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NASA completes full-power tests of small, portable nuclear reactor

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Re: NASA completes full-power tests of small, portable nuclear reactor

#61

Post by petra »

philipkglass wrote: post #45

Some background on this: NASA deep space missions have historically used radioisotope thermal generators powered by the decay of the exotic plutonium isotope Pu-238. This isotope has a good balance of lifetime (87.7 year half life) and specific energy (0.5 watts/gram). It is non-fissile -- no risk of criticality. It also decays solely by alpha emission, so there are no problems with shielding the rest of the systems…

The major fear that comes up with these is: what if the missile blows up while still in earth? How are these risks managed?

Re: NASA completes full-power tests of small, portable nuclear reactor

#62

Post by rbanffy »

asteli wrote: post #40
linuxguy2 wrote: post #32

Earlier quoted context omitted.

Isn't it actually designed for working on Mars? Convection in the sodium heat pipes wouldn't work in zero G. > generate heat that is carried to the Stirling converters via passive sodium heat pipes. https://en.wikipedia.org/wiki/Kilopower

The wiki page you linked specifically mentions deep space applications as a design goal. I'm not sure about the sodium heat pipes but the alcohol vapor heat pipes you see in electronics typically use an internal wick to return the working fluid.

Worst case scenario is you put the reactor next to the toilet, in the rotating part of the spacecraft, to solve the problem with convection...

These two things are enormously simplified when there is gravity.

Re: NASA completes full-power tests of small, portable nuclear reactor

#63

Post by philipkglass »

petra wrote: post #61
philipkglass wrote: post #45

Some background on this: NASA deep space missions have historically used radioisotope thermal generators powered by the decay of the exotic plutonium isotope Pu-238. This isotope has a good balance of lifetime (87.7 year half life) and specific energy (0.5 watts/gram). It is non-fissile -- no risk of criticality. It also decays solely by alpha emission, so there are no problems with shielding the rest of the systems…

The major fear that comes up with these is: what if the missile blows up while still in earth? How are these risks managed?

The Kilopower reactor presents far lower radiological risks than RTGs in the event of complete disassembly within Earth's environment.

Pu-238 has a specific activity of 634 billion Bq/g, nearly 8 million times that of the Kilopower reactor's U-235 fuel (80 thousand Bq/g). Once the reactor attains criticality, it produces fission products that have even higher specific activity than Pu-238. But the reactor can remain safely inactive until the risky launch phase is over. There is no way to inactivate the decay of Pu-238 for the launch phase.

The risks of RTG launch are handled by using designs with high mechanical/thermal robustness to encapsulate the plutonium ceramic. I think that they were already safe enough. But the Kilopower reactor is inherently low-radiotoxicity before criticality, which makes it safer yet during the launch phase.

Re: NASA completes full-power tests of small, portable nuclear reactor

#64

Post by jfrankamp »

linuxguy2 wrote: post #32
zaroth wrote: post #10

Earlier quoted context omitted.

That is in fact precisely what it is designed for.

Isn't it actually designed for working on Mars? Convection in the sodium heat pipes wouldn't work in zero G. > generate heat that is carried to the Stirling converters via passive sodium heat pipes. https://en.wikipedia.org/wiki/Kilopower

Would you mind backing up the assertion about convection being the mechanism of sodium transport? e.g. on earth are you saying the sterling part wouldn't work if the hot fluid had to traverse say down (a gravity gradient) instead of up?

Re: NASA completes full-power tests of small, portable nuclear reactor

#65

Post by rbanffy »

linuxguy2 wrote: post #32
zaroth wrote: post #10

Earlier quoted context omitted.

That is in fact precisely what it is designed for.

Isn't it actually designed for working on Mars? Convection in the sodium heat pipes wouldn't work in zero G. > generate heat that is carried to the Stirling converters via passive sodium heat pipes. https://en.wikipedia.org/wiki/Kilopower

The Stirling converters will need a heatsink which can connect to an actively circulated coolant even in zero g.

Thanks for the article.

Re: NASA completes full-power tests of small, portable nuclear reactor

#67

Post by mcguire »

petra wrote: post #61
philipkglass wrote: post #45

Some background on this: NASA deep space missions have historically used radioisotope thermal generators powered by the decay of the exotic plutonium isotope Pu-238. This isotope has a good balance of lifetime (87.7 year half life) and specific energy (0.5 watts/gram). It is non-fissile -- no risk of criticality. It also decays solely by alpha emission, so there are no problems with shielding the rest of the systems…

The major fear that comes up with these is: what if the missile blows up while still in earth? How are these risks managed?

Presumably, the same way Pu-238 RTGs manage the risks.

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

Re: NASA completes full-power tests of small, portable nuclear reactor

#68

Post by craftyguy »

killjoywashere wrote: post #20
craftyguy wrote: post #17

Earlier quoted context omitted.

Regardless of whether or not what you say is likely to happen (I think not, but anyways), if humans can even last long enough to see the Sun as a red giant, we'll have larger problems on our hands. In any case, we'll likely have destroyed ourselves or evolved into something else by that time.

The Moon orbits the Sun, and is only mildly perturbed by the Earth. Gas drag would decay the Moon's orbit, but it's not at all clear to me it would crash into the Earth. It seems far more likely it would simply fall into the Sun.

The Earth and Moon system orbit the Sun, but the Moon is very much in orbit around the Earth.

A more likely scenario is that the Earth's orbit will move outwards as the Sun expands, since stars in the red giant phase(s) lose a lot of mass. It's plausible that the Earth/Moon will not be destroyed, physically. But in that case all life would have been obliterated due to the immense temperatures associated with having the surface of the Sun within a few million miles (or less) of Earth.

Re: NASA completes full-power tests of small, portable nuclear reactor

#69

Post by mcguire »

netzone wrote: post #4

What would the effects be of releasing nuclear waste in space? Could we sent up all of our nuclear waste, put a rocket on it and just let it disappear into the distance?

"All told, the nuclear reactors in the U.S. produce more than 2,000 metric tons of radioactive waste a year..." (https://www.scientificamerican.com/article/nuclear-waste-let...)

Let's call it $20,000 / lb to orbit (http://www.businessinsider.com/spacex-rocket-cargo-price-by-...) and say double that for the "disappear into the distance" part.

That gives us $160B to loft the US's nuclear waste for one year into not-my-problem-ness.

That doesn't sound unreasonable, right?

P.S. Don't drop it.

Re: NASA completes full-power tests of small, portable nuclear reactor

#70

Post by rmah »

Thorazul wrote: post #34

They should team up with Tesla to develop storage batteries for the reactors in case a reactor fails you would still have some temporary emergency power.

Tesla doesn't make the batteries, Panasonic, Samsung and Hitachi does. IIRC, Panasonic is the firm building batteries at Tesla's new gigafactory facility. Tesla simply slaps their name on the battery packs.
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