Process for the production of spherical-shaped fuel elements for high temperature reactors

Nearly isotropic spherical fuel elements of high strength and high heavy metal content for gas cooled high temperature reactors consisting of a fissile and fertile fuel material containing nucleus and a fuel free shell are prepared by preliminarily compressing a molding powder consisting of a mixture of natural graphite and binder resin, synthetic graphite and binder resin or a mixture of both types of graphite powder and binder resin together with coated fissile and/or fertile fuel material particles with addition of a lubricant in a rubber mold with an ellipsoidally shaped cavity at low molding pressure three dimensionally to preform the nucleus of the fuel element, shaping the preformed nucleus into the required molding powder for the shell in a second rubber mold with ellipsoidally shaped cavity, preliminarily compressing this fuel element compact and subsequently final pressing and heat treating at up to about 2000.degree. C., the shaped fuel element in the rubber mold at a pressure of less than 200 kg/cm.sup.2 being precompressed three dimensionally to such an extent that the green graphite matrix of the so formed ellipsoidal compact has a density value of more than 65% of the theoretical green density and subsequently final pressing in a single dimension the preliminarily compressed fuel element body in a steel die between two cup shaped metal dies in the plastic temperature range of the binder resin at a pressure of less than 200 kp/cm.sup.2 to densities of more than 95% of the theoretical density of the green graphite matrix.

BACKGROUND OF THE INVENTION 
The invention is directed to a process for the production of nearly 
isotropic spherically shaped fuel elements having increased requirements, 
particularly high heavy metal content for improved gas cooled high 
temperature reactors by pressing a molding powder consisting of a mixture 
of natural graphite and binder resin, synthetic graphite and binder resin 
or a mixture of both types of graphite powder and binder resin together 
with coated fissile and fertile fuel material particles. 
Spherical fuel elements customarily consist of a fissile and fertile fuel 
material containing nucleus which is surrounded by a fuel free shell and 
is joined to it without transition (Hrovat, German OS No. 1,646,783). 
The graphite matrix, i.e., the graphite material, is identical in the 
nucleus of the sphere and in the fuel free shell. The fuel element 
diameter in general is 40 to 80 mm, by preference about 60 mm and the 
thickness of the shell is 2 to 20 mm, by preference about 5 mm. 
In the known spherical fuel elements the nucleus contains in homogeneous 
distribution the fissile and fertile fuel materials in the form of 
spherical heavy metal particles. To retain fission products the particles 
are provided with multiple coatings of pyrolytic carbon, in a given case 
with an intermediate layer of silicon carbide. 
As fissile fuel material there is normally employed uranium 235 and as 
fertile material thorium 232, the fissile and fertile fuel material being 
employed as the carbide or oxide. While the fissile and fertile fuel 
materials in the so-called THTR element, the standard spherical element of 
the thorium high temperature reactor, are jointly present in the same 
particles, they are provided for in the so-called Feed-Breed-Element 
separated in discrete particles mixed with each other. 
A series of requirements is placed on the spherical fuel elements: 
They must have high strength properties with the least possible modulus of 
elasticity and small thermal coefficients of expansion. During the reactor 
operation, particularly at start up and shut down of the reactor proceed 
as a result of temperature gradients thermal stresses which can only be 
partially relaxed by creep processes and therefore produce heavy 
mechanical stresses in the fuel element spheres. Since in the charging of 
the reactor core and circulation of the sphere heap the fuel elements drop 
from several meters high to the sphere heap surface, there are high 
additional mechanical stresses. Additionally in the disconnecting of the 
reactor operation the absorber rods get into the sphere heap directly 
which leads to a further considerable load on the individual fuel 
elements. In order to guarantee a sufficiently high service life of the 
fuel elements there are required high values for compressive, bending and 
tensile strength of the fuel element matrix. For the previously mentioned 
reasons there are added the requirements of a good drop and abrasion 
resistance and particularly of a high crushing load of the spheres. 
Besides they must have a high heat conductivity in order to hold the 
temperature gradients inside the sphere as small as possible. 
Furthermore, a good corrosion resistance against trace impurities is 
necessary, as for example against water vapor, CO, CO.sub.2 and H.sub.2 
which are contained in the helium cooling gas. 
Besides there is an increased heavy metal content of the spherical fuel 
elements. In the so-called THTR-Standard-Fuel-Element the heavy metal 
content is 11 grams per sphere. To raise the conversion rate (formation of 
uranium 233 from thorium 232) a substantially higher heavy metal content 
of the fuel elements for advanced high temperature reactors is required. 
Thereby in spite of the increased heavy metal content in the production 
the requirements of extremely low fractions of defective coated particles 
in the molded spherical fuel element are intensified. 
Besides a good irradiation behavior is necessary up to temperatures of 
about 1400.degree. C. and up to an exposure to fast neutrons (E&gt;0.1 MeV) 
of about 9.times.10.sup.21 neutrons/cm.sup.-2. This requirement assumes an 
as much as possible high crystallinity of the isotropic graphite matrix. 
For the production of spherical fuel element previously, processes have 
been proposed in which first the lower half of the fuel free shell is 
formed in a metallic pressing die, then the fuel containing spherical 
nucleus inserted and subsequently the upper shell half pressed on (German 
Patent No. 1,096,513). Since the bulk density of the molding powder 
mixture is relatively small (about 0.5 g/m.sup.3) and merely is densified 
in the axial direction about four times the volume, in the pressing there 
cannot be avoided a preferred orientation of the customarily anisotropic 
constructed graphite starting particles. This has as a result an 
inadmissible anisotropy of the matrix of the sphere. In such a sphere 
there occur in the irradiation with fast neutrons high irradiation induced 
stresses which can lead to the formation of cracks and therewith endanger 
the mechanical integrity of the fuel element. 
This disadvantage is avoided if in place of the die molding process with a 
steel tool there is used the semi-isostatic pressing in rubber molds of 
silicone rubber (Hrovat, German OS No. 1,646,783). The silicone rubber 
behaves in the pressing under pressure similarly to a liquid. Thereby 
there is attained an isotropic three-dimensional compressing of the 
molding powder. To take up the molding powder the rubber mold formed of 
two halves has a central, elliptic shaped cavity which is so proportioned 
that in the pressing there is formed a sphere having a diameter for 
example of about 60 mm. The prepared filled rubber mold is introduced into 
a steel die of the press and pressed together with the upper and lower 
punches. Because of the elastic behavior of the rubber there is used 
molding at room temperature and consequently a very high molding pressure 
is required. The fuel element spheres having a diameter of 60 mm are 
customarily compressed with a molding pressure of 3 metric tons/cm.sup.2 
which at the required rubber mold size corresponds to a very high pressing 
force of 400 tons (i.e., 400 metric tons). Therewith so that at this high 
molding pressure no particles bordering each other are mutually damaged 
the particles are encased in molding powder. In order that the spheres 
produced from the encased particles maintain a sufficient strength 
according to Hrovat German Patent No. 1,909,871 only a part of the molding 
powder needed for the nucleus is used to encase the particles, the 
remaining part mixed with the encased particles and the mixture pressed to 
the nucleus. In this way there are produced fuel element spheres with 
isotropic properties with a limit of up to about 11 grams heavy metal 
content. At higher heavy metal contents of for example 20 to 30 grams per 
sphere, however, there cannot be avoided the destruction of a part of the 
coated particles in the pressing. 
In German OS No. 2,246,163 (and related Rachor U.S. Pat. No. 3,912,798) to 
improve the course of the process there is proposed that the second 
pressing step in which the spherical nucleus embedded in a coating of 
graphite molding powder is pressed in a rubber mold is divided into two 
pressing stages wherein first there is carried out a preliminary pressing 
in a rubber mold at low pressure and then this preformed object is final 
pressed at high pressure. Here also at high metal content there cannot be 
avoided particle damage due to the high molding pressure. 
Furthermore, there has been proposed a process according to which there is 
first produced from the binder resin containing graphite molding powder 
mixture a granulate having isometrically constructed particles of high 
bulk density and then hot pressing this granulate together with the coated 
fuel particles in the plastic range of the binder resin at the relatively 
very low pressure of 100-200 kp/cm.sup.2 to molded articles (German Patent 
No. 2,104,431 and related Hrovat U.S. Pat. No. 4,017,567). Indeed with 
this process there can be prepared prismatic molded articles with an 
extensive isotropic structure and high heavy metal content on which there 
is placed no requirements as to the drop strength and crushing load but no 
spherical fuel elements can be considered for the above mentioned 
requirements. The decisive reason for this is a relatively poor bond of 
the smooth surfaces of the individual granulate particles which are 
already precompressed. Therefore, this process is unsuited for the 
production of fuel element spheres with the required drop strength and 
crushing load. 
The entire disclosures of the aforementioned German OS No. 1,646,783, 
German Patent No. 1,096,513, German Patent No. 1,909,871, U.S. Pat. No. 
3,912,798 and U.S. Pat. No. 4,017,567 are hereby incorporated by reference 
and relied upon. 
SUMMARY OF THE INVENTION 
The present invention therefore is based on the problem of working up a new 
process which avoids the above-mentioned disadvantages and permits the 
production of fuel element spheres having high heavy metal contents, e.g., 
20-40 grams per sphere which are isotropic and have good mechanical 
strength properties, particularly high crushing loads and high drop 
strength and consist of a fuel and fertile material containing nucleus and 
a fuel free shell. Fuel element spheres with heavy metal contents in the 
usual range of about 5 to about 15 grams heavy metal per sphere can be 
produced too. 
In the invention as fissile material there can be used, for example, oxides 
or carbides of U 235, U 233 and fissionable plutonium isotopes. As fertile 
fuel materials there can be used, for example, oxides and carbides of U 
238 or Th 232. As binder resins there can be used, for example, 
phenol-formaldehyde resin, xylenol-formaldehyde resin, cresol-formaldehyde 
resin or furfuryl alcohol resin. 
The problem was solved by preliminarily compressing a molding powder 
consisting of a mixture of natural graphite and/or synthetic graphite with 
a binder resin, together with coated fissile and/or fertile fuel material 
particles with addition of a lubricant in a rubber mold having an 
ellipsoidal cavity at low molding pressure three-dimensionally to preform 
the nucleus of the fuel element, shaping the nucleus into the required 
molding powder for the shell in a second rubber mold with ellipsoidal 
cavity, preliminarily compressing this fuel element and subsequently final 
pressing and heat treating at up to about 2000.degree. C., the improvement 
of the invention including precompressing the shaped fuel element in the 
rubber mold with ellipsoidal cavity at a pressure of less than 200 
kp/cm.sup.2 three dimensionally to such an extent that the green graphite 
matrix of the so formed ellipsoidal compact has a value of more than 65% 
of the theoretical green density and subsequently final pressing the 
preliminarily compressed fuel element in a steel die between two cup 
shaped metal punches in the plastic temperature range of the binder resin 
at a forming pressure of less than 200 kp/cm.sup.2 to densities of more 
than 95% of the theoretical density of the green graphite matrix. 
The most important advantages of the pressing technique of the invention 
are a lower forming pressure which permits a compacting of embedded coated 
fuel particles without particle breakage even at high heavy metal content 
in the fuel element, a good joining of the individual graphite starting 
particles to form the fuel element matrix with a high breaking load and 
good drop strength and an isotropic compressing. This is produced because 
the substantial reduction in volume of the molding mixture already takes 
place three dimensionally in the rubber molds so that in the subsequent 
one dimensional final pressing in the steel die there is avoided an 
inadmissible preferential orientation of the primary graphite particles. 
There have proven suitable as pressure ranges for the preliminary 
compressing of the fuel element values of 50-190 kp/cm.sup.2 through which 
there are produced densities of 65-85% of the theoretical density and for 
the final molding pressures of 100-190 kp/cm.sup.2 through which densities 
from 95% up to nearly 100% are attained. In the preliminary compressing 
there are advantageously used temperature of 20.degree. to 85.degree. C., 
in the final molding temperatures of 100.degree.-200.degree. C. 
To reduce the internal friction in the final molding and to cause an 
intensive lubrication of the matrices there is advantageously added a 
lubricant to the molding composition. It is known in the production of 
fuel elements to use, for example, stearic acid with a melting point of 
69.2.degree. C. as the lubricant. However, there can also be used other 
lubricants, the particular lubricant used not being critical. 
Furthermore, pressing experiments show that rubber molds produced from 
silicon rubber form absolutely elastically up to about 80.degree. C. and 
endure several thousand molding operations without wear. Therefore, it is 
particularly advantageous according to the invention to carry out the 
semi-isostatic pressing not at room temperature but at slightly elevated 
temperature at which the low melting lubricant begins to become fluid. 
This procedure has the advantage that already in the three dimensional 
presses in rubber molds there is produced a still stronger compression of 
the graphite matrix. The density values reached thereby correspond to up 
to 85% of the theoretical density. 
The process can comprise, consist essentially of or consist of the steps 
set forth and the compositions can comprise, consist essentially of or 
consist of the materials set forth. 
Unless otherwise indicated all parts and percentages are by weight and all 
measures are metric tons.

The process of the invention will be further explained in connection with 
the following examples. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
Production of a Spherical Fuel Element Having 30 Grams of Heavy Metal 
As fissile fuel particles there served spherical kernels of UC.sub.2 having 
a diameter of 200 .mu.m. These particles were provided with a triple 
coating, namely, with pyrolytically deposited carbon layers and an 
intermediate layer of silicon carbide. The total thickness of the coating 
was 205 .mu.m. The coated particles having an average diameter of 610 
.mu.m and a density of 2.17 g/cm.sup.3 contained 17 weight % uranium. 
The fertile material particles (ThO).sub.2) having a kernel diameter of 500 
.mu.m were twice provided with pyrolytically deposited carbon layers 
having a total thickness of 170 .mu.m. The coated particles with an 
average diameter of 840 .mu.m and a density of 3.39 g/cm.sup.3 contained 
54.3 weight % of thorium. 
As the graphite molding powder there was used a mixture consisting of 63.2 
weight % natural graphite, 15.9 weight % graphitized petroleum coke, 19.9% 
phenolformaldehyde resin as a binder and 1% stearic acid as a lubricant. 
If there is inserted for the graphite powder components the density of the 
graphite single crystal of 2.26 g/cm.sup.2, for the phenol-formaldehyde 
the ascertained density of 1.31 g/cm.sup.3 and for the stearic acid a 
density of 0.839 g/cm.sup.3 the theoretical density of the green graphite 
matrix is 1.93 g/cm.sup.3. 
Next, the fissile and fertile fuel material particles were encased with the 
graphite molding powder in separate processes in a revolving drum with 
addition of methanol. The amount of powder was uniformly so selected that 
there resulted an average over coating layer thickness of 130 .mu.m. 
For a fuel element nucleus the pressing charge is composed of 96 grams of 
encased coated fertile material particles, 13 grams of encased coated 
fissile fuel particles and 37 grams of graphite molding powder. 
The rubber mold used to preform the fuel element nucleus has an ellipsoidal 
cavity of 130 cm.sup.3 volume with an ellipsoid diameter of 57 mm and 
height of 79 mm. The rubber mold was filled with the homogeneous mixture 
of these components and was introduced into the steel die of the mold and 
pressed together with the upper and lower punches at room temperature 
under a pressure of 70 kp/cm.sup.2. Thereby the graphite molding powder 
was compressed three dimensionally isotropically from 0.5 g/cm.sup.3 to a 
matrix density of 1.3 g/cm.sup.3. The ellipsoidal body had a volume of 
about 75 cm.sup.3 with about 48 mm diameter and 62,5 mm height, axis ratio 
1.30. 
In a further operation the pre-pressed spherical nuclei were arranged in 
the center of a second rubber mold with the help of three spacers and the 
residual volume shape filled with graphite molding powder. The second 
rubber mold had an ellipsoidal cavity of 287 cm.sup.3 volume with 73 mm 
diameter and 105 mm height. Then there was carried out the preliminary 
pressing of the fuel element according to the invention at room 
temperature and at a pressure of 120 kp/cm.sup.2. The compressed body had 
about 157 cm.sup.3 volume and about 80 mm height and 61 mm diameter. The 
body had an axis ratio of about 1.31 and a density of the graphite matrix 
of 1.42 g/cm.sup.3. This value of 1.42 g/cm.sup.3 corresponds to 74% of 
the theoretical density. 
Subsequently the preliminarily pressed fuel element body was heated to 
180.degree. C. in a steel die and final pressed between two cup shaped 
dies to a sphere having a diameter of about 61 mm at a pressure of 120 
kp/cm.sup.2. The graphite matrix density under full load was about 1.91 
g/cm.sup.3, corresponding to 99% of the theoretical green density. 
To carbonize the resin binder the fuel element spheres were heated under 
argon gas in 18 hours to 840.degree. C. and after the cooling annealed in 
a further operation up to 1800.degree. C. in a vacuum (pressure &lt;10.sup.-3 
Torr). 
The finished fuel element spheres had the following properties: 
Geometrical density of the graphite matrix (g/cm.sup.3): 1.74 
Crushing load between two parallel steel plates (kp): 2,800 
Drop strength (number of drops): 350 
Anisotropic factor of the thermal expansion: 1.26 
Integrity of the coated particles: 
U free/U total.times.10.sup.6 : 26 
Th free/Th total.times.10.sup.6 : 10 
To determine the drop strength there was determined the number of drops 
from 4 meters high to the bed of spheres until there occured the first 
recognizable surface injury. 
To determine the integrity of the coated heavy metal particles the fuel 
elements were decomposed electrolytically and there were ascertained 
fluorimetrically in the electrolyte as well as in the decomposed graphite 
matrix the uranium and thorium found outside the coating. 
EXAMPLE 2 
Up to the preliminary pressing of the fuel element in the rubber mold the 
steps of the procedure were the same as in Example 1. After the molding of 
the preliminarily pressed nucleus the rubber mold filled with the graphite 
molding powder was heated to 75.degree. C. and pressed together in the 
steel die of the mold with the upper and lower punches at unchanged 
pressure of 120 kp/cm.sup.2. As a result of the increased temperature at 
which the lubricant (stearic acid) became liquid there could be produced 
already in the three dimensional compressing a relatively high graphite 
matrix density of 1.62 g/cm.sup.3. This value corresponds to 84% of the 
theoretical green density. After the final pressing the fuel element 
spheres were heat treated as described in Example 1 and their physical 
properties investigated. The results of the measurements showed a clear 
improvement of the isotropy. The anisotropic factor of the thermal 
expansion was 1.19. All the remaining properties agreed well with those 
reported in Example 1.