All-ceramic Core Research Articles

Influence of Background Material on 3 Veneered All-Ceramic Core Materials

To study the influence of background color on 3 veneered all-ceramic core materials, the color of all veneered samples that were put on noble metal, Ni-Cr alloy, Ag-Hg alloy and resin background was measured with chromatism instrument (and the chromatic aberration value among groups was Calculated). Under different backgrounds, there was significant difference in the L, a and b values among Empress II samples, there was significant difference in a and b values among In-Ceram Alumina samples, and there was no difference in the L, a and b values among In-Ceram Zirconium samples. After veneered, the influence of background color on Empress II was clear, on In-Ceram Alumina was less clear, and on In-Ceram Zirconium was void.

Surface roughness resulting from wear of lithia-disilicate-based posterior crowns

The objective of this study was to determine the surface roughness that occurred as a result of wear of all-ceramic crowns after 2 years of intraoral service. The ceramic material used in this study was a lithia-disilicate-based all-ceramic core ceramic and a glass-phase veneer ceramic. Posterior bilayer crowns were made for 30 subjects, who were recalled annually. At each recall time, the quality of crowns and adjacent gingival tissues were examined for acceptability, and impressions were made for replica models. The occlusal contacts of each ceramic crown were identified and surface roughness ( R a) measurement was performed on the replica model using a contact profilometer. Mean surface roughness values were analyzed using a repeated measure ANOVA. The wear patterns of contact surfaces were also characterized using a scanning electron microscope. From 30 ceramic crowns, 12 premolars and 13 molars were used for surface roughness measurements. For molar crowns, the surface roughness ( R a) at baseline was 1.76 ± 0.43 μm and the roughness significantly increased after 1 year and 2 years ( p < 0.05). For premolar crowns, the increase in roughness values was significantly different only after 2 years from 1.49 ± 0.22 to 1.9 ± 0.44 μm ( p < 0.05). Roughening of ceramic surfaces was observed over the entire occlusal area, especially in the contact zones. Loss of superficial ceramic material as a result of microfracture was the reason for surface roughening. In conclusion, the surface roughness of ceramic crowns was significantly increased after 2 years. The increase in surface roughness indicated the degradation of the ceramic crown surfaces over time that resulted from the wear process.

Influence of Background Color on the Chromatic Value of Four All-Ceramic System Core Materials

This study aims to investigate the effect of background color on the chromatic value of four all-ceramic system core materials at clinically appropriate thicknesses. Disc specimens with 15 mm in diameter and thicknesses ranging from 0.5±0.01mm In-Ceram zirconia core, Cercon base zirconia core, and Cercon base color zirconia core) to 0.8±0.01mm (Empress II) were fabricated. A colorimeter (CIE- 1976-Lab) measured the chromatic value of all samples placed on a high-noble metal-ceramic alloy and a visible light-cured dental composite resin (A2 color) background, after which the color difference was calculated. The color difference of Empress II samples was more than 1.5 among different background color groups, while that of Zirconia was less than 1.5. The influence of background color on the Empress II dentin was visible, such that it could be used on a tooth color post. Meanwhile, the influence of background color was invisible for the three kinds of Zirconia core materia1s, such that the materials developed excellent color-masking abilities and could be adapted for use on any color background.

Effect of two connector designs on the fracture resistance of all-ceramic core materials for fixed dental prostheses

Most all-ceramic fixed dental prostheses (FDPs) fail at the connectors. The purpose of this study was to determine the effect of 2 connector designs on the fracture resistance of core materials used for all-ceramic FDPs. Three materials were tested: (1) heat-pressed lithium disilicate glass ceramic (IPS e.max Press (Press)), (2) milled lithium disilicate glass ceramic (IPS e.max CAD (CAD)), and (3) milled yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) (IPS e.max ZirCAD (ZirCAD)). Specimens were made into 30 x 4 x 4-mm bars to represent 3-unit FDPs. Two connector designs, round (0.60 +/-0.01-mm radius of curvature) and sharp (0.06 +/-0.001-mm radius of curvature), with a 3.00 +/-0.05-mm cross-section for each connector, were studied (n=5). Each specimen was loaded to fracture in a universal testing machine with a crosshead speed of 0.1 mm/min. Data were analyzed with a 2-way univariate ANOVA and Tukey HSD test (alpha=.05). Mean (SD) failure loads for round connector designs were 684.2 (70.1) N for ZirCAD, 260 (7.8) N for CAD, and 172.9 (35.5) N for Press. Mean (SD) failure loads for sharp connector designs were 386.3 (51.5) N for ZirCAD, 87.9 (7.0) N for CAD, and 125.1 (15.1) N for Press. The 2-way univariate ANOVA indicated statistically significant differences (P<.005) for material and connector design, and, also, a significant interaction between material and connector design. Higher maximum failure loads were found for the round connector design when compared to the sharp connector design, for ZirCAD and CAD. However, this difference was not statistically significant for the Press groups. SEM subjective assessment of the fractured specimens revealed that the fracture initiated from the gingival surface (tensile) of the connector toward the pontic (central loading point). Fracture resistance of ceramic core materials is affected by fabrication technique and connector design. Connector design affected fracture resistance of the milled ceramic, but not the pressed ceramic.

Opalescence of all-ceramic core and veneer materials

Objectives The enamel of natural teeth is opalescent, where there is light scattering of the shorter wavelengths of the visible spectrum, giving a tooth a bluish appearance in the reflected color and an orange/brown appearance in the transmitted color. The objective of this study was to determine the opalescence of all-ceramic core, veneer and layered specimens with a color measuring spectrophotometer. Methods Colors of core (A2-corresponding shade), veneer (A2- and A3-corresponding shades) and layered (A2- and A3-layered) ceramics for all-ceramic restorations in clinically relevant thicknesses were measured in the reflectance and transmittance modes. The opalescence parameter ( OP), which was calculated as the difference in blue-yellow coordinate (Δ b *) and red-green coordinate (Δ a *), and the differences in blue-yellow coordinate (Δ b *) and in color ( Δ E a b * ) between the reflected and transmitted colors were calculated. One-way ANOVA was performed for the OP values of the core, veneer and layered specimens by the kind of materials. Regression analysis was performed between the OP and Δ b *, and the OP and Δ E a b * values. Results The range of the OP value was 1.6–6.1, 2.0–7.1, 1.3–5.0 and 1.6–4.2 for the core, veneer, A2- and A3-layered specimens, respectively, all of which were significantly influenced by the kind of materials ( p < 0.05). The OP and Δ E a b * values showed significant correlations within each of the core and veneer ceramics. Significance Opalescence varied by kind of ceramics. The OP values of ceramics were lower than those of tooth enamel. All-ceramic materials that can simulate the opalescence of natural teeth should be developed.

Difference in the translucency of all-ceramics by the illuminant

Objectives The objective of this study was to determine the differences in the translucency parameter (TP) of all-ceramic core, veneer and layered specimens relative to the CIE standard illuminant D 65, A and F2. Methods The A2-corresponding shade of seven all-ceramic core materials and one sintering ceramic as a reference were prepared in clinically relevant thicknesses ( n = 7). The A2- and A3-corresponding shades of each recommended veneer ceramics were fabricated so that the thickness of the layered specimen was 1.5 mm. The color of core, veneer and layered specimens was measured over white and black backgrounds relative to the illuminant D 65, A and F2 with a reflection spectrophotometer. TP value was obtained by calculating the color difference between the specimen over the white and that over the black background. Differences of TP values by the illuminant (ΔTP) were analyzed with three-way analysis of variance (ANOVA), with the fixed factors of the type of ceramic, the brand/thickness of ceramic within each type and the combination of illuminants ( α = 0.05). Results ΔTP values were influenced by three factors based on three-way ANOVA. Higher translucent materials showed higher difference in TP value by the illuminants. TP(D 65) was lower than TP(F2) and TP(A) values. Correlation coefficients between TP values relative to the different illuminants were higher than 0.994 ( p < 0.01). Significance Translucency of all-ceramic materials under daylight condition was lower than those under incandescent or fluorescent lamps. Therefore, this difference should be considered when shade matching translucent shades of all-ceramics.

Chromatic Study of All-ceramic Restorations: Relative Translucency of 9 All-ceramic Core Materials

Purpose: All-ceramic restorations are widely used in esthetic dentistry. Various materials have been used to improve ceramic core strength, but it is unclear whether they affect the opacity of all-ceramic systems. The aim of this study was to compare the translucency of 9 all-ceramic system core materials at clinically appropriate thickness.Materials and methods: A 360-degree round deep chamfer preparation for the all-ceramic crown was made for a maxillary right first molar tooth as the dental model. Coping of 0.5mm thickness were fabricated by Empress Esthetic (ETC1, EOC1, EO1, EO2), Empress2 (50) (IVOCLAR VIVADENT), Procera AllCeram, Procera AllZirkon (Nobel Biocare), LAVA (3M ESPE), and DentaCAD Systeme (Hint-Els). Abutments were made by black wax and white wax. Spectrophotometric measurements were made by a spectrophotometer. The final color of the central part of the buccal surface was measured and the L*a*b* color space was used to obtain the color values. Contrast ratios were calculated from the luminous reflectance (Y) of the specimens with a black (Yb) and a white (Yw) backing to give Yb⁄Yw with CIE illuminant D65 and 2-degree observer function (0.0=transparent, 1.0=opaque).Results: Contrast ratio in order of most translucent to most opaque were as follows: EO2 .41, EO1 .43, ETC1 .43 , EOC1 .44, Empress2 .50, AllCeram .63, AllZirkon .74, LAVA .76, DentaCAD.80. There was no significant difference(p<.05) between the AllZirkon, LAVA, DentaCAD or ETC1, EOC1, EO1, EO2.Conclusion: Within the limitations of this in-vitro study on 9 all-ceramic core systems, a significant range of translucency was identified across specimen groups.

Flexural strength and fracture toughness of dental core ceramics

Many different strengthened all-ceramic core materials are available. In vitro study of their mechanical properties, such as flexural strength and fracture toughness, is necessary before they are used clinically. The purpose of this study was to evaluate and compare the mechanical properties of 6 commonly used all-ceramic core materials using biaxial flexural strength and indentation fracture toughness tests. Specimens of 6 ceramic core materials (Finesse, Cergo, IPS Empress, In-Ceram Alumina, In-Ceram Zirconia, and Cercon Zirconia) were fabricated (n=25) with a diameter of 15 mm and width of 1.2 +/- 0.2 mm. For each group, the specimens were tested to compare their biaxial flexural strength (piston on 3 balls) (n=15), Weibull modulus, and indentation fracture toughness (n=10) (IF method). The data were analyzed with 1-way ANOVA test (a=.05). The Tamhane multiple comparison test was used for post hoc analysis. Mean (SD) of biaxial flexural strength values (MPa) and Weibull modulus (m) results were: Finesse (F): 88.04 (31.61), m=3.17; Cergo (C): 94.97 (13.62), m=7.94; IPS Empress (E): 101.18 (13.49), m=10.13; In-Ceram Alumina (ICA): 341.80 (61.13), m=6.96; In-Ceram Zirconia (ICZ): 541.80 (61.10), m=10.17; and Cercon Zirconia (CZ): 1140.89 (121.33), m=13.26. The indentation fracture toughness results showed that there were significant differences between the tested ceramics. The highest fracture toughness values (MPa x m(0.5)) were obtained with the zirconia-based ceramic core materials. Significant differences were found in strength and toughness values of the materials evaluated. Cercon Zirconia core material showed high values of biaxial flexural strength and indentation fracture toughness when compared to the other ceramics studied.

Layered color of all-ceramic core and veneer ceramics

Color of an all-ceramic restoration is the result of interaction between core and veneer ceramics. However, the influence on color of the different types of all-ceramic core and veneer combinations at clinically relevant thicknesses is not well understood. The purpose of this study was to measure the layered color of all-ceramic core and veneer combinations with the thickness of the core set as the clinically minimum thickness to mask the background, and the thickness of the veneer set as the remaining available thickness within a clinically allowable thickness. The A2-corresponding shade of 7 all-ceramic core materials, and 1 sintering ceramic and 1 alloy core as references, were prepared in clinically minimum thicknesses (0.4 to 0.8 mm) to mask the background (n=7). The A2 and A3 corresponding shades of each recommended veneer ceramic were used to fabricate specimens that were 1.5 mm thick. The color of the core, veneer, and layered specimens was measured with a reflection spectrophotometer. Two-way ANOVA with the independent variables of the types of core and veneer ceramics on the layered color was used to analyze the data (alpha=.05). The influence of the color coordinates in the core and veneering ceramics on the layered color was analyzed with multiple regression analysis. CIE L( *), a( *), b( *), and C( *)(ab) values of A2- or A3-veneer layered specimens were influenced significantly by the combination of core and veneer ceramics (P<.001). The CIE L( *) values of layered specimens were primarily influenced by the CIE L( *) values of the core ceramic (P<.001). The other 3 parameters were primarily influenced by each corresponding parameter of veneer ceramic, based on multiple regression analyses (P<.001). The layered color of all-ceramic core and veneer ceramics in the clinically allowable thickness was different even when the same shade, keyed to a VITA guide, of core and veneer ceramics were layered. The color difference between each pair of A2- or A3-veneered specimens by the combination of core and veneer ceramics ranged from 1.4 to 5.6 DeltaE( *)(ab) units.

Fluorescent Emission of All-Ceramic Core, Veneer and Layered Specimens

Fluorescent Emission of All-Ceramic Core, Veneer and Layered Specimens

Thin‐wall ceramic CAD/CAM crown copings: strength and fracture pattern

The strength and fracture pattern of posterior CAD/CAM-generated crown copings with 0.4 mm wall thickness were evaluated in vitro hypothesizing that fracture resistance of YTZP-zirconia copings might be independent of mode of cementation whether resin-bonded or cemented because of the high strength of YTZP-zirconia. Two sets of copings (n = 15) each were fabricated using CEREC inLab CAD/CAM from (i) lithiumdisilicate glass-ceramic, (ii) infiltration ceramic as controls and (iii) YTZP-zirconia. Copings (n = 15) of ceramics (i), (ii) and (iii) each were (a) zinc-phosphate cemented, (b) adhesively seated on resin-based composite dies and loaded until fracture. Load (N) data was analysed using anova and Scheffé tests. Crack pattern was evaluated on additional three sample cross-sections for each group at fracture-start. Radial cracks originated early at the cementation interfaces and cone cracks were observed finally at the loading sites. Mean load (N) values (+/-s.d.) of A-copings at fracture-start/-end (i) 804 +/- 195/862 +/- 162, (ii) 923 +/- 180/975 +/- 147, (iii) 697 +/- 110/1607 +/- 145, were all significantly (P < 0.01) lower when compared with their B-crown coping analogs (i) 1183 +/- 318/1919 +/- 326, (ii) 1621 +/- 165/1820 +/- 211, (iii) 731 +/- 115/1973 +/- 287 except for A3 and B3 at fracture-start. This confirmed our hypothesis at fracture-start (P > 0.05) but rejected it at fracture-end (P < 0.01). The A3 fracture-end data, even if significantly (P < 0.01) lower, came close to the B3 values by 18%. A3 was significantly (P < 0.001) stronger by 86/74% than A1/A2 at fracture-end. The data indicates that YTZP-zirconia copings have the potential to provide support for all-ceramic core crowns, which may be adequate for non-adhesive cementation.

オールセラミックレストレーションの色調に関する研究―支台歯色がコーピングの色調に及ぼす影響―

Recently, the use of all-ceramic crowns has spread widely in clinical applications to meet the demand for both functional and esthetically-pleasing restorations. In making all-ceramic crowns, it is necessary to reproduce the shape and color near to those of the natural teeth. However, the color shades of abutments might influence the color of the copings which are made of material with high transparency. This study examined the influence of the color shades of the abutments on the final color of copings for three kinds of all-ceramic core materials: Empress, Empress 2 (IVOCLAR VIVADENT), and Procera AllCeram (Nobel Biocare). Copings with 0.5 mm in thickness were fabricated by using Empress (TC1), Empress 2 (100), and Procera AllCeram (white) core materials for an upper-right central incisor. Abutments were made by using six kinds of die materials of the Empress system (ST1, ST2, ST3, ST5, ST8, ST9), gold-silver-palladium alloy, gold alloy, and experimental black body. Copings were inserted in each abutment and the final color of the central part of the buccal surface was measured using a spectrophotometer according to the L*a*b* color system. Regardless of the color shades of the abutments, the chroma values of the copings rose in the order of Empress, Empress 2, and Procera, and the values of lightness rose in the order of Empress, Procera, and Empress 2. When the final color of each coping measured under the wet and dry conditions were compared, the difference in chroma was great. Within the limitations of this study, the results suggest that the influence of the color shades of the abutments on the final color of the three kinds of copings is small in the order of Empress, Procera, and Empress 2. In clinical and dental laboratory operations, it is hoped to observe and measure the color of copings and restorations under the wet condition.

Relative translucency of six all-ceramic systems. Part II: Core and veneer materials

STATEMENT OF PROBLEM All-ceramic core materials with various strengthening compositions have a range of translucencies. It is unknown whether translucency differs when all-ceramic materials are fabricated similarly to the clinical restoration with a veneered core material. This study compared the translucency of 6 all-ceramic materials veneered and glazed at clinically appropriate thicknesses. Core specimens (n = 5 per group) of Empress dentin, Empress 2 dentin, In-Ceram Alumina, In-Ceram Spinell, In-Ceram Zirconia, and Procera AllCeram were fabricated as described in Part I of this study and veneered with their corresponding dentin porcelain to a final thickness of 1.47 +/- 0.01 mm. These specimens were compared with veneered Vitadur Alpha opaque dentin (as a standard), a clear glass disc (positive control), and a high-noble metal-ceramic alloy (Porc. 52 SF) veneered with Vitadur Omega dentin (negative control). Specimen reflectance was measured with an integrating sphere attached to a spectrophotometer across the visible spectrum (380 to 700 nm); 0-degree illumination and diffuse viewing geometry were used. Measurements were repeated after a glazing cycle. Contrast ratios were calculated from the luminous reflectance (Y) of the specimens with a black (Yb) and a white backing (Yw) to give Yb/Yw with CIE illuminant D65 and a 2-degree observer function (0.0 = transparent, 1.0 = opaque). One-way analysis of variance and Tukey's multiple-comparison test were used to analyze the data (P<.05). Significant differences in contrast ratios were found among the ceramic systems tested when they were veneered (P<.0001) and after the glazing cycle (P<.0001). Significant changes in contrast ratios (P<.0001) also were identified when the veneered specimens were glazed. Within the limitations of this study, a range of translucency was identified in the veneered all-ceramic systems tested. Such variability may affect their ability to match natural teeth. The glazing cycle resulted in decreased opacity for all test materials except the completely opaque In-Ceram Zirconia and metal-ceramic specimens.

Relative translucency of six all-ceramic systems. Part II: Core and veneer materials

Statement of Problem. All-ceramic core materials with various strengthening compositions have a range of translucencies. It is unknown whether translucency differs when all-ceramic materials are fabricated similarly to the clinical restoration with a veneered core material. Purpose. This study compared the translucency of 6 all-ceramic materials veneered and glazed at clinically appropriate thicknesses. Material and Methods. Core specimens (n = 5 per group) of Empress dentin, Empress 2 dentin, In-Ceram Alumina, In-Ceram Spinell, In-Ceram Zirconia, and Procera AllCeram were fabricated as described in Part I of this study and veneered with their corresponding dentin porcelain to a final thickness of 1.47 ± 0.01 mm. These specimens were compared with veneered Vitadur Alpha opaque dentin (as a standard), a clear glass disc (positive control), and a high-noble metal-ceramic alloy (Porc. 52 SF) veneered with Vitadur Omega dentin (negative control). Specimen reflectance was measured with an integrating sphere attached to a spectrophotometer across the visible spectrum (380 to 700 nm); 0-degree illumination and diffuse viewing geometry were used. Measurements were repeated after a glazing cycle. Contrast ratios were calculated from the luminous reflectance (Y) of the specimens with a black (Yb) and a white backing (Yw) to give Yb/Yw with CIE illuminant D65 and a 2-degree observer function (0.0 = transparent, 1.0 = opaque). One-way analysis of variance and Tukey's multiple-comparison test were used to analyze the data (P<.05). Results. Significant differences in contrast ratios were found among the ceramic systems tested when they were veneered (P<.0001) and after the glazing cycle (P<.0001). Significant changes in contrast ratios (P<.0001) also were identified when the veneered specimens were glazed. Conclusion. Within the limitations of this study, a range of translucency was identified in the veneered all-ceramic systems tested. Such variability may affect their ability to match natural teeth. The glazing cycle resulted in decreased opacity for all test materials except the completely opaque In-Ceram Zirconia and metal-ceramic specimens. (J Prosthet Dent 2002;88:10-5.)

Effects of Compression, Staging, and Braid Angle on Braided Rope Seal Performance

Future turbine engines and industrial systems will be operating at increased temperatures to achieve more demanding efficiency and performance goals. In the highest temperature sections of the engine new material systems such as ceramics and intermetallics are being considered to withstand the harsh thermal environment. Components constructed of these low expansion-rate materials experience thermal strains and a resulting reduction of life when rigidly attached to high expansion-rate, superalloy support structures. Seals are being designed to both seal and to serve as compliant mounts allowing for relative thermal growths between high temperature but brittle primary structures and the surrounding support structures. Previous seal research yielded several braided rope seal designs which demonstrated the ability to both seal and serve as a compliant mount. The hybrid seal was constructed of an all-ceramic (alumina-silica) core overbraided with a superalloy wire sheath (cobalt based superalloy). The all ceramic seal was constructed of an all-ceramic (alumina-silica) core overbraided with multiple ceramic (alumina-silica) sheath layers. Program goals for braided rope seals are to improve flow resistance and/or seal resilience. To that end, the current report studies the test results of: baseline and modified hybrid seals; two stage hybrid and two stage all-ceramic seal configurations; and single stage hybrid and single stage all-ceramic seal configurations for a range of seal crush conditions. Hybrid seal modifications include increasing the sheath braid angle and core coverage. For the same percent seal cross-sectional crush, results show that increasing the hybrid seal braid angle increased seal stiffness and seal unit load, resulting in flows approximately one third of the baseline hybrid seal flows. For both hybrid and all-ceramic seals, two stage seal configurations significantly outperformed single stage configurations. Two stage seal flows were at least 30% less than the single stage seal flows for the same seal crush. Furthermore, test results of single stage seals indicate that for both all-ceramic and hybrid seals, a specific seal crush condition exists at which minimum flows are achieved (i.e. increasing seal crush beyond a certain point does not result in better flow performance). Flow results are presented for a range of pressures and temperatures from ambient to 1300 F, before and after scrubbing. Compression tests results show that for both all-ceramic and hybrid seals, seal preload and stiffness increase with seal crush, but residual seal interference remains constant.

Steam Generator Materials Performance in High Temperature Gas-Cooled Reactors

High temperature gas-cooled reactor (HTGR) systems feature a graphite-moderated, uranium-thorium, all-ceramic core and utilize high pressure helium as the primary coolant. The steam generators in these systems are exposed to gas-side temperatures approaching 760°C (1400°F) and produce superheated steam at 538°C (1000°F) and 16.5 MPa (2400 psi). Thus, the design and development of steam generators for these systems require consideration of time-dependent materials behavior, corrosion, fretting, wear, and other related phenomena of concern in all steam generators.The prototype Peach Bottom Unit No. 1 40-MW (electric) HTGR was operated by the Philadelphia Electric Company for a total of 1349 equivalent full power days during a 7-yr period. Upon planned decommissioning of that plant, the forced-recirculation U-tube steam generators and other components were subjected to destructive properties tests and metallurgical examinations. These tests and examinations showed the steam generators to be in very satisfactory condition.The 330-MW(electric) Fort St. Vrain HTGR, owned and operated by Public Service Company of Colorado, and now in the final stages of startup, has achieved 70% power and generated more than 1.5 × 106 MWh of electricity. The steam generators in this reactor are once-through units of helical configuration, and their design and development required considering a number of new materials factors including creep fatigue. Also, because of the once-through design, water chemistry control needed special consideration.Current designs of larger HTGRs also feature steam generators of helical tube once-through design. Materials issues that are important in these designs include detailed consideration of time-dependent behavior of both base metals and welds, as required by current American Society of Mechanical Engineers Code rules, evaluation of bimetallic weld behavior, evaluation of the properties of very large tubesheet forgings, consideration of the gaseous corrosion effects of the primary coolant, and other related factors.

Development of gas-cooled reactors for power production

The paper reviews the possible development of gas-cooled graphite-moderated reactors for power production. It is shown that larger reactors of the Calder Hall type will produce electric power at a competitive cost in the United Kingdom. A very promising line of development is towards higher-temperature operation, since this increases both the heat rating of the reactor and the overall efficiency of the plant, although it requires the development of new materials, particularly for cladding the uranium. As higher temperatures are reached, interest is stimulated in the oxide and carbide forms of uranium, and for very high temperatures an all-ceramic reactor core has been suggested. The paper describes the use of recycled plutonium as a fuel, showing that under certain circumstances this is possible with a natural-uranium fuel. Comparison is also made of the relative merits of a number of possible coolant gases. It is concluded that, given research and development effort, the gas-cooled graphite-moderated reactor is capable of considerable development. For the base-load application, cheaper power should be possible, and the range of application may be extended to smaller power units tailored for specific purposes.