This design is thought to be possible of increasing the specific impulse to about seconds 9. Such a design could share design elements with a pebble-bed reactor , several of which are currently generating electricity. The project was canceled before testing. The pulsed nuclear thermal rocket, not to be confused with nuclear pulse propulsion which is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust , is a type of solid nuclear thermal rocket for thrust and specific impulse I sp amplification.
Because the residence time of the propellant in the chamber is short, an important amplification in energy is attainable by pulsing the nuclear core, which can increase the thrust via increasing the propellant mass flow. However, the most interesting feature is the capability to obtain very high propellant temperatures higher than the fuel and then high amplification of exhaust velocity. This is because, in contrast with the conventional stationary solid NTR, propellant is heated by the intense neutron flux from the pulsation, which is directly transported from the fuel to the propellant as kinetic energy.
By pulsing the core it is possible to obtain a propellant hotter than the fuel. However, and in clear contrast with classical nuclear thermal rockets including liquid and gas nuclear rockets , the thermal energy from the decay of fission daughters is unwanted. Very high instantaneous propellant temperatures are hypothetically attainable by pulsing the solid nuclear core, only limited by the rapid radiative cooling after pulsation. Liquid core nuclear engines are fueled by compounds of fissionable elements in liquid phase. A liquid-core engine is proposed to operate at temperatures above the melting point of solid nuclear fuel and cladding, with the maximum operating temperature of the engine instead being determined by the reactor pressure vessel and neutron reflector material.
The higher operating temperatures would be expected to deliver specific impulse performance on the order of to seconds A liquid-core reactor would be extremely difficult to build with current technology. One major issue is that the reaction time of the nuclear fuel is much longer than the heating time of the working fluid.
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If the nuclear fuel and working fluid are not physically separated, this means that the fuel must be trapped inside the engine while the working fluid is allowed to easily exit through the nozzle. An alternative liquid-core design is the nuclear salt-water rocket. In this design, water is the working fluid and also serves as the neutron moderator. The nuclear fuel is not retained which drastically simplifies the design. However, the rocket would discharge massive quantities of extremely radioactive waste and could only be safely operated well outside the Earth's atmosphere and perhaps even entirely outside earth's magnetosphere.
The final fission classification is the gas-core engine. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or nuclear lightbulb engine, where the gaseous nuclear fuel is contained in a super-high-temperature quartz container, over which the hydrogen flows.
The closed cycle engine actually has much more in common with the solid-core design, but this time is limited by the critical temperature of quartz instead of the fuel and cladding.
The Soviet RD went through a series of tests at the nuclear test site near Semipalatinsk. In October , Russia's Keldysh Research Center confirmed a successful ground test of waste heat radiators for a nuclear space engine, as well as previous tests of fuel rods and ion engines.
Twenty individual engines were tested, with a total of over 17 hours of engine run time. When NASA was formed in , it was given authority over all non-nuclear aspects of the Rover program. The NERVA program was intended to lead to the entry of nuclear thermal rocket engines into space exploration. Unlike the AEC work, which was intended to study the reactor design itself, NERVA's goal was to produce a real engine that could be deployed on space missions.
Progressively higher power densities culminated in the Pewee. During this program, the NERVA accumulated over 2 hours of run time, including 28 minutes at full power. A number of other solid-core engines have also been studied to some degree. It featured a split-nozzle that could be rotated to the side, allowing it to take up less room in the Shuttle cargo bay. A related design that saw some work, but never made it to the prototype stage, was Dumbo. The Dumbo reactor consisted of several large barrel-like tubes which were in turn constructed of stacked plates of corrugated material. The corrugations were lined up so that the resulting stack had channels running from the inside to the outside.
Some of these channels were filled with uranium fuel, others with a moderator, and some were left open as a gas channel. Hydrogen was pumped into the middle of the tube, and would be heated by the fuel as it traveled through the channels as it worked its way to the outside. The resulting system was lighter than a conventional design for any particular amount of fuel.
Advances in high-temperature metals, computer modelling and nuclear engineering in general resulted in dramatically improved performance. The reactor was not intended for flight and named after the flightless bird. The core was simply a stack of uncoated uranium oxide plates onto which the hydrogen was dumped. Two additional tests of the basic concept, A1 and A3, added coatings to the plates to test fuel rod concepts. The KIWI B series were fueled by tiny uranium dioxide UO 2 spheres embedded in a low- boron graphite matrix and coated with niobium carbide. Nineteen holes ran the length of the bundles, through which the liquid hydrogen flowed.
On the initial firings, immense heat and vibration cracked the fuel bundles. The graphite materials used in the reactor's construction were resistant to high temperatures but eroded under the stream of superheated hydrogen, a reducing agent. The fuel species was later switched to uranium carbide , with the last engine run in The fuel bundle erosion and cracking problems were improved but never completely solved, despite promising materials work at the Argonne National Laboratory.
The final engine in this series was the XE, designed with flight representative hardware and fired into a low-pressure chamber to simulate a vacuum. The series all generated MW, and many of the tests concluded only when the test-stand ran out of hydrogen propellant. The B run in February improved this to MW for 30 minutes. It was fired several times at MW in order to test coatings made of zirconium carbide instead of niobium carbide but Pewee also increased the power density of the system. The nuclear demonstration test will occur in late summer or early fall of The test will include connecting heat pipes and Stirling engines enclosed in a vacuum chamber sitting on the top of a critical experiment stand.
The critical experiment stand has a lower plate than can be raised and lowered. On this plate will be stacked rings of Beryllium Oxide BeO that form the neutron reflector in the reactor concept. A critical mass is achieved by raising the BeO reflector to generate fission in the reactor core.
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Once fission has begun, the BeO reflector will be slowly raised to increase the temperature in the system to degrees Centigrade. For the purpose of testing only, two of the eight Stirling engines will make electricity, the others will only discard heat. The data gained will inform the engineers regarding startup and shutdown of the reactor, how the reactor performs at steady state, how the reactor load follows when Stirling engines are turned on and off and how the system behaves when all cooling is removed.
This data will be essential to moving forward with a final design concept. Lessons learned from the kiloPower development program are being leveraged to develop a Mega Watt class of reactors termed MegaPower reactors. These concepts all contain intrinsic safety features similar to those in kiloPower, including reactor self-regulation, low reactor core power density and the use of heat pipes for reactor core heat removal.
Kilopower, Megapower reactors would revolutionize energy safety and space and military applications
The use of these higher power reactors is for terrestrial applications, such as power in remote locations, or to power larger human planetary colonies. The MegaPower reactor concept produces approximately two megawatts of electric power. The reactor would be attached to an open air Brayton cycle power conversion system. A Brayton power cycle uses air as the working fluid and as the means of ultimate heat removal. The development costs for more advanced reactor concepts are even less firm.
Kilopower, Megapower reactors would revolutionize energy safety and space and military applications. The Kilopower reactors are tiny nuclear reactors that have been redesigned for safe power. The full power runs for the test reactors will be done this month and the next two months. Brian Wang. He is known for insightful articles that combine business and technical analysis that catches the attention of the general public and is also useful for those in the industries.
He is also involved in angel investing and raising funds for breakthrough technology startup companies. He has been interviewed for radio, professional organizations. He was recently interviewed by the radio program Steel on Steel on satellites and high altitude balloons that will track all movement in many parts of the USA. He fundraises for various high impact technology companies and has worked in computer technology, insurance, healthcare and with corporate finance.
Past development approaches relied on sophisticated control systems to reduce or eliminate such a likelihood. Luckily, reactors also have an inherent ability to self-correct via negative temperature reactivity feedback; reactor power automatically decreases as core temperature increases, and vice versa. It has been known that strongly reflected small compact fast reactors, such as kiloPower, can be designed to maximize these mechanisms to a point of being totally self-regulating.
A scaled up 2 megawatt system would be expected to weigh about 35 metric tons. It would transportable by air and highway. To that end, multi-scale and multi-physics simulations are relied upon to perform high fidelity design studies that explicitly examined a how choices related to fabrication, alloying and bonding techniques would affect the internal crystalline structure of each nuclear component and in turn b how that morphology affects that components thermal, mechanical and nuclear performance at conditions of interest.
A key objective of the affordable strategy is that the nuclear components can be fabricated to the exacting tolerances demanded by the designers. This includes not only the physical dimensions, but also density and crystalline phase of the alloys. After several joint efforts, an exact replica of the kiloPower core was fabricated at Y with depleted uranium.
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