Clean lithium fission saltwater rocket

[Astronuc]

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!

 

[supermath]

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.

 

[Astronuc]

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.

 

[mfb]

The design is publicly accessible
I didn't find temperature and pressure values but the design parameters there allow reconstruction of temperature and pressure.

 

[Astronuc]

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.

 

[mfb]

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.

 

[supermath]

So can the reaction be "skimmed off" to power the neutron generator? Or does it only apply to pulsed designs?

skimming off

 

[x_engineer]

Lattice confinement fusion?

 

[Astronuc]

Lattice confinement fusion?

No. Even though NASA is researching it, doesn't mean it's worthwhile for propulsion.

https://www1.grc.nasa.gov/space/science/lattice-confinement-fusion/

 

[member 697307]

As a rough approximation, the cross-section for Li6(n,$\alpha$)t is 1 to 10 barn for MeV to 100 eV neutrons, increasing to 1kbarn for thermal neutrons (source). Let's take the 1 kbarn value as upper estimate.
If we want to shoot out fuel at 10 km/s (slow) with a reactor length of 10 meter (long), and want to get 1% fission efficiency (low), we need a fission timescale of 100 milliseconds. That needs a neutron flux of 10²²/(cm² s). Oops. That is about the neutron flux you get in a supernova. Nuclear reactors are 11 to 12 orders of magnitude below that. If you just get a fission rate of 10^-13 to 10^-14, the concept does not work.

I would like to point out that the nozzle speed and the passing-through-reactor speed can be different. It seems that mfb uses 10 km/s as the propellant flow speed in the reactor, which gives a little time for Lithium-6 to pass through the neutron flux, namely, 100 milliseconds.

I believe this hurdle can be overcome by multiplying flow channels in the reactor and lowering the average speed of Lithium sea salt to increase its exposure to the neutron flux.

 

[mfb]

Check the comment I replied to, the design had the reaction happen in the nozzle. If you want it to happen in a separate reaction chamber then you have to find out how to protect that reaction chamber from the unreasonable temperatures.

 

[member 697307]

Check the comment I replied to, the design had the reaction happen in the nozzle. If you want it to happen in a separate reaction chamber then you have to find out how to protect that reaction chamber from the unreasonable temperatures.

Well, gentlemen, it appears that you both (mfb and Astronuc) are right about lithium salt water rocket. It does not work if it is based on current commercial reactor technology. Basically, it lacks power density.

I didn’t focus on the wall protection of the reaction chamber because I thought it can be done by pumping water through small holes in the wall to create a boundary layer. Instead, I was interested in proving a concept from a required power point of view.
I made some calculations based on the size and performance of NERVA XE reactor. I replaced hydrogen with salted water.

Here are some numbers:
Flow rate 461 l/s, Flow speed 1.35 m/s, Reactor length 1.35 m, Neutron flux 10E+19 n/m2-s, LiOH solubility 129 kg/m3, Li6 enrichment 95%. These things can give 2.37E+09 Watts of power.
To get the exhaust velocity of 9801 m/s with water (that is a simplification) as a propellant 1.21E+10 – 3.16E+10 Watts is required. There are two figures because of two different estimates.
So, the only way to make the lithium saltwater engine run is to increase neutron flux at least by one order of magnitude compared to conventional reactors. HFIR can deliver 2.3E+21 n/m2-s, for example, which would be enough.

As I can see, there are two hurdles there. Chamber wall protection will require additional water, which will decrease Li6 concentration. Secondly, HFIR is a big facility, and I’m not sure that, let’s say, neutron flux of 3E+20 n/m2-s can be achieved with a compact lightweight Uranium reactor.
But! If Uranium salt water is used instead of Lithium one, then the engine can work within the current neutron flux constraints of commercial reactors. Basically, it can be a mix of LSWR and Zubrin’s NSWR concepts.

 

[Astronuc]

Secondly, HFIR is a big facility,

Actually, HFIR has a small compact core (about the size of a washing machine) comprised of highly enriched uranium dispersed in aluminum alloy plates. It sits in a large pool of water, but is not in a pressurized containment. I've been there.

The reactor core is cylindrical, approximately 2 ft (0.61 m) high and 15 inches (38 cm) in diameter. A 5-in. (12.70-cm)-diameter hole, referred to as the "flux trap," forms the center of the core.

https://neutrons.ornl.gov/hfir/core-assembly
https://en.wikipedia.org/wiki/High_Flux_Isotope_Reactor

https://en.wikipedia.org/wiki/High_..._Flux_Isotope_Reactor_Fuel_Assembly_Photo.jpg

Concepts for nuclear thermal rocket (NTR) propulsion include pumping hydrogen through the core. However, where the coolant does not contain fissionable material (energy generation in the coolant, as it is the case with combustion), the neutrons (and thermal energy) must come from the fuel, so as hot has the coolant might be, the fuel is much hotter.

Then there is the matter of hydrodynamics stability and erosion/corrosion, and if a phase change in the coolant (propulsive working fluid), choked flow.

In a conventional PWR, the coolant flow velocity is on the order of 5 m/s, which in a BWR, is about 2.5 m/s, and the temperatures are relatively low (280-323°C). Liquid metal and gas cooled reactors operate at higher temperatures. When considering coolant flow velocities at the speed of sound, or in some concepts, at hypersonic velocities, then one must consider the physics of such flows.

Edit/update: The average velocity of Na coolant in the highest power FFTF (fast reactor) assembly is 6.4 m/s, with a mass flow rate of 23.4 kg/s. Ref: A.E. Waltar, A.B. Reynolds, Fast Breeder Reactors, Pergamon Press, 1981, Chapter 10, Core Thermal Hydraulics Design, p. 354.

 

[Alex A]

Just pointing out that @mfb used flux density units with cm, not meters. So the flux density in the HFIR isn't one order of magnitude too low to make the rocket work, it's 5 orders too low.

 

[member 697307]

Just pointing out that @mfb used flux density units with cm, not meters. So the flux density in the HFIR isn't one order of magnitude too low to make the rocket work, it's 5 orders too low.

Hello, Alex.
Nope, Alex, it is not right. Operating at 85 MW, HFIR produces an average thermal neutron flux of 2.3 X 10^15 n/cm2 -seconds = 2.3E+21 n/m2-s
Source https://neutrons.ornl.gov/sites/default/files/High Flux Isotope Reactor User Guide 2.0.pdf

 

[Astronuc]

an average thermal neutron flux of 2.3 X 10^15 n/cm2 -seconds = 2.3E+21 n/m2-s

2.3 x 10¹⁵ n/cm²-s = 2.3 x 10¹⁹ n/m²-s, (100 cm = 1 m; 10⁴ cm² = 1 m²).

Besides 85 MW is not a lot of power for a propulsion system, and consider the core lifetime is approximately 23 days.

 

[member 697307]

Yes, I messed up squares with cubes, sorry.
As for 85MW, it is the HFIR power, and not the propulsion system's. Then, this is the proof that LSWR does not work with the HFIR neutron density either.