Patent Number: 050376063
Section: summary

The invention relates to nuclear fuel particles less than a few millimeters in size and to methods of making nuclear fuel compacts from such particles for use in nuclear reactors. More particularly, the invention relates to improved nuclear fuel particles having fission-product-retentive coatings which are able to withstand high pressures to which they may be subjected during the formation of dense, nuclear fuel compacts and to methods for producing compacts having few fractured particle coatings therein. BACKGROUND OF THE INVENTION Pyrolytic carbon coatings have been used to protect particles of nuclear reactor fuel, i.e., fissile and/or fertile materials, such as uranium, plutonium and thorium in the form of suitable compounds thereof. Coatings of aluminum oxide and other ceramic oxides have also been proposed. Examples of nuclear fuel particles employing pyrolytic carbon coatings include U.S. Pat. No. 3,325,363, issued June 13, 1967; U.S. Pat. No. 3,298,921, issued Jan. 17, 1968, and U.S. Pat. No. 3,361,638, issued Jan. 2, 1968. It is also known to incorporate one or more layers of refractory carbide materials, such as silicon carbide or zirconium carbide, to produce nuclear fuel particles having still better fission product retention characteristics, as disclosed in U.S. Pat. No. 3,649,472, issued Mar. 14, 1972. So long as these fission product retentive coatings remain intact, contamination exterior of the particles by the heavy metal fuel material and/or substantial spread of fission products exterior of the coatings is prevented. Such nuclear fuel particles are usually bonded together in some fashion to create what is termed in the art as a nuclear fuel compact, which is produced using a suitable binder and appropriate pressures. It has been found that fracture and/or cracking of the fission product retentive coatings often occurs during the formation of nuclear fuel compacts wherein these nuclear fuel particles are combined under high pressure with a binder material to produce a relatively dense "green" compact that is later subjected to high temperatures to produce the final nuclear fuel compact suitable for use in a nuclear reactor. It is also known to produce nuclear fuel compacts or nuclear fuel elements for a Pebble-Bed reactor or the like by blending such coated nuclear fuel particles with a carbonaceous thermosetting resin in a powder form and compressing the coated particle-resin mixture under pressures in excess of 20,000 psig to form "green" compacts, and sometimes these particles have been pre-treated with the resin. Nuclear fuel particles which can better tolerate such manufacturing processes are constantly being sought after. BRIEF SUMMARY OF THE INVENTION The invention minimizes the occurrence of fracture and/or cracking in the fission-product-retentive coatings by protecting coated particles by the use of appropriate overcoatings to allow them to achieve high loadings to meet overall nuclear fuel compact specifications. The employment of overcoating material having a density not greater than about 60% of its theoretical maximum density has been found to provide adequate protection for the more fragile fission-product-retentive layers during the green compacting steps when such coated particles are subjected to relatively high pressures. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Very generally, nuclear fuel particles are provided which have central cores of fissile or fertile material surrounded by multiple layers of materials designed to retain within the confines thereof substantially all of the fission products created during burnup of the fissile atoms to a reasonable level. Various layers of materials, such as pyrolytic carbon and silicon carbide, as are known in the art, or other comparable fission-product-retentive materials, can be employed which provide good structural and dimensional stability and fission-product retention even when exposed to high temperatures in high level irradiation for long periods such as will be encountered in the core of a nuclear power reactor. Other suitable fission-product-retentive materials can also be used as a part of the overall fission-product-retentive coating arrangement that surrounds the fissile or fertile cores while still obtaining the benefit the overcoating provides to avoid fracture and/or cracking. Although the central cores of nuclear fuel material may have different shapes, they are normally spheroidal in shape, and generally the diameter of the spheroid will be not greater than about 1 millimeter (1,000 microns). Usually, nuclear fuel will be in the form of spheroids between about 100 microns and about 500 microns in diameter. Preferably, fissile fuel cores have a diameter not greater than about 550 microns, and preferably fertile fuel cores are not greater than about 650 microns in diameter. Such so-called fertile fuel cores may contain mixtures of both fissile and fertile materials, for example, mixtures of uranium and thorium compounds. Core materials in the form of oxides or carbides or mixtures thereof are generally used, although other suitable forms, such as the nitride or the silicide, which are stable at relatively high temperatures, could alternatively be employed. Preferably, the fissile fuel cores are formed of mixtures of uranium oxide and uranium carbide; however, uranium oxide could be employed. On the other hand, fertile fuel cores should contain a suitable, high-temperature, stable thorium material, such as thorium oxide or thorium carbide; and a mixture of thorium carbide and thorium oxide or a mixture of thorium oxide and uranium oxide might be employed. Because nuclear fuel materials generally expand during high-temperature operation and create gaseous and metallic fission products during fissioning, it is well known to make provision to accommodate these effects in order to facilitate prolonged operation under exposure to nuclear flux. Because the density of the core material is usually dictated by other manufacturing process considerations and/or design criteria, cores are normally of relatively dense material and thus unable to accommodate the accumulation of such gaseous fission products within the core region itself. As a result, an initial layer of relatively low density material is provided near the surface of the core to accommodate expansion at a location interior of the outer coatings which constitute the pressure-tight shell and to also accommodate gaseous fission products. The layer which surrounds the core should also be chemically compatible with the core material, both in the environment in which it is deposited and within the nuclear reactor where levels of high neutron flux will be accommodated. Spongy, pyrolytic carbon, which is a soot-like amorphous carbon having a diffuse X-ray diffraction pattern, is well known in the art and commonly employed for this purpose. Such spongy pyrocarbon also attenuates fission recoils and prevents structural damage to the outer layers, and as such it is generally employed somewhere between 20 microns and about 130 microns in thickness, with a thickness of about 50 to 60 microns often being used. The exterior layers which create the pressure-tight shell often combine layers of relatively dense isotropic pyrolytic carbon and one or more layers of silicon carbide or zirconium carbide of sufficient thickness to provide good retention of metallic fission products. In general, dense, isotropic, pyrolytic carbon has good dimensional stability and, as such, is often provided both immediately interior of and exterior of the silicon carbide layer. The interior layer may be about 40-50 microns thick. Generally, a continuous layer of silicon or zirconium carbide between about 20 microns to 45 microns in thickness is employed to assure adequate containment of metallic fission products is achieved. Such silicon or zirconium carbide layers can be applied in any suitable manner to achieve satisfactory densities which are usually at least about 90% of the theoretical maximum density of the carbide material. Such layers can be advantageously deposited from a vaporous atmosphere in a fluidized bed coating apparatus or the like as, for example, that described in detail in U.S. Pat. No. 3,298,921. For example, silicon carbide can be directly deposited from a mixture of hydrogen and methyltrichlorosilane, which easily produces densities of about 99% of maximum theoretical density. Dense isotropic carbon has both good impermeability to gas and good dimensional stability during neutron irradiation, and generally its isotrophy should measure not more than about 1.2 on the Bacon scale. Such dense isotropic pyrolytic carbon can be deposited at relatively low temperatures, e.g., 1250.degree. to 1400.degree. C. or at temperatures at between about 1800.degree. to 2200.degree. C. At higher temperatures, a gas mixture containing about 10% by volume methane can be used, whereas at lower temperatures mixtures of about 20-40% propane or butane can be used. In general, about 25-50 microns of dense isotropic pyrolytic carbon is employed exterior of the metal carbide layer, and it should have a density of at least about 80% of the theoretical maximum density, e.g., about 1.85 to 1.95 g/cm.sup.3. The foregoing describes various of the multiple layer fission-product-retentive coating arrangements that can be used to provide a pressure-tight shell about a nuclear fuel material core, although, as indicated hereinbefore, other suitable fission-product-retentive arrangements can be employed. It is contemplated that these fission-product-retentive nuclear fuel particles should retain substantially all of the fission products generated therewithin throughout a burnup of up to about 30% of the fissile and/or fertile atoms present in the core. Very generally, the exterior dimension of the coated nuclear fuel particle will usually not exceed the range of about 3 to 5 millimeters, even if a nuclear fuel core as large as about 1 millimeter were employed. The protective overcoating is disposed exterior of the outermost layer of the fission-product-retentive shell and has a density of about 60% of its theoretical maximum density or less. As indicated above, normally the exterior surface of the fission-product-retentive arrangement, or a layer very close thereto, will have a density equal to at least about 80% of its theoretical maximum density, and it is this relatively brittle or fragile material to which the overcoating material affords mechanical protection during the ensuing fabrication process. The preferred overcoating material is pyrocarbon having a density not greater than about 1.4 grams per cm.sup.3, and preferably the pyrocarbon is isotropic pyrocarbon having a density between about 0.8 and about 1.4 grams per cm.sup.3. To afford adequate protection, it is believed that the thickness of the protective pyrocarbon should measure at least about 20 microns. Although there is no reasonable upper limit to the thickness of such a layer from the standpoint of affording protection, the necessity to provide adequate nuclear fuel loading within certain spatial parameters places constraints upon the maximum thickness of the overcoating as it does on the maximum thickness of the pressure-tight shell. For this reason, it is felt that a protective overcoating between about 20 and about 70 microns in thickness will be used, and preferably between about 30 and 60 microns of pyrocarbon is employed. More preferably, a pyrocarbon overcoating is used having a thickness of at least about 30 microns and a density between about 1.0 and about 1.3 grams per cm.sup.3. Although pyrocarbon is the preferred protective overcoating material, other chemically compatible substances having suitable nuclear properties might alternatively be employed. For example, aluminum oxide might be employed as a protective overcoating and when used as such might have a density between about 1.5 and about 2.0 grams per cubic centimeter. Of course, the exterior diameter of the coated nuclear fuel particle which includes the protective overcoating will vary depending upon the size of the core and the size of the pressure-tight shell surrounding the core. Preferably, however, the outer diameter of fertile nuclear fuel particles does not exceed about 1300 microns, and the outer diameter of particles having fissile fuel cores does not exceed about 1200 microns. To form the fuel compacts usable in a nuclear reactor, the coated fuel particles having these protective overcoatings are combined in precise amount with a flowable hardenable binder under pressure in a mold of the desired size and shape. Following the hardening of the binder, a nuclear fuel compact of the desired fuel loading is achieved. To achieve the desired fuel density within this compact, the particles and binder are subjected to relatively high pressure, and pressures of at least about 600 psig are commonly employed. Moreover, after the coated particles have been supplied to the mold and before the binder is supplied, the overcoated nuclear fuel particles are often subjected to pre-compacting pressures. For example, pressures between about 100 psig and about 600 psig may be employed. Suitable methods for forming nuclear fuel compacts from coated particles are disclosed in U.S. Pat. No. 4,024,209, the disclosure of which is incorporated herein by reference. Various binders can be used, including binders that are flowable as a result of being in a molten condition and which are hardened by cooling. More commonly, binders of pitch, such as petroleum pitch or coal tar pitch, particularly in mixture with a graphite powder or flour, are used. Suitable compositions of this type, including pitch and certain alcohol and fatty acid additives, are disclosed in U.S. Pat. No. 4,217,174, issued Aug. 12, 1980, the disclosure of which is incorporated herein by reference. Alternatively, other types of resins, such as phenolic resins or furfural resins, which can be carbonized may also be used. The preferred mixtures of petroleum pitch and graphite flour, which is relatively fine particle size graphite of less than about 40 microns, are hardened by heating to a temperature of at least about 1000.degree. C. Generally, so as not to unduly delay fabrication time and so as to assure that complete carbonization is achieved, temperatures of as high as about 2100.degree. C. may be employed. Following cooling to room temperature, the compacts are examined using tests to determine the extent of heavy metal (fissile or fertile) material which is leached from the compacts and to determine which particles suffered fracture damage such as to indicate a substantial loss of the fission-product-retention capability. The amount of contamination detectable from compacts made using features of the invention is a small fraction of that detected following the formation of comparable nuclear fuel compacts from coated nuclear fuel particles which are the same in all respects except for the absence of the protective overcoatings. Such tests show the effectiveness of the overcoatings in protecting the integrity of the pressure-tight shells during the compacting of the green material. Moreover, testing of these compacts following substantial neutron irradiation to a significant burnup of the nuclear fuel also shows equally significant improvement in fission-product retention over compacts made from particles without such protective overcoatings and confirms the test results are obtained by burning one of the compacts in order to ascertain the continued integrity of SiC layers.