Patent Number: 040175677
Section: summary

The invention is directed to the process of producing block fuel elements for gas cooled high temperature fuel reactors. The known block fuel elements are, for example, hexagonal prisms made of graphite and having a width over the flats of the hexagon of 360 mm and a height of 793 mm., see Docket 50267 -14(November 1969) page 3.4-1 Fort St. Vrain Nuclear Generating Station Final Safety Analysis Report. In the inside of the prism there are found in hexagonal arrangement about 320 bore holes running parallel to the longitudinal axis. Two thirds of these bore holes serve to receive the fuel containing cylinders and the remainder serves as channels for helium-cooling gas. The fuel cylinder consists of a carbon matrix in which the fuel and fertile material is embedded in the form of coated particles. The coated particles are spherical heavy metal oxide or carbide cores of several hundred microns diameter which preferably are coated several times with pyrolytically deposited carbon. In general as fuels there are used U 235, U 233 and fissionable plutonium isotopes. As fertile material there is employed thorium or uranium 238. The coating has the function of largely retaining the fission products formed in the particles. The total volume of the block fuel elements amounts to 89 liters. This is distributed as 18.5 volume percent cooling channels, 23.5 volume % fuel bore holes and 58 volume percent block graphite, which forms the fuel element structure. Furthermore, there are known, for example, fuel elements having a width over the flats of hexagon of 383 mm. and a height of 1050 mm. (see D.F.I. Bishop. Factors Affecting the Costs of Fabricating HTR Fuel. Dragon Project Fuel Symposium Paper, October 1969). The fuel element prism has only 18 hexagonally arranged fuel holes of 63 - 70 mm. diameter in which 36 graphite containers (two per bore hole) stand one on top of the other. Between the bore holes and graphite containers there is found a 5 mm. Wide annular gap for helium gas. The graphite container is a 500 mm. long tube in which 10 annular fuel containing compacts are piled up one on the other. The compacts consist of a graphite matrix with pressed in coated fuel particles. Of the 133liters of total fuel element volume 18.5 volume % is cooling channels for helium gas, 11 volume percent is fuel compacts and 70.5 volume percent is the structural graphite. The classification clearly shows that only 23.5 or 11 percent of the fuel element volume can be filled with fuel. In contrast the structural graphite requires the largest volume portion, i.e. 58 or 70.5 percent. In order to better utilize the fuel element volume it has been proposed to employ molded block fuel elements, Hrovat, U.S. Ser. No. 3284 filed Jan. 16, 1970, corresponding to German application P 19 02 994.8 filed Jan. 22, 1969. In contrast to the previously named types of fuel elements the molded block fuel element is a compact prism provided with cooling channels, which consist of only a homogeneous graphite matrix and coated fuel particles. It is essential that the graphite matrix in which the coated particles are impressed simultaneously form the fuel element structure. Consequently in relation to the portion of fuel particles, a far greater fuel volume is available. Besides there is eliminated the gap acting as heat flow barrier between the fuel zone and structural graphite. Additionally, at unchanged fuel element loading, the power density in the fuel zone is strongly reduced, the heat output considerably improved and correspondingly the temperature gradient and consequently the thermal and radiation induced stress greatly reduced. Moreover, the lower stress and the improved efficiency of the prism volume permits a several fold increase of the fuel and fertile material content in the fuel element, whereby the construction of the cooling channels (volume and surface area) can be adjusted without limitation of the sides of the fuel elements to the optimum cooling conditions. The increase fuel load considerably reduces the cost of producing the fuel element and simultaneously leads to higher powder density in the reactor core and also a lower capital cost, see R.C. Dahlberg "Comparison of HTGR Fuel Cycles for Large Reactors", Oak Ridge --Symposium April 1970, Paper No. 130, Session No. VI. The possibility of laying out the cooling channels without limitation reduces the helium pressure drop in the reactor core and accordingly the necessary pumping power for the helium cycle, which again reduces the cost of the generation of current. Besides the graphite matrix serves as moderator, heat conductor, secondary barrier for the fission products and protects the coated particles against a damaging corrosion by impurities which are present as traces in the helium cooling gas. A series of requirements are placed on the graphite matrix. 1. Good irradiation behavior up to temperatures of 1400.degree. C and to neutron exposure of about 7 .times. 10.sup.21 neutrons/cm.sup.2 (E&gt;o,1MeV). This requirement assumes an as much as possible high crystallinity of the isotropic graphite matrix. 2. Good thermal conductivity and an as low as possible coefficient of thermal expansion in order that entry of inadmissible thermal stresses in the block fuel element be avoided. 3. Good strength properties. 4. Good corrosion resistance Furthermore, in the production there is required a non destructive consolidation of the coated fuel particles into the graphite matrix. The present invention avoids the technological difficulties of the known processes and permits the production of a block fuel element of any size and shape satisfying all requirements. According to the invention there is first produced from molding powder as shown in example 1 by molding spheres in rubber molds at room temperature and at 3000 kg/cm.sup.2 and comminuting these spheres an isotropic graphite granulate of high density having a definite porosity. The molding powder for the production of granulates consists of a mixture of natural graphite and binder resin, synthetic graphite and binder resin, or a mixture of both types of graphite powder with binder resin. When a mixture of natural and synthetic graphite are employed, they can be used in any proportions, e.g. 1 to 99 percent of either by weight. The isotropic graphite granulate produced in the first step has an apparent density between 1.5 g/cm.sup.3 and 1.85 g/cm.sup.3 or as shown in example 1 even 1.9 g/cm.sup.3 and a porosity of 25 to 7.5 percent by volume. The molding pressure in the first step as shown in example 1 can be 3 t/cm.sup.2 (i.e., 3 metric tons/cm.sup.2). The temperature in the first step can be room temperature. The binder resin employed, for example, can be phenolformaldehyde, with a softening point of about 100.degree. C but phenolformaldehyde resins with other softening temperatures between 60.degree. and 120.degree. C or with addition of curing agents as for example hexamethylene tetramine or other formaldehyde resins for example on xylol or cresol base or furfurylalcohol resions can be used. The binder resin can be used in an amount of 10 to 30 percent of the graphite by weight. For the production of the isotropic granulate according to the invention in the first step, a fine graphite powder, e.g. about 20 microns in diameter, having a high crystallinity, is molded at high pressure with a binding agent additive, preferably phenol-formaldehyde resin, in a rubber mold to isotropic spheres. Subsequently the spheres are ground to granules having an average grain diameter of about 1 mm. The degree of fineness of the starting graphite powder is so chosen that on the average each granulate grain consists of several hundred thousand or even about 1,000,000 isotropically arranged graphite particles. For the production of the molding powder any graphite, independent of particle form is suited, for example, natural graphite powder, synthetic graphite powder or a mixture of the two. In another step the coated fuel particles in a rotating drum are overcoated with a molding powder of the same composition according to a kind of dragee process. As shown in example 2 to prepare fuel elements with a free zone in a second molding step the isotropic granulate is preliminarily molded to a block. The molding pressure in this step can be 30 kg/cm.sup.2, the temperature about 70.degree. C. The cooling channels can be bored out of the block either after the molding step or can be molded simultaneously with the molding of the block fuel element. As coated fuel particles there can be employed oxides or carbides of U 235, U 233 and fissionable plutonium isotropes a fuel materials in mixture with U 238 and/or Th 232 as fertile materials coated with multiple layers of pyrolytic carbon prepared in conventional manner. Conventional intermediate layers for example of SiC, ZrC or NbC can also be present in the coated fuel particles. The intermediate layers can be emitted.