Abstract: A fuel element with an ordered particle fuel particle packing to improve lifetime and retention. The fuel element includes an encapsulation matrix and a plurality of coolant channels formed in the encapsulation matrix. Fuel element further includes a plurality of fuel particle matrices disposed within the encapsulation matrix. Each of the fuel particle matrices is ordered in a vertically aligned geometry or a twisted geometry to: (a) substantially laterally surround a contour of a respective coolant channel, and (b) orient substantially longitudinally or substantially helically along the respective coolant channel. For example, each of the fuel particle matrices can include a plurality of fuel particles that are a cluster around the respective coolant channel. The encapsulation matrix can be a high-temperature matrix. A fabrication method for the fuel element includes three-dimensional printing the fuel element and placing the plurality of fuel particles in selected locations in the fuel element.

Abstract: Radioisotope power system and propulsion system technologies to increase the energy efficiency, mass efficiency, and duration capability of a vehicle during operation. A radioisotope power system includes a radioisotope power unit that emits a plurality of radiation particles and is configured to directly or indirectly provide power, propulsion, or both power and propulsion of a vehicle. The radioisotope power system can further include a thermoelectric generator coupled to the radioisotope power unit and configured for coupling to at least one thruster. The radioisotope power system can further include an optional radiation shield configured to block a first radiation particle of the plurality of radiation particles. The radioisotope power unit can include one or more radioisotopes. The one or more radioisotopes can include an alpha emitting isotope, a beta emitting isotope, a gamma emitting isotope, or a combination thereof.

Abstract: A nuclear reactor core includes a plurality of fuel elements and moderator blocks to form a nuclear reactor core inner portion and a nuclear reactor core outer portion. The nuclear reactor core inner portion includes an inner moderator matrix formed of a plurality of inner holes that include a plurality of inner fuel openings with one or more fuel elements disposed therein. The plurality of inner holes further include a plurality of inner coolant passages to flow a coolant. The nuclear reactor core outer portion includes an outer moderator matrix formed of a plurality of outer holes that include a plurality of outer fuel openings with one or more fuel elements disposed therein. The plurality of outer holes further include a plurality of outer coolant passages to flow the coolant. The outer coolant passage diameter exceeds the inner coolant passage diameter.

Abstract: A ceramic burnable absorber includes a first phase that includes a boride, a carbide, an oxide, a nitride, a silicide, a mixture, or a solid solution containing naturally occurring boron or enriched boron. The ceramic burnable absorber further includes at least one second phase which bonds to the first phase. Ceramic burnable absorber further includes a porosity that is interconnected and is at least 30 volume percent of the ceramic burnable absorber. In some implementations, the porosity can be open to an outer surface. Ceramic burnable absorber further includes a grain size and a grain contiguity that limit a diffusion distance for helium to less than 10 ?m. Ceramic burnable absorber further includes a compressive strength exceeding 30 MPa at approximately 0 to 100 degrees Celsius. Ceramic burnable absorber can be shaped as a pellet, cylinder, polyhedron, prism, spheroid, tube, pipe, ring, truncated portion thereof, or a combination thereof.

Abstract: Passive reactivity control technologies that enable reactivity control of a nuclear thermal propulsion (NTP) system with little to no active mechanical movement of circumferential control drums. By minimizing or eliminating the need for mechanical movement of the circumferential control drums during an NTP burn, the reactivity control technologies simplify controlling an NTP reactor and increase the overall performance of the NTP system. The reactivity control technologies mitigate and counteract the effects of xenon, the dominant fission product contributing to reactivity transients. Examples of reactivity control technologies include, employing burnable neutron poisons, tuning hydrogen pressure, adjusting wait time between burn cycles or merging burn cycles, and enhancement of temperature feedback mechanisms.

Abstract: A nuclear reactor system includes a nuclear reactor core disposed in a pressure vessel. Nuclear reactor system further includes control drums disposed longitudinally within the pressure vessel and laterally surrounding fuel elements and at least one moderator element of the nuclear reactor core to control reactivity. Each of the control drums includes a reflector material and an absorber material. Nuclear reactor system further includes a control drum controller with a counterweight to impart a reverse torque on the control drum. Control drum controller includes a driven pulley coupled to the counterweight, a tension member coupled to the driven pulley to rotatably control the driven pulley and apply torque to the driven pulley, and an actuator to apply a tension force to the tension member. The actuator counteracts the reverse torque with the applied tension force, and the tension member applies the torque in response to the tension force.