How the Curiosity Rover's Nuclear Battery Works • Hacker News Board

Re: How the Curiosity Rover's Nuclear Battery Works

#71

Post by uvdiv » Tue, Aug 07, 2012, 1:00 AM UTC

rogerchucker wrote: post #42

Burning question... can a miniature version of this ever be ported to consumer electronics devices? :-/

There's a prototype betavoltaic battery which can sustain 50 microwatts on 20 Curies of tritium [1] -- an efficiency of about 2%. Scaling linearly by 10^4, current technology could get 500 milliwatts on 200 kCi, or 20 grams, enough to charge a 5 watt-hour phone battery in 10 hours. The tritium cost would be on the order of $600,000 today [2]. If you could do this, the battery charge time would be a few decades.

[1] http://www.citylabs.net/index.php?option=com_content&vie...

[2] http://fire.pppl.gov/fesac_dp_ts_willms.pdf

Re: How the Curiosity Rover's Nuclear Battery Works

#72

Post by einhverfr » Tue, Aug 07, 2012, 1:01 AM UTC

ilamont wrote: post #37

What are the safety implications of launching 10 pounds of plutonium-238 on a rocket that could malfunction or explode before gaining escape velocity? The article says the plutonium would not explode, but what about plutonium particles or radiation entering the atmosphere and ocean?

Not significant. iirc, NASA's nuclear batteries are designed to survive re-entry and the resulting collision, meaning you aren't going to have to deal with vaporized plutonium in the air, and cleanup in the event of an accident wouldn't be too bad.

We are not talking about nuclear reactors here. Only basic nuclear decay. No criticality. The key thing is the packaging, to make sure you aren't contaminating an area when something goes wrong.

Re: How the Curiosity Rover's Nuclear Battery Works

#73

Post by pjscott » Tue, Aug 07, 2012, 1:31 AM UTC

bigiain wrote: post #64

ilamont wrote: post #37

What are the safety implications of launching 10 pounds of plutonium-238 on a rocket that could malfunction or explode before gaining escape velocity? The article says the plutonium would not explode, but what about plutonium particles or radiation entering the atmosphere and ocean?

You know coal typically contains 1-4ppm of both Uranium and Thorium, right? http://pubs.usgs.gov/fs/1997/fs163-97/FS-163-97.html And that the US burns around 1 billion tons of coal a year? https://en.wikipedia.org/wiki/Coal So coal fired powerstations send something like a few thousand tons of Uranium and Thorium into the environment every year (he said, blithely simplifying ppm into "proportion by weight"). Still wo…

You're right that the amount of radioactive material is too small to warrant serious worry, but please distinguish between isotopes; Plutonium-238, the isotope used in the rover, is a very powerful alpha emitter. You definitely don't want to inhale more than a trace amount of it. (On the plus side, a piece of paper can shield you against alpha particles, which makes this one of the easier-to-handle isotopes -- a good choice for radiothermal generators like this.)

Re: How the Curiosity Rover's Nuclear Battery Works

#74

Post by kondro » Tue, Aug 07, 2012, 1:31 AM UTC

nnnnnnnn wrote: post #27

mbreese wrote: post #21

Earlier quoted context omitted.

That's a good point... while in theory is probably could drive at night, it probably isn't a good idea to have a rover on a different planet driving around in the dark.

Is radio communication with the rover even possible at night? We only have line of sight to mars during the martian daytime.

It's also not possible to directly-drive the Rover anyway, with latency varying between 4 and 20 minutes depending on Mars/Earth orbit.

Curiosity drives itself.

Re: How the Curiosity Rover's Nuclear Battery Works

#75

Post by pjscott » Tue, Aug 07, 2012, 1:39 AM UTC

joshAg wrote: post #67

ktizo wrote: post #65

So, when can I get a nuclear car? And what's the insurance gonna cost?

when we start building "breeder" reactors again. Plutonium is a byproduct of the breeder reactors when they create the type of uranium that can be used for nuclear bombs (this is one of the reasons that breeder reactors aren't popular). I was talking with one of the scientists at Ames last night, and apparently this is pretty much the last of NASA supply, since the US has significantly cut down on their production of…

That's not really accurate, but rather than explaining why, I'll just give the abbreviated rundown on plutonium:

Plutonium 238 is a powerful alpha emitter with a half-life of 87.7 years, making it a great element for powering mars rovers. It's produced by exposing Neptunium 237 to neutron flux. You can get Np-237 out of nuclear waste from ordinary reactors. The US has mostly been buying Pu-238 from Russia, but we're running out, and starting up our own production again is kind of expensive. We can do it, though.

Plutonium 239 is the kind that gets used in bombs. It's fissile. It's produced by exposing Uranium 238 ("depleted uranium") to neutron flux in a nuclear reactor. It's tricky to make weapons-grade Pu-239, because it tends to be contaminated with Pu-240.

Plutonium 240 is annoying and nobody likes it.

Re: How the Curiosity Rover's Nuclear Battery Works

#76

Post by yk » Tue, Aug 07, 2012, 1:41 AM UTC

saraid216 wrote: post #41

brudgers wrote: post #40

Earlier quoted context omitted.

My understanding of the problem is that nuclear sources tend to be nasty stuff and not the sort of thing that one really wants lay people responsible for discarding.

Looking through the Wikipedia page on RTGs, there is a noted risk classified by the US-DOD that Beta-M RTGs could be used as dirty bombs, which is about what I expected. =/ I wonder if this risk extends to other RTGs in the wrong hands? I imagine so, but I honestly don't know.

Basically a RTG converts nuclear decays into power by absorbing the nuclear decay products, generating heat. This is quite similar to the process of radiation poisoning, where chemical bonds are destroyed by the decay products. So for a rather handwaving estimate, we can assume that the processes are the same.

The lethal dose of radiation is about 3-5 Sv, that is 3 to 5 J/ kg absorbed radiation (times some factors, which describe how the radiation is absorbed and which type of radiation). Therefore assuming full body irradiation and gamma particles, the lethal dose for an adult would be received in a matter of minutes ( 60kg irradiated by 1 W would be 1 Sv/minute). Note however, that a more realistic scenario for a dirty bomb from a RTG involves ingestion of alpha emitting particles for a rather large number of bystanders. ( And therefore the effects vary a lot by the details of the bomb and the radio isotopes used. )

So already with the radioactive material from a rather small RTG you can build a quite potent dirty bomb, rather independent of the exact type of the RTG used.

Re: How the Curiosity Rover's Nuclear Battery Works

#77

Post by Wingman4l7 » Tue, Aug 07, 2012, 1:55 AM UTC

DeepDuh wrote: post #69

Wingman4l7 wrote: post #66

Calling it a "battery" is disingenuous. An RTG doesn't fit the typical definition of a battery -- it doesn't store electrical energy. A better name would be "power supply".

Well it does - it stores nuclear binding energy that's unstable enough to be extracted and stable enough to last for x years. It's a nuclear battery just like convential ones are chemical batteries.

You're not clipping leads to the ends of it and getting current, though -- you're using thermocouples. It's more of a "heat battery", I suppose.

Re: How the Curiosity Rover's Nuclear Battery Works

#78

Post by msisk6 » Tue, Aug 07, 2012, 1:58 AM UTC

ilamont wrote: post #37

What are the safety implications of launching 10 pounds of plutonium-238 on a rocket that could malfunction or explode before gaining escape velocity? The article says the plutonium would not explode, but what about plutonium particles or radiation entering the atmosphere and ocean?

Well, there's been a few accidents involving these things, including Apollo 13. They're designed to survive reentry. http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_gen...

Re: How the Curiosity Rover's Nuclear Battery Works

#79

Post by msisk6 » Tue, Aug 07, 2012, 2:02 AM UTC

joaorj wrote: post #13

this article is wrong on many things. one being that the rover can drive during night. the small power output of the "nuclear battery" is not used to drive the rover. it used to recharge batteries during day and night so that the rover can operate on the batteries during the day.

They talked about this during a press conference today. Theoretically it could operate at night, but the power draw to heat the motors and joints would be excessive since it's so cold after sunset. So they don't and allow the MMRTG to charge the battery (and keep the heater for the electronics) while most of the machine is idle.

Re: How the Curiosity Rover's Nuclear Battery Works

#80

Post by yk » Tue, Aug 07, 2012, 2:17 AM UTC

jerf wrote: post #52

ilamont wrote: post #37

What are the safety implications of launching 10 pounds of plutonium-238 on a rocket that could malfunction or explode before gaining escape velocity? The article says the plutonium would not explode, but what about plutonium particles or radiation entering the atmosphere and ocean?

The same safety implications as launching 10 pounds of anything twenty miles in the air, over an ocean, and subjecting it to an explosion. 10 pounds divided by any realistic footprint is negligible. Plutonium is dangerous stuff, sure, but it isn't 10 orders of magnitude more dangerous than anything else the way some people act like it is. It's just dangerous, not imbued with an evil malevolent spirit that wants to ir…

Per wikipedia[1]:

  A commonly cited quote by Ralph Nader, states that a pound of plutonium dust spread into the atmosphere would be enough to kill 8 billion people. However, the math shows that one pound of plutonium could kill no more than 2 million people by inhalation.

So no need to worry.

[1]https://en.wikipedia.org/wiki/Plutonium#Toxicity