BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention related to a method for production of a nuclear
fuel element with oxide base, in which the nuclear fuel with oxide base is mixed
with chrome oxide and sintered into a solid body. The invention also relates to
a material with oxide base adapted to be sintered to a nuclear fuel element, which
comprises a nuclear fuel with oxide base and chrome oxide.
Methods and materials of the type mentioned above are already known
within the field of nuclear energy applications. The nuclear fuel with oxide base
may comprise UO2, ThO2, PuO2 or a mixture thereof,
and is provided as powder.
Different additions of further oxides, such as TiO2, Nb2O5,
Cr2O3, Al2O3, V2O5
and MgO have according to the prior art been added to the nuclear fuel with oxide
base for obtaining an increase of the grain size thereof in connection with the
sintering thereof, since said additions activate the crystalline growth of the
grains of the nuclear fuel during the sintering.
The increased grain size results in a need of a longer time for gas
enclosures in the grains to diffuse to grain boundaries and through these out of
the nuclear fuel when this is used during operation. The amount of such gases,
fission gases, outside the nuclear fuel element is accordingly reduced during normal
operation conditions thanks to the increased grain size of the nuclear fuel.
It may also be assumed that an increased corrosion resistance results
from an increased grain size of the nuclear fuel, since corrosion preferably starts
at the grain boundaries and the relationship between the total grain boundary area
and the volume of a nuclear fuel element is reduced, i.e. the total grain boundary
area is reduced, when the grain size increases. A good corrosion resistance is
desired, since the nuclear fuel may come into contact with steam or water during
the operation as consequence of damage on a surrounding cladding tube. Corrosion
products may then be spread further out in the plant, which should be avoided
for reasons known per se.
Besides the fact that the additions mentioned above result in a larger
grain size of the nuclear fuel and the advantages associated therewith, at least
some of them contribute to an increase of the density of the nuclear fuel element,
with respect to the weight of the very nuclear fuel, for example U, Th or Pu, in
relation to the volume of the nuclear fuel element. Thus, more power may be obtained
out of a given volume of the nuclear fuel.
At least some of said additions also result in an increase of the
plasticity of the nuclear fuel element sintered. This results in a smaller risk
of damage on surrounding cladding tubes at rapid power increases during operation,
and volume changes of the nuclear fuel element associated therewith, since the
fuel element with less power than otherwise acts on the cladding tube.
Cr2O3 is the addition of those mentioned above
that gives the mostly noticeable result. The prior art uses therefor preferably
Cr2O3 for obtaining the effects mentioned above, primarily
the increase of the grain size of the nuclear fuel. However, Cr2O3
to be considered as a poison in this context, since Cr has a comparatively large
neutron absorption cross section, which in its turn may have a negative influence
upon the power of the nuclear fuel element, which has been understood by the applicant.
According to the prior art 1000-5000 ppm Cr is added (separate or as Cr2O3)
with respect to the amount of the nuclear fuel with oxide base, for example UO2,
for obtaining the effects mentioned above.
WO-A-97/06535 discloses a sintered nuclear fuel element with oxide
base. The nuclear element comprises a nuclear fuel with oxide base such as uranium
dioxide and plutonium dioxide. In example 4 it is proposed to add particles comprising
one or more of Ti, Al, Nb, Cr and Mg. The proposed quantity of these metals is
0,01 - 1 %, i.e. 100 - 10000 ppm.
GB-A-1 334 391 discloses another sintered nuclear fuel element with
oxide base. The nuclear element comprises a nuclear fuel with oxide base, such
as uranium dioxide or plutonium oxide. It is proposed to add one further oxide
compound, selected among aluminium, titanium, magnesium, zirconium, colombium,
chromium, vanadium, iron and copper. The quantity of the added oxide compound
is also 0,01 - 0,1 %, i.e. 100 - 1000 ppm.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method which benefits
by the effects obtainable through addition of a further oxide, preferably chrome
oxide to the nuclear fuel with oxide base at the same time as the amount of chrome
oxide added is regulated while considering the negative consequences of the presence
of Cr in the nuclear fuel element.
The object is achieved by the method defined in claim 1. Such amounts
of added chrome oxide result in remarkably increased grain sizes of different nuclear
fuels with oxide base, such as UO2 in connection with the sintering
of the nuclear fuel element, at the same time as the amount Cr is kept at a lower
level than before, and has accordingly a reduced negative influence upon the efficiency
of the nuclear fuel element during operation, in spite of the comparatively high
neutron absorption cross section thereof. A still more preferred interval with
respect to the amount of Cr is 100-700 ppm.
The further metal oxide is preferably any of Nb2O5,
Al2O3 and MgO. Said additions are alone or in combination
with each other insufficient for obtaining the effects obtained by the chrome oxide,
but they function excellent as supplements to the chrome oxide. The chrome oxide
is preferably Cr2O3.
According to a further preferred embodiment of the method the further
metal oxide comprises Al2O3 and the amount of Al added is
≥ 20, and preferably ≤ 300 ppm. Below 20 ppm the effect of Al2O3
added is rapidly reduced. Above 300 ppm the further positive effects of Al2O3
get marginal with the chrome oxide proportion in question.
According to a further preferred embodiment of the method the further
metal oxide comprises MgO, in which the amount of Mg added is ≥ 20, and preferably
≤ 300 ppm. Below 20 ppm the positive effects of MgO upon the nuclear fuel with
oxide base are reduced rapidly. Above 300 ppm Mg the further positive effects
of MgO gets marginal with the chrome oxide proportion in question.
A further object of the invention is to provide a material with oxide
base adapted to be sintered to a nuclear fuel element, which through a content
of chrome oxide determined in advance and as a result of the composition thereof
gets the advantages in the form of higher density, larger nuclear fuel grains and
better plasticity resulting from the additions of said further oxides mentioned
in the introduction, at the same time as a neutron absorption cross section being
as low as possible is obtained for the nuclear fuel element.
This object is obtained by means of a material defined in claim 8.
A further metal oxide may be any of Nb2O5, Al2O3
and MgO already mentioned. Presence of several of these oxides jointly is also
possible in the material. The further metal oxide or oxides has a supplementary
object with respect to the chrome oxide, without increasing the total neutron absorption
cross section of the nuclear fuel element noticeably.
According to a further preferred embodiment said metal oxide comprises
aluminium oxide in the form of Al2O3, in which the amount
Al is ≥ 20, and preferably ≤ 300 ppm with respect to the amount nuclear fuel.
Below 20 ppm the effects of Al2O3 added are reduced. Above
300 ppm AI the further positive effects of Al2O3 gets marginal
with the chrome oxide proportion in question.
According to a further preferred embodiment the metal oxide comprises
magnesium oxide in the form of MgO, in which the amount Mg is ≥ 20, and preferably
≤ 300 ppm with respect to the amount nuclear fuel. The amount MgO is restricted
for the same reasons as for Al2O3.
Further advantages and features of the method and the material according
to the invention will appear from the detailed description following and the other
DETAILED DESCRIPTION OF AN EMBODIMENT
According to a preferred embodiment of the method according to the
invention one or a plurality of powders comprising Cr2O3,
Al2O3 and MgO is added to a powder comprising a nuclear fuel
with oxide base, in this case UO2.
The amount Cr2O3 added, where Cr is in the interval
50-1000 ppm (weight proportions with respect to the weight of UO2),
the amount AI added in the form of Al2O3 is in the interval
20-300 ppm and the amount Mg added in the form of MgO is in the interval 20-300
ppm. An adhesive and a lubricant are added separately or as a part of any of said
powders as already known per se.
The powders are then mixed in any way known per se so that a homogenous
mixture is obtained.
The homogenous powder mixture is then pressed into one or several
green bodies, by a pressure of 200-700 MPa.
The green body or bodies is after that sintered in a hydrogen atmosphere
with an addition of 0,1-5,0 % of CO2, as an alternative only in humidified
hydrogen without an addition of CO2. The sintering lasts for 1-6 hours
at a temperature of 1400-1800°C and under atmosphere pressure. A density very close
to the theoretical density is obtained thereby. The UO2 grains, which
at the beginning had a grain size in the order of 10 µm, have during the sintering
grown to ≥ 25 µm, i.e. they have become considerably larger.
The oxides Cr2O3, Al2O3
and MgO added have during the sintering formed a liquid phase which in the material
sintered and cooled, i.e. the fuel element formed, forms the matrix around the
UO2-particles present in the sintered body.
Variations of the preferred embodiment described will of course be
apparent to a man skilled in the art, but he will then not go around the scope
of protection defined by the appended claims with support by the description of
The method and the material according to the invention are well suited
for production of nuclear fuel elements in the form of fuel pellets, which are
positioned in cladding tubes and are used in compressed-water reactors and boiling
water reactors for extraction of nuclear energy through nuclear fission activated
by neutron irradiation.
It has to be pointed out that the ppm values mentioned relate to
weight metal/weight nuclear fuel with oxide base, for example weight Cr/weight