¿How would you design a reciprocating nuclear engine, a nuclear-powered analog of our petrol engine.

The higher the temperature and pressure of an engine is, the higher its thermodynamic efficiency. Nuclear power plants operate at low temperature, and are therefore less thermodynamically efficient than, for example, diesel engines.

Two ideas occur to me. One is to have control rods that oscillate in and out of an otherwise standard-ish fission power plant, producing bursts of high power. Two is to combine fission and fusion by having the temperature and pressure high enough for some of the fusion fuel (eg. lithium) to produce extra bursts of energy.

Assume the existence of a containment stronger and more effective than anything we can make. I’m looking into the far future here.

use a Stirling engine or seebeck effect to make electricity

use a Stirling engine or seebeck effect to make electricity

Another possibility is one that was called “tickling the tail of the dragon” during the original tests leading up to the first A-bomb. Bring two subcritical masses together (eg. one on piston and one on cylinder) to make a critical mass for a fraction of a second. If fast enough then you get a pulse of energy without a full-scale explosion.

mollwollfumble said:

Another possibility is one that was called “tickling the tail of the dragon” during the original tests leading up to the first A-bomb. Bring two subcritical masses together (eg. one on piston and one on cylinder) to make a critical mass for a fraction of a second. If fast enough then you get a pulse of energy without a full-scale explosion.

It doesn’t sound particularly safe

Cymek said:

mollwollfumble said:

Another possibility is one that was called “tickling the tail of the dragon” during the original tests leading up to the first A-bomb. Bring two subcritical masses together (eg. one on piston and one on cylinder) to make a critical mass for a fraction of a second. If fast enough then you get a pulse of energy without a full-scale explosion.

It doesn’t sound particularly safe

They ran the test. It didn’t work, for reasons I’ve never been told, but there was no massive explosion.

mollwollfumble said:

Cymek said:

mollwollfumble said:

Another possibility is one that was called “tickling the tail of the dragon” during the original tests leading up to the first A-bomb. Bring two subcritical masses together (eg. one on piston and one on cylinder) to make a critical mass for a fraction of a second. If fast enough then you get a pulse of energy without a full-scale explosion.

It doesn’t sound particularly safe

They ran the test. It didn’t work, for reasons I’ve never been told, but there was no massive explosion.

Damn, lose lose.

Peak Warming Man said:

mollwollfumble said:

Cymek said:

It doesn’t sound particularly safe

They ran the test. It didn’t work, for reasons I’ve never been told, but there was no massive explosion.

Damn, lose lose.

I thought they did it to measure radiation as the masses approached each other.

Tamb said:

Peak Warming Man said:

mollwollfumble said:

They ran the test. It didn’t work, for reasons I’ve never been told, but there was no massive explosion.

Damn, lose lose.

I thought they did it to measure radiation as the masses approached each other.

Looking it up on the web. It looks like there were two different experiments. The first one, in which Louis Slotin died because his screwdriver slipped, was done by hand to measure the radiation as the masses approached one another, and everything was steady-state.

The one I was thinking of was done later, everything was done automatically from a distance with larger masses, a subcritical mass was dropped on a slider to take it past the second mass. As the two masses passed, the system would go critical for a fraction of a second, but because the time was so brief the calculations showed that there would be no explosion.

mollwollfumble said:

Tamb said:

Peak Warming Man said:

Damn, lose lose.

I thought they did it to measure radiation as the masses approached each other.

Looking it up on the web. It looks like there were two different experiments. The first one, in which Louis Slotin died because his screwdriver slipped, was done by hand to measure the radiation as the masses approached one another, and everything was steady-state.

The one I was thinking of was done later, everything was done automatically from a distance with larger masses, a subcritical mass was dropped on a slider to take it past the second mass. As the two masses passed, the system would go critical for a fraction of a second, but because the time was so brief the calculations showed that there would be no explosion.

Thanks moll. Nice to know the old memory still works a bit.

mollwollfumble said:

Tamb said:

Peak Warming Man said:

Damn, lose lose.

I thought they did it to measure radiation as the masses approached each other.

Looking it up on the web. It looks like there were two different experiments. The first one, in which Louis Slotin died because his screwdriver slipped, was done by hand to measure the radiation as the masses approached one another, and everything was steady-state.

I remember reading about the above, not very good OH & S back then

Cymek said:

mollwollfumble said:

Tamb said:

I thought they did it to measure radiation as the masses approached each other.

Looking it up on the web. It looks like there were two different experiments. The first one, in which Louis Slotin died because his screwdriver slipped, was done by hand to measure the radiation as the masses approached one another, and everything was steady-state.

I remember reading about the above, not very good OH & S back then

My way of remembering that was (& no pun or disrespect intended) Slot in screwdriver

wookiemeister said:

use a Stirling engine or seebeck effect to make electricity

This would work very well.

Cymek said:

mollwollfumble said:

Looking it up on the web. It looks like there were two different experiments. The first one, in which Louis Slotin died because his screwdriver slipped, was done by hand to measure the radiation as the masses approached one another, and everything was steady-state.

I remember reading about the above, not very good OH & S back then

I’m not a huge fan of OH&S, but I personally was shocked by how bad OH&S was for that experiment. I would not approve anyone standing a couple of inches away from a near-critical mass of radiating uranium as Slotin did or in the same room, as his surviving colleagues did.

I think it would be cute if you could make a reciprocating engine powered by a nuclear fuel, especially if it was possible to get some fusion energy as well as fission energy out of it.

I a nuclear slug running at say 100 deg centigrade would be great for something using the see beck effect – it would easily run a Stirling motor

you could probably run a submarine on this principle where the cold of the water creates the temp differential

I a nuclear slug running at say 100 deg centigrade would be great for something using the see beck effect – it would easily run a Stirling motor

you could probably run a submarine on this principle where the cold of the water creates the temp differential

wookiemeister said:

I a nuclear slug running at say 100 deg centigrade would be great for something using the see beck effect – it would easily run a Stirling motor

you could probably run a submarine on this principle where the cold of the water creates the temp differential

Have you tried applying this to drones yet?

wookiemeister said:

I a nuclear slug

You shouldn’t be so hard on yourself.

> running at say 100 deg centigrade

Low temperature = low thermodynamic efficiency. That’s a problem with present nuclear power plants.

The very high temperature nuclear reactor is a type of reactor that can conceptually have an outlet temperature of 1000 °C.

But even that’s really small compared with typical combustion temperatures in car engines of about 2000 °C.

Uranium boils at 4131 °C.

The temperature in a fission bomb can reach more than 1,000,000 °C.

The critical ignition temperature for fusion (at ambient pressure) is about 4.5 × 10^7 °C.

So, what about this? Use a fuel injector into a cylinder that injects small droplets of liquid uranium (or other fissile element) that vapourises inside the cylinder. On compression of the uranium vapour you get a sudden increase in temperature as it approaches critical mass generating heat that drives the expansion of the cylinder. Almost the same as a petrol engine, but using uranium as fuel.

You could get even better compression from ionised gas than from un-ionised vapour.

the problem is the higher the temperature the faster small problems become big problems

a small unit running at a lower temp and lower efficiency isn’t a problem , there’s a heap of radioactive isotopes hanging around

mollwollfumble said:

So, what about this? Use a fuel injector into a cylinder that injects small droplets of liquid uranium (or other fissile element) that vapourises inside the cylinder. On compression of the uranium vapour you get a sudden increase in temperature as it approaches critical mass generating heat that drives the expansion of the cylinder. Almost the same as a petrol engine, but using uranium as fuel.

Sounds a bit like a piston version of the old US nuclear jet engine.

Direct cycle.

And Youtube

any kind of power unit involving nuclear should ideally be very simple , have as few moving parts as possible and be inherently safe eg never exceed a temperature that will mean structural breakdown of components

Spiny Norman said:

mollwollfumble said:

So, what about this? Use a fuel injector into a cylinder that injects small droplets of liquid uranium (or other fissile element) that vapourises inside the cylinder. On compression of the uranium vapour you get a sudden increase in temperature as it approaches critical mass generating heat that drives the expansion of the cylinder. Almost the same as a petrol engine, but using uranium as fuel.

Sounds a bit like a piston version of the old US nuclear jet engine.

Direct cycle.

And Youtube

I love it! It’s far more radical than anything I had in mind. More from link. “The US Aircraft Reactor Experiment (ARE) was a 2.5 MW thermal nuclear reactor experiment designed to attain a high power density for use as an engine in a nuclear-powered bomber. It used the molten fluoride salt NaF-ZrF4-UF4 (53-41-6 mol%) as fuel, was moderated by beryllium oxide (BeO), used liquid sodium as a secondary coolant and had a peak temperature of 860 °C. It operated for a 1000-hour cycle in 1954. It was the first molten salt reactor.”

> the problem is the higher the temperature the faster small problems become big problems. Never exceed a temperature that will mean structural breakdown of components.

Yes. When I was looking earlier into a super-high temperature nuclear reactor design that circulated liquid uranium, I found that corrosion (ie. chemical reactions) at high temperature was a very difficult problem. It was very difficult to get the convection needed for thermal uniformity without that convection exacerbating the corrosion problem. Not necessarily impossible, but far from trivial.

> any kind of power unit involving nuclear should ideally be very simple, have as few moving parts as possible and be inherently safe.

Not long ago I would have agreed with you. But think of the history of engines from the viewpoint of the ancient Romans. Hero’s steam turbine is very simple with a minimum of moving parts. A Wankel engine is relatively simple and has few moving parts. But we don’t use either these days. Instead we use piston engines that are very much more complicated and have many more moving parts.