Clean lithium fission saltwater rocket

[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/

 

[Yaroslav]

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.

 

[Yaroslav]

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.

 

[Yaroslav]

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.

 

[Yaroslav]

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.