clearly evident from the maximum measurements
~T3 and then T4, T6, etc.!. The closer to the interface,
the shorter the time delay and the stronger the influence
of the interface.
II.C.2. UF6 Temperatures in the Lower Part
of the Median Plane
The UF6 temperatures in the lower part of the median plane are as follows ~see Fig. 23!:
1. 0 , t , 540 s: As in the upper part, sudden
changes of temperature occur from the top to the bottom
until they reach Tsat ~P !. The phenomenon found for the
UF6 in the upper part seems to prevail right down to the
bottom of the container. We explain this by a sudden mass
transfer of UF6 vapor coming from the gaseous cavity
through porosities of solid UF6 in a preferential manner
along the tube supporting the thermocouples.
The liquid appears first at T11 ~370 s! and then at
T16 ~430 s!. This may also be explained by the fact that
the INCONEL temperature, and therefore the energy received by the steel, is higher at T11 ~Tinco : 8308C! than
at T16 ~Tinco : 8058C!.
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Fig. 21. Evolution of UF6 temperatures in the upper part of the median plane from 500 to 1100 s.
Fig. 22. Evolution of UF6 temperatures in the upper part of the median plane from 0 to 180 000 s.
2. t 5 540 s: At this time the collapse of the crust is
assumed to take place, with the result that the liquid confined between the solid and the steel is exposed to the
gas blanket. As this liquid is superheated @compared to
Tsat ~P !# , especially at the level of T11 and T16, there
will be sudden evaporation with cooling of the liquid until equilibrium is reached, i.e., until Tliq ' Tsat ~P !.
3. 540 , t , 760 s: In the neighborhood of 540 s,
practically all the thermocouples are at UF6 melting temperature and therefore all at solid-liquid equilibrium,
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which is not compatible with an energy balance.2 In fact,
the radiant heat flux received by the steel surface and
heat transfer at the steel-solid UF6 interface ~thermal contact resistance! are relatively well known. In this case,
energy transmitted to UF6 can be evaluated. The global
temperature of the solid mass at 540 s, given by the computation, would be of 428C, i.e., 68C above the initial temperature, instead of the 288C observed in the experiment.
At t 5 680 s, all the thermocouples located in the bottom
part seem to be in the liquid phase, except for T9, which
remains in the solid.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 23. Evolution of UF6 temperatures in the lower part of the median plane from 100 to 1400 s.
4. 760 , t , 1400 s: Near 760 s, the superheated
liquid undergoes a sudden temperature drop. Paradoxically, this drop occurs despite the fact that the flux at the
wall is still significant ~see the strong drop of the steel
temperatures in contact with the liquid in Fig. 12!, which
is not logical. We believe that the solid, which up to this
time had been attached to the tubes supporting the thermocouples, now sinks to the bottom of the container
~Fig. 24!. Other evidence supports this hypothesis, for
example, the temperature at TP8 that increases suddenly
~Fig. 25!, which suggests that TP8 initially in the solid is
now in the liquid bath ~see Fig. 24!.
With the fall of the solid UF6 block, the temperature
of the thermocouples decreases but nevertheless remains
higher than 648C ~Tmelt !, as though some liquid remained.
It can be assumed that liquid remains entrapped in the
tubes and that it is surrounded by solid UF 6 , as illustrated on Fig. 24. This assumption will be confirmed in
the next time period. Thermal equilibrium is then gradually attained. All the lower thermocouples have a uniform temperature that decreases gradually to stabilize at
;5 3 10 4 s, as illustrated on Fig. 26.
Looking at the values of these thermocouples, the
UF6 appears to be liquid. However, if this were the case,
stratification between different locations of UF6 thermocouples should occur as in the upper part of the container.
Moreover, there would be no reason for the temperature of T8 to rise above Tmelt . Thermocouple T8, in fact,
breaks away at ;25 000 s because the solid around it has
disappeared. Once freed from its solid prison, the thermocouple recovers the stratified medium of the liquid
~Fig. 27!. Moreover, an energy balance confirms that the
UF6 can be entirely in the liquid state.
Everything seems to indicate that the medium is liquid inside the tubes but solid outside the tubes. From T8
behavior, we conclude that there is still solid UF6 at the
bottom of the container ~after 7 h the solid is 30 cm in
height!.
II.C.3. The Temperatures on Comb 2 (Near the Wall)
The UF6 temperatures on comb 2 ~near the wall! are
as follows:
1. 0 , t , 400 s: The nearer to the wall, the higher
the temperature of the solid UF6 . This seems to indicate
that at the combs, the energy received by the UF6 comes
essentially from the steel.
Fig. 24. Sinking of the solid.
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2. t 5 400 s: Thermocouples TP5, TP6, and TP7 are
at the same temperature. Only TP8 indicates a lower
temperature.
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Fig. 25. Evolution of UF6 temperatures on comb 2 from 0 to 1100 s.
Fig. 26. Evolution of UF6 temperatures in the lower part of the median plane from 0 to 5 3 10 5 s.
3. 400 , t , 540 s: The temperature of TP5 breaks
away at 450 s and seems to reflect a liquid phase. With
melting of the crust ~at ;540 s!, a high evaporation is
observed that entails cooling of the liquid, as noted earlier. Thermocouples TP6 and TP7 evolve with Tsat ~P !
and TP8 continues to rise steadily.
Fig. 27. Freeing of T8 from its solid prison.
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4. 540 , t , 800 s: Thermocouples TP5, TP6, and
TP7 are in the liquid phase since their temperatures are
higher than melting temperature. They follow with
Tsat ~P ! : The liquid seems to be in equilibrium with the
vapor. This indicates significant agitation enabling the
liquid temperature to remain uniform.
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Pinton et al.
URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
least at Tsat ! and the first thermocouple 1 cm away from
the wall ~Fig. 25!. In contrast, between 1 and 5 cm from
the wall, the temperature is practically uniform. This confirms the assumption that the major input of energy from
the wall is taken up by the bubbly boundary layer ~on a
thickness lower than 1 cm at the level of the comb!.
The energy contained in this layer is carried directly
toward the liquid-gas interface. The convective motion
results in an arrival of hot UF6 that will then initiate the
stratification of the liquid in the upper part of the container ~see Fig. 28!.
II.D. Interpretation of the Pressure Evolution
5. 800 , t , 880 s: Nucleate boiling occurs gradually, and the generation of vapor is smaller. The liquid
agitation is less important, and a boundary layer bubble
regime is installed. This less dynamic boundary layer
transmits little energy to the liquid bath, leading to a temperature decrease.
We have two pressure sensors available, one with a
pressure range up to a maximum of 7 bars ~P1! and the
other up to 20 bars ~P2!. The supplier of these sensors
ensures that linearity is preserved to within 0.2% up to
150% of the full scale. Measurements are therefore usable up to 10.5 bars for P1 and 30 bars for P2. Note that
on Fig. 29, the sensor P2 gives a pressure always higher
than P1. Moreover, the difference between these two values increases with the pressure rise. It is logical to think
that at a low pressure, P1 gives a better measurement than
P2 because its range is limited to 7 bars, so it is more
accurate. An extrapolation of P1 with respect to P2 would
give a pressure at 1800 s of 25 bars instead of 29 bars as
indicated by P2, so a reduction of ;4 bars may be closer
to the correct value.
The pressure evolution is as follows:
6. 880 , t , 1080 s: The wall temperature at comb
level increases ~see A12 on Fig. 12!. There is a considerable temperature difference between the wall ~that is at
1. 0 , t , 180 s: The pressure slightly increases.
Sublimation occurs at the hot interface of the crust and
condensation on the colder solid interface at the bottom,
Fig. 28. Boundary layer and liquid agitation.
At t 5 760 s, TP8 ~5 cm from the wall! is in the
liquid phase, and its temperature rises suddenly. It almost instantaneously stabilizes at Tsat ~P !, indicating very
high heat transfer.
Fig. 29. Pressure evolution from 0 to 1800 s.
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to preserve thermodynamic equilibrium @the temperature
of the solid-gas interface must be equal to Tsat ~P ! over
its whole surface#.
2. t 5 180 s: A crack must appear in the crust that
allows infiltration of the gas trapped between the steel
and the solid, thereby increasing gas pressure.
3. t 5 240 s: The same phenomenon is observed.
4. 240 , t , 540 s: There is sublimation at the
inner surface of the crust that increases the mass of gas
in the gas blanket and therefore increases the pressure.
URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
icantly, thereby reducing the rise in pressure, or even
slightly decreasing the pressure.
8. 880 , t , 1080 s: Once nucleate boiling is established over all the surface of the container, a bubbly
thermal boundary layer forms along the inner metal surface. The energy absorbed by the liquid remains trapped
within this boundary layer from which it is released at
the liquid-gas interface ~Fig. 28!. Since the energy input
is at the interface, condensation on the interface is reduced, and consequently the pressure increases.
6. 700 , t , 800 s: There is a change to transition
boiling where the flux increases considerably ~Fig. 13!,
as well as the production of vapor. A larger quantity of
vapor reaches the gas pocket and so contributes to an increase in pressure.
9. 1080 , t , 4000 s ~see Fig. 30!: The heat transfer regime changes gradually to natural convection, where
the generation of vapor falls rapidly, which should lead
to a pressure drop. However, in natural convection, the
convective velocities of the liquid in the boundary layer
will be considerably less than during nucleate boiling.
Similarly, the motion induced in the liquid pool is reduced, which reinforces the stratification and then decreases the vapor condensation. Note that boiling close
to the free surface continues long after heating is stopped
due to the thermal inertia of the furnace and of the steel
at the top, producing the lateral thermal gradient described earlier ~Sec. II.B.1 and Fig. 15!.
Despite the fact that the quantity of vapor arriving in
the gas blanket decreases, the pressure continues to increase due to the simultaneous decrease in the quantity
of vapor condensed. However, the decrease in the quantity of vapor evaporated is more rapid than the decrease
in the quantity of vapor condensed, so that the pressure
does not rise so sharply and reaches at ;4000 s a maximum ;32 bars with P2 or 27 bars by extrapolating P1.
7. 800 , t , 880 s: Nucleate boiling replaces transition boiling, and the generation of vapor drops signif-
10. t . 4000 s : There is no more boiling, and
the pressure gradually decreases under the effects of
5. 540 , t , 700 s: At 540 s, the collapse of the
UF6 solid top crust allows film boiling to develop. In this
boiling regime, a relatively large quantity of vapor is generated, although pressure increases only slightly. This indicates that there must be considerable condensation of
the vapor produced at the boiling surface.
Because of gravity forces, in the lower part of the
container, the bubbles detach from the vapor film as shown
on Fig. 28. These bubbles condense in the subcooled liquid and on the solid, thereby releasing their energy to the
liquid. In contrast, in the upper part, the vapor film is
stable and a large quantity of vapor arrives in the gas
pocket. If the pressure does not increase, this means that
a large amount of this vapor condenses at the liquid-gas
interface.
Fig. 30. Pressure evolution from 0 to 10 5 s.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 31. Position of the thermocouples on the median plane.
condensation of the vapor at the interface and thermal
leakage from the furnace.
III. INTERPRETATION OF THE TEN4 TEST AND
COMPARISON WITH THE TEN2 TEST
The TEN4 test carried out on February 5, 1996, consisted of a high-temperature heating test for an internal
configuration of UF6 similar to the one existing just after
its filling. Indeed, the initial state of the UF6 during the
18-min TEN2 test did not correspond to this case because the container had already undergone a heating of
10 min 7 days previously.
As compared to the TEN2 test, the instrumentation
has undergone some modification since different interrogations have been raised ~symmetry, edge effect, etc.!.
Some measurements were displaced. Especially, the number of thermocouples on the median plane of the container was increased as indicated in Fig. 31.
Figures 32 through 35 show that the general evolution of the different measurements of TEN4 were identical to TEN2. Most of phenomena met with TEN2 were
reproduced in TEN4. For this reason, the detail of the
TEN4 interpretation is not resumed. Only some observed differences will be discussed. For example, there
is a small shift of ;200 s because of the initial temperature difference, i.e., colder in TEN4 ~178C! than in TEN2
~368C!, and a radiant flux slightly weaker in TEN4 than
in TEN2. This difference of flux seems to come from a
variation of the outer surface state of the container. In
fact, before being sanded, the container TEN2 was covered with a paint while TEN4 was in the manufactured
state. It is possible that paint particles have remained on
the steel, modifying the absorption of the infrared radiation of TEN2 when compared to TEN4. Note that
Figs. 32 and 33 show that up to 500 s of heating, the UF6
solid temperature has not practically increased. On the
other hand, from 500 to 700 s, the UF6 temperature has
increased 158C on average from the initial temperature.
This behavior may be due to internal tubes that disturb
measurements in the solid UF6 . At ;500 s, the pressure is such that the gas can penetrate the cracks existing in the solid and mainly near the tube. This explains
the observed increase locally. This would mean that
Fig. 32. Temperature evolution of the UF6 in the upper part of the median plane.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 33. Temperature evolution of the UF6 in the bottom part of the median plane.
Fig. 34. Evolution of the steel inner temperatures in the median plane.
temperatures measured in the solid do not reflect the UF6
global profile but remain local indications.
IV. INTERPRETATION OF TEN6 TEST AND COMPARISON
WITH TEN2 AND TEN4
The TEN6 test, undertaken June 13, 1996, concluded the Tenerife program. The container was heated
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to 8808C during 18 min. It had the objective of establishing a radiative flux close to that of an IAEA fire.
The physical phenomena are somewhat identical to
these of TEN2 and TEN4. This is why the interpretation
of TEN6 is not detailed. However, for TEN2 and TEN4,
the liquid rapidly wets all the metallic wall after rupture
of the upper crust, and transfer into the boiling regime
develops very rapidly. On the contrary, for TEN6, wetting of the wall is progressive.
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Pinton et al.
URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Fig. 35. Pressure evolution.
The quantity of vapor arriving in the gaseous region
is smaller, and the liquid stratification is delayed. A consequence is the delay in pressure increase reported in
TEN2 and TEN4 tests. This phenomenon can have several origins:
1. different initial configuration and solid internal
structure
2. variable contact resistance
3. nonhomogeneous radiative heat flux.
In the current state of the study, no leading hypothesis
can be identified.
on the steel wall. Reasons for this phenomenon are unknown at this time.
V. CONCLUSION
2. E. PINTON, “Modelisation du comportement d’un conteneur d’hexafluorure d’uranium soumis à un feu,” PhD Thesis,
Institut National Polytechnique de Grenoble ~Nov. 1996!.
A considerable improvement in the knowledge of heat
and mass transfer and internal physical phenomena has
been obtained thanks to tests carried out on containers
filled with UF6 . The main points are as follows:
1. The presence of a strong thermal contact resistance at the solid UF6-steel interface.
2. The rupture of the solid crust at a pressure near
1.5 bars ~triple point!. This leads to the initiation of boiling heat transfer and notably film boiling, followed by
transition boiling and nucleate boiling.
3. The appearance of the liquid stratification with
the appearance of nucleate boiling. It has the consequence of accelerating the rise in pressure because of the
reduction of mass transfer by condensation to the liquidgas interface. This stratification is preserved with the natural convection regime that replaces the nucleate boiling
after the end of the heating.
4. After rupture of the upper UF6 crust, the pressure
increase may be delayed by different wetting of the UF6
350
Also, these tests permitted building and validating a scenario that has been reproduced in the numerical model
developed by the CEA Grenoble ~France! about the behavior of a UF6 container under fire conditions.2
REFERENCES
1. “Recommendations for Providing Protection During UF6
Transport,” TEC-DOC0IAEA, International Atomic Energy
Agency.
3. E. PINTON, B. DURET, and G. BERTHOUD, “Behavior
of a UF6 Container During a Fire—II: Modeling and Validation,” Nucl. Technol., 127, 352 ~1999!.
4. SUZUKI et al., “An Experimental Study on Heat Transfer
of a UF6 Filled Vessel,” presented at UF6 Safe Handling Processing and Transporting Mtg., Oak Ridge, Tennessee, May
24–26, 1988.
5. J. SAROUL et al., “TENERIFE Program Experimental Results,” presented at UF6 Conf.: Handling, Processing, and Transporting, Paducah, Kentucky, November 28–December 1, 1995.
6. R. DE WITT, “Uranium Hexafluoride: A Survey of the
Physical-Chemical Properties,” GAT-280, Goodyear Atomic
Corp. ~1960!.
7. J. C. ANDERSON, C. P. KERR, and W. R. WILLAMS,
“Correlation of the Thermophysical Properties of UF6 ,”
ORNL0ENG0TM-51, Oak Ridge National Laboratory ~Aug.
1994!.
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URANIUM HEXAFLUORIDE BEHAVIOR IN FIRE
Eric Pinton ~Eng, heat transfer and energy systems, Institut Universitaire
des Systèmes Thermiques Industriels, France, 1992; PhD, physics and energy,
Institut National Polytechnique de Grenoble, France, 1996!, has studied the behavior of a UF6 container during a fire at the Commissariat à l’Energie Atomique
~CEA! of Grenoble; analysis and phenomenological interpretation of experimental results, numerical modeling, and validation of the DIBONA code; DIBONA
code input parameter sensitivity; and improvement of the code user-friendliness
at the French Institute for Protection and Nuclear Safety. Recently, he has been a
design and project engineer at Robatel, where he works on design, certification,
and manufacture of packaging and transporting of radioactive material casks.
Bernard Duret ~Eng, thermal and chemical process, Institut de Génie
Chimique de Toulouse, France, 1976! works in the thermohydraulic service of
CEA Grenoble on topics bound to nuclear safety studies ~molten UO 2 propagation, corium ablation, radioactive containers during fire, etc.!.
Georges Berthoud ~PhD, nuclear engineering, Institut National des Sciences et Techniques Nucléaires, France, 1973! has worked since 1976 in the Heat
Transfer Laboratory at CEA Grenoble mainly in the area of multiphase multicomponent modeling. Presently he is the head of the Multiphase Multicomponent Laboratory in the Thermal Hydraulics Department.
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