COMMISSIONER'S DECISION
OBVIOUSNESS Controlling Grain Size in Nuclear Fuel
Particle modification of uranium dioxide where the relative active
surface area of the fuel particles are reduced by wet attrition
milling while in the green state is shown in the cited art.
Final Action: Affirmed.
This decision deals with the applicant's request for review by the
Commissioner of Patents of the Final Action on application 293,717 (Class
31-88) filed December 22, 1977, assigned to Canadian General Electric
Company Limited entitled Method of Controlling Grain Size in Nuclear Fuel
Compacts. The inventors are Harvey Robert Lee, Adam Krawczyk and
Arnold K. Koch. The Examiner in charge issued a Final Action refusing to
allow the application on August 18, 1982.
The application relates to the preparation of nuclear fuel where uranium
powders originating from a solid and liquid reaction process involving
ammonium are prepared by a particle modification step where the relative
active surface area of the fuel particles are reduced by wet attrition
milling while in the green state.
In the Final Action, the Examiner rejected claims 1 to 7 in view of the
following references:
Publication
A.E.C, document, T.I.D.-7546, Book 2, Nov. 18-23, 1957, pp. 416-419,
453-466.
Canadian Patents
634,890 Jan. 16, 1962 Cope
656,281 Jan. 22, 1963 Moss
856,695 Nov. 24, 1970 Masselot
Masselot relates a process for manufacturing pellets of sintered nuclear
fuel comprising the steps of grinding uranium oxide into powder, adding a
sintering inhibitor, cold pressing and sintering.
Cope describes a method of making ceramic nuclear fuel comprising the steps
of ball-milling a mixture of uranium dioxide powder and gelatinous
plutonium polymer, pressing the mixture to form a green compact and
sintering the compact. Moss describes a process of densification of
uranium dioxide by ball milling and subsequent tumbling to granulate the
material. The publication describes methods for the preparation of uranium
dioxide fuel materials.
In the Final Action the Examiner stated (in part):
The rejection of claims 1 to 7 is maintained. The claims lack
invention in view of the cited art. The references teach
methods of manufacturing ceramic UO2 nuclear fuel bodies
comprising the step of milling the UO2 powder to obtain the
desired grain sizes. Contrary to applicant's arguments the
problem of obtaining a controlled and uniform grain size after
sintering is fully recognized in the prior art. The
publication on page 454, lines 1-4 and 30-36 discusses the
effects of ball milling on the sintering characteristics of
U02, at line 1 it is stated inter alia that "A more uniform
grain structure in the sintered pellet is achieved" and, at
line 4, that "it was found that wet milling is more efficient a
process than dry milling". Lines 30-36 of the same page also
emphasize the importance of UO2 powder structure in the
sintering characteristics of UO2, such as uniform grain
structure in the sintered material. Further evidence of the
recognition in the prior art of the importance of reducing
variations in grain size after sintering, can be found in the
publication on page 461 lines 5-7, page 463 line 16 and page
466 lines 16-20.
In response to the Final Action the applicant cancelled the claims on file
and replaced them with amended claims 1 to 5. That response stated
(in part):
The present invention comprises meritorious subject matter,
providing an inobvious solution to an existing problem.
In the CANDU R nuclear reactors which comprise Canada's major
contribution to this nuclear age the nuclear fuel is made up of
assemblies of fuel pellets encased in a thin zirconium sheath.
Under operating conditions of high temperature and high pressure
the long residence time of the fuel causes pellet growth to occur
against the wrapping constraint of the fuel sheath which is
collapsed in tight constraining relation against the pellets.
In the event that atypical crystal growth takes place there is a
strong likelihood of pellet failure occurring, which can lead
then to rupturing of the sheath and contamination of reactor
coolant by radioactive particles. The improved pellets made
possible by the present invention go a long way towards reducing
this probability. A further advantage of the present invention
is the simplified fuel processing made possible, wherein
preliminary pelletizing and sintering is avoided, and the wet
milling is carried out with powder in the green state. Whilst,
subsequent to the wet milling step for reducing the powder in
particle size and also in active surface area, the improved
powder may be satisfactorily pelletized without the step of
pre-compaction that otherwise would be provided.
It appears that the prior teachings cited by the Examiner are
defective, when viewed in light of the teachings of the present
application, because:
1) there is no recognition therein of the significant role
played by the reduction of active surfaces, by wet milling,
in controlling atypical grain growth;
2) the range of grain size obtained in the present process,
being about one order smaller than the sizes, where taught,
in the prior art (see Responsive letter of Jan. 16, 1981);
3) the effective cost reduction made possible in the overall
pelleting process, due to avoidance of having to fabricate
sintered pellets, prior to crushing and reprocessing the
material; and to avoidance of pre-compressing the pellets in
the green state, when pelletizing, by relying upon a single
compaction, made possible by the improved pouring
characteristics of the subject powder.
The consideration before the Board is whether or not the latest amended set
of claims are allowable over the art of record. Amended Claim 1 reads:
A process for providing a nuclear fuel green powder of reduced
susceptibility to atypical grain growth upon compaction, sintering
and prolonged hot soaking, including the step of diminishing the
active surface area of the fuel grains by wet milling to achieve a
grain size in the range substantially less than 25 micrometres.
Applicant maintains the improved pellet made by the present invention has
the advantage of simplified fuel processing wherein preliminary pelletizing
and sintering is avoided. Wet milling is carried out with powder in the
green state thereby reducing the powder in particle size so that it may be
satisfactorily pelletized without the step of pre-compaction previously
used. He argues that the cited prior art is defective because there is no
recognition of the significant role played by the reduction of active
surfaces by wet milling and the range of grain size obtained in the present
process being about one order smaller than the sizes in the prior art.
In the Final Action specific portions of the A.E.C. Publication were
detailed to show support for the refusal of the claims. Reference was made
to page 454 at lines 1 and 4 where studies of ball milling on the sintering
characteristics indicate that "a more uniform grain structure in the
sintered pellet is achieved" and "it was found that wet milling is more
efficient a process than dry milling". Reference is also made to lines 30
to 36 of the same page which emphasize the importance of UO2 powder
structure in the sintering characteristics of UO2 such as uniform grain
structure in the sintered material.
We note from the publication that particle size is detailed in several
locations. On page 417 it is stated that powder agglomerates broken up by
the action of a high-velocity fluid jet have been employed to prepare UO2
with particle diameters less than 1u. At page 454 particle size of less
than 3u are described. Figure 2 on page 416 shows particle sizes of UO2
powders ranging in size from 1 to 3u and figure 3 on page 418 shows similar
sized particles. Clearly then the range of size obtained by the
applicant's process is not smaller than shown in the prior art.
Grain size change associated with sintering is described on pages 454 to
466 inclusive of the A.E.C. Publication. Page 455 states there are "marked
differences between the microstructures of compacts sintered from
as-received MCW powder and from wet ball milled powder. The former are
heterogeneous both with respect to the spatial porosity distribution and
grain size, whereas the latter have uniform structures throughout (Figs
7 to 9)". At page 462 grain growth is related to time by a formula
utilizing mean grain diameter, a temperature constant and the exponential
characteristic of the material. Figure 17 on page 465 shows the rate of
grain growth utilizing cold pressed and sintered wet ball milled UO2
compacts. Page 477 states that the growth of grains will be controlled by
the activation energy which controls the disappearance of pores. Therefore
we conclude that the publication does recognize the role played by wet
milling in controlling grain growth.
The applicant argues that he has a method of preparing a nuclear fuel
pellet which reduces the probability of atypical grain growth, and provides
an effective cost reduction made possible in the overall process due to
avoidance of having to fabricate the sintered pellets material prior to
recrushing and reprocessing the material. In response to the Final Action
an amended set of claims was submitted but we are unable to find the argued
features in these claims. However, we note in the rejected set of claims
that refused claims 2, 6 and 7 when combined together would contain the
argued features and clear the cited art.
In view of the art of record therefore, we find that the amended claims do
not contain patentable subject matter and that the applicant's advance has
not been satisfactorily defined in any single claim of the rejected
claims.
We recommend that the decision in the Final Action to refuse the claims be
affirmed.
M.G. Brown S.D. Kot
Acting Chairman Member
Patent Appeal Board
I have reviewed the prosecution of this application and considered the
recommendation of the Patent Appeal Board. I concur with the reasoning and
the findings of the Board. Accordingly I refuse to grant a patent
containing either the amended claims, or the rejected claims as they
presently define the invention. The Applicant has six months within which
to appeal my decision under Section 44 of the Patent Act.
J.H.A. Gari‚py
Commissioner of Patents
Dated at Hull, Quebec
this 10th day of June 1987
R.A. Eckersley
1420 Dupont Street
Toronto, Ontario
M6H 2B2