The present invention relates to the production of fuel pellets for
There is an increasing trend to the use of uranium oxide fuel having
greater levels of enrichment to give higher fuel burn-up. The result of such a trend
is to increase the residence time of any given fuel pin in the reactor. Therefore,
it is necessary to produce fuel pins having a greater degree of confidence in their
Fuel pellets pressed from powders of uranium oxide and subsequently
sintered under controlled conditions of time, temperature and atmosphere are well
known. Such fuel pellets are normally generally right cylinders and many such pellets
are packed in axial alignment in metal tubes of, for example, Zircaloy (trade name),
an alloy of zirconium. The tubes are usually referred to as "cladding" and the filled
tube as a "fuel pin". Many fuel pins are grouped together, often in a parallel aligned
radial array to form a fuel element or fuel assembly.
In use, when the fuel elements are loaded into a reactor and producing
power, two distinct phenomena occur: "pellet cladding interaction" (PCI); and, the
generation of fission gas which ideally should be retained within the pellet oxide
structure and not released into the cladding tube, hence "fission gas retention"
(FGR). Both will be explained in more detail below.
During operation of a nuclear reactor, the high temperatures generated
result in the oxide fuel pellets expanding and the outer surface of the pellet coming
into contact with the inner surface of the cladding producing mechanical stress
(PCI). The mechanical stress is further exacerbated by the production of iodine
as a fission product during the nuclear reaction. The combination of mechanical
stress and iodine which acts as a stress corrosion agent attacking the cladding
material grain boundaries can cause rupture of the cladding in extreme cases. Generally,
a small grain size is beneficial in promoting creep of the oxide pellet at the surface
to give a stress relieving effect.
The second effect which occurs during the nuclear reaction is the
generation of many different fission gas products including iodine referred to above.
The fission gas products are created within the oxide structure as atoms which diffuse
to grain boundaries and nucleate thereat. Gas bubbles are eventually formed, the
bubbles coalescing with each other and being finally given off and known as "fission
gas release". It is undesirable for the fission gases to be released and fuel pellets
are generally produced with a large grain size to promote retention of the fission
gases (FGR). A large grain size promotes retention due to the fact that the fission
gas atoms have further to diffuse through the structure before they reach a grain
boundary where they can nucleate.
As will be noted, the requirements of promoting stress relief by a
small grain size to optimise creep and reduce PCI and, a large grain size to maximise
FGR are mutually conflicting.
European Patent Application 82305260.0 (EP 0076680) describes the
doping of uranium dioxide or ammonium diuranate as a precursor thereof with one
of a selected group of elements, including niobium, to produce a large grain size.
Similarly, Japanese Patent Application 04070594 describes the doping of uranium
oxide with Nb(IV) oxide, and Japanese Patent Application 03102292 describes the
doping of uranium oxide with an intermetallic compound or an oxide which may include
UK Patent Application 8517110 (GB2177249) describes the preparation
of large grain size materials using seed material composed essentially of single
crystals, but makes no reference to niobia doping. UK Patent Application 23791/78
(GB1600169) discloses the coating of nuclear fuels pellets with a getter material
selected from a group including metallic niobium, but no doping of the pellet material
itself is suggested.
Proposals have been made to produce fuel pellets having two distinct
regions: an outer annular region having a small grain size to minimise PCI; and,
an inner core region having a large grain size to maximise FGR. Such structures
are described in 89JP-311936; 87JP-262652; and, 83SU-600645. The pellets are generally
made by pressing an outer annulus component and a separate inner core component,
fitting the two together and then sintering. The pressing conditions are controlled
so that during sintering the outer annulus shrinks onto the core and diffusion bonding
of the two components occurs.
GB-B-2020641 of common ownership herewith describes the use of niobium
pentoxide (henceforth referred to as "niobia") as a dopant in uranium oxide fuel
pellets. The effect of the niobia is stated to be that of increasing the diffusion
rates during sintering so promoting the generation of a large grain size in the
pellet. It is also stated that the increased diffusion rate caused by niobia does
not also substantially affect the rate of diffusion of the fission gas products
within the grains. Thus, a large grain size may be achieved with no apparent adverse
effect of also increasing the diffusion rate of the fission gas products. However,
we have now found that the presence of niobia does adversely affect the diffusion
rate of fission gas.
It is an object of the present invention to provide a functionally
graded uranium oxide fuel pellet having improved creep stress relaxation properties
at the surface to minimise PCI and improved FGR in its core.
The present invention provides a uranium oxide fuel pellet comprising
an inner region and an outer region about the inner region, wherein the uranium
oxide of at least a portion of said outer region is rich in niobia relative to the
inner region. Other features and advantages of the invention are outlined in the
following description and the appended claims to which the reader is referred.
In this specification, the term "niobia" is used. The addition is
usually made in the form of niobium pentoxide, however, during processing such as
sintering, the composition may change due to the effects of a reducing atmosphere
or alternate reducing/oxidising atmospheres so that the "niobia" in the final material
as produced, or becomes in use in a reactor, may not be niobium pentoxide. Thus,
the term "niobia" used herein is intended to cover all forms of niobium and niobium
oxides which may be used or which are formed.
We have found that contrary to the teachings of the prior art, a uranium
oxide fuel material which nevertheless has a large grain size due to the increased
diffusion rates promoted by the addition of niobia actually has a greater creep
rate than does a material having a smaller grain size thus, reducing PCI.
We have also found that whilst a large grain size per se in a core
region of the pellet is beneficial for FGR compared with a smaller grain size in
the same material, it is preferable to produce a larger grain size by means other
than doping with niobia additions which we have now found may increase the diffusion
rate of fission gas within the structure. One such method of achieving a larger
grain size in the structure is by seeding of the powder used for pressing with uranium
dioxide (UO2) crystal grains of a desired size range as distinct from
the agglomerates of UO2 crystallites which form the principal bulk of
the powder. We have found that an average grain size of about 10 to 15µm in the
sintered pellets produces acceptable performance with regard to FGR but that an
average grain size of greater than about 25µm produces enhanced performance with
regard to FGR.
The content of niobia in the surface region may be in the range from
about 0.1 to 0.5 weight percent with a content of about 0.3 weight percent being
The average grain size of a prior art so-called "duplex" pellet having
distinct annulus and core regions as described above with reference to the prior
art may typically be in the range from about 0.25 to 2.5µm. In the pellet of the
present invention, the average grain size in the surface region may be greater than
about 25µm and typically in the range from about 35 to 50µm. It should be noted
that the method of measuring average grain size in this specification is by the
linear intercept method.
The pellets according to the present invention may be made having
the duplex structure as described above with a distinct outer annulus and a distinct
core. The pellets may be made by two stage pressing and assembling as described
or as a unitary construction comprising multiple pressing stages in a complex die
arrangement on a press of the type described, for example, in International Patent
Application Number WO93/18878.
Although the pellet may be made having two distinct regions produced
by pressing operations with two powder compositions constituting the outer and inner
regions, a homogeneous pellet having the surface regions enhanced in niobia content
would fall within the inventive concept of the present invention.
In a duplex type structure, the annulus and core regions may each
comprise about 50vol% of the total pellet volume for example.
In order that the present invention may be more fully understood,
examples will now be described by way of illustration only with reference to the
accompanying drawings, of which:
- Figures 1 and 2 show axial and radial cross sections through a schematic representation
of a fuel pellet according to the present invention, respectively; and,
- Figure 3 which shows a schematic representation of a pressing sequence for making
a fuel pellet according to the present invention on an integrated die assembly.
Figures 1 and 2 show a pellet generally at 10. The pellet comprises
an outer region 12 having a composition including a niobia addition and a core 14
which is substantially free of niobia. The axial end faces of the pellet are often
provided in a slightly dished form to accommodate expansion in the axial direction
in use and also to provide space for creep relaxation under stress at high temperatures
but this has not been shown in the interests of clarity.
In one method of manufacture the annular region 12 and core region
14 are pressed separately by known techniques. The two components are assembled
together and sintered under known conditions of about 1700/1750°C for about 4-5
hours under a hydrogen and moisture containing atmosphere. The green properties
of the pressed annulus cause it to shrink more than the core with the result that
under the sintering conditions the annulus grips the core and the two become diffusion
An alternative method of manufacture is illustrated with reference
to Figure 3 which show nine stages in the pressing of a duplex pellet on an integrated
die assembly. The die assembly comprises a single outer die 20 with a multiple inner
punch assembly, the components of which are enabled to move independently by means
of a hydraulically controlled matrix in a mechanical press for example. At Step
1 the die 20 has an outer punch 22 and a stepped inner punch 24 creating a cavity
26 for filling with powder 28 as shown in Step 2. A third punch member 30 compresses
the powder 28 to form a self supporting green compact 32 as shown in Step 3. The
punch member 30 is withdrawn and the stepped inner punch member 24 is lowered to
a position below the lowermost extent 34 of the green compact 32 as shown in Step
4. Powder 36 is filled into the core void so formed in Step 4 and as shown in Step
5. The outer punch 30 is replaced together with a coaxial top punch 38, punches
38 and 24 being advanced towards each other to compress the core powder filling
36 as shown in Steps 6 and 7 to create a unified green compact 40 comprising the
outer compacted annulus 32 and inner compressed core 42 as shown in Step 8. The
upper punches 30 and 38 are withdrawn and punches 20 and 24 are advanced simultaneously
to eject the green compact 40 from the die. The green compact 40 is then sintered.
In a first example of a sintered fuel pellet according to the present
invention, the core region 14 had an average grain size of 12-15µm and was substantially
free of niobia whereas the annulus portion 12 comprised a grain size in excess of
25µm and had an addition of 0.3 wt% niobia.
In the second example of a sintered fuel pellet according to the present
invention the core region 14 comprised a grain size in excess of 25µm being produced
by seeding with UO2 crystals and the annulus portion 12 had a grain size
in excess of 25µm and contained 0.3wt% of niobia.