Controlled atmosphere sintering process for urania containing silica additive

Improved method of sintering for the manufacture of nuclear fuel comprising a fissionable ceramic material including a silica containing additive. The method includes controlling the sintering atmosphere to impede loss through vaporization of the silica.

FIELD OF THE INVENTION
 This invention relates to the sintering process and conditions employed in
 the production of fissionable nuclear fuel comprising an oxide of uranium
 containing an additive having a silica constituent.
 BACKGROUND OF THE INVENTION
 Fissionable nuclear fuel for nuclear reactors typically comprise one of two
 principal chemical forms. One type consists of fissionable elements such
 as uranium, plutonium and thorium, and mixtures thereof, in metallic,
 non-oxide form. Specifically this category comprises uranium, plutonium,
 etc. metal and mixtures of such metals, namely alloys of such metals.
 The other principal type of nuclear reactor fuel consists of ceramic or
 nonmetallic oxides of fissionable and/or fertile elements comprising
 uranium, plutonium or thorium, and mixtures thereof. This category of
 ceramic or oxide fuels is disclosed, for example, in U.S. Pat. No.
 4,200,492, issued Apr. 29, 1980, and U.S. Pat. No. 4,372,817, issued Feb.
 8, 1983. Uranium oxides, especially uranium dioxide, have become the
 standard form of fissionable fuel in commercial nuclear power plants used
 for the generation of electrical power. However, minor amounts of other
 fissionable materials such as plutonium oxide and thorium oxide, and/or
 neutron absorbers, sometimes referred to as "poisons", such as gadolinium
 oxide, are sometimes admixed with the uranium oxide in the fuel product.
 Uranium oxide fuel is generally produced by converting uranium hexafluoride
 or uranium metal to oxides of uranium. The process includes a series of
 chemical and physical operations, including pressure compacting uranium
 oxide in particulate form into handlable pellets or physically integrated
 bodies of suitable size and configuration, then sintering the resultant
 pellets or bodies of compacted particles. Sintering at high temperature
 coalesces the compacted particles of each pellet or body into an
 integrated unit of high density, and produces other desired effects such
 as manipulating the molecular oxygen content of the material and removal
 of residual undesirable impurities, e.g. fluorides.
 Sintering processes are amply disclosed in the art, for example U.S. Pat.
 No. 3,375,306, issued Mar. 26, 1968; U.S. Pat. No. 3,872,022, issued Mar.
 18, 1975; U.S. Pat. No. 3,883,623, issued May 13, 1975; U.S. Pat. No.
 3,923,933, issued Dec. 2, 1975; U.S. Pat. No. 3,930,787, issued Jan. 6,
 1976; U.S. Pat. No. 4,052,330, issued Oct. 4, 1977; and U.S. Pat. No.
 4,348,339, issued Sep. 7, 1982.
 Fissionable nuclear fuel materials for commercial power generating, water
 cooled and/or moderated reactors, commonly comprising pellets of uranium
 oxide, are typically enclosed within a sealed container formed of an alloy
 of zirconium metal, such as zircaloy -2 (U.S. Pat. No. 2,722,964), or
 possibly stainless steel, to provide a fuel element. The container,
 sometimes referred to in the nuclear field as "cladding", generally
 comprises a tube-like or elongated enclosure housing fuel pellets stacked
 therein end-on-end to the extent of about 3/4 of the length of the
 containers.
 Fissionable fuel is enclosed and sealed in such containers for service in
 nuclear reactors to isolate it from contact with the coolant and/or liquid
 moderator. This precludes either any reaction between the fuel or fission
 products and the coolant or moderator media, or contamination of the
 coolant or moderator with escaping radioactive matter from the fuel or
 fission products.
 Experience has shown that after extensive exposure to the radiation in the
 core of an operating nuclear reactor, typical fuel elements consisting of
 the fissionable fuel sealed within a metal container are susceptible to
 failures due to breaching of their containers during or following rapid
 power increases. Fuel container breaching has been determined to be a
 result of a combination of conditions, namely, stress imposed upon the
 metal by thermal expansion of the contained fuel, embrittlement of the
 metal by prolonged exposure to radiation and stress corrosion cracking
 susceptibility by the presence of accumulated fission products from the
 fuel enclosed therein.
 Studies of this deleterious phenomenon have determined that three
 conditions contribute to produce such a failure of the metal fuel
 container, which is commonly referred to in the art as "intergranular
 stress corrosion cracking". First, the metal must be susceptible to stress
 corrosion cracking in the irradiation environment; second, a level of
 physical stress must be present; and, third, there must be exposure to
 aggressive corrosive agents. Metal failure due to stress corrosion
 cracking can be mitigated or even eliminated by alleviating any one or
 more of these three conditions.
 One effective means for deterring such failures in conventional fuel
 elements comprising zirconium alloy containers housing uranium oxide fuel
 has been to include a metallurgically bonded barrier liner of unalloyed
 zirconium metal over the inner surface of the alloy container substrate.
 The unalloyed zirconium metal of the barrier liner is more resistant to
 irradiation embrittlement than the alloy substrate whereby it retains its
 initial relatively soft and plastic characteristics throughout its service
 life notwithstanding prolonged exposure to irradiations, etc. Localized
 physical stresses imposed on such a barrier lined fuel container by heat
 expanding fuel during rapid power increases are moderated by the plastic
 movement of the relatively soft unalloyed zirconium metal of the liner.
 Moreover, the unalloyed zirconium metal has been found to be less
 susceptible than alloys to the effects of corrosive fission products. That
 is, the unalloyed zirconium has resistance to the propagation of cracks in
 the presence of corrosive fission products.
 The effectiveness of the unalloyed zirconium barrier liners in resisting
 the deleterious stress corrosion cracking phenomenon due to the
 interaction between the fuel pellets and the container in the presence of
 a corrosive environment of irradiation products, is achieved by mitigating
 the physical stress and stress corrosion crack propagation susceptibility
 of the zirconium barrier layer. Effective unalloyed zirconium metal
 barrier linings for nuclear fuel elements comprising fuel pellets enclosed
 within a container are disclosed in U.S. Pat. No. 4,200,492 and U.S. Pat.
 No. 4,372,817.
 Another approach to this problem of stress corrosion cracking as a cause of
 failure of fuel elements when subjected to frequent and drastic power
 increase has been to modify the physical properties of the uranium oxide
 fuel with the inclusion of additives. For example, aluminum silicates,
 derived from clays, when dispersed throughout the uranium oxide in amounts
 as low as a few tenths of one percent, have been demonstrated to be
 effective in increasing the plasticity of fuel pellets composed thereof,
 whereby the thermal expansion induced physical stress attributable to the
 fuel pellets is reduced. The aluminum silicate may also play a role in
 reducing the effectiveness and availability of the chemically aggressive
 fission products which promote stress corrosion cracking of the cladding
 tubes.
 Aluminum silicate additives blended with uranium oxide have been found to
 be effective in eliminating or mitigating two of the three conditions
 which must be simultaneously present to produce stress corrosion failures
 in the metal of a fuel container. An aluminum silicate additive
 substantially increases the creep rate of fuel pellets comprising oxides
 of uranium and thereby reduces the stress imposed on the container due to
 thermal expansion of the fuel material. The enhanced plastic deformation
 and deformation rates attributable to this additive enables the modified
 fuel to flow into its own void volume or other free space in the fuel rod
 within the interior of the fuel container, and thereby reduce the stress
 applied to the cladding. Thus high localized stresses are mitigated by
 increased distribution of their forces.
 Moreover, the aluminum silicate introduced into the fuel material reacts
 with fission products produced during irradiation. This reduces the
 concentration of aggressive fission products which, in the presence of
 physical stresses, are a cause of cracking in the metal of the fuel
 containers.
 The effects of additives comprising aluminum silicates upon fissionable
 nuclear fuels, including their relative quantities, are disclosed in U.S.
 Pat. No. 3,679,596; U.S. Pat. No. 3,715,273; U.S. Pat. No. 3,826,754; U.S.
 Pat. No. 3,872,022; and U.S. Pat. No. 4,052,330.
 However, experience in the processing or fabrication of aluminum silicate
 containing ceramic fuels comprising oxides of fissionable elements
 employing the conventional sintering procedures and conditions used for
 ceramic fuel has demonstrated the occurrence of distinctive shortcomings
 in the resulting products. Specifically, it has been found that there
 occurs inconsistencies in the concentrations of aluminum silicate added
 and in achieving the final fuel densities desired.
 The conventional sintering procedures and conditions commonly used in
 producing fuel with uranium oxides, such as disclosed in the foregoing
 patents, comprises employing reducing conditions to provide for an oxygen
 to metal ratio of the fuel material of near or at the desired
 stoichiometric composition of O/M=2.00 (UO.sub.2) during and following the
 sintering operation. For example, hydrogen or cracked ammonia sintering
 atmospheres with relatively low dew points, such as &lt;10 degrees C., or
 hydrogen/carbon dioxide gas mixtures or carbon monoxide/carbon dioxide gas
 mixtures with their ratios proportionally adjusted to produce near the
 stoichiometric UO.sub.2 compositions are typically used in sintering.
 Reducing conditions with high sintering temperatures, such as about 1600
 degrees C. or higher result in a relatively high vapor pressure of silicon
 monoxide (SiO) over silicon dioxide (SiO.sub.2) and aluminosilicate,
 amounting to as much as a few tenths of an atmosphere. See for instance
 "Graphical Displays of the Thermodynamics of High Temperature Gas-Solid
 Reactions and Their Application to Oxidation of Metals and Evaporation of
 Oxides" by Lou et al, Journal of the American Ceramic Society, Vol. 68,
 No. 2 February 1985, pages 49-58.
 Due to such high SiO vapor pressures, there is considerable volatilization
 of the silica bearing material from a uranium oxide material such as a
 fissionable fuel composition containing an aluminosilicate or silica
 bearing phase. Such a loss of silica material presents difficulties in
 controlling the amount of silica containing additives present in a fuel
 product. Moreover, because of the high vapor pressure of SiO over the
 silica containing additive phase, pores or voids formed within the
 additive phase are stabilized and achieving the desired final density is
 inhibited.
 The disclosed contents of the foregoing U.S. Pat. No. namely U.S. Pat. No.
 3,375,306; U.S. Pat. No. 3,679,596; U.S. Pat. No. 3,715,273; U.S. Pat. No.
 3,826,754; U.S. Pat. No. 3,872,022; U.S. Pat. No. 3,883,623; U.S. Pat. No.
 3,923,933; U.S. Pat. No. 3,930,787; U.S. Pat. No. 4,052,330; U.S. Pat. No.
 4,348,339; U.S. Pat. No. 4,578,229; U.S. Pat. No. 4,200,492; and U.S. Pat.
 No. 4,372,817, which illustrate the state of the art relevant to the
 invention disclosed and claimed herein, are each incorporated herein by
 reference.
 BRIEF SUMMARY OF THE INVENTION
 This invention comprises an improved method of producing nuclear fuel
 products comprising an oxide of uranium incorporating a silica containing
 additive. The invention includes a high temperature sintering procedure
 wherein the atmospheric composition is regulated to inhibit losses of the
 silica containing additive.
 OBJECTS OF THE INVENTION
 It is a primary object of this invention to provide an improved method of
 producing a fissionable nuclear fuel product comprising an oxide of
 uranium and a silica containing additive.
 It is also an object of this invention to provide an improved procedure for
 sintering a nuclear fuel composition comprising an oxide of uranium and a
 silica containing additive in the manufacture of fissionable fuel
 products.
 It is a further object of this invention to provide a production procedure
 for manufacturing nuclear fuel comprising uranium oxide with a silica
 containing additive which inhibits loss of the silica containing additive
 during sintering.
 It is an additional object of this invention to provide a method for
 manufacturing nuclear fuel comprising uranium oxide with an aluminum
 silicate additive which enables governing of the product density.
 It is a still further object of this invention to provide a means of
 impeding loss of SiO and in turn unwanted compositional changes during
 sintering.
 It is a yet further object of the present invention to provide a method for
 manufacturing nuclear fuel comprising uranium oxide with an aluminum
 silicate additive which allows control of the aluminum-silicate content of
 the product.
 DETAILED DESCRIPTION OF THE INVENTION
 This invention deals with nuclear fuel products produced from fissionable
 materials comprising oxides of uranium including a silica containing
 additive such as disclosed in the above patents. The fissionable material,
 in addition to the uranium oxide and silica containing additive, can also
 include oxides of plutonium or thorium, neutron absorbers or "poisons"
 such as gadolinia, and combinations thereof, among other ingredients
 disclosed in the above cited prior art. The oxides of uranium and other
 fissionable ceramics preferably have an oxygen to metal ratio (O/M) of
 approximately 2.00, namely substantially composed of uranium dioxide
 (UO.sub.2).
 The silica containing additives which are a fundamental component of this
 invention, likewise include those disclosed, and their amounts, as given
 in the above cited patents. Specific silica containing additives include
 silicon dioxide (SiO.sub.2), aluminum silicates (Al.sub.2
 O.sub.3.SiO.sub.2), natural minerals such as mullite (3Al.sub.2
 O.sub.3,.2SiO.sub.2), pyrophillites (Al.sub.2 O.sub.3.SiO.sub.2),
 kaolinite (Al.sub.2 (Si.sub.2 O.sub.3).(OH).sub.4), andalusite (Al.sub.2
 SiO.sub.3), sillimanite (Al.sub.2 SiO.sub.5), and cyanite (Al.sub.2
 SiO.sub.5), for example. It is also possible to employ a mixture of
 alumina powder and silica powder, wherein the alumina and silica are
 present in a ratio by weight from about 0.1 alumina to 0.9 silica to about
 0.9 alumina to 0.1 silica.
 Alternatively, it is possible to introduce each of the silicon and aluminum
 as a compound which decomposes to silica and alumina under the conditions
 of sintering. For example, the aluminum, or at least a portion of it, may
 be added as an organoaluminum compound, such as for example aluminum
 bistearate, diethylaluminum malonate or triphenyl aluminum. The aluminum
 compound, especially the bistearate, would act as a pressing die
 lubricant, and leave alumina when the hydrocarbon portion is volatilized.
 An organosilicon compound may be used for the silica addition, such as for
 example a volatile silicon compound that will vaporize early in the
 sintering process. Examples include silicobenzoic acid,
 triethylphenylsilicane, ethyltriphenylsilicane and methyltriphenyl
 silicane. The organosilicon compound would produce the fugitive silicon
 which would be converted to silica in the sintering furnace, and would act
 as a pore former to control the density and structure of the sintered
 pellets.
 The particle sizes of the alumina and silica powders may range from about
 0.01 micrometers to about 100 micrometers, more usually about 0.1 to about
 10 micrometers.
 The silica containing additives may be present in an amount of, for
 example, about 0.025 percent up to about 5.0 percent by weight of the
 overall fuel material. Generally the silica containing additives are
 present in an amount of about 0.025 percent up to about 1.0 percent by
 weight of the overall fuel material.
 With the sintering conditions commonly employed in the manufacture of
 uranium oxide fuel, the vapor pressure of SiO is strongly dependent upon
 temperature and oxygen free energy. The process is typically carried out
 at a temperature of at least about 1600 degrees C., more usually at least
 about 1700 degrees C. At 1700 degrees C., the SiO vapor pressure can range
 from approximately 10.sup.-6 (0.000001) to 10.sup.-1 (0.10) atmospheres,
 note "Review-Graphic Displays of the Thermodynamics of High Temperature
 Gas-Solid Reactions and Their Application to Oxidation of Metals and
 Evaporation of Oxides", by Lou et al, supra. At the typical sintering
 conditions used for urania based nuclear fuels, about 1600-1800 .degree.
 C., the vapor pressure of SiO is near 10.sup.-2 (0.01) atmospheres. Under
 such conditions, there can occur a considerable loss of any silica bearing
 material.
 In accordance with this invention, the oxygen free energy of the sintering
 atmosphere is increased during the sintering procedure. Such an increase
 of oxygen free energy has been determined to decrease the vapor pressure
 of SiO a significant amount, namely by several orders of magnitude. For
 instance, when the dew point of a cracked ammonia sintering atmosphere is
 increased from about 10 degrees C. up to about 120 degrees C., the SiO
 vapor pressure during sintering at about 1700 degrees C. decreases from
 approximately 0.1 atmospheres down to only approximately 0.0001
 atmospheres. The rate of volatilization of SiO from the sintering uranium
 ceramic is similarly decreased by about three orders of magnitude, thus
 mitigating the conditions substantially responsible for the problems of
 composition variations and density control due to SiO vaporization.
 Generally, in the present invention, the sintering process for uranium
 oxide based nuclear fuel materials containing silicon dioxide or aluminum
 silicate additives is performed in an atmosphere which produces a low SiO
 vapor pressure by providing and maintaining the partial molar free energy
 of oxygen therein of greater than -90 kilocalories per mole.
 Oxygen partial molar free energy can be regulated by manipulating the gas
 composition of the sintering atmosphere such as by applying specific gases
 and or by proportioning the ratios of mixtures of gases. For example, the
 sintering atmosphere conditions can be achieved through the application of
 wet hydrogen, wet cracked ammonia (or 25% nitrogen--75% hydrogen),
 mixtures of carbon monoxide/carbon dioxide gases and mixtures of
 hydrogen/carbon dioxide gases in appropriate ratios.
 Generally, sintering temperatures for the practice of this invention fall
 within a range of from about 1600 degrees C. up to about 2200 degrees C.
 More usually, the sintering is carried out within the range of about 1600
 degrees C. to about 2000 degrees.

The invention will now be described with reference to the following
 non-limiting example.
 EXAMPLE
 Alumina and silica powders in a weight ratio of 0.4 Al.sub.2 O.sub.3 /0.6
 SiO.sub.2 are blended with uranium dioxide powder to achieve a total
 addition of 0.25 wt % of the alumina/silica with 99.75% uranium dioxide.
 The blended powders are dry-pressed to a green density of approximately
 5.6 gm/cm.sup.3 to form powder compacts in the form of right circular
 cylinders for sintering to fuel pellets.
 The dry pressed pellets are sintered using a furnace feed gas of 75%
 hydrogen--25% nitrogen which has been moisturized by passing the gas
 through a water bubbler with the temperature of the water in the bubbler
 maintained at 55.degree. C. and a total furnace gas pressure of 1
 atmosphere (760 mm Hg). At 55.degree. C., the vapor pressure of water is
 118 mm Hg, the hydrogen and nitrogen gas pressures of the furnace feed gas
 are 481.5 and 160.5 mm Hg, respectively, and the H.sub.2 O to H.sub.2
 ratio of the furnace gas atmosphere is 118/481.5=0.245.
 The sintering furnace temperature profile is maintained to provide
 prolonged (.about.4 hours) sintering at 1750.degree. C. in the hot or
 working zone of the sintering furnace. At that sintering temperature, for
 the H.sub.2 O to H.sub.2 ratio noted above, the oxygen free energy in the
 hot zone of the sintering furnace is maintained at about -70 kcal/mole,
 the O/U ratio of the uranium oxide during the sintering operation is
 maintained at about 2.005, and the vapor pressure of SiO is maintained at
 about 10.sup.-5 (0.00001) atmospheres. For these sintering conditions, the
 desired final fuel pellet density of 10.5 gm/cm.sup.3 is achieved, and the
 aluminum and silicon contents of the final sintered pellets are within
 acceptable ranges of the initial amount added.
 While the invention has been described in connection with what is presently
 considered to be the most practical and preferred embodiment, it is to be
 understood that the invention is not to be limited to the disclosed
 embodiment, but on the contrary, is intended to cover various
 modifications and equivalent arrangements included within the spirit and
 scope of the appended claims.