A uranium-containing nuclear-fuel sintered pellet containing UO.sub.2, (U, Pu)O.sub.2, (U, Th)O.sub.2, (U, RE)O.sub.2, (U, Pu, Th)O.sub.2, (U, Pu, RE)O.sub.2, (U, Th, RE)O.sub.2 or (U, Pu, Th, RE)O.sub.2, wherein RE=rare earth, has a sintered-pellet surface layer being formed of at least 80% by volume of a chemical boron compound UB.sub.x or (U, . . . )B.sub.x, wherein x=2;4;6 or 12, and a remainder of the sintered pellet containing at most 5% by volume of the chemical boron compound. A nuclear-reactor fuel assembly has a fuel rod containing such a uranium-containing nuclear-fuel sintered pellet in a cladding tube with the boron as a burnable absorber for thermal neutrons. The surface layer having the chemical boron compound is obtained by treating the nuclear-fuel sintered pellet with boron or a boron-containing chemical compound at an appropriately high treatment temperature.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is a Continuation of International Application Ser. No. 
PCT/EP94/02470, filed Jul. 26, 1994, published as WO95/04994, Feb. 16, 
1995. 
BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The invention relates to a nuclear-fuel sintered pellet containing 
UO.sub.2, (U, Pu)O.sub.2, (U, Th)O.sub.2, (U, RE)O.sub.2, (U, Pu, 
Th)O.sub.2, (U, Pu, RE)O.sub.2, (U, Th, RE)O.sub.2 or (U, Pu, Th, 
RE)O.sub.2, wherein RE=rare earth. The invention also relates to a 
nuclear-reactor fuel assembly including a fuel rod having a cladding tube 
and such a uranium-containing nuclear-fuel sintered pellet in the cladding 
tube. The invention additionally relates to a method for treating such a 
uranium-containing nuclear-fuel sintered pellet. 
Published European Patent Application 0 239 843 A1, corresponding to U.S. 
Pat. No. 4,774,051, discloses a nuclear-fuel sintered pellet made of 
UO.sub.2, (U, Pu)O.sub.2 or (U, Th)O.sub.2. Boron is incorporated as a 
neutron poison in the chemical compound form UB.sub.x, wherein x=2; 4 
and/or 12 and/or B.sub.4 C, in a sinter matrix of that nuclear-fuel 
sintered pellet. That known nuclear-fuel sintered pellet is obtained by 
producing a mixture of uranium oxide powder or uranium mixed oxide powder 
with uranium boride powder or boron carbide powder and pressing it to form 
pellets which are subsequently sintered in a sintering furnace under a 
reducing sintering atmosphere to form nuclear-fuel sintered pellets. In 
those nuclear-fuel sintered pellets, the boron is thereby uniformly 
distributed throughout the sinter matrix. 
From the neutron physics point of view, boron in uranium-containing 
nuclear-fuel sintered pellets is a burnable neutron absorber which loses 
its property as an absorber for thermal neutrons after those nuclear-fuel 
sintered pellets have been used in a nuclear reactor for a certain period 
of time. 
Nuclear-reactor fuel assemblies having fuel rods that contain 
uranium-containing nuclear-fuel sintered pellets are used in a nuclear 
reactor, for example, during four sequential fuel assembly cycles, 
generally being of equal durations. At the end of a fuel assembly cycle, 
some of the nuclear-reactor fuel assemblies in the nuclear reactor are in 
each case replaced by fresh, unirradiated nuclear-reactor fuel assemblies. 
The fresh, unirradiated nuclear-reactor fuel assemblies would cause a 
comparatively high reactivity in the nuclear reactor relative to the 
nuclear-reactor fuel assemblies that are already irradiated. However, the 
boron in the nuclear-fuel sintered pellets of those fresh, unirradiated 
nuclear-reactor fuel assemblies at first moderates the reactivity due to 
those nuclear-reactor fuel assemblies by initially absorbing thermal 
neutrons. 
The nuclear fuel in fresh and unirradiated nuclear-reactor fuel assemblies 
gradually burns out in the nuclear reactor through nuclear decay, but a 
burnable neutron absorber that is present in that nuclear fuel 
simultaneously burns out gradually due to the physical effects of 
neutrons, so that finally, that neutron absorber absorbs no thermal 
neutrons or only very few. In that way, even unirradiated nuclear-reactor 
fuel assemblies being newly loaded into the nuclear reactor may cause 
approximately the same reactivity in the nuclear reactor during their 
entire residence time in the nuclear reactor, as the nuclear-reactor fuel 
assemblies which have already spent a fuel assembly cycle in the nuclear 
reactor. 
Boron is advantageously used as a neutron absorber in a nuclear fuel as 
compared to other burnable neutron absorbers such as rare earths if the 
fuel assembly cycles are comparatively long, i.e., for example, longer 
than 12 months, since accumulation of heat in the nuclear fuel is avoided 
with boron. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a 
uranium-containing nuclear-fuel sintered pellet, a nuclear-reactor fuel 
assembly having a uranium-containing nuclear-fuel sintered pellet and a 
method for treating a uranium-containing nuclear-fuel sintered pellet, 
which overcome the hereinafore-mentioned disadvantages of the 
heretofore-known products and methods of this general type and in which an 
excessively fast and excessively high increase in reactivity is not caused 
upon startup of a nuclear reactor if the nuclear-fuel sintered pellet is 
newly loaded, in the unirradiated state, in the nuclear reactor. 
Since the surface of the uranium-containing nuclear-fuel sintered pellets 
in the cladding tube of a fuel rod in the nuclear reactor is kept 
relatively constant at a substantially lower temperature than the rest of 
the sintered pellet by a coolant flowing past the exterior of the cladding 
tube, whereas, however, in the case of the uranium-containing nuclear-fuel 
sintered pellet according to the invention most of the boron is situated 
in a surface layer, chemical reactions between UB.sub.x and UO.sub.2 take 
place only to a limited extent in that surface layer, so that the boron 
cannot emerge from the uranium-containing nuclear-fuel sintered pellet 
according to the invention, and a reactivity increase having a rate and 
amplitude that is moderated is thereby guaranteed. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a uranium-containing nuclear-fuel sintered 
pellet containing UO.sub.2, comprising a sintered-pellet surface layer 
being formed of at least 80% by volume of a chemical boron compound 
UB.sub.x with at least one number x from a number set 2; 4 and 12, and a 
remainder of the sintered pellet containing at most 5% by volume of the 
chemical boron compound. 
With the objects of the invention in view, there is also provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, Pu)O.sub.2, 
comprising a sintered-pellet surface layer being formed of at least 80% by 
volume of a chemical boron compound (U, Pu)B.sub.x with at least one 
number x from a number set 2; 4 and 12, and a remainder of the sintered 
pellet containing at most 5% by volume of the chemical boron compound. 
With the objects of the invention in view, there is additionally provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, Th)O.sub.2, 
comprising a sintered-pellet surface layer being formed of at least 80% by 
volume of a chemical boron compound (U, Th)B.sub.x with at least one 
number x from a number set 4 and 6, and a remainder of the sintered pellet 
containing at most 5% by volume of the chemical boron compound. 
With the objects of the invention in view, there is further provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, RE)O.sub.2 
(RE=rare earth), comprising a sintered-pellet surface layer being formed 
of at least 80% by volume of a chemical boron compound (U, RE)B.sub.x with 
at least one number x from a number set 4; 6 and 12, and a remainder of 
the sintered pellet containing at most 5% by volume of the chemical boron 
compound. 
With the objects of the invention in view, there is also provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, Pu, 
Th)O.sub.2, comprising a sintered-pellet surface layer being formed of at 
least 80% by volume of a chemical boron compound (U, Pu, Th)B.sub.x with 
at least one number x from a number set 2; 4; 6 and 12, and a remainder of 
the sintered pellet containing at most 5% by volume of the chemical boron 
compound. 
With the objects of the invention in view, there is additionally provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, Pu, 
RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer 
being formed of at least 80% by volume of a chemical boron compound (U, 
Pu, RE)B.sub.x with at least one number x from a number set 2; 4; 6 and 
12, and a remainder of the sintered pellet containing at most 5% by volume 
of the chemical boron compounds. 
With the objects of the invention in view, there is further provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, Th, 
RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer 
being formed of at least 80% by volume of a chemical boron compound (U, 
Th, RE)B.sub.x with at least one number x from a number set 4 and 6, and a 
remainder of the sintered pellet containing at most 5% by volume of the 
chemical boron compound. 
With the objects of the invention in view, there is also provided a 
uranium-containing nuclear-fuel sintered pellet containing (U, Pu, Th, 
RE)O.sub.2 (RE=rare earth), comprising a sintered-pellet surface layer 
being formed of at least 80% by volume of a chemical boron compound (U, 
Pu, Th, RE)B.sub.x with at least one number x from a number set 4 and 6, 
and a remainder of the sintered pellet containing at most 5% by volume of 
the chemical boron compound. 
In accordance with another feature of the invention, there is provided at 
least 90% by volume of the chemical boron compounds in the sintered-pellet 
surface, and at most 2% by volume of the chemical boron compounds in the 
remainder of the sintered pellet. 
In accordance with a further feature of the invention, there is provided at 
least 98% by volume of the chemical boron compound in the sintered-pellet 
surface layer, and at most 1% by volume of the chemical boron compound in 
the remainder of the sintered pellet. 
In accordance with an added feature of the invention, the remainder of the 
sintered pellet is without a detectable boron content. 
In accordance with an additional feature of the invention, the 
boron-containing sintered-pellet surface layer has a thickness of from 2 
to 40 .mu.m. 
In accordance with yet another feature of the invention, the 
boron-containing sintered-pellet surface layer has a thickness of from 5 
to 20 .mu.m. 
In accordance with yet a further feature of the invention, the isotope 
B.sub.10 in the boron of the chemical boron compound is enriched relative 
to a natural isotopic composition. 
With the objects of the invention in view, there is also provided a 
nuclear-reactor fuel assembly, comprising a fuel rod having a cladding 
tube, and such a uranium-containing nuclear-fuel sintered pellet in the 
cladding tube. 
With the objects of the invention in view, there is additionally provided a 
method for treating a uranium-containing nuclear-fuel sintered pellet with 
boron or a boron-containing chemical compound, which comprises treating a 
uranium-containing nuclear-fuel sintered pellet with boron or a 
boron-containing chemical compound at a treatment temperature being high 
enough to form uranium-containing boride in a surface layer of the 
nuclear-fuel sintered pellet. 
In accordance with another mode of the invention, there is provided a 
method which comprises carrying out the treatment step in the presence of 
hydrogen-containing inert gas. 
In accordance with a further mode of the invention, there is provided a 
method which comprises carrying out the treatment step in the presence of 
at least one hydrogen-containing inert gas selected from the group 
consisting of helium, argon and nitrogen. 
In accordance with an added mode of the invention, there is provided a 
method which comprises embedding the uranium-containing nuclear-fuel 
sintered pellet in a boron and/or a boron-containing chemical compound. 
In accordance with an additional mode of the invention, the boron or 
boron-containing chemical compound contains an admixed catalyst. 
In accordance with yet another mode of the invention, the boron or 
boron-containing chemical compound is in the form of a powder. 
In accordance with yet a further mode of the invention, there is provided a 
method which comprises circulating the powder. 
In accordance with yet an added mode of the invention, the boron or 
boron-containing chemical compound is in molten form. 
In accordance with yet an additional mode of the invention, there is 
provided a method which comprises selecting the boron-containing chemical 
compound as at least one gas from the group consisting of borane, boron 
halide and boron alkyl. 
In accordance with again another mode of the invention, there is provided a 
method which comprises selecting the powder as at least one material from 
the group consisting of boron carbide, silicon boride and metal boride, 
preferably zirconium diboride. 
In accordance with again a further mode of the invention, there is provided 
a method which comprises setting the treatment temperature from 
850.degree. to 1600.degree. C. and preferably from 1100.degree. to 
1450.degree. C. 
In accordance with a concomitant mode of the invention, there is provided a 
method which comprises setting a treatment time of from 10 minutes to 6 
hours and preferably of from 1 to 4 hours. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
uranium-containing nuclear-fuel sintered pellet, a nuclear-reactor fuel 
assembly having a uranium-containing nuclear-fuel sintered pellet and a 
method for treating a uranium-containing nuclear-fuel sintered pellet, it 
is nevertheless not intended to be limited to the details shown, since 
various modifications and structural changes may be made therein without 
departing from the spirit of the invention and within the scope and range 
of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the steps of the method in detail, the following examples 
are described below: 
A plurality of ceramic nuclear-fuel sintered pellets made of UO.sub.2, 
which have the form of a solid cylinder with a diameter of 9.11 mm and a 
height of 10 mm were disposed next to one another to form a cylindrical 
column in an Al.sub.2 O.sub.3 boat on a ZrB.sub.2 powder bed that may, for 
example, contain at least one of the materials NH.sub.4 Cl, BaF.sub.2 
and/or KBF.sub.4 admixed as a catalyst. Each of the nuclear-fuel sintered 
pellets had a sinter density of between 10.38 and 10.44 g/cm.sup.3. The 
nuclear-fuel sintered pellets were also completely covered with ZrB.sub.2 
powder which may likewise, for example, contain NH.sub.4 Cl, BaF.sub.2 
and/or KBF.sub.4 admixed as a catalyst. 
The boat with the nuclear-fuel sintered pellets was then disposed in an 
Al.sub.2 O.sub.3 tube and heated inside this tube in an electrically 
heated tube furnace for three hours at 1400.degree. C. under a treatment 
atmosphere being formed of 5% H.sub.2 and 95% He. 
After cooling, measurement of the nuclear-fuel sintered pellets through the 
use of X-ray diffractometry showed that these nuclear-fuel sintered 
pellets, insofar as they were situated between the nuclear-fuel sintered 
pellets at each end of the column, had a surface layer of virtually 100% 
UB.sub.4 and UB.sub.2 by volume under their external surface. The 
thickness of this surface layer was determined as 12 .mu.m on average by 
using a microscope for a transverse and a longitudinal ground section of 
the nuclear-fuel sintered pellets. The variation between maximum value and 
minimum value of this thickness was 6 .mu.m. The rest of the sintered 
pellets were formed virtually only of unaltered UO.sub.2 without a 
detectable boron content. 
When boron powder was used instead of ZrB.sub.2 powder for the powder bed, 
surface layers of virtually 100% UB.sub.2 and UB.sub.4 by volume with a 
thickness of 21 .mu.m.+-.5 .mu.m were produced under the external surface 
of the nuclear-fuel sintered pellets. In this case again, the rest of the 
sintered pellets were formed virtually of unaltered UO.sub.2 without a 
detectable boron content. 
In a further exemplary embodiment, use was made of an Al.sub.2 O.sub.3 tube 
which was disposed with a horizontal longitudinal axis in an electrically 
heated tube furnace. Two thirds of the empty volume of this Al.sub.2 
O.sub. tube was filled with ZrB.sub.2 powder in which twelve ceramic 
nuclear-fuel sintered pellets made of UO.sub.2, likewise with sinter 
densities of between 10.38 and 10.44 g/cm.sup.3, were embedded. The 
nuclear-fuel sintered pellets likewise had the form of a solid cylinder 
with a diameter of 9.11 mm and a height of 10 mm. The Al.sub.2 O.sub.3 
tube was rotated about its longitudinal axis at one revolution per minute, 
so that the powder, together with the nuclear-fuel sintered pellets, was 
circulated. In this case, the powder and the nuclear-fuel sintered pellets 
were heated for three hours at a treatment temperature of 1400.degree. C. 
under a surrounding atmosphere in the tube furnace being formed of 5% 
H.sub.2 and 95% He. 
After cooling, the nuclear-fuel sintered pellets had a surface layer of 
virtually 100% UB.sub.2 and UB.sub.4 by volume under their entire surface. 
This surface layer had, below the external surface of the nuclear-fuel 
sintered pellets, a thickness of 16 .mu.m.+-.4 .mu.m and, at the two end 
surfaces, a thickness of 7 .mu.m.+-.3 .mu.m. The rest of the sintered 
pellets was unaltered UO.sub.2 without a detectable boron content. 
In a variant of this exemplary embodiment, fifteen ceramic nuclear-fuel 
sintered pellets made of UO.sub.2, which likewise had the form of a solid 
cylinder with a diameter of 9.11 mm and a height of 10 mm, were mounted 
without a powder bed in the Al.sub.2 O.sub.3 tube, and this tube was 
likewise rotated in the tube furnace about its horizontal longitudinal 
axis at one revolution per minute. A gas mixture of diborane B2H.sub.6 and 
H.sub.2 was fed through a duct into the internal space of the tube, which 
was closed at both ends in gas-tight fashion, and fed out again through 
another duct. The flow rate of the gas mixture was 10 liters per minute, 
and the composition was 99.9 mole % H.sub.2 and 0.1 mole % B2H.sub.6. The 
nuclear-fuel sintered pellets made of UO.sub.2 in this case were kept at a 
temperature of 1050.degree. C. in the tube furnace for 90 minutes. 
After cooling, these nuclear-fuel sintered pellets made of UO.sub.2 had a 
surface layer with a thickness of 8 .mu.m that was formed of 100% by 
weight UB.sub.2 and UB.sub.4 under their entire surface. The rest of the 
sintered pellets was unaltered UO.sub.2 without a detectable boron 
content. 
The surface layer containing UB.sub.2 and UB.sub.4 can also be formed in 
the nuclear-fuel sintered pellet made of UO.sub.2 by embedding this 
nuclear-fuel sintered pellet in boron and/or a boron-containing chemical 
compound, which are in the molten state. 
It is expedient if the isotope B.sub.10 in the boron is enriched relative 
to the natural isotopic composition of boron, in the boron being used or 
in the boron-containing chemical compounds being used. This can be 
achieved in a known manner, for example by cyclotron enrichment, diffusion 
enrichment or separation nozzle enrichment. It is this isotope B.sub.10 
that essentially absorbs the thermal neutrons. By virtue of the fact that 
it is enriched in the boron that is situated in the surface layer of the 
uranium-containing nuclear-fuel sintered pellet, the thickness of this 
surface layer can be selected to be comparatively small. 
In a similar way, it is even possible to treat uranium-containing ceramic 
nuclear-fuel sintered pellets that contain at least one of the chemical 
compounds (U, Pu)O.sub.2, (U, Th)O.sub.2, (U, RE)O.sub.2, (U, Pu, 
Th)O.sub.2, (U, Pu, RE)O.sub.2, (U, Th, RE)O.sub.2 and (U, Pu, Th, 
RE)O.sub.2, since the other heavy metals in these mixed oxides all form 
borides structured identically or similarly to that which uranium forms. 
The rare earths RE may, in particular, be gadolinium, samarium, europium, 
erbium and dysprosium, which are all neutron poisons, but can exhibit a 
burnout behavior due to the physical effects of neutrons which is 
different from that of boron, and therefore can advantageously influence 
the reactivity control in a nuclear reactor, in combination with boron. 
It is advantageous to fit the uranium-containing nuclear-fuel sintered 
pellets according to the invention in a cladding tube of a fuel rod, 
wherein the cladding tube is generally made of a zirconium alloy or 
stainless steel, and to seal this cladding tube. This fuel rod is 
expediently a component of a nuclear-reactor fuel assembly for a nuclear 
reactor. Advantageously, such a nuclear-reactor fuel assembly is intended 
for a light water nuclear reactor, in particular for a pressurized water 
nuclear reactor or a boiling water nuclear reactor. 
Tests simulating the conditions in a nuclear reactor and being carried out 
with such a cladding tube showed not only that the boron-containing 
surface layer of the uranium-containing nuclear-fuel sintered pellets is 
firmly anchored in the crystal structure of these nuclear-fuel sintered 
pellets, but also that the boron does not escape from this surface layer, 
even at temperatures of 500.degree. C. and above.