Development of a process to lower the thermal expansion of hot pressed niobium--graphite composites

Technical Report · Thu Aug 01 00:00:00 EDT 1968

DOI:https://doi.org/10.2172/4421631· OSTI ID:4421631

Blay, J S; Ellenburg, G W

Declassified 30 Aug 1973. Recent analysis of the NERVA hot end support system revealed that the high with-grain (WG) thermal expansion and the high creep deformation of NRX-A6 NERVA-type 75 wt% niobium carbide -- 25 wt% graphite hot pressed composite material may not be entirely satisfactory in future designs. Design requirements have shown the need for a better match between the normally high average coefficient of thermal expansion ( alpha ) of the composite material and the fueled graphite. A manufacturing engineering program was initiated to investigate methods for reducing the difference in the coefficient of thermal expansion (CTE) between the fuel and the carbide composite and, in addition, methods for reducing the creep rate. These objectives were targeted to be accomplished on 75--25 wt% composite material. It was demonstrated that the coefficient of thermal expansion can be r by about 50 percent of the difference between the coefficient for fueled graphite and that for the NRX-A6 type of composite material without compositional changes. In addition, by a slight modification of the carbide-- graphite ratio, the process should permit the matching of the CTE of fueled graphite and composite material. Improvements can be accomplished by a compressive heat treatment process in which the billet is subjected to a laterally unrestrained axial compression at an elevated temperature. The process can be accelerated by the substitution of calcined- petroleum needle coke in place of the graphite flour. A reduction of approximately 50% of the difference between the NRX-A6 and fueled graphite CTEs was accomplished with a compressive heat treatment without a die at 2700 deg C and 2000 psi for 60 minutes. A further reduction of 10% was accomplished when these pararmeters were changed to 2500 deg C and 3000 psi for 120 minutes. In addition to the reduction in the CTE that can be accomplished by the inclusion of the compressive heat treatment and the use of coke as a carbon source, the creep rate of the material is significantly reduced from the former 3 to 8% to a new range of from 2 to 4%; WG flexural strength seems to be more uniform and generally higher than NRX-A6 material properties. It has been further concluded that this combination of process changes and, in particular, the compressive heat treatment also reduces the spread within a billet and from billet to billet. The experiments also predicted that the CTE of the fueled graphite might possibly be matched by changing the carbide/graphite composite composition ratio to 65--35 wt%. A process for accomplishing this consists of a compressive heat treatment cycle of 2500 deg C and 3000 psi for 120 minutes on the 65--35 wt% hot-pressed billet with a 50--50 ratio of --100 + 150 mesh and --150 + 320 mesh needle coke as the graphite source. (auth)

• Research Organization: Westinghouse Electric Corp., Pittsburgh, PA (United States). Astronuclear Lab.
• Sponsoring Organization: USDOE
• DOE Contract Number: SUBNP-1
• NSA Number: NSA-29-003002
• OSTI ID: 4421631
• Report Number(s): WANL-TME-1801
• Resource Relation: Other Information: Declassified 30 Aug 1973. Orig. Receipt Date: 30-JUN-74
• Country of Publication: United States
• Language: English

Related Subjects

N50120* -Metals
Ceramics
& Other Materials-Ceramics & Cermets-Preparation & Fabrication
N77800 -Reactors-Space
Mobile
Propulsion
Transportation & Package Reactors
N79200 -Reactors-Reactor Components & Accessories
*COMPOSITE MATERIALS- THERMAL EXPANSION
*GRAPHITE- THERMAL EXPANSION
*NERVA REACTOR- REACTOR MATERIALS
*NIOBIUM CARBIDES- THERMAL EXPANSION
COMPRESSION
CREEP
FLEXURAL STRENGTH
HEAT TREATMENTS
HIGH PRESSURE
HOT PRESSING
MECHANICAL PROPERTIES
SUPPORTS
TIME DEPENDENCE
VERY HIGH TEMPERATURE
NESDPS Office of Nuclear Energy Space and Defense Power Systems