Thursday, September 3, 2015

Small Fission Reactors for Interstellar Probes

As noted in another post, it may be that fusion reactors cannot be built small. It is certainly possible that a thousand years of bright aliens doing engineering on fusion reactors gets them nice and small and efficient, but for this post, let’s be pessimistic. No small fusion, so fission is the player for any small vessel traveling to another star.

We are talking about interstellar probes, but probes for alien civilizations could be of several types. One could be for the purpose of preparing for emigration. It would be a follow-up to observations of a potentially habitable planet, probably a sweet spot planet but maybe something a lot worse, and the desire would be to take closer pictures of it, many scientific measurements from orbit, and perhaps put some robots down on the surface to get even more data.

It is more efficient and faster as well to carry along a transmitter and send back the collected data. If the distance is less than 100 light years or so, this is quite reasonable, based on what we know now about the engineering of communications systems. Probably another millennium of research will make it even more efficient, but for a ballpark estimate, let’s say the whole payload package is 100 tons. So the reactor has to propel a 100 ton payload up to 0.01 c or even 0.1 c. It has to last for the travel time of perhaps 100 to 1000 years, plus a short stint while in orbit around the potential new colony planet.

The power profile would be propulsion power for a period to be determined, hotel load to keep the probe alive for the travel time of up to 1000 years, propulsion power for deceleration for the same length of time as acceleration, some orbital maneuver power, and then hotel load plus communication power for a period to be determined. If it was planned to watch the rovers run around the planet for as long as 10 years, then the communications power would have to last that long or longer. Rover power is something to be discussed elsewhere.

This is a very strange power profile, as compared to anything useful for a planetary power station. A power station produces roughly the same amount of power for its life cycle. It is also not like something that would be used for an industrial purpose, either on a home planet near a distant mine, or on another planet in the solar system where a small colony might be located to search for or gather rare elements.

It is less strange if we consider a continuously accelerating probe, which then immediately turns around mid-course and begins decelerating. This is a somewhat slower trajectory that one in which maximum speed is achieved early on, but it is less demanding of the power source. Longevity is still there, however. How can a reactor be designed to run for a thousand years? Other design specifications include the need for minimal maintenance, probably shadow shielding for the payload and perhaps the propulsor, and very high reliability. The reactor must also operate in zero gravity, which eliminates some designs, such as liquid cores, at least as they are currently envisioned.

It would be nice, from the minimal maintenance and high reliability specifications, to have a single fuel load that lasts the whole flight profile. Having to automatically replace other components, such as the power extraction system, the supports, and the shielding would also be a negative.

One problem with long-lived reactor fuel elements is that as the fuel fissions, it poisons itself. U235 is the fissionable isotope in all reactors today, and many of its fission products absorb neutrons, and therefore interfere with the reactivity of the core. These neutron poisons build up with time. Some of them, for example the worst one, Xe135, will burn up and also decay, and come into equilibrium rather quickly, and others of lesser impact will do the same, but more slowly. The usual solution to the poisoning problem involves replacing the cores after 3% or so of the U235 has been consumed, leading to the spent fuel buildup that everyone seems to be aware of. This means that the core is much heavier than if a larger fuel consumption fraction were used, and for a spaceship, the heavier the core, the heavier the whole power plant must be, which means the propulsor must be larger and the fuel load larger, both of which have a multiplier effect on themselves and the power plant as well.

This means power plant design may not be the right paradigm to use for a probe’s nuclear power plant. In addition, power plants use only about 3 to 5% enrichment, meaning the majority of the core weight is useless U238. Research reactors used to be designed with high enrichment uranium, meaning their core weight was much less. These have been replaced with low enrichment fuel for safety and security reasons, but for a reactor which will be assembled in space and which will spend most of its life light years away from its home planet, high enrichment may be the most feasible solution to the core and reactor weight problem.

To deal with the high degree of reactivity control that long life and poison build up will entail, control rods might not be the best solution; instead, movable core pieces may be, as has been used in some research reactors. The same dangers of overheating and meltdown occur with this control method as with control rods, and the question of how to do this automatically with equipment that will last a thousand years is very challenging.

All the materials near the core will be bathed in neutrons, and if it is desired to not replace them, they need to be designed to withstand a long-term neutron flux. Certain parts can be shielded, such as the control electronics, or whatever replaces electronics when starships are being built. This leaves structural components and shielding that has to have extraordinary longevity. One way would be to manufacture all these components only from isotopically separated materials, with the isotope chosen to have a very low neutron cross-section. This would mean that the neutrons which penetrated the material would either pass out, or be captured and eventually decay to protons. The design of the materials would also have to be done to allow a buildup of hydrogen deep inside the materials without producing cracking or other structural damage.

For a first glimpse of what a star probe’s reactor might look like, we can imagine a spindly structure with the reactor core separated by as much distance as possible from the heat conversion components and the rest of the ship components. The core would be high enriched uranium or possibly other isotopes, with control methods in place to allow the core pieces to be moved to control reactivity. All the structural elements around the core would be made of uniquely enriched isotopes, with treatment of the metal, ceramic, or whatever other materials were used to prevent hydrogen embrittlement.

The only technology that is included here that has not been much explored on Earth is isotope enrichment of structural materials. This might be difficult or not, but alien civilizations reaching asymptotic technology would most likely have mastered it at some point in their climb to the heights of technology. So we assume it can be done.


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    1. The problem with artificial gravity is not so much that we don't know yet how to do it, but that we do know it will be incredibly energy-consuming. The amount of energy necessary to accelerate and decelerate is huge, but even more is needed for artificial gravity. So, since antimatter storage is barely enough for sublight speeds, artificial gravity is just out of the question.

  6. ...interstellar travel (thousands G of constant acceleration)... gravitation, dimensions and inertia: the Matter... The electromagnetic radiation: quanta (pieces) of energy in discrete amounts (minimum and independents), photons, which move in waves... Why energy quanta (photons) are attracted by gravitational fields of "stellar lenses" curving their trajectories and move on?...go ahead because they have inertia...and are attracted because the Energy also has Gravitation, but does not manifest because it no emits Graviton, or emits in imperceptible degree, because it has to be condensed enough like in matter for that, but it is sensitive to Gravitation from Matter emits... Energy and Gravitation "associated"...and...the Gravitational Force is another manifestation, unknown yet, from Energy...(no "curvature" of that called "space-time" relativistic)... When was finished contraction, F=G((m1*m2)/d²), the pre-early Universe; (no relativistic "singularity", but it was a Sphere with dimensions and outside the Infinite Empty Space); wass need have as its starting point towards the expansion, one first action: when all Universe´s Matter was almost infinitely compressed and hot with all subatomic particles in direct contact without empty space among them, then →Gravity← becomes ←Antigravity→, those hyper-dense Matter becomes Energy ("infinites": Temperature + Energy + suddenly Antigravity = Big-Bang)...already again how a New Universe in expansion cooling, appears again the Matter from Energy...and again antigravity becomes normal Gravity... Its particle exchange, the theoretical Graviton yet...maybe "quanta of gravitational energy"... The Energy (e=mc²) is sensitive to Gravitation but hardly emits Graviton... The Matter, which is condensed Energy (m=e/c²), emits now Gravitons proportionally to its density... The Matter with normal density, naturally, emits few Gravitons (weak intensity of gravitational force). But...if someday could transform some of matter, not in Energy by annihilation with antimatter but entirely in Gravitational Force with an exponential Gravitons (or antigravitons) emission...would have a way to amplify the Gravitational Force a certain amount of mass... Light (photons) can be directed with a mirror... Gravity (gravitophotons) perhaps also can be directed someday the same such as light in a car´s headlight placed in ceiling looking downwards... This would be necessary for having artificial gravitational attraction only toward the ceiling in all the floors of the ship, each down´s floor must have less own gravity-power because receives gravitational attraction from all up´s floors. This can be difficult to observe in Universe because, although with unequal intensity, there are Gravitational Forces actuating from all directions. But in...gravitational transformers...for spacecrafts to thousands G of constant acceleration...

    1. Another way to think about it is that large masses curve space-time, and light photons follow the equivalent of straight lines. This might be easier to intuit than photons with gravitational mass. The difficulty with thousands of g's of acceleration is the construction of matter objects which can withstand it without crushing; how to build such a ship when you don't have any materials that are suitable? Gravitational photons don't do what you think they might; they don't provide a uniform field of gravity, but instead carry energy out to large distances. Artificial gravity needs some different trick.