Clean lithium fission saltwater rocket

  • #26
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3He + T gives 4He + D and 4He + p + n to about 50% each (plus 9.5 and 12.1 MeV respectively).
Various endothermic reactions have a probability to release D.
Where is the point?
 
  • #27
3He + T gives 4He + D and 4He + p + n to about 50% each (plus 9.5 and 12.1 MeV respectively).
Various endothermic reactions have a probability to release D.
Where is the point?
A low-energy-barrier exothermic fission reaction which releases D could potentially do so at high enough particle energy to result in 6Li + D fusion with decay of the resulting 8Be to 2 4He at 22 MeV. Such a fuel could be mixed into the 6Li saltwater and serve as the trigger for the more energetic reaction.
 
  • #28
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The probability that a released D nucleus does fusion is tiny, it does not matter where it got its speed from.
 
  • #29
sevenperforce, by now you should have read "Fission-fragment rocket" wikipedia page.

It has a "Dusty Plasma" section, where it describes an engine which does not look like it has immediate show-stoppers.

Roughly, there is a ball of fissile dust kept levitating by e.g. electrostatic forces. This dust cloud is a leaky fast reactor. About half of neutrons and fission products escape (you need to capture at least half of neutrons, otherwise "reactor" won't be able to stay critical; and if 50% neutrons are captured, so are about the same percentage of fission fragments fail to escape the cloud).

About half of escaping neutrons and fission products escape "in the right direction" to become exhaust. The rest impinges onto the ship and can be used for power generation etc.

Thus, ~1/4 of fission becomes exhaust.

If this can be made to work, and with no further losses, such engine gives Isp of up to 0.01c, which is enough even for some types of interstellar missions.

IOW. I don't understand your "quest" for unobtanium-class fission fuel. The problems, of course, exist, but they are not about lack of suitable fuel. They are in detailed thermal and neutronic design of such "dust reactor" and other systems around it, and consequently in finding a source of funding for this R&D.
 
  • #30
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Can the helium-4 exhaust be collected(MHD) to power the neutron generator?
 
  • #31
Astronuc
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Can the helium-4 exhaust be collected(MHD) to power the neutron generator?
In short, NO! One is leaving out a lot of details, e.g., the stream in which the He is present. Extracting/collecting the He, if used in a propellant stream, would defeat the purpose of using it for propulsion.

This discussion is about 4.5, almost 5 years old, and I had forgotten about my participation. There is a lot wrong with the discussion on the part of the original poster, who seems not to have a good grasp on engineering or physics.

The discussion is a prime example of someone who takes an reaction equation or concept (single piece of physics) and builds a faulty case for a complex system (multiphysics).

A single reaction, e.g., 6Li + n => T + α + energy is a single reaction that would take place in a population of 6Li, depending on the atomic density of the Li and the neutron flux. The physics and engineering get very complex depending on the various aspects such as propellant mass flow rate (thrust) and power generation. Note that the propellant is consumed, so somewhere, there is a mass of stored propellant that must be introduced into the neutron flux.

In nuclear systems, only a tiny fraction of the fuel (target material) is consumed at any given time. So one cannot simply take a single reaction equation and declare, Voila!, we have thrust. Rather, one must consider that reaction takes place in the presence of other atoms that do not experience the same reaction so that the energy of the one reaction is distributed to billions, trillions, . . . . 1014 - 1020 other atoms, depending on the density of the matter (in an engineered system) in which the reaction takes place. Natural systems like stars can achieve conditions (i.e., pressures, and mass and energy densities) well out of reach of human engineered systems.

Nuclear salt water systems are not practical for propulsion, period!
 
  • #32
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In short, NO! One is leaving out a lot of details, e.g., the stream in which the He is present. Extracting/collecting the He, if used in a propellant stream, would defeat the purpose of using it for propulsion.

This discussion is about 4.5, almost 5 years old, and I had forgotten about my participation. There is a lot wrong with the discussion on the part of the original poster, who seems not to have a good grasp on engineering or physics.

The discussion is a prime example of someone who takes an reaction equation or concept (single piece of physics) and builds a faulty case for a complex system (multiphysics).

A single reaction, e.g., 6Li + n => T + α + energy is a single reaction that would take place in a population of 6Li, depending on the atomic density of the Li and the neutron flux. The physics and engineering get very complex depending on the various aspects such as propellant mass flow rate (thrust) and power generation. Note that the propellant is consumed, so somewhere, there is a mass of stored propellant that must be introduced into the neutron flux.

In nuclear systems, only a tiny fraction of the fuel (target material) is consumed at any given time. So one cannot simply take a single reaction equation and declare, Voila!, we have thrust. Rather, one must consider that reaction takes place in the presence of other atoms that do not experience the same reaction so that the energy of the one reaction is distributed to billions, trillions, . . . . 1014 - 1020 other atoms, depending on the density of the matter (in an engineered system) in which the reaction takes place. Natural systems like stars can achieve conditions (i.e., pressures, and mass and energy densities) well out of reach of human engineered systems.

Nuclear salt water systems are not practical for propulsion, period!
Thanks for the reply, Astronuc.

I actually meant magnetic nozzle not MHD. The magnetic nozzle's magnetic field will be used to generate electricity, similar to z-pinch designs which make electricity for the capacitor bank.

I was under the impression Zubrin's NSWR was feasible, but too Dr. Strangelove to be made.
 
  • #33
Astronuc
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I was under the impression Zubrin's NSWR was feasible, but too Dr. Strangelove to be made.
No, it's not feasible. By invoking Dr. Strangelove, is one implying 'fictional'. If so, I would agree.

A Wikipedia article states "One design would generate 13 meganewtons of thrust at 66 km/s exhaust velocity (or exceeding 10,000 seconds ISP . . . ," however, there are no calculations. I'd have to look at the calculations, but based on experience, I'm skeptical. I'd want to see the temperature and pressure of the propellant in the nozzle throat.

There is another claim, "In a NSWR the nuclear salt-water would be made to flow through a reaction chamber and out of an exhaust nozzle in such a way and at such speeds that critical mass will begin once the chamber is filled to a certain point; however, the peak neutron flux of the fission reaction would occur outside the vehicle." I consider such a claim to be nonsense.

In a nuclear propulsion system, whatever energy is extracted from the propellant to maintain (or power) the system is then unavailable for propulsion. Rocket propulsion engineers know this.
 
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  • #35
Astronuc
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The design is publicly accessible
I didn't find temperature and pressure values but the design parameters there allow reconstruction of temperature and pressure.
I found the link through the Wikipedia page on the NSWR.

From the abstract: "When the plenum has filled to a certain point, the fluid assembly within it exceeds critical mass and goes prompt supercritical, with the neutron flux concentrated on the downstream end due to neutron convection."

This is insane! The neutron flux will not be concentrated downstream, but will be more or less an isotropic source, with some fraction streaming upstream to the source. The paper does not mention the initiating neutron source.

In section 4, an example: "Taking the velocity of a thermal neutron as 2200 m/s, this implies that the fluid velocity needs to be 66 m/s. Since this is only about 4.7% of the sound speed of room temperature water, it should be possible to spray the water into the plenum chamber at this velocity. The total rate of mass flow through the chamber is about 196 kg/s."

The energy content of the detonating fluid is then 3.4 x 109 J/kg. Assuming a nozzle efficiency of 0.8, this results in an exhaust velocity of 66,000m/s, or a specific impulse of 6,730 seconds. The total jet power output of the engine is 427,000 MW (427 GWt), and the thrust is 12.9 MN or 2.9 Mb.

The 427 MWt is equivalent to 122 3500 MWt LWR nuclear plants! Into 196 kg/s?!? Really!?

From 66 m/s to 66,000 m/s!? Imagine the shock waves traveling back through the engine and spacecraft.

The paper goes on to other mission scenarios, rather than focus on the details of the propulsion process.
 
  • #36
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This is insane! The neutron flux will not be concentrated downstream, but will be more or less an isotropic source, with some fraction streaming upstream to the source.
It doesn't claim an anisotropic flux, it claims there are more neutrons in one place (downstream) than another (upstream).
The 427 MWt is equivalent to 122 3500 MWt LWR nuclear plants! Into 196 kg/s?!? Really!?
What's the specific problem here? Sure, controlling that will be a challenge.
 
  • #37
16
0
So can the reaction be "skimmed off" to power the neutron generator? Or does it only apply to pulsed designs?

skimming off
 

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