-Ceramics The problem of diffuser failure when subjected t o rapid cycling has been solved principally by design changes of the diffuser, best illustrated by comparing Figures 3 and 4. Without a change in the material used, the diffuser tip shown in Figure 4 is virtually indestructible. The reason for such a striking improvement can also be recognized by comparing the two figures. The part of the diffuser subject to failure has simply been removed. Of interest should be the possible use of recently developed high temperature metal-ceramice as materials of construction for the diffuser and other parts of the burner. Oxidation rate of most high temperature metals and carbides is rapid, but can be reduced materially by incorporating varying amounts of refractory oxides that form an impervious protective coating. Fabricating parts by ceramic methods from metal and oxide powder and subsequent sintering has advanced to the point where shapes of intricate design can be produced in quantity to close tolerances. The principal advantages of these materials are high heat conductivity, high strength a t elevated temperatures, and relatively low oxidation rate. Very recent newcomers in this field are molyb-
and Glass-
denum and silicon, together with chromium and titanium carbide. Some of these recent developments are really not new. More than 50 years ago many investigators and inventors proposed metal-ceramics incorporating substantially identical components as those of today. However, there were few, if any, needs for materials of construction for severe temperature fluctuation a t that time. Chromium and silicon ( 5 ) are being recognized as valuable metals for high temperature service, especially from the standpoint of low oxidation rate at elevated temperature. A ductile form of silicon (8) has also been reported. LITERATURE CITED
(1) E v a n s , M. W., Chem.Revs.,51, No. 3,363 (1952). (2) Gen. Elect. Rev.,56, No. 1, 14 (1953). (3) McAdam, D. J., Jr., J. Research Natl. Bur. Standards, 28, 693 (1942). RECEIVED for review April 14, 1953.
ACCEPTED August 28, 1963.
for Nuclear L. R. MCCREIGHT, Knolls Atomic Power Laboratory, .
I
I
General Electric Co., Schenectady, N . Y .
The nonmetallic inorganic materials that comprise the field of ceramics have been associated directly and indirectly with atomic energy work since the beginning of the Manhattan Project. Today, ceramic materials are being investigated to an even greater extent than during the war for possible applications in a reactor that will produce useful power. Such applications are many and varied. They include the use of conventional electrical and thermal insulations, refractories, and ceramic coatings. Other less conventional ceramics are being studied for use in reactors as fuels, moderators, poisons, shields and bearings, and in other special applications.
T
HE five major interior components of a power-producing nuclear reactor are listed in Table I, with the general
properties and types of material required for each application. These are illustrated schematically in Figure 1 (6).
TABLE I. COMPONENTS OF POWER-PRODUCING NUCLEAR REACTOR Fuel Moderator Shielding Coolant structural materials
Under oes fission U236 PuzZ9 or U2as Slows $own neutions b Be ‘etc. Prevents esca e df ‘radioactivity. Hydrogenous material to agsorb neutrons. Heavy metals t o absorb beta and gamma radiation Heat transfer air, water, and liquid metals TO support other components
Several reactors (3) have been built using a ceramic fueluranium dioxide-as part or all of their fuel. The very first reactor, which wm built in the west stands of Stagg Field at the January 1954
University of Chicago in 1942, had about 10 tons of uranium metal and 40 tons of uranium dioxide for fuel. It also had a ceramic moderator-graphite. The second British reactor, nicknamed “Gleep” (for graphite, low energy, experimental pile), also uses uranium metal and uranium oxide (12 and 21 tons, respectively) as fuel and graphite as a moderator. The French reactor, “Zoe” (zero, oxyde d’urane, eau lourde), uses about 30 tons of purified sintered uranium dioxide rods for fuel. Another reactor similar to Zoe is reported to be under construction by Norway and Belgium. The construction of nuclear reactors for the generation of useful power will probably result in higher and higher operating tempere tures. When the operating temperature of the fuel is to be more than 1000’ C., ceramic fuel elements will be almost a necessity. Even below this temperature there is considerable interest in ceramic materials for fuels. Among the significant research work being done is the preparation of phase diagrams of uranium dioxide with other refractory oxides such as magnesium oxide, beryllium oxide, calcium oxide, aluminum oxide,
INDUSTRIAL AND ENGINEERING CHEMISTRY
185
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Figure 1. Schematic View of Nuclear Reactor of Heterogeneous Type
The use of any of these combinations of fuel and diluent involves fundamental and applied research on phase diagrams, thermal conductivities, thermal expansion, effects of radiation, effects of impurities, chemical separation processes to recover the unused fuel after partial burnup in the reactor, and, finally, the fabrication technique to make use of the materials in the reactor. One of the major problems in the use of ceramic materials is their inability to withstand sudden temperature changes and uneven mechanical loads because of a marked lack of ductility. While this does not entirely prevent their use in reactors, it may limit it to a certain extent. These problems are being investigated at several laboratoiies under the sponsorship
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CER4MIC RESISTANT TO MOLTEI' SODIUM
Ceramic materials of high stability to liquid r= metal corroGon are needed for such uses as bearings and electrical and thermal insulations. These applications vary from complete immersion t o only accidental contacts with the metallic coolants. At the Knolls lltomic Pon-er Laboratory, the primary interest has been on sodium Materials suitable for constant immersion include sintered and cemented carbides. For occasional contact, dense pure oxide ceramics for instrument insulation and some of the commercial ceramic thermal insulations for use as duct coverings arp suitable. High density is of primary importance in obtaining good corrosion resistance. An rxample i s titanium carbide, which disintegrates if porous under conditions when the dense material shows only 0.025 mg. per sq. cm. loss of weight per month Corrosion rates from a screening test performed at the Knolls Atomic Power Laboratory on one sample of each material in static sodium are presented in Table 11. After a screening test such as this, numerous other samples of the more promising materials are given further static and dynamic tests under simulated service conditions before they can be specified for use in the reactor system. 4
January 1954
,
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Q (illustrated in Figures 6 and 7 ) which are analogous to induction wound motors ( 2 ) . -4duct ill carry the metal coolant between the windinns of the Duma. Althoueh all the disadvantages of the usual pumps are eliminated, the electromagnetic pump introduces a new one of its own. The windings must be insulated with a high temperature insulation or they must be cooled. As the latter method involves equipment of considerable weight and space, the desire to d i i n i i i ; i r e it is great. This nccessitites tlie devclopnwi,t oi iii.dition nnteri:ils w1iic.h mu.: nittist:,ii I "npc~arut-eshigher t h n n thc best oi
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Figure 4. Equilibrium Diagram of UOs-2.1-
Figure 3. Diagram of Uranium DioxideCalcium Oxide Phase ( I )
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Pit for Wosk?s, Water and Air Lines
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~i~~~~ 6 . Model of ~ lpump of ~i~~~~ ~ Induction Alternating Current Type
INDUSTRIAL AND ENGINEERING CHEMISTRY
187
tion of refractory liners for vessels in which to carry out reductions of special metals, and general service or consultant assistance on insulations, crucible materials, and protective coatings. SUMMARY
There are two general fields for the application of ceramic materials to nuclear power reactors: inside and outside the reactor. Security restrictions limit description of the research necessary to make the former application possible, but somewhat greater detail can be presented on a few of the exterior applications. In general, ceramic materials offer the reactor designers higher operating temperatures as well as other properties not available from metallic materials. Until some of the disadvantages of ceramic objects, particularly the lack of ductility, are overcome by further research development on the materials and on the design of the shapes, reactors are not likely to be constructed primarily from ceramic materials. They will continue, however, to have an important place in reactor construction and in the supporting research work on other materials.
Figure 7 . Top View of Lower Stator of Alternating Current Linear Induction Pump Ready for Installation of Coolant Duct Insulated windings as they emerge from slots can be seen along right-hand edge. METAL-CERAMIC SEALS
Because of the inability of organic insulations, as well as glassto-metal seals, to withstand high temperatures, high pressures, thermal shock, and liquid metal corrosion, considerable work has been done on metal-to-ceramic seals at the Knolls Atomic Power Laboratory. This has resulted in the successful development of several types of seals which will meet the required operating conditions. These seals are tested for mechanical strength as well as vacuum tightness and must not leak when measured by a mass spectrometer leak detector which can locate leaks as small as 10-8 (STP) cc. per second, which is equivalent to 3 cc. in 10 years. One type of seal is shown in Figure 8. CERAMIC COATINGS FOR REDUCING DIFFUSION OF HYDROGEN
Among the ceramic coatings for reducing diffusion of hydrogen has been the application of refractory ceramic coatings to a vacuum furnace part which has t o operate at temperatures around 750” C. with a hydrogen atmosphere on the exterior. The coatings were applied at the University of Illinois for testing a t the Knolls Atomic Power Laboratory. Results of the tests indicate that ceramic coatings will reduce the diffusion rate of hydrogen through stainless steel by factors of 15 to 250. Ceramic-coated metals should prove useful for many similar applications. MISCELLANEOUS CERAMIC APPLICATIONS
Other activities in the ceramic field being performed for various laboratory personnel at the Knolls Atomic Power Laboratory include the application and preparation of refractory oxide coatings for crucibles used in metal melting, fabrication of crucibles especially of commercially unavailable materials or sizes, fabrica-
Figure 8. Metal-Ceramic Seal for 8-2 Mineral-Insulated Cable 1. 2. 3. 4. 5.
6. 7. 8. 9.
Unfired ceramic blank Machined and fired ceramic Copper adapter Fernico or Kovar inner sleeve Titanium hydride-coatcd ceramic piece Copper outer spinning Completed ceramic seal Sectioned seal Ceramic seal mounted on mineral-insulated cable LITERATURE CITED
(1) Alberman, K.B., Blakey, R. C., and Anderson, J. A,, J . Chem. SOC.(London),26,1352 (1951). (2) Cage, J. F., Jr., “ElectromagneticPumps for High Temperature Liquid Metal,” Convention of American Society of Mechanical Engineers, Paper 52A66,New York, December 1952. (3) Koshuba, W. J., and Calkins, V. P., Metal Progr., 62, 97-114 (July 1952). (4) Lambertson, W.A., and Mueller, M. H., J. Am. Ceram. SOC., 36, 329-332 (1953). (5) Norton, F. H., “Refractories,” 3rd ed., p. 644, New York, McGraw-Hill Book Co., 1949. RECEIVED for review March 23, 1953
ACCEPTEDReptember 1 , 1953.
END OF SYMPOSIUM s
188
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 1
