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crosscountry
2008-Jan-27, 06:10 PM
How can we power small things with nuclear power?

Isn't most energy we get from heat that boils water and then turns turbines just like any other power plant?


Even submarines have power plants.


How can we get electrical energy from nuclear material?

trinitree88
2008-Jan-27, 06:17 PM
How can we power small things with nuclear power?

Isn't most energy we get from heat that boils water and then turns turbines just like any other power plant?


Even submarines have power plants.


How can we get electrical energy from nuclear material?

Hi Cross. It isn't practical to get too small with a nuke. I think the experimental Phoebus 6B reactor was ~ the size of an office desk for several megawatts. Thr Russians made thermoelectric nukes that used the temperature difference of the ends of a bimetallic strip to power satellites. One landed in ~ North Conway, NH in a farm pond. :shifty: pete

Noclevername
2008-Jan-27, 06:18 PM
http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator

crosscountry
2008-Jan-27, 06:19 PM
I've heard of using a reverse thermocouple. Is that what you're talking about?

alainprice
2008-Jan-27, 06:24 PM
Do you mean to say a Pelletier junction(or device)?

crosscountry
2008-Jan-27, 06:34 PM
I'm asking you.

Ronald Brak
2008-Jan-27, 06:38 PM
Except in certain special cases such as space probes it is much cheaper and easier and safer to use something else rather than an RTG. This isn't likely to change anytime soon.

antoniseb
2008-Jan-27, 07:16 PM
How can we power small things with nuclear power?
There is something called a Stirling Cycle engine which is more efficient than the thermocouples. These can be used with RTGs or any other heat source, such as a fission reactor.

Thermocouples and RTGs are nice because they have no moving parts, and no chance of doing anything especially explosive. The worst case would be for the device to burn up in atmospheric reentry, because Plutonium is chemically very poisonous.

On a slightly larger scale, and with moving parts, you could use various methods on a small fission reactor (the Prometheus Platform would be something along these lines). The reactor could be a heat source for Stirling Cycle engines, or perhaps something that would generate energy in some form other than electric potential.

alainprice
2008-Jan-27, 07:19 PM
I'm asking you.

You're asking me what you meant to say?

A Pelletier device is a junction that uses a temperature difference to establish a voltage and create a current. Likewise, a current through the device will setup a temperature difference with a hot and cold side.

It can be used to create current from a temperature diffence, but you still need to setup a temperature differential.

JustAFriend
2008-Jan-27, 09:51 PM
The moon buggys and nearly every deep-space probe is powered by small nuclear-heat powered thermocouple generators.

The US military and the Russians have used them for 50years to power remote sensors and small stations (especially in the polar bases.)

Look around, there's plenty of info on them.

crosscountry
2008-Jan-27, 10:00 PM
I thought there were bans on sending nuclear powered instruments into space.

Van Rijn
2008-Jan-27, 10:24 PM
I thought there were bans on sending nuclear powered instruments into space.

Not at all. There is a treaty against nuclear weapons testing in space, however.

Here's another way to get power from nuclear material. The tritium based beta battery:

http://en.wikipedia.org/wiki/Atomic_battery#Betavoltaics

Nowhere Man
2008-Jan-28, 04:23 AM
The lunar rovers used batteries. Some of the ALSEP dinguses used radioisotopes. I don't know what the Lunokhods used.

Fred

neilzero
2008-Jan-28, 12:44 PM
There have been some recent claims of much improved pn juctions to convert low grade heat to electriciy. If these reach the market we may have better RTGs = radio-active thermal generators? The plutonium isotope typical used for RTGs is comparitively safe. I presume there are other isotopes, which would make RTGs smaller and lighter per watt, but far more dangerous. Neil

Ronald Brak
2008-Jan-28, 12:55 PM
This article on silicon thermoelectric stuff may be of interest.

http://www.lbl.gov/Science-Articles/Archive/MSD-silicon-nanowires.html

But it certainly could have been written better. How about a simple efficiency percentage for those of us who have been remiss in our thermoelectric materials study?

antoniseb
2008-Jan-28, 01:14 PM
There have been some recent claims of much improved pn juctions to convert low grade heat to electriciy. If these reach the market we may have better RTGs = radio-active thermal generators? The plutonium isotope typical used for RTGs is comparitively safe. I presume there are other isotopes, which would make RTGs smaller and lighter per watt, but far more dangerous. Neil

There aren't too many choices that are better than Plutonium... though one big price factor is that Plutonium isotopes are available from the isotope separation process to make nuclear weapons.

Gadolinium 148 -> Samarium 144 gives more heat per unit mass, and has only a slightly shorter half life... AND neither element is especially toxic. The problem is that it is pretty expensive to make enough quantity of the Gd.

joema
2008-Jan-28, 01:28 PM
I thought there were bans on sending nuclear powered instruments into space.
Not only are there no bans on nuclear powered devices in space, full-fledged nuclear fission reactors have been launched many times.

Russia launched over 30 spacecraft with fission reactors, the U.S. just one (SNAP-10A).

http://en.wikipedia.org/wiki/SNAP-10A
http://www.uic.com.au/nip82.htm

NASA planned on using a nuclear reactor on the Jupiter Icy Moons Orbiter, part of Project Prometheus. It has since lost funding:

http://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Orbiter
http://en.wikipedia.org/wiki/Project_Prometheus

The upcoming Mars Science Laboratory (a large rover) will likely be powered by a nuclear RTG: http://en.wikipedia.org/wiki/Mars_Science_Laboratory

crosscountry
2008-Jan-28, 02:40 PM
cool. I'm learning every day.

Ilya
2008-Jan-28, 02:58 PM
There have been some recent claims of much improved pn juctions to convert low grade heat to electriciy. If these reach the market we may have better RTGs = radio-active thermal generators? The plutonium isotope typical used for RTGs is comparitively safe. I presume there are other isotopes, which would make RTGs smaller and lighter per watt, but far more dangerous. Neil

Very few isotopes are practical for RTG's. The longer the half-life, the less power output, and vice versa. Half-life of Pu-238 is 88 years. That means its power output will not drop appreciably over the lifetime of a typical space probe, even including the time plutonium sample sits in a lab before being installed into an RTG.

A much shorter half-life means RTG will lose power that much faster. Half-life of Strontium-90 is 29 years -- and that much time can easily pass from strontium being made and an outer planet probe reaching its target. Not good. Whereas half-life more than 100 years or so means such feeble power output per kilogram, RTG becomes too heavy to fly. 50-80 years half-life is ideal; and that does not leave many choices (http://ie.lbl.gov/toi/nucSearch.asp). Moreover, alpha-decay is required (anything else needs too much shielding), which means Gd-148 and U-232 are really the only options besides Pu-238. Uranium-232 ought to be a better choice than plutonium because its decay product is also an alpha-emitter with much shorter half-life, so you get twice the energy per kg. I do not know why it is not used -- cost?

kzb
2008-Jan-28, 06:43 PM
Ilya, just a quick thought, U-232 decays to a whole chain of shortlived radionuclides, and they will grow in quite quickly as the first daughter is Th-228, half life 1.9 years. Some of the daughters are hard gamma emitters.

Pu-238 decays to U-234, and then Th-230, both long half-lives so the daughters do not grow in much.

Ilya
2008-Jan-28, 07:47 PM
You are probably right.

TrAI
2008-Jan-28, 10:36 PM
The lunar rovers used batteries. Some of the ALSEP dinguses used radioisotopes. I don't know what the Lunokhods used.

Fred

Hmmm... I beieve the Lunokhod used photovoltaics for power, though they did have a polonium-210 radiothermal heater to help keep the insides warm...