6 ms·

Almost any time nuclear is mentioned, I find this link to be relevant: https://en.wikipedia.org/wiki/Nuclear_lightbulb https://en.wikipedia.org/wiki/Nuclear_lig

Almost any time nuclear is mentioned, I find this link to be relevant: https://en.wikipedia.org/wiki/Nuclear_lightbulb https://en.wikipedia.org/wiki/Nuclear_lightbulb The thing is, even if you got the system small enough (lightbulb reactor or otherwise), you'd have one hell of a time with cooling. It works with rockets because they need to dump massive amounts of heat into propellant and generate a lot more thrust with it but airplanes fly slower and generate force through lift. If the money were there though, using something like the lightbulb for synthetic jet fuel could massively change their environmental impact.

chrisseaton 9y ago
Why do you need to cool the reactor? I thought the whole point was the reactor was hot - that was how you got useful energy from it. Doesn't using the heat energy dissipate it?
- corndoge 9y ago
  You have to keep it cool enough to sustain a chain reaction in order to prevent runaway, what the media refers to as a meltdown
  - deleted 9y ago
    [deleted]
  - deleted 9y ago
    [deleted]
- nine_k 9y ago
  Also, if you care about mechanical stability (an engine, not a bomb), you either cool the reactor down to temperatures of its containment material (all attainable solid materials are limited to a few thousand K), or use something like magnetic containment, which is not always an option (you need everything inside electrically charged, and a low neutron flux).
ridgeguy 9y ago
The nuclear lightbulb is new to me, thanks for the link. Has anybody thought through the engineering of those silica reaction vessels (I haven't searched)? I don't think it can work. If I understand the concept shown in the Wikipedia page, it depends on uranium hexafluoride operating at 25,000°C in fused silica containers which transmit hard UV that heats hydrogen external to the silica containers and is exhausted through nozzles, propelling the vehicle. Silica has very poor thermal conductivity, which means it can't be cooled readily by conduction. This means that the inner container wall will melt even if the outer wall coolant is liquid hydrogen because the wall material can't transport the heat flux. One might be able to keep a 10µm silica shell from melting, but nothing thicker. Too thin to be mechanically sound. Also, the uranium hex will be completely dissociated and ionized at 25K°C. This means the inner silica wall will be exposed to atomic fluorine, and will corrode away immediately. Uranium at 25k°C probably reacts with SiO2, but I don't know that. Uranium silicides and oxides are known compounds, so it's likely. It's a horrific corrosion environment. At temperatures like this, everything reacts with everything else. In short, this nuclear lightbulb can't be built with any material we currently know of. We'll have to wait for Scotty's transparent aluminum. Perhaps the MIT drone could use a Stirling engine with a Pu238 RTG heat source and air as the working fluid. If so, it could stay up forever. I'd bet on that before the nuclear light bulb.
- walrus01 9y ago
  > Perhaps the MIT drone could use a Stirling engine with a Pu238 RTG heat source and air as the working fluid. If so, it could stay up forever. Radioisotope thermoelectric generators only output something like 250W up to a max of 600W. Not nearly enough for sustained flight of something this size. Also, if you think the licensing and permitting process is hard for a regular drone, take a look at the amount of paperwork and security related to all of the RTG powered spacecraft/probes launched in the last 30 years. Pu238 is, to put it mildly, a somewhat controlled substance.
  - leot 9y ago
    > up to a max of 600W. Not nearly enough for sustained flight of something this size. Which size? Why? Human powered airplanes exist. Max sustained human output is around 400W. Are RTGs significantly heavier than humans?

chrisseaton 9y ago

Why do you need to cool the reactor? I thought the whole point was the reactor was hot - that was how you got useful energy from it. Doesn't using the heat energy dissipate it?

corndoge 9y ago
You have to keep it cool enough to sustain a chain reaction in order to prevent runaway, what the media refers to as a meltdown
- deleted 9y ago
  [deleted]
- deleted 9y ago
  [deleted]
nine_k 9y ago
Also, if you care about mechanical stability (an engine, not a bomb), you either cool the reactor down to temperatures of its containment material (all attainable solid materials are limited to a few thousand K), or use something like magnetic containment, which is not always an option (you need everything inside electrically charged, and a low neutron flux).

ridgeguy 9y ago

The nuclear lightbulb is new to me, thanks for the link. Has anybody thought through the engineering of those silica reaction vessels (I haven't searched)? I don't think it can work. If I understand the concept shown in the Wikipedia page, it depends on uranium hexafluoride operating at 25,000°C in fused silica containers which transmit hard UV that heats hydrogen external to the silica containers and is exhausted through nozzles, propelling the vehicle. Silica has very poor thermal conductivity, which means it can't be cooled readily by conduction. This means that the inner container wall will melt even if the outer wall coolant is liquid hydrogen because the wall material can't transport the heat flux. One might be able to keep a 10µm silica shell from melting, but nothing thicker. Too thin to be mechanically sound. Also, the uranium hex will be completely dissociated and ionized at 25K°C. This means the inner silica wall will be exposed to atomic fluorine, and will corrode away immediately. Uranium at 25k°C probably reacts with SiO2, but I don't know that. Uranium silicides and oxides are known compounds, so it's likely. It's a horrific corrosion environment. At temperatures like this, everything reacts with everything else. In short, this nuclear lightbulb can't be built with any material we currently know of. We'll have to wait for Scotty's transparent aluminum. Perhaps the MIT drone could use a Stirling engine with a Pu238 RTG heat source and air as the working fluid. If so, it could stay up forever. I'd bet on that before the nuclear light bulb.

walrus01 9y ago
> Perhaps the MIT drone could use a Stirling engine with a Pu238 RTG heat source and air as the working fluid. If so, it could stay up forever. Radioisotope thermoelectric generators only output something like 250W up to a max of 600W. Not nearly enough for sustained flight of something this size. Also, if you think the licensing and permitting process is hard for a regular drone, take a look at the amount of paperwork and security related to all of the RTG powered spacecraft/probes launched in the last 30 years. Pu238 is, to put it mildly, a somewhat controlled substance.
- leot 9y ago
  > up to a max of 600W. Not nearly enough for sustained flight of something this size. Which size? Why? Human powered airplanes exist. Max sustained human output is around 400W. Are RTGs significantly heavier than humans?