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March 11, 2010

Dusty Plasma Based Fission Fragment Nuclear Reactor

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Dusty Plasma Based Fission Fragment Nuclear Reactor (7 page pdf)

We propose an innovative nuclear power generation system design using dusty radioactive (fissile or not) material plasma as a fuel. The fission fragments or decay products accelerated during the disintegration process to velocities of 3-5% of the speed of light are trapped and collected in a simple combination of electric and magnetic fields resulting in a highly efficient (90%), non-Carnot, DC power supply. In a conventional nuclear reactor this high kinetic energy of the fission fragments is dissipated by collisions to generate heat, which is converted to electrical power with e±ciencies of no more than 50%. Alternatively, the fission fragments produced in our dusty plasma reactor can be used directly for providing thrust. The highly directional fission fragment exhaust can produce a special impulse of one million seconds resulting in burnout velocities several thousand times those attainable today. Previous concepts su®ered from impractical or inadequate methods to cool the fission fuel. In this work the heating problem is overcome by dividing the solid fuel into small dust particles and thereby increasing the surface to volume ratio of the fuel. The small size of the fuel particle allows adequate cooling to occur by the emission of thermal radiation.




Further improvements in nuclear propulsion system efficiency beyond nuclear-electric (NEP) are possible. The fission process accelerates the fission fragments to velocities between 3-5% of the speed of light, far faster than the 0.027% achieved by NEP, which uses a conventional nuclear reactor to convert the kinetic energy of the fission fragments into heat, the heat into electricity, and the electricity back into Xe ion kinetic energy with eficiencies much less than 40%. In the fission fragment reactor, the high-speed fragments are used directly as the rocket exhaust after charge neutralization. Therefore the fission fragment rocket can produce a specific impulse (Isp) greater than one million seconds.

Previous concepts su®ered from impractical or inadequate methods to cool the fission fuel. In this work the heating problem is overcome by dividing the solid fuel into small dust particles and thereby increasing the surface to volume ratio of the fuel. The small size of the fuel particle allows adequate cooling to occur by the emission of thermal radiation.

In direct conversion the kinetic energy of the charged fission fragments is extracted by deceleration in an electrostatic field to directly produce electrical energy bypassing the Carnot thermodynamic cycle. Thus energy conversion efficiencies achievable with direct conversion methods approach 90%. In the case of propulsion, the fission fragments are used as the rocket exhaust after charge neutralization. The usual performance criterion for rocket propulsion is specific impulse (Isp), which is the exhaust velocity divided by 9.8 m/s2. The fission fragment rocket could produce Isp of 10^6 seconds compared to 350-450 s for chemical rockets or 3000-10000 s for ion engines. As a result, burnout velocities several thousand times those attainable today would be possible.

In our concept of a fission fragment reactor , the fuel consists of a cloud of nano-particle dust (< 100 nm diameter) composed of fissile material. This configuration of the fuel allows the fission fragments to escape from the fuel particle with a high probability. In addition, the large surface to volume ratio of the fuel particles enables them to transfer heat effectively by radiation directly into the space environ- ment. The fuel particles and the fission fragments in the core of the reactor form a dusty plasma cloud. The significant difference in both the energy per charge and the mass per charge ratios between the fuel particles and the fission fragments allows the fissile dust to be electrostatically or magnetically contained within the reactor core while the more energetic fission fragments are extracted for power or thrust. The electrical conversion unit is in the exhaust chamber, which operates on the principle of direct collection of charged particles. Another interesting feature of this system should be noted: by adjusting the strength of the magnetic mirror, the system can be adjusted to produce either high Isp thrust or electrical power or both. Field strengths between 0.33 and 0.63 Tesla-meters are required, which can be achieved with current magnet technology.


Scaling Chapline's moderator mass by 0.25 therefore, resulted in a 6 ton moderator, to which we added 2 tons for radiators and liquid metal cooling, 1 ton for magnets, power recovery, and coils, for a dry weight of 9 tons. Supposing an engineering and scientific payload of another ton, gives us an order-of-magnitude estimate of a 10 ton spacecraft. A 10 year trip to the gravitational lens point 550AU distant from the sun, would take a delta-V of about 2% the speed of light. We assumed that the fission fragments had an exhaust velocity of 0.05 c (Isp=1.5 million), to obtain a fuel fraction 3% that of the rocket. We then added the mass of the fuel to the mass of the rocket to get the total mass, and multiplied by the acceleration implied by the mission profile to get the thrust required. This thrust had to be provided by fission fragments, which gave us the power level of the reactor, assuming some 46% of the fragments provided thrust. From these considerations we could estimate the power required by the fission fragment rocket to enable various missions.

A 10 year mission to the 550AU gravitational lens point would require only 180kg of nuclear fuel, and a 350MW reactor power, well within the calculated thermal limit of 1GW. A 30 year trip to the Oort cloud at 0.5 Ly is more strenuous, requiring a 5.6 GW reactor. And a 50 year trip to Alpha Centauri, 4 Ly distant, is probably not feasible, requiring a 208 GW reactor, and consuming 240 tons of fission fuel.

Fission fragment reactors have several substantial benefits over other reactor designs including higher electrical efficiency and higher specific impulse thrust. Previous designs had difficulties with keeping the reactor core cool, which we propose to overcome by using dusty plasma fuel.


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