CA1240497A - Method of controlling grain size in nuclear fuel compacts - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Satisfactory nuclear fuels made of natural uranium powders originating with a solid and liquid reaction process involving ammonium are prepared by a particle modification step wherein the relative active surface area of the fuel particles is reduced by wet attrition milling while in the green state. The grain size of the ceramic resulting therefrom is more uniform than for the unmodified powder, and undue grain growth of isolated particles is substantially eliminated, thus precluding or largely avoid-ing undue internal stresses and consequent pellet cracking under operating conditions.

Description

Case 2586 This inven-tion is directed to a system for the preparation of improved nuclear fuel and in particular to the provision of fuel pellets of enhanced operational characteristics.
The prepara-tion of uranium dioxide, so-called "natural uranium", may be effected by a variety of processes.
Owing to the formation of certain undesired by-products from certain of the well known liquid-to-liquid processes there has been a trend towards the adoption of liquid to solid processes for preparing nuclear fuel wherein a solid phase fuel component is always present during each step of the process. One such process, known as the AU process disclosed in Canadian Patent No. 1,059,729, issued August 7, 1979, T~W. Zawidzki reIies upon the interaction of ammonium nitrate with solid U03~
It is known that the presence of some impurity either ammonia (N~IS), Sulphur (S) or even U02 formed in the process from a different precursor at some time during the course of preparation of nuclear fuel leads to deleterious grain growth which occurs in the subsequent life of the material in fuel fabrication and irradiation.
Furthermore, for this or other undetermined reasons, fueI powder hatches from the AU process have given to be significantly inconsistent, and difficult to process into a consistent product.
Experimental fabrication of fueI pellets has been carried out using powder prepared by thb noted AU process, including the usual procedures involving pre-pelleting the powder in so-called "green'l form; sintering the green pellet to ceramic; crushing the sintered pellets to particles and powder; compac~ing the crushed ma-terial at high pressure to increase pellet densi-ty, a~d resintering -the pellet. I-t has been ~ound that, despi-te the extensive process involvedr such . !.

Case 2586 pellets have lacked consistency indensity, suffer a large variation in diametral shrinkage, and are prone to generating internal cracks, upon submitting them to a protracted hot soak.
The generation of such crack reduces the handleability of pellets during processing and is generally considered to be disadvantageous in -the operation in nuclear reactors.
Micrographic examination of the cracked pellets has revealed the presence of large grains, with evidence of undue grain growth in some of the grains.
It is postulated that cracking of the pellets is the end result of such a typical grain growth, and that this in turn stems from the presence of the afformentioned impurities, introduced during the course of preparation of the fuel.
A process has been evolved wherein the content of active surfaces of green fuel powder is reduced by the step of wet milling the powder within an attrition mill.
A wet milling process using passified steel balls has been evolved, disclosed in Canadian Patent Application No. 285,138 - August 19, 1977 - Lee et al, which does not form part of the present invention, but is suitable for providing the required powder attrition.
A dry attrition process for purposes of fuel powder densification is disclosed in Canadian Patent No. 656,281 -Moss - January 22, 1963.
The improved attrited fuel powder has been Eound to be very pourable, to the extent that in preparing pel:Let compacts in green form it has been found possible to achieve sufficiently high final density in the pellets without providing a pre-compac-tion step by the pellet press. ~rhus, -the ~tep of pre compa~tion has been obviated, ancl the pellet makin~

proceC3s correc3pond:ingly shortened from -the usual time ~ 7 Case 2586 requirement, in the provision of a green pe:Llet by means of a single compaction.
The pellet is then comple-ted, by sintering the green pellet. Pellet densities of the sintered pellet in excess oE 90% theoretical density have been achieved . Thus in practicing the present invention there is no need for the previous practice of further granulation, recompaction and re-sintering.
Examination of microscopic sections of pellets made in accordance with the improved process reveal a significant uniformity of grain size, with grains having a mean value of about 8.0 micrometres and a standard deviation o~ less than 1.5 micrometres.
Upon carrying out hot soak tests of the improved pellets it has been found that while there was substantial grain growth as expected, the growth was substantially uniform, leading to a mean grain si2e of about 21 micrometres, and a standard deviation of less than about 4.0 micrometres after hot soaking for 90 hours at 1973 K.
Of the grains lying outside the standard deviation, none were seen to exhibit the atypical grain growth pre-viously obser~ed in the cracked fuel, as prepared by the prior process. The observed results for a series of eleven such tests are tabulated below.
Thus, there is provided a process of preparing, from green powder susceptible of atypical grain ~rowth, an improved nuclear fuel formulation, comprising ~.he steps of;
wet attriting the green powder, in order to reduce the content of active surace o the powder; pouring the attrited powder into a mold; compressing the powcler -to a prede-termined state o~ compaction wi-thin the mold, and sin-tering -the powder to a ceramic p~LIe-t.

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~40 49~7 Case 2586 In -the foregoing process the step of compressiny the powder within the mold is a single step. Furthermore, no pre sintering of the material i5 provided, so that for general purposes a single sintering step only is used.

Claims (4)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. The method of preparing a nuclear fuel pellet having a density in excess of 90% theoretical when sintered, using as base stock a fuel powder normally subject to random grain growth after sintering, upon subjection to extended hot soak, including the steps of wet ball milling said powder to a predetermined grain size, to diminish the content of active surfaces thereof, compressing said milled powder to a compact by means of a single compaction step within a mold, and sintering the compact in a single sintering step, whereby upon subjecting said pellet to a prolonged hot soak the grain size is limited to a range generally not in excess of 25 microns.

2. The method as claimed in claim 1 wherein said wet ball milling step is effected prior to pelleting said powder.

3. The method as claimed in claim 1 or 2 wherein said grain size subsequent to said hot soak is maintained substantially uniform and predominantly in the range of 17-25 microns.

4. The method as claimed in claim 1 or 2 wherein said fuel is derived from the reaction of an ammonium based material with solid UO3.