2005 Joint Propulsion Conference Tucson, AZ July 10-13, 2005 Candidate Near-Term Fuel Options for Conventional and Bimodal NTR Engines J. A. Halfinger DeWayne Husser John M. Kerr

Topics Updated Bimodal Reactor Concept Basic Fuel Selection Criteria Near-term Fuel Options Development/Qualification Methodologies Schedule Considerations

Updated Bimodal Design Small Compact Size –15,000 lb thrust –25 kWe Update of NERVA design –Graphite core –Carbide-based fuel

Bimodal NTR Concept Uncooled Skirt Pressure Vessel Nozzle Dome

Bimodal NTR Concept Core Insulator-Slat Region Manifolds Control Drums Reflector Actuator Shield Core Support Structure

Reflector Control Drums Insulator- Slat Region Pressure Vessel Bimodal NTR Concept Core

241 Support Elements 564 Fuel Elements Core Power 317 MWt Rocket Mode 500 KWt Power Mode Bimodal NTR Concept

Pattern is Symmetric in all 6 Sectors Rocket Power Key Bimodal NTR Concept Support ElementFuel Elements Support Cup Inner Tie Tube YH Moderator Miniarches Outer Tie Tube

Flow Director Shield Support Weldment Shield Reflector Dome Bimodal NTR Concept Seals Core Support Plate

Header Orifice & Filter Screen Housings Inlet Duct Feed Tubes Rocket Header Bimodal NTR Concept

Header Orifice & Filter Screen Housings Inlet Duct Feed Tubes Power Feed Header Bimodal NTR Concept

Feed Manifolds Dual Exhaust Manifold Exhaust (Rocket) Feed (Rocket) Feed (Power) Exhaust (Power) Bimodal NTR Concept

Fuel Selection Criteria Time at temperature Fuel swelling, rate, and fission gas release rate Fuel mechanical properties Fuel/clad/fission product/coolant compatibility –Fuel erosion in thrust mode –Fission product accommodation in power mode Non-catastrophic off-normal performance capabilities Fabricability Development time or risk

High temperature fuels are associated with longer time to flight development Development Time vs. Operating Temperature

Fuel & Matrix Options KernelKernel CoatingMatrixMatrix Coating UCxZrCGraphiteZrC (U, Zr)C x ZrCGraphiteMixed Carbide (U, Zr)C x (Zr, Nb)CGraphiteCarbide/Moly (U, Nb)C x NbCGraphiteCarbide (U, Nb)C x (Zr, Nb)CGraphiteCarbide (U, Zr, Nb)C x None (U, Zr, Nb)C x (Zr, Nb)CGraphiteCarbide UO 2 W or noneWW or W/Re

UO 2 Temperature: ~Thermally stable at temperatures in excess of 2000 K. Performance:Performance demonstrated to high burnup. Compatibility: Stability demonstrated with a variety of materials. Feasibility Issues: low thermal conductivity, lower uranium density,

Cermet Fuels Refractory Metals W/Re Ta possibly others depending upon chemistry Temperature: ~Thermally stable at temperatures in excess of 2000 K. Performance:Higher thermal conductivity that base fuel component and lower swelling and FGR. Database not fully established. Compatibility: Stability demonstrated with a variety of materials. Feasibility Issues: results in heavy core and lower uranium density.

ZrC Coated UC 2 Temperature: Potentially stable at temperatures in excess of 3000 K. Performance:Reduced FGR relative to UC 2. Compatibility: Stability demonstrated with a tungsten and rhenium. Feasibility Issues: swelling and FGR still an issue, narrow composition range, fabrication process control requirements

Development Methodology Historical programs have developed and tested fuels for NTR and power generation No testing of bimodal fuel system Historical fabrication facilities inoperable or have not produced fuel for an extended period of time –Lack of production for >6-12 months requires process requalification Utilize existing facilities for initial development –Fabrication –Testing Utilize nuclear and non-nuclear testing

Development Methodology Develop Performance Criteria Select Candidate Materials Fuel Clad Structures Controls Develop Manufacturing Techniques Fabricate Samples “Development” Techniques Clad Fuel Etc. Nuclear Test Non-Nuclear Test Meet Criteria? N Scale-Up Fabrication Capabilities Y

Develop Manufacturing Techniques Fabricate Samples Nuclear Test Non-Nuclear Test Development Methodology Meet Criteria? N Fabricate Assemblies Y Nuclear Test Non-Nuclear Test Meet Criteria? N Process Qualification

Objective –Ensure process capability prior to fabrication of production hardware Methodology –Fabricate statistically significant quantity of components to demonstrate process capabilities –Begin qualification after development complete Fuel Clad Matrix? Structural materials Other core materials –Non-nuclear testing –Nuclear confirmation

Process Qualification Qualify Material Suppliers Qualify Matrix Fabrication (if necessary) Qualify Fuel Fabrication Qualify Other Components as necessary Non-nuclear Test Qualify Element Fabrication Non-nuclear Test Nuclear Test Confirmation

Timeline for BNTR System Years Systems Definition Conceptual Design Preliminary Test and Evaluation Hardware Build Irradiation Capsule Tests Post Irrad Capsule Exams Preliminary Design Assembly Performance Test and Evaluation Test Hardware Build Irradiation Test Post Irradiation Exam Systems Performance & Safety Test Test Hardware Build Critical Experiment Physics Test Post Irradiation Test Final Design Process Qualification & Integrated Test and Evaluation Test Hardware Build Ground Test Flight Engine Assembly & Test Start Flight Hardware Build Materials Testing Element/Component Testing System Testing Ground Testing Non-Nuclear Tests Non-Nuclear Test Process Qualifications Early Component start

Conclusions Updated NERVA concept to bimodal –317 MWt in thrust mode –500 kWt in power mode Design details need to be investigated –Thermal management –Neutronic design –Structural integrity Fuel performance has been demonstrated on historical programs –NTR –Power –Need to demonstrate bimodal performance capabilities Oxide and carbide based systems appear feasible to support NTR or bimodal concepts –Carbide system will take longer to develop and qualify due to fabrication complexities –Carbide systems can operate at higher temperatures, improving the thrust performance Significant Development/qualification program needed –~11-13 year program to begin flight hardware