JoumalofNuclearMaterials
North-Holland, Amsterdam
171
153 (1988) 171-177
SPECIFICATION AND CHARACTERIZATION
BREEDER REACTORS
OF DENSE FUELS FOR LIQUID METAL COOLED FAST
H. BLANK
Commission of the European Communities, Joint Research Centre, Karlsruhe Establishment, European Institutefor
Transuranium Elements, Postfach 2340, D-7500 Karlsruhe, Fed. Rep. Germany
The specifications of a given nuclear fuel depend on the general requirements of the fuel cycle and especially on the pin design
and the reactor operation conditions.
For the dense LMFBR fuels these requirements have changed several times in the course of the development of the LMFBRs
since 1950. By following the course of this development it is shown how one arrived at the fuel specifications and pin design which
are in agreement with the current fuel cycle requirements.
1. Introduction
When at the beginning of the fifties the concept of the
fast breeder reactor was conceived the natural choice for
the fuel of this new energy source appeared to be alloys of
uranium and plutonium. In fact, from the point of view
of breeding and neutron economy such a dense nuclear
fuel would have been optimum. However, from this starting point the history of the development of (dense)
LMFBR fuels has taken many unexpected and even dramatic changes in which, however, the density of any type
of fuel was always compared to the highest possible density of the uranium-plutonium
alloys. In recent years it
has become customary to call carbide, nitride and alloy
fuels for the LMFBR “dense” in contrast to the oxide fuel
with its lower density of heavy metals, see table 1.
To understand the present role and specifications of
these “dense” LMFBR fuels and their possible future role
it is necessary to review their history over the past 35 years.
This history should be seen in context with the current
oxide fuel technology for LWRs and LMFBRs which is
Table 1
Theoretical heavy metal densities, dM, in potential LMFBR fuels
(1960)
& Wcm3)
ff-U
u + 2O%Pu a)
(U,Pu)N
(U,Pu)C
MOz
19.07
5 18.6
13.53
12.95
9.75
2. The role of dense fuels 1950-1985
At the beginning of the nuclear aera, i.e. in the fifties
and sixties, it seemed to be quite clear that in the future
breeders and thermal reactors would be operated in a
complementary way and that good breeding would be
indispensable.
2.1. The metal area 1950-1960
When in the fifties neutron physicists and engineers
started to develop the reactors for obtaining access to the
‘) Cast alloy.
022-3 115/88/$03.50 0 Elsevier Science Publishers
(North-Holland
Physics Publishing Division)
the result of more than 40 years of research and development in four fields:
- development of economic fabrication methods
- requirements for safe and economic fuel performance
in the various reactor types,
- operation experience with fuels and reactors,
- compatibility of the fuel with an established reprocessing technique (i.e., the PUREX process in Western
Europe).
For the LMFBR reprocessing is indispensable, hence any
new optimized fast reactor fuel must be developed within
the boundary conditions fixed by all steps involved in the
closed fuel cycle, see fig. 1.
The development of the closed fuel cycle depends very
much on the energy scenario in a given economical environment, hence the conditions for its realisation are different in different parts of the globe. For obvious reasons
the situation in Western Europe will be mainly considered in this paper without, however, neglecting the important milestones achieved elsewhere.
B.V.
H. Blank/Speci’cation
172
and characterization of dense LMFBR fuels
gonne National Laboratory (USA) in conjunction with
the EBR II. It appeared attractive to leave the metallic
fission products in the melt as “fissium” alloying additions and improve thus the fuel properties. This process
has been used for uranium-fission alloys with EBR II since
about 1964 up to the present [ 11.
2.2. Dense ceramic fuels versus oxides 1960- I965
WRSTE CONOlTIONlNG
Fig.
I. The main steps in the LMFBR fuel cycle.
vast energy potential contained in the world resources of
natural uranium by the transformation of the non-fissile
238Uinto the fissile 239Pu a first obstacle in the use of the
U-Pu alloys was encountered. The strongly anisotropic
crystal structures and the many phase transformations
made the metallurgical processing difficult and the irradiation behaviour complicated. The natural solution of
the problems seemed to be to modify and improve the
metallurgical and irradiation properties of the required
U-Pu alloys by suitable alloying additions. In the past such
a procedure had been very successful in the metallurgy of
iron.
In the fifties large research programmes were funded
mainly in the USA, UK, France and USSR to explore systematically the binary and certain ternary alloy systems
of U and Pu. Apart from the scientific (and military) interest in the unique metallurgical properties of plutonium
and its alloys the properties of several liquid (! ) and solid
alloy systems for fast reactor applications
were
investigated.
The driver fuels of the first generation of fast test reactors (EBR I, EBR II and DFR and the first prototype
commercial Enrico Fermi Reactor) were all equipped with
metal fuel based on uranium alloy systems. The small fast
reactor BR 5 made an exception as it was equipped with
Pu02 fuel. In addition the pyrometallurgical reprocessing
of the uranium based fuel alloys was developed by Ar-
In spite of progress in the metallurgy of U- and U-Pualloys the problems of fission gas swelling could not be
solved satisfactorily at the beginning of the sixties. Even
stabilizing the cubic y-phase of uranium at lower temperatures in U-MO and U-Pu-MO alloys could not reduce
the strong fission gas swelling in these high density fuels
to tolerable values. In addition it was feared, that the relatively low melting temperatures of these alloys might
limit the thermal development potential of the fast power
reactor and that the risk of eutectic formation between
fuel and clad during a temperature transient might be
rather high with the maximum coolant (and clad) outlet
temperatures of 650°C envisaged.
Hence among the many alternative fuels proposed and
discussed at the time (see e.g. the papers and discussions
on LMFBR fuels in ref. [ 21) the refractory dense U-Pu
carbide and nitride appeared to be a most promising
choice, see table 1. However, in contrast to the metallurgy
of U and Pu the techniques for producing these ceramics
into dense bodies was not available and nothing was
known about their irradiation performance. Contrary to
this situation, the fabrication of U02 fuel, its properties
and irradiation behaviour were already rather well established [ 31.
In a discussion of three possible candidates for the driver
fuel of the RAPSODIE reactor in 1960, the ternary alloy
U-Pu-MO, the mixed carbide (U,Pu)C and the mixed
oxide ( U,Pu)02 were compared [ 41. The subsequent decision was that the oxide fuel became the immediate
choice and the carbide was further investigated as the attractive LMFBR fuel for the future.
In the period from 1960 to 1965 the interest in metal
fuel decreased and that in ceramic fuels increased, as documented by the corresponding papers presented at the
Plutonium Conferences 1960 in Grenoble [ 21 and 1965
in London [ 51. All countries with FBR projects started
research and development programmes on carbide fuel in
view of its attractive nuclear properties, its compatibility
with the coolant and its refractory properties. There was
less interest for the nitride because of difficulties in fabrication and a slightly lower breeding gain because of the
in-pile reaction 14N(n,p) 14C.The better known oxide was
used by all FBR projects as the unproblematic first fuel
H. Blank/Specificationand characterizationof dense LMFBR fuels
for direct use in the current development of the LMFBR
in spite of its lower breeding potential and its chemical
interaction with the coolant since it required less research
and development work as compared with the carbide.
2.3. Why was the progress in carbide fuel development so
slow between 1960 and 19 77?
The carbide development
was started with two
premises:
(a) In the sixties the estimated increase in world power
demand and the known uranium resources seemed to
indicate a likely shortage of fissile material before the
end of the century. Thus a short compound doubling
time ( < 15 yr) in the FBR fuel cycle, i.e. a high
breeding gain appeared highly desirable. With this
boundary condition the breeder required a dense fuel
which should be operated at relatively high linear rating ( N 100 kW/m) to moderate bum-ups ( I 100
GWd/ton( U)).
(b) Unlike to oxide dense ceramic fuels cover a wide range
of possible chemical compositions within the pseudoquaternary system M-C-N-O,
(M = 8O%U + 20%
Pu). Carbide and nitride form a continuous range of
solid solutions, the carbide may dissolve a considerable amount of oxygen and, depending on the fabrication procedure, it will either contain as little oxygen
as I500 ppm and considerable amounts of higher
carbides or, vice versa, it may be nearly single phase
but hold w 3000 ppm of oxygen and contain MO1 as
a second phase.
A priori it was not known what specifications are required for a high density carbide fuel with regard to
chemical composition, structure, pin design and in-pile
operation in order to show tolerable swelling up to the
desired target bum-up. A large variety of fabrication procedures with resulting different fuel compositions and
structures and different pin design was tested. The basic
in-pile mechanisms which determine the fuel performance of the dense MX-type fuels (X = C, N, 0) were little
understood and pin failures occurred relatively often during this period. As regards the pin concepts, He-bonding
with various initial gap sizes, Na-bonding, Na-bonding
with shroud, and vented pins were tried. The fuel was used
in the form of solid pellets containing different porosities,
annular pellets and vibrated particles.
For example in 1975 the three European FBR projects
had each a different reference fuel and pin concept, Nabonded MC of high density, He-bonded MC of low density and vibro-compacted particles of M(C,O) respectively. The general irradiation experience obtained with
173
carbide, carbonitrides and nitride in 1977 [B] could be
summarized as follows.
As compared with the oxide irradiation performance
the performance of the dense fuels is more sensitive to
- fuel specifications (composition, structure, density),
- pin design parameters (bonding, fuel diameter, smear
density, properties of clad material),
- reactor operation (fuel temperatures, linear rating, bumup).
Hence the solution of the problem requires to define
technologically feasible sets of parameters under the
boundary conditions of a given fuel cycle scenario [ 71.
This was only achieved during and at the end of the period between about 1977 and 1983. The progress was
mainly due to three factors:
(a) The original request, that a “dense” fuel should be
operated in pins with high smear density and high
linear rating was relaxed at the end of the seventies
on the basis of accumulating irradiation experience
and because a high breeding gain was no more the
primary aim of the LMFBR.
(b) Systematic property studies of the dense ceramic fuels
carbide, carbonitride and nitride and systematic postirradiation analyses had led to a better understanding
of the relevant in-pile mechanisms which determine
the performance of these fuels under irradiation [ 81.
(c) The result of a large technological carbide irradiation
programme became known mainly between 1977 and
1983 [9-l 11. In this programme one type of carbide
fuel was irradiated under the conditions of a large
range of pin design parameters.
3. The development of dense fuels since 1983
At the end of the seventies, beginning of the eighties the
general energy scenario in the developed countries and
the conditions for the introduction
of commercial
LMFBR’s had changed profoundly with respect to the
sixties.
- The increase in energy demand was considerably less
than previously predicted.
- There will be no shortage of Iissile material until after
the turn of the century and high breeding gain is no more
the objective of the first generation of commercial fast
reactors.
- Instead, every effort has to concentrate on developing
the fast reactor fuel cycle for better economy and competitiveness with the LWR fuel cycle.
Consequently the requirements for the properties and
performance of an FBR fuel are now quite different from
what was required in the sixties (see beginning of subsec-
tion 2.1). The fuel cycle economy calls for cheap fuel element fabrication, and a bum-up as high as possible ( > 150
twit)
at moderate rating ( I70 kW/m ). During the
Long in-pile time of the fuel the Lossof reactivity should
be as low as possible in order to avoid the necessity of
large reactivity corrections by the control rods. This requires high core breeding and hence a high density of the
heavy metals in the fuel.
Thus after more than 25 years the question of a dense
LMFBR fuel and its competition with the oxide fuel is
posed again. Yet meanwhile considerable progress has
been made in the development and operation experience
of the oxide and in the development of the metallic and
ceramic dense fuels. In order to understand the resulting
fuel spe~i~~atio~s and pin concept it is necessary to show
how this progress has been achieved, see table 2.
Xn the second column of table 2 the theoretical densities, dM, of the heavy metals U and Pu are shown for the
four, presently most important FBR fuels and for a-U.
Thirty years ago these heavy metal densities were regarded as “ideal” values for LMFBR fuels. In the third
column of this table the corresponding smear densities of
the heavy metals are shown as they are used or recommended today in optimized pin designs for each fuel type.
For alloy fuels these are Na-bonded pins with 55 to 7.4
mm outer diameter and for carbide and nitride He-bonded
pins with 8.7 mm outer diameter or more. The striking
rest.& is, that today both alloys and dense ceramic fuels
are operated with practically the same heavy metal. smear
density in spite of the different pin designs and the widely
differing melting temperatures.
In the following discussion fuel swelling and the range
of operating temperatures of the three fuels alloys, carbide and nitride will be compared on a common basis.
This is conveniently done by dividing for each fuel j the
in-pile operating temperatures T by its melting temperature T,,,, and plotting thus all operating temperatures Ton
the reduced temperature scale kp= TIT,, see fig. 2.
From metallurgy the rule of thumb is known that in solids with similar and not too complicated crystal structures the thermal activated sel~di~usion based on the
vacancy mechanism becomes effective at reduced temperatures 8r 0.5. The thermal activated migration of existing vacancies prevails at @< 0.5 but essentially stops at
8 c 0.35. h closer study of the fission gas swelling in nuclear fuels shows that the @-values of 0.5 and 0.35 are
also relevant for the fission gas diffusion in dense fuels. A
limited bum-up dependence, especially of the upper value
is possible, but does not affect the present discussion.
Therefore the two values have beer marked on the G-scale
in fig. 2.
In the dense (i.e. metallic conducting) fuels the mechanism of fission gas swelling consists essentially of three
parts: (a) single gas atom diffusion through the fuel matrix by a vacancy mechanism; (b) precipitation of part of
the gas atoms into intra- and intergranular gas bubbles
leading to bubble growth and a certain rate of swelling;
(c) precipitation of part of the gas atoms into the open
porosity of the fuel structure, leading to gas release and
reducing the rate of swelling. If the fission gas diffusion is
fast and the open porosity sufficiently large, the rate of
fission gas swelling may become effectively zero. From
this picture two extreme cases for the fission gas swelling
can be identified at high and low fuel temperatures from
three temperature ranges shown in tig. 2.
(a) Fuel temperatures i9> 0.5 result in fast fission gas
diffusion. This leads to a high rate of swelling if the
fuel structure contains little or no open porosity.
However, for 2 20% open porosity the rate of fission
gas release may surpass the rate of gas production and
swelling stops.
(b) At fuel temperatures 0.35 < 8 < 0.5 fission gas diffusion is slow as well as the growth of gas bubbles and
has a weaker tem~rature dependence than at e33 0.5.
Thus the rate of swelling will be low, and also the rate
Table 2
Theoretical and smear densities of heavy metals in LMFBR fuels (1985)
U+lSPuflQZr*’
(U,fQJ)C
(U,Pu)N
(U,Pu)Oz-<
ru”U
a) Weight %.
‘) Solidus temperatures.
dM
Smear density
@m3f
G&m’)
14.13
12.95
13.53
9.75
19.07
10.6
10.4
LO.8
8.0
Melt. temp. ‘)
(K)
Smear density
(%I
- 1300
2700
3050
2950
1405
15
80
80
82
H. Blank/Specificationand characterizationof dense LMFBR fuels
06
175
1 to 3% burn-up depending on the linear rating).
In summary, the swelling problems of the dense fuels
(alloys, carbide and nitride) which prevented their application as LMFBR fuels in the early sixties have in principle been solved. The solution consists of two parts. The
first part is common to all three fuels and consists of a
considerable reduction of the smear densities; see table 2.
The second part has to take account of the particular
properties of each fuel in conjunction with the pin concept, the reactor operation conditions and the fuel specifications. In each case the corresponding parameters have
to be optimized for use in the respective fuel cycle.
4. Specification and characterization of MC and MN
Q301421.5
1780
-
1915
1
0.3
Fig. 2. Typical operating temperatures of the dense LMFBR fuels
a-U+Fs, U+Pu+Zr, MC and MN shown on a common temperature scale 8 = TIT,. The symbols L, A, and C are explained
in the text.
of gas release, even if a large amount of open porosity
exists.
(c) At fuel temperatures 8~0.35 a thermal activated
migration of metal lattice vacancies is practically nonexistent. Atoms are stochastically moved by high energy atomic collisions with a fission rate induced diffusion coefficient D* - 1O-‘8 cm’s_‘. A biased point
defect migration does not exist. However, at high
burn-up when a very strong supersaturation of nonsoluble fission product atoms exists in the lattice small
fission product clusters are expected to develop.
The two cases (a) and (b) + (c) illustrate the concepts
of the “hot fuel” and the “cool fuel”.
For a first qualitative understanding of the fission gas
swelling in a given nuclear fuel it is sufficient to compare
the mechanisms of fission gas swelling and gas release with
the location of its working temperatures on the reduced
temperature scale.
One finds then from fig. 2 that the a-U-based alloy fuels
are clearly working under the “hot fuel” concept in spite
of the Na-bonding and the low absolute operating temperatures. The Na-bonded ceramic fuels MC and MN on
the contrary operate under the concept of the “cool fuel”
[ 12,13 1, temperature ranges marked L in fig. 2. The Hebonded ceramic fuels finally operate in a mixed hot- cool
fashion for which the relevant in-pile mechanism have
been recently analysed [ 141. In fig. 2 the temperature
ranges marked A for MC and MN pertain to the beginning of irradiation in the He-bonded pins and those
marked C to the rest of the irradiation history when the
initial gap between fuel and clad is closed (roughly after
4.1. The present state of the LMFBR development
The general enthusiasm for a rapid introduction of the
breeder as an important energy source which prevailed in
the sixties and early seventies is now replaced by cautious
reflections to find the most economic way in developing
a competitive commercial LMFBR fuel cycle which should
be available at the beginning of the next century.
The European LMFBR technology is presently well advanced (operation of PX, PFR and SPX 1, construction
of SNR 300 finished, a developed reprocessing technology for oxide fuel with the PUREX process available).
There is sufficient know-how about carbide and nitride
fuel available to make use of these fuels in relatively short
time.
In the USA two fast test reactors EBR II and FFTF are
in operation. Long standing experience exists with the
closed fuel cycle of EBR II with U-Fs metal fuel. There is
good fabrication and irradiation experience with oxide
and carbide but except for basic know-how no commercially developed reprocessing technology for ceramic fuels,
In the USSR a steady development is taking place with
oxide fueled LMFBRs (operation of BOR 60, BN 350 and
BN 600 and beginning of the construction of BN 800).
The reprocessing technique is under development and
carbide and nitride fuel has been and is being tested,
respectively.
Japan and India have started the development of
LMFBRs more recently with the test reactors already in
operation.
4.2. Oxide and/or dense ceramic fuels in Europe
Since about 1984 in Europe and somewhat earlier in
the USA the situation of the LMFBR has been reconsidered in order to adapt its further development to the
176
H. Blank/Specijication and characterization of dense
changed political, economic and energy situation on the
basis of the accumulated operation experience. In principle three routes appear possible to provide commercial
fast reactors as an abundant and safe non-fossile energy
source to satisfy the growing energy demand during the
next century and replace fossile energy production.
(A) Further improvements in reactor design, fuel fabrication and reprocessing within the frame of the existing oxide fuel cycle.
(B) Replacement of the oxide by a dense ceramic fuel under the condition that the new fuel is to a large extent
compatible with the established technology of the
oxide fuel cycle.
(C) Installation of a new fuel cycle, preferably with a
dense fuel and a new reactor design.
Because of the different situations in Europe and in the
USA it was attractive for the latter to decide for route (C)
whereas Europe will choose rather between (A) and (B).
However, to put this choice on a sound basis the irradiation performance of carbide and nitride has to be tested
in a pin design optimized for the future boundary conditions of the closed fuel cycle. This led to the start of a
limited but well defined fabrication and irradiation programme of these two fuels.
In principle both fuels could be used in the operating
European fast reactors with little or no change in the core
design. Against the oxide carbide and nitride show three
advantages:
- higher heavy metal density,
- metallic thermal conductivity,
- compatibility between fuel and coolant.
If full use of these advantages is made they can be
transformed into a series of important improvements in
the fuel cycle, i.e. cheaper fuel, simplified core design and
easier reactor operation.
Eventually, there remains the decision between carbide
and nitride. Provided the general irradiation performance of both fuels turns out to be equivalent in the adopted
pin design the carbide would have a slight advantage inpile because of somewhat lower breeding in the nitride
caused by the neutron capture in the “N(n,p)‘4C reaction. However, when judging the complete fuel cycle this
is amply compensated (a) by a better chemical stability
of the nitride against oxygen and moisture during fabrication and out-pile handling and (b) by easier dissolution in nitric acid in the head end of the PUREX process.
The “‘C-produced during irradiation is not regarded as a
serious problem for reprocessing since it should be liberated as CO? in the cover gas and can thus be transformed
into a solid waste form.
LMFBRfuels
Table 3
Pin design and operation conditions for optimized dense ceramic fuels
Bonding
Clad
Pin outer diameter (mm)
Clad thickness (mm)
Smear density (o/o)
Max. lin. rating (kW/m)
Max. burn-up (at%)
He
Low swelling, carburisation
resistant compatible with
reprocessing technique
8.5-9.5
-0.5
180
175
>I2
4.3. Specification for MN and MC
The specifications for an optimized pin design are
practically the same for carbide and nitride fuel. They are
collected in table 3. In order to lower the cost of fabrication fat pins with He-bonding are employed. The cladding material must be compatible with the head end of
the PUREX process, i.e. it must not dissolve together with
the fuel. The smear density is sufficiently low to provide
space for accommodation of swelling and will be 80% or
slightly below. The maximum linear rating is moderate
but the bum-up should be high, e.g. 15% of the heavy atoms or even more.
The specification of the fuel properties are shown in
table 4. The carbide should contain as little oxygen impurity as possible, I500 ppm, in order to provide good
thermal and chemical in-pile stability of the as-fabricated
fuel structure [ 141. In order to achieve this 5 to 10 ~01%
of sesquicarbide can easily be tolerated.
Previous irradiation experience with nitride fuel shows
that high oxygen contamination increases the swelling
considerably above about 1000°C. Nevertheless the limits of tolerable oxygen contamination may be somewhat
higher than in the case of the carbide. On the contrary,
the carbon contamination left from the carboreduction
should be as low as possible in the nitride since large
amounts of carbon would complicate the dissolution process in the head end of the aqueous reprocessing. FurtherTable 4
Composition of optimized dense fuels
MC
Pu/(U+Pu)
Oxygen (wtl)
Carbon (wt%)
Porosity (%)
MN
-0.2
10.01
5.08-5.65 ”
<O.Ol
<O.Ol
15*2
‘) Corresponding to 5 to 10 ~01%M2C3.
H. BlankDpeciJication and characterization of dense LMFBR
more, the carbon contamination increases in the nitride
during irradiation by the formation of 14C unless the nitride is produced with the nitrogen isotope “N [ 151.
The fuel porosity will be 15% or slightly higher. The
fabrication of the fuel should be such that this porosity
possesses sufficient thermal stability so that no densification occurs during the first short period of irradiation,
when the fuel/clad gap is still open and the fuel temperatures are high in the He-bonded pins.
5. Conclusions
The dense ceramic LMFBR fuel, carbide and nitride,
could not be used at the beginning of the development of
the LMFBR’s 30 years ago because their fabrication technology did hardly exist and their irradiation performance
was insufficient. Therefore up to the present the development of the LMFBR was carried out with the easier to
handle oxide in spite of several drawbacks of this fuel.
In the meantime the dense fuels have been developed
to such a degree that it appears possible to substitute the
oxide in a future FBR fuel cycle which has been optimized for economy and is competitive with the LWR fuel
cycle. However, dense ceramic fuels with the corresponding specifications and pin design have not been irradiated
in a prototype fast reactor up to now, hence their irradiation performance has to be demonstrated. A suitable fabrication and irradiation programme has been started in
Europe recently [ 16- 181.
References
[ 1] L.C. Walters, B.R. Seidel and F.H. Kittel, Nucl. Technol. 65
(1984) 179.
[2] E. C&on, W.B.H. Lord, R.D. Fowler, Editors, Plutonium
1960 (Cleaver-Hume Press, Ltd., London, 196 1).
[ 31 J. Belle, Editor, Uranium Dioxide: Properties and Nuclear
Applications (USAEC, Washington, 196 1).
[4] P. Bussy and C.P. Zaleski, in: Plutonium
1960
( Cleaver-Hume Press, Ltd., London, 1961) p. 589.
[5] A.E. Kay and M.B. Waldron, Editors, Plutonium 1965
(Chapman and Hall, London, 1967).
[6] J. Leary and H. Kittel, in: Proc. Topical Meeting on Advanced LMFBR Fuels, Tucson, Arizona, October 10-13,
1977 (ANS, 1977).
[ 71 H. Blank, ibid. ref. [6], p. 482.
[ 81 Hj. Matzke and C. Ronchi, ibid. ref. [ 61, p. 2 18.
fuels
111
[9] J.O. Simmons, J.A. Leary, J.H. Kittel and C.M. Cox, ibid.
ref. [ 61, p. 2.
[lo] R.B. Matthews and R.J. Herbst, Nucl. Technol. 63 (1983)
9.
[ 111 P.J. Levine, N.P. Nayak and A. Boltax, Trans. 7th Int. Conf.
on SMiRT, Chicago, 1983, Vol. C5/2, p. 161.
[ 121 H. Blank, J. Less-Corn. Met. 121 (1986) 583.
[ 131 M. Colin et al., Nucl. Technol. 63 (1983) 442.
[ 141 H. Blank et al., in: Proc. Int. Conf. on Reliable Fuels for
Liquid Metal Reactors, Tucson, Arizona, 7-l 1 Sept., 1986
(ANS, 1986) p. 7-15.
[ 151 H. Blank and H. Bokelund, in: Advanced fuel technology
and performance, IAEA-TECDOC-352 (1985) p. 189.
[ 161 H. Bailly, in: Advanced fuel technology and performance,
IAEA-TECDOC-352, Vienna, 1985, p. 95.
[ 171 K. Richter and H. Blank, in: Nuclear Europe VII,
March/April 1987, p. 19.
[ 181 Y. Guerin and J. Ronault, in: Proc. Int. Conf. on Reliable
Fuels for Liquid Metal Reactors, Tucson, Arizona 7-l 1 Sept.
1986 (ANS, 1986) p. 7-28.
Discussion
K. Kummerer (KfK Karlsruhe, Fed. Rep. Germany): It might be
justified to assume that the operating range of oxide fuel in the
LMFBR lies well above TITr=0.5 in contrast to carbide and
nitride?
H. Blank (TUI Karlsruhe, Fed. Rep. Germany): On account of
the much lower thermal conductivities of the oxides the range of
their working temperatures is much larger than that of metal, carbide and nitride while the pin power is much lower. The surface
temperatures of the oxide pellets in the LMFBR occur around
0.3 1 to 0.368 and the central temperatures around 0.80 to 0.908
with 8= T/T, which means that roughly one third of the fuel
volume is operated at 8 < 0.5 and roughly two thirds are operated at 8> 0.5. However, it should be considered in addition that
the dynamic effects of the fission events (in-pile creep, dissolution of fission gas bubbles, etc.) in the oxide are stronger by approximately one order of magnitude than in the dense metal,
carbide and nitride nuclear fuels. This must be taken into account when a comparison is made of operating temperatures and
irradiation behaviour of oxide and dense nuclear fuels on the reduced temperature scale.
J. Rousseau (CEA, Cadarache, France): Not a question, but only
a remark:
1. I fully agree with the analysis of Dr. Blank.
2. The French program on advanced LMFBR fuel is now entirely
concentrating on (U,Pu)N.