Nils J. Diaz

Ultrahigh temperature vapor-core reactor - Magnetohydrodynamic system for space nuclear electric power

This article presents the conceptual design of a nuclear space power system based on the ultrahigh temperature vapor-core reactor (UTVR) with magnetohydrodynamic (MHD) energy conversion. This UF4-fueled gas-core cavity reactor operates at a maximum core temperature of 4000 K and 40 atm. Potassium fluoride working fluid cools the reactor cavity wall and mixes with the fissioning fuel in the core. Neutron transport calculations with specialized high temperature gas-core cross-sectional libraries indicate criticality at core radii of 60-80 cm, with BeO reflector thicknesses of —50 cm. The heated core exhaust mixture is directed through a regeneratively cooled nozzle into a disk MHD channel to generate electrical power. The MHD generator operates at fluid conditions below 2300 K and 1 atm. Fission fragment ionization enhances the electrical conductivity in the channel significantly, allowing an overall conversion efficiency of 20%. The mixture is condensed in heat exchangers, and pumped back to the core in a MHD-Rankine thermodynamic cycle. Heat rejection temperatures of 1500-2100 K lead to compact heat exchangers and an overall specific weight of ~1 kg/kWe for 200 MWe. Material experiments, performed with UF4 up to 2200 K (to date), show acceptable compatibility with tungsten-, molybdenum-, and carbon-based materials. This article discusses the supporting nuclear, fluid flow, heat transfer and MHD analysis, materials experiments, and fissioning plasma physics.

Nuclear design of a vapor core reactor for space nuclear propulsion

Neutronic analysis methodology and results are presented for the nuclear design of a vapor core reactor for space nuclear propulsion. The Nuclear Vapor Thermal Reactor (NVTR) Rocket Engine uses modified NERVA geometry and systems which the solid fuel replaced by uranium tetrafluoride vapor. The NVTR is an intermediate term gas core thermal rocket engine with specific impulse in the range of 1000–1200 seconds; a thrust of 75,000 lbs for a hydrogen flow rate of 30 kg/s; average core exit temperatures of 3100 K to 3400 K; and reactor thermal powers of 1400 to 1800 MW. Initial calculations were performed on epithermal NVTRs using ZrC fuel elements. Studies are now directed at thermal NVTRs that use fuel elements made of C‐C composite. The large ZrC‐moderated reactors resulted in thrust‐to‐weight ratios of only 1 to 2; the compact C‐C composite systems yield thrust‐to‐weight ratios of 3 to 5.

Gas core reactor concepts and technology - Issues and baseline strategy

Results of a research program including phenomenological studies, conceptual design, and systems analysis of a series of gaseous/vapor fissile fuel driven engines for space power platforms and for thermal and electric propulsion are reviewed. It is noted that gas and vapor phase reactors provide the path for minimum mass in orbit and trip times, with a specific impulse from 1020 sec at the lowest technololgical risk to 5200 sec at the highest technological risk. The discussion covers various configurations of gas core reactors and critical technologies and the nuclear vapor thermal rocket engine.

Heat transfer analysis of fuel assemblies in a heterogeneous gas core nuclear rocket

Heat transfer problems of a heterogeneous gaseous core nuclear rocket were studied. The reactor core consists of 1.5-m long hexagonal fuel assemblies filled with pressurized uranium tetrafluoride (UF4) gas. The fuel gas temperature ranges from 3500 to 7000 K at a nominal operating condition of 40 atm. Each fuel assembly has seven coolant tubes, through which hydrogen propellant flows. The propellant temperature is not constrained by the fuel temperature but by the maximum temperature of the graphite coolant tube. For a core achieving a fission power density of 1000 MW/cu m, the propellant core exit temperature can be as high as 3200 K. The physical size of a 1250 MW gaseous core nuclear rocket is comparable with that of a NERVA-type solid core nuclear rocket. The engine can deliver a specific impulse of 1020 seconds and a thrust of 330 kN.

Graphic System Code Analysis of the Ultrahigh Temperature Vapor Core Reactor-MHD Nuclear Space Power System

A system analysis code has been developed for the Ultrahigh Temperature Vapor Core Reactor (UTVR) with MHD energy conversion. The UTVR-MHD power system uses UF/sub 4/ vapor as the fissioning fuel and KF alkali metal fluoride as the working fluid in a closed Rankine cycle. The code calculates the system power output, specific mass, T-s diagram and thermodynamic state points, core neutronic performance, MHD performance, and numerous system parameters of interest. The codes calculational results are presented graphically on the screen, superimposed on a schematic of the system. The code is highly flexible in allowing the user to change the values of many of the system parameters right on the screen with simple mouse or cursor movements. This allows immediate evaluation of the effect of the changes on all other system parameters. A screen display of the T-s diagram is included. Expanded output includes fluids properties, individual component masses, constants used in the models, etc. The baseline UTVR-MHD system design produces 200 MWe net electrical power at a specific mass of 0.5 kg/kWe. This is accomplished with a 3 m/sup 3/ reactor core providing a working fluid mixture maximum outlet temperature of 4000 K at a pressure of 40 atm. The working fluid inlet temperature to the reflector is 1500 K. The MHD generator operates at inlet Mach number of 3 and swirl of 2, and a plasma conductivity of 60 mho/m.

Design Optimization of Nuclear Vapor Thermal Rocket Core — A Thermo‐Mechanical Study

Fuel structural materials for the Nuclear Vapor Thermal Rocket (NVTR) are exposed to very high temperature vapor fuel in the fuel channel and to high temperature but cooler propellant in the coolant channel. This temperature difference leads to thermal stress in the fuel element. There is also a mismatch in the value of coefficients of thermal expansion between the fuel element material and the coating material that could lead to failure of the coating. The stress in the coating and the fuel element material is dependent on the power density of the core and also on the arrangement of fuel and coolant channels. In order to achieve higher power density, the fuel element design has to be optimized to yield lower stress. Analytical studies found that carbon/carbon composite hexagonal fuel elements employing a square lattice arrangement of multiple UF4 fuel and hydrogen coolant channels yield maximum stress intensities well below fuel element materials stress limit.

Nuclear vapor thermal reactor propulsion technology

The conceptual design of a nuclear rocket based on the vapor core reactor is presented. The Nuclear Vapor Thermal Rocket (NVTR) offers the potential for a specific impulse of 1000 to 1200 s at thrust‐to‐weight ratios of 1 to 2. The design is based on NERVA geometry and systems with the solid fuel replaced by uranium tetrafluoride (UF4) vapor. The closed‐loop core does not rely on hydrodynamic confinement of the fuel. The hydrogen propellant is separated from the UF4 fuel gas by graphite structure. The hydrogen is maintained at high pressure (∼100 atm), and exits the core at 3,100 K to 3,500 K. Zirconium carbide and hafnium carbide coatings are used to protect the hot graphite from the hydrogen. The core is surrounded by beryllium oxide reflector. The nuclear reactor core has been integrated into a 75 klb engine design using an expander cycle and dual turbopumps. The NVTR offers the potential for an incremental technology development pathway to high performance gas core reactors. Since the fuel is readily ...

Ultrahigh temperature vapor core reactor-MHD system for space nuclear electric power

The conceptual design of a nuclear space power system based on the ultrahigh temperature vapor core reactor with MHD energy conversion is presented. This UF4 fueled gas core cavity reactor operates at 4000 K maximum core temperature and 40 atm. Materials experiments, conducted with UF4 up to 2200 K, demonstrate acceptable compatibility with tungsten-molybdenum-, and carbon-based materials. The supporting nuclear, heat transfer, fluid flow and MHD analysis, and fissioning plasma physics experiments are also discussed.

Heterogeneous Gas Core Reactor For Space Nuclear Power

The Heterogeneous Gas Core Reactor (HGCR) departs significantly from most other gas core reactor concepts in that the majority of the system moderation occurs within the core rather than in an external reflector. This leads to significant advantages over other gas core reactor concepts operating at comparable gas temperatures and neutron flux levels. Particularly important for space nuclear power systems is the improved neutron economy which leads to very compact, high power reactors. Calculations show that cylindrical HGCRs less than one meter in diameter by one meter in height can yield power densities of 1000 w/cc and power levels of hundreds of MWt for reactor specific powers of a few hundred kWt/kg.