Is it possible to construct a functioning miniaturized nuclear reactor?

[John Harmonic]

Or does a nuclear reactor require too much power for it to be miniaturized. I was told that they use miniature nuclear reactors in US submarines.

[StringJunky]

A nuclear reactor is basically hot metal with some coolant taking the heat away. A lot of the bulk is with materials and equipment to contain and use the nuclear material safely.

[OldChemE]

Depends on how small you mean by miniature.  The reactor in the research submarine NR-1 is about 6 ft in diameter and not a lot taller.  But then you need the control rod drives, recirc pumps and, of course, some way to use the power (traditionally a heat exchanger and steam turbine).

[John Cuthber]

You can't make one with less than a critical mass of fuel.

[Kevin Mulisa Isaya]

4 hours ago, John Harmonic said:

Or does a nuclear reactor require too much power for it to be miniaturized. I was told that they use miniature nuclear reactors in US submarines.

A nuclear reactor is basically hot metal with some coolant taking the heat away. A lot of the bulk is with materials and equipment to contain and use the nuclear material safely I agree with stinky

[Janus]

14 hours ago, John Harmonic said:

Or does a nuclear reactor require too much power for it to be miniaturized. I was told that they use miniature nuclear reactors in US submarines.

With a typical nuclear power plant, the fuel has been enriched to 3.5% - 5%

To make a reactor as small as the ones they use in submarines, they have to enrich the fuel to as high as 90%, which is weapons grade.  So you can make relatively small reactors as long as you are willing to produce and deal with such high grade fuel.

You still have to have enough fuel to maintain the reaction while keeping it spread out enough to allow you to control the reaction with control rods. So even with high grade fuel, there would be a minimum size for a reactor.

 

 

[John Cuthber]

If you don't have a critical mass, then you don't have a reactor.

If you suddenly assemble much more than a critical mass, you have a reactor- briefly.

 

So there's a pretty stringent lower limit to the size of a reactor.

However, with the right "fuel" that mass is under an ounce.

https://www.euronuclear.org/info/encyclopedia/criticalmass.htm

[Eise]

On 9/25/2018 at 3:24 AM, John Harmonic said:

Or does a nuclear reactor require too much power for it to be miniaturized. I was told that they use miniature nuclear reactors in US submarines.

Depends. Some space probes have a very small plutonium reactor (e.g. Voyager), but they are not based on nuclear fission, but the natural decay of plutonium.  Plutonium-238 is so highly radioactive that the heat of this decay is enough to produce electricity with it.

Then there are other types of fission reactors, e.g. aqueous homogeneous reactors, that are pretty small:

But they also work just with highly enriched Uranium-235.

In the article the KEMA Suspension Test Reactor is also mentioned ("the reactor consisted of a reactor vessel (ø310 mm, content 18.3 liter"):

My father worked at KEMA (but he was not associated with the reactor laboratory). You brought back a few melancholic associations with your question...

And last but not least, there are experiments in Germany with nuclear power for smart phones. Sorry, I only found a German video of its promo, but I think you can understand what it is about, just looking at the video. It is worth it!

https://www.youtube.com/watch?v=9-nezImUP0w

 

Edited September 26, 2018 by Eise

[Carrock]

1 hour ago, Eise said:

And last but not least, there are experiments in Germany with nuclear power for smart phones. Sorry, I only found a German video of its promo, but I think you can understand what it is about, just looking at the video. It is worth it!

https://www.youtube.com/watch?v=9-nezImUP0w

 

At last, an offsite youtube that's worth watching!

[Strange]

25 minutes ago, Carrock said:

At last, an offsite youtube that's worth watching!

The toast function had me laughing out loud!

[Eise]

2 hours ago, Strange said:

The toast function had me laughing out loud!

You spoiled my joke! 

[Strange]

1 hour ago, Eise said:

You spoiled my joke! 

Sooory! But it’s still brilliant!

[Janus]

6 hours ago, Eise said:

Depends. Some space probes have a very small plutonium reactor (e.g. Voyager), but they are not based on nuclear fission, but the natural decay of plutonium.  Plutonium-238 is so highly radioactive that the heat of this decay is enough to produce electricity with it.

 

Technically, I would be hesitant to call a radioisotope thermoelectric generator a reactor. As you noted, it depends on heat of decay rather than a fission reaction.

The main concerns for choosing an isotope in this case is its half-life and, in the case of spacecraft, it's power to density ratio. This, in turn, can depend on the mode of decay. Alpha decay is more energetic than beta decay.   Thus Plutonium-238 is a good choice for a space probe, while Strontium-90 would work in cases where you aren't as concerned with the mass. (The former Soviet Union used Sr-90 RTGs to power some remote lighthouses and navigation beacons)

[mistermack]

Would it be possible to have a core that is well below critical mass, that you stimulate using neutrons from a neutron generator? So that there is no possibility at all of an accidental explosion and the overall size is smaller?

[swansont]

1 hour ago, mistermack said:

Would it be possible to have a core that is well below critical mass, that you stimulate using neutrons from a neutron generator? So that there is no possibility at all of an accidental explosion and the overall size is smaller?

Depends on how many neutrons your neutron generator gives you. Realistically, this probably isn't viable, since the number you might reasonably be able to add means the reactor would be close to criticality in the first place. Unless you aren't generating much power, meaning your neutron density could be small.

On 9/24/2018 at 9:30 PM, StringJunky said:

A nuclear reactor is basically hot metal with some coolant taking the heat away. A lot of the bulk is with materials and equipment to contain and use the nuclear material safely.

Since it's probably using thermal neutrons, you have to add a moderator to this bulk. Though the coolant and moderator can be the same material (e.g. water)

[mistermack]

1 minute ago, swansont said:

Depends on how many neutrons your neutron generator gives you. Realistically, this probably isn't viable, since the number you might reasonably be able to add means the reactor would be close to criticality in the first place.

Wouldn't the neutrons from the generator cause more neutrons to be produced, magnifying the effect of the neutron beam? 

(although I agree that it probably isn't viable, just on the basis that it would have already been done) 

[StringJunky]

6 minutes ago, swansont said:

Since it's probably using thermal neutrons, you have to add a moderator to this bulk. Though the coolant and moderator can be the same material (e.g. water)

Yeah, I was being absolutely simplistic. What are thermal neutrons; are there other types? I presume it's how they are generated that defines their type..

Edited September 26, 2018 by StringJunky

[swansont]

4 hours ago, StringJunky said:

Yeah, I was being absolutely simplistic. What are thermal neutrons; are there other types? I presume it's how they are generated that defines their type..

Thermal means in thermal equilibrium, so they have the same temperature as the reactor. Generally speaking, this means they have much less energy than when they were generated, which is typically in the MeV range, which are called fast neutrons. Thermal energy would be less than an eV.

 

4 hours ago, mistermack said:

Wouldn't the neutrons from the generator cause more neutrons to be produced, magnifying the effect of the neutron beam? 

(although I agree that it probably isn't viable, just on the basis that it would have already been done) 

Yes, but if the system is subcritical, on average less than one neutron per fission induces another fission. The external source is making up that deficit. 

[mistermack]

14 hours ago, swansont said:

Yes, but if the system is subcritical, on average less than one neutron per fission induces another fission. The external source is making up that deficit. 

So that would mean that the reaction would gradually die away, without the input from the neutron generator. That would make it inherently safe and controllable. The big question would be how powerful a neutron generator could be included, and that would influence how close to a critical mass you would need in the reactor. But I guess that if it's below critical, then it's not possible for it to go to a runaway chain reaction, so close to critical would not be inherently any more dangerous than half-critical mass.

I suppose the danger of a melt-down would be higher though, but the reaction would be dying down at the same time.

[swansont]

1 hour ago, mistermack said:

So that would mean that the reaction would gradually die away, without the input from the neutron generator. That would make it inherently safe and controllable. The big question would be how powerful a neutron generator could be included, and that would influence how close to a critical mass you would need in the reactor. But I guess that if it's below critical, then it's not possible for it to go to a runaway chain reaction, so close to critical would not be inherently any more dangerous than half-critical mass.

I suppose the danger of a melt-down would be higher though, but the reaction would be dying down at the same time.

"Below criticality" is something that does not just depend on the neutron sources. It is also e.g. temperature dependent. You may have a system that ceases to be critical while at operating temperature, but after the temperature has dropped, and the water gets denser (and water is your moderator), the system could become critical again (or worse, prompt critical). The danger here is that the fission rate can increase dramatically without the temperature changing appreciably, and by the time the system starts to heat up, the power overshoot is huge — part of the fuel melts, and/or you have a steam explosion, before this "inherent safety" shuts it back down.

 

[mistermack]

I was surprised to find that the smallest reactors are not the ones in submarines, or destroyers, but a few land-based ones. Smallest in terms of output anyway. The ships obviously would be keeping peak performance in reserve, to extend the working life, as they wouldn't need a huge output 24/7. 

There's a lot of nuclear powered ships about. It makes you wonder what sort of environmental damage would happen in a war, when presumably those ships would be targeted just like all the others. It's not a pleasant thought.

[swansont]

12 hours ago, mistermack said:

I was surprised to find that the smallest reactors are not the ones in submarines, or destroyers, but a few land-based ones. Smallest in terms of output anyway.

There are university research reactors whose output is on the order of watts. Air cooled, and possibly moderated by paraffin

12 hours ago, mistermack said:

The ships obviously would be keeping peak performance in reserve, to extend the working life, as they wouldn't need a huge output 24/7. 

Big enough to provide power for systems and the needs of ~100 people, 24/7. Several MW of power just for that. But you're right, the reserve is mostly for being able to move at top speed, whatever that happens to be.