One proposed design of a nuclear thermal rocket uses pebble-like fuel containers in a fluidized bed to achieve extremely high temperatures
They are real and they are more numerous (in design) than the general public might think. Micro nuclear reactors could be used to power large vessels, production facilities (e.g. water purification, or mines), or small (remote) villages...and maybe aircraft?
A traveling-wave reactor, or TWR, is a type of fourth-generation nuclear reactor that nuclear engineers anticipate can convert fertile material into fissile fuel as it runs using the process of nuclear transmutation. TWRs differ from other kinds of fast-neutron and breeder reactors in their ability to, once started, reach a state whereafter they can achieve very high fuel utilization while using no enriched uranium and no reprocessing, instead burning fuel made from depleted uranium, natural uranium, thorium, spent fuel removed from light water reactors, or some combination of these materials. The name refers to the design characteristic that fission does not happen in the entire TWR core, but takes place in a fairly localized zone that advances through the core over time. TWRs could theoretically run, self-sustained, for decades without refueling or removing any used fuel from the reactor.
The TerraPower team includes scientists and engineers from Lawrence Livermore National Laboratory, the Fast Flux Test Facility, Microsoft, and various universities, as well as management with experience at Siemens A.G., Areva NP, the ITER project, and the U.S. Department of Energy.
Gen4 Energy Inc
(formerly Hyperion Power Generation, Inc.) is a privately held corporation formed to construct and sell several designs of relatively small (70 MW thermal, 25 MW electric) nuclear reactors, which they claim will be modular, inexpensive, inherently safe, and proliferation-resistant. According to news coverage, these reactors could be used for heat generation, production of electricity, and other purposes, including desalinization.
The company is currently attempting to license their technologies through the U.S. Nuclear Regulatory Commission. Hyperion intends to pursue the licensing of the uranium nitride, lead-bismuth small reactor with the U.S. Nuclear Regulatory Commission (NRC), though the firm's deployment schedule - the target date for deployment will be by the end of 2013 - as well as indications from senior personnel within Hyperion indicates that perhaps the reactor will bypass the normal NRC process for commercial reactors - as it takes many years - and will instead be initially deployed by the U.S. Department of Energy or the U.S. Department of Defense, not subject to NRC regulation, or that Hyperion will seek a 10CFR50.21 Class 104 Research and Development reactor license from the NRC. As of May 2010. Hyperion expects to apply to the NRC for regulatory approval "within a year."
Nereus and HTR-10
The pebble bed reactor (PBR) is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative. Like other VHTR designs, the PBR uses TRISO fuel particles, which allows for high outlet temperatures and passive safety. Berkeley professor Richard A. Muller has called pebble bed reactors "in every way... safer than the present nuclear reactors, and arguably safer than the global-warming danger posed by fossil fuels".
The basic design of pebble bed reactors features spherical fuel elements called, naturally, pebbles. These tennis ball-sized pebbles are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro fuel particles called TRISO particles. These TRISO fuel particles consist of a fissile material (such as 235U) surrounded by a coated ceramic layer of silicon carbide for structural integrity and fission product containment. In the PBR, thousands of pebbles are amassed to create a reactor core, and are cooled by a gas, such as helium, nitrogen or carbon dioxide, which does not react chemically with the fuel elements.
This type of reactor is claimed to be passively safe; that is, it removes the need for redundant, active safety systems. Because the reactor is designed to handle high temperatures, it can cool by natural circulation and still survive in accident scenarios, which may raise the temperature of the reactor to 1,600 °C. Because of its design, its high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (up to 50%) and has the additional feature that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids.
The Toshiba 4s
Currently Toshiba, together with its Westinghouse subsidiary, is in the preliminary design review stage of the Design Certification process before the United States Nuclear Regulatory Commission (USNRC). Application for certification of the design is currently planned for 2012 when the standardized Design Certification application will be filed for the 4S. The most recent meeting with the NRC took place on August 8, 2008, at which time the NRC's staff met with representatives of Toshiba and Westinghouse for a pre-application presentation of a Phenomena Identification and Ranking Table (PIRT) for the Toshiba 4S (Super-Safe, Small and Simple) reactor. Lawrence Livermore National Laboratory recently released an interesting study on the Toshiba 4S design, which provides an overview of the 4S design and suggests that certain goals may be easier to meet if lead is used as the coolant rather than sodium, due to lead's high transparency to neutrons and low transparency to gamma radiation, though lead has a higher melting point than sodium does.