TECHNICAL FIELD

The present subject matter relates to examples of nuclear thermal propulsion systems and nuclear reactor systems. The present subject matter also encompasses passive reactivity control of nuclear thermal propulsion reactors.

BACKGROUND

Conventional chemical based propulsion systems commonly deployed in rockets rely on an oxidizer, such as oxygen, to generate a chemical reaction in order to create thrust. Nuclear thermal propulsion (NTP) systems have the potential to deliver thrust values that far exceed chemical based fuels. Typically, this is done by heating a propellant, typically low molecular weight hydrogen, to over 2,6000 Kelvin by harnessing thermal energy from a nuclear reactor.

NTP is an appealing technology with prospects for becoming the propulsion system of choice for human missions beyond low earth orbit. Numerous mission architectures call NTP the preferred approach for a 2030s human Mars mission for its ability to produce significant amounts of thrust while operating at a high specific impulse.

The design of NTP systems dates back to the Nuclear Engine for Rocket Vehicle Applications (NERVA) work done by NASA. The NERVA design typically consists of a small nuclear fission reactor, turbopump assembly (TPA), nozzle, radiation shield, assorted propellant lines, pressure vessel, and support hardware. Thermal energy gained by the propellant during an expansion cycle is used to power the rocket.

In conventional NTP designs, control drums are used to adjust reactor reactivity. Unfortunately, reactivity control of NTP reactors through the use of the control drums can be problematic in terms of operating requirements and effect on the performance of the propulsion system.

SUMMARY

Hence, there is room for further improvement in NTP systems and devices that incorporate such NTP systems. The passive control technologies disclosed herein simplify the control of an NTP system and improve the overall performance during operation. With some of the passive control technologies, little to no active mechanical movement of circumferential control drums will be required for the majority of NTP operation.

The passive control technologies mitigate the effect of the build-up of135Xe, a powerful neutron poison, during operation. While the technologies disclosed are utilized with low-enriched uranium (LEU) NTP systems (e.g., graphite composite fuel and tungsten (W) CERMET fuel), the technologies have application to any moderated NTP system, including highly-enriched uranium (HEU) graphite composite fueled NTP reactor systems. An LEU system has 20% or lower235U. A ceramic and metal matrix (CERMET) fuel can include (U, PuO2), (enriched U, UO2), or other fuels.

In an example, a nuclear thermal propulsion system comprises a nuclear reactor core. The nuclear reactor core includes an array of fuel elements and an array of tie tubes adjacent the array of fuel elements. Each tie tube includes a propellant supply passage to flow a propellant, an inner tie tube layer surrounding the propellant supply passage, a moderator sleeve surrounding the inner tie tube layer, a propellant return passage surrounding the moderator sleeve, and an outer tie tube layer surrounding the propellant return passage.

A burnable poison can be dispersed in the nuclear reactor core. For example, the burnable poison is dispersed in the array of fuel elements or the array of tie tubes. The burnable poison can include Gadolinium (Gd). The Gd can be dispersed in an alloy that forms the outer tie tube layer. The propellant can be hydrogen. The Gd dispersed in the outer tie tube layer alloy can be in a quantity greater than 0 parts per million (ppm) and less than one-thousand (1,000) ppm. The Gd in the outer tie tube layer alloy can be a natural isotopic composition or an enriched157Gd isotope. The outer tie tube layer alloy can further include nickel, chromium, and iron. The outer tie tube layer alloy can further include molybdenum, niobium, cobalt, manganese, copper, aluminum, titanium, silicon, carbon, sulfur, phosphorus, and boron. The outer tie tube layer alloy can also be formed of zirconium or zirconium carbide.

The array of fuel elements can include a graphite composite fuel formed of low-enriched uranium (LEU) having 20% or lower235U. The array of fuel elements can include a tungsten ceramic and metal matrix (CERMET) fuel formed of low-enriched uranium (LEU) having 20% or lower235U. The moderator sleeve can include a solid hydride. The Gd dispersed in the outer tie tube layer alloy can be in a quantity of 20 parts per million (ppm) to 200 ppm.

Each tie tube can further comprise an inner gap between the inner tie tube layer and the moderator sleeve, a graphite insulation layer surrounding the outer tie tube layer, a medial gap between the outer tie tube layer and the graphite insulation layer, an outer gap surrounding the graphite insulation layer, a graphite sleeve surrounding the outer gap, and a coating formed of zirconium carbide or niobium carbide surrounding the graphite sleeve. The graphite insulation layer can include zirconium carbide (ZrC), titanium carbide, silicon carbide, tantalum carbide, hafnium carbide, ZrC—ZrB2composite, or ZrC—ZrB2—SiC composite.

The propellant supply passage, the inner tie tube layer, the moderator sleeve, the propellant return passage, and the outer tie tube layer can be radially arranged. A respective radial wall thickness of the outer tie tube layer can be less than the inner tie tube layer. The array of fuel elements can be interspersed with the array of tie tubes and each of the tie tubes can be in direct or indirect contact with at least one fuel element. A plurality of circumferential control drums can surround the array of fuel elements and the array of tie tubes to change reactivity of the nuclear reactor core.

In one example, the burnable poison can include Gadolinium (Gd) and the Gd can be dispersed in an alloy that forms the inner tie tube layer. In on example, the burnable poison can include Gadolinium (Gd) and the Gd can be dispersed in each of the fuel elements. In one example, the moderator sleeve can be formed of zirconium hydride and the burnable poison can include Gadolinium (Gd). The Gd can be dispersed in the zirconium hydride that forms the moderator sleeve.

In one example, the nuclear reactor core can further comprise at least one wire formed of an alloy that is housed inside the nuclear reactor core and separate from the array of fuel elements and the array of tie tubes. The burnable poison can include Gadolinium (Gd). The Gd can be dispersed in the at least one alloy wire.

The nuclear thermal propulsion system can further comprise a propellant tank to store the propellant, a nuclear reactor core inlet directly or indirectly connected to the nuclear reactor core, and a propellant line directly or indirectly connected to the propellant tank to flow the propellant to the nuclear reactor core inlet. The nuclear thermal propulsion system can further comprise a propellant density control valve system directly or indirectly connected between the propellant line and the nuclear reactor core inlet to regulate density of the propellant flowing into the nuclear reactor core.

The nuclear thermal propulsion system can further comprise a turbopump assembly comprising at least one turbopump that includes a turbine and a pump. The pump can be configured to cool the nuclear reactor core during a burn cycle by flowing the propellant from the propellant tank through the propellant line to the propellant density control valve system, and then through the nuclear reactor core inlet.

The nuclear reactor core inlet can include a tie tube inlet to flow the propellant to the array of tie tubes and can be directly or indirectly connected to the propellant density control valve system and a respective propellant supply passage of a respective tie tube in the array of tie tubes. The pump can be further configured to flow the propellant through the tie tube inlet, through the respective propellant supply passage of the respective tie tube in the array of tie tubes, and then through a respective propellant return passage of the respective tie tube.

The nuclear thermal propulsion system can further comprise a propellant heating line directly or indirectly connected to the turbine and the respective propellant return passage of the respective tie tube and a second propellant density control valve system directly or indirectly connected to the turbine and the nuclear reactor core inlet to regulate density of the propellant flowing back to the nuclear reactor core inlet. The pump can be configured to flow the propellant returned by the respective propellant return passage to the turbine via the propellant heating line. The turbine can be configured to flow the propellant back to the second propellant density control valve system and the nuclear reactor core inlet.

The nuclear reactor core inlet can further include a fuel element inlet to flow the propellant to the array of fuel elements and can be directly or indirectly connected to the second propellant density control valve system and a respective propellant passage of a respective fuel element of the array of fuel elements. The turbine can be configured to flow the propellant to the second propellant density control valve system, through the fuel element inlet, and then through the respective propellant passage of the respective fuel element of the array of fuel elements.

The nuclear thermal propulsion system can further comprise a pressure vessel housing the nuclear reactor core that can be directly or indirectly connected to the propellant density control valve system. The pressure vessel can include a pressure vessel inlet connected to the periphery of the pressure vessel that is outside of the nuclear reactor core and a pressure vessel outlet to return the propellant passing through the periphery of the pressure vessel. The nuclear thermal propulsion system can further comprise a propellant heating line directly or indirectly connected to the turbine and the pressure vessel outlet. The pump can be further configured to flow the propellant to the pressure vessel inlet, through the periphery of the pressure vessel, and then flow the returned propellant from the pressure vessel outlet to the turbine via the propellant heating line.

The nuclear thermal propulsion system can further comprise a nozzle, a plurality of control drums, a neutron reflector, and a second propellant density control valve system. The second propellant density control valve system can be directly or indirectly connected between the propellant line and the pressure vessel inlet to regulate density of the propellant flowing into the nozzle, the periphery of the pressure vessel, the neutron reflector, and the plurality of control drums.

The propellant density control valve system can include a regulator valve to adjust density of the propellant passing through the regulator valve from an initial density when entering the regulator valve to a regulated density when exiting the regulator valve. The propellant density control valve system can also include an actuator to actuate the regulator valve by adjusting the regulated propellant density upwards during each subsequent burn cycle to maintain constant reactivity at a beginning of each of the subsequent burn cycles. The actuator can be electric, mechanical, thermal, magnetic, or a combination thereof.

The propellant density control valve system can further include a flow control circuit to control speed of the actuator. The flow control circuit can be a bleed-off circuit, a meter-out circuit, or a meter-in circuit.

The regulator valve and the actuator can form a solenoid valve or an electrohydraulic servovalve. The regulator valve and the actuator can form the solenoid valve and the solenoid valve can be controlled by electric signals conveyed from an external computer, a digital circuit, an analog circuit, or a combination thereof.

The propellant can be hydrogen and the regulator valve can be configured to adjust density of the hydrogen from the initial density to the regulated density by varying pressure between 100% and 200% of nominal hydrogen pressure, inclusive, to increase reactivity of the nuclear reactor core. The nominal hydrogen pressure can be 5 to 15 megapascals (MPa).

Additional objects, advantages and novel features of the examples will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by production or operation of the examples. The objects and advantages of the present subject matter may be realized and attained by means of the methodologies, instrumentalities and combinations particularly pointed out in the appended claims.

