Jeffrey C. King

Review of Refractory Materials for Alkali Metal Thermal-to-Electric Conversion Cells

Refractory alloys are being considered as structural materials in multitube, vapor-anode alkali metal thermalto-electric conversion (AMTEC) cells for future use in radioisotope space electric power systems. In these power systems, the AMTEC cells would operate at a heat source temperature of » 1150 K and a radiator temperature of » 550 K, for a 7‐15 year mission lifetime. In addition to high strength, low density, and low brittle-to-ductile transition temperature, suitablematerialsmustbecompatiblewith thesodium working e uid and havelowthermal expansion and low vapor pressure ( 900 K). C-103 (niobium‐10% hafnium‐1% titanium‐0.5% zirconium ) is also suitable, particularly for thecell’ s colderstructurebecause of its higher strength andlowerthermalconductivity.However,thecompatibilityoftheseniobiumalloyswithsodiumattypicaloperating temperatures and in the presence of minute amounts of oxygen (>5‐10 ppm) for up to 15 years needs further evaluation. Despite the limited availability of rhenium, Mo ‐Re alloys, with a rhenium content of 14 ‐45%, are also good choices as structural materials in vapor anode AMTEC cells. However, their relatively higher density and thermal conductivity could lower the cell’ s performance and increase its specie c mass.

Thermal‐Hydraulic Analyses of the Submersion‐Subcritical Safe Space (S∧4) Reactor

Detailed thermal‐hydraulic analyses of the S∧4 reactor are performed to reduce the maximum fuel temperature of the Submersion‐Subcritical Safe Space (S∧4) reactor to below 1300 K. The fuel pellet diameter is reduced from 1.315 cm to 1.25 cm, decreasing the thermal resistance of the pellets and each of the 1.54 cm diameter coolant channels in the reactor core are replaced with several 0.3 cm ID channels to increase the effective heat transfer area and to encourage mixing of the flowing helium‐28% xenon coolant. The calculated maximum fuel temperature decreased from more than 1900 K to 1302 K and the relative pressure drop across the reactor core increased from 1.98% to 2.57% of the inlet pressure. Moving the concentric inlet and outlet pipes 1 cm towards the center of the reactor core encouraged more flow through the center region, further reducing the maximum fuel temperature by 14 degrees to 1288 K, with a negligible effect on the core pressure losses.

A Methodology for the Neutronics Design of Space Nuclear Reactors

A methodology for the neutronics design of space power reactors is presented. This methodology involves balancing the competing requirements of having sufficient excess reactivity for the desired lifetime, keeping the reactor subcritical at launch and during submersion accidents, and providing sufficient control over the lifetime of the reactor. These requirements are addressed by three reactivity values for a given reactor design: the excess reactivity at beginning of mission, the negative reactivity at shutdown, and the negative reactivity margin in submersion accidents. These reactivity values define the control worth and the safety worth in submersion accidents, used for evaluating the merit of a proposed reactor type and design. The Heat Pipe‐Segmented Thermoelectric Module Converters space reactor core design is evaluated and modified based on the proposed methodology. The final reactor core design has sufficient excess reactivity for 10 years of nominal operation at 1.82 MW of fission power and is ...

Solid‐Core, Gas‐Cooled Reactor for Space and Surface Power

The solid‐core, gas‐cooled, Submersion‐Subcritical Safe Space (S∧4) reactor is developed for future space power applications and avoidance of single point failures. The Mo‐14%Re reactor core is loaded with uranium nitride fuel in enclosed cavities, cooled by He‐30%Xe, and sized to provide 550 kWth for seven years of equivalent full power operation. The beryllium oxide reflector disassembles upon impact on water or soil. In addition to decreasing the reactor and shadow shield mass, Spectral Shift Absorber (SSA) materials added to the reactor core ensure that it remains subcritical in the worst‐case submersion accident. With a 0.1 mm thick boron carbide coating on the outside surface of the core block and 0.25 mm thick iridium sleeves around the fuel stacks, the reflector outer diameter is 43.5 cm and the combined reactor and shadow shield mass is 935.1 kg. With 12.5 atom% gadolinium‐155 added to the fuel, 2.0 mm diameter gadolinium‐155 sesquioxide intersititial pins, and a 0.1 mm thick gadolinium‐155 sesqu...

Spectral Shift Absorbers for Fast Spectrum Space Nuclear Reactors

The space nuclear reactors being considered to support the Jupiter Icy Moons Orbiter (JIMO‐1) mission‐1 sometime in the next decade are compact and fast spectrum with void fractions ranging from 20–40%. In order to secure launch approval, it has to be demonstrated that these reactors will remain sufficiently subcritical when submerged in water or wet sand and subsequently flooded with water, following a launch abort accident. The resulting shift in the neutron spectrum towards thermal increases reactivity, potentially making the reactors supercritical. Incorporating “Spectral Shift Absorbers” (or SSAs), elements such as boron, europium, gadolinium or rhenium, which have significantly higher absorption cross‐sections for thermal versus fast neutrons, can offset the reactivity increase. It has always been the assumption that the worst‐case submersion accident is with a fully flooded reactor; however, this work shows that, depending on the type and amount of SSA in the reactor, a submerged but unflooded reac...

A review of refractory materials for vapor-anode AMTEC cells

Recently, refractory alloys have been considered as structural materials for vapor-anode Alkali Metal Thermal-to-Electric Conversion (AMTEC) cells, for extended (7–15 years) space missions. This paper reviewed the existing database for refractory metals and alloys of potential use as structural materials for vapor-anode sodium AMTEC cells. In addition to requiring that the vapor pressure of the material be below 10−9 torr (133 nPa) at a typical hot side temperature of 1200 K, other screening considerations were: (a) low thermal conductivity, low thermal radiation emissivity, and low linear thermal expansion coefficient; (b) low ductile-to-brittle transition temperature, high yield and rupture strengths and high strength-to-density ratio; and (c) good compatibility with the sodium AMTEC operating environment, including high corrosion resistance to sodium in both the liquid and vapor phases. Nb-1Zr (niobium-1% zirconium) alloy is recommended for the hot end structures of the cell. The niobium alloy C-103, w...

S∧4 Reactor: Operating Lifetime and Estimates of Temperature and Burnup Reactivity Coefficients

The S∧4 reactor has a sectored, Mo‐14%Re solid core for avoidance of single point failures in reactor cooling and Closed Brayton Cycle (CBC) energy conversion. The reactor is loaded with UN fuel, cooled with a He‐Xe gas mixture at ∼1200 K and operates at steady thermal power of 550 kW. Following a launch abort accident, the axial and radial BeO reflectors easily disassemble upon impact so that the bare reactor is subcriticial when submerged in wet sand or seawater and the core voids are filled with seawater. Spectral Shift Absorber (SSA) additives have been shown to increase the UN fuel enrichment and significantly reduce the total mass of the reactor. This paper investigates the effects of SSA additions on the temperature and burnup reactivity coefficients and the operational lifetime of the S∧4 reactor. SSAs slightly decrease the temperature reactivity feedback coefficient, but significantly increase the operating lifetime by decreasing the burnup reactivity coefficient. With no SSAs, fuel enrichment is...

Reactivity Control Schemes for Fast Spectrum Space Nuclear Reactors

Several different reactivity control schemes are considered for future space nuclear reactor power systems. Each of these control schemes uses a combination of boron carbide absorbers and/or beryllium oxide reflectors to achieve sufficient reactivity swing to keep the reactor subcritical during launch and to provide sufficient excess reactivity to operate the reactor over its expected 7–15 year lifetime. The size and shape of the control system directly impacts the size and mass of the space reactors reflector and shadow shield, leading to a tradeoff between reactivity swing and total system mass. This paper presents a trade study of drum, shutter, and petal control schemes based on reactivity swing and mass effects for a representative fast‐spectrum, gas‐cooled reactor. For each control scheme, the dimensions and composition of the core are constant, and the reflector is sized to provide

Structural analyses of a PX-type, Nb-1Zr/C-103 vapor-anode AMTEC cell

5 of cold‐clean excess reactivity with each configuration in its most reactive state. The advantages and disadvantages of each configuration are discussed, along with optimization techniques and novel geometric approaches for each scheme.

Radiation Shielding Options for the Affordable Fission Surface Power System

Nb-1Zr/C-103 PX-type AMTEC cells were proposed for future space power applications and shown to have a specific mass of 19.7 g/We. In this paper, linear stress and buckling analyses of these cells, while mounted in the radioisotope generator configuration proposed by AMPS, are performed. The stress and buckling analyses are performed using the Algor finite element analysis software. The stress analyses results indicated that the total induced Von Mises stresses in the structural support members of the cells will not exceed 86% of the temperature dependent yield stresses, suggesting that no permanent deformation should be expected during launch. The buckling analysis indicated the need to use reinforcing ribs on the cell wall and to decrease the pressure of the inert cover gas in the generator to 125 kPa. The total mass of the ribbed Nb-1Zr/C-103 cell increased by 5.0% from 163.4 g to 171.5 g, while its specific mass at the predicted maximum electric power of 8.0 W/sub e/ is 21.3 g/W/sub e/.